B.E. (Civil Engineering) Third Semester (C.B.S.) A Notebook Of rd B.E 3 Semester C.B.S [Civil Engineering] Subject:- Concrete Technology Name:Class:Section:Roll No.:College:- Page | 1 Concrete Technology By SHAH SIR (7888280720) B.E. (Civil Engineering) Third Semester (C.B.S.) SHAH ENGINEERING CLASSES (A Classroom of Poly/B.E/B.Tech/M.Tech/GATE/SSC/RRB/MPSC) ( ALL BRANCHES/ ALL SEMESTERS/ ALL SUBJECTS ) CIVIL ENGINEERING By SHAH SIR Weekly Classes Available Venue-1 Venue-2 SHAH ENGINEERING CLASSES (63,Cosmos Town, Trimurtinagar,NearHingna T-Point,Nagpur) Contact No.:- +91 7888280720 (SHAH SIR) SHAH ENGINEERING CLASSES (Main Square Friends Colony, Katol Road,Nagpur) Contact No. :- +91 7888280720 (SHAH SIR) Page | 2 Concrete Technology By SHAH SIR (7888280720) B.E. (Civil Engineering) Third Semester (C.B.S.) Performance of charity is the key of attaining the estate of a philanthropist Bachelor of Engineering (B.E) In ( Civil Engineering ) 3rd SEMESTER C.B.S HELPFUL NOTES Based On RTMNU, Nagpur Syllabus Subject- Concrete Technology BY Md Shahjada Alam (Shah Sir) B.E (Civil Engineering), M.Tech (Environmental Engineering) Page | 3 Concrete Technology By SHAH SIR (7888280720) B.E. (Civil Engineering) Third Semester (C.B.S.) Disclaimer This Helpful Notes does not claim any originality and cannot be used as a substitute for prescribed textbooks. This Notes Complete done through with the help of many sources (like Internet Sources, Textbook, My Engineering Notes, Faculty Guidelines etc.). Specially, this Notes prepared for Job Seeker Students & also a big treat for weak students. Further; this Notes Claims not used for Commercial purposes. यह सहायक नोट्स ककसी भी मौलिकता का दावा नह ीं करता है और इसे ननर्ााररत पाठ्यपस् ु तक के ववकल्प के रूप में उपयोग नह ीं ककया जा सकता है । यह नोट्स कई स्रोतों (जैसे इींटरनेट स्रोत, पाठ्यपस् ु तक, मेर इींजीननयररींग पस् ु तक, सींकाय ददशाननदे श आदद) की सहायता से परू ा ककया गया है । ववशेष रूप से यह नोट्स जॉब सीकर छात्रों के लिए तैयार ककया गया है और कमजोर छात्रों के लिए एक बडा उपचार है । आगे की; इस नोट्स का दावा वाणिज्ययक उद्दे श्यों के लिए नह ीं ककया गया है । About Writer Md Shahjada Alam (Shah) former lecturer at Nuva College of Engineering & Technology, Nagpur. He completed his Graduation degree in Civil Engineering at the age of 20 years from Nagpur University. (Batch 2014-18) His Teaching style & hand written Notes are amazing & very helpful for students. This achievement comes from his valuable students. Only at the age of 14 years, he has been rewarded to Qualify Delhi-CET. Page | 4 Concrete Technology By SHAH SIR (7888280720) B.E. (Civil Engineering) Third Semester (C.B.S.) Dedicated To My Respected Grandfather Late Haji Mohammad Abbas Page | 5 Concrete Technology By SHAH SIR (7888280720) B.E. (Civil Engineering) Third Semester (C.B.S.) CONCRETE TECHNOLGY (80/20 Marks) Theories:- 80 Marks Numericals:- 0 Marks Syllabus Unit – I Cement: Chemistry of Cement, Main constituents of cement Hydration of cement, Water required, Physical properties and testing of cement, Soundness test. Hardening and compressive strength Grades and different types of cements. Ordinary Portland cement, Rapid Hardening Cement, B.B. Blast furnace slag cement, Low heat Portland cement, Portland pozzolones cement, Portland flyash cement, Sulphate resisting cement. Field test, Aggregates : Sources of aggregates, classification and nomenclature. Coarse and fine aggregate, normal weight (light and heavy weight aggregates). Aggregate characteristics and their significance in strength, workability, placement and compartion of concrete.Sampling. Particle shape and texture, Bond of aggregate, size & grading of aggregate strength of aggregates Mechanical properties and test-Specific gravity, Bulk density, porosity absorption of aggregates, moisture content of aggregate, bulking of sand abrasion test, impact value. Sieve analysisDeleterious substances in aggregates, organic impurities class and other fine material etc. Water : Quality of water for concrete mixing, suitability Unit – II Fresh concrete : Batching, Mechanical mixers, automatic batching and mixing plants. Efficiency of mixing. Workability Measurement – Slump cont test, compacting factor test, flow table, Vee-Bee consistometer, Factor affecting workability, setting time. W/C Law Significance of w/c ratio cohesiveness.Segregation, bleeding, voids, permeability.Hot weather concreting.Underwater concreting, Conveyance of concrete, Placing of concrete.Compaction-vibrators. Curing of concrete Significance, methods of curing, Temperature effect on during& strength gain. IS code on curing.Maturity of concrete. Unit - III Strength of concrete: Gain of strength, Wet ratio, Factor affecting compressive strength w/c ratio. Type of cement, air entrainment, aggregates, mixing water, Admixtures, curing conditions. Tensile and flexural strength.Relation between cracking in compression.Impact strength fatigue strength. Shear strength, Bond between concrete & reinforcement. Modulus of elasticity, Poison’s ratio. Testing of Hardness of Concrete: Compression test-cube strength & cylinder strength their relation, effect of aspect ratio on strength. Flexural strength of concrete, Determination of tensile strength.Indirect tension test.Splitting test.Abrasion resistance.Accelerated curing test. Page | 6 Concrete Technology By SHAH SIR (7888280720) B.E. (Civil Engineering) Third Semester (C.B.S.) Unit – IV Mix Design – Process, Statistical relation between mean & characteristic strength, Variance, Standard deviation. Factor affection mix properties. Grading of aggregate, aggregate/ cement ration etc. Degree of quality control. Design mix by Road note no. 4 (BS). IS:10262:2009. Additives and Admixtures: Types of admixtures, Natural products-DietomaceousearthBy products Pozzolones. Fly ash, silica fume, rice husk ash, , G.G. blast furnace slag. Admixturers-air encraining, water reducing, acceletors, retarders, plasticizers & Super plasticizers, permeability reducing,surface harderners.Corrosion inhibitors & water proofing agents. Unit – V Special concrete : Self compacting concrete, High performance concrete, fiber reinforced & polymer concrete, Ferro cement, Shortcrete pumped concrete, Free flow concrete. Shrinkage-Early volume changes, drying shrinkage, mechanism of shrinkage.Factor affecting shrinkage. Influence of curing & storage conditions. Differential shrinkage.Carbonation shrinkage.Creep-Factors influencing.Relation between creep & time, nature of creep, effect of creep. Unit – VI Durability of concrete-significance water as an agent of deterioration. Permeability of concrete, Efflorescence. Distress in concrete structures and its causes, cuses of deterioration of concrete. Cracks in concrete: Causes, types, prevention, repairs of craqcks – materials and methods Non Destructive tests. Instructions 1. For Topper Read All Questions. 2. For On paper Read All Compulsory & Semi-Compulsory Questions. 3. For Passing Read Only Compulsory Questions. व्यावसाययक उपयोग के लिए कडाई से यिषिद्ध है Page | 7 Concrete Technology By SHAH SIR (7888280720) B.E. (Civil Engineering) Third Semester (C.B.S.) Unit-01 (14 Marks) Cement, Aggregates, Water Theories:- 14 Marks Ques No.01:- Give the oxide composition of cement & state the effect of each on the properties of cement. Or Explain Bogue’s Compound Composition. Or What are the ingredients of the cement? Or Describe the cement composition. Or What are the functions of different cement ingredients? (Compulsory For 7 Marks) Ans:- Oxide composition of cement: The raw materials used for the manufacture of cement consist mainly of lime, silica, alumina and iron oxide. These oxides interact with one another in the kiln at high temperature to form more complex compounds. The relative proportions of these oxide compositions are responsible for influencing the various properties of cement; in addition to rate of cooling and fineness of grinding. Table 1: Approximate Oxide Composition Limits of Ordinary Portland Cement S.No. Oxides Formula Contents (%) 1. Calcium oxide (lime) CaO 60–67 2. Silicon dioxide (silica) SiO2 17–25 3. Aluminum oxide (alumina) Iron oxide Sulfur trioxide Alkalies (Potash And Soda ) Magnesium oxide (Magnesia) Calcium Sulfate Al2O3 3.0–8.0 Fe2O3 SO3 ( K2O, Na2O) 0.5–6.0 1.3–3.0 0.4–1.3 MgO 0.1–4.0 CaSO4 0.1-0.5 4. 5. 6. 7. 8. Page | 8 Concrete Technology By SHAH SIR (7888280720) B.E. (Civil Engineering) Third Semester (C.B.S.) Functions of Cement Ingredients:The main features of these cement ingredients along with their functions and usefulness or harmfulness are given below: 1. Lime: Lime is calcium oxide or calcium hydroxide. Presence of lime in a sufficient quantity is required to form silicates and aluminates of calcium. Deficiency in lime reduces the strength of property to the cement. Deficiency in lime causes cement to set quickly. Excess lime makes cement unsound. Excessive presence of lime cause cement to expand and disintegrate. 2. Silica: Silicon dioxide is known as silica, chemical formula SiO2. Sufficient quantity of silica should be present in cement to dicalcium and tricalcium silicate. Silica imparts strength to cement. Silica usually presents to the extent of about 30 percent cement. 3. Alumina: Alumina is Aluminium oxide. The chemical formula is Al2O3. Alumina imparts quick setting property to the cement. Clinkering temperature is lowered by the presence of the requisite quantity of alumina. Excess alumina weakens the cement. 4. Magnesia: Magnesium Oxide. Chemical formula is MgO. Magnesia should not be present more than 2% in cement. Excess magnesia will reduce the strength of the cement. 5. Iron oxide: Chemical formula is Fe2O3. Iron oxide imparts color to cement. It acts as a flux. At a very high temperature, it imparts into the chemical reaction with calcium and aluminum to form tricalcium alumino-ferrite. Tricalcium alumino-ferrite imparts hardness and strength to cement. 6. Calcium Sulfate: Chemical formula is CaSO4 . This is present in cement in the form of gypsum(CaSO4.2H2O) It slows down or retards the setting action of cement. 7. Sulfur Trioxide: Chemical formula is SO3. Should not be present more than 2%. Excess Sulfur Trioxide causes cement to unsound. Page | 9 Concrete Technology By SHAH SIR (7888280720) B.E. (Civil Engineering) Third Semester (C.B.S.) 8. Alkaline: Should not be present more than 1%. Excess Alkaline matter causes efflorescence. Major Compounds of Portland Cement (Bogue’s Compound Composition):S.NO NAME Formula Abbreviation 1. 2. 3. 4. Tricalcium silicate Dicalcium silicate Tricalcium aluminate Tetracalcium alumino 3CaO.SiO2 2CaO.SiO2 3CaO.Al2O3 4CaO.Al2O3 C 3S C 2S C 3A C4AF Properties of cement compounds:These compounds contribute to the properties of cement in different ways: Tricalcium aluminate, C3A:It liberates a lot of heat during the early stages of hydration, but has little strength contribution. Gypsum slows down the hydration rate of C3A. Cement low in C3A is sulfate resistant. Tricalcium silicate, C3S:This compound hydrates and hardens rapidly. It is largely responsible for portland cement’s initial set and early strength gain. Dicalcium silicate, C2S: C2S hydrates and hardens slowly. It is largely responsible for strength gain after one week. Ferrite, C4AF: This is a fluxing agent which reduces the melting temperature of the raw materials in the kiln (from 3,000o F to 2,600o F). It hydrates rapidly, but does not contribute much to strength of the cement paste. Characteristics of Hydration of the Cement Compounds:S.NO. Compounds Reaction Rate Amount of Liberated Strength Heat Liberation 1. 2. C 3S C 2S Moderate Slow Moderate Low High Low 3. 4. C 3A C4AF Fast Moderate Very high Moderate High Low initially, high later Low Low Page | 10 Concrete Technology Very high Moderate By SHAH SIR (7888280720) B.E. (Civil Engineering) Third Semester (C.B.S.) Ques No.02:- List different types of cements. Explain any two type in detail. Or Explain in brief "sulphate Resisting Portland Cement". (Compulsory For 7 Marks) Ans:- Cement: Cement, any material that hardens and becomes strongly adhesive after application. Types of Cement:Following are the different types of cement used in construction works:1) 2) 3) 4) 5) 6) 7) 8) 9) 10) 11) 12) 13) 14) 15) 16) Ordinary Portland Cement, Rapid hardening cement, Ultra-rapid hardening cement, Low heat cement, Quick setting cement, High alumina cement, Blast furnace slag cement, Pozzolana cement, White cement, Hydrophobic cement, Super sulfate cement, Low alkali cement, Water proof cement, Air entraining cement, Expansive cement, Colored cement. 1. Ordinary Portland Cement: In usual construction work, Ordinary Portland Cement is widely used. The composition of Ordinary Portland Cement: Argillaceous or silicates of alumina (clay and shale) Calcareous or calcium carbonate (limestone, chalk, and marl) Uses: It is used for general construction purposes. It is also used in most of the masonry works. 2. Rapid Hardening Cement: It is also known as High-Early-Strength cement. It is manufactured with such adjustments in the proportion of raw materials. So that the cement produced attains maximum strength with-in 24-72 hours. Rapid hardening cement is mostly used where rapid construction is needed like the construction of pavement. Page | 11 Concrete Technology By SHAH SIR (7888280720) B.E. (Civil Engineering) Third Semester (C.B.S.) Uses: It is a special purpose cement. It is used in that types of projects, where quick hardening is required. 3. Low Heat Cement: It is that type in which a very low amount of heat or hydration is liberated during setting and hardening. Mostly it is used in massive concrete structures like Dams etc. The initial setting time shall not be less than 60 minutes. And the final setting time shall not be less than ten hours. Uses: It is mostly used in mega projects construction like DAMS. If we use ordinary Portland cement instead of low heat cement in such structures, Cracks will develop in such structures due to the great amount of heat liberated during setting and hardening. And a DAM with cracks is a useless structure. But when low heat cement is used, this danger (development of cracks) can be eliminated. It is also used for the construction of chemical plants. 4. High Alumina Cement: These Types of Cement contains alumina in considerably larger proportions (average 40 percent) than normal cement. High Alumina cement is manufactured by calcining a well-proportioned mixture of Limestone and Bauxite (Al 2O3, n H2O). No other raw material is added, not even gypsum is mixed with the clinker during grinding. The total Alumina content is generally above 32 percent. Uses: Unfortunately, it is more costly. Therefore it is used only in those situations where resistant against corrosion is required. It is commonly used in construction work near and along sea-shore. 5. White Cement: It may also be defined as a special type of Portland cement when used it gives a milky or snow-white appearance. White cement is manufactured from pure limestone (chalk) and clay that are totally free from iron oxides and any other pigments like manganese and chromium. There strength and setting time is similar to ordinary Portland cement. Uses: White cement is the most favored material for use in making highways curbs and for a variety of ornamental work. They are also used widely for making cast stones of appealing appearance. White cement is comparatively a costly cement type and is, therefore, used only selectively. It is usually used in decorative work. It can also use for traffic barriers, tile grouts, swimming pools, roof tiles patching materials and terrazzo surfaces. Page | 12 Concrete Technology By SHAH SIR (7888280720) B.E. (Civil Engineering) Third Semester (C.B.S.) 6. Air Entraining Cement: This cement type is manufactured by adding some indigenous air entraining agents such as glues, resins, sulfates, etc., during the grinding stage of clinker. Uses: They are used to improve the workability of concrete with smaller water-cement ratio and they also improve the frost resistance of concrete. 7. Quick Setting Cement: Quick setting cement is the cement which sets in a very short time. The initial setting time is 5 minutes and the final setting time is 30 minutes. The composition of Quick Setting Cement: Clinker Aluminium sulphate (1% to 3% by weight of clinker) The aluminium sulphte increase the hydration rate of silicate. Uses: It is used in underwater construction. It is also used in rainy & cold weather conditions. It is used a higher temperature where water evaporates easily. Used for anchoring or rock bolt mining and tunnelling. 8. Hydrophobic Cement: To resist the hydration process in the transportation or storage stage, clinkers are grinded with water repellent film substance such as Oleic Acid or Stearic Acid. These chemicals form a layer on the cement particle and do not allow water to mix and start the hydration process. When cement and aggregate are thoroughly mixed in the mixer, protective layers break and start normal hydration with some air-entrainment which increase workability. Uses: Usually, it is used in the construction of water structures such as dams, spillways, or other submerged structures. It is also used in the construction of underground structure like tunnel etc. Sulphate Resisting Portland cement (SRC): Sulphate Resisting Portland Cement is a type of Portland cement in which the amount of Tricalcium aluminate (C3A) is restricted to lower than 5 % and (2 C3A + C4AF) is lower than 25%. SRC can be used for structural concrete wherever OPC or PPC or Slag Cement is usable under normal conditions. The use of SRC is particularly beneficial in such conditions where the concrete is exposed to the risk of deterioration due to sulphate attack. The use of SRC is recommended for following applications: 1. Foundations, piles. 2. Basements and underground structures. 3. Sewage and Water treatment plants. 4. Chemical, Fertilizers and Sugar factories. Page | 13 Concrete Technology By SHAH SIR (7888280720) B.E. (Civil Engineering) Third Semester (C.B.S.) 5. Food processing industries and Petrochemical projects. 6. Coastal works. 7. Also for normal construction works where OPC is used. 8. Construction of building along the coastal area within 50 km from sea. Ques No.03:- List important physical tests of cement? Explain any two in brief? Or What are the field tests for judging the preliminary quality of cement? Or Explain field test on cement? Also Explain soundness test on cement. Or Explain field test on cement & also list down the different types of lab test conducted on cement. (Compulsory For 7 Marks) Ans:- Physical tests of cement:The physical tests which are generally performed to determine the acceptability of cement are: 1) 2) 3) 4) 5) 6) 7) Fineness Test Consistency Test Setting time Test Soundness Test Strength Test Heat of Hydration Test Specific Gravity Test 1. Fineness Test: Fineness is the mean size of cement grain. It is done to measure the mean size of cement grain. The finer the cement the surface area for hydration will be large and it increases the strength of cement. But the fineness varies in different types of cement. One of the Following three methods can be applied to test the fineness of cement:1) Sieve method 2) Air Permeability method 3) Sedimentation method Importance of Fineness of Cement:1) 2) 3) 4) It can decrease bleeding. It can increase the final strength of cement. Fineness can also affect workability. Fineness test indirectly measures the surface area of the particles of cement per unit mass. Page | 14 Concrete Technology By SHAH SIR (7888280720) B.E. (Civil Engineering) Third Semester (C.B.S.) 5) Fineness increases the strength development in the cement principally during its first seven days. 6) It increases the rate of hydration. More is the fineness of cement particles more will be the rate of hydration. 1. Sieving Method:Apparatus: 1) 2) 3) 4) IS Sieve (90 µm) with lid. A balance of 0.01 gm sensitivity. A pure bristle or nylon brush. Sieve Shaking Machine (optional). Procedure: 1. Take 1000 grams (1 Kg) of cement for the test sample and name it as (w1). 2. Rub the cement particle well with your hands so that no lumps are left. 3. Now pour the 1 Kg cement content in the sieve and close it perfectly with the sieve lid. 4. If you have a sieve shaking machine then it’s awesome, now you just need to put the sieve in the shaking machine and start the machine for 15 minutes. Anyway, if you don’t have the sieve shaking machine then no problem, you can shake it also with your hands. But you need to make sure that the sieving operation is done in all the direction for a minimum of 15 minutes. 5. Brush the sieve base gently with the bristle brush so that nothing is left on the sieve surface. 6. Now just weight the retained amount of cement on the sieve and note it as (w2). 7. Now you need to find the percentage of the weight of cement-retained on the 90 µm sieve. 8. For calculation the formula is; Percent of cement retained on sieve = (w2/w1) x 100. 9. Repeat the test for at least three times to minimize the risk of errors. Result: According to Indian Standard, the amount of cement-retained on the 90 µm sieve shall never exceed 10%. 2. Soundness Test: Soundness of cement is the ability of a hardened paste to retain its volume after setting. A cement is said to be unsound (i.e., having lack of soundness) if it is subjected to delayed destructive expansion. Unsoundness of cement is due to presence of excessive amount of hard-burned free lime or magnesia. Soundness of cement indicates quality to expand on the setting. Unsound cement expands too much on setting and develops cracks in the structure. The test used for determining soundness of cement is known as “Le chatelier apparatus test.” Page | 15 Concrete Technology By SHAH SIR (7888280720) B.E. (Civil Engineering) Third Semester (C.B.S.) By soundness of cement, it is understood that, its capacity to form a non-disintegrating, hard uniformly strong mass on the setting. The soundness of cement means it doesn’t undergo large volume change after setting. Large changes in volume produce cracks, disintegration, and distortion, ultimately leading to failure. So it is very important to test the soundness of cement. This depends on its original composition, proper burning, and grinding. Presence of lime in the free state, an excess of sulfate or magnesia and a very fine size may be the cause of soundness in cement. The Indian Bureau of Standards has specified the le chatelier apparatus test – the same as adopted by British and American Standards for soundness test of cement. To test the soundness two methods can be applied:1) Le-Chatelier method 2) Autoclave method Le Chatelier Apparatus Test:Apparatus: It consists of a small, split, metallic cylinder having an internal diameter of 30 mm. It is 30 mm in height, with the thickness of wall being 0.5 mm. It is fixed with two indicators, one fixed on either side of the split. The distance from the end of the indicator to the center of the cylinder is 165 mm. Other apparatus used in the Le Chatelier Apparatus test are: 1) 2) 3) 4) 5) 6) 7) 8) Balance. Measuring box. Water bath. Caliper. Le Chatelier Apparatus. Glass Sheets. Enamel Tray. Trowel. Specimen: The cement to be tested is made into a paste of specified consistency. In common practice, 100 gm of neat cement is mixed with water calculated at 0.72 (P) where “P” is the percentage of water required for making a paste of normal consistency. Procedure: 1. Before commencing setting time test, do the consistency test to obtain the water required to give the paste normal consistency (P). 2. Prepare a paste by adding 0.78 times the water required to give a paste of standard consistency (i.e. 0.78P). 3. Lightly oil the Le-chatelier mold and place it on on a lightly oiled glass sheet. Page | 16 Concrete Technology By SHAH SIR (7888280720) B.E. (Civil Engineering) Third Semester (C.B.S.) 4. The paste, of the known consistency, is filled in the cylindrical mold, covered with glass sheet and kept under water (at 27 degrees to 32-degrees centigrade) for 24 hours. 5. After which the cylinder is taken out, and the distance between the two indicators is measured (say it is x). It is again immersed in the water. 6. The water is then brought to the boiling point in 15-30 minutes and kept boiling for 1 hour after which the distance between the two indicators is again measured (say it is now y). 7. The difference of y-x gives the expansion of the cement specimen on the setting. 8. It is specified that this distance shall not exceed 10 mm for the cement to qualify as SOUNDNESS CEMENT. Field Tests of Cement for Civil Engineers:1. Date of Manufacturing: As the strength of cement reduces with age, the date of manufacturing of cement bags should be checked. 2. Cement Color: The color of cement should be uniform. It should be typical cement color i.e. gray color with a light greenish shade. 3. Whether Hard Lumps are Formed: Cement should be free from hard lumps. Such lumps are formed by the absorption of moisture from the atmosphere. 4. Temperature Inside Cement Bag: If the hand is plunged into a bag of cement, it should be cool inside the cement bag. If hydration reaction takes place inside the bag, it will become warm. 5. Smoothness Test: When cement is touched or rubbed in between fingers, it should give a smooth feeling. If it felt rough, it indicates adulteration with sand. 6. Water Sinking Test: If a small quantity of cement is thrown into the water, it should float some time before finally sinking. 7. The smell of Cement Paste: A thin paste of cement with water should feel sticky between the fingers. If the cement contains too much-pounded clay and silt as an adulterant, the paste will give an earthy smell. 8. Glass Plate Test: A thick paste of cement with water is made on a piece of a glass plate and it is kept under water for 24 hours. It should set and not crack. 9. Block Test: A 25mm × 25mm × 200mm (1”×1”×8”) block of cement with water is made. The block is then immersed in water for three days. After removing, it is supported 150mm apart and a weight of 15kg uniformly placed over it. If it shows no sign of failure the cement is good. Page | 17 Concrete Technology By SHAH SIR (7888280720) B.E. (Civil Engineering) Third Semester (C.B.S.) Ques No.04:- What are the sources of aggregates. Explain the classification's of Aggregate. Or Give the classification of aggregate according to their source, shape & size. Or What are the various Aggregates used in cement concrete work ? Explain about the surface texture of Aggregates. (Semi-Compulsory For 7 Marks) Ans:- Aggregate: Aggregates are the most widely used products during the construction. Aggregates are the granular materials used as ingredients to make up the proportion of concrete. These materials include gravel, sand, or crushed stone mixed with water which is all added to the cement mixture to create concrete. The most common size of aggregate used in the concrete mix is 20 mm for coarse aggregate and particle size less than 4.75 mm for fine aggregate. Aggregates are the important constituents of the concrete which give body to the concrete and also reduce shrinkage. Aggregates occupy 70 to 80 % of total volume of concrete. According to nature of the formation of aggregates, they are classified as:1) Natural Aggregates 2) Crushed Aggregates 1. Natural Aggregates: Natural – Aggregates taken from natural sources, such as riverbeds, quarries and mines. Sand, gravel, stone and rock are the most common, and these can be fine or coarse. 2. Crushed Aggregates: Processed – Also called ‘artificial aggregates’, or ‘by-product’ aggregates, they are commonly taken from industrial or engineering waste, then treated to form construction aggregates for high quality concrete. Common processed aggregates include industrial slag, as well as burnt clay. Processed aggregates are used for both lightweight and high-density concrete mixes. Aggregates are classified according to shape into the following types:1) 2) 3) 4) 5) 6) Rounded aggregates Irregular or partly rounded aggregates Angular aggregates Flaky aggregates Elongated aggregates Flaky and elongated aggregates Page | 18 Concrete Technology By SHAH SIR (7888280720) B.E. (Civil Engineering) Third Semester (C.B.S.) 1. Rounded Aggregate: The rounded aggregates are completely shaped by attrition and available in the form of seashore gravel. Rounded aggregates result the minimum percentage of voids (32 – 33%) hence gives more workability. They require lesser amount of water-cement ratio. They are not considered for high strength concrete because of poor interlocking behavior and weak bond strength. 2. Irregular Aggregates: The irregular or partly rounded aggregates are partly shaped by attrition and these are available in the form of pit sands and gravel. Irregular aggregates may result 35- 37% of voids. These will give lesser workability when compared to rounded aggregates. The bond strength is slightly higher than rounded aggregates but not as required for high strength concrete. 3. Angular Aggregates: The angular aggregates consist well defined edges formed at the intersection of roughly planar surfaces and these are obtained by crushing the rocks. Angular aggregates result maximum percentage of voids (38-45%) hence gives less workability. They give 10-20% more compressive strength due to development of stronger aggregatemortar bond. So, these are useful in high strength concrete manufacturing. 4. Flaky Aggregates: When the aggregate thickness is small when compared with width and length of that aggregate it is said to be flaky aggregate. Or in the other, when the least dimension of aggregate is less than the 60% of its mean dimension then it is said to be flaky aggregate. 5. Elongated Aggregates: When the length of aggregate is larger than the other two dimensions then it is called elongated aggregate or the length of aggregate is greater than 180% of its mean dimension. Page | 19 Concrete Technology By SHAH SIR (7888280720) B.E. (Civil Engineering) Third Semester (C.B.S.) 6. Flaky and Elongated Aggregates: When the aggregate length is larger than its width and width is larger than its thickness then it is said to be flaky and elongated aggregates. The above 3 types of aggregates are not suitable for concrete mixing. These are generally obtained from the poorly crushed rocks. Aggregates are classified into 2 types according to size:1) Fine aggregate 2) Coarse aggregate 1. Fine Aggregate: When the aggregate is sieved through 4.75mm sieve, the aggregate passed through it called as fine aggregate. Natural sand is generally used as fine aggregate, silt and clay are also come under this category. The soft deposit consisting of sand, silt and clay is termed as loam. The purpose of the fine aggregate is to fill the voids in the coarse aggregate and to act as a workability agent. According to sources, fine aggregates can be further classified as:a) Natural Sand: It is the fine aggregate resulting from the natural disintegration of rock. It is deposited by streams or glacial agencies. b) Crushed Gravel Sand: It is the fine aggregate which is produced by crushing hard gravel. c) Crushed Stone Sand: It is the fine aggregate which is produced by crushing natural stone. 2. Coarse Aggregate: When the aggregate is sieved through 4.75mm sieve, the aggregate retained is called coarse aggregate. Gravel, cobble and boulders come under this category. The maximum size aggregate used may be dependent upon some conditions. In general, 40mm size aggregate used for normal strengths and 20mm size is used for high strength concrete. the size range of various coarse aggregates given below. According to sources of coarse aggregates, it can be further classified as: (a) Uncrushed Gravel or Stone: It results from natural disintegration of rock. Page | 20 Concrete Technology By SHAH SIR (7888280720) B.E. (Civil Engineering) Third Semester (C.B.S.) (b) Crushed Gravel Stone: It results from the crushing of gravel or hard stone. Ques No.05:- Explain impact value test on Aggregate. Or Explain impact value test on Aggregate with diagram. (Future For 7 Marks) Ans:- Impact value test on Aggregate: The property of a material to resist impact is known as toughness. Due to movement of vehicles on the road the aggregates are subjected to impact resulting in their breaking down into smaller pieces. The aggregates should therefore have sufficient toughness to resist their disintegration due to impact. This characteristic is measured by impact value test. The aggregate impact value is a measure of resistance to sudden impact or shock, which may differ from its resistance to gradually applied compressive load. PURPOSE OF TEST: Aggregate Impact Value test determines the Aggregate Impact Value (AIV) of aggregates which provides a relative measure of the resistance of an aggregate to sudden shock or impact. Resistance of the aggregates to impact is termed as toughness. Aggregates used in the pavement should be able to resist the effect caused by the jumping of the steel tyred wheels from one particle to another at different levels causes severe impact on the aggregates. Lower the aggregate crushing value greater will be the “resistance to to sudden shock or impact (toughness)”. Procedure to determine Aggregate Impact Value:1) The cup of the impact testing machine should be fixed firmly in position on the base of the machine and the whole of the test sample placed in it and compacted by 25 strokes of the tamping rod. 2) The hammer should be raised to 380mm above the upper surface of the aggregates in the cup and allowed to fall freely onto the aggregates. The test sample should be subjected to a total of 15 such blows, each being delivered at an interval of not less than one second. APPLICATIONS OF AGGREGATE IMPACT VALUE: Aggregate having AIV % less 50% is better to use in construction where as those having value more than 50% are bad for construction. Page | 21 Concrete Technology By SHAH SIR (7888280720) B.E. (Civil Engineering) Third Semester (C.B.S.) It is reliable to calculate the AIV % of the aggregate use as road construction as it will give us the capacity of road to bear the load on it. Aggregate having AIV % equal or greater than 35 % is considered bad to use in road construction. Diagram:- RESULTS: i) The sample should be removed and sieved through a 2.36mm IS Sieve. The fraction passing through should be weighed (Weight ‘B’). The fraction retained on the sieve should also be weighed (Weight ‘C’) and if the total weight (B+C) is less than the initial weight (A) by more than one gram, the result should be discarded and a fresh test done. ii) The ratio of the weight of the fines formed to the total sample weight should be expressed as a percentage. Aggregate impact value = B/A x 100% Page | 22 Concrete Technology By SHAH SIR (7888280720) B.E. (Civil Engineering) Third Semester (C.B.S.) iii) Two such tests should be carried out and the mean of the results should be reported. A sample proforma for the record of the test results is given in the below table. ***** Complete By Shah Sir (Civil Engineering Tutor) Mob- +91 7888280720 Page | 23 Concrete Technology By SHAH SIR (7888280720) B.E. (Civil Engineering) Third Semester (C.B.S.) SHAH ENGINEERING CLASSES (A Classroom of Civil Engineering Department) [Poly/B.E/B.Tech/M.Tech/JE Govt./GATE] By SHAH SIR Weekly Classes Available Venue:Trimurtinagar, Near Hingna T-Point, Nagpur Contact Us:+91 7888280720 (SHAH SIR) Page | 24 Concrete Technology By SHAH SIR (7888280720) B.E. (Civil Engineering) Third Semester (C.B.S.) Unit-02 (13 Marks) Fresh concrete Theories:- 13 Marks Ques No.01:- What is workability and what are the factors affecting it? Or Explain various factors affecting workability of concrete. Or What is workability? List out the tests for workability measurements. compare volume Batching & weight Batching. Or What do you mean by Workability of concrete? Is it necessary to determine the workability of concrete. Explain. Or Define workability. Enlist various test to measure workability. Explain compaction factor test in detail. (Compulsory For 7 Marks) Ans:- Workability: Workability is a property of freshly mixed concrete. Concrete is a mixture of cement, aggregates, water, and admixtures. The properties of concrete, whether in the fresh state or hardened state, are affected by its ingredients and their proportions. Workability is a purely physical property of freshly mixed concrete. Workability of concrete simply means the ability to work with concrete. Fresh concrete is said to be workable if it can be easily transported, placed, compacted, and finished without any segregation. The ease of placing, compacting, and finishing of concrete in the desired manner is called its workability. ACI (American Concrete Institute) defines it as ‘that property of freshly mixed concrete or mortar which determines the ease and homogeneity with which it can be mixed, placed, consolidated and finished’. ASTM (American Society for Testing and Materials) defines it as “that property determining the effort required to manipulate a freshly mixed quantity of concrete with minimum loss of homogeneity”. Page | 25 Concrete Technology By SHAH SIR (7888280720) B.E. (Civil Engineering) Third Semester (C.B.S.) Elaborating More a Concrete is Said to be Workable If: 1) It can be placed without loss of homogeneity. i.e. concrete ingredients never get dispersed while placing and remain uniformly distributed across cross section. 2) It can be handled without segregation. i.e. concrete ingredients never get separated during transport or placing. 3) It can be compacted with specified effort. i.e. with a normal vibrator or even rodding with bamboo or a rod. 4) It can be finished easily. Workability Vs. Strength of Concrete:The following figure explains the relation between workability and compressive strength of concrete: (Fig: Workability Vs. Strength of Concrete) Factors Affecting Workability of Concrete:1) Water content: The increase in water content increases the fluidity of the concrete thus providing greater lubrication. This helps to increase the workability of the concrete. Increasing the water content should be the last resort to improve the workability in the concrete as this will seriously affect the strength of the concrete. Even if more amount of water is to be added, more cement also should be added so that the water/ cement ratio remains the same and hence the strength of the concrete remains unaffected. 2) Size of aggregates: The surface area of bigger aggregates is less and hence less amount of water is required for lubricating the surface to reduce the friction. Thus the concrete having large sized aggregate is more workable (of course, within certain limits). 3) Mix proportions: Aggregate/ cement ratio is the measure of how lean or rich the concrete is. If aggregate/ cement ratio is higher, the concrete becomes leaner. In lean concrete less paste is available for the lubrication of the aggregate, while in rich concrete with low a/c ratio, more paste is available which makes the mix more cohesive and hence provides better workability. Page | 26 Concrete Technology By SHAH SIR (7888280720) B.E. (Civil Engineering) Third Semester (C.B.S.) 4) Shape of aggregates: Rounded aggregates have considerably less surface area and less voids in comparison to angular or flaky aggregates which provide better possibility of overcoming the frictional resistance. Further, angular and flaky aggregates make concrete very harsh. 5) Surface texture of aggregates: The aggregates having smooth or glossy texture have less surface area compared to rough textured aggregates. This provides better workability as less amount of water is required for lubricating effect. But, taking into account the poor interlocking action provided by the glossy textured aggregate, its use is generally discouraged in high strength concrete. 6) Grading of aggregates: Well graded aggregate is the one with least amount of voids in a given volume. If the grading of aggregate is good, the voids will be less and hence higher the workability. 7) Use of admixtures: Use of admixtures in concrete is the major factor that affects the workability. The use of plasticizers and super-plasticizers amply increase the workability of the concrete. Air entraining agents produce air bubbles which act as rollers between particles and provide better mobility thus improving the workability. 8) Time and temperature: Fresh concrete gets stiffened as the time flows. This is because some of the water used to mix the concrete gets evaporated and some gets absorbed by the aggregates. Thus the workability of concrete reduces with time. This loss of workability with time is known as slump loss. The effect of temperature on workability of concrete is noteworthy. As the temperature increases, the workability of the mix reduces. DIFFERENT TEST METHODS FOR WORKABILITY MEASUREMENT:Depending upon the water cement ratio in the concrete mix, the workability may be determined by the following three methods:1) Slump Test 2) Compaction Factor Test 3) Vee-bee Consistometer Test 1. SLUMP TEST: It is the most common method for measuring the workability of freshly mixed concrete. It can be performed both in lab and at site. Uniformity of the concrete regarding workability and quality aspects can be assessed from batch to batch by observing the nature in which the concrete slumps. It is not very suitable for very wet or very dry concrete. A steel mould in the form of frustum of cone is used in slump test which has the top diameter of 100 mm, bottom diameter of 200 mm and the height is 300 mm. According to Indian standard specification, the maximum size of the aggregate in concrete that can be used to perform slump test is restricted to 38 mm. The mould is cleaned and freed from any surface moistures and then the concrete is placed in three layers. Each layer is tamped 25 times with a standard tamping rod (16 mm dia, 0.6 meter length). Immediately after filling, the cone is slowly lifted and the concrete is allowed to subside. The decrease in the height of the center of the slumped concrete is called slump and is measured to the nearest 5mm. Page | 27 Concrete Technology By SHAH SIR (7888280720) B.E. (Civil Engineering) Third Semester (C.B.S.) If the concrete subsides evenly all round, the slump measured is true slump. If one half of the cone slides down an inclined plane, a shear slump is said to have taken place and the test has to be repeated. Too wet mix shows collapsible nature of slump. SUITABILITY: This method is suitable only for the concrete of high workability. This test is carried out with a mould called slump cone whose top diameter is 10 cm, bottom diameter is 20 cm and height is 30 cm. Diagram:- Page | 28 Concrete Technology By SHAH SIR (7888280720) B.E. (Civil Engineering) Third Semester (C.B.S.) PROCEDURE: The test is performed in the following steps: 1) Place the slump mould on a smooth flat and non-absorbent surface. 2) Mix the dry ingredients of the concrete thoroughly till a uniform colour is obtained and then add the required quantity of water in it. 3) Place the mixed concrete in the mould to about one-fourth of its height. 4) Compact the concrete 25 times with the help of a tamping rod uniformly all over the area. 5) Place the mixed concrete in the mould to about half of its height and compact it again. 6) Similarly, place the concrete upto its three-fourth height and then up to its top. Compact each layer 25 times with the help of tamping rod uniformly. For the second and subsequent layers, the tamping rod should penetrate into underlying layer. 7) Strike off the top surface of mould with a trowel or tamping rod so that the mould is filled to its top. 8) Remove the mould immediately, ensuring its movement in vertical direction. 9) When the settlement of concrete stops, measure the subsidence of the concrete in millimeters which is the required slump of the concrete. LIMITATIONS OF SLUMP TEST:Following are the limitations: 1) 2) 3) 4) Not suitable for concrete containing aggregates larger than 40 mm. Not suitable for concrete of dry mix. Not suitable for very wet concrete. Not reliable because slump may be of any shape. 2. COMPACTION FACTOR TEST: According to this test, the workability may be defined as the amount of applied work required to compact the concrete to its maximum density. Page | 29 Concrete Technology By SHAH SIR (7888280720) B.E. (Civil Engineering) Third Semester (C.B.S.) SUITABILITY: This method is adopted for determining the workability of concrete mix in laboratories. It gives fairly good results for concrete of low workability. Diagram:- Page | 30 Concrete Technology By SHAH SIR (7888280720) B.E. (Civil Engineering) Third Semester (C.B.S.) PROCEDURE: 1) 2) 3) 4) The test is performed in the following steps: Clean and dry the internal surface of the mould. With the help of hand scoop, place the concrete in upper hopper A. Open the trap door of hopper in order to facilitate the falling of the concrete into lower hopper B. the concrete sticking to the sides of the hopper A, should be pushed downward with the help of a steel rod. 5) Open the trap door of the hopper B and allow the concrete to fall into cylinder C. 6) Remove the surplus concrete from the top of the cylinder with the help of a trowel. Wipe and clean the outside surface of the cylinder. 7) Weigh the cylinder with partially compacted concrete nearest to 10 g. 8) Fill in the cylinder with fresh concrete in layers not exceeding 5 cm in thickness and compact each layer till 100 percent compaction is achievd. 9) Wipe off and clean the outside surface of the cylinder and weigh the cylinder with fully compacted concrete nearest to 10 g. 10) Calculate the value of compaction factor using the following formula. Compaction factor = weight of partially compacted concrete/weight of fully compacted concrete ADVANTAGES OF COMPACTION FACTOR TEST:Following are the advantages: 1) 2) 3) 4) Suitable for testing workability in laboratories Suitable for concrete of low workability Suitable to detect the variation in workability over a wide range Its results are more precise and sensitive. Ques No.02:- What is batching ? Compare Volume Batching and Weight Batching. (Future For 6 Marks) Ans:- The following steps are involved in the concreting:1. Batching 2. Mixing 3. Transporting 4. Placing & 5. Compacting. Batching: In general batching is the process of measuring and combining the ingredients of concrete (cement, water, sand, aggregates)…as per the mix design. Batching is the process of measuring and combining required ingredients of concrete by either by weight or by volume as per the mix design and introducing them into a mixture to produce a uniform quality of concrete. The proper and accurate measurement of all the material used in concrete making is necessary to ensure uniformity of proportions in succeeding batches. Page | 31 Concrete Technology By SHAH SIR (7888280720) B.E. (Civil Engineering) Third Semester (C.B.S.) Compare Volume Batching and Weight Batching:S.No. 1. Volume Batching The concrete ingredients are batched / proportioned based on volume. Batching by volume achieved by using a constructed gauge boxes (Farmas) for proportioning the materials according to various mix proportions and based on the quantity of material needed per batch. In small works gamellas are also used for batching. But it is not recommended for batching of concrete. 3. Nominal mix is batched by mass using regularly maintained and well-calibrated equipment, therefore high degree of uniformity in proportion of the fresh and hardened state concrete can be achieved. 4. Materials batched by mass are exact in measurement taking cognisance of the differences in the material properties. Water for mixing is weighed out precisely as a function of the weight of cement used for each mix proportion. Hence uniform water-cement ratio can be maintained, which plays an important role in achieving strength of concrete. Each batch is weighed which makes it easy to detect overruns or incorrect dosages. The slump of concrete batched by mass is higher than batched by volume for same mix proportion and water cement ratios. i.e. slump varies from medium to very high slump. For same mix proportion, the compressive strength of concrete batched by mass is higher than that batched by volume at all w/c ratios and for curing ages 7 to 28 days. When batching by volume is used, possible sources of error could lead to variation in the amount of aggregate in a specific volume and errors in measured volume. These errors often lead to variations in the fresh and hardened properties of concrete as against specified characteristics properties. Variations in quantity are very much likely for materials. Ultimately affect strength of concrete. The volume of water required for mixing is not often specified. Sometime extra water is added to make concrete workable. This makes concrete non uniform and reduces the strength. 2. 5. 6. 7. 8. Page | 32 Weight Batching The concrete ingredients are batched/ proportioned based on weight. Batching by mass is achieved by using weigh batching machine/ plant. Concrete Technology Difficult to detect overruns or incorrect dosages. Slump varies from low to high. For same mix proportion, the compressive strength of concrete batched by volume is lower than that batched by mass at all w/c ratios and for curing ages 7 to 28 days. By SHAH SIR (7888280720) B.E. (Civil Engineering) Third Semester (C.B.S.) Batching can be done by two methods, volume batching and weight batching. Batching should be done properly to get quality concrete mix. Ques No.03:- What is curing? Explain various types of curing with their suitability? Or Why curing is necessary ? Explain various methods of curing. Or Explain significance of concrete curing? Also explain maturity of concrete. Or What is curing of concrete? How do you decide the period of curing. Explain. (Semi-Compulsory For 7 Marks) Ans:- Curing: The curing of concrete is the process of keeping the concrete damp or moist and warm after the initial setting of concrete or removal of formwork. Proper curing of concrete maintains the satisfactory moisture content and favorable temperature inside the concrete so that hydration of cement may continue until the desired properties are developed. The curing of concrete must be continued for a reasonable period of time to achieve its desire strength and durability. Curing of Concrete is a method by which the concrete is protected against loss of moisture required for hydration and kept within the recommended temperature range. Curing will increase the strength and decrease the permeability of hardened concrete. Curing is also helps in mitigating thermal and plastic cracks, which can severely impact durability of structures. A curing practice involves keeping the concrete damp or moist until the hydration of concrete is complete and strength is attained. Curing of concrete should begin soon after initial setting time of concrete or formwork/shuttering is removed and must continue for a reasonable period of time as per the specified standards, for the concrete to achieve its desired strength and durability. It is important to make sure an uninterrupted hydration of PCC & RC after concrete is placed and finished in its position. Uniform temperature ought to be maintained throughout the concrete-section depth to avoid thermal shrinkage cracks. Why Curing of Concrete is Necessary:1) 2) 3) 4) Enhance Hydration of Concrete to achieve desired Strength Improved durability of concrete by reducing cracks Higher serviceability performance by increasing abrasion resistance Improved microstructure by developing better hydrate gels and solid mass. Page | 33 Concrete Technology By SHAH SIR (7888280720) B.E. (Civil Engineering) Third Semester (C.B.S.) Significance of concrete curing: Curing has a strong influence on all properties of concrete and therefore it should not be taken lightly. Properly cured concrete has better surface hardness and can better withstand surface wear and abrasion apart from the basic strength. Curing also makes concrete more impermeable, which prevents moisture and water-borne chemicals from entering into the concrete, thereby increasing durability and service life. Proper curing helps to prevent crazing, dusting, surface disintegration and scaling.Adequate curing reduces shrinkage, gives better resistance to wear and improves long-term appearance. Without proper curing, the chemical process of hydration is not complete. Without proper curing, you will never be able to get the designed strength of concrete and please note that this is a loss of strength for a lifetime and there is no easy alternative to strengthen such poorly cured weak concrete. So always be careful and cure with care. METHODS USED FOR CURING OF CONCRETE:There are various methods of curing. The adoption of a particular method will depend upon the nature of work and the climatic conditions. The following methods of curing of concrete are generally adopted: 1) 2) 3) 4) 5) 6) 7) Shading concrete work Covering concrete surfaces with hessian or gunny bags Sprinkling of water Ponding method Membrane curing Steam curing Spraying or Fogging Method 1. SHADING OF CONCRETE WORK: The object of shading concrete work is to prevent the evaporation of water from the surface even before setting. This is adopted mainly in case of large concrete surfaces such as road slabs. This is essential in dry weather to protect the concrete from heat, direct sun rays and wind. It also protects the surface from rain. In cold weather shading helps in preserving the heat of hydration of cement thereby preventing freezing of concrete under mild frost conditions. Shading may be achieved by using canvas stretched on frames. This method has a limited application only. 2. COVERING CONCRETE SURFACES WITH HESSIAN OR GUNNY BAGS: This is a widely used method of curing, particularly for structural concrete. Thus exposed surface of concrete is prevented from drying out by covering it with hessian, canvas or empty cement bags. The covering over vertical and sloping surfaces should be secured properly. These are periodically wetted. The interval of wetting will depend upon the rate of Page | 34 Concrete Technology By SHAH SIR (7888280720) B.E. (Civil Engineering) Third Semester (C.B.S.) evaporation of water. It should be ensured that the surface of concrete is not allowed to dry even for a short time during the curing period. Special arrangements for keeping the surface wet must be made at nights and on holidays. 3. SPRINKLING OF WATER: Sprinkling of water continuously on the concrete surface provides an efficient curing. It is mostly used for curing floor slabs. The concrete should be allowed to set sufficiently before sprinkling is started. The spray can be obtained from a perforated plastic box. On small jobs sprinkling of water may be done by hand. Vertical and sloping surfaces can be kept continuously wet by sprinkling water on top surfaces and allowing it to run down between the forms and the concrete. For this method of curing the water requirement is higher. 4. PONDING METHOD: This is the best method of curing. It is suitable for curing horizontal surfaces such as floors, roof slabs, road and air field pavements. The horizontal top surfaces of beams can also be ponded. In the ponding method, small rectangular or square artificial ponds are built with using bunds of clay or lean mortar or sand across and along the concrete surface. Water is filled in small rectangular or square ponds two or three times per day depending upon the atmospheric conditions. This method is very efficient but the amount of water required is huge. Page | 35 Concrete Technology By SHAH SIR (7888280720) B.E. (Civil Engineering) Third Semester (C.B.S.) 5. MEMBRANE CURING: The method of curing described above come under the category of moist curing. Another method of curing is to cover the wetted concrete surface by a layer of water proof material, which is kept in contact with the concrete surface of seven days. This method of curing is termed as membrane curing. A membrane will prevent the evaporation of water from the concrete. The membrane can be either in solid or liquid form. They are also known as sealing compounds. Bituminised water proof papers, wax emulsions, bitumen emulsions and plastic films are the common types of membrane used. Whenever bitumen is applied over the surface for curing, it should be done only after 24 hours curing with gunny bags. The surface is allowed to dry out so that loose water is not visible and then the liquid asphalt sprayed throughout. The moisture in the concrete is thus preserved. It is quite enough for curing. This method of curing does not need constant supervision. It is adopted with advantage at places where water is not available in sufficient quantity for wet curing. This method of curing is not efficient as compared with wet curing because rate of hydration is less. Moreover the strength of concrete cured by any membrane is less than the concrete which is moist cured. When membrane is damaged the curing is badly affected. 6. STEAM CURING: Steam curing and hot water curing is sometimes adopted. With these methods of curing, the strength development of concrete is very rapid. These methods can best be used in pre cast concrete work. In steam curing the temperature of steam should be restricted to a maximum of 750C as in the absence of proper humidity (about 90%) the concrete may dry too soon. In case of hot water curing, temperature may be raised to any limit, ay 1000C. At this temperature, the development of strength is about 70% of 28 days strength after 4 to 5 hours. In both cases, the temperature should be fully controlled to avoid non-uniformity. The concrete should be prevented from rapid drying and cooling which would form cracks. 7. Spraying or Fogging Method: Curing by spraying of water or fogging is one of the conventional methods of curing concrete in the construction industry. It can be an efficient method for curing by supplying additional moisture during hot weather. By using nozzles or sprayers, water is sprayed on the concrete members, which are to be cured. Page | 36 Concrete Technology By SHAH SIR (7888280720) B.E. (Civil Engineering) Third Semester (C.B.S.) What is the Right Time for Concrete Curing? The right time of curing of concrete depends on several factors. Like, temperature, the evaporation rate, moisture content in the air, etc. According to ACI-308, three phases of concrete curing should be done: (1) Initial Curing: This process is also called Bleeding of Concrete. After placing of concrete, the water starts bleeding from concrete and rises from its pores. After rising of water, evaporation take place, and the water starts disappearing from the surface due to evaporation. Bleeding of concrete depends on many factors like thickness, length, temperature, etc. So, to reduce the loss of water and prevent shrinkage, initial curing of concrete is required. Evaporation reducers can be used for this process. (2) Intermediate Curing of Concrete: This process is done when finishing work is finished before the final setting of cement. As you know, water plays an important role in the strength of concrete. Therefore, the intermediate curing of concrete is required. (3) Final Concrete curing: After the final setting of concrete, it should be cured so that to prevent the loss of more water and increase the strength of concrete. Ques No.04:- Explain in Brief segregation, Bleeding & Water Cement ratio. Or Explain significance of water cement ratio. What is segregation and bleeding in concrete. Or What are various causes of bleeding and segregation in concrete ? (Compulsory For 7 Marks) Ans:- Segregation: Segregation is the “Separation of constituent materials in concrete.” Segregation of concrete is the separation of cement paste and aggregates of concrete from each other during handling and placement. Segregation also occurs due to over-vibration or compaction of concrete, in which cement paste comes to the top and aggregates settles at the bottom. Segregation of concrete affects strength and durability in structures. In a good concrete, all concrete aggregates are evenly coated with sand and cement paste and forms a homogeneous mass. Page | 37 Concrete Technology By SHAH SIR (7888280720) B.E. (Civil Engineering) Third Semester (C.B.S.) Diagram:- In concrete technology, segregation is of three types:1) Separation of Coarse aggregate from the concrete mixture, 2) Separation of Cement pastes from the concrete during its plastic stage. 3) Separation of water from the concrete mix (Bleeding in concrete) Causes of Segregation of Concrete:a) Use of high water-cement ratio in concrete. This general happens in case of concrete mixed at site by unskilled workers. b) Excessive vibration of concrete with mechanical needle vibrators makes heavier particles settle at bottom and lighter cement sand paste comes on top. c) When concreting is done from height in case of underground foundations and rafts, which causes concrete to segregate. How to minimize segregation in concrete:1) Segregation can be controlled by maintaining proper proportioning the mix. 2) By proper handling, placing, transporting, compacting and finishing of concrete. 3) Adding air entraining agents, admixtures and pozzolanic materials in the mix segregation controlled to some extent. Page | 38 Concrete Technology By SHAH SIR (7888280720) B.E. (Civil Engineering) Third Semester (C.B.S.) Bleeding: Bleeding is a form of segregation in which water present in the concrete mix is pushed upwards due to the settlement of cement and aggregate. The specific gravity of water is low, due to this water tends to move upwards. Bleeding ordinarily occurs in the wet mix of concrete. The Prime factor for bleeding in concrete is the high dosage of Water cement ratio. Higher water-cement ratio weakens concrete and leads to excessive bleeding. The bleeding in concrete is not harmful if the rate of evaporation of water is equal to the rate of bleeding. Normal bleeding is quite good, and it enhances the workability of concrete. When the concrete is fully plastic, bleeding may not cause much harm. However, concrete is still in the plastic stage later it is subsidized and compacted. Causes of bleeding in Concrete:a) High water-cement ratio leading to highly wet mix. b) Badly proportioned and insufficiently mixed concrete. How the bleeding in concrete is controlled:a) Bleeding in concrete is controlled by Adding minimum water content in the concrete mix. b) Encouraging the use of air en-training admixtures in the mix. c) By adding more cement in the mix. Effects Of Bleeding:a) Due to bleeding concrete loses its homogeneity. b) Bleeding is responsible for causing permeability in concrete. c) As far as safety is concerned, water that accumulates below the reinforcing bars, reduces the bond between the reinforcement and concrete. d) In the process of bleeding the accumulation of water creates a water voids and reduces bond between the aggregate and cement paste. e) Due to bleeding pumping ability of concrete is reduced. f) Increase in the water-cement ratio at the top. g) The accumulation of water at the top, results in delayed surface finishing. Water Cement ratio: Water-Cement Ratio - The ratio of the amount of water to the amount of cement by weight is termed the water-cement ratio. The strength and quality of concrete depend on this ratio. The quantity of water is usually expressed in litre per bag of cement. if water required for one bag of cement is 30 litres, the water-cement ratio is equal to 30/50 = 0.6. Water cement ratio can be defined as the ratio of the volume of water to the volume of cement used in a concrete mix. Water has a great role on the strength and workability of concrete. After lots of experiments it has been found that for a specific proportion of materials in a concrete mix, there is a certain amount of water that gives maximum strength. Page | 39 Concrete Technology By SHAH SIR (7888280720) B.E. (Civil Engineering) Third Semester (C.B.S.) A slight change in the amount of water causes much more differences in the strength of concrete. If less water is used, the resultant concrete will be nearly dry, hard to place in the form and may create difficulties in compaction. Besides this, with less water proper setting will not be guaranteed and thus the strength of concrete get reduced considerably. On the other hand, if water is used more, it may develop larger voids and honey-combing in the set concrete, in this way decreasing its density, durability, and strength. Hence, water cement ratio attends an important role in producing concrete of required strength. The lower the ratio, the greater is the strength of concrete. Duff Abrams’ Law for Water-Cement Ratio: American Researcher Duff Abrams’ gave a concept on water/cement ratio which known is as “Abrams’ water/cement ratio law”. As per Duff Abrams’ law strength of concrete is inversely related to the W/C ratio. If w/c ratio increases, the strength of concrete decreases. Compaction of concrete is also important because, If concrete is not properly compacted it contains large voids, which leads to porosity. Thus, at a low water/cement ratio, where fully compacting is hard to achieve, Abrams’ law is not valid. Decrease in compressive strength with an increase in water/cement ratio illustrated graphically below, Water-cement ratio is the most important factor which affects the properties of concrete. Selection of water-cement ratio depends on various factors like requirements of strength, workability, concreting techniques, nature of work etc. General criteria why the low watercement ratio is recommended are as below. Page | 40 Concrete Technology By SHAH SIR (7888280720) B.E. (Civil Engineering) Third Semester (C.B.S.) Following reasons why high water-cement ratio is not recommended: 1) 2) 3) 4) 5) 6) 7) 8) Chances of segregation and bleeding Less durability Development of voids Decrease in strength of concrete Decrease in life of structure Leakage and chances of corrosion Increased carbonation due to voids and hence reduce life Freeze-thaw effect etc. Following major reasons why the low water-cement ratio is recommended: 1) 2) 3) 4) 5) 6) Increase in strength Increase in durability Lower permeability No leakage or dampness Greater freeze-thaw durability Better bond with reinforcement etc. Ques No.05:- Write a short note on hot weather concreting and underwater concreting. Or Explain under water concreting by the use of Tremie Pipe method. Or Explain hot whether concreting & underwater concreting. Or Explain hot weather concreting. (Compulsory For 7 Marks) Ans:- Under water concreting by the use of Tremie Pipe method: The degradation of concrete under water is a serious and troublesome matter. Underwater concreting is necessarily adopted in marine works and in deep foundations. Due to the continuous loss of cement and segregation of concrete, placing of concrete under water becomes very difficult to unreinforced construction. There are several methods to carry out underwater concreting such as tremie method, pumping methods, preplaced aggregate concrete etc. The underwater concreting techniques designed mostly to prevent cement washout. These methods did not obtain the full purpose of avoiding cement wash out at early stages of using under water concreting apart from cases where large masses of concreting were employed. Methods of Underwater Concreting:Following are the methods of underwater concreting: Page | 41 Concrete Technology By SHAH SIR (7888280720) B.E. (Civil Engineering) Third Semester (C.B.S.) 1) 2) 3) 4) 5) 6) 7) 8) 9) Tremie method Pumping technique Hydro valve method Pneumatic valve method Skip method Tilting pallet barge method Preplaced aggregate concrete Toggle bags method Bagged concrete method 1. Tremie Method of Underwater Concreting: Tremie method is one of the most common methods that is used for concreting under water. In this method, a long steel pipe (named as tremie) having a diameter of 15 to 30 cm is inserted vertically into the water. The pipe should be long enough that it reaches to the bed of water keeping its one end above the water level. The tremie is then fitted with a hopper at the upper end for pouring concrete inside the pipe. The lower end of the tremie pipe must be closed with a check valve before inserting it into the water. After that, freshly mixed concrete is poured with the help of hopper. When the concrete is poured, it displaces the air and water present in the pipe and finally reaches to the bed. Diagram:- Page | 42 Concrete Technology By SHAH SIR (7888280720) B.E. (Civil Engineering) Third Semester (C.B.S.) Advantages: If the work is executed properly, this method has the following advantages: 1. The concrete is not affected by the flowing water, except the top layer. The top layer may be cut off to remove the affected concrete at the end of the whole operation. 2. Under water concrete dos not need compaction as concrete automatically gets compacted by the hydrostatic pressure of water. 3. The concrete is of such a high consistency which normally does not require compaction. Disadvantage: Concreting by tremie method requires high water/cement ratio for obtaining high consistency, which reduces the strength of concrete. But now, with the use of some plasticizer a concrete with as low w/c ratio as 0.3 or even less can be placed by tremie method. Hot weather concreting: We know that cement possesses faster rate of hydration in hot weather. The rate of hydration of cement increases with the increases in temperature. Generally, 10° C – 27° C is considered as most suitable for hydration. At higher temperature, the concrete may start to set before placing and compacting in position. Therefore the concrete should be placed as fast as possible after mixing. During concreting in hot climatic condition, the concrete may have to be cooled to protect from ill effects. To achieve this, cooled water and aggregates can be used while preparing the concrete. The concrete placed at an atmospheric temperature above 40C is considered as hot weather concreting.At this temperature certain special problems are usually encountered. They are: 1) 2) 3) 4) 5) 6) 7) 8) Rapid rate of hydration of cement, quick setting and early stiffening Rapid evaporation of mixed water. Greater plastic shrinkage. Less time for finishing. Reduced relative humidity. Absorption of water from the concrete by the sub grade andformwork. Difficulty in continuous and uninterrupted curing. Difficulty in incorporation of Air entertainment. Precautions: 1) Aggregate should be stack piled in shade frequent sprinkling of water should be done to lower the temperature. 2) Cold water may be used to lower the temperature and concrete or use of ice can be done while mixing. 3) Reinforcement formwork and subgrade should be wetted with cold water prior to planning. Page | 43 Concrete Technology By SHAH SIR (7888280720) B.E. (Civil Engineering) Third Semester (C.B.S.) 4) It is better to do the concreting operations during the evening time when temperature is low. 5) Continuous wet curing is necessary Hot weather has a significant effect on all stages of concrete production and placement procedures such as increasing rate of hydration and movement of moisture from within and surface of the concrete. It affects long term strength and durability. Hot weather along with relative humidity and wind speed also have a significant influence. Basic Guides for Hot Weather Concreting:1) 2) 3) 4) 5) 6) 7) 8) 9) 10) 11) 12) Plan in advance. Have equipment and materials ready before the hot weather arrives. Keep the subgrade and forms moist so they do not absorb water from the mix. Keep sunshades and windbreaks available and use them whenever possible. Have everything prepared before the ready-mix truck arrives. Don’t make the truck wait for you. Keep in constant communication with the ready-mix concrete provider. Coordination between the contractor and the producer is key. Concrete should be placed, struck off and darby immediately. Use evaporation retardants, fogging or misting with water, or cover with a vapor-proof sheet after screeding. This will help prevent rapid drying, crusting, plastic shrinkage and rubber sets. Temporary covers, like continuously moistened burlap, may be placed over the fresh concrete and removed in small sections immediately ahead of the finishers. Substituting a burlap drag or broom finish will eliminate other high-risk finishing practices, such as smooth trowel finish. Curing should take place when the surfaces are hard enough to resist marring. Seal with a good, quality sealer for a minimum of 30 days after placing and curing. Remember to protect the crew in high temperatures. Drink plenty of fluids and be careful with long exposure to the sun. ***** Complete By Shah Sir (Civil Engineering Tutor) Mob- +91 7888280720 Page | 44 Concrete Technology By SHAH SIR (7888280720) B.E. (Civil Engineering) Third Semester (C.B.S.) SHAH SIR’S TEACHING SUBJECT:(A) Poly (Civil Engineering): 3rd Semester: MOS 4th Semester: TOS, Hydraulics 5th Semester: DSS, Traffic Engineering 6th Semester: Estimation & Costing, DRS (B) B.E/ B.Tech (Civil Engineering): 1st Semester: M-1 2nd Semester: M-2, EM 3rd Semester: M-3, SOM 4th Semester: SA-1, Survey-1, GT-1, TPE-1 5th Semester: SA-2, RCC, FM-1, GT-2, HWR 6th Semester: SS, Survey-2, FM-2, ENV-2 7th Semester: ACS, E&C, Traffic Eng. ,CML 8th Semester: PAD, Adv. RCC, CEF, IE Note:Regular/Weekly/Crash Classes Available Contact Us :7888280720 (SHAH SIR) Page | 45 Concrete Technology By SHAH SIR (7888280720) B.E. (Civil Engineering) Third Semester (C.B.S.) Unit-03 (13 Marks) Strength of concrete, Testing of Hardness of Concrete Theories:- 13 Marks Ques No.01:- What are the factors which affect the strength of concrete? Explain one of them in brief. Or Explain factor affecting strength of concrete. Or Explain the various properties which affect the quality of concrete. (Semi-Compulsory For 7 Marks) Ans:- Factors which affect the strength of concrete:1) Water – Cement Ratio: Water cement ratio is one of the most important factors which govern the strength of concrete. Water cement ratio is the weight of water in proportion to weight of cement expressed in terms of percentage. Workability increases if more water is added. But if large amount of water is added then paste will become dilute and it will not gain its strength. Adequate amount of water should be added so as to attain high workability and strength. 2) Size of Aggregates Used: For large size aggregates, the surface area is less and less cement paste is required to cover the surface area. But if extra-large size aggregates are used then cracks may appear. 3) Shape of Aggregates: Rounded aggregates give more workability to concrete. Angular or pointed aggregates make concrete harsh. 4) Mix Proportion: High mixes of concrete give more workability to concrete as more cement gives more paste of cement for lubricating the concrete. 5) Admixtures: Presence of admixtures such as pozzolana or plasticizers increases the workability and strength of concrete. 6) Grading of Aggregates: If proper grading of aggregates is done then presence of void is reduced. Therefore aggregates must be properly graded and then used so as to increase the workability and strength of concrete. 7) Surface Texture of Aggregates: Aggregates having irregular or hard surface require more cement to cover them. Hence aggregates must be smooth so as to give proper covering of cement to aggregates and provide workability to concrete. 8) Quality of Aggregates: Good quality aggregates prevents the concrete from creep. Page | 46 Concrete Technology By SHAH SIR (7888280720) B.E. (Civil Engineering) Third Semester (C.B.S.) 9) Creep: Creep of concrete is the permanent strain induces in concrete due to sustained load over a long period of time. Poor mix proportion shows more creep and water-cement ratio is the major factor which affects creep in concrete. 10) Age of Concrete: With age of concrete the rate of creep also increases in concrete due to sustained load for a long period of time. 11) Segregation: Segregation is the separation of constituent materials of concrete. Segregation may appear due to bad quality of aggregates used or due to voids present in cement paste. 12) Bleeding: Bleeding is a form of segregation in which water comes at top of the concrete. Bleeding is also called water gain and is more problematic in wet mixes. 13) Shrinkage: Concrete shrinks in volume due to loss of water or absorption of water. Major cause of shrinkage is bleeding due to which water from surface evaporates rapidly causing shrinkage cracks or uneven surface finish. Shrinkage can also appear due to hydration reaction. Ques No.02:- Explain Poison's ratio of concrete. (Compulsory For 4 Marks) Ans:- Poison's ratio of concrete: The ratio of the relative contraction strain (transverse, lateral or radial strain) normal to the applied load - to the relative extension strain (or axial strain) in the direction of the applied load. Poisson's ratio varies between 0.1 for high strength concrete and 0.2 for weak mixes. It is normally taken as 0.15 for strength design and 0.2 for serviceability criteria. Poisson's Ratio can be expressed as; μ = - εt / εl where; μ = Poisson's ratio εt = transverse strain (m/m, ft/ft) εl = longitudinal or axial strain (m/m, ft/ft) Ques No.03:- Explain Modulus of elasticity of concrete. (Future For 4 Marks) Ans:- Modulus of elasticity of concrete: Modulus of elasticity (also known as elastic modulus, the coefficient of elasticity) of a material is a number which is defined by the ratio of the applied stress to the corresponding strain within the elastic limit. Physically it indicates a material’s resistance to being deformed when a stress is applied to it. Modulus of elasticity also indicates the stiffness of a material. Value of elastic modulus is higher for the stiffer materials. Page | 47 Concrete Technology By SHAH SIR (7888280720) B.E. (Civil Engineering) Third Semester (C.B.S.) Modulus of Elasticity of Concrete can be defined as the slope of the line drawn from stress of zero to a compressive stress of 0.45f’c. As concrete is a heterogeneous material. The strength of concrete is dependent on the relative proportion and modulus of elasticity of the aggregate. Modulus of elasticity of concrete(Ec) is defined as the ratio of the applied stress to the corresponding strain. Not only does it demonstrate the ability of concrete to withstand deformation due to applied stress but also its stiffness. In other words, it reflects the ability of concrete to deflect elastically. Modulus of elasticity of concrete is sensitive to aggregate and mixture proportions of concrete. It is defined as the ratio between the normal stress to normal strain below the proportional limit of a material called modulus of elasticity Ec. Modulus of elasticity = unit stress/unit strain With the compressive strength test on the concrete specimen (cylinder of 15 cm diameter and 30 cm length having a volume 15 cm cube), the modulus of elasticity of concrete is calculated with the help of stress and strain graph. According to ACI codes, the modulus of elasticity of concrete can e measure with the formula, And with normal density or weight of concrete, these two relationships can be simplified as, #Where Ec = Modulus of elasticity of concrete. f’c = Compressive strength of concrete. Ques No.04:- Write short notes on Accelerated curing test. (Compulsory For 4 Marks) Ans:- Accelerated curing test: Accelerated Curing Method is used to get early high compressive strength in concrete. This method is also used to find out 28 days compressive strength of concrete in 28 hours. (As per IS 9013-1978-Method of making, curing and determining compressive strength of accelerated cured concrete test specimens). OBJECTIVE: Normally, the strength of concrete is found out after 7 days and 28 days. For some construction activities, it may be too late and need to know the strength earlier. Page | 48 Concrete Technology By SHAH SIR (7888280720) B.E. (Civil Engineering) Third Semester (C.B.S.) REFERENCE STANDARDS: IS: 9013-1978-Method of making, curing and determining compressive strength of accelerated cured concrete test specimen. EQUIPMENT & APPARATUS: 1) Accelerated curing tank 2) Compression Testing Machine PROCEDURE: 1) Prepare the specimen and store it in moist air of at least 90% relative humidity and at a temperature of 27+2 0C for 23 hrs + 15 minutes. 2) Lower the specimen, into a curing tank with water at 100 0C and keep it totally immersed for 3 ½ hours + 5 minutes 3) The temperature of water shall not drop more than 3 0C after the specimens are placed and should return to boiling within 15 minutes. 4) After curing for 3 ½ hours + 5 minutes in the curing tank, the specimen shall be removed from the moulds and cooled by immersing in cooling water 27+2 0C for a period of at least one hour. 5) Read my post compressive strength test of concrete for further steps. CALCULATION: The corresponding strength at 28 days can be found out from the following correlation. (It is however suggested that a new specific correlation should be developed for the specific concrete used at site.) R28 (Strength at 28 days) = 8.09 + 1.64 Ra Where, Ra = Accelerated Curing Strength in MPa. SAFETY & PRECAUTIONS: 1) Use hand gloves, safety shoes, while removing containers from curing tank after switching off the curing tank & place sample in machine properly. 2) Equipment should be cleaned thoroughly before testing & after testing. ***** Complete By Shah Sir (Civil Engineering Tutor) Mob- +91 7888280720 Page | 49 Concrete Technology By SHAH SIR (7888280720) B.E. (Civil Engineering) Third Semester (C.B.S.) Crash Course Starts From 28th October 2019 # B.E 3rd Semester Civil Engineering M-3/SOM # B.E 5th Semester Civil Engineering SA-2/RCC/FM-1/GT-2/HWR # B.E 7th Semester Civil Engineering ACS/E&C/ATE By SHAH SIR Venue:- SHAH ENGINEERING CLASSES Trimurtinagar, Near Hingna T-Point, Nagpur Contact Us:- 7888280720 (SHAH SIR) Page | 50 Concrete Technology By SHAH SIR (7888280720) B.E. (Civil Engineering) Third Semester (C.B.S.) Unit-04 (13 Marks) Mix Design, Additives and Admixtures Theories:- 13 Marks Ques No.01:- Explain concept of Mix design & Factor affecting mix proportion. Or Enlist various methods of mix design and discuss various factors affecting mix proportions. Or Explain mix design process ? What are the factors affecting mix properties ? Also explain aggregate cement ratio. Or What are the objective of concrete mix design? Draw a flow chart for concrete mix design. Why mix design is required? (Compulsory For 7 Marks) Ans:- Concrete mix design: Cement concrete is a very useful constructional material whose properties can be modified readily at will. The desired properties of concrete can be obtained by using ingredients in a certain proportion. Thus determining the relative amount of materials is known as mix design. Hence mix design can be defined as the process of selecting suitable ingredients of concrete and determining their relative quantities for producing concrete of certain minimum properties as strength, durability and consistency etc., as economically as possible. Objective of concrete mix design:Following are the objectives of concrete mix design:a) b) c) d) e) f) g) h) To achieve a specified compressive strength for a specified grade. To maintain workability of concrete mix throughout work. For achieving durability. To achieve economy by selecting appropriate concrete ingredients. To obtain maximum possible yield per bag of cement. To avoid honey combing and bleeding. To comply with various standards. To reduce wastage of concrete by correct proportioning. Page | 51 Concrete Technology By SHAH SIR (7888280720) B.E. (Civil Engineering) Third Semester (C.B.S.) Advantages of Concrete Mix Design:1) 2) 3) 4) 5) Required Proportions of Each ingredient Quality Concrete Mix Economical Concrete Mix Best Use of Locally Available Material Desired Properties of Mix Requirements of concrete mix design:The requirements which form the basis of selection and proportioning of mix ingredients are : 1) The minimum compressive strength required from structural consideration 2) The adequate workability necessary for full compaction with the compacting equipment available. 3) Maximum water-cement ratio and/or maximum cement content to give adequate durability for the particular site conditions 4) Maximum cement content to avoid shrinkage cracking due to temperature cycle in mass concrete. Factors Affecting the Choice of Mix Proportion:According to IS 456-1978 and IS-1345-1980, the design of concrete should be based on the following factors: 1) 2) 3) 4) 5) 6) 7) 8) Grade designation Type of cement Max nominal size of aggregate Grading of combined aggregates Water-cement ratio Workability Durability, and Quality control. Methods of Mix Design:There are various methods of mix design as follows:1) 2) 3) 4) 5) 6) 7) 8) 9) 10) 11) 12) Arbitrary method Minimum void method Maximum density method Water-cement ratio method Fineness modulus method Standard deviation method American method of mix design Graphic or Road Note 4 method of mix design Indian Road Congress IRC-44 method High strength concrete mix design Mix design based on flexural strength ACI committee 211-1 method Page | 52 Concrete Technology By SHAH SIR (7888280720) B.E. (Civil Engineering) Third Semester (C.B.S.) 13) 14) 15) 16) Department of Environment (DOE) mix design method Mix design for pumpable concrete Indian standard recommended (IS 10262-1982) method, and Rapid method of mix design. Nowadays out of the above methods following methods are commonly used: 1) 2) 3) 4) ACI committee 211 method DOE method Indian Standard method, and Rapid method of mix design. Aggregate cement ratio: Aggregate cement ratio is the ratio of weights of aggregate to the weight of cement. If this ratio is more, that implies aggregates are more and cement is less and if this ratio is less, that implies weight of aggregate is less and weight of cement is more (relatively). When the weight of cement is more, i.e. aggregate cement ratio is less, the concrete has more cement paste required to coat aggregates and fill the voids between them. This more cement paste makes the concrete relatively easy to mix, place and compact as it reduces the friction between aggregates and allows its smooth movement. Thus the workability of concrete increases. When the weight of cement is less, i.e. aggregate cement ratio is more, then there will be very less cement paste to coat aggregate surfaces and fill the voids, thus, mixing, placing and compacting of concrete will be more harder than previous case. This implies, the workability of concrete will be less than previous case. Ques No.02:- What are admixtures ? Explain air entrained concrete. Or List various types of Admixtures used in concrete ? Explain plasticizers and superplasticizers. Or Write a short note on Air entraining admixtures ,plasticizers and super plasticizers. Or Types of admixtures and factors affecting admixture (any two). Or Write down the role of admixture as a component of concrete & Explain plasticizers. Or Explain various admixtures used to improve workability of concrete. Or Write short note on Accelerators and Retarders. (Compulsory For 7 Marks) Page | 53 Concrete Technology By SHAH SIR (7888280720) B.E. (Civil Engineering) Third Semester (C.B.S.) Ans:- Admixtures: As per BIS (IS – 9103: 1999) Page No.1, Concrete Admixture is defined as a material other than water, aggregates and hydraulic cement and additives like Pozzolana or slag and fiber reinforcement, used as on ingredient of concrete or mortar and added to the batch immediately before or during its mixing to modify one or more of the properties of concrete in the plastic or hardened state. A material other than water, aggregates, or cement that is used as an ingredient of concrete or mortar to control setting and early hardening, workability, or to provide additional cementing properties. Admixtures of concrete are generally used to alter the properties of concrete (such as increased workability or reduced water content, acceleration or retardation of setting time, acceleration of strength development, and improved resistance to weather and chemical attacks) to make it more suitable for a particular purpose. For example, calcium chloride can be used to accelerate strength development in mass concrete during winter. Air-entraining admixtures (inexpensive soaps, detergents, etc.) entrained air which greatly improves the workability of concrete and thus permits the use of harsher and more poorly graded aggregates and also those of undesirable shapes. When Concrete Admixtures Used? 1) 2) 3) 4) 5) When properties cannot be made by varying the composition of basic material. To produce desired effects more economically. Unlikely to make a poor concrete better. Not a substitute for good concrete practice. Required dose must be carefully determined and administered Types of Concrete Admixtures:Concrete admixtures are of different types and they are as follows: 1) 2) 3) 4) 5) 6) 7) 8) 9) 10) 11) 12) 13) 14) 15) Water Reducing Admixtures (Plasticizers) Retarding Admixtures Accelerating Admixtures Air entraining concrete admixture Pozzolanic Admixtures Damp-proofing Admixtures Gas forming Admixtures Air detraining Admixtures Alkali Aggregate Expansion Inhibiting Admixtures Anti-washout Admixtures Grouting Admixtures Corrosion Inhibiting Admixtures Bonding Admixtures Fungicidal, Germicidal, Insecticidal Admixtures Coloring Admixtures Page | 54 Concrete Technology By SHAH SIR (7888280720) B.E. (Civil Engineering) Third Semester (C.B.S.) 1. Water Reducing Admixtures (Plasticizers): Water reducing admixtures, the name itself defining that they are used to minimize the water demand in a concrete mix. Workability is the important property of concrete which is improved with the addition of water but if water is added more than required the strength and durability properties of concrete gets affected. In addition to increase in workability it also improves the strength of concrete, good bond between concrete and steel, prevents cracking, segregation, honeycombing, bleeding etc. Water reducing admixtures are also called as plasticizers and these are classified into three types namely plasticizers, mid-range plasticizers and super plasticizers. Normal plasticizer reduces the water demand up to 10%, mid-range plasticizers reduce the water demand up to 15% while super plasticizers reduce the water demand up to 30%. Calcium, sodium and ammonium lignosulphonates are commonly used plasticizers. Some of the new generation super plasticizers are acrylic polymer based, poly carboxylate, multicarbovylatethers etc. Applications of Retarding Concrete Admixture:1) Where long transportation of ready mixed concrete is required then premature setting can be usefully avoided by this type admixture. 2) When concrete is being placed or transported under conditions of high ambient temperature. 3) In case of large concrete pours 4) Concrete construction involving sliding formwork 2. Retarding Admixtures: Retarding admixtures slow down the rate of hydration of cement in its initial stage and increase the initial setting time of concrete. These are also called as retarders and used especially in high temperature zones where concrete will set quickly. The quick setting in some situations may lead to discontinuities in structure, poor bond between the surfaces, creates unnecessary voids in concrete etc. Retarders are useful to eliminate this type of problems. Commonly used retarding admixture is calcium sulphate or gypsum. Starch, cellulose products, common sugar, salts of acids are some other retarders. Most of water reducing admixtures are also acts as retarding admixtures and they are called as retarding plasticizers. 3. Accelerating Admixtures: Accelerating admixtures are used for quicker setting times of concrete. It provides higher early strength development in freshly cast concrete. Accelerating admixtures are used to reduce the initial setting time of concrete. They speed up the process of initial stage of hardening of concrete hence they are also called as Page | 55 Concrete Technology By SHAH SIR (7888280720) B.E. (Civil Engineering) Third Semester (C.B.S.) accelerators. These accelerators also improves the strength of concrete in it early stage by increasing the rate of hydration. Earlier hardening of concrete is useful in several situations such as early removal of formwork, less period of curing, emergency repair works, for constructions in low temperature regions etc. Some of the accelerating admixtures are triethenolamine, calcium formate, silica fume, calcium chloride, finely divided silica gel etc. Calcium chloride is the cheap and commonly used accelerating admixture. Disadvantages of Accelerating Concrete Admixtures:1) 2) 3) 4) It has increased drying shrinkage It offers reduced resistance to sulphate attack CaCl2 high risk of corrosion of steel – not permitted in reinforced concrete It is more expensive and less effective Main uses of Accelerating Concrete Admixtures:1) These admixtures are suitable for concreting in winter conditions 2) During any emergency repair work 3) In case of early removal of formwork 4. Air Entraining Concrete Admixture: Air entraining admixtures are one of the most important inventions in concrete technology. Their primary function is to increase the durability of concrete under freezing and thawing conditions. When added to concrete mix, these admixtures will form millions of noncoalescing air bubbles throughout the mix and improves the properties of concrete. Air entrainment in concrete will also improve the workability of concrete, prevents segregation and bleeding, lower the unit weight and modulus of elasticity of concrete, improves the chemical resistance of concrete and reduction of cement or sand or water content in concrete etc. Most used air entrainment admixtures are vinsol resin, darex, Teepol, Cheecol etc. These admixtures are actually made of Natural wood resins, alkali salts, animal and vegetable fats and oils etc. 5. Damp-proofing Admixtures: Damp proofing or water proofing admixtures are used to make the concrete structure impermeable against water and to prevent dampness on concrete surface. In addition to water proof property, they also acts like accelerators in early stage of concrete hardening. Damp proofing admixtures are available in liquid form, powder form, paste form etc. The main constituents of these admixtures are aluminum sulfate, zinc sulfate aluminum chloride, calcium chloride, silicate of soda etc. which are chemically active pore fillers. Page | 56 Concrete Technology By SHAH SIR (7888280720) B.E. (Civil Engineering) Third Semester (C.B.S.) 6. Super Plasticizers or High Range Water-Reducing Admixtures in Concrete: These are the second generation admixture and also called as Superplasticizers. These are synthetic chemical products made from organic sulphonates of type RSO3, where R is complex organic group of higher molecular weight produced under carefully controlled condition. The commonly used superplasticizer are as follows: a) Sulphonated melamine formaldehyde condensate (S M F C) b) Sulphonated napthalene formaldehyde condensate (S N F C) c) Modified ligno-sulphonates and other sulphonic esters, acids etc., Advantages of Super Plasticizer Admixtures:1) The concrete using this admixture can be placed with little or no compaction and is not subject to excessive bleeding or segregation. 2) They can be used as high dosages became they do not markedly change the surface tension of water. 3) It does not significantly affect the setting of concrete except that when used the cements having a very low C3A content. 4) They do not influence shrinkage, creep modulus of elasticity or resistance to freeing to thawing. Disadvantage:1) The only real disadvantage of superplasticizer is their relatively high cost. Applications of Super Plasticizer Admixtures:2) In very heavily reinforced sections, in inaccessible areas in floor or road slabs. 3) Where very rapid placing is desired. Ques No.03:- Explain in detail concrete mix Design by IS : 10262 : 2009. Or Explain I. S. method of concrete Mix Design. Or Write the step by step procedure adopted for the method of mix design as per IS. Or Explain concrete mix design procedure as per IS:10262. (Compulsory For 6 Marks) Ans:- Concrete mix Design by IS : 10262 : 2009: Procedure for concrete mix design calculation as per IS 10262-2009 based on strength and durability, workability, economy . To produce concrete of required strength and properties, selection of ingredients and their quantity is to be found which is called concrete mix design. Page | 57 Concrete Technology By SHAH SIR (7888280720) B.E. (Civil Engineering) Third Semester (C.B.S.) Concrete Mix Design Procedure as per is 10262 – 2009:Procedure for concrete mix design requires following step by step process:1) 2) 3) 4) 5) 6) 7) 8) Calculation of target strength of concrete Selection of water-cement ratio Determination of aggregate air content Selection of water content for concrete Selection of cement content for concrete Calculation of aggregate ratio Calculation of aggregate content for concrete Trial mixes for testing concrete mix design strength Step 1: Calculation of Target Strength of Concrete: Target strength is denoted by ft which is obtained by characteristic compressive strength of concrete at 28 days (fck) and value of standard deviation (s) ft = fck + 1.65 s Standard deviation can be taken from below table: Grade of concrete Standard deviation (N/mm2) M10 M15 M20 M25 M30 M35 M40 M45 M50 3.5 3.5 4.0 4.0 5.0 5.0 5.0 5.0 5.0 Step 2: Selection of Water-Cement Ratio: Ratio of the weight of water to weight of cement in the concrete mix is water-cement ratio. It is the important consideration in concrete mix design to make the concrete workable. Water cement ratio is selected from the below curve for 28 days characteristic compressive strength of concrete. (Fig: Selection of Water-Cement Ratio for Concrete Mix Design) Page | 58 Concrete Technology By SHAH SIR (7888280720) B.E. (Civil Engineering) Third Semester (C.B.S.) Similarly, we can determine the water-cement ration from the 7-day concrete strength, the curves are divided on the basis of strength from water cement ratio is decided. Which is observed from the below graph. (Fig: Concrete Compressive Strength vs. Water Cement Ratio) Step 3: Determination of Aggregate Air content: Air content in the concrete mix is determined by the nominal maximum size of aggregate used. Below table will give the entrapped air content in percentage of volume of concrete. Nominal maximum size of aggregate 10mm 20mm 40mm Air content (% of volume of concrete) 5% 2% 1% Step 4: Selection of Water Content for Concrete: Select the water content which is useful to get required workability with the help of nominal maximum size of aggregate as given in below table. The table given below is used when only angular shaped aggregates are used in concrete as well as the slump should be 25 to 50mm. Nominal maximum size of aggregate 10mm 20mm 40mm Page | 59 Concrete Technology Maximum water content 208 186 165 By SHAH SIR (7888280720) B.E. (Civil Engineering) Third Semester (C.B.S.) If the shape of aggregate or slump value is differing from above, then some adjustments are required as follows. Condition Adjustment Sub angular aggregate Gravel with crushed stone Rounded gravel Using plasticizer Using superplasticizer For every increment of 25mm slump Reduce the selected value by 10% Reduce the selected value by 20kg Reduce the selected value by 25kg Decrease the selected value by 5-10% Decrease the selected value by 20-30% Increase the selected value by 3% Step 5: Selection of Cement Content for Concrete: Water – cement ratio is determined in step2 and quantity of water is determined in step -4. So, we can easily calculate the quantity of cement from these two conditions. But, the value obtained should satisfy the minimum conditions as given in the below table. The greater of the two values is decided as quantity of cement content. Cement Content for Plain Cement Concrete: Exposure Mild Moderate Severe Very severe Extreme Plain Cement Concrete (P.C.C) Minimum Cement Max Free Water – Content Kg/m3 Cement Ratio 220 0.6 240 0.6 250 0.5 260 0.45 280 0.4 Minimum Grade of Concrete – M15 M20 M20 M25 Cement Content for Reinforced Concrete: Exposure Mild Moderate Severe Very severe Extreme Reinforced Cement Concrete (RCC) Minimum Cement Max Free Water – Content Kg/m3 Cement Ratio 300 0.55 300 0.5 320 0.45 340 0.45 Minimum Grade of Concrete M20 M25 M30 M35 360 M40 0.4 Step 6: Calculation of Aggregate Ratio: For the given nominal maximum size of aggregate, we can calculate the ratio of volumes of coarse aggregate and volume of total aggregates for different zones of fine aggregates from the below table Page | 60 Concrete Technology By SHAH SIR (7888280720) B.E. (Civil Engineering) Third Semester (C.B.S.) Nominal maximum size of aggregate 10mm 20mm 40mm Ratio of volume of coarse aggregate and volume of total aggregate for different zones of fine aggregate Zone – 1 Zone – 2 Zone – 3 Zone – 4 0.44 0.46 0.48 0.50 0.6 0.62 0.64 0.66 0.69 0.71 0.73 0.75 Step 7: Calculation of Aggregate Content for Concrete: We already determine the coarse aggregate volume ratio in the total aggregate volume. So, it is very easy that, 1 – volume of coarse aggregate will give the volume of fine aggregate. Alternatively, there are some formulae to find the volume of fine and coarse aggregates as follows. Mass of fine aggregate is calculated from below formula: Similarly, mass of coarse aggregate is calculated from below formula: Where, V = volume of concrete W = water content C = cement content Gc = sp. Gravity of cement P = aggregate ration obtained in step6 F.A & C.A = masses of fine and coarse aggregates Gf & Gca = sp. Gravities of fine and coarse aggregates. Step 8: Trial Mixes for Testing Concrete Mix Design Strength: Based on the values obtained above, conduct a trail test by making at least 3 cubes of 150mm size as per above standards. Test that cubes and verify whether the required strength is gained or not. If not, redesign the mix with proper adjustments until required strength of cube occurs. The following points should be remembered before proportioning a concrete mix a per IS-102622009:a) This method of concrete mix proportioning is applicable only for ordinary and standard concrete grades. b) The air content in concrete is considered as nil. c) The proportioning is carried out to achieve specified characteristic compressive strength at specified age, workability of fresh concrete and durability requirements. ****** Page | 61 Concrete Technology By SHAH SIR (7888280720) B.E. (Civil Engineering) Third Semester (C.B.S.) Crash Course (Regular Winter 2019 Examinations) 10 Days Workshop (Starts from 28th Oct to 10th Nov) # B.E 3rd Semester Civil Engineering Applied Mathematics-3 (M-3) Strength of Materials (SOM) By SHAH SIR Fees:M-3 @3000/M-3+SOM @5000/With ENV-1 & EG (Numericals) @ Free/- Venue:- SHAH ENGINEERING CLASSES Trimurtinagar, Near Hingna T-Point ,Nagpur Contact Us:- 7888280720 (SHAH SIR) Page | 62 Concrete Technology By SHAH SIR (7888280720) B.E. (Civil Engineering) Third Semester (C.B.S.) Unit-05 (13 Marks) Special concrete, Shrinkage, Creep Theories:- 13 Marks Ques No.01:- Explain in brief self compacting concrete and high performance concrete. Or What is high performance concrete. Also explain its procedure. Why this concrete is used in the field. Or Write short notes on:i) Self compacting concrete ii) High performance concrete (Compulsory For 7 Marks) Ans:- Self compacting concrete: Self compacting concrete (SCC) can be defined as fresh concrete that flows under its own weight and does not require external vibration to undergo compaction. It is used in the construction where it is hard to use vibrators for consolidation of concrete. Filling and passing ability, segregation resistance are the properties of self compacting concrete. SCC possess superior flow ability in its fresh state that performs self compaction and material consolidation without segregation issues. Advantages of Self Compacting Concrete:The main advantages of self compacting concrete are: a) b) c) d) e) f) g) The permeability of the concrete structure is decreased SCC enables freedom in designing concrete structures The SCC construction is faster The problems associated with vibration is eliminated The concrete is placed with ease, which results in large cost saving The quality of the construction is increase The durability and reliability of the concrete structure is high compared to normal concrete structures h) Noise from vibration is reduced. This also reduce the hand arm vibration syndrome issues Page | 63 Concrete Technology By SHAH SIR (7888280720) B.E. (Civil Engineering) Third Semester (C.B.S.) Disadvantages of Self Compacting Concrete:SCC construction face the following limitations: a) b) c) d) e) There is no globally accepted test standard to undergo SCC mix design The cost of construction is costlier than the conventional concrete construction The use of designed mix will require more trial batches and lab tests The measurement and monitoring must be more precise. The material selection for SCC is more stringent Applications of Self Compacting Concrete:The major applications of self compacting concrete are: a) b) c) d) Construction of structures with complicated reinforcement SCC is used for repairs, restoration and renewal construction Highly stable and durable retaining walls are constructed with the help of SCC SCC is employed in the construction of raft and pile foundations High performance concrete: High performance concrete is a concrete having high strength, high durability, high resistance to chemical attack and high workability. In other words all the mechanical properties of concrete are superior in a high performance concrete. High performance concrete is a concrete mixture, which possess high durability and high strength when compared to conventional concrete. This concrete contains one or more of cementious materials such as fly ash, Silica fume or ground granulated blast furnace slag and usually a super plasticizer. The term ‘high performance’ is somewhat pretentious because the essential feature of this concrete is that it’s ingredients and proportions are specifically chosen so as to have particularly appropriate properties for the expected use of the structure such as high strength and low permeability. Hence High performance concrete is not a special type of concrete. It comprises of the same materials as that of the conventional cement concrete. The use of some mineral and chemical admixtures like Silica fume and Super plasticizer enhance the strength, durability and workability qualities to a very high extent. High Performance concrete works out to be economical, even though it’s initial cost is higher than that of conventional concrete because the use of High Performance concrete in construction enhances the service life of the structure and the structure suffers less damage which would reduce overall costs. ADVANTAGES OF HPC:a) b) c) d) e) Reduction in size of the columns Speed of construction More economical than steel concrete composite columns Workability and pumpability Most economical material in terms of time and money Page | 64 Concrete Technology By SHAH SIR (7888280720) B.E. (Civil Engineering) Third Semester (C.B.S.) f) g) h) i) j) k) l) m) n) o) p) Increased rentable\useful floor space Reduced depth of floor system and decrease in overall building height Higher seismic resistance, lower wind sway and drift Improved durability in aggressive environment Wearing resistance, abrasion resistance Durability against chloride attack Increased durability in marine environment Low shrinkage and high strength Service life more than 100 years High tensile strength Reduced maintenance cost LIMITATIONS OF HPC:a) High Performance Concrete has to be manufactured and placed much more carefully than normal concrete. b) An extended quality control is required c) In concrete plant and at delivery site, additional tests are required. This increases the cost d) Some special constituents are required which may not be available in the ready mix concrete plants. Ques No.02:- Write short notes on :(i) Ferrocement. (ii) Shotcrete. Or Explain shotcrete pumped concrete. (Semi-Compulsory For 6 Marks) Ans:- (i) Ferrocement: Ferrocement is a construction material consisting of wire meshes and cement mortar. Applications of ferrocement in construction is vast due to the low self weight, lack of skilled workers, no need of framework etc. “Ferro cement is a type of thin wall reinforced concrete, commonly constructed of hydraulic cement mortar, reinforced with closely spaced layers of continuous and relatively small size wire mesh. The mesh may be made of metallic or other suitable materials”. Properties of Ferrocement:a) Highly versatile form of reinforced concrete. b) It’s a type of thin reinforced concrete construction, in which large amount of small diameter wire meshes uniformly throughout the cross section. c) Mesh may be metal or suitable material. d) Instead of concrete Portland cement mortar is used. e) Strength depends on two factors quality of sand/cement mortar mix and quantity of reinforcing materials used. Page | 65 Concrete Technology By SHAH SIR (7888280720) B.E. (Civil Engineering) Third Semester (C.B.S.) Constituent Materials for Ferrocement:a) b) c) d) e) f) g) h) Cement Fine Aggregate Water Admixture Mortar Mix Reinforcing mesh Skeletal Steel Coating Advantages and Disadvantages of Ferrocement:Advantages: a) b) c) d) e) f) g) h) High ductility High resistance to cracking width Ability to undergo large deflection Improved impact resistance and toughness Good fire resistance Good impermeability Low strength to weight ratio Low maintenance costs Disadvantages: a) b) c) d) e) f) Structures made of it can be punctured by collision with pointed objects. Corrosion of the reinforcing materials due to the incomplete coverage of metal by mortar. It is difficult to fasten to Ferrocement with bolts, screws, welding and nail etc. Large no of labors required. Cost of semi-skilled and unskilled labors is high. Tying rods and mesh together is especially tedious and time consuming. Process of Ferrocement Construction:a) b) c) d) Fabricating the skeletal framing system. Applying rods and meshes. Plastering. Curing Applications of Ferrocements in Construction:a) b) c) d) e) f) g) h) Marine applications Water supply and sanitation Agricultural Residential buildings Rural energy Other structures Boats, fishing vessels, barges, cargo tugs, catamarans, yachts and flotation buoys Key criteria for marine applications: light weight, impact resistance, thickness and water tightness Page | 66 Concrete Technology By SHAH SIR (7888280720) B.E. (Civil Engineering) Third Semester (C.B.S.) (ii) Shotcrete: Shotcrete is defined as “mortar or concrete pneumatically projected at high speed onto a surface” (American Concrete Institute, 1990). There are two basic types of shotcrete—dry mix and wet mix. In dry mix shotcrete, the dry cement, sand, and coarse aggregate, if used, are premixed with only sufficient water to reduce dusting. Shotcrete is a method of applying concrete projected at high velocity primarily on to a vertical or overhead surface. The impact created by the application consolidates the concrete. Although the hardened properties of shotcrete are similar to those of conventional cast-in-place concrete, the nature of the placement process results in an excellent bond with most substrates, and rapid or instant capabilities, particularly on complex forms or shapes. The shotcrete process requires less formwork and can be more economical than conventionally placed concrete. Shotcrete is applied using a wet- or dry-mix process. The wet-mix shotcrete process mixes all ingredients, including water, before introduction into the delivery hose. The dry-mix shotcrete process adds water to the mix at the nozzle. Shotcrete is used in new construction and repairs and is suitable for curved and thin elements (ACI 506R). Shotcrete, also called (trademark) Gunite, concrete applied by spraying. Shotcrete is a mixture of aggregate and portland cement, conveyed by compressed air to the nozzle of a spray gun, where water is added. The wet mixture is then sprayed in place and may be carved or troweled almost immediately. For structural uses, shotcrete is usually applied over a framework of reinforcing bars and steel mesh. Because it can take any shape, is easily coloured, and can be sculptured after application, shotcrete is used for a variety of fancy concrete structures, including artificial rock walls, zoo enclosures, canopy roofs, refractory linings, pools, and dams. It is sometimes used in tunneling to bind the walls of the tunnel to prevent leaks and fragmentation. Shortcreting is define as pneumatically applied concrete or motor placed directly on to the surface. Shortcreting can be placed by either the dry mix or wet mix process. Page | 67 Concrete Technology By SHAH SIR (7888280720) B.E. (Civil Engineering) Third Semester (C.B.S.) Dry mix process consist of :a) b) c) d) Thoroughly mixing the dry material. Feeding of these material into mechanical feeder or gun. Carrying the material by compress air through a nose to special nozzle. Introducing water at nozzle point and immediately mixing it with other ingredient at the nozzle. e) Jetting the mixture from the nozzle at high velocity on to the surface to receive the shortcrete. Wet mix process shall be consist of :a) b) c) d) Thoroughly mixing all ingredient with the expecting of the accelerating admixture if used. Feeding the mixture into the delivery equipment. Delivering the mixture by positive displacement or compressed air to nozzle. Setting the mixture from the nozzle at high velocity on the surface to receive the shortcrete. If specification fibre at steel polypropylene or other material as may be specified could also be used together with the admixture to modify the structural properties at the concrete motor being placed in position. Ques No.03:- Explain in brief polymer concrete & fibre reinforced concrete. Or Write a short note on:i) Fiber Reinforced & Polymer concrete. (Future For 6 Marks) Ans:- Fibre reinforced concrete: Fiber Reinforced Concrete can be defined as a composite material consisting of mixtures of cement, mortar or concrete and discontinuous, discrete, uniformly dispersed suitable fibers. Fiber reinforced concrete are of different types and properties with many advantages. Continuous meshes, woven fabrics and long wires or rods are not considered to be discrete fibers. Fiber is a small piece of reinforcing material possessing certain characteristics properties. They can be circular or flat. The fiber is often described by a convenient parameter called “aspect ratio”. The aspect ratio of the fiber is the ratio of its length to its diameter. Typical aspect ratio ranges from 30 to 150. Fiber reinforced concrete (FRC) is concrete containing fibrous material which increases its structural integrity. It contains short discrete fibers that are uniformly distributed and randomly oriented. Fibers include steel fibers, glass fibers, synthetic fibers and natural fibers. Within these different fibers that character of fiber reinforced concrete changes with varying concretes, fiber materials, geometries, distribution, orientation and densities. Fibre-reinforcement is mainly used in shotcrete, but can also be used in normal concrete. Fibre-reinforced normal concrete are mostly used for on-ground floors and pavements, but can be considered for a wide range of construction parts (beams, pliers, foundations etc) either alone or with hand-tied rebars. Page | 68 Concrete Technology By SHAH SIR (7888280720) B.E. (Civil Engineering) Third Semester (C.B.S.) Necessity of Fiber Reinforced Concrete:a) b) c) d) It increases the tensile strength of the concrete. It reduce the air voids and water voids the inherent porosity of gel. It increases the durability of the concrete. Fibres such as graphite and glass have excellent resistance to creep, while the same is not true for most resins. Therefore, the orientation and volume of fibres have a significant influence on the creep performance of rebars/tendons. e) Reinforced concrete itself is a composite material, where the reinforcement acts as the strengthening fibre and the concrete as the matrix. It is therefore imperative that the behavior under thermal stresses for the two materials be similar so that the differential deformations of concrete and the reinforcement are minimized. f) It has been recognized that the addition of small, closely spaced and uniformly dispersed fibers to concrete would act as crack arrester and would substantially improve its static and dynamic properties. Polymer concrete: Polymer concrete is an ordinary concrete produced with OPC ( Ordinary portland cement) wet cured and inseminated with liquid or vaporous chemical compound (Methyl methacrylate monomer) and polymerized by gamma radiation or with chemical initiated implies, i.e by utilizing thermal catalytic method (Adding 3% Benzoyl peroxide) to the monomer as a catalyst. The impregnation is helped by drying the concrete at an extreme temperature by evacuations and absorbing the monomer under limited pressure. TYPES OF POLYMER CONCRETE: Polymer concrete can be classified in following three categories: a) Polymer impregnated concrete (PIC). b) Polymer cement concrete (PCC). c) Polymer concrete (PC). ADVANTAGES OF POLYMER CONCRETE: a) b) c) d) It has high impact resistance and high compressive strength. Polymer concrete is highly resistant to freezing and thawing. Highly resistant to chemical attack and abrasion. Permeability is lower than other conventional concrete. APPLICATION OF POLYMER CONCRETE: Polymer concrete is broadly utilizing in several circumstances as following: a) b) c) d) e) f) Nuclear power plants. Kerbstones. Prefabricated structural element. Precast slabs for bridge decks. Roads. Marine Works. Page | 69 Concrete Technology By SHAH SIR (7888280720) B.E. (Civil Engineering) Third Semester (C.B.S.) g) h) i) j) k) Prestressed concrete. Irrigation works. Sewage works. Waterproofing of buildings. Food processing buildings etc. Ques No.04:- Explain in detail shrinkage of concrete. Or Define Shrinkage ? Explain in brief various classification of shrinkage. Or Define and classify Shrinkage in concrete. How it can be controlled? Or Different types of shrinkage and factors affecting shrinkage. Or Explain the factors affecting creep and shrinkage of concrete. Or Explain Differential shrinkage. Or Explain plastic shrinkage and drying shrinkage. (Compulsory For 7 Marks) Ans:- Shrinkage: The volumetric changes of concrete structures due to the loss of moisture by evaporation is known as concrete shrinkage or shrinkage of concrete. It is a time-dependent deformation which reduces the volume of concrete without the impact of external forces. Concrete is subjected to changes in volume either autogenous or induced. Volume change is one of the most detrimental properties of concrete, which affects the long-term strength and durability. To the practical engineer, the aspect of volume change in concrete is important from the point of view that it causes unsightly cracks in concrete and called concrete shrinkage. TYPES OF SHRINKAGE: The types of concrete shrinkage are listed below: 1. PLASTIC SHRINKAGE: Plastic shrinkage occurs very soon after pouring the concrete in the forms. The hydration of cement results in a reduction in the volume of concrete due to evaporation from the surface of concrete, which leads to cracking. 2. DRYING SHRINKAGE: The shrinkage that appears after the setting and hardening of the concrete mixture due to loss of capillary water is known as drying shrinkage. Drying shrinkage generally occurs in the first few months and decreases with time. Page | 70 Concrete Technology By SHAH SIR (7888280720) B.E. (Civil Engineering) Third Semester (C.B.S.) 3. CARBONATION SHRINKAGE: Carbonation shrinkage occurs due to the reaction of carbon dioxide (Co2) with the hydrated cement minerals, carbonating Ca(Oh)2 to CaCO3. The carbonation slowly penetrates the outer surface of the concrete. This type of shrinkage mainly occurs at medium humidities and results increased strength and reduced permeability. 4. AUTOGENOUS SHRINKAGE: Autogenous shrinkage occurs due to no moisture movement from concrete paste under constant temperature. It is a minor problem of concrete and can be ignored. FACTORS AFFECTING SHRINKAGE: The shrinkage of concrete depends on several factors which are listed below. 1. WATER-CEMENT RATIO: shrinkage is mostly influenced by the water cement ratio of concrete. It increases with the increases in the water-cement ratio. 2. ENVIRONMENTAL CONDITION: It is one of the major factors that affect the total volume of shrinkage. Shrinkage is mostly occurred due to the drying condition of the atmosphere. It increases with the decrease in the humidity. 3. TIME: The rate of shrinkage rapidly decreases with time. It is found that 14-34% of the 20 years shrinkage occurs in two weeks, 40-80% shrinkage occurs in three months and the rest 66-85% shrinkage occurs in one year. 4. TYPE OF AGGREGATE: Aggregates with moisture movement and low elastic modulus cause large shrinkage. The rate of shrinkage generally decreases with the increase of the size of aggregates. It is found that concrete made from sandstone shrinks twice than the concrete of limestone. 5. ADMIXTURES: The shrinkage increases with the addition of accelerating admixtures due to the presence of calcium chloride (CaCl2) in it And it can be reduced by lime replacement. Other Factors: a) b) c) d) e) The type and quantity of cement. Granular and microbiological composition of aggregates. The strength of concrete. The method of curing. The dimension of elements etc. Page | 71 Concrete Technology By SHAH SIR (7888280720) B.E. (Civil Engineering) Third Semester (C.B.S.) Ques No.05:- Define creep. Explain various factors affecting creep. Or Define creep and write down the factor affecting creep. Or What do you understand by Creep? What are various factors affecting it? Or What do you mean by creep in harden concrete? Describe the relation between creep & age of concrete. (Compulsory For 7 Marks) Ans:- Creep: Concrete creep is defined as: deformation of structure under sustained load. Basically, long term pressure or stress on concrete can make it change shape. This deformation usually occurs in the direction the force is being applied. Like a concrete column getting more compressed, or a beam bending. Creep does not necessarily cause concrete to fail or break apart. When a load is applied to concrete, it experiences an instantaneous elastic strain which develops into creep strain if the load is sustained. FACTORS AFFECTING CREEP OF CONCRETE: The factors that affect creep of concrete are similar to the factors affecting shrinkage, which are as following: 1. WATER-CEMENT RATIO: The rate of creep is increased with increasing water cement ratio. 2. HUMIDITY: It is influenced by humidity and drying condition of the atmosphere. Page | 72 Concrete Technology By SHAH SIR (7888280720) B.E. (Civil Engineering) Third Semester (C.B.S.) 3. AGE OF CONCRETE: The rate of creep rapidly decreases with time. The time taken by a concrete structure to attained creep is 5 years. 4. AGGREGATE: Aggregates with moisture movement and low elastic modulus cause a large amount of creep. The rate of creep generally decreases with the increase of the size of aggregates. 5. ADMIXTURES: Some admixtures (mainly accelerators) are also responsible for causing creep in concrete. 6. OTHER FACTORS: a) Types of cement: Fineness of cement affects the strength development at early ages & thus influence of creep. b) Entrained air. c) Concrete strength. ****** Complete By Shah Sir (Civil Engineering Tutor) Mob- +91 7888280720 Page | 73 Concrete Technology By SHAH SIR (7888280720) B.E. (Civil Engineering) Third Semester (C.B.S.) PDF PAID NOTES Upgrade Version Shah Sir Notes Poly (Civil Engineering) 3rd Sem @ All Subjects 499/4th Sem @ All Subjects 499/5th Sem @ All Subjects 499/6th Sem @ All Subjects 499/- B.E (Civil Engineering) 3rd Sem @ All Subjects 499/4th Sem @ All Subjects 499/5th Sem @ All Subjects 499/6th Sem @ All Subjects 499/7th Sem @ All Subjects 499/8th Sem @ All Subjects 499/Research Paper @ 3000/- per paper Final year projects @ 10000/- Approx. M.Tech (Environmental & Structure Engineering) 1st Sem @ All Subjects 499/2nd Sem @ All Subjects 499/3rd Sem @ All Subjects 199/Research Paper @ 3000/- per paper Final year projects @ 10000/- Approx. Payment Mode:- Gpay/PhonePay/Paytm :- 7888280720 (SHAH SIR) Page | 74 Concrete Technology By SHAH SIR (7888280720) B.E. (Civil Engineering) Third Semester (C.B.S.) Unit-06 (14 Marks) Durability of concrete, Cracks in concrete Theories:- 14 Marks Ques No.01:- Explain permeability of concrete. (Compulsory For 7 Marks) Ans:- Permeability of concrete: Concrete is a composite material comprising of Cement, Sand & coarse aggregate. Every material has pores, which contains voids in it. Aggregates have a more substantial void ranging from 1mm to 10mm which cement paste fills these. Even cement has voids ranging from 1micron to 10micron. Due to this interconnected and continuous link to fill one void by other material concrete is prone to permeate fluid or gases into it. In simple words, Presence of voids in concrete makes permeable which in turn allows water or gas to flow into it. The permeability of concrete is the ability of concrete to resist the water flow or any other substance into it when the external force is applied. Permeability of concrete is defined as the property that controls the rate of flow of fluids into a porous solid. It largely depends on the size of pores, connectivity of pores, and how tortuous the path is for the permeating fluid. Permeability is the ease with which liquids or gases can travel through concrete. The coefficient of permeability, K, of concrete (simply called as “permeability” in concrete technology) for steady-state flow of water through a sample of concrete is determined from Darcy’s expression: dq/dt = K (A . ∆h/L) Where; dq/dt = rate of flow of water through the sample A = cross-sectional area of the sample ∆h= drop in hydraulic head through the sample L= thickness of the sample Significance:a) Permeability of concrete plays an important role in durability because it controls the rate of entry of moisture that may contain aggressive chemicals and the movement of water during heating or freezing. Higher the permeability lesser will be the durability Page | 75 Concrete Technology By SHAH SIR (7888280720) B.E. (Civil Engineering) Third Semester (C.B.S.) b) Permeability of concrete is of interest also in relation to the water-tightness of liquidretaining structures. Higher the permeability lesser will be the water-tightness Factors Influencing Concrete Permeability:There are three major factors that influence the permeability of concrete : 1. Water-Cement ratio: Excessive water is added to the concrete mix to increase the workability of concrete. This additional mixing of water, more than required increases the porosity in concrete and degrades the durability of concrete. To resist the entry of water into the concrete 0.4 water-cement ratio is adopted. Experiments proved that taking a water-cement ratio of 0.4 makes concrete impermeable. 2. Improper compaction of concrete: Improper compaction in concrete is the major problem for porosity in concrete. Concrete should be adequately compacted using hand compaction method or machine compaction methods. Poorly compacted concrete leads to the formation of honeycomb which ultimately makes steel to corrode and forms surface cracks. 3. Improper Curing: Concrete should be adequately cured by considering the atmospheric weather. Improper curing in concrete leads to the formation of cracks and in turn, it increases the permeability of concrete. Ques No.02:- Explain in brief non destructive test of concrete. Or Explain in brief the following Non-Destructive testing of concrete. i) Rebound Hammer method. ii) Ultrasonic Pulse velocity testing method. Or Describe ultra sonic pulse velocity test. its suitability & its significance. Or Explain ultra sonic pulse velocity test on concrete. (Compulsory For 14 Marks) Ans:- Non destructive test of concrete: Non destructive test is a method of testing existing concrete structures to assess the strength and durability of concrete structure. In the non destructive method of testing, without loading the specimen to failure (i.e. without destructing the concrete) we can measure strength of concrete. Now days this method has become a part of quality control process. This method of testing also helps us to investigate crack depth, micro cracks and deterioration of concrete. Non destructive testing of concrete is a very simple method of testing but it requires skilled and experienced persons having some special knowledge to interpret and analyze test results. Page | 76 Concrete Technology By SHAH SIR (7888280720) B.E. (Civil Engineering) Third Semester (C.B.S.) Purposes of Non-destructive Tests:1) 2) 3) 4) 5) 6) 7) 8) 9) 10) 11) Estimating the in-situ compressive strength Estimating the uniformity and homogeneity Estimating the quality in relation to standard requirement Identifying areas of lower integrity in comparison to other parts Detection of presence of cracks, voids and other imperfections Monitoring changes in the structure of the concrete which may occur with time Identification of reinforcement profile and measurement of cover, bar diameter, etc. Condition of prestressing/reinforcement steel with respect to corrosion Chloride, sulphate, alkali contents or degree of carbonation Measurement of Elastic Modulus Condition of grouting in prestressing cable ducts Methods of Non-Destructive Testing of Concrete:Following are different methods of NDT on concrete: 1) 2) 3) 4) 5) Penetration method Rebound hammer method Pull out test method Ultrasonic pulse velocity method Radioactive methods i) Rebound Hammer method: Rebound Hammer test is a Non-destructive testing method of concrete which provide a convenient and rapid indication of the compressive strength of the concrete. The rebound hammer is also called as Schmidt hammer that consist of a spring controlled mass that slides on a plunger within a tubular housing. The operation of rebound hammer is shown in the fig.1. When the plunger of rebound hammer is pressed against the surface of concrete, a spring controlled mass with a constant energy is made to hit concrete surface to rebound back. The extent of rebound, which is a measure of surface hardness, is measured on a graduated scale. This measured value is designated as Rebound Number (rebound index). A concrete with low strength and low stiffness will absorb more energy to yield in a lower rebound value. Objective of Rebound Hammer Test:1) To determine the compressive strength of the concrete by relating the rebound index and the compressive strength 2) To assess the uniformity of the concrete 3) To assess the quality of the concrete based on the standard specifications 4) To relate one concrete element with other in terms of quality Page | 77 Concrete Technology By SHAH SIR (7888280720) B.E. (Civil Engineering) Third Semester (C.B.S.) (Fig.1.Operation of the rebound hammer) Principle of Rebound Hammer Test: Rebound hammer test method is based on the principle that the rebound of an elastic mass depends on the hardness of the concrete surface against which the mass strikes. The operation of the rebound hammer is shown in figure-1. When the plunger of rebound hammer is pressed against the concrete surface, the spring controlled mass in the hammer rebounds. The amount of rebound of the mass depends on the hardness of concrete surface. Thus, the hardness of concrete and rebound hammer reading can be correlated with compressive strength of concrete. The rebound value is read off along a graduated scale and is designated as the rebound number or rebound index. The compressive strength can be read directly from the graph provided on the body of the hammer. Advantages and Disadvantages of Rebound Hammer Test:The advantages of Rebound hammer tests are: 1) 2) 3) 4) Apparatus is easy to use Determines uniformity properties of the surface The equipment used is inexpensive Used for the rehabilitation of old monuments Page | 78 Concrete Technology By SHAH SIR (7888280720) B.E. (Civil Engineering) Third Semester (C.B.S.) The disadvantages of Rebound Hammer Test: 1) The results obtained is based on a local point 2) The test results are not directly related to the strength and the deformation property of the surface 3) The probe and spring arrangement will require regular cleaning and maintenance 4) Flaws cannot be detected with accuracy ii) Ultrasonic Pulse velocity testing method: Ultrasonic testing of concrete or ultrasonic pulse velocity test on concrete is a nondestructive test to assess the homogeneity and integrity of concrete. With this ultrasonic test on concrete, following can be assessed: 1) Qualitative assessment of strength of concrete, its gradation in different locations of structural members and plotting the same. 2) Any discontinuity in cross section like cracks, cover concrete delamination etc. 3) Depth of surface cracks. Ultrasonic pulse velocity test consists of measuring travel time, T of ultrasonic pulse of 50 to 54 kHz, produced by an electro-acoustical transducer, held in contact with one surface of the concrete member under test and receiving the same by a similar transducer in contact with the surface at the other end. With the path length L, (i.e. the distance between the two probes) and time of travel T, the pulse velocity (V=L/T) is calculated. Higher the elastic modulus, density and integrity of the concrete, higher is the pulse velocity. The ultrasonic pulse velocity depends on the density and elastic properties of the material being tested. Ultrasonic pulse velocity tests have a great potential for concrete control, particularly for establishing uniformity and detecting cracks or defects. Its use for predicting strength is much more limited, owing to the large number of variables affecting the relation between strength and pulse velocity. Procedure for Ultrasonic Pulse Velocity:i) Preparing for use: Before switching on the ‘V’ meter, the transducers should be connected to the sockets marked “TRAN” and ” REC”. The ‘V’ meter may be operated with either: 1) The internal battery, 2) An external battery or 3) The A.C line. Page | 79 Concrete Technology By SHAH SIR (7888280720) B.E. (Civil Engineering) Third Semester (C.B.S.) ii) Set reference: A reference bar is provided to check the instrument zero. The pulse time for the bar is engraved on it. Apply a smear of grease to the transducer faces before placing it on the opposite ends of the bar. Adjust the ‘SET REF’ control until the reference bar transit time is obtained on the instrument readout. iii) Range selection: For maximum accuracy, it is recommended that the 0.1 microsecond range be selected for path length upto 400mm. iv) Pulse velocity: Having determined the most suitable test points on the material to be tested, make careful measurement of the path length ‘L’. Apply couplant to the surfaces of the transducers and press it hard onto the surface of the material. Do not move the transducers while a reading is being taken, as this can generate noise signals and errors in measurements. Continue holding the transducers onto the surface of the material until a consistent reading appears on the display, which is the time in microsecond for the ultrasonic pulse to travel the distance ‘L’. The mean value of the display readings should be taken when the units digit hunts between two values. Pulse velocity=(Path length/Travel time) v) Separation of transducer leads: It is advisable to prevent the two transducer leads from coming into close contact with each other when the transit time measurements are being taken. If this is not done, the receiver lead might pick-up unwanted signals from the transmitter lead and this would result in an incorrect display of the transit time. Table: 1 – Concrete Quality based on Ultrasonic Pulse Velocity Test PULSE VELOCITY CONCRETE QUALITY >4.0 km/s 3.5 – 4.0 km/s 3.0 – 3.5 km/s <3.0 km/s Very good to excellent Good to very good, slight porosity may exist Satisfactory but loss of integrity is suspected Poor and los of integrity exist. Ques No.03:- What are the different types of cracks. Occur in concrete also Explain its causes? Explain the procedure of repairs. Or What are types of cracks in concrete ? Explain various causes of cracks in concrete. Or Explain various causes of cracks in concrete. Or Explain the methods on repairs of crack in concrete. Or Page | 80 Concrete Technology By SHAH SIR (7888280720) B.E. (Civil Engineering) Third Semester (C.B.S.) Explain the material and methods of Repairs of cracks in concrete. Or Explain Repairs of cracks – materials & methods. (Compulsory For 7 Marks) Ans:- Types of cracks in concrete:1) 2) 3) 4) 5) 6) Plastic shrinkage concrete cracks Expansion concrete cracks Heaving concrete cracks Settling concrete cracks Concrete cracks caused by overloading the slab Concrete cracks caused by premature drying 1. Plastic shrinkage concrete cracks: Plastic shrinkage cracking (Fig.1) occurs when subjected to a very rapid loss of moisture caused by a combination of factors which include air and concrete temperatures, relative humidity, and wind velocity at the surface of the concrete. These factors can combine to cause high rates of surface evaporation in either hot or cold weather.” When moisture evaporates from the surface of freshly placed concrete faster than it is replaced by bleed water, the surface concrete shrinks. Since plastic shrinkage cracking is due to a differential volume change in the plastic concrete, successful control measures require a reduction in the relative volume change between the surface and other portions of the concrete. These measures include the use of fog nozzles to saturate the air above the surface and the use of plastic sheeting to cover the surface between finishing operations. (Fig.1: Typical Plastic Shrinkage Cracks in Concrete) Page | 81 Concrete Technology By SHAH SIR (7888280720) B.E. (Civil Engineering) Third Semester (C.B.S.) 2. Settlement Cracks in Concrete: After initial placement, vibration, and finishing, concrete has a tendency to continue to consolidate. During this period, the plastic concrete may be locally restrained by reinforcing steel, a prior concrete placement, or formwork. This local restraint may result in voids and/or cracks adjacent to the restraining element (Fig.2). When associated with reinforcing steel, settlement cracking increases with increasing bar size, increasing slump, and decreasing cover (Dakhil et al. 1975). The use of the lowest possible slump, and an increase in concrete cover will reduce settlement cracking. (Fig.2: Settlement Cracks in Concrete) 3. Expansion Cracks: It is so apparent that in hot weather a concrete slab, the same as anything else, will enlarged. Expansion in concrete is another reason for concrete cracking. This can cause great stress on a slab. As the concrete expands, it pushes against any object in its path, such as a brick wall or an adjacent slab of concrete. If neither has the ability to flex, the resulting force will cause something to crack. Two separate surfaces can be subdivided by an isolation joint or expansion joint as a boarder. Its entire depth is filled with some type of compressible material such as tar-impregnated cellulose fiber, closed-cell poly foam, or even lumber. Various causes of cracks in concrete:Common cause of cracking in concrete is restrained drying shrinkage. 1) 2) 3) 4) 5) 6) 7) 8) Shrinkage Temperature Change Cracks due to Thermal Stresses. Cracks due to Chemical Reaction. Weathering Cracks. ... Corrosion of Reinforcement. Poor Construction Practices. Construction Overloads. Page | 82 Concrete Technology By SHAH SIR (7888280720) B.E. (Civil Engineering) Third Semester (C.B.S.) 9) Errors in Design and Detailing. 10) Elastic Deformation and creep 1. SHRINKAGE: Shrinkage is one of the major causes of cracking in hardened concrete. In drying shrinkage, the volume of concrete is gradually decreased and if the component is restrained against free movement, tensile stresses are developed which causes cracks. 2. TEMPERATURE CHANGES: The temperature variation in concrete results in the differential volume change. When the tensile strain capacity of concrete exceeds due to the differential volume change, it will crack. 3. CHEMICAL REACTION: Due to the alkaline nature of cement, it reacts with the carbon dioxide (CO2) present in the atmosphere resulting in an appreciable increase in the volume of the materials which finally leads to cracking. 4. POOR CONSTRUCTION PRACTICES: Poor construction practices such as adding excessive water to the mix, lack of curing, poor compaction, using low-grade materials, unreasonable placements of construction joints etc. are also responsible for cracking in concrete. 5. ERRORS IN DESIGN & DETAILING: Errors in design and detailing such as an inadequate amount of reinforcement, improper design of foundation, precast members and slabs, improper selection of materials, lack of sufficient contraction joints etc may result in excessive cracking. 6. CONSTRUCTION OVERLOADS & EARLY FORMWORK REMOVAL: The load induced in the structure during construction can also lead to cracking especially at the younger stage when the formwork is removed earlier. 7. ELASTIC DEFORMATION AND CREEP: The different components of the building such as wall, column, beam. slab etc undergo elastic deformation when loaded. The deformation of concrete depends on the type of building materials used in the construction such as bricks, cement concrete blocks etc. This unusual deformation of concrete results in cracking. 8. CORROSION OF CONCRETE: The corrosion of steel develops a huge amount of iron oxides and hydroxide that have a much greater volume than the volume of metallic iron. Hence the volume is increased and cracks. Various methods on repairs of crack in concrete: There are several methods of concrete crack repair such as epoxy injection, routing and sealing, grouting, stitching, drilling and plugging, gravity filling of cracks in concrete. 1. Stitching of Concrete: In this technique, the crack is bridged with U-shaped metal units called stitching dogs before being repaired with a rigid resin material. A non- shrink grout or an epoxy resin based adhesive should be used to anchor the legs of the dogs. Stitching is suitable when tensile Page | 83 Concrete Technology By SHAH SIR (7888280720) B.E. (Civil Engineering) Third Semester (C.B.S.) strength must be re -established across major cracks. Stitching dogs should be of variable length and orientation. (Fig. Concrete Crack Repair by Stitching) Benefits Of Cracked Stitching: 1) 2) 3) 4) Quick, simple, effective and permanent. The grout combination provides an excellent bond within the substrate. Masonry remains flexible enough to accommodate natural building movement. Non-disruptive structural stabilization with no additional stress 2. Epoxy Injections: Epoxy injection method of concrete crack repair may be used to bond the cracks having greater than or equal to 0.05mm opening. This method is not suitable for crack is active and if the cracks are large in number, or when the water leakage cannot be controlled. In this method, the cracks on exposed concrete surfaces are sealed by injecting epoxy under the concrete crack. But, it is essential to find and fix the major cause of cracks before injecting the epoxy into the cracks otherwise the cracks will keep on emerging again and again. Problems such as leakages and silt contamination damage the efficiency of epoxy, therefore it is recommended to fix these issues before injecting the epoxy to make it last longer. Epoxy injection method of crack repairing requires a lot of discipline and skillful execution so you need to be sure that the person executing the repairs knows his work. 3. Drilling and Plugging Method: Drilling and plugging a crack consists of drilling down the length of the crack and grouting it to form a key (Fig.6). Page | 84 Concrete Technology By SHAH SIR (7888280720) B.E. (Civil Engineering) Third Semester (C.B.S.) (Fig.6: Concrete Crack Repair by Drilling and Plugging) This technique is only applicable when cracks run in reasonable straight lines and are accessible at one end. This method is most often used to repair vertical cracks in retaining walls. A hole [typically 2 to 3 in. (50 to 75 mm) in diameter] should be drilled, centered on and following the crack. The grout key prevents transverse movements of the sections of concrete adjacent to the crack. The key will also reduce heavy leakage through the crack and loss of soil from behind a leaking wall. If water-tightness is essential and structural load transfer is not, the drilled hole should be filled with a resilient material of low modulus in lieu of grout. If the keying effect is essential, the resilient material can be placed in a second hole, the fiat being grouted. Ques No.04:- Write a note on distress in concrete structure and its cause. (Future For 7 Marks) Ans:- Distress in concrete structure: Distress in concrete members occurs with age due to corrosion in reinforcement, loading, settlement of foundations etc. This distress in in building can be found by development of cracks in concrete members such as slabs, beams, columns etc. The cracking of concrete in building is developed in three stages: 1. Stage – I: Volume of rust formed due to corrosion of reinforcement is increased about 2.5 times the volume of steel. With the result of corroded reinforcement bar presses the concrete outwards. Since concrete is poor in tension, longitudinal cracks are developed along the reinforcement bar. 2. Stage – II: Longitudinal cracks in RCC provide wide access to oxygen, carbon dioxide, and moisture with result excessive carbonation starts and structural damages starts. Fear in the mind of users starts. Page | 85 Concrete Technology By SHAH SIR (7888280720) B.E. (Civil Engineering) Third Semester (C.B.S.) 3. Stage – III: In this stage cover comes out and causes danger to the life of structure and structure becomes unserviceable. Remedial Measures for Distressed Concrete: 1. At design stage: 1) Adequate cover to concrete should be planned. 2) Adequate thickness of structural members particularly non-structural members e.g. chajjas, parapets, pergolas and fins etc. should be provided. 3) Proper detailing of reinforcement specially at junctions should be designed to avoid congestion and to ensure smooth placement of concrete. 2. At the construction stage: 1) Concrete should be workable with minimum water cement ratio (< 0.45). It should be well compacted by vibrator. Thus we should try to achieve highest density with minimum void. 2) Ensure proper grading and quality of aggregate free from deleterious material. 3) Use potable water only. 4) Leak proof and properly designed from work should be used. Ensure proper mixing, placement, compaction and curing of concrete. No segregation, honeycombing is allowed. 5) Use of plasticizers and super-plasticizers to achieve workability for controlled water-cement ratio in placement of concrete in congested conditions. 6) Binding wires to be turned inside (should not touch formwork). G.I. wires to be used in aggressive environment. 7) Proper cover with dense concrete / mortar is must. 8) Provision of drip course for projections should be made. 9) Proper cement to be used to prevent sulphate and chloride attack. 10) Cement not more than 3 months old should be used. 3. Protection of reinforcement bars: Protection to reinforcement bars against corrosion can also be provided by: (i) Using corrosion resistant steel Constituents of steel alloy are adjusted to resist corrosion. (ii) Fusion bonded epoxy coating This process has good results in protecting reinforcement bars from corrosion. Powder epoxy is fusion bonded to bar at about 2500C temperature. This is being used in coastal areas particularly in Mumbai in prestigious projects like Bridges, Flyovers, Shipyards and Jetties. Loss of bond is about 20%. IS:13620 – 1993 provides the specification for “Fusion Bonded Epoxy Coated Reinforcement Bars”. (iii) Passive coating with polymer based cement slurry: In this process, reinforcement bars are cleaned for rust and freshly prepared polymer cement slurry is applied by brushes. Page | 86 Concrete Technology By SHAH SIR (7888280720) B.E. (Civil Engineering) Third Semester (C.B.S.) (iv) Protective epoxy coating: Certain epoxy coating are also available in the market. Firstly rust is removed by wire brush or sand blasting. Thereafter, epoxy as per manufacturer’s specifications are applied by spray / brush. Loss of bond may be upto 30%. ***** Complete By Shah Sir (Civil Engineering Tutor) Mob- +91 7888280720 Page | 87 Concrete Technology By SHAH SIR (7888280720) B.E. (Civil Engineering) Third Semester (C.B.S.) 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