CONCRETE SCOPE This specification establishes the material, mixing, placing, curing, etc., of all types of cast – in - situ concrete to be used in foundations, underground and above ground structures, floors, etc., Any special requirement as shown or noted on the drawings shall govern over the provisions of this specifications. All work should be carried out with proper scientific knowledge and practice and it is obligatory that the contractor shall employ a qualified and competent Engineer for the said purpose. MATERIALS CEMENT (IS : 269) Cement shall be ordinary Portland cement with a 7 days strength of not less than 220 Kg / cm2 unless otherwise specified, obtained from approved suppliers and shall be stored in a waterproof / weatherproof shed in a manner approved by the Engineer. No cement that has been allowed to deteriorate in quality or that has become caked or has perished by dampness or otherwise shall under any circumstances be used on the works. The concrete mix proportions shall be used on the weight of cement bags as delivered at the mixer and not on their theoretical weight. The contractor should allow in his rates for losses in weight of cement bags in transit and handling as indicated in special terms of contract part I. STORAGE The contractor shall follow accepted good practice in handling and storing cement. Cement may be stored on site in moisture proof bulk containers, which shall be equipped with venting arrangements. Cement delivered in bags shall be stored off the ground (at lest 15 cms.) in dry, well ventilated, weather – proof / water proof sheds, arranged in separate consignments as received from the manufacturer so that consumption of cement is ensured in the order of receipt. I.e., ‘First in First Out’ rule. The stacks of cement shall be such that there is easy access for proper inspection and identification. The bags shall be piled not more than 15 bags per pile and placed close together in the pile to reduce circulation of air. Each stack of cement shall be covered with good water proof tarpaulin or thick polyethylene sheets. USE Cement shall be used in the order in which it is received. Cement in bags in storage for more than 3 months shall be rest before use. TESTING Testing shall be carried out on cement delivered to the site for fineness, initial and final setting time, and compressive strength (IS : 4031) and the results should be approved by the Engineer before use of the cement in permanent works. Samples shall be taken immediately on receipt of cement at site. The methods and procedure of sampling shall be in accordance with IS : 3535. The Engineer may specify other forms of sampling and tests, if in his opinion the cement is of doubtful quality, the costs of such additional tests shall be borne by the contractor, if supplied by him. FINE AGGREGATE (SAND) (IS : 383) It shall be of river or pit sand or, if permitted by the Engineer, crushed stone sand (produced from crushing stone suitable for concrete aggregates) and all fine aggregates shall be sharp, free from excess fines, loam earth, vegetable matter, soluble salts and other harmful chemical organic impurities and shall be clean. If considered by the Engineer as necessary, the same shall be washed. Washing shall be done at least one day before using it in concrete. The aggregate should be stored in such a manner as to avoid contamination. Fine aggregates acceptable for the works shall normally have a grading which falls within the following limits GRADING LIMITS FOR FINE AGGREGATES (IS : 383) IS Sieve Designation Equivalent B.S. Sieve No. Natural Sand or crushed gravel sand Crushing stone sand 4.75 mm 3/1600 90 – 100 90 – 100 2.36 mm No. 7 75 – 100 60 – 95 1.1.8 mm No. 14 55 – 90 30 – 70 600 Micron No. 25 35 – 60 15 – 35 300 Micron No. 52 5 – 30 5 – 20 150 micron No. 100 0 – 10 0 – 12 If grading of fine aggregates can be improved by mixing two varieties of sand, the Engineer may at his discretion specify such mixing, and may permit the use of crushed sand as one of the two sands forming the mixture. The provisions of two types of sand and their mixing in the specified proportions shall be done at the contractors cost. a) The fine aggregate shall not contain more than a total 4% by weight of loam, dust silt and any. b) The amount of fine particles shall not exceed 3% by weight for natural sand. COARSE AGGREGATES (IS : 383) Coarse aggregate for the works shall be river gravel or crushed stone obtained from sources approved by the Engineer. It shall be clean and free from clay or loamy admixture or quarry refuse or vegetable matter. The pieces of aggregates shall be cubical or rounded in shape and shall have granular or crystalline or smooth (but not glossy non - powdery surfaces. Aggregates shall be properly screened and it necessary washed and cleaned before use. Coarse aggregates containing flat or flaky pieces or mica shall be cleaned of such impurities before use. The grading shall conform to the following as far as possible : GRADING LIMITS FOR SINGLE SIZED COARSE AGGREGATE (IS : 383) Passing Sieve (%) Nominal size of aggregat e 80 m m 63 mm 63 mm 100 40 mm 40 mm 20 mm 85 – 100 0 – 30 - 100 20 mm - 12 mm 10 mm 12 mm 10 mm 4.75 2.36 0 – 50 - - - - 85 – 100 0 – 20 - 0–5 - - - 100 85 – 100 - 0 – 20 0–5 - - - - 100 85 – 100 0 – 45 0 – 10 - - - - - 100 85 – 100 0 – 20 - Limits for use of single sized coarse aggregate in various types of structures shall be as follows : USE SIZE Slab and walls 200 mm or more in 40 mm to 10 mm thickness Columns and Girders with least 40 mm to 10 mm dimension of 300 mm or more Slab and walls under 200 mm in 20 mm to 10 mm thickness Columns and Girders with at least 20 mm to 10 mm dimension under 300 mm Note : Note : The sieve sizes of 80mm, 40mm, 20mm, 12mm, and 10mm will correspond roughly to B.S. Sieve size of 3”, 21/2”. ¾” and 3/8” respectively. Limit for impurities in Aggregates (IS : 383) TEST FINE AGGREGATE COARSE AGGREGATE Clay, Silt & Fine dust in natural sand or crushed Gravel (by weight) 4% 1% Organic impurities The solution obtained by immersing the samples in 3 percent solution of sodium Hydroxide for 24 hrs. should be light colour. Coarse and fine aggregates shall be batched separately. All in aggregate may be used only where specifically permitted by the Engineer. WATER Before commencing the trial mix design, and in course of the work whenever the source of water changes, or whenever there is in the opinion of the Engineer reason to suspect a change in water quality, the contractor shall get the water to be used for mixing, tested in an approved laboratory. The water in general shall comply with the following requirements. Water to be used for mixing and curing concrete shall be clean, fresh and free from organic or inorganic matter in solution or suspension in such amounts as may impair the strength or durability of concrete or steel. Water which is potable is normally considered satisfactory for concrete mixing. Sea water or water from excavations shall not be used. In case of doubt suitable checks shall be made using distilled water as the basis of comparison. Where water can be shown to contain an excess of acid, alkali, sugar or salts, the Engineer may refuse to permit its use. As a guide the following concentrations represent the maximum permissible values. a) To neutralize 200 ml sample of water, using phenolphthalein as an indicator, it should not require more than 2 ml of 0.1 Normal NaOH. The details of test shall be as given in section 13 of IS : 3025. b) To neutralize 200 ml sample of water, using methyl - orange as an indicator; it should not require more than 10ml of 0.1 HCL, the details of test shall be as given in section 19 of IS : 3025. c) Percentage of solid when tested in accordance with IS : 3025 shall not exceed the following : TABLE Permissible Limit Max. d) Organic 200 mg/l Inorganic 3000 mg/l Sulphate (as SO4) 500 mg/l Chlorides (as CL) 1000 mg/l Suspended matter 2000 mg/l The pH value of water shall be 6 - 10. TRIAL MIXES The contractor shall be entirely responsible for the design of the concrete mixes. The design is however to be approved by the Engineer before commencing any concreting in the works. The contractor shall make trail mixes using samples of coarse aggregates, sand, water and cement, typical of those to be used in the works, and which have been tested in an approved laboratory. A clean dry mixer shall be used and the first batch discarded. The cement contents of different grades of concrete and the required average strength at 28 days for which the mixes shall be designed are specified below TABLE : 2 Characteristic Grades Strength thk. Designation At 28 days Estimated minimum cement content Target average cube strength at 7 days Target average cube strength at 28 days (Kg / cm2) (Kg / cm2) 100 140 M 10 100 (Kg / cm3) 225 M 15 150 300 150 210 M 20 200 350 195 275 M 25 250 400 235 335 (Kg / cm2) The estimated cement contents indicated above are for the contractors guidance only. They are for 20 mm maximum nominal size aggregates, and for other sizes different cement contents may be necessary to get the specified strength. For 20 mm aggregates, the above are cement contents with which it should be possible to obtain the required target mean strengths provided the aggregate and other ingredients are of satisfactory quality and grading. In case however the contractor is unable to obtain the required average strengths with these cement contents he will be required to provide increased cement content at his own cost. On the other hand, cement contents less than estimated minimum are not permitted. Where the mix is not designed following proportions shall be used for the concreting : (by Volume) M-10 M-15 M-20 M25 - (1:3:6) (1:2:4) (1:1.5:3) (1:1:2) R.C.C WORK IN CONTROLLED CONCRETE Strength of concrete of different grades and their proportions for controlled concrete shall be as under Grade of Concrete Specified Characteristic compressive cube strength at 28 days in Kg / Sq. cm Total Qty. by weight dry aggregate (course + 50 kg cement) Proportion of fine to coarse aggregates Qty. of water per 50 kg. Cement (Ltrs) M–5 50 800 Generally 60 M – 7.5 75 625 1 : 2 but 45 M – 10 100 480 Subject 34 M – 15 150 350 To upper 32 M – 20 200 300 Limit 30 250 1 : 1.5 & Lower limit 1 : 2.5 30 M – 25 250 Note : 1. M - 5 and M - 7.5 grades need not be designed for mixes. 2. Coarse aggregate above means graded coarse aggregates. 3. If for obtaining workability etc., quantity of water has to be increased, then the cement content shall also proportionately increased so that specified water cement ratio is not exceeded. The cost of such extra cement shall be deemed to be included in the item rates / prices. 1, 2, 3 and 4 samples for quantity of concrete poured daily not exceeding 5 Cu. M, 16 - 39 Cu.M & 31 - 50 Cu. M respectively shall be taken or as directed by the Engineer. From each sample six test cubes shall be made (150 x 150 x 150 mm ) size, three for testing at seven days and three for testing at twenty eight days. The strength at seven days for use of normal Portland cement shall not be less than two thirds of twenty eight days strength. All tests shall be carried out as per IS : 456, IS: 516. The results of test in any recognized laboratory shall be final and binding on the contractor. All arrangements for testing concrete cubes shall be made by the contractor at his own cost. All the materials and labour for making, curing and transporting the cubes for carrying out necessary tests including the fees of testing whether at site or elsewhere shall be at the cost of the contractor. Normally cube strength s substantially in excess of those specified are expected. In case the compressive strength obtained from the test cubes of concrete is less than the minimum specified in acceptance criteria as per IS : 456 – Clause 15 the work is liable to be rejected. If however, the strengths are higher than the specified due to good control or otherwise, the contractor shall have no claim for extra payment. CONCRETE MIX DESIGN (IS : 01262) Concrete mixes for various specified design strengths shall be worked out by the contractor, generally as per the requirements of IS : 456. a. The mix proportion shall be selected to ensure that the workability of the fresh concrete is suitable for the conditions of handling and placing, so that after compaction it surrounds all reinforcements or tendons and ducts and completely fills the formwork. When the concrete is hardened, its quality shall be such as to comply with the strength, durability and other requirements taking into account the conditions to which it will be exposed. b. Any standard method of mix design may be used. For determining the required strength the probability of any test result falling short of the specified minimum shall be taken as per IS : 456. Unless the contractor can prove from his past experience that he is capable of achieving a high degree of control, a fair control should be assumed in the initial mix design, before arriving at the average strength values due regard shall be given to the ‘Criteria of Acceptance’ of the site concrete, which shall be as here under. Laboratory cube strength for mix design shall be 25% higher than the targeted characteristic strength e.g. for M - 20 mix, laboratory 28 day strength for 150 mm size cube should be 250 Kg / Sq. cm so as to get 200 Kg/ Sq. cm field strength. c. The mixes designed by the contractor shall be used on works only after obtaining a written approval of the Engineer. It is to be understood that the mix design shall be entirely the responsibility of the contractor and such approval by the Engineer shall not relieve the contractor of his responsibility in respect thereof. d. The contractor shall prepare all calculations, tabulations, graphs etc., pertaining to concrete mix designs and / or test results and supply copies of such calculations, tabulations, graphs etc., as required by the Engineer. e. Not withstanding the mix design prepared by the contractor, the absolute minimum quantity of cement which must be used for various grades of concrete shall be as follows MIX K.G. PER CU. M M 15 300 M20 350 M 25 400 f. Either ordinary Portland or Portland pozzolana cement will be supplied. No deviation either in rate or schedule of work will be entertained on this account. The mix design should be based on the use of ordinary Portland cement / Portland pozzalana cement. Due regard should also be taken regarding minimum cement content specified as above. g. The contractor shall furnish the cement content assumed for various mixes for approval by the Engineer within on (1) week after award of contract. In case it becomes necessary to use Portland pozzolana cement during the course of the work, the contractor shall design fresh mixes and the difference in the consumption of cement due to use of pozzolana cement shall be taken into account for reconciliation purpose also. In all cases the contractor shall make trial cubes from each consignment of cement and test the same before actually using in the work. h. The ‘Criteria Acceptance’ for concrete shall be as follows: The Criteria of acceptance of site concrete shall be as per clause 15 of IS : 456. Concrete which does not fulfill requirements specified in clause 15.1 of IS : 456. but has a strength greater than that required by clause 15.2 is liable for rejection at all consequences of rejection of concrete shall be totally at the contractors expense. For each grade a total of 18 cubes shall be made. Of these 18 cubes made not more than 6 may be made on any day and further. Of the 6 cubes made in one day not more than 2 cubes may be from any single batch. 9 of these cubes. Each representing a different batch of concrete shall be tested at the age of 7 days and the remaining 9 cubes shall be tested at the age of 28 days. The making of the cubes, their curing, storing, transporting and testing shall be in accordance with Indian Standard IS : 516. The test shall be carried out in a laboratory approved by the Engineer. ACCEPTANCE CRITERIA (IS : 456) The contractor shall submit test results in statistical form as described in IS : 456. Standard Deviation calculations shall be on a set consisting of not less than 30 samples. Concrete will be considered acceptable if The average strength of set is not less than Not more than 5% of the samples of the set fall below characteristic strength of the concrete. Lowest samples strength value in a set does not fall below greater of the following two values. If the concrete produced at site does not satisfy the above strength requirements, the Engineer will reserve the right to require the contractor to improve the methods of batching, the quality of the ingredients and redesign the mix with increased cement content if necessary. The contractor shall not be entitled to claim any extra cost for the extra cement used for the modifications stipulated by the Engineer for fulfilling the strength requirements specified. If from the test results it appears that some portion of the works has not attained the required strength, the Engineer may order the testing of the suspected as well as adjacent portions of the structure. Such testing shall be at the contractor’s cost. The Engineer may also reject the work and order its demolition and reconstruction at the contractor’s cost. If the strength of concrete in any portion of the structure is lower than the required strength, but it is considered never the less adequate by the Engineer so that demolition is not necessary, the contractor shall be paid a lower rate for such lower strength concrete as determined by the Engineer. If the average strength of the concrete cubes falls below the required strength fresh preliminary mixes for that grade shall be made as before, until the trial mixes yield cubes of compressive strength at 28 days greater than the required average strength at that age. When there is a significant change in the quality of any of the ingredients of concrete, the Engineer may at his discretion order the carrying out of fresh trial mixes. All costs for trial mixes and tests shall be to the contractor’s account and held to be included in the contract rates. Before commencing the works the contractors shall submit to the Engineer, for approval full details of all preliminary trial mixes and tests. When the proportions of a concrete mix have been approved by the Engineer, the contractor shall not vary the quality or source of the materials or the mix without the written approval of the Engineer. CONCRETE CUBE TESTS The quantity of hardened concrete will be verified by the following procedure The engineer shall select random batches of concrete for examination without warning the contractor and sampling will generally be done at the point of discharge from the mixer. From the batches thus selected 6 concrete cubes shall be made in accordance with Indian Standards. However not more than 2 cubes may be made from any single batch. Of these 6 cubes thus made 3 cubes (each representing concrete of different batches) shall be tested 7 days and the remaining 3 cubes shall be tested at 28 days. All cubes shall be made, cured, stored, transported and tested in accordance with Indian Standards. The tests shall be carried out in a laboratory approved by the Engineer. For concrete mix above M - 15 grade, at least 6 cubes shall be made on each day’s concreting until 60 cubes have been made for each grade of concrete. This is the initial period. After the initial period, subject to the acceptance of the Engineer. The frequency at which the cubes shall be made, may be reduced to 1 set for every 40 Cu.M. or part thereof for each day’s concreting (1 set = six cubes, each pair of cubes representing concrete from a different batch). If concrete is batched at more than one point simultaneously the above frequency of making cubes shall be followed at each point of batching. The test strength of sample shall be the average of the strength of three cubes. The individual variation should not be more than 15% of the average. QUALITY CONTROL The contractor shall exert proper quality control at the various stages of concrete production and placement. As frequently as the Engineer may require, testing shall be carried out in the field for 1. Moisture content of sand 2. Moisture content of aggregates 3. Silt content of sand 4. Grading of sand The contractor shall provide and maintain at all times, until the works are completed, equipment and staff required for carrying out these tests. The contractor shall grant the Engineer or his representative full access to this laboratory at all times and shall produce on demand complete records of all tests carried out on site. Alternatively, the contractor may also avail of the services of a local testing laboratory with prior approval of the Engineer. CONCRETE BATCHING 1. The aggregates shall always be measured by weigh batching making due allowance for the water contained in aggregates. Volume batching of aggregates shall be permitted in exceptional cases only, after the Engineer is satisfied that adequate control can be exercised by the contractor and densities of the aggregates are fairly uniform. 2. The water shall be controlled by direct measurement and due allowance shall be made for water in the aggregates. The quantity of water contained in the aggregates shall be determined by a method approved by the Engineer. 3. Where cement concrete is specified by strength cement shall always be used on actual weight basis with necessary addition to compensate for weight of gunny bag and deficiency in weight of gunny bag and deficiency in weight of cement in the particular bag. Where nominal mixes viz. Mud-mat / blindage are specified, use of cement may be permitted on basis of number of bags as may be decided by the Engineer. In addition to the accurate control of water / cement ratio, slump tests shall be adopted to check that there is no significant variation in the workability of the mix. 4. All tests shall be carried out at regular intervals and records of the results shall be kept at site. Copies of records shall be sent to the Engineer for reference. 5. To ensure that the grading of the aggregate remains the same as the grading to which the mix design is based, sieve analysis shall be carried out regularly and charts showing the results shall be prepared. Copies of this information shall be kept at site and supplied by the contractor as instructed. If a change in grading is unavoidable the mix shall be redesigned and got approved. 6. Record of theoretical and actual consumption of cement shall be maintained by the contractor for each work separately and regularly. MIXING Concrete shall be machine mixed, in a batch mixer of approved type, which will ensure a uniform distribution of the material through the mass, so that the mixture is uniform in colour and homogeneous. The mixer shall be cleaned at frequent intervals while in use. The mixing of each batch shall continue not less than 11/2 minutes after all the materials and water are in the mixer during which time the mixer shall turn at a speed of about 15-20 rounds per minute. The quantity of the mixed materials per batch shall not exceed the manufacture’s capacity of the mixer. When hand mixing is authorized by the Engineer, it shall be done on a water tight platform. The materials shall be turned at least three times after the water is added and until the batch is homogeneous in appearance and colour. Concrete shall be poured and consolidated in its final position within half an hour of mixing. Tampering with concrete, which has partially hardened, i.e., re - mixing with or without additional cement, aggregate or water, shall not be permitted. PLACING CONCRETE Before placing concrete, all equipments for mixing and transporting the concrete shall be cleaned and all debris shall be removed from the place to be occupied by the concrete. All form and soil surface shall be finished to desired level and shall be thoroughly wetted immediately prior to placing concrete. Concrete shall be handled from the place of mixing to the place of final deposit as rapidly as practicable by methods which will prevent the segregation or loss of any of the ingredients. If segregation does occur during transport, the concrete shall be mixed before being placed. The concrete shall be placed and compacted before setting commences and shall not be subsequently disturbed. The concrete shall in no case be dropped from a height over 1.5 m and it shall be carefully laid in position. Before depositing the concrete, all debris, and dirt shall be removed from the space to occupy by concrete. Concreting shall not be done unless the formwork conforms to the shapes, lines and dimensions as shown in the drawing. The form work shall be sufficiently rigid. During the placing and compaction of concrete, care shall be taken to ensure that there is no loss of water from concrete and no segregation takes place. The method of placing and compaction employed in any particular section of the work shall be to the entire satisfaction of the Engineer. To ensure bond and water tightness between old concrete surface and the concrete to be placed, the surface should be cleaned and roughened to initial green cut may be done after 6 hours of placing concrete in order to facilitate the work. Before plastering, the surface shall be thoroughly hacked. The bonding of old and new concrete should be done by applying the cement slurry after thoroughly watering the old concrete surface and removing all loose particles. Unless otherwise approved, concrete shall be placed in single operation to the full thickness of slabs, beams and similar members and shall be placed in horizontal layers but exceeding 1m. deep in walls, columns and similar members. Concrete shall be placed continuously until completion of the part of the work between construction joints or as directed by the Engineer. Concreting shall not be started unless the Electrical conduits or any other piping wherever required are laid by the concerned agency. The civil contractor shall afford all the facilities, to maintain coordination of work with other agencies engaged in electrical and such other works as directed by the Engineer. Before concreting, the contractor shall provide fabricate and lay in proper position metal inserts, anchor bolts, pipes etc., (which are required to be embedded in concrete members as per relevant drawings and directions of the Engineer. Where concrete is placed on soil, it shall be placed only on firm undisturbed ground. Any concrete that is place on well compacted fill shall have the prior approval of the Engineer. Concrete shall not be placed in standing water on sub - grade or in foundation excavation. The concrete after being laid shall be compacted by means of vibrators of approved type under proper supervision as directed by the Engineer-inCharge. Vibration shall not be confined only to the top surface, but the whole mass of concrete shall be well vibrated until the dense mass assumes jelly like appearance and consistency with water just appearing on the surface. Over vibration or vibration of vary wet mixes shall be avoided. Care should be taken to avoid segregation and accomplished by means of ‘spud’ type internal vibrators with flexible shaft of 6000 vibration / min. The vibrator shall not be left in any position for more than 5 seconds. The whole process starting from the mixing of concrete to the placing and compaction shall not take more than 20 min. the process shall be completed before the initial setting takes place. All chutes, pipes and other placing equipment shall be kept clean and free from coatings of hardened concrete by cleaning and thoroughly flushing, with water after each run, and water used for flushing shall be discharged clear of the concrete already in place. No concrete shall be deposited until the Engineer has inspected the forms, reinforcing steel, inserts hollow clay tile units, sleeves etc., and given permission to place. Concrete, shall be deposited only in the presence of a representative of the Engineer. CURING Curing shall be accomplished in accordance with IS : 456 by keeping the concrete covered with a layer of sacking canvas, hessian or similar absorbent materials and kept constantly wet for atleast ten days from the date of placing of concrete unless otherwise specified. The approval of the Engineer shall be obtained for method of curing which the contractor proposes to use on the work. In very hot weather precaution shall be taken to see that temperature of wet concrete does not exceed 38oC while placing. Heavy loads shall not be placed on or moved across floor slabs until curing is complete. Care shall be taken to prevent floor surface from being marred during curing period. For freshly laid concrete formwork shall not be jarred. Concrete placed under water shall be protected form falling earth during and after placing. Walking on concrete shall not be permitted for at least twenty – four hours after it has been placed in the forms and for such additional length of time as the Engineer may direct. CONSTRUCTION JOINTS Construction joints shall in general conform to the relevant clauses of IS : 456. When the placing of concrete is interrupted and a construction joint is formed, provision shall be made for interlocking with the succeeding layer by the embodiment or saturated wooden blocks or strips, smoothened on four sides to facilitate their removal. Prior to the next pour the wooden pieces shall be loosened and removed in such a manner as to avoid damage to the concrete. Such construction joints, as the contractor feels, are necessary shall be approved by the Engineer and shall be so located and formed as to least impair the strength and the appearance of the structure. They shall be made in the positions as specified or elsewhere as approved. Such joints shall be truly vertical or horizontal as the case may be except that in an inclined or curved member the joint shall be strictly at right angles to the axis of member. Construction joints shall be rebated to an approved profile and an approved water stop shall be inserted in the joints when specified. Construction joints shall be made horizontally in the foundations and 75 mm below the lowest beam soft at the head of columns. Concrete in the ribs and slabs of small tee and other beams shall be placed in one operation but for large beams concrete in the rib up to first. Concrete in haunches or splays on beams or braces and concrete in the head of adjoining position of the column shall be placed at the same time junction of walls and slabs shall be placed at the same time as that in the slab. Construction joints in slab and beams shall be located at 1/3 of span and keyed and doweled as specified. FINISHING OF CONCRETE On skirting the formwork all blowholes and honey combing observed shall be brought to the notice of Engineer-in-Charge. The Engineer may at his discretion allow such honey combing or blowholes to be rectified by necessary chipping and packing or grouting with concrete or cement mortar. If mortar is used, it shall be (1:3) mix, or as specified by Engineer-in-Charge. However, if honey combing or blowholes are of such extent as being undesirable, the Engineer-in-Charge may reject the work totally and his decision shall be final and binding. No extra payment shall be made for rectifying these defects. All humps and uneven faces shall be rubbed smooth with the help of carborundum stone. The surface of non - shuttered faces shall be smoothened with a wooden float to give a finish equal to that of the rubbed down shuttered faces. Concealed concrete faces shall be left as from the shuttering except that honeycombed surface shall be made as detailed above. The top faces of slabs but intended to be surfaced levels or falls shown in the drawings or elsewhere. The floating shall not be executed to the extent of bringing excess fine materials to surface. The top faces or slabs intended to be covered with screened, granolithic or similar faces, shall be made rough when wet with wire brush. Faces of concrete intended to be plastered shall be roughened by approved means to form a key. PROTECTION OF CONCRETE Care shall be exercised to protect the completed concrete from damage by subsequent construction operations. No equipments shall be run over the complete slabs until they are at least two weeks old, special case shall be taken on concreting in hot weather. The forms must be thoroughly wetted first before the concrete is placed and the exposed surface of the concrete shall be kept continually damp, by sprinkling for two days. CRACKS If cracks, which in the opinion of the Engineer may be detrimental to the strength of the construction, develop in concrete construction, the contractor at his own expense shall test the slab or other construction as specified in special conditions. If under such test loads the cracks develop further, the contractor shall dismantle the construction, carry away the debris, replace the construction and carry out all consequential work thereto, at his cost. In any cracks develop in the concrete construction. Which in the opinion of the Engineer have suffered damages either in appearance or stability owing to such cracks. The Engineer’s decision as to the extent of the liability of the contractor in the above matter shall be final and binding. DEFECTIVE CONCRETE Should any concrete be found honey combed or in any way defective, such concrete shall on the instruction of the Engineer be cut out by the contractor and made at his own expenses. EXPOSED FACES, HOLES AND FIXTURES On no account shall concrete surface be patched or covered up or damaged concrete rectified or replaced until the Engineer or his representative has inspected the works and issued written instructions for rectification. Further to observe this procedure will render that portion of the works liable to rejection ; in which case it will be treated as rejection which has failed to meet specified strength requirements. APPROVAL BEFORE CONCRETING No concreting shall be carried out by the contractor until the Engineer or his representative has inspected formwork and reinforcement and certified in writing that concreting may proceed. Any concrete poured without such prior written approval shall be cut out and removed by the contractor at his own cost. CONCRETE FOR FLOORING ON GRADE Concrete for flooring on grade shall be over well packed stone metal or no earth as specified with or without reinforcement, placed in alternate bays not exceeding more than 6 m x 6 m or as specified including hacking the joints or adjacent bays. The water cement ratio shall not exceed 0.4 and cement content shall not be less than 320 Kg/ Cu.M. of finished concrete. The stiff mix shall be thoroughly vibrated and finished to receive the floor finish. PRECAST CONCRETE All provisions, not specifically excluded and not in conflict with provisions of Section 2 : Concrete and Formwork. Section 4 : Reinforcement and Section 7 : Structural Steel work shall apply to pre cast concrete. CASTING All pre - cast units shall be cast on suitable bed or platform with firm foundation and free from wind. The contractor shall be responsible for the accuracy of the level or shape of the bed or platform. EMBODIMENTS Embedded items shall be placed and maintained in correct position while concreting. Embedded items shall be properly anchored to develop required factored loads. STRIKING FORMS Side shutters shall not be struck in less than 24 hours after depositing concrete and no pre - cast unit shall be lifted until the concrete reaches strength of at least twice the stress to which the concrete may be subjected to at the time of lifting. CURING All pre-cast work shall be protected from the direct rays of the sun for at least 7 days after casing an during that period each unit shall be kept constantly watered or preferably be completed immersed in water if the size of the unit so permits. MODE OF MEASUREMENT (IS : 1200) Payment for all concrete work shall be made on cubic metre basis. The measurements shall be as per relevant part of IS : 1200. The rate shall be inclusive of labour, equipments, material, curing etc., complete. Reinforcement and formwork shall be paid separately.