Aggregate crushing value and 10% fines- BS 812: Part 110/1: 1990 This test is used to measure the strength of the coarse aggregates. This test will give a relative measure of an aggregate to crushing under a gradually applied load. This test will be applied to aggregates passing the 14mm sieve and retained at 10mm sieve. Procedure 1. The cylinder of the yest apparatus will be placed in position on the base plate. The aggregates will then be placed into the cylinder in three layers, each layer to be compacted by 25 strokes of the tamping rod. 2. The surface of the cylinder will be leveled and a plunger inserted so that it rests horizontally on the surface of the cylinder. 3. The apparatus with the aggregates will be placed between the plates of the compression testing machine. A force will then be applied gradually so that the required force of 400kN will be reached in the required 10 minutes. 4. The force will be released and the crushed material released from the cylinder. The mass will be recorded as A. 5. The whole material will be sieved with the 2.36mm sieve. The weight of the sample passing this sieve will be recorded as B, and the mass of the mass retained at the same sieve will be recorded as C. 6. The ACV will then be calculated as π΅ π΄πΆπ = × 100% π΄ The Ten percent fine value (TFV) test will be carried out after the ACV. This will be determined by measuring the load required to crush the prepared aggregate sample to give 10% of the material passing a specified sieve after crushing. From the weights that will be measured during the ACV test, the TFV for materials having an ACV ranging from 75% - 12.5% will be calculated as follows, ππΉπ = 14π₯ π¦+4 Where, οΌ X- Maximum loading that will be recorded from the ACV test οΌ Y- The percentage of material passing the 2.36mm sieve at the maximum force. 3.2. Concrete mix design This procedure involves the process of finding right proportions of cement, sand and aggregates for concrete to achieve specific properties. The properties that will be specified are: ο· ο· The workability of the fresh concrete: this will be measured using two alternative tests, slump test which is appropriate for higher workability mixes, and***** The compressive strength at a specified age. ο· The durability, by means of specifying the minimum cement content and the maximum free water cement ratio and in some cases, requiring the use of selected types of materials. 3.2.1. Definitions Free-water The total water in concrete mix consists of water absorbed by the aggregate to bring it to a saturated surface dry condition, and the free water available for the hydration of the cement and for the workability of fresh concrete. It is expressed as mass per unit volume of concrete and a major factor influencing the workability of concrete. The free-water content required to produce a concrete of a specified slump value or Ve-be time depends upon the characteristics of the aggregates. Generally, it is assumed that uncrushed aggregates require less water compared to crushed aggregates to make concrete of equal workability, and that the smaller the maximum size of the aggregate the higher the water content required. Mix parameters Durability A durable concrete is one which gives a satisfactory performance during an adequate life in a given environment; including protection of steel against corrosion in reinforced concrete and prestressed concrete. This is dependent on free-water/ cement ratio. Type and strength class of cement Different types and strength class of cement produce concrete having different rates of strength development. The cement adopted for this research will be******** Strength margin As a result of the variability of concrete in production it is necessary to design the mix to have a mean strength greater than the specified characteristic strength by an amount termed as the margin. Thus ππ = ππ + ππ Where ππ = π‘βπ π‘πππππ‘ ππππ π π‘πππππ‘β ππ = π‘βπ π ππππππππ πβπππππ‘ππππ π‘ππ π π‘πππππ‘β ππ = π‘βπ ππππππ, π€βππβ ππ π‘βπ πππππ’ππ‘ ππ: π = π‘βπ π π‘ππππππ πππ£πππ‘πππ π = π ππππ π‘πππ‘ K is derived from normal distribution and increases as the proportion of defectives is decreased, thus K for 10% defectives= 1.28 K for 5% defectives= 1.64 K for 2.5% defectives= 1.96 K for 1% defectives= 2.33 For 5% defective level specified in BS 5328= 1.64 and thus ππ = ππ + 1.64π 3.2.2. Stages in mix design 3.2.3. Trial mixes ********** 3.2.4. Trial mixes test The tests on the fresh concrete, the making of the test specimens, the curing and their method of test will be accordance with the relevant part of the BS 1881 as follows ο· ο· ο· ο· Slump test- BS 1881: Part 102 Making test cubes- BS 1881: Part 108 Accelerated curing- BS 1881: Part 112 Compression test of cubes- BS 1881: Part 116 1. Slump test – BS 1881: Part 102 Slump test has been used extensively in construction works to detect variation in the uniformity of mix of given proportions. An increase on slump may mean that the moisture content of the aggregate has increased or a change in grading of the aggregate, such as a deficiency of fine aggregate. Too much or too low slump gives an immediate warning and enables the mixer operator to remedy the situation. Apparatus a. A standard mould which is a frustrum of a cone complying with BS 1881- 102: 1983. b. A standard flat base plate preferably steel. c. A standard tamping rod. d. A standard graduated steel rule. e. A scoop approximately 100mm wide. Procedure 1. The inside surfaces of the mould will be cleaned and oiled to prevent adherence of fresh concrete on the surfaces. 2. The mould will be placed on the base plate and firmly held. 3. The cone will then be filled with fresh concrete in three layers with each layer compacted with 25 strokes of the tamping rod. 4. After filling the mould, the top surface will be struck off by rolling action of the tamping rod. 5. After filling the mould, the cone will be slowly and carefully lifted. 6. Immediately after the removal of the mould the slump of the unsupported concrete will be measured and recorded. 2. Split Tensile Strength – ASTM C496-11 Fresh concrete will be placed in a cylinder of 100mm diameter by 200mm height and left for 24 hours before being removed and cured for 28 days, then the test will be done. The test will be done to determine the tensile strength of the cylinders. The cylinders will be placed in a horizontal position and the load will be applied gradually and the value will be recorded when the cylinder split into two halves or when the cylinder will fail while applying the load on it. 3. Flexural Strength – ASTM C78 Flexural strength is a measure of tensile strength of concrete. It is a measure of an unreinforced concrete beam to resist failure to bending. 100mm x 100mm x 500mm unreinforced beam will be casted and compacted using a tamping bar in three layers and will be tested by loading according to ASTM C78. 4. Compression test of cubes- BS 1881: Part 116 Casting of cubes The specimen will be cast in iron moulds which are 150mm cubes. The moulds surface will be cleaned and oiled on their inner surfaces in order to prevent the concrete from sticking on the mould. The moulds will be assembled and tightened using bolts and nuts to prevent leakage of the cement paste. The moulds will be filled with concrete in three layers, each layer will be compacted using a tamping rod to remove as much entrapped air as possible. The moulds will be filled to the brim and excess concrete will be removed by sawing action of a steel rule. The specimen will be left in the mould undisturbed for 24 hours. Curing of cubes Curing is used to promote the hydration of cement. The objective of curing is to keep concrete saturated, until the originally water- filled space in the fresh concrete paste is filled with the desired extent by the products of hydration of cement. The temperature during curing also controls the rate of progress of the reactions of hydration. In this project the curing temperature will be at room temperature in a curing tank filled with water. Before placing cubes in the basin, they will be marked with a waterproof marker. Details on the cubes will be mainly the types of mix, the date of casting and cube identification number. Compressive Test After curing the cubes for the specified period, they will be removed and wiped to remove surface moisture in readiness for compression test. The cubes will be placed with the cast faces in contact with the platens of the testing machine. The load will then be applied at a constant rate. The readings on the dial gauge will be recorded for each cube.
