There can only be one optimum value for the fine aggregate content. An excess of coarse aggregate will increase the void content, and an excess of fine aggregate will lower the bulk density. The optimum ratio gives the smallest amount of gaps (voids) between the larger particles of aggregate, decreasing void content and maximising bulk density. Also the fine aggregate reduces the porosity of the concrete mix. One of the major influential factors of bulk density is aggregate shape. The shapes of the aggregate influence the workability and performance.
Rounded shape aggregates are good for workability while multi cubical shape provide the best bond for concrete. Similarly moisture content of the aggregate influences the composition of the concrete. The aggregate moisture content is required to determine the amount of water required for concrete mix. The reason for finding this optimum value is to increase the compressive strength of concrete. In maximising the density of the raw materials (of which aggregates is the major constituent), the density of the concrete is maximised, as is its strength. This means that less cement is needed in the mix.
Overall, this leads to greater workability, lower shrinkage, reduced heat of hydration, improved durability, and lower levels of segregation and costs. The main reason for blending fine and coarse aggregates in making concrete mixes is to achieve cheaper economy as well as durability and stability. Aggregates provide dimensional stability acting as a skeleton of the concrete reducing cement paste, therefore, decreasing shrinkage and creep. The particle distribution of both fine and coarse aggregates improve long-term durability, filling the voids and decreasing the density of cement paste. II. IV Conclusion
The test for compacted bulk density found that the optimum fine aggregate content, that is, to produce minimum void content and maximum bulk density, is 60%. This means that a fine: coarse aggregate ration of 60:40 will lead to the most efficient concrete. III. Sieve Analysis Objectives To determine the particle size distribution of the comprising aggregates of a sample by sieving. Equipment * Electronic Balance * Set of Sieves (AS Compliant) Test method 1. Oven dry aggregates at 105°C for 24 hours 2. Stack the sieves (with the pan at the bottom) in order from smallest to largest aperture size. . Take a sample of the aggregate to be tested and place in the top sieve. 4. Place the lid on the sieve stack, then hold the stack by the base and the lid and shake it to and fro at about 100 strokes per minute. The sieve stack is turned through one-sixth of a turn every 25 shakes. 5. Figure [ 1 ] Stack of sieves with pan fitted Figure 1 Stack of sieves with pan fitted This will continue until no more aggregates passes through the sieves. Aggregates are removed from the stack one by one and weighed what remains in each sieve. Results – Data & calculations