Grading of aggregates consists of proportioning the fine and coarse aggregates in such a ratio, so as to get strongest and densest mix with the least amount of cement. Grading the aggregates is so graded as to have minimum voids when mixed with all ingredients, and water should render a concrete mass of easy workability.
The grading of aggregates are done by the following methods
- By trail – In this method, proportioning of aggregates as to give heaviest weight for same volume, yield the densest concrete
- By finesse modules method (sieve analysis method): in this method, the samples of both coarse and fine aggregates are passed through a set of nine standard sieve and the percentage of sample retained on each of the said sieves is determined. The total of these percentages divided by 100 gives the finesses modulus of sample
- By minimum voids method: This method is based on the fact, that so obtain dense concrete the quantity of cement should also be slightly in excess of voids more that the fine aggregates. In this method the voids in the fine and coarse aggregates are separately found out with the help of graduated cylinder and water. The percentage of voids I aggregate, “X” given by the equation.
X = (V1 – V2) x 100
Where v1, volume of water filled
Where v2, volume of aggregates.
- By arbitrary standards: It is a commonly adopted method of propitiating the aggregates in a concrete mix for small works of moderate importance. This method is not recommended for large works or important works in this method, the volume of cement, sand and coarse aggregates are taken in the proportion of 1:n:2n respectively. The quantity of water to be used a varied suit the workability descried.
Ex: 1:1:2 M250 rich mix for columns, beams
1:1:3 – M200 Water retaining structures etc
1:3:6 – M150 slab’s columns roads etc
1:3:6 – M100 – foundations,
1:4:8 – For mass concrete.