Proportioning of Concrete Ingredients
The main ingredients of concrete are cement, fine aggregates, coarse aggregate, and water. The mix of each quantity in correct proportions will give a quality and best concrete mix. "An ideal concrete is that concrete mix with greater strength and solidity at the least cost".
Quantity estimation of the concrete mix has to be carried out by keeping in mind that the voids that are formed by coarse aggregates are filled by the fine aggregates. And the voids of fine aggregates are filled with cement.
Methods of Proportioning Concrete Mixes
Bringing a concrete mix with higher density and strength employing least amount of cement is the concept behind proportioning the concrete mixes. The methods involved in proportioning of concrete are:
- Arbitrary Standard Method
- Minimum Voids Method
- Fineness Modulus Method
- Maximum Density Method
- Water/Cement Law
Arbitrary Standard Method in Concrete Proportioning
The concept of Arbitrary Standard Method: Sufficient fine aggregate must be available to fill up the voids that are created by the coarse aggregate. Also, sufficient cement to fill the voids of fine aggregate. Experiments and experience tells that fine aggregate: cement ratio to get a dense mix lies between
1: 1/2 and 1: 2(1/2 ).
Based on experience it is possible to fix the ratio of F.A and C.A (fine aggregate and coarse aggregate), in the form of 1: n: 2n. These ratios are by volume. This means if n parts of F.A have to be added to one part of cement, then C.A should be added in 2n, irrespective of the actual requirements. Based on this method 1: 1: 2, 1: 1(1/2) : 3, 1: 2 : 4; 1 : 3: 6; 1: 4 : 8 ratios of cement concrete have been fixed.
1: 3 :6, 1: 2: 4, 1: 1(1/2) : 3 and 1: 1 : 2 mix ratios are for M100, M150, M200 and M250 Concrete.
Minimum Voids Method in Concrete Proportioning
With the help of a graduated cylinder and water, the voids in fine aggregate and coarse aggregate are determined. After determination of voids, the F.A and C.A are proportioned slightly greater than the volume of voids. Cement is taken extra by an amount of 10 %. While Fine aggregate is taken by an extra amount of 15% more than the voids in F.A and C.A.
To make the mix workable, sufficient amount of water is added. This method does not give satisfactory results, as the formation of extra voids are tend to happen because of presence of water in cement and F.A. This would separate C.A. Higher strength concrete mix cannot be obtained by this method.
Fineness ModuluS Method in Concrete Proportioning
Knowing the fineness modulus of F.A. and C.A separately, it is possible to get a mix of designed fineness modulus that can give a concrete of higher strength. The determination is as follows:
Let P = desired F.M of the mix
P1 = F.M of F.A
P2 = F.M of C.A
w = % proportion of F.A that is to be added to 100 parts of C.A
Maximum Density Method In Concrete Proportioning
This method involves filling up a box of a fixed volume with varying proportions of fine and coarse aggregates. The option of proportion that gives the highest density is used; as that mix will be a denser concrete mix.
The method was developed by Mr. Fuller. The determination of grading of materials that will give the highest density can be done by the following expression:
Where D= Maximum size of coarse aggregate; d = maximum size of fine aggregate: M = Percentage by weight of material finer than d.
W/C Rati Method in Concrete Proportioning
The w/c ratio law states that the strength of well-compacted concrete with a good workability is dependent on the water-cement ratio (w/c ratio). The workability of the concrete depends upon the water used in the mixture other than the factors of grading, proportioning, aggregates, and cement proportion.
According to Abraham, the strength of rich concrete is more because of more cement due to the decrease of w/c ratio. The strength and w/c ratio for the concrete relationship is given by the formula.
S28 = 984 / 4x
S28 = cylindrical crushing strength of concrete in kg/cm2 after 28 days of curing
x = w/c ratio by volume