Concrete is a heterogeneous, brittle and the most widely used construction material. Factors governing the strength of concrete are many and are discussed below.
- Water/cement ratio
In 1918 AD Duff Abram made some experiments with tweaking the amount of water added to mix the concrete. He found that lower the amount of water higher will be the strength.
Though, we need to add some minimum quantity of water for full hydration of the cement.
Generally, the 38 percent to 40 percent of water is sufficient to fully hydrate the cement.
Again, the water cement ratio has hyperbolic relation if we plot the graph between W/C ratio and compressive strength of concrete. So, we could approximate it to a linear graph for ease of use.
- Ratio of cement to aggregate
Adequate cement paste should be available to fill the gaps between aggregate. This imparts role in transferring load to the aggregate.
- Grading of aggregate
The grading of aggregate should be as per IS:10262-82
- surface texture and shape of aggregate
Aggregate having smooth surface do not provide a good bond with aggregate.
On the other hand aggregate having angular shape gives ease to interlocking and thus higher bond strength consequently higher strength of concrete. Aggregate having round surface provides workability but on cost of reduction in strength. So, round shape aggregate should be avoided.
- strength and stiffness of aggregate
Higher the strength of aggregate higher will be the strength of concrete.
- Maximum size of aggregate
If we choose the bigger size of aggregate the need of cement for bond will be less because of lower surface area. Thus we can adopt lower W/C ratio.
But this is not true. As low bond strength leads to lower strength of concrete. Also, the bigger size of aggregate provides more heterogeneous concrete, in which stress distribution is not uniform.
- Proper curing
Proper curing will lead to higher strength.