The strength of concrete can be increased, keeping in mind the factors on which the strength of concrete depends.
Strength of concrete depends upon the following factors
1- Type and age of cement: The variation of the strength of concrete with the type of cement is shown below
With age, the strength of cement decreases due to more moisture absorption from the atmosphere. The quality of cement also depends upon the storage conditions of the cement and hence the strength of concrete is also dependent on the storage conditions of the cement as shown-
2- Cement-Aggregate ratio: With the increase in the cement-aggregate ratio, the strength of concrete increases to some extent, provided the other factors are kept constant. This is shown as follows-
3- Water-cement ratio: With the increase in the water-cement ratio, the strength of concrete decreases, which is as depicted below
4- Degree of compaction: Strength of concrete also depends on the degree of compaction as improper compaction will lead to the presence of air voids in the concrete which will certainly reduce its strength. It has been observed that if there is 5% void, the strength of concrete decreases by as much as 30%.
5- Mixing time: Strength of concrete increases with an increase in mixing time up to 2 minutes, beyond which no significant increase in the strength is observed. This becomes much more clear with the following figure.
6- Curing: Effect of curing can be understood with the following figure-
Curing is essential for optimum strength gain and durability. It is necessary for maintaining the required moisture content as well as the temperature conditions.
7- Type and size of the aggregate: Degree of packing is the main factor to be considered here. Greater the degree of packing, lesser is the void content and more is the strength. Spherical or rounded aggregates provide much more degree of compaction than the irregular shaped ones. This is the reason as to why they provide more strength. There is a common disbelief that greater the maximum size of the aggregate, smaller will be it’s surface area, lower will be the w/c ratio and consequently greater will be the strength. But this is not so, the smaller surface area will lead to a smaller surface area for gel bonds, which results in less strength.
