Properties of the hardened concrete are as follows
The strength of concrete is basically referred to as compressive strength and it depends upon three factors.
i- Paste Strength
ii- Interfacial Bonding
iii- Aggregate Strength
i- Paste strength
It is mainly due to the binding properties of cement that the ingredients are compacted together. If the paste has higher binding strength, higher will be the strength of concrete.
ii- Interfacial bonding
Interfacial bonding is very necessary regarding strength. Clay hampers the bonding between paste and aggregate. The aggregate should be washed for better bonding between paste and aggregate.
iii- Aggregate strength
It is mainly the aggregate that provides strength to concrete especially coarse aggregates which act just like bones in the body. Rough and angular aggregate provides better bonding and high strength.
Factors affecting the Strength of concrete
Following are the factors that affect the strength of concrete:
1- Water-Cement ratio
2- Type of cementing material
3- Amount of cementing material
4- Type of aggregate
5- Air content
1- Water-Cement ratio
It is the water-cement ratio that basically governs the property of strength. Lesser the water-cement ratio, greater will be a strength.
2- Type of cement
Type of cement affects the hydration process and therefore strength of concrete.
Amount of cementing material: it is the paste that holds or binds all the ingredients. Thus a greater amount of cementing material greater will be a strength.
3- Type of Aggregate
Rough and angular aggregates are preferable as they provide greater bonding.
Chemical admixtures like plasticizers reduce the water-cement ratio and increase the strength of concrete at the same water-cement ratio. Mineral admixtures affect the strength at a later stage and increase the strength by increasing the amount of cementing material.
Concrete is subjected to changes in volume either autogenous or induced. Volume change is one of the most detrimental properties of concrete, which affects long-term strength and durability. To the practical engineer, the aspect of volume change in concrete is important from the point of view that it causes unsightly cracks in concrete.
Hardened Concrete undergoes in three types of shrinkage that are important with respect to its dimensional stability:
1- Plastic shrinkage.
2- Drying shrinkage.
3- Thermal shrinkage.
1- Plastic Shrinkage
This is the shrinkage that the freshly placed concrete undergoes until it sets completely. It may also be called initial shrinkage.
Such a volumetric change is due to loss of water from the fresh concrete due to evaporation, bleeding, seepage and soaking by formwork.
Excessive shrinkage at initial stages may develop extensive cracking in the concrete on the setting. Therefore, all precautions should be taken to avoid excessive loss of water due to evaporation.
2- Drying Shrinkage
As the concrete has completely set and hardens, some further shrinkage may result because of the contraction of gel-structure due to further loss of moisture, or drying.
This kind of shrinkage is practically an essential and irreversible property of concrete. It has to be met with by careful design of reinforcement to avoid its ill effects.
3- Thermal Shrinkage
This may be due to a fall in temperature of concrete from the time it is laid till it sets completely. Thus, when concrete laid at 30°C cools down to 15°-18°C, some shrinkage may be expected. It may be negligible on its own account. But when added to drying shrinkage, it becomes necessary.
Environmental forces such as weathering, chemical attack, heat, freezing, and thawing try to destroy concrete. The period of existence of concrete without getting adversely affected by these forces is known as durability. Generally dense and strong concretes have better durability. The cube crushing strength alone is not a reliable guide to the durability. Concrete should have adequate cement content and should have a low water-cement ratio.
This is the resistance of concrete to the flow of water through its pores. Excess water during concreting leaves a large number of continuous pores leading to the permeability. Since the permeability reduces the durability of concrete, it should be kept very low by using low water-cement ratio, dense and well-graded aggregates, good compaction and continuous curing at low-temperature conditions. The cement content used should be sufficient to provide adequate workability with a low water-cement ratio and the available compaction method.