Monday, January 6, 2014

What causes concrete to crack


What is concrete?

Concrete is a mixture of cement, sand and aggregate mixed in certain proportions determined by the strength required. When water is added to the mixture a chemical reaction occurs by which concrete becomes solid. Water cement ratio (amount of water per bags of cement) is very important. If more water is added concrete losses its compressive strength. However, add to little and the concrete is not workable. Additional compounds usually known as admixtures are also added to concrete mixture to improve the workability of the concrete.  Concrete is strong in compression but weaker in tension. Therefore, to improve its tensile strength steel reinforcement is added.

What causes cracks?

Cracks in concrete are inevitable. Although good workmanship and quality control can avoid disasters. However, they cannot prevent cracks from developing over period of time. Cracks in the concrete are a result of many different reasons. Few of them can be!

1. SHRINKAGE CRACKS

Concrete in its plastic state contains water, when water evaporates during curing of concrete, it shrinks and stresses develop. To release these stresses cracks develop in concrete. These cracks can develop anywhere on concrete which can be unsightly. Therefore, a good practise is to construct control joints (not expansion joints) at regular intervals to control where the cracks should develop so that they don't appear ugly.

2. EXPANSION CRACKS

Concrete like any other material expands and contracts as per variations in temperature. When concrete expands and pushes against the adjoining material, stresses develop resulting in development of cracks. Expansion joints are required at regular distances and various other locations to prevent these cracks.

3. CRACKS CAUSED BY HEAVING

Ground conditions also result in development of cracks in the concrete. Certain types of soils expands when wet and contracts when dry. Furthermore, frost will make the soil to expand and while thawing the soil contracts. This cycle of expansion and contraction causes concrete to crack.

4. SETTLEMENT OF GROUND

All buildings settle little bit after getting constructed. If the ground underneath was not compacted properly, differential or irregular settlement can occur resulting in development of cracks.

5. OVERLOADING

Concrete is designed to take certain amount of load. If it is loaded beyond that limit cracks might appear. Furthermore, the ground below the concrete may not be compacted properly or is wet, making it to settle under load and as a result cracking the concrete.

6. PREMATURE DRYING

If the concrete dries to quickly then the crazing cracks appear on the surface.