Concrete shrinking is the shrinking and often therefore cracking of concrete over time. Concrete is a structural material composed of water, cement and aggregates. Not all cement is the same, however compounds commonly found in cement are often composed of calcium, aluminium and silicon. The term aggregates denotes coarse particles often of about 20mm in diameter, usually gravel, slag and sometimes recycled concrete, which acts as a structural filler.
There are four main types of concrete shrinkage which affect concrete structures most:
Drying Shrinkage:
This occurs in in concrete in which gel molecules are used (due to how they make the concrete “self-healing”) and is due to the reduction in the volume of the gel molecules due to water being lost. The smaller the gel molecule, the greater the rate of drying shrinkage due to the increased surface are to volume ratio.
2. Plastic Shrinkage:
This is one of the most common types of concrete shrinkage. It occurs as the concrete is drying in its mould, what is known as its plastic phase, and occurs as a result mainly of evaporation of water from the surface of the concrete, but also due to the absorption of water by some aggregates. To reduce the effects of plastic Shrinkage, the water / cement ratio must be carefully calculated and the drying concrete can be covered by a polythene sheet.
3. Carbonation Shrinkage:
This occurs when carbon dioxide reacts with compounds in the cement causing them to crystallise and subsequently dissolve, reducing the volume of the concrete. The rate of Carbonation Shrinkage can be controlled by regulating the moisture content. If the moisture in the surrounding air is greater than 50% and the conditions are dry, this is when carbonation shrinkage occurs most.
4. Basic / Autogenous Shrinkage:
This is caused by reactions between the cement and water, and does not usually have any large impacts providing the water / cement ratio is above 0.4.
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