In simple way ,when fresh concrete has been placed in forms, concrete undergoes a volumetric contraction while it is in plastic state (before the concrete has set). This is known as Plastic Shrinkage,
Plastic Shrinkage are so-called because they form while the concrete is still plastic, ie has not set. Rapid drying of the surface of the plastic concrete causes it to shrink and crack.They rarely occur near the edges of a slab as at those locations the concrete is usually free to move.It can starts after 30 minutes of concrete pouring or during finishing.Plastic shrinkage cracking rarely impairs the strength of a concrete element.
Main factors affecting plastic SHRINKAGE
- The main reason behind plastic shrinkage cracking is considered to be rapid and excessive surface water evaporation of the concrete element in the plastic stage (freshly cast concrete) which in turn leads to the so-called plastic or capillary shrinkage1. Water/cement ratio: Low water cement ratio resulting high strength mixes resulting in early plastic shrinkage crack.2. Additives(Effect of superplasticizer on cracking): Higher doses of SP resulting in early crack. 3. Fines Content: More fines is liable to get plastic shrinkage crack.
4. Depth of the concrete section: Deeper concrete section is less prone to plastic shrinkage cracking
5. Curing measures- Poor or inadequate curing
6. Effect of coarse aggregate content on cracking – Reducing the amount of the coarse aggregate in concrete mix leads to more plastic shrinkage and higher cracking risk.
7. Type of cement – OPC 53 grade cement is very fine grinding and need more water to hydrate & liable to get crack.
Practice To Minimize The Plastic Shrinkage Crack
- To minimize the incidence of plastic shrinkage cracking: Dampen the subgrade and formwork , ensuring that any excess water is removed prior to placing concrete.
- In hot weather, lower the temperature of the fresh concrete by using chilled mixing water or any suitable method.
- Do the concrete work , when temperature is below 35˚ C
- Protect concrete surfaces from drying out by gunny bags or suitable means
- Commence curing regime promptly after finishing and continue for the specified period.
- Revibrate the concrete or use towel with pressing into the concrete and make finish before hardening the concrete.