1.Plastic Shrinkage Crack
They occur after 1-6 hours after casting, they are upto 3mm wide, 20-50 mm deep and 50 mm to 3 meter in length, in some cases in extend upto whole depth, they are at 45o approximately to the direction of casting, and they are roughly parallel to each other. They don’t extent to the edges of slab.
i. Control over evaporation.
ii. Curing within the first of casting is required.
iii. Polypropylene fiber is used.
i. Low viscosity resins is injected.
ii. Brush dry cement (dampen later).
2.Early Thermal Contraction Crack
Hydration of cement release heat and is the cause of early thermal contraction crack. In walls casted over strip footing, these cracks start at the base and runs up vertically.
i. Limit the temperature change.
ii. Use sufficient reinforcement to avoid cracking.
iii. Cracks are wider near corners.
It is present in the slab cast on the ground, this is different from the drying shrinkage becausewater is lost only in one direction, hence moisture gradient is develop which cause curling.
It is the network of cracks; they are generally 0.1 mm wide forming closing polygons. They are few mm deep they does not effect the concrete durability, surface dries out more quickly then inner mass and goes into tension.
5.Long Term Drying Shrinkage
Concrete generally contain more water for compaction and workability, water in excess goes into pores, moisture loss result in drying shrinkage, these types of cracks are more obvious near corners of concrete. It is due to the change in volume, it occurs in between several months to 3-4 years.