Hair Cracks

Hair cracks are partly due to the unequal shrinkage of the surface concrete and the mass behind it. Delayed finishing and final floating of concrete, up to a certain limit, avoids surface cracks.  Surface cracks are also caused by surface dressing with a mortar too rich in cement, too much water, insufficient curing or from over troweling.  Such hair cracks do not affect the strength of the concrete. One method of avoiding such hair cracks is to ,remove the surface skin of the concrete by brushing it with a stiff brush soon after setting. Fine superficial cracking is known as "crazing". 

Contraction of Concrete 

Contraction of concrete is more harmful than expansion as it sets up tensile stresses in the structure, particularly those with a large surface area and thus form cracks.  Such contraction cracks may be prevented by inserting reinforcement near ~e surface.  Closely spaced reinforcement of small diameter and near the surface is more effective than large diameter bars further apart and further from the surface. Contraction or shrinkage of concrete in hot dry areas is much greater than that in damp and cool climates. 

Causes of Cracks after Hardening of Concrete 

Cracks can develop by tensile stresses which are produced by restraining deformations caused by strains due to rising or falling temperatures or due to shrinkage and creep of the concrete. Interposed deformations like differential settlement between foundations can also cause cracks. 

Cracks in prestressed concrete bridges have taught us that they were mainly due to temperature differences produced by sunshine and following cooling by rain or night.