Cracks in concrete are caused due to following factors.
One of the main causes of cracks in concrete is the cooling and contraction due to setting of concrete. Volume change and stresses due to shrinkage are independent of any external load or stress applied.
Cracks may develop in a smaller section attached to a large section due to differential expansion and contraction. Therefore a joint should be provided at the change of section. There is more possibility of cracking of fixed members than those which are free to expand and contract as simply supported beams.
Repeated expansion and contraction or alternate wetting and drying which may result in gradual disintegration of poor concrete.
Rapid drying due to hot weather and high speed winds or absorption of water from the concrete by wooden forms also a cause of cracking in concrete. Therefore the form work on which fresh concrete is placed must be damped, or it should be waterproof so that it does not absorb water from fresh concrete.
Loose form work can also lead to cracks in concrete. so form work should be of adequate strength to bear the pressure of the wet concrete without swelling, spreading or any movement.
Concentration of tensile reinforcement at square openings or re-entrant angles (as in corners of door and window openings) causes cracks. This can be avoided by suitably placing reinforcements having adequate covering. Sufficient thickness of concrete should be given at the points where bars are bent up and anchored.
Minute cracks on the tension side of a reinforced concrete member are unavoidable due to poor tensile strength of concrete as compared to steel and which must crack when steel reinforcement taken its load. Those cracks, however, should be fine enough for moisture penetration to prevent corrosion of the reinforcement.
Hair cracks are the result of unequal shrinkage of the surface concrete and the mass behind it. Delayed finishing and final floating of concrete can avoid these cracks up t o a certain limit.
Surface cracks are also caused by surface dressing with a mortar having too rich in cement. Too much water, insufficient curing, or from over trowelling. 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.
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 contracting cracks may be prevented by inserting reinforcement near the surface. Closely spaced reinforcement of small diameter and near the surface is more effective than large diameter bars further apart from the surface.