WHAT CAUSES CRACKS IN CONCRETE AND HOW TO PREVENT IT?

Causes and remedies for cracks in concrete

Cracks in concrete are caused by the following factors.

1. One of the main causes of cracks in concrete is the cooling and contraction caused by the setting of the concrete. The volume change and stress due to shrinkage are independent of any external load or applied stress.
2. Due to differential expansion and contraction, cracks may develop in the smaller section connected to the larger section. Therefore a joint should be provided on change of section. Permanent members are more likely to break than members that are free to expand and contract only as supported beams.
3. Repeated expansion and contraction or alternate wetting and drying may result in gradual disintegration of poor concrete.
4. Rapid drying due to hot weather and strong winds or the absorption of water from concrete by wooden forms is also a cause of cracks in concrete. Therefore the form work on which the fresh concrete is placed must be damp, or it must be waterproof so that it does not absorb water from the fresh concrete.
5. Loose form work can also cause cracks in the concrete. Therefore the form work should be of such strength as to withstand the pressure of the wet concrete without swelling, spreading or any movement.
6. Concentration of tensile reinforcement at square openings or re-entry angles (such as at the corners of door and window openings) causes cracking. This can be avoided by suitably placing reinforcement with adequate cover. Adequate thickness of concrete should be given at the points where the bars are bent and anchored.
7. Minute cracks on the stress side of a reinforced concrete member are inevitable due to the poor tensile strength of concrete compared to steel and must break when the steel reinforcement takes its load. However, those cracks must be fine enough for moisture to penetrate to prevent corrosion of the reinforcement.
8. Hair cracks are the result of uneven shrinkage of the surface concrete and the mass behind it. These cracks can be avoided to a certain extent by delayed finishing and final floating of the concrete.
9. Surface cracks are also caused by surface dressing with a mortar rich in cement. From too much water, inadequate treatment, or over-troweling. One way to avoid such hair cracks is to remove the concrete surface skin by brushing it with a stiff brush immediately after setting.
10. Shrinkage of concrete is more harmful than expansion because it sets up tensile stresses in the structure, especially those with large surface area and thus creates cracks. Such shrinkage cracks can be prevented by laying reinforcement near the surface. Reinforcement of smaller diameter and distance closer to the surface is more effective than bars of larger diameter separated from the surface.