When concrete is placed it generally contains a higher amount of water than is required for the hydration of cement. As it hardens, excess water is lost and shrinkage begins. Unrestrained concrete will have no cracks, however it is extremely difficult to support a structure without restraint. Therefore the issue of cracking is complex and is caused by a wide variety of factors such as; shrinkage rate, climate such as hot and windy days, tensile strength, creep, tensile strain, elasticity, amount of restraint, etc.
While cracking can be an indicator of major issues structurally, most cracking does not affect the stability or durability of the structure. Extreme cases can reduce the structural stability and serviceability of the structure and thus cracking must be adequately addressed and kept to a minimum.
Noting cracking as a major concern to consumers, concrete manufacturers have begun addressing the issue in the laboratory by conducting drying shrinkage tests. By conducting tests in a multitude of scenarios and conditions manufacturers are able to give consumers assurance in applications of concrete in a scenario closely related to theirs.
Drying shrinkage, a major cause of concrete cracking is influenced by many factors; water to cement ratio, cement fineness and composition, stiffness, grading of aggregate, surface texture, size, shape, humidity, and temperature.
Common Causes of Cracking
A Survey has been carried out revealing the common causes of concrete cracking by percentage and can be attributed due to the following:
Supervision and Construction 36%
Design defects 27%
Climate (temperature, humidity, moisture, etc) 21%
Material Quality 16%
Identifying Cracks and Preventative Measures
Shrinkage cracks: Prevent by contraction joints.
Shrinkage cracks caused by stress concentrated in corners: Prevent by using reinforced steel or expansion joints.
Shrinkage cracks at door or windows: Avoid by using reinforced steel or by use of low-slump concrete in concrete walls.
Shrinkage cracks in feathered sections: Prevent by avoiding narrow feathered sections.
Settlement cracks caused by movement of sub-grade or footings.
Cracks due to heaving under the slab through poor drainage of sub-grade.
Expansion cracks: Prevent by placing expansion joints.
Plastic shrinkage cracks: Due to loss of water to dry sub-grade or to the atmosphere.
The best way to prevent cracking is to at time of placing and curing of concrete to monitor water/cement ratio, constantly water concrete evenly as often as possible to keep hydration or to spray with a chemical sealant that withholds the water content within the concrete slab.
Summary
Generally cracking should not be of major concern unless there is a pattern across the slab which may indicate structural damage. Majority of the time it is a cosmetic issue due to drying conditions. If you require a concrete slab cracking inspection please contact us.