Cracks in Concrete
Concrete, when placed, is a mass containing more water than is required for hydration of the cement it contains and for subsequent curing. When the concrete hardens and starts to lose the excess water, shrinkage begins. Unrestrained concrete should not experience cracking due to drying shrinkage, however, it is virtually impossible to support a structure of any appreciable size without some restraint.
The cracking phenomenon is complex and depends upon a number of things - rate and amount of drying, drying shrinkage, tensile strength, tensile strain, creep, elasticity, degree of restraint and other factors. Types and causes of cracking can be divided into categories: Cracks occurring before and during hardening, occurring after hardening, cracking resulting from structural design or accident.
Shrinkage cracks cannot always be prevented, but they can be controlled by making planes of weakness to establish the direction of cracking when contraction occurs. This is done by cutting grooves one third the thickness of the slabs, and is done as soon as the concrete is hard enough to resist damage by the saw.
Types of Cracks
- Shrinkage cracks - avoid by cutting contraction joints
- Shrinkage cracks caused by stress concentration at corners - prevent by placing expansion joint or by using reinforced steel
- Settlement crack caused by movement of subgrade or footings
- Cracks due to heaving under slab through poor drainage of subgrade
- Expansion cracks - prevent by placing expansion joints
- Shrinkage cracks in feathered sections. Narrow feathered sections should be avoided
- Plastic shrinkage cracks, due to quick loss of water to subgrade or the atmosphere
- Shrinkage cracks at door or window corners - avoid by using reinforced steel or (in solid concrete walls) by careful placement of low slump concrete.
Cracking
The majority of cracks occur within 72 hours after concrete has been placed. These are preventative measures that will minimise cracking in this period.
Check that the subgrade is well compacted
Check that form work is firm
Ensure that subgrade and form work are moist before placing concrete
Do not add water to ready-mixed concrete at the point of placement
Adequately compact the concrete
Cut sufficiently contraction joints to allow for shrinkage an/or preventative crack inducers to control location of cracking at early ages
Provide expansion joints where necessary
Start curing as soon as possible
Maintain proper curing for an adequate period