Cracking
Concrete, when placed, is a mass containing more water than is required for cement bonds to form. When the concrete hardens and starts to lose the excess water, shrinkage begins. The cracking phenomenon is very complex and depends on a number of factors:
- Rate and amount of drying
- Drying shrinkage
- Tensile strength
- Tensile strains
- Creep
- Elasticity
- Degree of restraint and other factors.
Most cracks occur within 72 hours after the concrete has been placed. Preventative measures which will minimise cracking in this period include:
- Ensuring the sub-grade is well compacted
- Checking that form work is firm
- Ensuring that sub-grade and form work are moist before placing
- Not adding water to plant-batched, ready-mixed concrete
- Adequately compacting the concrete
- Cutting sufficient contraction joints to allow for shrinkage or using crack inducers
- Providing expansion joints where necessary
- Maintaining constant proper curing for an adequate period
- Using curing compounds if required.
Do not add water to on-site plant batched concrete
The most common mistake customers make is to add water to their delivery of concrete – often with the view to making it easier to spread and work.
So remember: Wharehine Concrete is expertly and freshly mixed. Do not add water after it is delivered to your site.
Only about half of the water within the concrete mix acts as a lubricant to produce flow and workability mixed. The rest is needed to hydrate (set) the cement.
When this balance is upset, the concrete is diluted and may lead to:
- A severe loss in strength
- Excessive cracking resulting from high shrinkage and low-tensile strength
- Dusting and crazing cracking of slabs caused by excessive bleeding and fines floating up to the surface
For more information visit www.nzrmca.org.nz or www.cca.org.nz