Concrete is a durable material, but it is not immune to wear and tear. Cracks, chips, and stains can all occur over time, but there are steps you can take to prevent them. To avoid stress cracking, make sure that the slab is built on a uniformly compacted and well-drained subgrade and that it is thick enough to withstand the type of use it will receive. In residential concrete, 4 inches is the minimum thickness for walkways and patios.
Plastic shrinkage cracks can be controlled by windbreaks, shading, and surface treatments. Vertical cracks can be caused by the settlement of concrete around the reinforcing bars. To reduce or prevent cracking, vibration and revibration should be used. Relatively small movements of formwork in the early stages of hardening will lead to cracks.
Swelling or bulging of wood, nail jumping, clumsy or excessive use of vibrators are the common causes of movement of the shape that irregular cracks occur. Wide cracks, which sometimes extend through the slab, occur when the subgrade decreases before the concrete. This can happen because the subgrade is not level and well compacted, or is muddy or otherwise unstable. Shrinkage cracks cannot always be prevented, but can be controlled by making planes of weakness to establish the direction of cracking when shrinkage occurs.
This is done by cutting grooves one-third of the thickness of the slabs, and is done as soon as the concrete is hard enough to withstand the damage of the saw. For a crack control joint to be effective, it must be ¼ deep as the slab is thick. That is, in a typical 100 mm thick slab, the joints should not be less than 25 mm deep; a 150 mm thick slab would require joints 38 mm deep, etc. To minimize the chances of early random cracks, these joints should be placed as soon as possible after pouring the concrete. Expansion joints or insulation joints usually consist of pre-molded tar material 10 mm thick, as deep as the slab and as long as the slab. They should be placed where paths or driveways meet, and where the slabs meet the base of a column, wall, or any other mass that resists expansion in that direction. If you use a saw to cut the joints, the cuts should be made as soon as the concrete can withstand sawing without fraying the edges of the saw cut.
For smaller projects such as concrete vases and other decorative items, you can add wire mesh during pouring to improve strength and reduce cracking. The covers must be kept continuously moist and a film of water should remain on the surface of the concrete throughout the curing period. Read here about methods to cure concrete and understand how your contractor will cure concrete. Today, Holcim continues to supply essential building materials including aggregates, sand, ready-mix concrete, engineered precast concrete and prestressed concrete solutions throughout Australia. While it takes 28 days for concrete to fully cure, the steps you take in the first few days after pouring are the most important to ensure a strong, crack-free surface.
