Controlling cracks in concrete

Why does concrete crack? How do I control it?

Concrete cracks typically result from failure to cure the concrete properly. However, the cause may also be the result of poor site preparation and attention to detail in mixing, placing and finishing concrete with the inclusion of appropriate control joints. The prevailing weather conditions may also contribute to cracking, with the risk most likely on warm days, dry days with low humidity and/or windy days.

To control the incidence of concrete cracks, the following advice is offered:

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  1. Prepare the site properly, including where control joints will be. Ensure correct placement of steel reinforcement if used. Spread a 10-mm base layer of sand or fine gravel over the formed area to act as a slip-joint between the ground and the concrete. Ensure that formwork is level, firm and properly fixed in place.
  2. If mixing concrete on site, mix the concrete correctly. Use a container, not a shovel, to measure each part of materials used in the mix. Add only enough water to obtain an even, workable mix.
  3. Place the concrete carefully.
  4. Ensure thorough compaction of the concrete during placement.
  5. Start early. Avoid concreting in the middle of the day. Make sure there are sufficient able-bodied helpers to assist in placing, compacting, and finishing the concrete. Work at a steady pace until the entire job is complete.
  6. Avoid, if possible, executing major household concreting projects on warm sunny days, dry days with low humidity, or cold and/or windy days. Heed the prevailing weather conditions: if the weather is forecast to be hot, dry or windy - dampen the subgrade (ground) and formwork before placing the concrete. Make sure the work is adequately prepared because there will be less time to place, compact, finish and cure the concrete in weather conditions described above.
  7. The concrete MUST be finished correctly - from initial screeding to final finish; including properly positioned and constructed control joints.