FREQUENTLY ASKED QUESTIONS
Concrete can experience two kinds of cracking: shrinkage cracking and flexural cracking. Flexural cracking occurs when the concrete deflects under load. For DIY projects, general concrete will be strong enough for all expected loads, so you only need to be concerned with shrinkage cracking.
Concrete literally shrinks as it dries. The volume of water lost to evaporation usually causes about 1/16-inch shrinkage per 10 lineal feet of concrete, and this creates tension within the concrete. If the tensile forces become great enough, the concrete will crack.
When pouring concrete in a large area, such as a driveway slab, the use of reinforcing mesh will help distribute tensile forces. After pouring, concrete must be cured. This means its surface temperature and humidity must be controlled so it can properly set. For three or more days after it is poured, concrete should be kept damp by wetting it with a hose. Direct sunlight should be avoided; cover the concrete if necessary.
Even when these steps are followed, cracking cannot be completely eliminated – it must be controlled. Cutting joints into the concrete’s surface every four to six feet using a tool known as a ‘groover’ will give potential cracks a path to follow, ensuring that they do not mar the concrete’s finished surface.
Environmental and material temperatures above 70°F (21°C) may speed setting time and increase the rate of strength gain. Higher temperatures will have a more pronounced effect. To compensate for warm temperatures, keep material cool, use chilled mix water or use an approved retarder.
Cold weather can dramatically slow hydration which the concrete will need more time to set. Pouring concrete in weather below 50 degrees Fahrenheit (10 degrees Celsius) should be avoided.
Too much water will result in weaker concrete that is prone to spalling or chipping.
Dry concrete is better than wet concrete because it creates a stronger finished product. However, concrete that appears crumbly or chunky during mixing needs to have water added, little by little, until all ingredients are thoroughly wetted but not runny.
In addition to commercially available chemical cleaners, stains can be removed by: sandblasting, shot-blasting, grinding, scabbing (removing a thin layer of concrete with a chipping machine), planing and scouring.
It means the concrete can withstand 8,000 pounds of load per square inch when mixed, placed and cured properly.
Exposing the concrete’s own aggregates can result in a pleasant finished surface. Exposed aggregate surfaces can either be left rough or be ground and polished to a high sheen. Concrete may be colored by adding pigments, either before or after the concrete is in place, or by using chemical stains. Patterns can be imprinted into the concrete before it is set, or the surface can be textured using a trowel, a float, or even a broom.