Cracking occurs in concrete after it has solidified for a number of reasons. The two most common forms of cracking in hardened concrete are: Madness And Drying Shrinkage Cracking,
It is described as a network of very fine cobweb-like or crocodile-skin cracks that appear on the concrete surface after exposure to the atmosphere for some time. They occur in surface cement mortars and are usually more common in surfaces with highly steel-trowel or burnish finishes because the additional time required for finishing exposes the surface for a longer period before curing begins.
Insanity generally does not extend below the top 2 to 3 mm, nor does it cause durability or other serious problems; It generally affects only appearance.
because of the madness
The main reasons for this form of cracking are:
- rapid drying (by troweling) of a highly compacted surface while the concrete is still weak (between troweling and the start of curing);
- Expanding and shrinking of the concrete surface during alternating cycles of wetting and drying;
- Steel troweling the concrete before the water sheen is gone and therefore troweling water that bleeds into the concrete surface has resulted in increased shrinkage and weakening of the surface mortar;
- using cement-containing mixtures on the surface as ‘dryers’ to remove excess water (these can create a surface layer with properties quite different to the concrete below, eg high shrinkage);
- overworking the surface and bringing additional mortar to the surface (again creating a surface layer that may have increased shrinkage characteristics);
- adding extra water to the mixture;
- Inadequate or inconsistent treatment.
To avoid craziness on troweled surfaces:
- Avoid mixing too wet and do not add excess water to the site.
- Do not use a ‘dryer’ such as cement, pigment or color hardener to absorb bleeding water.
- Do not pour the bleed water over the surface, but wait until the water starts to shine. Excess bleed water can be drawn from the surface with a hose.
- Do not overwork the concrete by unnecessary troweling of the surface.
- Avoid wetting and drying cycles and excessive ‘wet wiping’ of the surface where water is sprayed onto the surface to aid in troweling.
- Protect surfaces from rapid drying during finishing.
- Start treatment immediately.
These are caused by moisture in the concrete that dries out over time causing the concrete to shrink. If the concrete is free to move this is not a major problem, but, if tempered, tensile stresses can develop in the concrete and if these exceed the tensile strength of the concrete, it can cause cracking Is.
The amount of water in the mixture is the major factor affecting the drying shrinkage. Other factors that can affect the risk of cracking in hardened concrete include restrictions, curing conditions, aggregate size and material, geometry, and details of construction practices.
Ways to reduce the risk of cracking due to drying shrinkage include:
- Do not add excess water to the concrete on site.
- Compact the concrete enough to achieve maximum density.
- Provide adequate treatment and start immediately.
- Place joints in correct locations, provide correct geometry (size/shape of slab panels) and ensure proper construction detail to limit concrete cohesiveness.
- Use good construction practices when pouring concrete.
- Provide adequate reinforcement and ensure correct placement.