Type of Cracks in Concrete Slab

Posted by Civil Engineering on

Thursday 30 April 2020

There are three main types of cracks in concrete. Each has its own cause and strategies to prevent or minimize.

Plastic shrinkage cracks. These occur during the first few hours when the concrete is still in a “plastic” state. They are caused when the surface moisture evaporates too quickly, usually during hot or windy weather. Synthetic fiber additives can help reduce this type of cracking, but do little once the concrete has cured.

Drying shrinkage cracks. These occur as moisture leaves the concrete after the slab has hardened. The main cause is concrete that is too wet, referred to as a “high-slump” mix. The best solution is to use less water in the concrete mix. Concrete suppliers sometimes add water to make the concrete easier to work with, but this weakens the concrete.

Welded wire mesh can also help reduce shrinkage cracking, but only if it is placed in the middle or upper half of the slab, but at least 2 inches below the surface. Wire mesh also helps keep small cracks from growing. In too many cases, however,  the wire mesh ends up on the bottom of the slab where it does nothing.

Shrinkage cracking can be managed by the use of control joints placed in the slab. Some contractors cut or form a grid of small grooves in the slab to keep the shrinkage cracks in an orderly grid, which looks better than random cracks, but functions the same way. If you are placing tile on the slab, it’s important the control joint line up with a control joint in the tile easier said than done. So random cracking might be a better approach for tile.

Structural cracks. Concrete can support a lot of weight in compression, but is weak in tension.  For example, a concrete wall can support tons of weight from above, but will crack easily if pushed sideways forcing it to bend. Similarly, a slab will crack if too much weight is placed in one spot, or if the soil settles unevenly, bending the slab.

The best protection against structural cracking in residential structures is good compaction of the soil and gravel underneath the slab. In addition, rebar should be placed in the footings around the perimeter of the slab and at post bases within the slab.

Labels: Construction, Prestressed Concrete, Reinforced Concrete

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