Tech Lightweight Concrete

Lightweight Concrete Variables

Concrete cracks even when using the guidelines of the American Concrete Institute. It is in the nature of concrete to crack. Lightweight concrete will crack. This is not a workmanship issue; it is a concrete issue called heaving. If this happens, the lightweight concrete can heave and an elevation change can occur. Increased cracking can result when uncontrollable conditions such as heavy winds or extremes of heat or cold prevail. These factors could contribute to more superficial cracking than usual.

Secondly, concrete does not dry uniformly. A moisture gradient exists from top to bottom on any given slab. The surface is drier than the bottom, so shrinkage will be greater at the top. This results in the slab forming a cupped, or curled, shape. Where curling is constrained – for example, where a slab joins a perimeter footing – stresses on the surface of the slab are greater than where the slab was free to curl, so cracks may occur. Curling cannot be prevented and shrinkage cracks in concrete slabs can only be minimized – not prevented.

Third, to make concrete, a huge amount of water is added to promote plasticity. In a relatively short period of time this huge amount of water dissipates which lends itself to plastic shrinkage cracking.

Hydration and curing: When water is added to cement, it acts as a catalyst, causing the mix to harden and gain strength. This is called hydration. Microscopic fibers begin to grow and inter lock. For optimum hydration to continue, however, the moisture should remain for some time in the concrete to help the fibers to continue to form.

Initial hardening occurs immediately, but a longer process called “curing” is needed for the concrete to gain real strength and ability to withstand more PSI (pounds per square inch) of pressure. Curing also causes the material to shrink slightly.

Adding color in the concrete will not be consistant.

Please call our office if you have any further questions.

Thank you,

Terry Borek

President