What are concrete shrinkage cracks?
Thursday, May 28, 2020
The addition of water to a dry concrete mix activates a process called hydration, the chemical reaction in which water is absorbed to create a rock-like crystalline structure. It continues slowly for months after the inital set, and the concrete shrinks in size slightly over this time. Enough water must be used to cause the hydration, but too much water weakens the final product and leads to more shrinkage.
Because the shinkage is normal, some amount of it is always expected and hairline shrinkage cracks can appear. Reinforcing steel bars or mesh in the concrete makes it stronger against tensile (bending) stress, but does not significantly reduce the hairline shrinkage cracks. Two ways that they can be controlled are addition of fibers to the mix or post-tensioning the slab, both of which you can read about at What is fiber reinforced concrete? and Why is there a "WARNING! POST-TENSION SLAB" sticker in my house?
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