Why does concrete shrink as it sets and hardens?
Sunday, September 26, 2021
Concrete is created when cement, water, and rock aggregate are mixed together. The cement and water mixture turns into a paste that bonds with the aggregate as it sets. This happens through a curious process called hydration: the cement chemically cracks open the water molecules and uses one hydrogen atom and one oxygen atom to create the crystalline structure that forms as the material hardens. This process creates some initial heat, and also leftover hydrogen atoms are released as a gas into the atmosphere.
But a side-effect of the breaking apart of the H2O molecules and reconfiguration of the atoms is a reduction in the volume of the concrete. This shrinkage happens gradually over time, with most of it occurring within the first 28 days after the material is mixed and begins to set up. It creates tension within the concrete mass, which leads to cracking, releasing some of the tension.
The cracks are usually hairline, but can be larger if too much water is added to the mix. The cement can only use so much water in the hydration process, and the excess evaporates away, causing even more shrinkage.
Because concrete shinkage is normal, some amount of it is always expected. 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?
Also, shallow score lines cut into a concrete floor slab shortly after placement are often used to manage—but not eliminate—the cracks. It makes them likely to occur at the bottom of the score slot, where they are less visible. This is often done for garage floors. Go to Why are there score line grooves in the concrete floor of the garage? for details.
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