December 12, 2012 Webinar: 3:00 p.m. CST
Why Concrete Shrinks: What You Can And Can’t Do About It
Presented by Dr. Ken Hover, Cornell University
When concrete cools or dries it shrinks, and when concrete gets wet or gets warmer it expands. Controlling concrete temperature and moisture content therefore have a lot to do with controlling volume change, but even if we keep concrete temperature constant and prevent all water-loss, some amount of shrinkage is unavoidable. A 300-foot long concrete slab is likely to experience a total shrinkage of about 2 inches just due to drying and hydration of cement, in addition to a length change of about ± 1 inch for a 50 degree swing in concrete temperature between casting and in-service conditions. But shrinkage by itself is not really the problem. When concrete shrinkage is resisted by the subgrade, rebar, dowels, columns, or walls, restrained shrinkage turns into stress, and stress leads to cracking. When concrete strength develops faster than shrinkage stress, we get shrinkage without cracking. When shrinkage stress outpaces strength development, cracks appear. We can improve the odds in the stress-vs.-strength horse race by getting a few hot tips such as:
- How concrete mix design and ingredients influence shrinkage
- Why drying leads to shrinkage
- How we can get shrinkage even without drying
- How concrete temperature, air temperature, humidity, and wind speed affect drying.
- Delaying shrinkage and giving strength a boost through curing
- Putting the shrinkage cracks where we want them by sawcutting
- Controlling the inevitable cracks
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