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Re: Slab-on-Grade
While it is true that concrete continues to gain strength over time it is usually impractical to wait for it to reach its ultimate strength. Concrete used in residential construction is usually 3,000 psi (pounds per square inch compressive strength). That is the 28-day strength of the concrete. After 7 days the concrete will probably be 2/3 of its 28-day strength (2,000 psi for 3,000 psi concrete). The ultimate strength will be marginally higher but could take years to reach.
When concrete is made water is added to cement and course and fine aggregate (gravel and sand). The addition of water starts the hydration process that causes the concrete to harden. The hydration process proceeds very quickly at first and slows down after a while but goes on for a long time. If you plotted the compressive strength of concrete over time it would be a graph with a very steep curve over the first few days that then flattens out; beyond 28 days the curve rises only slightly. That's why you can walk on a freshly poured slab or driveway not long after the concrete has been placed but you have to wait a few days before it is strong enough to drive on.
If you need higher strength concrete it may be cheaper to pay extra for it rather than pouring 3,000 psi concrete and waiting for it to strengthen beyond 3,000 psi (i.e., why not just go ahead and pour 3,500 or 4,000 psi concrete or use "high early" concrete instead of paying interest, insurance, property taxes, etc. while waiting several months or years for 3,000 psi concrete to gain strength beyond its 28-day strength).
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"Baseball is like church. Many attend but few understand." Leo Durocher
Bruce Breedlove
www.avaloninspection.com
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