Q: I'm a distributor of concrete repair materials that are often used in steel manufacturing plants. The concrete in the plants is often exposed to extremely high temperatures. To what temperature can you expose normal portland-cement concrete before it starts to deteriorate?
A: The behavior of concrete at high temperatures is influenced by several factors, including the rate of temperature rise and the aggregate type and stability. Abrupt temperature changes can cause cracking and spalling due to thermal shock, and aggregate expansion can also produce distress within the concrete.
High temperatures also affect the compressive strength of concrete. Above 212º F, the cement paste begins to dehydrate (loses chemically combined water of hydration), which gradually weakens the paste and paste-aggregate bond.
The temperature that concrete has reached often can be determined by observing color changes in the aggregate. For example, limestone aggregates turn pink when they reach about 570º F, which can result in substantial loss of compressive strength.
- ASTM C 856, "Standard Practice for Petrographic Examination of Hardened Concrete," ASTM, West Conshohocken, Pa., 1998.
- Steven H. Kosmatka and William C. Panarese, Design and Control of Concrete Mixtures, Portland Cement Association, Skokie, Ill., 1988.