When determining the true cost of a concrete repair project, owners must consider the financial impact of shutting down a structure to perform the work. The money saved by reducing the downtime of in-service structures has created a big market for concrete repair materials that gain strength quickly. An owner often is willing to pay a substantial premium for a product that allows a structure to be returned to service in a few hours instead of a few days.
When faster strength gain is required, it can be provided by calcium-aluminate cements, magnesium phosphate, and other proprietary cement formulations that reach design strengths in less than 4 hours. This article describes products that reach a compressive strength of at least 500 psi in 3 hours and thus meet the compressive-strength requirement for rapid-hardening concrete as outlined in ASTM C 928. Most products exhibit good freeze-thaw durability without requiring a separate air-entraining agent. Because some rapid-hardening concrete reaches initial set in as little as 5 minutes, manufacturers recommend mixing times as short as 60 seconds.