Hardening Retarders for Massive Concrete

Abstract

The heat of hydration in massive concrete structures can raise the temperature to a level where thermal cracks can pose a problem. Hardening retarders are admixtures that lower the rate of hydration, distribute the heat release over time, and lower maximum temperature in concrete. Such admixtures will inherently lead to lower early strength, but should lead to comparable 28-day strength to reference concrete. Relative large amounts of urea works, in particular when the additional retardation of setting (not hardening) is counteracted by the set accelerator calcium nitrate. However, these dosages are high, and urea will also slowly decompose to ammonia that may limit the urea application to outdoor use, if any. The latest potential admixtures are combinations of minor amounts of strong setting retarders like organic acids (0.1-0.3 %) with the setting accelerator calcium nitrate (1-3%), where a true synergy between the two leads to hardening retardation.