Effect of high curing temperatures on the strength, durability and potential of delayed ettringite formation in mass concrete structures

The Florida Department of Transportation (FDOT) has in recent years recorded high core temperatures of 170°F-200°F during curing of mass concrete elements. Frequent reports of such high temperatures have raised concerns of the strength and durability of concrete cured at such high temperatures. Additional concerns have been raised of the possibility of expansions of the hardened concrete from delayed ettringite formation (DEF) and its subsequent deterioration. FDOT specifies a maximum differential temperature of 35°F between the core and exterior of mass concrete elements during curing to avoid cracking from high thermal stresses and a shorter service life of the structure. However no limit is specified for the maximum curing temperature. This dissertation investigated the effects of high temperatures on the strength, durability and potential of delayed ettringite formation in mass concrete mixes.; Using typical FDOT Class IV mass concrete mixes it was found that elevated curing temperatures resulted in lower later-age strengths. Blending the cement with fly ash and slag resulted in increased strength and durability when compared to the plain cement mixes for all curing durations and temperatures.; Investigating the potential of delayed ettringite formation in the concrete mixes cured at elevated temperatures and the effect of permeability on the onset and amount of ettringite formed showed that: (1) At room temperature curing no ettringite was observed when samples were examined microscopically using a scanning electron microscope (SEM) at 7, 28 and 91 days, notwithstanding having the highest permeability values. (2) At elevated curing temperatures of 160 and 180°F, the plain cement mixes had high permeability values and microscopic examination showed ettringite crystals in void spaces at 28 days. At 91 days these samples showed voids almost filled with the crystals. (3) Concrete mixes containing 18% fly ash and 50% slag and cured at the elevated temperatures resulted in much lower permeability. The low permeability of the blends delayed the onset of ettringite formation as well as the amount formed when compared to the plain cement mixes. This was particularly evident in the slag mixes.