Term: Accelerated curing
– **Mechanism**:
– At heightened temperatures, hydration process accelerates.
– Formation of Calcium Silicate Hydrate crystals is quicker.
– Gel and colloid formation speed up.
– Diffusion rate of the gel increases.
– Optimum temperature for accelerated curing is 65-70°C.
– **Delay period**:
– High temperatures in accelerated curing cause thermal stresses.
– Pore pressure and thermal stresses induce tensile stress in concrete.
– Immediate curing after pouring leads to microcracks and delayed ettringite formation.
– Delay period allows concrete to gain minimum tensile strength.
– Delay period typically equals initial setting time.
– **Excessive temperatures**:
– High temperatures reduce compressive strength due to the crossover effect.
– Lower temperatures improve compressive strength but lengthen cycle time.
– Trade-off exists between cycle time, cost savings, and compressive strength.
– Cycle time and concrete mix design are adjusted based on project needs.
– **Role of pozzolanic material**:
– Pozzolona increases later age strength by reacting with calcium hydroxide.
– Portland pozzolona cements have lower activation energy than OPC.
– Accelerated curing techniques enhance strength gain rates.
– Steam curing improves 1-day compressive strength values significantly.
– Useful in precast concrete industry for formwork removal.
– **References**:
– Erdem (2003) emphasizes setting time as crucial for delay period determination.
– ACI 517.2 R-87 discusses accelerated curing of concrete.
– Turkel & Alabas (2005) study the effect of excessive steam curing on concrete.
– Paya et al. (1995) focus on early-strength development of cement mortars.
– Yazici et al. (2005) explore the effect of steam curing on high-volume fly ash concrete.
Accelerated curing is any method by which high early age strength is achieved in concrete. These techniques are especially useful in the prefabrication industry, wherein high early age strength enables the removal of the formwork within 24 hours, thereby reducing the cycle time, resulting in cost-saving benefits. The most commonly adopted curing techniques are steam curing at atmospheric pressure, warm water curing, boiling water curing and autoclaving.
A typical curing cycle involves a preheating stage, known as the "delay period" ranging from 2 to 5 hours; heating at the rate of 22 °C/hour or 44 °C/hour until a maximum temperature of 50−82 °C has been achieved; then maintaining at the maximum temperature, and finally the cooling period. The whole cycle should preferably not exceed 18 hours.