Meaning
Chemical volume reduction in a cementitious mixture occurs during hydration without loss of moisture to the surrounding environment. This volume change takes place as the cement hydrates and consumes internal water, leading to self desiccation in mixtures with low water to cement ratios. It must be accounted for in high strength concrete design to prevent early age cracking.
Internal Strain
Volumetric reduction driven by self desiccation creates tension in the capillary pores of the hardening paste. This internal pressure pulls the solid structure inward, generating microcracks if the tensile strength of the young concrete is exceeded before sufficient curing has occurred. Tensile stress from this phenomenon can diminish the long term structural integrity of high performance concrete elements.
Hydration Mechanism
Water consumption during the chemical reaction of cement creates empty pores that lower internal relative humidity. As chemical reactions proceed, the meniscus in each pore curves and exerts a tension that shrinks the paste. This action continues until the internal moisture falls below critical hydration levels.
Mitigation Protocol
Internal curing using pre wetted lightweight aggregate or superabsorbent polymers provides a reservoir of water to maintain high internal humidity. This slow release of moisture counteracts the capillary tension and reduces the overall volume change. Properly cured concrete maintains its dimension and resists premature cracking.
