What Is Mass Concrete?
Mass concrete is any large-volume concrete placement where the heat of hydration can cause damaging temperature differentials — typically elements 3+ feet thick (mat foundations, dams, bridge piers, transfer girders). The core heats up as cement hydrates while the surface cools, and the differential expansion cracks the concrete. Mass concrete is designed and placed as a thermal management problem, not just a structural one.
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The thermal problem
Cement hydration generates heat — in a thick section, the core can reach 150-180°F while the surface sits at ambient. As the core later cools and contracts against the already-cool exterior, tensile stresses develop that exceed the young concrete's strength: through-cracking results. The damage is internal and structural, not cosmetic — thermal cracks compromise durability and, in dams and water structures, watertightness.
Control measures
ACI 207 guides the practice: limit cement content (use slag, fly ash, or other SCMs that hydrate slower and cooler), pre-cool ingredients (chilled water, ice, liquid nitrogen), place in lifts with cooling intervals, embed cooling pipes circulating chilled water in the largest placements, and insulate the surface to reduce the core-to-surface differential. Thermal modeling predicts the peak temperatures and differentials before the pour — the mix and placement plan are designed from the model.
Specifications and monitoring
Mass concrete specifications set maximum placement temperature, maximum core-to-surface differential (typically 35°F), and maximum peak core temperature. Thermocouples embedded during placement feed real-time data; if differentials exceed limits, protection (insulation, heating) adjusts immediately. The thermal control plan is a submittal the engineer of record approves — it is as important as the structural calculations.
The bottom line
Mass concrete (3+ ft thick) cracks from hydration heat differentials — control with low-heat mixes, pre-cooling, lift placement, and insulation. Model thermally, monitor with thermocouples, and enforce differential limits.
Torch Engineering PLLC · (480) 490-0064 · info@torchengineering.com — PE stamping backed by Apex Grid Engineering.