Mass concrete & thermal cracking: the heat problem in thick pours

What makes concrete “mass” concrete

Cement generates heat as it hydrates. In a thin slab that heat escapes to the air almost as fast as it is made, so the concrete barely warms. But in a thick element, the heat is trapped in the middle — and the core can climb 50 to 100°F above where it started. Once a pour is big enough that this internal heat has to be managed, it is mass concrete, governed by ACI 207.

The common rule of thumb: any element with a least dimension of about 3 feet or more. Mat foundations, large footings and pile caps, thick walls, piers, columns, and transfer beams are the usual suspects — though a rich, high-cement mix can behave like mass concrete at smaller dimensions.

Why the heat cracks it: the 35°F rule

The problem is not the heat itself — it is the difference in temperature across the pour. The hot core expands. The cooler surface, exposed to the air, does not. The core's expansion is restrained by the cool skin around it, and that restraint puts the surface in tension. When the temperature difference between core and surface exceeds about 35°F (roughly 20°C), that tension cracks the concrete.

So mass-concrete thermal control plans track two numbers: a peak core temperature (often capped near 160°F to avoid delayed internal problems) and the core-to-surface differential (kept under about 35°F). These thermal cracks are different from the surface-drying plastic shrinkage cracks you fight on flatwork — they come from within.

How to control it

The strategy is two-sided: make less heat, and let the surface cool slowly so it stays close to the core.

Make less heat. Use low-heat cement and replace some cement with supplementary cementitious materials — fly ash and slag hydrate slower and cooler. Pre-cool the mix with chilled water, ice, or liquid nitrogen so it starts lower. On the largest pours, run cooling pipes through the concrete to pull heat out actively.

Slow the cooling. Insulate the surface — blankets, insulated forms, leaving forms on longer — so the skin does not shed heat faster than the core. Counterintuitively, keeping a mass pour warm on the outside is what protects it, because it keeps the differential small.

Monitoring a mass pour

You cannot manage what you cannot see, so mass pours use embedded temperature sensors — often the same maturity sensors used to track strength gain — reading core and surface temperatures continuously. The crew watches the differential and adjusts insulation to keep it under the limit, sometimes for a week or more as the pour slowly releases its heat.

Ambient conditions still matter

The weather sets your starting point and your surface-cooling rate. A hot placement raises the peak core temperature; a cold snap accelerates surface cooling and widens the differential. Knowing the conditions your pour is heading into helps you plan pre-cooling and insulation before the truck arrives — which is exactly what PourDay tracks for your job site.