What causes thermal cracking in mass concrete
Three things combine: the concrete generates heat through cement hydration, that heat cannot escape evenly in a thick section, and the resulting temperature gradient generates stress the young concrete cannot yet resist.
Two variables that matter — not a single pass/fail number
Core-to-surface differential. A differential in the region of 20°C is a commonly used starting point, but it is not a hard boundary. When the hot core expands and the cooler surface restrains it, tensile stress builds at the face — how much stress the section can actually tolerate depends on the mix, section geometry, reinforcement, and restraint conditions, and should be set by the project specification following a CIRIA C766 assessment, not read off as a fixed threshold.
Peak core temperature.A range of roughly 70–75°C is a commonly cited caution point for delayed ettringite formation (DEF) risk, but this is also a starting point rather than an absolute ceiling. BRE Digest and CIRIA guidance note that mixes with a high proportion of GGBS can tolerate meaningfully higher peak temperatures — in some cases well above 80°C — before DEF becomes a practical concern, because GGBS shifts the chemistry that drives DEF onset. That is not a reason to design toward the upper end routinely; it illustrates that the number itself isn't the pass/fail criterion.
In the end, whether a pour cracks is not decided by either number alone. It comes down to how thermal behaviour, restraint, reinforcement, and material properties interact — temperature is one input into that picture, not the whole picture.
Differential drives early surface cracking within days of the pour.
Peak temperaturedrives a longer-term durability risk (DEF) that may not show up until much later in the structure's life.
Both are performance-driven starting points, not hard boundaries — actual crack risk depends on how they interact with restraint, reinforcement, and the specific mix.
What raises the risk in a mass pour
How to control thermal cracking in mass concrete
| Measure | How it helps |
|---|---|
| Lower-heat binder | GGBS or fly ash to cut peak temperature and slow heat release |
| Control placement temperature | Chilled water, ice, or aggregate cooling to lower the starting temperature |
| Insulate, don't shock | Keep forms on and insulate the surface so the face cools with the core |
| Stage the pour | Lift heights and pour sequencing to limit heat build-up and restraint |
| Simulate before you pour | Model peak temperature, differential, and crack risk against the actual mix and geometry before concrete is ordered |
| Monitor the differential live | Embedded sensors at core and surface with alerts before the limit is approached |
The first four measures reduce the heat generated or slow the gradient before it forms. Pre-pour simulation is what turns those measures from guesswork into a plan with actual certainty behind it — running the mix, geometry, and pour sequence through a thermal model before ordering concrete shows whether the plan holds within limits, rather than finding out after the pour. Live monitoring then catches a developing problem in time to act, which matters because by the time a crack is visible, the intervention window has already passed. See concrete curing temperature for how temperature more broadly drives strength and cracking risk, and concrete thermal control plan for what a documented plan needs to contain.
Catch the differential before it cracks
ConcreteAI's Thermal Crack Management solution runs pre-pour simulation of peak temperature, core-to-surface differential, and predicted crack width from the actual mix design, geometry, and pour sequence — so the plan can be adjusted before concrete is ordered. During the pour, SmartHub places sensors at the core and surface and streams the live differential to a web dashboard 24/7, with alerts before the specified limit is approached, while also tracking maturity-based in-place strength from the same sensor data.
A free Simple Thermal Crack Check tool is available for initial screening of a planned pour.
Planning a mass pour with thermal crack risk?
ConcreteAI's engineering team can run a pre-pour thermal simulation for your specific mix, geometry, and site conditions.