Technical Guide

Thermal Cracking in Concrete — Causes, Assessment, and Prevention

Thermal cracking in concrete occurs when early-age temperature differentials between a pour's core and surface — or between the element and its restraints — generate tensile stresses that exceed the concrete's tensile capacity. It is most common in mass concrete, thick slabs, transfer structures, pile caps, and retaining walls, and can be assessed and controlled before a single cubic metre is placed.

CIRIA C766BS EN 1992-3Mass ConcretePre-pour Simulation
The mechanism

How early-age thermal cracking develops

Cement hydration is exothermic: the chemical reaction between cement and water releases heat. In large concrete elements, this heat cannot dissipate quickly, and the internal temperature rises — often to 60–75°C in mass pours — while the surface loses heat to the environment. The resulting temperature differential causes the hotter interior to expand while the cooler exterior restrains it, generating tensile stresses at the surface and at restrained faces. If those stresses exceed the concrete's early-age tensile strength, cracking occurs.

Two distinct mechanisms drive early-age thermal cracking:

Through-section crackingoccurs when the temperature differential between the core and the surface of a single element is large enough to develop tensile stresses at the surface that exceed the concrete's tensile capacity. This typically governs in thick unreinforced or lightly reinforced sections and in elements where the geometry limits heat escape.

Restrained-base cracking occurs when a new pour is placed against an existing element — a previously cast slab, pile cap, or foundation — and the new concrete wants to contract as it cools from its peak temperature back toward ambient. The existing element restrains this contraction, generating tensile strain in the new pour that can cause vertical through-cracks. This is addressed in CIRIA C766 Control of Cracking in Concrete Structures (2018) — the primary design reference for thermal crack control in the UK and Commonwealth markets.

CIRIA C766 updated and replaced CIRIA C660 (2007) as the authoritative guidance document. Both provide methods for estimating crack widths under restrained and through-section thermal loading. For crack width limits in water-retaining or liquid containment structures, BS EN 1992-3 provides additional requirements.

Risk factors

Which elements are most susceptible to thermal cracking

Mass concrete elements
Pile caps, raft foundations, transfer slabs, thick mat slabs. Any section where the minimum dimension is large enough that heat of hydration cannot escape before significant temperature buildup — commonly >500mm, but depends on mix, formwork, and insulation. Both core temperature and differential must be managed.
Restrained walls and slabs
Retaining walls, basement walls, and slabs cast against existing structures. As the new pour cools after peak hydration, contraction is restrained by the existing element, generating tensile strain. Crack risk is governed by the degree of end restraint and the temperature change from peak to ambient.
High-cement-content mixes
Higher cement content (or higher OPC proportion) means more heat generated per cubic metre. Replacing OPC with GGBS or fly ash reduces heat of hydration substantially — GGBS at 50–70% replacement can halve the peak temperature rise — making SCM selection a primary thermal crack mitigation tool.
Water-retaining structures
Sewage treatment tanks, reservoir walls, tunnels (DTSS, MRT), basements. Crack width limits are tighter — typically 0.20mm or 0.10mm under BS EN 1992-3 — making thermal control critical. A crack that would be cosmetically acceptable in a column becomes a waterproofing failure in a liquid-retaining wall.
Prevention measures

How to control thermal cracking — before and during the pour

Effective thermal crack management starts before any concrete is placed. The key variables — peak temperature, core-to-surface differential, and tensile strain at restrained faces — are functions of decisions that must be made at the design and planning stage.

Mix design: Replacing OPC with GGBS (commonly 50–70% for mass pours) substantially reduces heat of hydration and lowers peak temperatures. Fly ash and other SCMs also reduce peak temperature, though typically less effectively than GGBS at equivalent replacement rates.

Placing temperature: Lower fresh concrete temperature at placement reduces the starting point for temperature rise. Ice-water mixing, night-time pours, and cool aggregate storage are common mitigation measures in hot climates.

Insulation and formwork: Insulating blankets or extended formwork retention slows surface heat loss, reducing the core-to-surface differential. For some geometries, surface cooling (embedded pipes) is used to manage the core temperature directly.

Pour sequence and geometry: Dividing large pours into smaller lifts or bays — with adequate time between pours for cooling — reduces the heat volume generated at any one time. Restricting pour depth also limits maximum core temperature.

Pre-pour simulation: Before specifying any of these measures, a thermal simulation of the planned pour using actual mix design parameters, geometry, and site conditions predicts peak temperatures, differentials, and crack risk — so engineers can adjust the plan before the concrete is ordered.

Typical thermal crack criteria

Peak core temperature:Commonly <70°C (project-specific — verify against specification)

Core-to-surface differential: Limit defined by mix, restraint, and section — no universal value; governed by project specification and CIRIA C766 assessment

Crack width limits: Structural concrete typically 0.30mm; water-retaining 0.20mm or 0.10mm (BS EN 1992-3)

Always confirm limits against the project specification and design basis

ConcreteAI's approach

Simulate, assess, verify, document — before and during the pour

ConcreteAI's Thermal Crack Management solution covers the full workflow from pre-pour assessment to post-pour verification:

Simulate: Input mix design parameters, pour geometry, placing temperature, insulation, formwork type, pour sequence, and restraint conditions. The simulation predicts peak temperature, core-to-surface differential, tensile strain profile, and estimated crack width — before any concrete is placed. Scenario comparison allows engineers to evaluate different mix designs or pour sequences side by side.

Assess:Review predicted outputs against the project's thermal control criteria and acceptance limits. Identify which variables, if adjusted, bring the pour within limits.

Verify: During and after the pour, SmartHub embedded sensors log actual core, mid-depth, and surface temperatures continuously, allowing comparison against the simulated profile. If actual temperatures deviate from the prediction, the site team is alerted immediately via the ConcreteAI web dashboard and WhatsApp notifications.

Document: All temperature data, differential readings, and comparison against the thermal control plan are exported as a QA report — ready for submission and sign-off.

A free Simple Thermal Crack Check tool is available at thermal-public-stg.concreteai.app for initial screening. For project-specific simulation with full scenario analysis, contact ConcreteAI directly.

For projects requiring both thermal crack management and concrete strength monitoring — most mass concrete and infrastructure pours — SmartHub provides both functions from the same embedded sensor. See also mass concrete temperature monitoring for how temperature differential monitoring works alongside strength development tracking.

Assess your pour

Have a mass concrete or restrained element with thermal crack risk?

ConcreteAI's engineering team can run a pre-pour thermal simulation for your specific mix, geometry, and site conditions — and provide scenario comparison before the pour is planned.

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FAQ

Frequently asked questions

Thermal cracking in concrete is caused by temperature differentials that develop during early-age cement hydration. The exothermic cement-water reaction raises the internal temperature of a pour — often 40–70°C above placing temperature in mass elements — while the surface loses heat to the environment or formwork. This differential causes the hotter interior to expand relative to the cooler surface, generating tensile stresses. If those stresses exceed the concrete's early-age tensile capacity, cracking occurs. In restrained elements (e.g. a new wall cast against an existing slab), cracking also occurs as the pour cools from peak temperature and contraction is prevented by the existing structure.
Elements most at risk include mass concrete pours (pile caps, raft foundations, transfer slabs, thick mat slabs), retaining walls and basement walls cast against existing foundations, and water-retaining structures such as sewage tanks, reservoir walls, and tunnel linings. Risk increases with section thickness, cement content, OPC proportion, placing temperature, and degree of external restraint from adjacent existing elements. GGBS replacement is the most effective single mix design measure for reducing risk — 50–70% GGBS can substantially lower peak temperatures.
CIRIA C766 Control of Cracking in Concrete Structures (2018) is the primary design reference for thermal crack control in the UK and Commonwealth markets, including Singapore and Australia. It updated and replaced CIRIA C660 (2007). CIRIA C766 provides methods for estimating crack widths under restrained and through-section thermal loading, and sets out the design approach for controlling crack risk through mix design, pour geometry, and restraint assessment. For crack width limits in water-retaining or liquid containment structures, BS EN 1992-3 Eurocode 2 Part 3 applies.
The primary controls are: (1) Mix design — specifying high GGBS or fly ash content substantially reduces heat of hydration and peak internal temperature; (2) Placing temperature — reducing fresh concrete temperature at placement lowers the starting point for temperature rise; (3) Formwork and insulation — retaining formwork longer or adding insulation reduces the core-to-surface differential by slowing surface heat loss; (4) Pour sequence — dividing large pours into smaller lifts or bays limits total heat generated at any one time; (5) Pre-pour simulation — modelling the planned pour before placement predicts peak temperature and differential, allowing engineers to adjust parameters before any concrete is ordered.
There is no single universal limit — the allowable core-to-surface temperature differential depends on the element geometry, mix design, restraint conditions, and the project specification. Project specifications and thermal control plans typically define the limiting criteria for each pour. CIRIA C766 provides the assessment framework for estimating whether a given differential will lead to cracking under the specific restraint and geometry conditions. Always confirm the applicable limit with the project specification and design basis rather than applying a generic value.
Yes. Pre-pour thermal simulation uses inputs including mix design (cement type, SCM content, water-cement ratio), element geometry, placing temperature, insulation, formwork type, pour sequence, and restraint conditions to predict peak core temperature, core-to-surface differential, and tensile strain at restrained faces. The simulation outputs allow engineers to identify which parameters need adjustment and compare scenarios before committing to a mix or pour plan. ConcreteAI's Thermal Crack Management solution includes pre-pour simulation, and a free Simple Thermal Crack Check tool is available at thermal-public-stg.concreteai.app for initial screening.