Thermal cracking in mass concrete develops when the heat released by cement hydration creates a temperature differential between the core and surface of a pour that generates tensile stresses beyond the concrete's early-age tensile strength. In large pours, core temperatures commonly climb to 60-75°C while the surface cools against the formwork or ambient air. Once that gap gets too steep, the hotter core expands while the cooler skin restrains it — and the surface cracks. The same mechanism applies at restrained joints, where a new pour cast against an existing element cannot contract freely as it cools, producing vertical through-cracks. For concrete placed in pile caps, raft foundations, transfer slabs, and retaining walls, this risk needs to be assessed before the pour — not discovered afterwards.

Which elements are most at risk of thermal cracking?

Not every pour needs the same level of scrutiny. Risk concentrates in a few recognisable element types:

  • Mass concrete elements: pile caps, raft foundations, transfer slabs and thick mat slabs where the minimum section dimension — commonly around 500mm, though this depends on mix design, formwork and insulation — is large enough to trap hydration heat.
  • Restrained walls and slabs: retaining walls, basement walls and slabs cast against an existing structure, where contraction during cooling is held back by the older element.
  • High-cement-content mixes: more cement (or a higher OPC proportion) per cubic metre means more heat generated and a steeper temperature rise.
  • Water-retaining structures: sewage treatment tanks, reservoir walls, tunnels and basements, where crack width limits are typically tighter — around 0.20mm or 0.10mm under BS EN 1992-3 — so a crack that would be cosmetic elsewhere becomes a waterproofing failure here.

Two distinct failure mechanisms sit behind these risk categories. Through-section cracking occurs when the core-to-surface differential within a single element generates surface tensile stress beyond the concrete's capacity — this typically governs thick, unreinforced or lightly reinforced sections where geometry limits heat escape. Restrained-base cracking occurs at the interface between new and existing concrete, where the new pour's contraction during cooling is held back by the element it's cast against, generating tensile strain that can split the new concrete vertically.

What does CIRIA C766 require for crack control?

The design reference for thermal crack control in UK and Commonwealth markets is CIRIA C766, Control of Cracking in Concrete Structures, published in 2018. It replaced CIRIA C660 (2007) as the authoritative guidance document, which itself had updated the original CIRIA R91 from 1992 to align with reinforced concrete design rules in BS EN 1992-1-1. The guide provides methods for estimating crack-inducing strain and crack width under both restrained and through-section thermal loading, together with reinforcement design guidance where cracking is predicted. For water-retaining or liquid containment structures, BS EN 1992-3 adds its own crack-width requirements on top of the general framework.

"Cracking occurs when the tensile strain, arising from either restrained contraction or the strain differential within the concrete section, exceeds the tensile strain capacity of the concrete."

— P B Bamforth, CIRIA C766

How can you reduce heat of hydration before the pour?

Effective thermal crack management starts in the mix design stage, well before the first load reaches site. Cement content and cement type are the primary levers: higher cement content, or a higher proportion of ordinary Portland cement, means more heat released per cubic metre. Substituting part of that CEM I content with supplementary cementitious materials cuts the heat load substantially — ground granulated blast-furnace slag (GGBS) at 50-70% replacement can halve the peak temperature rise compared with a straight OPC mix. That makes SCM selection the single most effective thermal crack mitigation tool available to a mix designer, ahead of formwork insulation or curing-stage interventions.

On site, this translates into three practical checks before you commit to a pour programme for mass sections:

  • Confirm the specified cement replacement level (GGBS or fly ash) is actually reflected in the delivered mix, not just the design mix.
  • Check the minimum section dimension against the thresholds that trigger mass-concrete treatment — thick pile caps and raft slabs around 500mm or above need temperature monitoring, not just standard cube testing.
  • For any pour against an existing element — a basement wall on an existing slab, a new lift on a retaining wall — factor in the degree of end restraint, since this governs crack risk as much as the temperature itself.

Practical take-away: treat heat of hydration as a mix-design decision, not a curing-stage fix. Specify the SCM replacement level in the mix design, confirm the minimum section dimension against mass-concrete thresholds, and tighten crack-width tolerances early for any water-retaining structure — retrofitting crack control after the pour is placed is far more costly than getting the mix right first.