Mass Concrete Temperature Differential: Limits, Cracking Risk and Sensor Placement

As cement hydrates it releases heat. In a thin element that heat escapes almost as fast as it forms. In a thick raft, pile cap, transfer beam or dam block it builds up in the core while the surfaces lose heat to the air and formwork. ACI 207.1R gives a sense of scale: a 150 mm wall becomes thermally stable in about 1.5 hours, a 1.5 m wall takes about a week, and a 15 m section takes about two years.

The resulting temperature gradient, together with restraint from the subgrade, earlier pours or the element's own cooler outer layer, can crack the concrete before any structural load is applied. This guide covers why that happens, the limits used to control it, and where to place sensors to prove the pour stayed within them.

What counts as mass concrete?

ACI defines mass concrete by behaviour, not by size: any volume of concrete large enough that measures must be taken to cope with the heat of hydration and the volume change that follows, to minimise cracking (ACI 116R, ACI 207.1R).

Under ACI 301-16 the contract documents must designate which elements are mass concrete. The ACI 301-16 checklist gives a general trigger of a least dimension of 1.2 m or more. Many project specifications use a lower trigger of about 0.9 m.

Size alone does not decide it. The same ACI checklist says thinner placements with high early heat, such as mixes with Type III cement, accelerators or high cementitious contents, should also be considered, as should placements that trap heat, such as stacked lifts poured too quickly. If the documents are silent, ask the engineer of record before the pour.

Why does mass concrete crack?

Mass concrete cracks when tensile stress from restrained thermal movement exceeds the concrete's tensile capacity at that age. There are two mechanisms, and they can occur in the same element.

• Internal restraint (early-age surface cracking). The hot core expands while the surface stays cooler. The core stretches the surface, and the surface can crack while the core is still heating up or near its peak. The core-to-surface differential limit is intended to control this.
• External restraint (through-cracking on cooling). An element cast onto rock, piles or an earlier pour expands as it heats and contracts as it cools, and the base resists both movements. Cooling from peak temperature to ambient while restrained can open full-depth cracks days or weeks later. The peak temperature and the rate of cooling matter more here than the differential alone (ACI 207.2R).

Young concrete gains tensile strength as it hydrates, so cracking risk depends on both temperatures and maturity. That is why thermal limits are paired with rules for when protection may come off.

What temperature limits are specified?

Two limits are standard: a maximum peak temperature and a maximum core-to-surface temperature difference. When the contract documents designate mass concrete but give no values, ACI 301-16/-20 set a peak of 70 °C and a differential of 19 °C. The project specification always governs.

The 19 °C figure is a default, not a physical constant. The ACI 301-16 checklist notes that a higher differential may be acceptable depending on concrete properties, placement dimensions and reinforcement, and that the right value for a given mix can be found by numerical simulation (ACI 207.2R). Concrete with high thermal expansion or strong external restraint may need a tighter limit.

Regional codes

• ACI (ACI 301-16/-20), the basis of many Gulf project specifications: for designated mass concrete, a thermal control plan is required, with defaults of 70 °C peak temperature and 19 °C core-to-surface differential unless the specification states other values. The upper limit on placing temperature is set in the thermal control plan; the 35 °C hot-weather limit applies to ordinary concrete, not mass concrete.
• Saudi Arabia (SBC 304-2007, 5.13): fresh concrete no hotter than 35 °C at placing, discharge within 2 hours with retarders, and curing for at least 7 days. It sets no peak or differential limit for mass concrete.
• UAE (Dubai Building Code 2021, ADIBC 2013): no mass-concrete temperature limits; these come from the project specification. In Dubai, reinforcement in members that restrain shrinkage and temperature movement must be checked to CIRIA C766 or the ACI Structural Journal method referenced in DBC F.8.5.

Why is peak temperature limited? Delayed ettringite formation

The peak temperature is limited mainly to avoid delayed ettringite formation (DEF). This is an internal sulfate reaction that can expand and crack concrete months or years after construction. ACI 301-16 also cites possible reductions in ultimate strength.

When the concrete gets hot enough at early age, ettringite, which normally forms during hydration, does not form in the usual way. It can form later inside the hardened paste in the presence of moisture, and the expansion damages the concrete. The damage appears long after the thermal records have been filed, so the peak-temperature limit is preventive: it cannot be fixed afterwards.

The risk depends on the binder: cement chemistry and the amount of fly ash or slag both affect it. That is why ACI 301 allows the engineer to set a different limit or other mitigation methods. Treat the specified peak as a hard limit unless the engineer has approved otherwise in writing.

What goes into a thermal control plan?

A thermal control plan is the contractor's documented method for keeping the pour within the specified limits. It is submitted before the first mass pour. ACI 301-16 requires one for each mass placement unless the specification says otherwise, and ACI PRC-207.4-20 covers cooling and insulating systems in more depth.

ACI 301-16 (8.1.4) requires the plan to include:

1. Mix proportions. The ACI 301-16 default is a low- or moderate-heat cement, or portland cement with Class F fly ash or slag, and no Type III cement. Its checklist gives typical mass-concrete blends of 40–50 % Class F fly ash or 65–75 % slag, and warns that below 65 % slag the temperature rise can be higher than with no slag at all. Check this against mandated mixes such as Dubai's DBC Table F.3.
2. Adiabatic temperature rise of the mix, calculated or measured.
3. Upper limit for concrete temperature at placing, and how it will be achieved (chilled water, ice, liquid nitrogen, pour timing). Methods are covered in our hot-weather concreting guide.
4. Measures to keep the peak below the limit, and the calculated peak under expected conditions.
5. Measures to keep the differential below the limit, and the calculated maximum differential: insulation, blankets, insulated formwork or tenting, and cooling pipes for very large or high-heat placements.
6. Monitoring equipment and procedures, and a drawing of sensor locations.
7. Format and frequency of temperature data provided to the engineer.
8. Actions if temperatures or differentials become excessive.
9. Curing materials, methods and duration. The ACI 301-16 default for mass concrete is to keep forms in place and to use no water curing unless the plan shows the limits will still be met.
10. Formwork removal procedure that keeps the differential at newly exposed surfaces within the limit, and how curing continues.

If the mix proportions change, the plan must be updated.

The action part matters as much as the predictions. If the differential approaches the limit at 2 a.m., the plan should say who is alerted and what they do: add insulation, cover exposed faces, or delay stripping. If a limit is exceeded, ACI 301-16 (8.3.1.2(b)) requires immediate action under the plan, and no further mass concrete may be placed until the cause is identified and corrections are accepted.

Where should temperature sensors be placed?

ACI 301-16 (8.3.1.2(a)) sets a default layout unless the specification or plan says otherwise:

• one sensor at the centre of the largest portion of the placement
• one sensor 50 mm from the centre of the nearest exterior surface
• a backup sensor at each location
• one sensor in a shaded location for ambient temperature

Pair each core sensor with a surface sensor so the differential is calculated from two points on the same heat path. Taking the maximum of all core readings minus the minimum of all surface readings can overstate the differential for the element.

Unless otherwise specified, ACI 301-16 requires sensors that read from 0 to 100 °C to an accuracy of ±1 °C, checked as working before the pour. Temperature readings must be recorded at least hourly.

How long should monitoring continue?

ACI 301-16 (8.3.1.2) requires temperature control until the core has passed its peak and cooled so that the difference between the average daily ambient temperature and the internal temperature is below the differential limit when protection is removed. The thermal control plan states the exact end criteria.

How should the data be used during the pour?

Watch the core-to-surface trend, not just the latest value, and act before the limit is reached.

The differential usually grows over the first days as the core heats. A differential still rising steeply towards the limit calls for action before it is reached. Set alert thresholds for this in the thermal control plan. Cold nights, wind and rain on exposed faces, and early removal of formwork or blankets are the usual causes of sudden jumps. Removing forms from a hot core on a cool day causes "thermal shock" surface cracking (ACI 207.1R).

Temperature data can also feed the maturity method to estimate in-place strength. Two cautions from ASTM C1074 apply:

• very high early temperatures can reduce long-term strength, and maturity functions do not capture this (5.3)
• before stripping forms or post-tensioning on the basis of maturity, other tests must confirm that the concrete has the expected strength potential (9.5)

Site checklist for mass pours (rafts, pile caps, transfer beams)

How SHM360 helps

SHM360 provides temperature monitoring for mass pours. Each SHM360-Node has three probe channels plus an ambient temperature sensor, so a single node can log a core, mid-depth and surface point with ambient air. Two Nodes cover the ACI 301 default layout, including backups. It sends readings over LTE without a gateway and stores them locally if the signal drops. SHM360-Probe sensors (±0.5 °C accuracy, within the ±1 °C that ACI 301-16 requires; cable lengths from 1 m to 30 m) are tied to reinforcement and remain in the concrete. The SHM360 platform shows live charts, sends threshold alerts when a reading crosses a limit you set, and produces PDF reports and CSV exports for the thermal control record. See concrete temperature and strength sensors for details.