Mass Concrete Thermal Control Per ACI 207

Why this matters

Any concrete element where the heat of hydration produces a temperature differential that risks cracking is classified as mass concrete and falls under ACI 207. The threshold is not size alone; ACI 301-20 defines mass concrete as any structural concrete element with a minimum dimension equal to or greater than 36 inches, OR any element where measures must be taken to control the differential temperature. Large commercial column footings, transfer-beam mats, foundation piers under high-rise cores, and machinery foundations routinely fall into this category. A flatwork contractor pricing a 5x5x5 foot column footing as a normal placement misses the temperature monitoring, the mix design constraints, and the cooling strategy that drive the real cost.

Why thermal control matters

When portland cement hydrates, it releases roughly 80 to 100 BTU per pound of cement. In a 36 inch or larger pour the inside of the element cannot shed heat to ambient fast enough; the core temperature rises while the surface cools. ACI 207.2R recommends a maximum temperature differential between core and surface of 35 degrees Fahrenheit; differentials beyond that cause thermal cracking as the cooler surface contracts against the still-warm core. The maximum core temperature is typically capped at 158 degrees Fahrenheit (70 C) by specification because higher temperatures can cause delayed ettringite formation (DEF), a long-term durability problem.

Mix design strategies

Strategies to reduce the heat-of-hydration peak include:

• Substitute supplementary cementitious materials (SCMs) for a portion of the portland cement. Class F fly ash at 25 to 40 percent substitution and ground granulated blast-furnace slag (GGBS) at 50 to 70 percent substitution are typical. Both have lower heat of hydration than portland cement.
• Use Type II or Type IV cement (Type IV is "low heat" per ASTM C150 and is rare; Type II "moderate heat" is widely available).
• Use a coarser cement grind (lower Blaine fineness) when the supplier offers a mass-concrete grade.
• Increase aggregate gradation efficiency to reduce paste volume; the heat comes from the cement paste, not the aggregate.

The mix submittal should include the calculated adiabatic temperature rise (typically by Schmidt's method or a calibrated heat-of-hydration test per ASTM C186) and the projected peak temperature based on placement temperature and element geometry.

Pre-cooling the mix

Lowering the fresh concrete temperature at placement reduces the eventual peak. Strategies include: chilled mix water (chilled to 35 to 40 F before batching), ice substitution for a portion of the mix water (one pound of ice absorbs 144 BTU melting), and aggregate pre-cooling by water spray or air blast. Liquid nitrogen injection directly into the mixer is used on premium projects for placement temperatures below 50 F.

Post-placement cooling

Embedded cooling pipes (1 inch ID PEX or steel tubing on a grid of 36 to 48 inches each way through the element) circulate chilled water or ambient water to extract heat from the core during the first 7 to 14 days. The flow rate and water temperature are designed by the mix-design engineer; field instrumentation (thermocouples cast into the element at core and surface locations) confirms the temperature differential stays below the 35 F limit.

Insulation

Counter-intuitively, the surface of a mass placement is insulated, not exposed, during the first week. Insulating the surface keeps the surface warm so the differential to the core stays small. Insulation blankets (R-3 to R-10) cover the top of the pour; side forms are usually left in place for 5 to 7 days for the same reason.

Instrumentation

Thermocouples cast into the placement at known locations report core and surface temperatures to a data logger every 15 minutes. The data file becomes part of the closeout submittal. The field engineer monitors the differential daily; if the differential approaches the 35 F limit, additional insulation is added or the cooling water flow is reduced.

Acceptance criteria

The specification usually adopts: maximum core temperature 158 F (or lower as specified), maximum differential 35 F, and a specified rate of cooling once the peak is past (typically 5 F per day maximum so the element does not contract too quickly). The mix-design engineer issues the thermal control plan; the contractor implements and monitors.

References

• ACI 207.1R Guide to Mass Concrete
• ACI 207.2R Report on Thermal and Volume Change Effects on Cracking of Mass Concrete
• ACI 207.4R Cooling and Insulating Systems for Mass Concrete
• ACI 301 Specifications for Structural Concrete
• ASTM C150 Standard Specification for Portland Cement
• ASTM C186 Standard Test Method for Heat of Hydration of Hydraulic Cement
• ASTM C1582 Admixtures to Inhibit Chloride-Induced Corrosion of Reinforcing Steel in Concrete