Mass Concrete Cooling

What counts as mass concrete

ACI defines mass concrete by behaviour rather than by a single dimension: any volume of concrete large enough that measures must be taken to deal with the heat it generates and the resulting volume change. In practice a least dimension around three feet is the point where most engineers start treating a placement as mass concrete, but the governing factor is the specification in front of you, not a rule of thumb.

Typical candidates are mat foundations, bridge piers and abutments, pile caps, LNG and cryogenic tank bases, wind turbine foundations, transfer slabs, and heavy equipment pedestals.

The two numbers that govern the pour

Nearly every mass concrete specification turns on two limits. Get familiar with both, because they fail in different ways.

• Peak internal temperature. A cap on how hot the core of the element is allowed to get. A limit around 158°F is commonly specified, tied to the risk of delayed ettringite formation, an internal expansive reaction that can crack concrete years after it cures.
• Maximum temperature differential. A cap on the gap between the hot core and the cooler surface, commonly around 35°F. The core expands while the surface does not, the surface goes into tension, and it cracks.

Why heat of hydration is the problem

Cement hydration is exothermic. In a thin slab that heat escapes to the air almost as fast as it is produced. In a thick section the concrete insulates itself, heat accumulates faster than it can leave, and the core climbs well above the temperature the concrete was placed at.

The critical point is that peak temperature is built on top of placement temperature. The heat of hydration adds a rise that is largely fixed by the mix, the section, and the ambient conditions. If you start hot, you end hotter. Lowering the temperature of the concrete as placed lowers the entire curve, which is why placement temperature shows up as a hard number in so many specifications.

Where cooling fits among the controls

Cooling the mix is one tool among several, and a good thermal control plan usually combines them:

• Mix design. Supplementary cementitious materials such as fly ash and slag generate less heat and generate it more slowly. This is the cheapest lever and it belongs to your engineer, not to us.
• Lowering placement temperature. Chilled mix water, ice replacement, aggregate cooling, or liquid nitrogen injection. This is where we work.
• Insulation and surface protection. Counter-intuitively, keeping the surface warm reduces the differential by letting the core and surface converge.
• Embedded cooling pipes. Circulating water through the element after placement. Effective on very large structures and expensive to install.
• Placement scheduling and lift height. Pouring at night, and breaking the element into thinner lifts where the design allows.

If your engineer can get inside the limits with mix design and a night pour, that is the cheaper path and you should take it. Cooling earns its place when the arithmetic does not work without it. Our comparison of cooling methods walks through how far each one can actually take you.

What we bring to it

We supply the nitrogen, the equipment, and the field crew to lower the temperature of the concrete as placed, plus the temperature record that supports your documentation. We are not your ready-mix supplier, we do not write your thermal control plan, and we do not perform your acceptance testing. We handle the cooling scope and we do it on the pour date.

The scope is sized to the placement rather than sold as a package. See nitrogen supply and project support for how we scope, schedule, and document a job.