Thermal Control Plans Help Control Cracking of Mass Concrete

Many state DOTs require contractors to provide thermal control plans for bridge elements and other mass concrete. Thermal control plans ensure that the differential between the core temperature and the surface temperature is never high enough to cause cracking. The plan can either use a maximum allowable temperature differential of 35 °F or demonstrate that the imposed stresses never exceed the cracking strength of the concrete.

Thermal modeling predicts the core temperature of the concrete over time. The core temperature depends on the concrete mixture proportions – how much of each type of cementitious material is present – and the initial placement temperature.

Insulation alters the surface temperature to keep the temperature differential within acceptable limits. The thermal model can determine what type of insulation is necessary and when it can be removed. It can also calculate the maximum acceptable placement temperature.

If need be, the concrete can be cooled after placement using internal cooling pipes. However, judicious mixture proportioning, low initial placement temperature, and insulation.may make it possible to avoid this relatively expensive option.

The massive piers of the St. Croix Crossing Bridge between Minnesota and Wisconsin were placed without the need for internal cooling. Photo credit: MnDOT.

Supplementary cementitious materials help control heat

All cementitious materials evolve heat on hydration. Portland cements evolve the most heat, while supplementary cementitious materials such as fly ash and slag cement evolve less heat over a longer time. Because concrete does not conduct heat well, concrete in thick sections grows warmer in the hours and days following placement.

Like most materials, concrete expands on heating and contracts on cooling. If the core gets too much hotter than the surface, the concrete will crack at the plane of restraint, typically on the edge — and these are not small, tight cracks.

Even if the concrete doesn’t crack, curing at high temperatures is not desirable, as it results in weaker concrete and a more open, permeable pore system. Permeable concrete is more vulnerable to deterioration due to cycles of freezing and thawing and other forms of degradation.

High curing temperatures can also cause delayed ettringite formation (DEF). Normally ettringite forms as a product of cement hydration. But at higher temperatures it’s not stable, so monosulfate forms instead. When the concrete cools, the monosulfate reacts with sulfates in the pore solution to form ettringite, an expansive reaction. DEF does not normally occur without some other process to cause cracking and requires the presence of liquid water for the reaction to take place.

As part of thermal control plans, supplementary cementitious materials not only help to control heat, but also mitigate the effects of elevated-temperature curing.

Beton designs concrete for mass placements

Because portland cement generates the most heat in the first hours after placement, minimizing the cement content is essential. There are different ways to do this. Fly ash and/or slag cement in place of some of the portland cement will reduce the total heat evolved and slow the rate, giving more time for the heat to dissipate. We look at all the requirements for the concrete– control of reactive aggregates, compressive strength, durability to a harsh environment – and the availability of materials to make the best choice.

Beton also pays attention to the grading of the aggregates. We optimize the proportions of the various particle size fractions to fill the space efficiently so that less cement paste – and therefore less cement – is needed. The concrete is easier to mix, place, and consolidate when it’s fresh; generates less heat as it cures; and maintains better dimensional stability in service.

Note: ACI 207 defines mass concrete as, “any volume of concrete with dimensions large enough to require that measures be taken to cope with the generation of heat from hydration of the cement and attendant volume change to minimize cracking.” Historically, mass concrete meant dams. Nowadays, though, other types of concrete members are also subject to excessive heat and potential cracking. Bridge elements such as footings, piers, caps, and abutments, radiation shielding, and even airfield pavements can all require measures to control heat of hydration.