Temperature Management of Mass Concrete Structures
By: Jon Poole, Kyle Riding, Ralph A. Browne, Anton Schindler
Thermal gradients in bridge elements were historically ignored in the United States, since the columns, footings, and bent caps were often relatively small. But as element size has increased for structural, traffic, and aesthetic reasons, thermal gradients and thermal cracking have become serious concerns for bridge engineers. Many mass concrete projects specify a 35° F maximum temperature differential, and some limit the maximum internal temperature (usually 160°F). Contractors and engineers on these projects are increasingly called upon to develop a temperature control plan to meet these specifications.
Unfortunately, even though the intent of the specification is well understood, the validity of the 35° F maximum temperature differential is questionable. Remember, the goal of these specifications is to minimize and control cracking. Thermal analysis of mass concrete elements alone will not directly control cracking risk. Clearly, a more unified approach to engineering mass concrete to prevent thermal cracking and improve long-term performance is needed. Research* suggests that the cracking risk of mass concrete can be lowered by a variety of methods, including:
- Reduction of the fresh concrete temperature
- Use of a larger maximum size aggregate
- Use of aggregate with a low coefficient of thermal expansion
- Use of crushed aggregate instead of smooth, round aggregate
- Replacement of cement with fly ash, slag, or other suitable supplementary cementitious materials (SCMs)
- Entrained air
- Reduction of cement content and paste content
Unfortunately, there hasn't been a good way to quantify the effects of each of these methods. There is no easy answer when a contractor asks, “If I use a crushed limestone and reduced cement content in my mixture, will this meet the placement temperature specification?” Being able to answer such a question would improve the performance and economics of mass concrete.