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.
The TxDOT solution
In October 2001, the Texas Department of Transportation (TxDOT) was interviewed by Concrete Construction magazine for an article on mass concrete. After publication of that article, TxDOT took a critical look at both the existing mass concrete specification and ACI Committee 207's guidelines for mass concrete construction. TxDOT recognized the need for a tool to easily predict the performance of concrete mix designs for mass concrete applications.
In September 2002, TxDOT initiated a research project to develop software to perform the temperature analysis of mass concrete elements. Within six weeks, the project grew to encompass a full-blown concrete mixture design, analysis, and performance prediction program named Concrete Works. The investment in the project grew from $ 400,000 to $1,300,000, and the duration from three years to five years. The goal of Concrete Works was to give laboratory technicians, engineers, and contractors one tool that combines concrete design, analysis, and performance predictions to improve and guide TxDOT to better designs. One additional outcome of the research may be a change to the 35° F maximum temperature differential criteria dictated in the specification. The change could be switching to a gradient in lieu of the differential restriction and relaxation of the differential or gradient due to various material influences on cracking. The results and suggested modification to the specification will be presented to the TxDOT specification committee at the completion of this research project.
The Concrete Durability Center located at the University of Texas at Austin (UT) submitted the winning proposal for this project. The research team is composed of Dr. Kevin Folliard (UT, principal investigator), Dr. Anton Schindler (Auburn University), Dr. Maria Juenger (UT), Dr. Mike Thomas (University of New Brunswick, Canada), and Dr. Loukas Kallivokas (UT).
*Springenshmid, R., and R. Breitenbücher, “Influence of Constituents, Mix Proportions and Temperature on Cracking Sensitivity of Concrete,” Prevention of Thermal Cracking in Concrete at Early Ages, Edited by R. Springenschmid, RILEM Report 15, EF Spon, London, 1998, pp. 40–50.