Kevin R. Pruski and Ralph Browne, Texas Department of Transportation
Over the past 30 years, the Texas Department of Transportation (TxDOT) has transitioned from building bridges supported by multi-column piers to using single column piers for many bridges built in urban locations. This minimizes site restrictions, enhances aesthetics, and improves the hydraulic function. Thus, the use of more massive concrete members, required to accommodate the single pier approach, became mainstream. With this trend, construction practices required modification to ensure quality concrete construction.
Mass Concrete Challenge
The push for accelerated construction combined with large members brought new challenges. Because of specification changes, Type I or Type I/II cement began to behave more like Type III cement with respect to rate of strength gain, set time, and heat of hydration. Also, contractors began providing higher strength concrete to reduce formwork cycling times and to ensure that specified strengths were exceeded. The faster strength gain and the higher strength concrete placed in large members resulted in higher heat of hydration leading to higher concrete temperatures. This resulted in observed thermal cracking. At least one structure showed signs of possible delayed ettringite formation distress.
Specifications
It became clear to TxDOT that mass concrete placement controls were practically ineffective and needed modification. Prior to the current TxDOT standard specification, mass concrete members were not specifically identified on the plans or differentiated from other similar structural concrete that did not meet the mass concrete parameters. Now, TxDOT specifically identifies members requiring mass concrete temperature controls in the plans as well as differentiates the mass concrete as a separate bid item to allow the contractor to capture the costs.
The specifications require any concrete member with a least dimension of 5 ft (1.5 m) or more to comply with the mass concrete requirements. A plan must be submitted showing that the temperature differential between the core and the surface will not exceed 35°F (19°C) and the maximum core temperature will not exceed 160°F (71°C). In addition, the concrete temperature at start of a placement must not exceed 75°F (24°C) and all formwork must remain in place for a minimum of 4 days.
Field instrumentation, consisting of two recording temperature probes, is required for all designated mass concrete members to verify that the proposed plan adequately complies with the temperature limits. Also, understrength concrete penalties were adjusted to a less punitive, more logical assessment of actual damage and financial impact to TxDOT. The intention was to eliminate the industry practice of adding an extra quantity of cement to avoid the potential penalties.
Implementation
Calculating thermal rise and temperature differentials in concrete members is not a simple matter. It was soon discovered that the method outlined in ACI 207 overwhelmed the construction personnel. TxDOT worked with the University of Texas in Austin to develop a computer-based method that would simplify this issue substantially. The endeavor resulted in a computer program named ConcreteWorks, which may be downloaded from www.texasconcreteworks.com. The simplicity and versatility of this program allows for rapid analyses to be performed on typical bridge elements based on current field conditions. This facilitates construction operation scheduling more suitably than the traditional approach and also promotes better owner and contractor cooperation. The program allows for detailed material property input or the user may select the default values for a variety of the usual concrete constituents. A significant amount of field verification testing was done showing the program accurately predicts heat generation. In addition, verification testing continues as results from field instrumentation are compared with the analysis results.
Practice
Twenty percent of the total pier concrete let in the past year had the mass concrete designation. There was no substantial difference in the price bid for the mass concrete compared to other concrete. Though not shown in the bids, there is a cost associated with meeting the mass concrete provisions. The most significant expense is providing the concrete with a temperature not exceeding 75°F (24°C) at placement compared with 95°F (35°C) allowed for normal pier concrete. Replacing mix water with ice is the most common method for cooling but there has also been a trend for producers supplying concrete for large construction projects to use liquid nitrogen.
The other cost is associated with instrumentation and data collection for the mass concrete members, which is approximately $200/member. The most common method of instrumentation uses concrete maturity sensors that are self-contained time and temperature collectors. After the critical heat generation period is complete, a handheld device retrieves the data, which is then transferred to a computer.
One particular situation revealed that the standard concrete mix design provisions limiting the amount of fly ash in the concrete made it necessary to pursue more substantial measures to keep the maximum core temperature from exceeding 160°F (71°C). It was decided that the maximum limit of 35 percent Class F fly ash should be increased to 45 percent for mass concrete members. The high volume fly ash mixture was tested and used and the temperature provisions were satisfied. The specifications for concrete mix design have been updated to now allow up to 45 percent Class F fly ash for all mass concrete placements.
Future
TxDOT believes the implementation of ConcreteWorks for mix design development will significantly improve the concrete used for mass concrete placements and possibly other concrete as well. One feature that shows great promise is mix design optimization. The user can clearly see the benefits of using supplementary cementitious materials and uniformly graded aggregates to reduce the cement requirement, which subsequently reduces heat generation. With the aim to more carefully control concrete temperatures, TxDOT likely is getting more durable concrete as well.