John Staton, Michigan Department of Transportation and Dave Juntunen, Michigan Department of Transportation
The Michigan Department of Transportation (MDOT) recently completed the “Fix on 96”, which was a $148 Million project that reconstructed 7 miles of urban freeway and rehabilitated 37 bridges from Newburgh Road to Telegraph Road on I-96 in metropolitan Detroit.
Michigan’s strategic goal is to be an “Innovation Hub” where engineers work to achieve greater efficiencies and value added outcomes. A cornerstone for innovation includes efforts to continually strive to incorporate more durable products into our transportation infrastructure. One of these innovations used on the “Fix on 96” was to implement the use of High Performance Concrete (HPC) for bridge deck application. The HPC mixture, specified by MDOT as Grade DM Concrete, is a concept which had its inception in the HPC pavement arena over 15 years by MDOT material engineers.
In addition to the concrete’s ability to sustain long-term repetitive loading of Michigan’s heavy trucks, the primary criterion for high performance hinges on the concrete’s ability to resist Michigan’s harsh wet-freeze environment where a substantial amount of deicer salts are applied to the roadway surface in efforts to facilitate safe travel. Hence, it was established that the Michigan HPC must represent the following durability characteristics:
- the entrained air-void system within the concrete mortar must be well distributed and of sufficient quality to provide frost resistance.
- the concrete must be of low permeability in efforts to minimize the ingress of water and deicer chemicals, and
- the amount of cementitious paste must be reduced and optimized in efforts to reduce the overall shrinkage, thus reducing the risk of shrinkage-related cracking.
A well-established entrained air void system is critical toward protecting the mortar fraction of the concrete against freeze-thaw damage. The quality of this air-void system depends greatly on the chemical interaction between each of the components of the concrete mixture, as well as other outside influences (handling and placement, temperature of the concrete mixture). In efforts to ensure the overall quality and stability of an air-void system, smart selection of compatible materials is crucial, as well as employment of a strategy for process control which encompasses the need for periodic air content checks at set points in the process as the concrete moves from batching through transport to final placement into the forms. Short of the use of advanced testing devices and test methods developed to actually quantify the air bubble size distribution in the field, it can be expected that if the fresh concrete can be handled with minimal loss of total air content, the “locked-in” air-void system of the hardened in-situ concrete will be of sufficient quality to protect the concrete from freeze-thaw damage for many years to come.
Keeping the water and aggressive deicing salts from getting in to the concrete is a premiere aspect of MDOT’s HPC bridge decks in Michigan’s wet-freeze environment. Moderate replacement of the Portland cement (25 to 40 percent) with a Supplemental Cementitious Material (SCM – fly ash and slag cement) cement will work in harmony with the Portland cement to produce a secondary chemical hydration reaction during hydration that, in turn, contributes toward further densification of the cementitious paste. The result is a significant reduction in the overall permeability of the hardened concrete. This keeps the salt and water from getting into the concrete, reducing the potential for freeze-thaw and deicer-related damage as well as corrosion of the reinforcing steel. SCM’s have also proven to successfully mitigate the potential for Alkali-Silica Reaction (ASR).
Since the cement paste is the weakest component of a normal concrete matrix, it makes sense to strive to minimize its presence in the mixture. Additionally, since the vast majority of mass loss (shrinkage) in the hardened concrete occur in the paste component of the matrix, it makes sense to, again, keep the paste volume toward a minimum. Granted, sufficient paste volume is still necessary to ensure ease of placing and finishing, however, adopting a well-graded distribution of aggregates in the matrix will greatly enhance the concrete’s ability to be placed and finished, while shaving 100 pounds per cubic yard of cementitious material from the mixture. To add a synergistic flavor to the mix, incorporating well-graded aggregates in conjunction with an SCM further enhances the ability of the concrete mixture to be placed and finished while accommodating the overall reduction in cementitious material content.
Finally, since over 70 percent of the volume of the concrete is comprised of coarse aggregates, there should be an emphasis toward ensuring that high quality freeze-thaw resistant aggregates are specified for the HPC.
To date, MDOT has demonstrated that it can be both innovative and cost effective to produce HPC for highway applications using current materials and methods. With approximately 11,300 cubic yards of HPC bridge deck mixture placed on this project, there is now compelling testimony that it can easily be done. The feedback from this project was encouraging in the sense that the local concrete producers and material suppliers were heavily vested in making sure that all aspects of the Grade DM concrete were successfully accomplished. Initially, there were concerns about the level of efficiency that would be gained when attempting to optimize standard bin aggregates. However, to the satisfaction of all, the combined effects of the final aggregate gradation blend and the SCM produced a fresh concrete that was easily pumped, could be placed and finished with less hand work, was much more able to maintain a stable entrained air-void system, and ultimately contributed to a reduction in deck cracking.