InTrans / Sep 12, 2016
ISU pavement center, West Des Moines testing a modified concrete mixture
Workers paved a West Des Moines street last year with a modified concrete mixture.
A new industrial development in West Des Moines, Iowa, is providing a unique opportunity to test a modified concrete mixture on several pavement sections. The city is especially interested in monitoring the performance of the pavements’ joints.
The challenge
In recent years, especially in cold weather states, some joints in concrete pavements have begun deteriorating relatively soon—5 to 10 years—after pavement construction.
“The occurrences are limited,” says Gordon Smith, president of the Iowa Concrete Paving Association, “but concrete associations and roadway agencies in northern states are eager to know what’s causing them and how to prevent them in the future.”
Through studies by National Concrete Pavement Technology Center (CP Tech Center) and its partners across the country, premature joint distress has been associated with two primary phenomena: (1) expansive pressures in pavements treated with certain salts and (2) freeze-thaw pressures in saturated pavements.
The CP Tech Center is administered and housed at the Institute for Transportation at Iowa State University.
In cold weather states, calcium chloride and magnesium chloride are sometimes placed on concrete pavements as a deicing or anti-icing strategy. However, these salts react with one of the products of cement-water hydration, calcium hydroxide, resulting in the compound calcium oxychloride. Calcium oxychloride is expansive and can damage the cement paste. This can have a significant impact on durability.
Joints can be “collector areas” for highly concentrated salt solutions, making the joints especially susceptible to damage.
Freeze-thaw damage occurs when water trapped in cement paste freezes and expands, exerting force on the paste and aggregate in the concrete. To mitigate the pressure, it is common practice to specify that concrete mixtures include strong, durable aggregates and a well distributed system of air bubbles, or air voids. Still, freeze-thaw damage is occurring in mixtures with durable aggregates and adequate air-void systems. Researchers have discovered a common factor: saturated concrete. When concrete exceeds approximately 85 percent of the void space, freeze-thaw damage occurs.
The solution
A goal for the West Des Moines pavement project, constructed in the fall of 2015, was to mitigate these two problems.
A specification for a modified quality management concrete was developed. Cement was replaced with Class C fly ash at a rate of 30–35 percent (a standard Iowa DOT mix is 20 percent) to reduce risk of chemical attack by salts. A higher fly ash content results in more durable concrete, with a system chemical composition that is more resistant to oxychloride formation, lower permeability (potentially lowering saturation rates) and greater long-term strength.
Other mix modifications included a minimum of 6 percent air behind the paver and a target water-to-cementitious-materials (w/cm) ratio of 0.40 (maximum 0.42) (the standard Iowa DOT mix is about 0.45). The low w/cm ratio can contribute to reduced permeability in concrete (reducing saturation rates and the potential for freeze-thaw damage).
According to Andy Denker of CTI, the paving contractor, issues related to the modified mixture presented some challenges. “The water-to-cement ratio was quite low compared to normal, and that resulted in a stiffer mix. Sometimes we had to make adjustments, like increasing the water reducer.”
The West Des Moines project was designed and constructed using best practices, not only in placing the concrete but in providing good drainage and protecting the concrete from water and chemical infiltration. The contractor also experimented with more aggressive use of pavement sealers. The joints were sealed after sawing, and the slab was sealed after a period of time.
Ongoing performance testing
ISU’s CP Tech Center undertook testing of the mixture before and during construction. Laboratory tests were conducted on samples cast in the field to investigate freeze-thaw damage resistivity, air structure, and potential for joint deterioration. Field tests were conducted to investigate the robustness and consistency of the modified mix proportions. Core tests were conducted to compare field samples with lab test results.
The performance of the pavement sections and joints will be closely monitored for several years.
According to H.R. Green’s Jeremy Huntsman, who managed the paving project, it’s unusual to have such a large-scale field test section in one location. “We could control the variables,” he said, “like the type of concrete mix in each pavement and what types of subgrades and sealers were used.” Because of their close proximity, the pavement sections also experience the same environmental conditions.
“Now,” said Ben McAlister, principal engineer for the City of West Des Moines, “we just have to wait and see how it works for the long term.”
ISU”s Institute for Transportation administers 15 centers and programs that are leaders in transportation research, outreach, education, and workforce development.
Contact: Marcia Brink, InTrans Communications, mbrink@iastate.edu, 515-294-9480