Void all checks

March 14, 2005

In the early 1980s, after years of studying D-cracking in concrete pavements, the Iowa DOT figured out how to minimize the problem. Materials engineers at the Iowa DOT developed a classification system that would group aggregates into durability classes and, as a result, protect pavements from early deterioration.

In the early 1980s, after years of studying D-cracking in concrete pavements, the Iowa DOT figured out how to minimize the problem. Materials engineers at the Iowa DOT developed a classification system that would group aggregates into durability classes and, as a result, protect pavements from early deterioration.

But then, in 1990, staining and cracking were discovered in a pavement that was built in 1987. The problem wasn’t specifically an aggregate issue. It was a new phenomenon. What was causing this? How could it be prevented in the future? Over the next seven years, numerous studies, reports, tests and discussions with experts led the Iowa DOT to realize the problem was caused by a combination of issues related to aggregate gradation, material compatibilities, cementitious materials, concrete workability, air entraining and paving equipment.

As a result, in the late 1990s, the Iowa DOT and other states started to investigate all aspects of concrete paving that could contribute to durability problems. The Midwest Concrete Consortium then began formulating a concept for its 10 member states to join together in one large-scale project to find better ways to characterize materials and test concrete.

This effort resulted in the pooled-fund study “Material and Construction Optimization for Prevention of Premature Pavement Distress in PCC Pavements” (known as the MCO project). The MCO project is led by the Center for Portland Cement Concrete Pavement Technology (PCC Center) at Iowa State University and includes 17 states to date.

The objective of the MCO project is to establish an optimal method of ensuring long-life, durable concrete pavements. This research has the potential to transform the concrete paving industry by providing practical methods of integrating best material and construction practices and controlling the quality of concrete pavement during construction.

The door to the suite

The MCO project team is in the process of evaluating and refining new tests that will allow practitioners to better understand and monitor the materials and construction processes that result in concrete pavement. The tests are being evaluated at three periods in the construction process (mix design, preconstruction mix verification and construction quality control) and with regard to five important concrete properties (workability, strength, air system, permeability and shrinkage). For tests to be included in the suite of tests they must be able to be conducted with relative ease and accuracy in the field. Many of the tests being evaluated in the MCO project have been modified from standard laboratory methods into new field test procedures. Rapid results from these tests will allow real-time adjustments to be made to materials, mixtures and construction so that corrections can be made as the pavement is constructed.

Future star

One of the most promising yet challenging test devices to be evaluated in the project is the air-void analyzer (AVA). The AVA is the only test equipment able to measure the complete air-void system (volume, size and distribution) of fresh concrete. An adequate air-void system is critical to providing concrete pavement protection from freeze-thaw damage.

Previously, the standard for determining the air system of concrete (ASTM C 457) required a core to be cut from hardened concrete. This method is very time-consuming and does not provide results in time for quality control adjustments to be made during the construction.

The AVA, on the other hand, is a real-time test. The AVA provides immediate information about the air system of plastic concrete. Unfortunately, the equipment is sensitive to vibrations, making field application very difficult.

While designing the mobile lab that would provide the project with field testing facilities, Bob Steffes, PCC research engineer with the PCC Center, believed he could overcome the AVA’s limitations related to sensitivity. He designed the lab with a trapdoor in the floor. Before running an AVA test, the base of a tripod stand is placed through the hole to rest on the ground. The AVA then sits on a plate on top of the tripod stand.

This custom design for the AVA has been successful. Test results have not been affected by movements including wind or people walking in the lab. Making the AVA a field-worthy test device may be the biggest breakthrough of this research.

Easy flow

During the initial phase of research, each state was interviewed to document their current procedures and testing methods. This not only helped establish a baseline of commonly used tests but also uncovered unique test methods that could be evaluated and further developed for use elsewhere.

From these interviews, it was found that South Dakota successfully used a flow-table test for evaluating mortar samples. The MCO project team responded by including the test in the suite of tests to be evaluated.

The flow-table lab test had to be adapted into a field test because the properties of mortar samples change quickly with time and excess handling. Obtaining a sample, transporting it to a laboratory and screening off the mortar involve a significant amount of time and handling. Therefore, the MCO project team developed a way to mount the flow table onto the ball hitch on the back of a truck. The flatbed of the truck acts as a table on which to run the field tests.

Obtaining a mortar sample was the next challenge. After experiencing the way AVA mortar samples are obtained, the idea arose to make a larger unit using the same principle. A cylindrical cage was fabricated and mounted onto a vibrator.

Concrete is placed into the cage and mortar is gathered as it moves through the spaces in the cage. This device, put into the field at the end of the season, already seems very promising and will be further evaluated in future demonstration projects. As the MCO project produces advancements in concrete pavement materials selection, mix design and construction technology, many efforts are being made to transfer these advancements into the hands of practitioners who can benefit from the new information and technologies and help transform the industry.

Current MCO project findings are being compiled in a user-friendly best practices manual, Integrating Materials and Construction Practices for Durable Concrete Pavements: A State-of-the-Practice Manual. The manual will provide information on identifying and solving mixture problems during design and production, and when and how to adjust concrete mixtures based on weather and other factors.

This manual will be field-focused and contain a troubleshooting guide to help resolve common problems in the field.

Lab on tour

Current MCO project activities include field visits to each of the 17 states participating in the project. A mobile concrete research lab was purchased and equipped to allow the researchers to evaluate the tests right on project sites. The mobile lab also provides a way to demonstrate the test equipment and procedures to field personnel in each state, which supports the technology transfer goal of the project.

The lab visited five states in 2004: Missouri, Kansas, Michigan, Wisconsin and North Carolina. While in Michigan, the MCO project team had the opportunity to demonstrate the AVA test. The city of Madison Heights had a concern about construction practices and the durability of pavements based on experiences with early deterioration. The city asked if the team would test a sample of concrete from a paving project that was under way during the time the lab was in the area. Samples were delivered to the lab and the project team ran the AVA tests. The results showed the spacing factor to be within the commonly accepted limits. This was good news to the city and was certainly an opportunity to fulfill the mission of the mobile lab to provide demonstrations of the capabilities of the test equipment.

The MCO project is a model for how the most can be done when federal and state government, private industry, academia all come together around a central goal—in this case generating improvements to concrete paving material, construction and quality control practices.

The MCO research is sponsored by the Federal Highway Administration (FHWA) and 17 state departments of transportation:

•Iowa (lead state)
•New York
•North Carolina
•North Dakota
•South Dakota

The Mobile Concrete Research Lab was funded by the American Concrete Pavement Association (ACPA), state concrete paving associations and Iowa State University. The Integrating Material and Construction Practices manual is sponsored by FHWA and is being prepared with contributions from Iowa State University, Construction Technologies Laboratories, ACPA, Portland Cement Association and many other organizations.

With these public-private, regional-national partnerships, duplication of effort is reduced and much greater progress is made toward common goals.

About The Author: Grove is PCC paving engineer at Iowa State University’s PCC Center and leads the Material and Construction Optimization for Prevention of Premature PCC Pavement Distress pooled-fund study. Anderson-Wilk is editor for the PCC Center.

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