On the road testing roads: FHWA's lab-on-wheels bridges the gap between research and field implementation of new technologies for concrete pavements. Mobile Concrete Laboratory - Federal Highway Administration's FHWA - Brief Article

Public Roads, July-August, 2002 by Gary L. Crawford, Leif Wathne, Jon I. Mullarky

Few things draw a crowd like an ice cream truck. For engineers, the truck that delivers treats to their neighborhood is the Federal Highway Administration's (FHWA) Mobile Concrete Laboratory. Rather than beckoning with ice cream, the Mobile Concrete Lab attracts pavement industry professionals eager to learn about the newest nondestructive and durability-related concrete testing equipment.

In the late 1980s, FHWA recognized a significant gap between the state-of-the-art technologies emerging from public and private research laboratories and the implementation of these technologies by State highway agencies and the highway construction industry. To bridge the gap between research and implementation, FHWA launched the Mobile Concrete Laboratory-a state-of-the-art concrete testing facility on wheels. As the ice cream truck of the highway construction industry, the Mobile Concrete Lab brings innovative concrete technologies-and the know-how to use them-straight to your doorstep.

Contained within a 15-meter (48-foot) trailer and pulled behind a semi-truck, the Mobile Concrete Lab is fully equipped with both new and conventional testing equipment. FHWA staffs the lab with engineers and technicians skilled in the latest techniques and technologies for testing concrete materials. Lab personnel are available to provide State highway agencies with everything from equipment evaluations and loans to classroom and field demonstrations of specific concrete technologies.

The lab includes enough office space, laboratory workspace, and storage facilities to serve as a highly effective on-site testing facility for extended periods of time. And with its own truck-mounted generator, the lab can operate even in the most remote areas of the country far from power lines.

Classroom on Wheels

For more than a decade, Dr. Ken Hover, a professor of civil and environmental engineering at Cornell University, has relied on the mobile lab to provide invaluable assistance during a highway materials course developed and presented by the National Highway Institute.

During 1 week of the 6-week course, the instructors have a truckload of fresh portland cement concrete brought in. The 30 engineer-students use the concrete to create more than 160 specimens to employ in a range of tests. In some cases, the tests can take more than 3 hours to complete, and the collected data can include air content (pressure- and volume-meter), unit weight, temperature, slump, and strength.

"The Mobile Concrete Lab provides the equipment and instruction we need to get the tests done quickly and to ensure that the data are valid," Hover says. "The lab personnel act as roving instructors, overseeing all the test procedures and helping the students get a handle on how concrete test values vary within the batch and over time."

New Focus for Concrete Technology

Durability is the name of the game when it comes to concrete highway structures. In the past, engineers focused on how to improve concrete's mechanical properties or how to reduce the initial costs. But with an aging interstate system, an urgent need for repair and reconstruction, and a sharp rise in rehabilitation costs, most of the total available highway dollars in the United States are spent on keeping the existing highway system in service--not on expanding the network or its capacity.

Because concrete durability is synonymous with a long service life, engineers are becoming more aware of the importance of designing and building with durability in mind, and many States now are beginning to design pavements with 50- to 60-year life spans.

To double the conventional service life of pavements, engineers need to monitor critical parameters during construction to ensure that they achieve the desired concrete properties indicative of long-term durability--such as proper air void structure, low permeability, proper water content, and low susceptibility to cracking. Equally important is the deployment of innovative technologies that evaluate the in situ properties of concrete quickly and accurately.

Two of the Mobile Concrete Lab's most important capabilities include techniques for testing fresh concrete properties needed to ensure durability and techniques for testing the properties of in-place hardened concrete.

Durability-Related Tests

The first durability-related test is an air void analyzer. To ensure the durability of concrete exposed to the cyclic freeze-thaw conditions common in most northern and mountainous States, quality control personnel need to determine the amount of air trapped in moist concrete. Standard field tests to measure air content focus on the total fresh air content (entrapped and entrained), but the tests do not provide information about the size of the air bubbles or their spacing, which is critical in an air void system for concrete exposed to freeze thaw cycles.

In the United States when unexpected freeze-thaw deterioration occurs, engineers use the American Society for Testing and Materials (ASTM) method C 457, Standard Test Method for Microscopical Determination of Parameters of the Air-Void System in Hardened Concrete, to determine the size and spacing of air bubbles in concrete. A significant disadvantage of this method is that it is conducted on hardened concrete, when it is too late to make adjustments to the mixture during placement.