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.

The air void analyzer, however, enables quality control personnel to determine the volume of entrained air, as well as bubble size and spacing, from samples of fresh concrete in the field in less than 30 minutes. With these timely test results, personnel can make adjustments to ensure that the concrete has an adequate air void system.

Results to date indicate that the air void analyzer is a good measure of the in-place air void system. Kansas currently is evaluating the potential for including this test in its concrete pavement specifications.

Durability Testing in Chicago

In spring 2001, FHWA staff from the Mobile Concrete Lab visited Chicago to conduct air entrainment testing using the air void analyzer on a specialized concrete mix for a bridge deck project on Wacker Drive. The city wants to achieve a 100-year life for a new post-tensioned bridge deck between Randolph and Michigan Avenues, and the Chicago Department of Transportation is employing specialized, high-performance concrete mix designs. Over several days, the mobile lab team took concrete samples during deck pours and conducted air entrainment testing to determine how well the mix was doing.

Tim Schmidt, a quality assurance engineer with Chicago-based engineering firm Alfred Benesch & Company, met with personnel from the Mobile Concrete Lab and was impressed with the team's technical knowledge and breadth of experience.

"Personnel from the Mobile Concrete Lab bring to the table a diversity of perspectives and experiences gathered from around the country," Schmidt says. "With durability as a major goal, the city specified a quad mix with four cement components for the new bridge deck. After talking with lab personnel, we learned that this is a pretty rare mix design in the United States."

Rapid Chloride Permeability Test

Although road salt, applied to melt snow and ice during the winter, helps make roadways safer for drivers, it also leads to spalling (chipping) of the concrete, potholes, corrosion of steel in bridge decks, and problems with other applications that involve regular- or high-performance concretes. The Rapid Chloride Permeability Test (RCPT) can help identify how well different concretes will protect reinforcing steel from corrosion.

The test measures a concrete's ability to pass an electrical charge, which relates to its ability to resist chloride penetration. The RCPT functions on the principle that when a concrete core or cylinder is placed in a test cell and voltage is applied between electrodes on each end, chloride ions move toward the positive electrode. The amount of charge transmitted during a 6-hour test is an indication of the chloride permeability of the concrete - the lesser the charge passed, the lower the permeability of the concrete.

The RCPT method, in most cases, is suitable for evaluation of materials and additives for the mix design of regular- or high-performance concretes, as well as for testing the permeability of cores taken from in-place concrete.

Microwave Water Content Test

Water in excess of the amount specified by the mixture design can be detrimental to the performance of concrete, yet construction personnel rarely measure the water content of concrete prior to placement, relying instead on batch tickets to confirm that the percentage of water is within limits. This approach does not account for any water that may be left in the mix trucks after washing, nor does it take into account any water added to the truck after batching, or undetected changes in the moisture content of the aggregate.

Several States have adopted the microwave water content test to determine the amount of free water in concrete. The test quickly and accurately measures the water content of fresh concrete by drying a known mass of fresh concrete in a microwave oven for several intervals. Once the mass change is negligible, the free water as the change in mass can be calculated. Given an accurate cement content of the concrete, the water-cement ratio then can be calculated. Both Minnesota and New Hampshire are using the method to monitor the water content of fresh concrete supplied to their jobs.

HIPERPAV

As demonstrated by FHWA's Long-Term Pavement Performance (LTPP) program, early-age cracking of concrete pavements is linked to reduced long-term pavement performance. Material, environmental, construction, or design factors may lead to excessive concrete stresses in the first 72 hours. To address early-age concrete cracking, FHWA's concrete research team at the Turner-Fairbank Highway Research Center sponsored research that resulted in the development of a software program, HIPERPAV, that simulates the behavior of early-age pavements. (See "Paving the Way" on page 20.)

HIPERPAV enables field personnel to evaluate the cracking potential of concrete pavements based on a given set of circumstances, which may include placement timing, construction procedures, concrete mixture designs, pavement geometry, and environmental factors. Once these parameters are defined, HIPERPAV analyzes the input values using a series of prediction equations that compares the stress and strength development of the concrete pavement over the initial 72-hour period after placement.

Several State highway agencies, including the California and Indiana departments of transportation, are considering ways to implement HIPERPAV during their design and construction processes to minimize the potential for early-age cracking in their pavements.

In-Place Testing

In-place testing, which involves technologies that measure an existing concrete's properties in situ, often provides an effective assessment of a concrete's performance. In-place testing accounts for field placement, consolidation, and curing factors - all factors that can vary from project to project. But in conventional quality control testing, these factors remain constant and controlled. The Mobile Concrete Lab offers a selection of both conventional and nondestructive in-place testing capabilities.

The variety of nondestructive tests provided by the Mobile Concrete Lab provide specialized equipment and procedures not available at many facilities around the country. Each of Cornell professor Hover's student teams, for example, casts a 1.2-meter by 1.2-meter by 20.3-centimeter (4-foot by 4-foot by 8-inch) concrete slab that simulates a concrete bridge deck. Lab personnel, in addition to extracting core samples to conduct standard tests, show the students how to do nondestructive tests, such as the ultrasonic pulse velocity, maturity, Windsor probe, rebound hammer, and pull-out tests.

"Validating the utility of nondestructive tests shows engineers how you can go out and evaluate a structure in-place," Hover says. "It is one thing to evaluate fresh concrete from a truck, but it can be even more valuable to learn how to evaluate the strength of an existing concrete structure nondestructively in the field."

Maturity

One in-place nondestructive test for concrete, the maturity test, is a well-established technique for estimating strength gain based on the concrete's temperature-time history. Using temperature-time data collected by a maturity meter, a highway engineer can convert the actual temperature-time history of the concrete to a maturity index indicative of strength development. The maturity method is based on the assumption that samples of a given concrete mixture attain equal strengths if they achieve equal values on the maturity index.

Applying the maturity method involves three steps: (1) developing the maturity-strength relationship in the laboratory for the concrete mixture to be used on a project, (2) monitoring the maturity of the in-place concrete during construction, and (3) using the lab-generated curve to predict the strength of the in-place concrete. The test is extremely helpful in determining when the concrete has attained the required strength for early opening to traffic or for stripping forms.

Several States, including Indiana, Iowa, Minnesota, Pennsylvania, and Texas, have adopted maturity as a technique to determine the in-place strength of new concretes in projects that call for early opening to traffic.

Impact-Echo Method

Verifying the presence of suspected internal voids, cracks, honeycombing, or delaminations in a concrete structure is often difficult, as hammer soundings and chain drag techniques only detect near-surface flaws and do not provide quantifiable measures of the type or depth of discontinuity. Random coring is time-consuming, expensive, and often does not provide adequate data.

Enter impact-echo. The nondestructive impact-echo method, which uses a mechanical impact source to create sound waves that travel through the concrete to detect discontinuities, is highly effective at locating voids and determining the thickness of concrete slabs. Because the impact-echo method requires access from only one side of the pavement, highway engineers can use this technique to identify and determine the depth to discontinuities or boundaries in existing concrete pavements and bridge decks.

Several States, including Indiana, Nebraska, and Tennessee, are employing the impact-echo technique experimentally as a nondestructive tool to measure pavement thickness.

Tensile Bond Test

To ensure a long service life for a rehabilitated concrete surface, the repair material or overlay needs to be well bonded to the underlying concrete. Prior to rehabilitating a concrete surface, deteriorated concrete needs to be removed and the substrate must be properly prepared. The tensile bond strength (pull-off) test is a quick and simple method for determining how well the surface has been prepared and how well the repair material or an overlay bonds to underlying concrete.

Highway engineers can use the tensile bond test to determine the need for surface preparation, detect relative differences in the potential surface strength over an area to be repaired, and determine the adequacy of surface preparation. The test involves making a 5-centimeter (2-inch) diameter core through an overlay into the underlying concrete. By gluing a metal disk to the surface of the core and pulling it out in tension with a loading device, the tensile or bond strength between the old concrete and the overlay can be determined easily. The test can also estimate the expected service life of overlays by measuring the degradation of bond strength over time. South Dakota and Virginia have used the tensile bond test to assess the bond quality of bridge deck overlays.

Emerging Technologies

In addition to demonstrating and implementing existing technologies, the Mobile Concrete Lab also keeps tabs on new and emerging technologies under development at FHWA or by State highway agencies, private industry, or international research organizations. Among the most promising emerging technologies is the rapid migration test.

Developed through a pooled-fund study administered by FHWA's concrete research team, the rapid migration test expands on the RCPT for measuring chloride penetration into concrete. The advantage of the rapid migration test over the RCPT method is that it measures the depth of chloride penetration into the sample, while the RCPT simply measures the total electrical charge passed during the test, which relates its ability to resist chloride penetration. The Mobile Concrete Lab and researchers elsewhere are evaluating the rapid migration test as a new test procedure that promises to help design concrete mixtures that protect reinforcing steel from corrosion.

Another emerging technology is the vibrating slope apparatus. Understanding the workability - or ease of placement - of portland cement concrete during placement and compaction is extremely important for successful construction of concrete pavement. Workability provides an indication of how much energy will be required to place, consolidate, and finish the concrete. Technicians traditionally use slump to measure the flow properties of freshly mixed concrete, but slump does not work well to determine the properties of very stiff concrete used for slipform paving. The Mobile Concrete Lab is working with researchers at the Turner-Fairbank Highway Research Center to evaluate a prototype vibrating slope apparatus, a device that could provide more usable information about flow properties of low-slump paving mixtures and better quantify how the material would react under vibration.

Predicting the behavior of concrete in response to temperature changes requires knowing the concrete's coefficient of thermal expansion (CTE), or the rate at which the concrete expands or contracts in response to temperature changes. CTE values vary significantly from one concrete mixture to another. LTPP data indicate that the CTE is linked to the longevity of concrete pavements, and both HIPERPAV and the 2002 Guide for Design of New and Rehabilitated Pavement Structures use concrete CTE as inputs.

FHWA's Concrete Research Team recently developed a test method that allows for the consistent and accurate measurement of concrete CTE. The American Association of State Highway and Transportation Officials also adopted this emerging test method as a provisional standard (TP60-00).

On the Road Again

Traveling around the country, lab personnel share with Federal, State, and local transportation professionals the experience, data, and conclusions garnered from innovative field projects. The lab spends about 60 percent of the year on the road visiting active project sites for 2 to 4 weeks at a time, demonstrating procedures, hosting tours, and putting on workshops covering everything from HIPERPAV to admixtures. In 2001, the lab logged highway miles from California and Texas to Illinois and Pennsylvania.

According to Brian Egan, a field operations engineer in the Materials and Testing Division of the Tennessee Department of Transportation, the Mobile Concrete Lab was very flexible in making time to visit Nashville in August 2001 to supplement the department's research into using ground granulated blast furnace slag as a replacement for portland cement.

"Typical construction delays kept forcing us to change the date," Egan says, "but FHWA and the Mobile Concrete Lab personnel went out of their way to accommodate our fluctuating schedule and be on site to help us out."

Through the Mobile Concrete Lab, State highway agencies can sample the menu of available concrete technologies before investing. Lab personnel assist with everything from "hands-on" training and demonstrations of new technologies to participating in field projects to gather data on technologies of interest to an individual State. The Mobile Concrete Lab even loans selected equipment to State agencies for training, evaluation, and research projects.

In January 2002, the Mobile Concrete Lab visited Grantville, PA, for the concrete industry's Third Annual Concrete Seminar. More than 200 individuals, representing the Pennsylvania Department of Transportation, the Pennsylvania Turnpike Commission, and academia, as well as contractors, consultants, and materials suppliers, toured the lab.

"The Mobile Concrete Lab sparked some good discussions, especially about nondestructive testing devices like the maturity meter," says John Becker, regional director with the American Concrete Pavement Association's Northeast Chapter. "In Pennsylvania, some of our contractors are very interested in using maturity devices on street and local road projects. Having the laboratory showcase the latest testing devices at the seminar was a very good educational experience for us."

The Mobile Concrete Lab seeks active partnerships with State highway agencies, manufacturers, contractors, industry associations, and academia to maximize the lab's value and impact. Current focus areas include high-performance concrete for pavements and bridges, nondestructive testing, and evaluation of performance-related specifications.

 

---

Gary L. Crawford is a concrete quality engineer in FHWA's Office of Pavement Technology. He currently works as the project manager in charge of the Mobile Concrete Laboratory. He also has been involved in the development and delivery of workshops on concrete durability, nondestructive testing techniques, concrete mixture design, and HIPERPAV. He holds a bachelor's degree in civil engineering and has more than 18 years of experience in nondestructive testing, construction, and concrete materials.

Leif Wathne is a materials engineer for Soil and Land Use Technology, Inc., a Beltsville, MD-based consultant to FHWA's Office of Pavement Technology. He currently works as the project engineer for FHWA's Mobile Concrete Laboratory and was previously the concrete research engineer assigned to FHWA's concrete pavement research team at the Turner-Fairbank Highway Research Center. Much of his experience relates to concrete pavements and bridges, specifically pertaining to performance measurement, innovative materials testing, and nondestructive examination. He received his master's degree in civil engineering from Pennsylvania State University and is a licensed professional engineer.

Jon I. Mullarky is a senior project engineer with Soil and Land Use Technology, Inc. Currently he is a member of the concrete team with FHWA's Office of Pavement Technology. This team is responsible for the Agency's concrete pavement research and technology transfer activities including the FHWA Mobile Concrete Laboratory. Mullarky is a registered professional engineer. He holds bachelor's and master's degrees and has more than 30 years of experience in construction and concrete materials.

For more information about the Mobile Concrete Lab and its services, or to participate in an upcoming project, visit www.fhwa.dot.gov/pavement/mcl.htm or contact one of the authors at FHWA's Office of Pavement Technology: Gary Crawford at 202-366-1286, gary.crawford@fhwa.dot.gov; Leif Wathne at 202-366-1335, leif.wathne@fhwa.dot.gov; or Jon Mullarky at 202-366-6606, jon.mullarky@fhwa.dot.gov.