|Accelerating Infrastructure Innovations|
|Federal Highway Administration > Publications > Focus > September 2005 > Air Void Analyzer Offers More Accurate Concrete Testing|
|September 2005||Publication Number: FHWA-HRT-05-030|
Air Void Analyzer Offers More Accurate Concrete Testing
In States such as Kansas, Iowa, and New York, the promise of a new concrete testing method known as the air void analyzer (AVA) is paying off. Since 1993, the Federal Highway Administration (FHWA) has been conducting studies and pilot projects exploring the advantages of using the AVA equipment. "The AVA represents a more accurate method for determining the susceptibility of concrete materials to freeze-thaw deterioration," says Gary Crawford of FHWA.
Freeze-thaw deterioration is caused by water expanding in concrete under freezing temperatures. This damage can be mitigated by the presence of a system of small air bubbles (voids) in the concrete mix. Before the advent of the AVA method, core samples of the hardened concrete were necessary to verify that this system of air voids was in place. The AVA method represents an improvement, according to Crawford, because it can be performed on fresh concrete before it sets, and the test only takes 20 to 25 minutes to produce results. This allows the contractor to make adjustments during the concrete batching process to ensure that air voids are spaced properly, thus avoiding the costly process of removing concrete that has an inadequate air void system. A 20 ml (.68 oz) sample of fresh concrete is required for testing. While other methods of testing fresh concrete exist, notes Crawford, they only measure the total volume of air and do not accurately measure the size and spacing of the air voids, which is a factor that has become more important with the development of new concrete mixtures in recent years.
FHWA has been demonstrating the AVA technology to States since 1993. "The equipment is very easy to use and is portable, but it is sensitive, and so tests must be conducted in an enclosed building," Crawford says. "Implementation has also been slowed somewhat because many State agencies don't have the manpower to implement this new technology as quickly as they might like."
Kansas is one State that has taken the lead in championing the new technology, including working with the American Association of State Highway and Transportation Officials' (AASHTO) Technology Implementation Group (TIG) to make it a TIG focus technology in 2002. The Kansas Department of Transportation (KDOT) had discovered that its concrete pavements less than 10 years old had begun to crack and deteriorate at the joints, even though the aggregate was sound and the total air contents met specifications. Examination of core samples showed poor spacing factors in the concrete paste. Starting in 2001, KDOT began using the AVA technology to monitor concrete paving projects, and was able to immediately detect and rectify any shortcomings in the pavement onsite. The agency estimates that for pavements built in 2001-2002, future cost savings will amount to more than $1 million, based on reduced repair costs and more durable pavements.
The New York State Department of Transportation (NYSDOT), meanwhile, has purchased two AVA units. According to Donald Streeter, concrete program manager with NYSDOT's Materials Bureau, the units have been used on this season's concrete projects. "Our situation is different than that in Kansas," says Streeter. "We're primarily looking at bridge decks, because we've been having problems with scaling. New York anticipates developing a specification addressing the size and spacing of air voids, rather than the traditional total air content. Using the AVA so far has been very positive."
He also notes that New York has been showing the technology to its precast concrete suppliers. "Historically, we've required suppliers to provide us periodic cores for quality assurance," says Streeter. "AVA can save them from all that destructive testing. We work jointly with them, lending our AVA units to manufacturers and building a database of the results to verify the benefits. For the smaller suppliers, a few cores might not be that much of a hassle, but we're hoping to persuade the large-volume suppliers to buy their own AVA units to replace submitting core samples."
FHWA is working with AVA equipment manufacturer Germann Instruments to develop a more rugged AVA unit for testing in the field. According to Germann's Claus Peterson, "The AVA presently produced, AVA-2000, requires a solid foundation and no wind to perform the recording. We are working on a revised model, the AVA-3000. In this model, the water tank acts as a ballast for the AVA measuring unit itself, to minimize disturbance from exterior vibrations, and the riser column has at its top a wind shield. The basic operation of the AVA-3000 is, however, exactly the same as the AVA-2000." The revised model is expected to be ready for production in 2 to 3 months.
In the meantime, other efforts have been undertaken to improve the AVA technology's performance in the field. Iowa State University's Center for Portland Cement Concrete (PCC) Pavement Technology, for example, has developed a Mobile Concrete Research Lab (see November 2004 Focus). The lab was largely designed around the AVA testing equipment, according to Jim Grove, PCC paving engineer at the center. The mobile lab is supporting a pooled-fund study on preventing premature distress in PCC pavements. The study is sponsored by FHWA, 17 State transportation departments, the American Concrete Pavement Association, and numerous State/regional concrete paving associations. To date, the trailer has been used for projects in nine of the participating States.
"Our goal in the design of the lab was to bring the AVA technology out to the project site," says Grove. "We've designed a trapdoor in the middle of the trailer floor, which measures approximately 0.6 m by 0.6 m (2 ft by 2 ft), where we can set the AVA tripod on the ground for a stable base, with a skirt to prevent the wind from blowing in. This has worked very well." Grove reports that lab personnel have successfully conducted concrete analysis while trains passed by the lab. "We've been very pleased with the AVA technology's performance in the lab," he says.
Looking to the future, Crawford hopes to organize an AVA user group to share in-formation. He also expects that AASHTO will approve a provisional standard test method for the AVA.
For more information on the AVA, contact Gary Crawford at FHWA, 202-366-1286 (email: firstname.lastname@example.org). For additional information on States' use of the technology, see contacts listed in sidebar.
United States Department of Transportation - Federal Highway Administration