Gary Crawford, Federal Highway Administration and Leif Wathne, Soil and Land Use Technology
For over 50 years, researchers have known that concrete is susceptible to freeze-thaw deterioration unless a system of air bubbles is present in the concrete to protect it. The size and spacing of these air bubbles or voids are important. If the air voids are too large or too far apart, water cannot reach an air void to relieve expansive pressure as the water freezes. As the concrete continues to cycle through freezing and thawing, micro-cracking occurs and eventually the concrete sustains significant damage.
The most common methods used to measure air content in fresh concrete today are the pressure and volumetric methods. Both methods measure the volume of air only and not the size or spacing between the voids known as the spacing factor. Nevertheless, these methods have worked well in the past, since the volume of air has been a successful surrogate measure of the spacing factor, and indirectly the concrete freeze-thaw durability. But, the ingredients and processes used to make concrete have changed over the years, and the traditional relationship between volume of air, air-void system, and freezethaw durability may no longer be valid. Consequently, methods are needed to measure size and spacing of the air-voids in the fresh concrete. The Air Void Analyzer (AVA) was developed in Europe during the 1980s to meet this need.
How does the AVA work?
A 0.68 fl oz (20 mL) mortar sample is obtained from the fresh concrete using a vibrating wire cage and a syringe. The sample is then injected into the bottom of a transparent cylinder or riser column filled at its base with a viscous liquid and topped with water. The sample is then stirred for 30 seconds releasing the air bubbles and allowing them to rise through the liquids. The rate of rise depends on the bubble size. The air bubbles are collected under a submerged dish attached to a balance. The change in suspended mass of the dish is recorded for 25 minutes after the stirring period. A computer algorithm uses the weight change with time to calculate the void size distribution, total air volume, spacing factor, and specific surface.
How does the AVA compare to other tests?
The Federal Highway Administration (FHWA) first purchased an AVA test unit in 1993. The equipment was used on a variety of projects throughout the United States. Results showed that the spacing factor was consistent with results obtained on hardened concrete using ASTM C 457, but the AVA tended to report smaller void sizes when compared to the ASTM C 457 examination.
After upgrading the equipment in 1999, the FHWA has used the AVA on a variety of field projects in nine different states. Projects have included pavements, precast sheet piles, foundation elements, and bridge decks. For six of these projects, accompanying hardened air content tests were also performed.
Data show that the difference in observed spacing factors between the AVA and ASTM C 457 test is relatively small, and fall well within the range of averages for between-laboratory precision for two test results reported in ASTM C 457. More importantly, in 9 of the 14 cases where the concrete met the total air volume requirements based on pressure meter tests (ASTM C 231), it did not meet generally accepted durability criteria limits based both on AVA and ASTM C 457 spacing factor results. This means that in approximately 65 percent of the cases where a deficient concrete was delivered, it was deemed adequate by the current test practice of total air volume. Implementing the use of the AVA can, therefore, significantly improve the quality of concrete placed in the United States from a freezethaw perspective.
What’s next?
Several state highway agencies are evaluating the use of the AVA and the American Association of State Highway and Transportation Officials (AASHTO) included the AVA as a focus technology in its 2002 Technology Implementation Group (TIG) program. The TIG has provided leadership and technical assistance to promote implementation of the AVA technology by the states.
The primary benefit of the AVA is that it measures the air content, spacing factor, and specific surface of fresh concrete in about 30 minutes, allowing for the timely detection of concrete that will not be resistant to freeze-thaw cycles. Adjustments can then be made to minimize the delivery of fresh concrete with a deficient air-void structure. The FHWA field experience confirms the ability of the AVA to detect substandard air-void systems with accuracy comparable to that of ASTM C 457 results for hardened concrete.
Further Information
For more information on the TIG’s activities go to www.aashtotig.org. For other details, contact the authors [email protected] or 202-366-1286 or [email protected] or 202-366-1335.