Express air measurement: sophisticated software provides the link to predict concrete durability
Concrete specifications for high-durability concrete may soon take on a new look thanks to an air void analyzer (AVA) being demonstrated by the Federal Highway Administration (FHWA). The device, reminiscent of a lava light with a PC interface, was developed more than a decade ago by the European firm Dansk Beton Teknik (DBT). It gives much more data about air entrained in concrete than the standard pressure pot or roll-a-meter, which give only the total percentage of entrained air. The AVA is one of several technologies included in the FHWA’s Mobile Concrete Lab scheduled to travel throughout the United States.
Air in concrete
Air is a natural component of concrete. Larger air voids, where the air is entrapped, are undesirable. But some amount of entrained air, consisting of tiny bubbles dispersed evenly throughout the concrete, is desirable for freeze/ thaw resistance. Although entrained air improves workability, it also reduces compressive strength, so it must be kept under control. For example, entrained air content in pavement is typically kept in the range of 6 to 10 percent by volume.
Air-entraining agents can be provided already ground into the cement. These air-entraining cements are designated as Types IA, IIA and IIIA. Air-entraining admixtures also may be added to the concrete as it is being mixed. The desired result in either case is to have extremely small air bubbles, well-dispersed throughout the concrete.
Entrained air is typically included in pavement and marine structures because it increases the durability of concrete by providing localized stress relief for freezing and thawing. Because it also helps increase impermeability and improves workability, air entrainment is generally recommended for all exposed concrete.
Comparing the data from petrographic examination of hardened concrete with its field performance has shown that the size of the entrained air bubbles and their spacing–reported as specific surface and spacing factor, respectively–most influence concrete’s durability. However, traditional tests on fresh concrete measure only the total air content without any indication of its bubble size or distribution (see Traditional specs and checks).
Furthermore, concrete samples for air tests typically are taken before it has been placed, and sometimes before it has been transported. Thus the effects on air content of transit mixer agitation, pumping, and vibration, which can be substantial, are not measured. Finally. by the time petrographic tests can be done, the concrete is already in place.
“New” test and parameters
The DBT air void analyzer allows a sample of fresh concrete, taken from the slab behind the paver for example, to be analyzed within a half hour so that adjustments can be made to subsequent batches, if needed. The system consists of a sample extraction device, the test device, and a personal computer (PC) with DBT’s proprietary software.
The 20-cubic-centimeter sample is taken with a vibrating wire cage driven through a plastic template into the placed concrete by a high-speed drill. The cage excludes larger aggregate, allowing in only the concrete mortar. The sample is transferred into a 20-milliliter syringe, which allows it to be introduced into the base of the AVA.
Stirring the mortar sample for 30 seconds releases entrained air bubbles into a viscous liquid at the base of a column of water. The liquid is specifically formulated with a particular viscosity and hydrophilic character to preserve the bubbles’ original size and also prevent them from either coalescing or disintegrating.
The air bubbles rise through the viscous liquid and the water column above it according to Stokes’ law, which describes the rates at which spherically shaped particles (in this case, voids) settle (in this case, float). The air bubbles are collected at the top of the water column by a submerged, inverted bowl connected to a balance. Data on the increasing buoyancy of the bowl are collected over the time of the test.
Using the data, an algorithm based on Stokes’ law determines the spacing factor and specific surface of the entrained air bubbles released from the mortar sample. That algorithm and the software to apply it enable the AVA test results to correlate very well with results of ASTM C 457 “Microscopical Determination of Parameters of the Air-Void System in Hardened Concrete.”
Standardization and use
Tests of the AVA system by the FHWA and the Kansas Department of Transportation (DOT) have shown that AVA testing yields spacing factor and specific surface data that are very similar to petrographic analysis results of the same concrete.
Kansas DOT specifications now provide for the use of the AVA. This performance specification is likely to be incorporated in other concrete projects requiring high durability, such as highways and bridge structures, as well as waterfront structures.
The AVA will not replace traditional field measurement devices. Rather, it will be used in support of ongoing field testing to make sure the air bubble parameters are right, or to allow changes to he made quickly in the field. The pressure pot, roll-a-meter, and gravimetric tests will then continue to confirm the total air.
A standard test method specification is being prepared by the American Association of State Highway and Transportation Officials (AASHTO). They too will offer a package of descriptive material, due to be released this fall.
More information on the air void analyzer is available at AASHTO’s Technology Implementation Group Web site, www.aashtotig.org. Facts on the Mobile Concrete Laboratory can be found at www.fhwa.dot.gov/pavement/mcl.htm.
RELATED ARTICLE: Traditional specs and checks.
Traditionally, air content has been specified as a percent of the total volume of the concrete.
Confirmation has been through testing the fresh concrete as well as later petrographic examination of the hardened concrete.
The three standard fresh concrete test methods are pressure, volumetric, and gravimetric, each requiring specific equipment. The pressure and volumetric tests are suitable for field use, but the gravimetric test is best done in the lab.
The pressure method (ASTM C 231) subjects a concrete sample to an applied pressure. The air in the concrete compresses, and the air content is calculated based on measuring the change in volume of the sample. The testing device for this test is often referred to as a pressure pot.
The volumetric method (ASTM C 173) starts with a sample of known volume. Water is added to a certain level, the container sealed, and the sample agitated to remove the air. The air content is determined based on the sample’s decrease in volume. Because one of the prescribed modes of agitation is rolling the testing device, it is frequently called a roll-a-meter.
The gravimetric method (ASTM C 138) consists of comparing the weight of a consolidated sample with the theoretical weight of the components (other than air) that were mixed to obtain the sample. It requires a balance or scale, accurate to within 0.03% of the load, and a good bit of raw data, such as the weight of the aggregates in the condition used.
Petrographic examination of hardened concrete, covered by ASTM C 457, can occur only after the concrete has hardened sufficiently to prepare a sample, which can take as long as 14 days. The testing standard describes two methods of examination that yield more descriptive information about the entrained air void system in the hardened concrete, including the specific surface, void frequency, spacing factor, and paste-air ratio.
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