Muscle fiber

Nov. 13, 2008

In the past few years, the Arizona Department of Transportation (AzDOT) together with Arizona State University (ASU) and the local concrete industry constructed and tested several thin whitetopping (TWT) pavement sections to evaluate their laboratory and field performance. Earlier projects included a major intersection in the city of Cottonwood in Yavapai County and a highway ramp on the Casa Grande-Tucson I-10 in Pinal County. Both projects utilized glass fibers, polypropylene fibers and crumb rubber as additives to the concrete.

In the past few years, the Arizona Department of Transportation (AzDOT) together with Arizona State University (ASU) and the local concrete industry constructed and tested several thin whitetopping (TWT) pavement sections to evaluate their laboratory and field performance. Earlier projects included a major intersection in the city of Cottonwood in Yavapai County and a highway ramp on the Casa Grande-Tucson I-10 in Pinal County. Both projects utilized glass fibers, polypropylene fibers and crumb rubber as additives to the concrete. The fibers were added at the normal dosage rate of 3 lb/cu yd. The crumb rubber was added at 50 lb/cu yd to provide additional ductility to the concrete.

The fiber-reinforced concrete plays a major role in increasing the ductility of TWT concrete pavement structures. It allows ultrathin and thin sections, normally 2 to 5 in., supported by the existing asphalt concrete layer, to act as a structural load-bearing system. Cracks or microcracks are intersected by random fibers that provide for the energy absorption and toughening. This effect is normally measured in the laboratory by evaluating the post-peak region of the load-deformation responses.

While all of the aforementioned test sections performed reasonably well, the polypropylene test sections performed the best in the laboratory testing program and the field. A third recent project was constructed on a very busy highway ramp of the Kingman-Seligman I-40 and was dedicated exclusively to evaluating the effects and benefits of utilizing different dosages of the polypropylene fibers in the TWT mix. Four different mixtures were evaluated in this third study: a control mix with no fibers and three mixtures with 3, 5 and 8 lb/cu yd of polypropylene fibers. For each mix, field samples during construction were collected and subjected to a laboratory testing program that included compression, three-point bending and round-panel tests.

Divine intervention

The study project is located in the Kingman-Seligman, Andy Divine interchange off the I-40 highway. The project included milling 5 in. of the existing asphalt surface and placing three TWT sections with the variable fiber dosages. A full-depth section with a control (no fiber) mix was constructed adjacent to the TWT sections.

Approximately 413 sq yd of pavement was paved using these concrete sections. TWT pavement thicknesses were 5 in. for the fiber-reinforced sections. The section poured with the control mix had a 10-in. thickness after the complete removal of the original AC material. An approximate total of 70 cu yd of concrete were placed.

All laboratory specimens were cast on the jobsite. The next day, the samples were transferred to a curing room at AzDOT facilities and kept for 28 days. The samples were then brought to ASU for testing at the structural laboratory. Several load frames, ranging from load capacities of 20 to 110 kps, equipped with servohydraulic closed-loop testing controllers, were used. Closed-loop testing allows monitoring and control of the response of a system during the test. Deflections were measured using linear variable differential transducers.

Prismatic specimens, 4 x 4 x 18 in., (width, depth, length) were used for the three-point bending (flexural) tests. The deformation across the tensile cracks was measured and used as the feedback signal to the test machine. Cylindrical specimens, 3 in. in diameter and 6 in. long, were used for the compression test. The round-panel test specimens were 24 in. in diameter and 3 in. thick. The test yields a load-deflection record and the energy absorbed.

Test results were as follows:

Compression test

Both simple compression and closed-loop tests were conducted on the various mixes. Axial and radial deformations were recorded during the test. The control mixture showed the highest strength in both compression tests. The difference within the fiber-reinforced mixtures was insignificant and was confirmed by statistical analysis.

Flexural test

The tests also were performed under simple and closed-loop control with tensile displacement as the controlled variable. The deflection of the specimen was measured to compute the energy absorbed throughout the test. The cyclic test provides the post-peak response, which allows the calculation of the material toughness. For the control mix, no cyclic loading could be obtained, because the specimens failed quickly after a peak load was reached. This failure was attributed to the brittleness of the material. The peak responses for the fibrous mixtures were very similar, but when the toughness values are compared, it can be observed that toughness increased as the fiber content increased.

Round-panel test

The ASTM C1550-03a Standard Test Method for Flexural Toughness of Fiber Reinforced Concrete Using Centrally Loaded Round Panel test method also was utilized in this study. All mixes reached a very similar average peak load value. However, it was clear that the toughness values increased with the increase of the fiber. The mix with 8 lb/cu yd of fibers had the highest energy absorption capacity.

A residual strength analysis was conducted on both the flexural and round-panel tests. The flexural test strength increments were modest of 9.5, 11.5 and 13% for the mixes with 3, 5 and 8 lb/cu yd, respectively. However, the round-panel test results showed increments of 23, 37 and 42% for the 3, 5 and 8 lb/cu yd mixes, respectively. These results better show the benefits of the fibers added to the concrete mix. The increments between the mixes (23, 14 and 5%) also suggest that there is a 14% increase in value between the 3 and 5 lb/cu yd mixes and a 5% increase in value between the 5 and 8 lb/cu yd mixes. Based on these percentages, it was determined that a 5 lb/cu yd fiber dosage has the best value-added benefit to the mix.

Almost two years after their construction, a field survey of the test sections showed that they are all performing very well with no signs of cracking or any other distress.

Thin air conditioning

There are several thermophysical properties that affect the pavement maximum and minimum temperatures. These include albedo, thermal diffusivity, thermal conductivity, emissivity, density and volumetric heat capacity. Work conducted at ASU has shown that both albedo (better with lighter-colored surfaces) and emissivity (controls the far-infrared re-radiation from the surface back to the sky) have positive responses in the reduction of the pavement temperatures. Albedo affects the pavement maximum temperature more than it does the minimum temperature. On the other hand, emissivity is of a greater factor to the minimum temperature than the maximum. In addition, a common trend in pavement temperature variations with respect to thickness has been observed. Thick pavements conduct and store more heat; whereas thin pavements have lower thermal mass and heat storage capacity. The increase in the layer thickness results in an increase in the pavement thermal mass, which is affected by the incident sunlight. This causes the pavement to have a higher heat storage capability, in other words it is able to absorb more heat per unit rise in temperature.

Thin and ultrathin whitetopping pavements have a great advantage in mitigating pavement surface temperatures and heat storage capacity. This is especially desirable at urban intersections where higher surface temperatures are observed due to the added built infrastructure (buildings, parking lots, etc.). Infrared imaging demonstrates the cool surface temperatures of an ultrathin whitetopping pavement constructed as a parking area in Rio Verde, Ariz.

TWT pavements will continue to gain popularity in the pavement community because of their good field performance and favorable role in mitigating the urban heat island effect. They should be viewed as “cool strategies.” With this added benefit, they may encourage future wider implementation.

The authors would like to acknowledge the following AzDOT personnel for their valuable assistance in this research study: James Delton, Paul Burch, Ali Zareh, George Way, Alex Durazo, Scott Weinland, Russell DiVincenzo and Michael Kondelis. Thanks also are due to Larry Scofield from the American Concrete Pavement Association and Joby Carlson of the National Center of Excellence on SMART Innovations at ASU (www?.asusmart.com).

About The Author: Kaloush and Rodezno are in the Department of Civil and Environmental Engineering at Arizona State University.

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