S20 / CONCRETE PROGRESS www.ROADSBRIDGES.com

Kamil E. Kaloush, Ph.D., P.E., and Maria Carolina Rodezno

II n the past few years, the Arizona Department of Trans-

portation (AzDOT) together with Arizona State University

(ASU) and the local concrete industry constructed and

tested several thin whitetopping (TWT) pavement sec-

tions to evaluate their laboratory and fi eld 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 fi bers,

polypropylene fi bers and crumb rubber as additives to the con-

crete. The fi bers 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 fi ber-reinforced concrete plays a major role in increas-

ing 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-bear-

ing system. Cracks or microcracks are intersected by random

fi bers that provide for the energy absorption and toughening.

This effect is normally measured in the laboratory by evaluat-

ing 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 fi eld. 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 benefi ts of utilizing different dos-

ages of the polypropylene fi bers in the TWT mix. Four differ-

ent mixtures were evaluated in this third study: a control mix

with no fi bers and three mixtures with 3, 5 and 8 lb/cu yd of

polypropylene fi bers. For each mix, fi eld samples during con-

struction were collected and subjected to a laboratory testing

program that included compression, three-point bending and

round-panel tests.

Divine interventionThe 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 fi ber dosages. A full-depth sec-

tion with a control (no fi ber) 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 fi ber-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. Defl ections were

measured using linear variable differential transducers.

Additives give concrete whitetopping strengthAdditives give concrete whitetopping strength

Prismatic specimens, 4 x 4 x 18 in., (width, depth, length)

were used for the three-point bending (fl exural) tests. The de-

formation 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-defl ection record and the

energy absorbed.

Test results were as follows:

Compression testBoth simple compression and closed-

loop tests were conducted on the vari-

ous mixes. Axial and radial deformations

were recorded during the test. The con-

trol mixture showed the highest strength

in both compression tests. The difference

within the fi ber-reinforced mixtures was

insignifi cant and was confi rmed by sta-

tistical analysis.

Flexural testThe tests also were performed under

simple and closed-loop control with ten-

sile displacement as the controlled vari-

able. The defl ection 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 fi brous mixtures were very similar, but when the tough-

ness values are compared, it can be observed that toughness

increased as the fi ber content increased.

Round-panel testThe ASTM C1550-03a Standard Test Method for Flexural

Toughness of Fiber Reinforced Concrete Using Centrally Load-

ed Round Panel test method also was utilized in this study. All

mixes reached a very similar average peak load value. How-

ever, it was clear that the toughness values increased with the

increase of the fi ber. The mix with 8 lb/cu yd of fi bers had the

highest energy absorption capacity.

A residual strength analysis was conducted on both the

fl exural and round-panel tests. The fl exural test strength incre-

ments 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 ben-

efi ts of the fi bers 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 fi ber dosage has the best value-added benefi t to the mix.

CONCRETE PROGRESS / S21

Almost two years after their construction, a fi eld survey of

the test sections showed that they are all performing very well

with no signs of cracking or any other distress.

Thin air conditioningThere are several thermophysical properties that affect the

pavement maximum and minimum temperatures. These in-

clude albedo, thermal diffusivity, thermal conductivity, emissiv-

ity, density and volumetric heat capacity. Work conducted at

ASU has shown that both albedo (better with

lighter-colored surfaces) and emissivity (con-

trols the far-infrared re-radiation from the sur-

face back to the sky) have positive responses

in the reduction of the pavement temperatures.

Albedo affects the pavement maximum tem-

perature more than it does the minimum tem-

perature. 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 pave-

ments conduct and store more heat; whereas

thin pavements have lower thermal mass and

heat storage capacity. The increase in the lay-

er thickness results in an increase in the pave-

ment 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 ad-

vantage in mitigating pavement surface temperatures and heat

storage capacity. This is especially desirable at urban intersec-

tions where higher surface temperatures are observed due to

the added built infrastructure (buildings, parking lots, etc.). In-

frared 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 pave-

ment community because of their good fi eld performance and

favorable role in mitigating the urban heat island effect. They

should be viewed as “cool strategies.” With this added benefi t,

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 Dura-

zo, Scott Weinland, Russell DiVincenzo and Michael Kondelis.

Thanks also are due to Larry Scofi eld from the American Con-

crete Pavement Association and Joby Carlson of the National

Center of Excellence on SMART Innovations at ASU (www

.asusmart.com).

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

Almost two years after

their construction, a

fi eld survey of the test

sections showed that

they are all perform-

ing very well with no

signs of cracking or

any other distress.

LearnMore! For more information related to this article, go to:

www.roadsbridges.com/lm.cfm/rb110807

•