Arizona State University Study

In coordination with FORTA® Corporation and the City of Tempe, Arizona, an asphalt mixture overlay was placed at Evergreen Drive (East of the Loop 101 and North of University Drive) in Tempe. The designated road section within the construction project had two main asphalt mixtures: a control mix with no fibers added and a mixture that contained 1-lb of fibers per ton of asphalt mixture. Mixtures for laboratory testing were sampled during construction and brought back to the Arizona State University (ASU) laboratories. Mixture preparation included compaction of 150 mm diameter gyratory specimens for triaxial testing, and beam specimens prepared and compacted according to AASHTO TP8 test protocols. The target air void levels for the test specimens were those typically achieved in the field (about 7%). Rice gravities were determined for the loose mixtures, as well as thickness and bulk densities measured in preparation of the testing program. Laboratory experimental programs included: triaxial shear strength, dynamic (complex) modulus, and repeated load for permanent deformation characterization; flexural beam tests along with flexural toughness tests for fatigue cracking evaluation; indirect diametral tensile tests for thermal cracking mechanism evaluation; C* Integral test for crack growth and propagation evaluation. The data was used to compare the performance of the fiber-reinforced mixtures to the control mixture.

Binder Characterization

FORTA-FI® is a proprietary blend of several materials substantially affecting the binder characteristics as well as reinforcing the final mixture. The viscosity-temperature susceptibility relationship at lower temperatures showed no changes from virgin binder, which is positive and desirable, but at high temperatures, improved properties were observed to have higher viscosities. The modified binder is far less susceptible to viscosity change with increased temperatures, and it was used as the control mix in all additional tests.

Triaxial Shear Strength Test

The FORTA-FI® concrete mixture showed higher residual energy compared to the control mix. This indicated that the fiber-reinforced mixture shows higher resistance to crack propagation than the mixture without reinforcing fibers. While various ratios of materials in FORTA-FI® were tested to determine the optimum mix across a wide range of applications, it was determined that the 1 lb/Ton dosage of FORTA-FI® blend would yield the best performance based on triaxial shear strength laboratory tests.

Permanent Deformation Tests – Static Creep / Flow Time Test

Two important characteristics were observed for the FORTA-FI® mixture when compared to the control mixture. One was the endurance of the secondary stage, and the second was gradual/less accumulation of permanent strain. Both were attributed to the presence of the reinforcing fibers in the mixture, as this behavior is not typically observed in conventional mixtures. The fiber-reinforced mixture had higher Flow Time values than the control mixture (over 900% higher), and 700% lower slope values. These indicate that the FORTA-FI® mixture has a much higher potential to resist permanent deformation than the control mixture.

Repeated Load Flow Number Test

The Flow Number for 1 lb/Ton dosage of FORTA-FI® mixture was 1,150% higher than the control mix. The FORTA-FI® mixture had a much lower strain slope compared to the control mixture and lower values of strain slope during the tertiary stage (when the sample has already failed due to higher shear stress). This indicates that the mixture has higher potential to resist shear failure and shows a lower rate of permanent deformation and rutting. The reinforcing fibers in the mixture provide unique resistance to shear failure beyond the tertiary flow point. Due to the lower strain slopes of the fiber-reinforced mixture, the mixture is capable of storing more energy than conventional mixtures before and during tertiary flow.

Dynamic (Complex) Modulus Test

The modulus of the 1 lb/Ton mixture was higher than the control mixture, which indicates that the reinforcing fibers in the mixture enhance the modulus of the mixture and therefore its resistance to permanent deformation. While various ratios of materials in the FORTA-FI® blend were tested to determine the optimum mixture across a wide range of applications, it was determined that the 1 lb/Ton dosage of FORTA-FI® mixture would be best, or optimum, for improved moduli properties.

Fatigue Cracking Testing

Comparing the initial stiffness for the FORTA-FI® mixture at 70°F (21°C) and 250 micro-strains, it was noted that the fiber-reinforced mixture showed higher stiffness values compared to the control mixture. Comparing the fatigue life for the FORTA-FI® mix at 150 micro-strains, it was observed that the fiber-reinforced 1 lb/Ton mixture had the highest fatigue life.

Flexural Strength Test

The flexural strength and corrected flexural strength are increased by 14% and 25% respectively with the addition of the 1 lb/Ton dosage of the FORTA-FI® blend. While various ratios of materials in the FORTA-FI® blend were tested to determine the optimum mixture across a wide range of applications, it was determined that the 1 lb/Ton dosage of the FORTA-FI® blend would be best, or optimum, in terms of flexural properties.

Indirect Diametral Tensile Test

The tensile strength of 1 lb/Ton dosage of FORTA-FI® mixture increased as the temperature decreased. Also when comparing the results of the tensile strength for both the control and 1 lb/Ton dosage of the FORTA-FI® blend, it was observed that the fiber-reinforced asphalt mixture had about 150% higher strength than the control mixture. Traditionally, lower thermal cracking would be expected for mixtures with higher tensile strength values. In essence, the FORTA-FI® blend of reinforcing fibers in the mixture play a vital role in resisting thermal cracking in the HMA mixture. At all temperatures tested, (50°F, 32°F, 14°F)(10°C, 0°C, -10°C), the 1 lb/Ton dosage of FORTA-FI® mixture showed energies averaging 200% higher than the control mixture. At the lowest temperature of 14°F (-10°C), where susceptibility of crack initiation and propagation is the highest, it was observed that the fiber-reinforced asphalt mixture had higher energy values than the control mixture. In general, lower thermal cracking should be expected as the energy at failure, or fracture energy, is increased.

Crack Propagation Test – C* Integral

After the relationships between crack growth rates as well as slopes of crack growth rates for the 1 lb/Ton dosage of FORTA-FI® mixture and the control mixture were developed, it was noted that the fiber-reinforced mixture had higher C* and slope values than the control mixture. This means that the fiber-reinforced asphalt mixture has a higher potential to resist crack propagation because of the reinforcement effect provided by the reinforcing fibers.

(dU*=Change in energy rate for a load P and a crack extension dC, B=thickness)

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