Effect of fiber-reinforced concrete in thin overlays

Cracking and debonding are important considerations for pavement maintenance because they are linked with the service life of pavement structures. Concrete overlay pavements are expected to have reduced crack widths and reduced debonding rates when the concrete mixture contains fibers. The age-dependent changes in flexural and fracture properties of fiber-reinforced concrete (FRC) between 3 and 90 days were experimentally investigated. Compared to plain unreinforced concrete, steel and polymeric macro-FRC of up to 1% by volume of fibers were confirmed to have no significant effects on compressive strength, free drying shrinkage, or coefficient of thermal expansion. For both steel-fiber-reinforced concrete (SFRC) or polymeric-fiber-reinforced concrete (PFRC) mixtures, fracture properties as used in the FEA model were found, through wedge splitting testing, to increase with age. However, the property currently used in the FRC overlay pavement design is the residual strength ratio, which was found to decrease with age for both SFRC and PFRC. A simple crack width equation was developed to predict the crack width of thin FRC overlays based on the addition of fibers to the concrete. The predicted crack widths were validated against data from a field project on variable joint spacing and subjected to temperature and humidity variations. Both the tensile and shear bond between an aged concrete and a newly cast fiber-reinforced mortar were investigated. The tensile interfacial energy between the fiber-reinforced mortar cast against the aged and sand-blasted concrete was higher than that of plain unreinforced mortar. It was found that this tensile interfacial energy was proportional to the physical number of fibers located near the interface surface, particularly because some of the fracture path went through the mortar layer and was bridged by these fibers. No statistical trend could be found between the peak strengths associated with either the tensile or the shear bond and the addition of fibers in the overlay mixture. In addition, a performed finite element analysis (FEA) study indicated that, as expected, crack width, vertical liftoff, and debonding length all decreased as the fracture energy of the FRC increased or as the interfacial tensile bond increased. The developed crack width equation and finite element model were found to predict the crack widths within 0.19 mm (or 26%) compared to actual pavement. The previously developed FEA model was modified to resemble 150 mm thick pavement in order to to compare FRC pavement responses to that of unreinforced concrete containing dowels. Compared to completely unreinforced pavement, it was found that dowel reinforcement reduced crack widths by 3 times, while a typical 0.5% volume fraction of PFRC reduced crack widths by only 1.3 times. Dowel bars are used only in thick pavements rather than thin pavements, so among thin overlays, FRC is a good option to reduce crack width, debonding length, and vertical deflection.