11 Transportation Research Record: Journal of the Transportation Research Board, No. 2591, Transportation Research Board, Washington, D.C., 2016, pp. 11–18. DOI: 10.3141/2591-02 Field overlay and bridge deck studies have suggested that vertical curling deflections and debonding can be reduced when the overlay contains fiber reinforcement. This study investigates the tensile and shear bond between aged concrete and fiber-reinforced mortar. Three macrofibers commonly used in fiber-reinforced concrete pavement overlays or bridge decks were investigated. The macrofibers were a stretched synthetic, a textured synthetic, and a hooked-end steel fiber. The tensile fracture energy within the fiber-reinforced mortar material and an interfacial tensile bond energy between the fiber-reinforced mortar cast against the aged and sandblasted concrete were all higher than that of plain unreinforced mortar. The peak loads associated with tensile or shear bond failure were not statistically affected with the addition of fibers in the overlay mixture. Overall, the interfacial tensile bond energy did improve as the fiber volume content increased, especially because some of the fracture path occurred through the mortar layer and was bridged by the fibers near the interface surface. One of the key requirements for thin pavement rehabilitation overlays is to have adequate bond between the existing substrate and new overlay. When the bond is intact, the slab is expected to move mono- lithically as a larger composite structure, with lower stresses in the slab when subjected to temperature or humidity-induced curling. Temperature fluctuations produce differential shrinkage stresses within the concrete overlay, which is expected to cause debonding (1, 2). Silfwerbrand and Paulsson identified the significant param- eters to improve the bond as cleanliness of substrate, absence of microcracks, overlay compaction, and overlay curing (3). A rough surface macrotexture, especially that created by milling of the existing substrate, has been noted to be necessary to ensure a good bond (4–9). In experimental and field testing, steel fiber-reinforced concrete (FRC) overlays were found to have better bond strengths than un- reinforced or synthetic FRC overlays (10–12). Granju reported that fibers do not show direct effect on the increased bond strength itself, but were expected to cause a delay in debonding (13). Quite a number of test specimen configurations have been proposed to evaluate the bond strength in the field and the laboratory, as shown in Figure 1 (14). Of these configurations, the wedge splitting test (Figure 1m) is expected to provide tensile opening or liftoff fracture characterization of the bonded layers and has been reported to pro- duce low variability in bond properties because of the larger sam- ple fracture area relative to the sample volume (15, 16). A similar sample to the wedge split tension test is used to measure shear bonding performance, but in a bisurface shear test (Figure 1i). The bisurface shear test originally proposed by Momayez et al. is expected to also produce a low coefficient of variation (less than 10%) among concrete specimens (17 ). The objective of this study is to address whether fibers contrib- ute to interfacial shear bond, interfacial tensile bond, or bulk fracture resistance when a composite section of FRC is cast on an existing pavement. Different fiber types and volume fractions of fibers are studied. A mortar mixture is selected to represent the overlay layer to reduce any dispersion issues associated with addition of fiber to concrete. A wedge splitting test and bisurface shear test are selected to assess the interface bond strength in tension and shear, respectively. Detailed test variables, test materials, experimental procedures, and results are described in the following sections. RESEARCH SIGNIFICANCE Applying fibers in thin concrete overlays is becoming popular because the fiber-reinforced material is found to exhibit reduced cracking and improved load-carrying capacity. The bond performance between the old substrate and FRC overlay is commonly assumed to not be affected by the addition of fibers, yet some field studies indicate that curling deflections and debonding rates can be reduced with the addition of fibers. It is useful to determine whether there is any effect on the measured interfacial bonding properties from adding fibers in the new overlay concrete. This study aims to identify the interfacial bond between an aged concrete substrate and a new overlay with common fiber types and reasonable volume contents. Tensile and shear bond strength at the interface between concrete and fiber-reinforced mortar (FRM) are measured. EXPERIMENTAL INVESTIGATION Materials and Test Variables Three types of fibers and two dosage rates for each fiber were con- sidered in this study (Table 1). The volume fraction dosage rates of each fiber were 0.40% and 0.78% for a stretched synthetic fiber (Y1), 0.5% and 1.0% for a textured synthetic fiber (Y2), and 1.0% and 2.0% for a hooked-end steel fiber (T). These fiber types and respective dosage ranges were selected to represent the potential applications of slabs on ground, pavements, and bridge decks. The hooked-end steel fibers were expected to produce the highest tensile fracture resistance, and the textured synthetic fibers were expected to have a slightly higher tensile performance than the stretched synthetic fibers (18). Fiber Effect on Interfacial Bond Between Concrete and Fiber-Reinforced Mortar Min Ook Kim and Amanda Bordelon M. O. Kim, MCE 116, and A. Bordelon, MCE 2038, University of Utah, 110 Central Campus Drive, Salt Lake City, UT 84112. Corresponding author: M. O. Kim, tominookkim@gmail.com.