International Concrete Abstracts Portal

This paper presents the time-dependent interfacial behavior of near-surface-mounted (NSM) carbon fiber-reinforced polymer (CFRP) strips bonded to a concrete substrate using inorganic resins. Four types of bonding agents (mortar, polyester-silica, ultra-high-performance concrete [UHPC], and geopolymer) are tested to appraise the potential for NSM application with a focus on rheological and mechanical performance during a curing period of 28 days. Unlike the case of the mortar and geopolymer resins, the rheological resistance of the polyester-silica and UHPC resins increases within 30 minutes, owing to an evolved setting process. The hydration of mortar continues for up to 28 days of curing in line with assorted chemical reactions. The compressive strength of polyester-silica gradually ascends to 35 MPa (5076 psi) at 28 days, while that of UHPC rapidly rises to 95.3 MPa (13,822 psi) at 3 days. Contrary to the stabilized interfacial capacity of the specimens with mortar and geopolymer after 7 days, the capacity of the specimens with polyester-silica steadily develops until 28 days. Unlike the failure mode of other cases over time, a shift in the plane of failure is noticed for the mortar-bonded interface. The post-peak response and energy dissipation of the interface are controlled by the resin type and curing period. Analytical modeling quantifies the level of hazard and clarifies the functional equivalence of the interface with the inorganic resins against conventional organic epoxy resins.

This paper presents the behavior of concrete confined by basalt fiber-reinforced polymer (BFRP) and BFRP-polyester hybrid grids exposed to elevated temperatures ranging from 25 to 150°C (77 to 302°F). The functionality of organic (epoxy) and inorganic (geopolymer) resins is studied comparatively. A total of 75 cylinders are monotonically loaded in compression to examine the axial capacity, post-peak deformability, and failure modes. Ancillary tests indicate that the thermal degradation of the geopolymer resin is not as significant as that of the epoxy resin and that the strength of BFRP and polyester grids dwindles with temperature. The geopolymer resin outperforms its epoxy counterpart in terms of confining efficacy under thermal distress, leading to an insignificant reduction in the cylinder capacity. The residual load-carrying mechanism of the confined concrete is enhanced by the hybrid grids along with an increase in energy dissipation. The thermal loading changes a confining pressure distribution so that the failure of the confined concrete with the epoxy resin entails irregular fiber rupture at temperatures exceeding 100°C (212°F). Owing to the high rupture strain of the polyester grids (over 7%), disintegration of the hybrid-confined concrete is impeded. Analytical modeling characterizes the capacity degradation rate and reliability of the test specimens.

Externally bonded carbon fiber-reinforced polymer (CFRP) composites are an effective method for flexural and shear strengthening and repair of reinforced concrete (RC) beams. In externally bonded CFRP strengthening, epoxy-based CFRP wet layup systems are predominately used. CFRP composites pre-impregnated with polyurethane (PU) resin and PU primers were demonstrated as an effective alternative in previous studies yet have seen limited applications. Due to the broad range of PU primer and laminate properties, bond between PU-CFRP system and concrete substrate can be potentially promoted. This paper experimentally characterizes the performance of six different PU-CFRP systems for external flexural strengthening of RC beams and investigates the relationship between bond-slip properties and the target flexural responses. Twenty-one lap shear specimens, 30 concrete flexural beams, and seven RC flexural girders were tested and compared with a conventional epoxy-based wet layup system and non-CFRP control specimens. Results indicated that PU adhesive and laminate can be tailored to obtain desired flexural performance, with bond cohesive energy and shear slip being the parameters that best predict the flexural beam strength, deformability, and failure modes. Comparable flexural capacity improvement was also observed between the PU-CFRP and the epoxy-based CFRP wet layup systems.

Several cases of premature damage of wind turbine tower foundations constructed using the anchor-ring method have been reported to cause towers’ uplift. Sludge buildup (in the form of crushed concrete) was evident along the periphery of the damaged base towers and was found to originate mainly from the concrete-steel interface around the steel anchor foundation that is embedded in concrete, due to the cyclic movement of the anchor steel and the induced water pressure at the interface. This study experimentally investigated possible repair and reinforcing methods to mitigate this problem. Cyclic load tests were carried out on three steel anchor mockup specimens. In the first two specimens, the anchor steel was retrofitted with a nonshrink mortar and epoxy resin, respectively, whereas in the other specimen, the anchor steel was further restrained against uplift by additional anchorage bars. The results indicate that the non-shrink mortar was not effective in preventing water ingress into the interface and could only delay the extent of concrete damage and the progress of bond deterioration during cyclic loading. Epoxy resin was found to perform much better in this regard. In addition, the epoxy layer could also help to dissipate the induced bearing stresses under cyclic loading, thereby making it the best candidate for repair and reinforcing materials in such applications. The use of additional anchorage bars was also found to be effective, although it still requires additional measure to prevent moisture ingress.

Concrete beams reinforced by short composite macrofibers uniformly distributed in their volume were tested mechanically in bending. The short composite macrofibers were a few centimeters long and less than 2.5 mm (0.01 in.) in diameter. Macrofibers were manufactured impregnating glass or carbon-fiber tows by epoxy resin, forming unidirectionally oriented composite material rods later cut in short pieces. Such fibers were designated in the framework of the paper as macrofibers. The length-to-diameter ratios L/d of the glass and carbon macrofibers were equal to 22.9 and 28.2, respectively. The beams were loaded until the opening of the macrocrack reached 5 mm (0.02 in.). The macrofibers bridging the crack were pulled out during opening of the crack. Low-, medium-, and high-strength concretes in the range of 40 to 120 MPa (5800 to 17,405 psi) were used in the experiments. Pullout tests with single fibers were carried out. The volume fraction of the fibers in concrete was 1.5%. Two types of fiber-reinforced concrete beams with glass and carbon fibers were manufactured and tested, and the data obtained were compared with experimental results for steel fiber-reinforced concrete beams. The potential of the composite fibers was analyzed.