International Concrete Abstracts Portal

This paper presents the results of an experimental investigation under-taken for evaluating the cyclic lateral load-drift response of rectangular reinforced concrete (RC) columns which were damaged due to large drift reversals, but then repaired for upgrading the bond strength of the spliced reinforcement within the critical hinging region. The original specimens consisted of full-scale unconfined and fiber-reinforced polymer (FRP) confined columns having a relatively high section aspect ratio of 2.0. These original specimens were subjected to large drift reversals until complete bond degradation of the spliced reinforcement within the hinging zone and complete loss of flexural strength of the columns8. The repair procedure consisted of removing the deteriorated concrete within the damaged/splice zone and casting new concrete. Two types of concrete confine-ment for improving the bond strength and flexural capacity were investigated and compared, namely, internal confinement by transverse steel ties and external confinement using carbon fiber-reinforced polymer (CFRP) jackets. It was found that repairing the bond-damaged zone through concrete confinement leads to substantial regain of flexural strength up to or exceeding the strength of the original specimens, lower structural damage associated with concrete fracturing and bond degradation, and considerable improvement of the energy dissipation capacity under cyclic loading. Confinement by external FRP jackets was relatively more effective than confinement by internal steel ties. However, unlike columns with continuous reinforcement, columns with spliced reinforcement within the hinging region experienced significant bond and strength degradation beyond drift ratios between 3 and 4%, irrespective of the type and amount of confinement used. The experimental results are discussed, and a design expression for estimating the thickness of the FRP jacket required for seismic bond strengthening is presented and compared with the test data.

Four reinforced concrete T-beams were tested to study their behavior when strengthened for shear using near-surface mounted (NSM) bars. The objectives were to study the effects of the type of bars used (carbon fiber-reinforced polymer (CFRP) and conventional reinforcing steel) and the effects of the load level at which the bars are installed. It was observed that the CFRP bars bonded at zero load increased the shear capacity by up to 92%, while those bonded when the precracked beam was under load increased the capacity by 77%. For the steel bars, these values were 75% and 57%, respectively. The CFRP strengthened regions showed a slightly more favorable response than those strengthened with conventional steel bars. Strengthening provided an improved control of the diagonal crack width. In the beams with closer spacing of NSM bars, the increase in shear strength allowed the beam to fail after considerable flexural deformations.

Short-fiber reinforcement is commonly added to cement-based materials to improve various aspects of their durability and life-cycle performance. Effective designs of Fiber Reinforced Cement Composites (FRCC) depend not only on material composition, but also on their methods of processing. In particular, the distribution of fibers within a structural component can significantly affect its resistance to cracking and, therefore, its durability when exposed to severe environments. Probability-based analyses can be used to accommodate such factors in life-cycle performance evaluation, in which the relevant performance measures are described by probability distributions and their evolution over time. This paper concerns the simulation of FRCC materials using lattice models, in which the individual fibers are explicitly modeled within the material domain. This approach facilitates the study of non-uniform fiber dispersions and their potential effects on structural performance.

This paper summarizes a series of tests performed on strain hardening High-Performance Fiber-Reinforced Concrete (HPFRC) coupling beams with span length-to-depth ratios (ln/h) of 1.75 and 2.75. These tests show that incorporating HPFRC simplifies the detailing required to ensure a stable response of coupling beams subjected to earthquake induced displacement reversals. Results from five tests of precast coupling beams, three with ln/h = 1.75 and two with ln/h = 2.75, are reported herein. Strategies for embedding the precast HPFRC coupling beams into the structural walls without interfering with boundary element reinforcement were explored. Test results confirm that HPFRC can reliably confine diagonal reinforcement and ensure stable hysteresis behavior. HPFRC was also found to significantly increase shear strength, thereby forcing a flexurally dominated failure mode with modest stiffness degradation and excellent energy dissipation. A revised coupling beam design philosophy is outlined in order to ensure ductile flexural behavior.

Previous studies using pullout-type tests comprising monotonic, unidirectional cyclic, and reversed cyclic loads have shown that bond between reinforcing bars/prestressing strands and concrete can be significantly enhanced by replacing the conventional concrete with high-performance fiber-reinforced cement composites (HPFRCCs). This is attributed to the fact that, compared to plain concrete and conventional fiber-reinforced concrete (FRC), HPFRCCs exhibit a strain-hardening response under tension up to large strains, thereby preventing the concrete from deterioration under bond action. Pullout test results provide the bond stress versus slip relationship that can be considered the constitutive property of the steel-to-HPFRCC interface. Since the post-cracking tensile stress and strain of fiber-reinforced cement composites are the fundamental characteristics that distinguish them from conventional concrete, the HPFRC tensile stress-strain response obtained from direct tensile tests was used to derive the local bond stress-slip models presented in this paper. It is shown that the proposed models are more concise than previous models suggested for FRC and give good agreement with test results.