International Concrete Abstracts Portal

Reinforced concrete walls are commonly used in low- and mid-rise construction because they provide high strength, stiffness, and durability. In regions of low and moderate seismicity, ACI 318 Code requirements for minimum reinforcement ratio and maximum reinforcement spacing typically control over strength-based requirements. However, these requirements are not well-supported by research. The current study investigates requirements for the amount and spacing of reinforcement using experimentally validated nonlinear finite element modeling. For lightly reinforced concrete walls subjected to out-of-plane loading: 1) peak strength is controlled by concrete cracking; and 2) residual strength depends on the number of curtains of steel. Walls with very low steel-fiber dosages were also studied. Results show that fiber, rather than discrete bars, provides the most benefit to wall strength, with fiber-reinforced concrete walls achieving peak strengths more than twice that of identically reinforced concrete walls.

The geometry of arched (vertically curved) reinforced concrete (RC) members contributes to the development of additional stresses, affecting their flexural and shear strengths. This aspect of curvilinear RC members reinforced with glass fiber-reinforced polymer (GFRP) bars has not been reported in the literature. In addition, no specific design recommendations consider the effect of curvilinearity on the flexural and shear strengths of curved GFRP-RC members. This study has performed pioneering work in developing models to predict the flexural and shear strengths of curvilinear GFRP-RC members, with a focus on precast concrete tunnel lining segments. Eleven full-scale curvilinear GFRPreinforced tunnel segment specimens were tested under bending load as the experimental database. Then, a model was developed for predicting the flexural strength of curvilinear GFRP-RC members. This was followed by the development of two shear-strength prediction models based on the Modified Compression Field Theory (MCFT) and critical shear crack theory (CSCT). After comparing the experimental and analytical results, a parametric study was performed to evaluate the effect of different parameters on the flexural and shear strengths of curvilinear GFRP-reinforced members. The results indicate that neglecting the curvilinearity effect led to a 17% overestimation of the flexural strength, while the proposed models could predict the flexural strength of the specimens accurately. The proposed models based on the MCFT—referred to as the semi-simplified Modified Compression Field Theory (SSMCFT) and the improved simplified Modified Compression Field Theory (ISMCFT)—predicted the shear strength of the specimens with 28% conservativeness. In addition, the modified critical shear crack theory (MCSCT) model was 10% conservative in predicting the shear strength of curvilinear GFRP-RC members.

Extensive experimental verification has shown that the use of fiber-reinforced polymer (FRP) anchors in combination with externally bonded FRP composites increases the flexural capacity of existing reinforced concrete (RC) structures. Thus, a rational prediction model is introduced in this study so that fiber splay anchors may be accurately designed for practical strengthening applications. Simplified structural mechanics principles are used to build this model for capacity prediction of a group of fiber splay anchors used for FRP flexural strengthening. Three existing test series using fiber splay anchors to secure FRP-strengthened T-beams, block-scale, and one-way slabs were used to calibrate and verify the accuracy and applicability of the present model. The present model is shown to yield very accurate predictions when compared to the results of the block-scale specimen and eight different one-way slabs. The proposed model is also compared with the predictions of a design equation adapted from the case of channel shear connectors in composite concrete-steel construction. Results show a very promising correlation.

This study addressed a critical knowledge gap by examining the influence of staggering on the bond strength of lapped glass fiber-reinforced polymer (GFRP) bars in concrete members. It involved a comprehensive investigation of new-generation GFRP bars with varying staggering configurations in nine large-scale GFRP-reinforced concrete (RC) beams with a rectangular cross section of 300 x 450 mm (11.8 x 17.7 in.) and a length of 5200 mm (204.7 in.). The tests investigated splice strength with three staggering distances: 0, 1.0, and 1.3 times the splice length (ls) from center-to-center of two adjacent splices, and three splice lengths of 28, 38, and 45 times the bar diameter (db). Results revealed a slight improvement in ultimate load-carrying capacity (less than 10%) for partially and fully staggered splices compared to non-staggered ones, with the latter exhibiting a more ductile failure mode. The effect of staggering was consistent across different splice lengths, demonstrating that splice length was not a factor. Although staggering reduced flexural crack width, it increased the total number of cracks due to expanded splice regions. Bond strength improved with staggering, with gains of 4.0% and 8.0% for partially and fully staggered splices, respectively. ACI CODE- 440.11-22 provides more accurate predictions of the bond strength of lap-spliced GFRP bars than the other design codes, showing an average test-to-prediction ratio of 1.03 for non-staggered splices. Nevertheless, it requires some reconsiderations when it comes to staggered splices. To address this, a proposed modification factor was introduced to account for staggering conditions when calculating bond strength and splice length in ACI CODE-440.11-22.

The weakness of concrete in tension can be mitigated by developing fiber-reinforced concrete (FRC) to induce pseudo-ductility. However, enhancing the intrinsic tensile strength of the matrix in FRC has received little attention. In this regard, nanofibers, which can improve the intrinsic tensile properties of the matrix, were used in conjunction with microfibers to enhance intrinsic tensile strength. Different volumes of nanofibers (0.0–0.6%) and microfibers (0.0–2.0%) were tested, and various fresh and hardened properties were analyzed. Test results show that the superplasticizer dosage increased with both nanofiber and microfiber volume and that strength increased with microfiber volume, reaching an optimum point at a certain nanofiber dosage. Moreover, incorporating nanofibers and microfibers to develop multiscale FRC (MSFRC) significantly improved direct tensile strength and energy absorption. The synergy between nanofibers and microfibers was revealed both qualitatively and quantitatively, contributing to the advancement of FRC.