International Concrete Abstracts Portal

This investigation aims to study the effect of different types and volumes of synthetic fibers (SFs) on the cyclic behavior of rubberized beam-column joints. Different self-consolidating rubberized concrete (SCRC) mixtures with different percentages of crumb rubber (CR) and SFs were tested. The main parameters were the percentage of CR (0, 15, and 25% by volume of sand), coarse aggregate size (10 and 20 mm [0.39 and 0.79 in.]), concrete type (SCRC and vibrated rubberized concrete), type of SF (microsynthetic fibers [MISFs] and macro-synthetic fibers [MASFs]), length of SFs (19, 27, 38, 50, and 54 mm [0.75, 1.06, 1.5, 1.97, and 2.13 in.]), and volume of SFs in the mixture (0, 0.2, and 1%). The structural performance of the tested beam-column joints was assessed based on load-deflection response, initial stiffness, load-carrying capacity, first cracking load, cracking behavior, failure mode, rate of stiffness degradation, displacement ductility, brittleness index, and energy dissipation. The results indicated that using MISFs slightly improved the structural performance of beam-column joints, while using MASFs had a significant effect on enhancing the load-carrying capacity, ductility, stiffness, and energy dissipation of tested joints. The highest improvement in the cyclic performance of beam-column joints was noticed when 38 mm (1.5 in.) MASFs were used. The results also showed that adding a high percentage of SFs (1%) to joints with a high percentage of CR (25%) compensated for the reduced load-carrying capacity caused by the high percentage of CR and helped to develop the joint with significant improvements in ductility, cracking behavior, brittleness index, first crack load, and energy dissipation.

Two full-scale concrete masonry walls were repaired with three horizontally aligned 20 in. (508 mm) wide unidirectional carbon fiber sheets using different commercially available epoxies. Twenty years later, the carbon fiber-reinforced polymer concrete masonry unit (CFRP-CMU) bond was determined through selective pulloff tests that were preceded by detailed nondestructive evaluation. Results showed that despite superficial damage to the top epoxy coating and debonding along masonry joints, the residual CFRP-CMU bond for the wall surface was largely unaffected by prolonged exposure to Florida’s harsh environment. Therein, over 90% of the failures were in the concrete substrate. Although bond was poorer at mortar joints because the CFRP was well bonded to the masonry surface, its impact on structural performance of the repair was expected to be minimal. Overall, the repairs proved to be durable with both epoxy systems performing well.

In this study, a proposed three-dimensional (3-D) grid strut-and-tie model approach is verified using the experimental capacity from 78 reinforced concrete pile caps, 19 slab-column joints, and 60 torsional beams obtained by others. The analysis results were compared with those obtained using the BS 8110, EC2, CRSI, fib, AASHTO-LRFD, and ACI 318 design provisions. In addition, the design of a pier cap subjected to multiple load combinations with longitudinal and lateral loads was conducted to illustrate the proposed approach. The design results from the pier cap example were also compared with those obtained using ACI 318-14. The analysis results agreed well with experimental results.

The strut-and-tie model approach has now been incorporated in current U.S. design codes and guidelines for the design of disturbed regions in structural concrete elements. However, more work is needed to extend the approach to design of three-dimensional (3-D) structural concrete. It is also important to consider its verification with experimental evidence. The application to 3-D design situations of this approach brings more uncertainties with its proper application and limitations. To reduce uncertainty and assist designers in the application of the strut-and-tie model to 3-D situations, the authors present in this paper a 3-D grid strut-and-tie model approach consisting of three key steps: 1) grid elements to construct a 3-D strut-and-tie model; 2) triaxial failure model of concrete to determine effective strengths of concrete struts and nodal zones; and 3) iterative technique to evaluate the axial stiffness of struts and ties. In this paper, the authors also incorporate in the strut-and-tie model a new concept of maximum cross-sectional areas of struts and ties to examine the strut-and-tie model’s geometrical compatibility. The approach is illustrated with the redesign of a deep pile cap with tension piles available in the literature. In a subsequent paper, the authors will evaluate the approach with test results of 157 specimens tested to failure. The tests include 78 reinforced concrete pile caps, 19 slab-column joints, and 60 beams subjected to torsion.

Earlier studies have shown that post-installed reinforcing bars are capable of developing bond behavior equivalent to cast-in-place reinforcing bars. Therefore, connections with post-installed starter bars using qualified adhesives can be designed according to conventional anchorage design provisions. However, this approach often results in development lengths which cannot be accommodated in structural joints without hooks due to the limited thickness of members. The results of 16 full-scale tests presented in this paper show the beneficial effect of increased bond strengths due to column moment loading and the adverse effect of reduced bond strengths due to cyclic loading on the seismic performance of column-to-foundation connections. The quantification of both effects is a prerequisite for the development of a new design methodology for these joints, which are based on current bonded anchor design provisions. The proposed design approach potentially allows a reduction of the development length to the extent that hooks may not be required.