International Concrete Abstracts Portal

To reduce CO2 emissions of concrete, a slag-based zero-cement concrete (ZC) of high strength (60 MPa [8.70 ksi]) was developed. In the present study, cyclic loading tests were conducted to investigate the seismic performance of full-scale interior precast beamcolumn joints using the new ZC. One monolithic portland cementbased normal concrete (NC) beam-column joint and two precast ZC beam-column joints were tested. The test parameters included concrete type, fabrication method, and beam bottom bar anchorage detail. The structural performance was evaluated, including the strength, deformation capacity, damage mode, and energy dissipation. The test results showed that the structural performance of the precast ZC beam-column joints could be equivalent, or superior, to that of the monolithic NC beam-column joint. Although the reinforcement details of the ZC joints do not satisfy the seismic design requirements of special moment frames in ACI 318-19, the seismic performance of the ZC joints satisfied the requirements of ACI 374.1-05 and AIJ 2002 Guidelines.

This paper presents the behavior of unreinforced cylindrical concrete elements confined with a hybrid system, consisting of an ultra-high-performance concrete (UHPC) jacket and basalt fiber-reinforced polymer (BFRP) grids. For exploring the feasibility of the proposed strengthening scheme, a series of tests are conducted to evaluate material properties and to obtain results related to interfacial bond, load-bearing capacity, axial responses, and failure modes. To understand the function of the individual components, a total of 57 cylinders are loaded under augmented confining conditions, including plain cores with ordinary concrete (CONT), plain cores with UHPC jackets (Type A), and plain cores with UHPC jackets plus BFRP grids (Type B). By preloading the cores at up to 60% of the control capacity (60%fc′) before applying the confinement system, the repercussions of inherent damage that can take place in vertical members on site are simulated. The compressive strength of UHPC rapidly develops within 7 days, whereas its splitting strength noticeably ascends after 14 days. The adhesion between the ordinary concrete and UHPC increases over time. While the Type B specimens outperform their Type A counterparts in terms of axial capacity by more than 18%, reliance on the BFRP grids is reduced with the growth of UHPC’s strength and adhesion because of the interaction between the hardened UHPC and the core concrete. The adverse effects of the preloading are noteworthy for both types, especially when exceeding a level of 30%fc′. The BFRP grid-wrapping alleviates the occurrence of a catastrophic collapse in the jacketed cylinders, resulting from a combination of the axial distress and lateral expansion of the core. Analytical models explain the load-carrying mechanism of the strengthened concrete, including confinement pressure and BFRP stress. Through parametric investigations, the significance of the constituents is clarified, and design recommendations are suggested.

Recent research data was evaluated with the aim of extending the applicability of using deformed steel fiber-reinforced concrete (SFRC) to enhance the shear strength of beams and one-way slabs. Experimental results were assessed for influences on the shear strength of SFRC members that do not contain stirrups of factors, including size effect, concrete density (normalweight and lightweight) and compressive strength, fiber-volume fraction (Vf), and the longitudinal steel reinforcement ratio. Estimates of steel stresses in longitudinal bars at the time of shear failure were carried out to identify differences in members with distinct longitudinal steel ratios and bar grades, consistent with the range of flexural design parameters in ACI 318-19. Results of these analyses and a reliability investigation of design equations applicable to members without fibers were used for proposing new provisions for the shear design of SFRC beams and one-way slabs based on the ACI 318-19 shear-strength model.

This paper proposes a series of empirical modifications to an existing three-step analytical model used to derive the cyclic shear capacity of circular RC columns considering corrosive conditions. The results of sixteen shear-critical RC columns, artificially corroded to various degrees and tested under quasi-static reversed cyclic loading, are used for model verification. The final model is proposed in a stepwise damage-state format relative to the measured damage of the steel reinforcement. New empirical decay coefficients are derived to determine the degraded material properties based on an extensive database of over 1,380 corroded tensile tests. An additional database of 44 corroded RC circular piers is collected to assist in the modification of ductility-based parameters. Compared to the shear-critical test specimens, the model results indicate that the peak shear capacity can be predicted well across a range of deterioration severities (0 to 58.5% average transverse mass loss), with a mean predictive ratio of ± 8.60 %. As damage increases, the distribution of the corrosion relative to the location of the shear plane becomes a critical performance consideration, increasing predictive variance.

Noncorrosive fiber-reinforced polymer (FRP) reinforcement presents an attractive alternative to conventional steel reinforcement, which is prone to corrosion, especially in harsh environments exposed to deicing salt or seawater. However, FRP rebars’ lower axial stiffness leads to greater crack widths when FRP reinforcing bars elongate, resulting in significantly lower flexural stiffness for FRP-reinforcing bar-reinforced concrete members. The deeper cracks and larger crack widths also reduce the depth of the compression zone. Consequently, both the aggregate interlock and the compression zone for shear resistance are significantly reduced. Additionally, due to their limited tensile ductility, FRP reinforcing bars can rupture before the concrete crushes, potentially resulting in sudden and catastrophic member failure. Therefore, ACI Committee 440 states that through a compression-controlled design, FRP-reinforcing bar-reinforced concrete members can be intentionally designed to fail by allowing the concrete to crush before the FRP reinforcing bars rupture. However, this design approach does not yield an equivalent ductile behavior when compared to steel-reinforcing bar-reinforced concrete members, resulting in a lower strength reduction, ϕ, value of 0.65. In this regard, using FRP-reinforcing bar-reinforced ultra-high-performance concrete (UHPC) members offers a novel solution, providing high strength, stiffness, ductility, and corrosion-resistant characteristics. UHPC has a very low water-to-cementitious materials ratio (0.18 to 0.25), which results in dense particle packing. This very dense microstructure and low water ratio not only improves compressive strength but also delays liquid ingress. UHPC can be tailored to achieve exceptional compressive ductility, with a maximum usable compressive strain greater than 0.015. Unlike conventional designs where ductility is provided by steel reinforcing bars, UHPC can be used to achieve the required ductility for a flexural member, allowing FRP reinforcing bars to be designed to stay elastic. The high member ductility also justifies the use of a higher strength reduction factor, ϕ, of 0.9. This research, validated through large-scale experiments, explores this design concept by leveraging UHPC’s high compressive ductility, cracking resistance, and shear strength, along with a high quantity of noncorrosive FRP reinforcing bars. The increased amount of longitudinal reinforcement helps maintain the flexural stiffness (controlling deflection under service loads), bond strength, and shear strength of the members. Furthermore, the damage-resistant capability of UHPC and the elasticity of FRP reinforcing bars provide a structural member with a restoring force, leading to reduced residual deflection and enhanced resilience.