International Concrete Abstracts Portal

In performance-based seismic design, buckling and fracture of longitudinal steel in reinforced concrete columns are damage limit states that may be considered for damage control and near collapse, respectively. This study evaluates the progression of buckling instability, which eventually leads to bar fracture, based on bending strains measured in buckled bars of cyclic quasi-static column tests. Buckling-induced bending strains are calculated with bare bar fiber models and experimental buckled shapes of longitudinal reinforcement in the column data set. This study proposes an empirical equation that calculates the buckling-induced bending strain based on column displacement ductility, low-cycle fatigue, and column design parameters for grade 60 steel. This study also identifies the buckling-induced bending strains that trigger transverse steel yielding, visual bar buckling, and brittle bar fracture.

The fatigue tension-softening constitutive model of concrete is a crucial material property for the nonlinear analysis of fatigue crack propagation processes. However, existing models are derived and calibrated based on concrete with a single strength grade, which limits their applicability. To address this issue, this study develops a fatigue tension-softening constitutive model applicable to normal-strength grade concrete. First, based on the fracture test results of three-point bending (TPB) beams, the relationship between the external work and the energy consumed for fatigue crack propagation is established using the principle of energy conservation. The second-order derivative of this relationship is then used to determine the cohesive stress under fatigue loading. It is found that the cohesive stress decreases with the increase in both fatigue crack opening displacement and the number of fatigue cycles. For a given fatigue load level, the higher the tensile strength of the concrete, the slower the degradation rate of cohesive stress. Subsequently, by introducing the number of fatigue cycles, crack opening displacement, and tensile strength as key parameters, the fatigue tension-softening constitutive model for normal-strength concrete is formulated. Finally, the model is validated by using it to predict the fatigue crack propagation length, fatigue life, and stress intensity factor at the fatigue failure of TPB beams and comparing these predictions with experimental results. The model proposed in this study provides essential parameters for evaluating the fatigue fracture performance of concrete.

This paper presents the implications of creep-fatigue interactions for the long-term behavior of bulb-tee bridge girders prestressed with either steel strands or carbon fiber-reinforced polymer (CFRP) tendons. A large amount of weigh-in-motion data incorporating 194 million vehicles are classified to realistically represent live loads. Computational simulations are conducted as per the engagement of discrete autonomous entities in line with time- dependent material models. In general, the properties of CFRP tendons vary insignificantly over 100 years; however, the stress range of CFRP responds to fatigue cycles. Regarding prestress losses, the conventional method with initial material properties renders conservative predictions relative to refined approaches considering time-varying properties. The creep and fatigue effects alter the post-yield and post-cracking responses of steel- and CFRP-prestressed girders, respectively. From deformational capability standpoints, steel-prestressed girders are more vulnerable to fatigue in comparison with CFRP-prestressed ones. It is recommended that the fatigue truck and the compression limit of published specifications be updated to accommodate the ramifications of contemporary traffic loadings. Although the operational reliability of both girder types is satisfactory, CFRP-prestressed girders outperform their steel counterparts in terms of fatigue safety. Technical findings are integrated to propose design recommendations.

ACI 318 permits the use of mechanical couplers for Grade 60 (420 MPa) bars in hinge regions, but not for higher-grade bars. This restriction was introduced due to limited testing of mechanical couplers under inelastic strain demands and is hindering the use of higher-grade bars in seismic regions. Eleven mechanical couplers splicing Grade 80 (550 MPa) bars through varying connection details were tested in a uniaxial testing machine to evaluate their performance compared to bare bars under reversed cyclic inelastic strain demands, akin to those experienced in hinge regions of special seismic systems. The low-cycle fatigue life of coupled subassemblies is compared to those of the bare bars tested under the same loading protocol. Results indicate that some coupled bars can have equivalent fatigue life to the bare bars, while others can have substantially reduced fatigue life. A qualification test is proposed to qualify mechanical splices for use in seismic hinge regions of special concrete systems.

A comprehensive laboratory testing program, field-testing program, numerical analysis, and life-cycle cost analysis were conducted to evaluate the beneficial effects of incorporating shrinkage-reducing admixture (SRA), polymeric microfibers (PMFs), and optimized aggregate gradation (OAG) into internally cured concrete (ICC) mixtures for rigid pavement applications. Results from the laboratory program indicate that all the ICC mixtures outperformed the standard concrete (SC) mixture. All the ICC mixtures showed a decrease in drying shrinkage compared to the SC mixture. Based on the laboratory program, three ICC mixtures and one SC mixture were selected for the full-scale test and subjected to a heavy vehicle simulator for accelerated fatigue testing. Extensive testing and analysis have shown that ICC mixtures incorporating SRA, PMFs, and OAG can be beneficially used in pavement applications to achieve increased pavement life.