International Concrete Abstracts Portal

Three large-scale reinforced concrete rectangular slender structural walls were subjected to cyclic displacement demands to establish whether, and under what conditions, mechanical splices can be used with Grade 100 (690) bars where yielding is expected. These tests were conducted because ACI 318-19 prohibits both lap splices and mechanical splices for high-strength longitudinal reinforcement (Grade 80 (550) and higher) in special structural walls where yielding is expected. Three mechanical splices were used that differed in connection type (one type per wall) and overall splice length. The mechanical splices were all placed starting 2 in. (50 mm) from the wall base. Mechanical splices satisfying the specified minimum tensile strength criterion of ACI 318-19 Type 2 mechanical splices resulted in better wall behavior than reported for lap splices, but satisfying Type 2 requirements alone did not prevent bar fractures at the mechanical splice. Thus, Type 2 mechanical splice requirements are not recommended as the sole qualification criteria where yielding is expected. Test results also showed that mechanical splices with a strength not less than the actual bar tensile strength, such that bars systematically fail in direct tension tests away from the splice and therefore develop their actual uniform elongation, perform well, and are recommended for use where yielding is expected in special structural walls.

The reduction in the shear capacity using recycled coarse aggregate made from crushed concrete is evaluated in terms of tensile cracking and fracture surface characteristics. An experimental investigation is presented into the fracture and flexure-shear behaviors of recycled aggregate concrete (RAC). Replacing natural aggregate in concrete proportioned for 30 MPa compressive strength with recycled coarse aggregate results in lower compressive and tensile strengths. The tensile fracture surface characteristics vary between RAC and natural aggregate concrete (NAC). While the surface area created in the tensile fracture of RAC is larger than that of NAC, the fracture surface profile in RAC has a smaller roughness than that of NAC. In the flexure-shear response of reinforced concrete beams, the dilatancy determined from the slip and crack opening displacements measured across the shear crack is smaller in RAC than NAC. The failure in the reinforced beam is due to the frictional stress transfer loss across the primary shear crack. There is a larger decrease in the shear capacity with the use of RAC than indicated by the reduction in compressive strength. The reduced shear capacity of reinforced RAC is due to the combined influences of reduced tensile strength and crack surface roughness. The design provisions require calibration for crack surface roughness when using RAC in structural applications.

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 study investigates the behavior of recycled steel fibers (RSFs) recovered from waste tires and industrial hooked-end steel fibers (ISF) in two single and hybrid reinforcement types with different volume content, incorporating microstructural and macrostructural analyses. Scanning electron microscopy (SEM) is used to study the microstructure and fractures, focusing on crack initiation in the fiber interface transition zone (FITZ). The macrostructural analysis involves using digital image correlation (DIC) software, Ncorr, to analyze the split tensile behavior of plain and fiber reinforced concrete (FRC) specimens, calculating strain distribution and investigating crack initiation and propagation. The SEM study reveals that, due to the presence of hooked ends, industrial fibers promoted improved mechanical interlocking; created anchors within the matrix; added frictional resistance during crack propagation; significantly improved load transfer; and had better bonding, crack bridging, and crack deflection than recycled fibers. RSFs significantly delay crack initiation and enhance strength in the pre-peak zone. The study suggests hybridizing recycled fibers from automobile tires with industrial fibers as an optimum strategy for improving tensile performance and using environmentally friendly materials in FRC.

The limitations of known fracture mechanics (FM) models are noted, and to overcome them, a specific FM model was proposed based on modeling the stress distribution only along the crack fracture process zone (FPZ), with elastic concrete behavior out of the FPZ. This peculiar stress distribution was called physically verisimilar stress (PVS), and it was accepted as the basis of the proposed PVS model, which used three material parameters: maximum stress, intrastructural linear size a, and dimensionless value n, taking into account the plastic properties of the material. The relationships for determining the PVS model parameters were suggested for concrete. Strength problems were solved by the modified method of sections, in which the PVS was applied along the FPZ. The FM model based on PVS and the modified method of sections led to an acceptable in-practice method of strength design, which was considerably simpler than the known phase-field method. The proposed PVS model and design method allow for the prediction of the cracks development, considering their stable growth up to the critical (ultimate) values of the crack length and load. The unstable cracks propagation is considered also. The paper provides examples of designs and shows sufficient theoretical strength relative to the experimental one.