Shear Strength and Shear-friction Strength of RC Walls with Grade 550 MPa Reinforcing Bars

Abstract

In the construction of nuclear power plants, a number of large diameter reinforcing bars are used in massive reinforced concrete walls, which significantly affects the constructability and economical efficiency. After the recent flurry of earthquakes, the structural safety requirements for nuclear power plants have increased, which has further increased the number of bars required for reinforced concrete walls. Thus, to enhance the constructability and economical efficiency, and to satisfy the increased safety requirement of nuclear power plant walls, the use of high-strength reinforcing bars needs to be considered. However, the yield strength of shear reinforcement is limited in current design codes, to ensure yielding of shear reinforcement before shear failure, and to control the width of potential diagonal shear cracks. In this study, an extensive range of experimental studies was performed, to provide evidence for use of high-strength reinforcement in RC walls, potentially leading to modification of the current design provision of shear reinforcement for reinforced concrete walls. Because of the high safety requirement in nuclear power plant walls, the shear reinforcement ratio is generally close to the permissible maximum shear reinforcement ratio specified by the current design codes. Considering the unconservative safety margin for heavily reinforced concrete members, the validity of the maximum shear reinforcement ratio needs to be verified, when higher-strength reinforcing bars are used for shear reinforcement. Walls with aspect ratios of 1.0, 2.0, and 0.5 were tested under cyclic lateral loading to investigate the effect of Grade 550 MPa reinforcing bars on the shear strength. The test parameters included grade of shear reinforcement, shear reinforcement ratio, failure mode, concrete compressive strength, shape of wall cross-section, and the presence of boundary confinement hoops. The ratios of the test shear strength to the prediction by ACI 349 (i.e., strength ratios) were 1.45-2.61 and 1.11-1.74 for the general and seismic provisions, respectively. The test results of walls with Grade 550 MPa re-bars were comparable to those of walls with Grade 420 MPa re-bars for several evaluations: failure mode, strength ratio, strains of shear reinforcement, shear deformation, deformation contribution, average crack width, and energy dissipation. In addition to the limited yield strength of shear reinforcement, current design codes require minimum shear reinforcement to assure the safety of reinforced concrete walls against brittle shear failure. In actual design of walls, the amount and placement of shear reinforcement are often governed by the minimum shear reinforcement ratio, which significantly affects the economical efficiency and constructability. This is a probable case even in nuclear power plant walls, depending on various design condtions. Therefore, when high-strength reinforcing bars are used for shear reinforcement, it should be considered whether the minimum shear reinforcement ratio required by current design codes may be decreased. To investigate the effect of high-strength re-bars on the shear strength and minimum shear reinforcement, slender walls (aspect ratio of 2.5) with Grade 500 MPa shear reinforcement were tested under cyclic lateral loading. The test parameters were failure mode, the grade and ratio of shear reinforcement, concrete compressive strength, and axial compression. The test results of walls with Grade 500 MPa re-bars were directly compared with those of walls with Grade 400 MPa re-bars, which is currently permitted in the current design codes. The grade of shear reinforcement did not significantly affect failiure mode, strength ratio, strains of shear reinforcement, deformation contribution, average crack width, and energy dissipation and lateral stiffness. Under repeated cyclic loading, squat walls with aspect ratio smaller than 0.5, which are commonly used for nuclear power plants, are vulnerable to shear sliding at a construction joint. Thus, generally in the design of squat walls, the number of vertical re-bars is determined by shear sliding rather than other failure mechanisms such as flexural yielding and shear failure. Thus, for the use of high-strength reinforcing bars for nuclear power plant walls, it is crucial to investigate the effect of high-strength re-bars on the shear-friction strength. Low-rise walls were tested to verify the applicability of Grade 550 MPa reinforcing bars to the design of shear sliding. The test parameters were the grade of re-bars, aspect ratio, reinforcement ratio, and surface condition of the construction joint, axial compression, presence of additional shear-friction reinforcement. The test results showed that the specimens were susceptible to sliding failure and the stress of Grade 550 MPa shear-friction bars was not reached to the yield strength. Particularly, the shear-friction strengths under cyclic loading were smaller than those subjected to monotonic loading reported in previous studies. The applicability of current design methods was evaluated for the shear-friction design of walls with Grade 550 MPa bars. The design equations predicted by fib Model Code 2010 and Eurocode 8, which consider both the shear-friction and dowel action, were also used for the evaluation. Based on the test results of specimens with significant sliding deformation, dowel resistance-slip relationship of reinforced concrete squat walls with a construction joint was proposed. As such, the seismic resistance of reinforced concrete walls varies according to many parameters: aspect ratio, failure mode, reinforcement ratio, grade of reinforcing bars, concrete compressive strength, shape of wall cross-section, and presence of boundary elements. Although previous design equations have been used to predict the shear strength and shear-friction strength of RC walls, the strength predctions show huge scatter. To accurately predict failure mechanism and shear capacity of reinforced concrete shear walls regardless of material properties such as grade of reinforcing bars (and corresponding reinforcement ratio) or concrete compressive strength, existing strain-based methods were adopted and proposed: 1) diagonal tension failure, 2) web crushing failure, and 3) sliding failure mechanisms, which can be commonly observed in reinforced concrete walls. The load-drift ratio relationships of test specimens in the present study were compared with those predicted by the strain-based method. A set of database of reinforced concrete walls was collected including previous test results reported by others, as well as the present test results, and was used for vertification of the strain-based strength prediction. The predictions by the strain-based method yield most uniform results among existing equations, with a minimum value of coefficient of variance. The predictions were not affected by effective re-bar strength, concrete compressive strength, and axial load ratio.