Reinforced high strength concrete beams under shear

• Main Author: Lim, Darren Tze Yang
• Other Authors: Tan Kang Hai
• Format: Theses and Dissertations
• Language: English
• Published: 2019
• Subjects: DRNTU::Engineering::Civil engineering
• Online Access: https://hdl.handle.net/10356/90124; http://hdl.handle.net/10220/48381
• Institution: Nanyang Technological University

This thesis presents a discussion on the shear behaviour of reinforced high strength concrete beams and new proposed methods to predict the shear strength of reinforced normal and high strength concrete beams. High strength concrete is an advanced construction material with increased tensile strength and modulus of elasticity compared to normal strength concrete. Concrete structural elements with higher tensile strength for concrete material could delay the formation of the first crack and increase the cracking load. An experimental programme consisting of eleven full-scale beam specimens made of high strength concrete was conducted. The objectives of the experimental programme were: (1) to investigate the influence of effective depth on large high strength concrete beams; (2) to investigate the influence of maximum aggregate size in high strength concrete beams; (3) to study the influence of shear reinforcement ratio on size effect in high strength concrete beams; (4) to find out the minimum shear reinforcement ratio required to eliminate size effect on the shear strength, should shear reinforcement be able to do so; (5) to evaluate the efficacy of existing codes, such as ACI 318-14 (2014) and Eurocode 2 (2004), in predicting the shear strength of high strength concrete beams with varying depths; and (6) to propose new methods to predict the shear strength of large sized high strength concrete beams with and without shear reinforcement. Three parameters were studied in the eleven beam specimens to achieve the above-mentioned objectives. These three parameters were: (1) the depth of the beam specimens; (2) the maximum aggregate size; and (3) the shear reinforcement ratio. The depths of the beam specimens ranged from 450 to 1800 mm. The maximum aggregate sizes used in the concrete material varied between 10 and 20 mm. The shear reinforcement ratio provided in the two shear spans of each beam specimen was different. Four shear reinforcement ratios were provided for each beam depth (0%, 0.13%, 0.20%, and 0.30%). Shear spans with shear reinforcement ratios of 0% and 0.13% formed one beam specimen, and shear spans with shear reinforcement ratios of 0.20% and 0.30% formed another beam specimen. The shear span with lesser shear reinforcement was weaker and failed first. This failed shear span was strengthened to resume the test until failure of the other shear span. This arrangement enabled the collection of data from twenty-two shear span specimens in the eleven beam specimens. The specimens with no shear reinforcement and depths of 1350 and 1800 mm were among the largest reinforced high strength concrete beams tested to date. Together with the results of smaller specimens (with no shear reinforcement), it was shown that effective depth can influence the shear strength of high strength concrete beams in the same manner as normal strength concrete beams. The results from this study indicated that aggregate size plays a less important role in high strength concrete compared to normal strength concrete. The experimental results of the specimens with shear reinforcement showed that while shear reinforcement increases the shear strength of the specimen, it does not eliminate size effect between specimens of different depths. Yet, shear reinforcement could mitigate the influence of size effect between two specimens of different depths. The experimental results were used to evaluate the accuracy of the shear strength calculated from code equations in ACI 318-14 and Eurocode 2. Two new approaches to predict shear strength of beams with shear reinforcement were proposed. The first approach is an analytical truss method, based on softened truss model, with proposed inclusion of size effect and dowel action. These two elements were not considered in Hsu and Mo’s method (2010). Without considering size effect, the predicted shear strength from the truss model would be un-conservative for large-size concrete beams. Without considering the shear strength contribution from longitudinal reinforcement, the predicted shear strength would be very conservative for concrete beams with high amount of longitudinal reinforcement. The second approach is a simplified truss method that predicts shear strength in the form of Vn = Vc + Vs. The motivation to develop a second approach is to obtain the inclination angle of the diagonal truss similar to that obtained from the first approach, but with shorter computation procedures. Using the inclination angle of the diagonal truss, the shear strength contribution from the shear reinforcement can be obtained. In addition, an empirical equation is proposed as part of second approach to calculate the shear strength contribution from concrete, and added to the contribution from shear reinforcement to obtain the total shear strength. The novelty of the simplified truss method (second approach) is the ability to obtain both the inclination angle of the diagonal truss and shear strength of the beam without going through the iterative procedure of the first approach. The two proposed approaches were verified using the experimental results and existing published test data. The results and subsequent discussions indicated that the two approaches for predicting shear strength of a shear reinforced beam can provide reasonable accurate predictions, and at the same time is simple to apply. Keywords: high strength concrete; shear; reinforced concrete beam; size effect; shear reinforcement; aggregate size.