SHEAR STRENGTH OF REINFORCED CONCRETE EXPOSED TO ELEVATED TEMPERATURES

Abstract

Most of the concrete structures are subject to ambient temperature conditions. However, there are some crucial situations where these structures are exposed to elevated temperatures. Examples of these situations are building fires, industrial applications where concrete is in the vicinity of a furnace, and in new generation reactors where concrete elements are subject to long-term steady-state elevated temperatures. After the outbreak of fire, a structure may require strengthening and retrofitting to ensure its smooth functioning and the safety of the occupants. Moreover, the design of reinforced concrete elements for industrial applications requires the extent of flexure or shear capacity degradation caused by the exposure temperature. Several studies exist in the literature that reports the flexural behaviour of reinforced concrete beams after exposure to elevated temperatures. Investigations on the residual shear strength of reinforced concrete exposed to high temperatures are barely found in the literature. The present study attempts to address the two aspects of shear viz. shear transfer strength of concrete and shear strength of reinforced concrete beams exposed to elevated temperatures. Firstly, a simple analytical model for the estimation of shear transfer strength of uncracked concrete at ambient temperature was proposed. The model is based on the combination of cohesion resistance offered by concrete and frictional resistance provided by the reinforcement crossing the interface. The coefficient of friction was derived from the experimental data of one hundred forty-six cracked push-off specimens reported in the literature. Analysis of these specimens showed that a relationship exists between the coefficient of friction and the normal stress provided across the interface. Twelve uncracked push-off specimens with different normal stress across the shear plane and cast with a concrete mix of target compressive strength of 40 MPa were also tested. The derived relationship of the coefficient of friction was used to work out a relationship between cohesion and concrete compressive strength using the experimental results of twelve uncracked push-off specimens tested in the present study. The proposed model was compared with those available in the literature and the provisions of AASHTO LRFD-14 and CSA A23.3-14. The comparative analysis revealed that the proposed model produced uniform and accurate predictions over the entire range of clamping stress. Secondly, shear transfer strength of uncracked concrete after elevated temperatures of 350oC, 550oC, and 750oC was investigated. Thirty-six uncracked push-off specimens with different normal stress across the shear plane were cast with the same concrete mix as used in the Abstract ix previous exercise. Specimens were heated for different temperatures in an electric furnace and tested for STS after natural cooling. Results revealed that loss in STS was maximum for the specimens without transverse reinforcement. An increase in transverse reinforcement resulted in a decrease of loss in STS after exposure to elevated temperature. Elevated temperatures resulted in the reduction of stiffness of the shear stress-crack deformation curves and also increased the crack deformation corresponding to peak shear stress. An equation for the prediction of STS after the elevated temperature was also suggested from the experimental results and validated on the specimens reported in the literature. Thirdly, two simple approaches modified Zia failure analysis, and a simple calculation method (SCM) was suggested for the prediction of shear strength of concrete after high temperatures. Modified Zia failure analysis is a graphical method for the estimation of shear transfer strength of uncracked concrete at ambient temperature, which was based on the failure envelope constructed by Zia, which combines the two failure theories, viz. Rankine's maximum stress theory and Coulomb's internal friction theory. In the simple calculation method, an ambient temperature model was used along with the residual material properties for the prediction of shear transfer strength of uncracked concrete exposed to high temperatures. Modified Zia failure analysis provided precise estimates of shear strength for all the temperature levels. The simple calculation method was found to be precise for shear strength predictions up to an exposure temperature of 550oC. For an exposure temperature of 750oC, the simple calculation method was found to be unconservative by 20-30%. Fourthly, a database on shear strength of seven-hundred nineteen RC beams without transverse reinforcement was generated from experimental results published between 1952 and 2018. Well-known shear strength models available in the literature were summarized and evaluated through comparison with the experimental results of the generated database. It was found that models proposed by Niwa et al. (1986) and Zararis and Papadakis (2001) predicted shear strength of RC beams more accurately than the other models. The former model gives an average strength ratio of 1.05 with a coefficient of variation of 29%, while the latter yielded an average strength ratio of 1.10 with a coefficient of variation of 29%. A detailed evaluation in various ranges of parameters revealed that both the models give unconservative results of shear strength for beams with concrete compressive strength > 90 MPa. The parametric study of the two models within the individual test series showed that the model of Niwa et al. does Abstract ix not capture the size effect in RC beams very effectively. In contrast, the model of Zararis and Papadakis yielded uniform results over the whole range of experimental data. Lastly, forty-eight reinforced concrete beams without transverse reinforcement cast with a concrete mix of target compressive strength of 40 MPa were tested at ambient and after exposure to high temperatures. Beams were heated to high steady-state temperatures of 350oC, 550oC, and 750oC in an electric furnace and were tested after cooling down to ambient temperatures. Besides exposure temperature, shear-span-to-depth ratio (a/d) and percentage of longitudinal reinforcement (ρ) were the principal variables. In the beams tested at ambient and after the elevated temperature of 350oC, beam action was dominant, and failure occurred due to a diagonal tension crack. When beams were exposed to 550oC and 750oC, beam action almost vanished, and the beams failed due to the failure of a compression strut joining the loading point and the support. Shear strength loss was more significant in beams with a lower percentage of longitudinal reinforcement. Stiffness of the load-deflection curved was reduced, and deflection corresponding to the ultimate load was increased after exposure to high temperatures. A model is also proposed for the prediction of shear strength of RC slender beams after exposure to elevated temperature.