Elsevier

Engineering Structures

Volume 225, 15 December 2020, 111305
Engineering Structures

Effect of elevated temperatures on the shear-friction behaviour of concrete: Experimental and analytical study

https://doi.org/10.1016/j.engstruct.2020.111305Get rights and content

Highlights

  • Shear-friction behaviour of concrete is investigated after elevated temperatures.

  • Decrease in shear strength and increase in crack deformation was observed.

  • Two methods for the prediction of residual shear strength were proposed.

Abstract

Thirty-two uncracked shear-friction specimens were tested to investigate the effect of elevated temperatures on the shear-friction behaviour of concrete. Exposure temperature and the shear reinforcement were the principal variables. Specimens were exposed to high temperatures of 350 °C, 550 °C, and 750 °C in a programmable insulated closed electric furnace and were cooled down to room temperature before testing. Shear-friction specimens were also tested at the ambient temperature. Exposure of shear-friction specimens to high temperatures resulted in the decrease of shear strength of concrete. An increase in shear reinforcement reduced the loss in shear strength when specimens were subjected to high temperatures. The post-ultimate strength of the specimens was not reduced up to an exposure temperature of 550 °C. After experiencing high temperatures, crack slip and crack width at the shear plane increased significantly at all the load levels. Two simple approaches, modified Zia failure analysis and a simple calculation method (SCM) were also suggested for the prediction of shear strength of concrete after high temperatures. Modified Zia failure analysis provided precise estimates of shear strength for all the temperature levels. SCM was found to be precise for shear strength predictions up to an exposure temperature of 550 °C. For an exposure temperature of 750 °C, SCM was found to be unconservative by 20–30%.

Introduction

Shear-friction is a mechanism by which shear is transmitted through an interface between two members that can slip relative to one another. The interface on which the shear acts is called the shear plane or slip plane. It may be noted that in this case, the shear failure is constrained to occur along a definite plane. In reinforced concrete beams, the location of the critical inclined crack across which shear has to be transferred is not fixed. Some common examples of these interfaces might be a vertical plane at the junction of corbel and column, the junction of a pre-cast girder and a cast-in-place slab, bearing in a ledger beam and construction joints. Such interfaces must be designed prudently so that their shear capacity is greater than the diagonal-tension capacity of the adjoining members, which is achieved by providing reinforcement, generally perpendicular to the shear plane [1], [2], [3]. Numerous studies [4], [5], [6], [7], [8], [9], [10], [11], [12], [13], [14], [15], [16], [17], [18], [19], [20], [21], [22], [23], [24], [25], [26], [27], [28], [29] performed in the past six decades on shear-friction of concrete shows that the interface of concrete studied was either monolithic or cold jointed. Monolithic interface simulates the junction of a corbel and column or the bearing of a ledger beam. Cold-jointed interface simulates construction joints in a shear wall or interface of the pre-cast girder and cast-in-place slab. A brief overview of the studies performed on the shear-friction behaviour of concrete is given in Table 1.

When concrete comes in contact with high temperatures, various physical and chemical changes take place, which results in the degradation of its strength and stiffness. Temperature increase causes loss of free moisture followed by loss of physically adsorbed water and, finally, chemically bound water from the hydrated products. Latter is responsible for the significant loss in strength. Also, heating of concrete leads to its microcracking due to shrinkage of cement paste and expansion of aggregates, hence causing a further loss in strength [30], [31], [32], [33], [34]. Also, the properties of reinforcing steel reduce when the temperature increases beyond 500 °C [35], [36], [37], [38]. Table 1 indicates that studies on shear-friction of concrete at ambient temperature conditions are adequate, and most of the parameters have been studied. Few studies [18], [22], [28] have addressed the influence of high temperatures on the shear-friction of concrete. Smith et al. (2011) [18] tested normal-strength concrete specimens over a temperature range of 20–622 °C. The concrete compressive strength and restraining stress across the interface were kept constant. It was found that an increase in temperature leads to a decrease in ultimate strength and stiffness of the shear plane. Xiao et al. (2014) [22] tested high-strength-concrete specimens with a constant restraint provided across the shear plane over a temperature range of 20–800 °C. Specimens were cast with concrete having compressive strengths of 64.7 and 94 MPa. The shear strength was found to be decreased, while the crack deformation increased with an increase in exposure temperature. The loss in shear strength was found to be more for the specimens cast with concrete of higher compressive strength. Ahmad et al. (2018) [28] tested normal-strength concrete for shear-friction with a constant restraining stress after exposure to 250 °C and 500 °C. The findings of the experimental investigation were similar to Smith et al. (2011). A simplified procedure was also developed for the computation of the shear strength of concrete exposed to high temperatures.

The restraint provided at the shear plane was kept constant in all three studies. RILEM TC -200 HTC [39] suggests that accidental fires may expose structures up to 750 °C. Residual compressive strength of concrete severely decreases when exposure temperature increases over 600 °C, which may also result in an abrupt reduction of shear-friction in concrete. Therefore, this study is intended to have an insight into shear-friction behaviour concrete with different confinements up to an exposure temperature of 750 °C. Hofbeck et al. (1969) [4] proposed a graphical method for the estimation of shear strength in concrete at ambient temperature, which was based on the failure envelope constructed by Zia [40]. The method is often called as the modified Zia failure analysis. Changes in the modified Zia failure analysis were also suggested to make its use for the estimation of shear strength of concrete after elevated temperatures. Another approach is also suggested in which an ambient temperature model incorporating residual strength of concrete and steel is used for the prediction of residual shear strength of concrete after elevated temperature.

Section snippets

Research significance

It is important to evaluate the residual shear-friction in concrete after high temperatures so that the strengthening and retrofitting of a structure may be performed after an event of a fire. In this study, the results of an experimental investigation on the shear-friction behaviour of concrete after exposure to high temperatures are reported. Necessary changes in modified Zia failure analysis are also suggested to make its use possible for the estimation of shear- friction in concrete exposed

Materials used

43 grade (min. avg. compressive strength of three mortar cubes was 43 MPa) Portland cement, complying with IS 8112-2013 [41], was used in the experimental investigation. Thermo-mechanically treated bars of diameter 12 mm and 8 mm were used as the reinforcement. Reinforcing bars were tested in tension using a displacement controlled UTM as per IS 1608:2005 [42]. The yield and ultimate strength of reinforcing bars were found to be 567.2 MPa and 648 MPa, respectively. Coarse sand obtained from a

Thermal behaviour

The heating-cooling curves for furnace and different locations in the specimens are illustrated in Fig. 5. The temperature of thermocouples embedded in specimens increased at a slower pace than the furnace owing to the lower thermal conductivity of the concrete. It may be observed that the temperature of the thermocouple T1 increased earlier than that of T2 since T1 is nearer to the upper heating face. The temperature of the thermocouple T2 increased earlier than that of T3 because T2 is closer

Failure theories

Failure theories are of great importance for engineering applications and the effective utilisation of the materials. Mainly for the design of structures, a reliable strength prediction method for various combinations of stresses is always essential. For the estimation of concrete strength under combined stresses, various theories of failure are suggested in the literature for which failure envelopes are constructed [40], [54], [55], [56]. Failure will occur if a Mohr’s circle representing a

Modified Zia analysis

The method explained in Section 5.1 is used to draw the failure envelopes for different temperatures using the temperature-dependent properties of concrete. The temperature-dependent properties (f′cT, ftT, and φT) were obtained by using the methods explained in 5.2 Residual compressive and tensile strength of concrete after elevated temperatures, 5.3 Angle of internal friction of concrete (ф. Estimated (v, ρfy) relationships for different temperature levels and the corresponding experimental

Conclusions

From the experimental and analytical studies performed in this paper, the following conclusions can be drawn:

  • 1.

    The ultimate shear strength of the specimens was found to be decreased as the exposure temperature was increased. An increase in transverse reinforcement up to 5.5 MPa reduced the shear strength loss at all the temperature levels.

  • 2.

    Exposure temperature up to 550 °C did not show any reduction in the post-ultimate strength of the specimens. Post-ultimate strength of the specimens was found

CRediT authorship contribution statement

Subhan Ahmad: Conceptualization, Methodology, Formal analysis, Investigation, Writing - original draft. Pradeep Bhargava: Resources, Supervision, Funding acquisition. Ajay Chourasia: Visualization. Asif Usmani: Writing - review & editing.

Declaration of Competing Interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

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