1. Introduction
Comprehensive methods can be employed to increase the bear capacity of reinforced concrete (RC) structures, such as structural retrofitting [
1,
2,
3,
4] and the use of high-performance materials [
5,
6,
7,
8,
9,
10,
11,
12,
13,
14,
15]. Recently, a newly arising bolted side-plating (BSP) technique, i.e., attaching steel plates to the side faces of reinforced concrete (RC) beams using anchor bolts, has become increasingly popular all over the world [
16,
17,
18,
19]. The BSP technique not only has the advantages such as minimal space occupation and easy installation, but also avoids serious debonding and peeling failures that are common in RC beams strengthened by adhesively bonded steel plates [
20] or fibre-reinforced polymers (FRP) [
21,
22,
23,
24,
25]. A variety of theoretical and experimental studies have been conducted to investigate the strengthening effects and mechanical behaviours of BSP beams. Oehlers et al. [
26] established the relationship between the degree of transverse partial interaction and the properties of anchor bolts. Based on this model, Nguyen et al. [
27] derived the relationship between longitudinal and transverse partial interactions as well as the distribution of slip strain, slip and neutral-axis separation. Su et al. [
28,
29,
30] conducted experimental and numerical studies on BSP beams, which showed that even small slips on the steel-concrete interface could significantly affect the overall response. Su and Siu [
31,
32,
33] proposed numerical procedures for predicting the nonlinear load-deformation response of bolt groups as well as the longitudinal and transverse slip in BSP beams. Li et al. [
34,
35,
36,
37,
38,
39,
40] conducted comprehensive experimental, numerical and theoretical studies on the BSP technique, and found that the flexural strength, shear strength, stiffness and ductility of RC beams could be effectively improved. All studies show that the BSP method is feasible and effective to rehabilitate RC beams in existing buildings and infrastructures.
However, most existing studies are focused on the flexural performance of BSP beams, studies on the shear behaviour are still limited: Barnes et al. [
41] compared the shear strengthening effect of fixing steel plates to the side faces of RC beams by using adhesive bonding or bolting. Su and Zhu [
28] investigated the shear performance of BSP-strengthened coupling beams and found that small uneven slips on the steel–concrete interface would cause serious loss in shear strengthening effect. Su and Cheng [
16] investigated the shear performance of coupling beams retrofitted by bolted steel plates with or without buckling restraining device, and considerable improvement in deformability and energy dissipation were found. Li et al. [
42,
43] conducted an experimental study on BSP beams to investigate the shear strengthening effect under the room temperature and proposed a simplified analytical model based on the force equilibrium and deformation compatibility of the beam segment in the shear span.
Furthermore, available research outcomes in the literature have mainly focused on the mechanical behaviour of BSP beams under room temperature; the fire resistance and the post-fire residual capacities have not yet been studied comprehensively. However, building fire is one of the most frequent and threatening disasters for building structures. For instance, the total number of fires reported in UK was 212,500 in the year 2013, 19% of which happened in dwelling buildings [
44]. Moreover, Jiang and Li [
45,
46] have found that the fire exposure can influence the bearing capacity, the stiffness, the ductility factor, and the energy dissipation capacity of the RC structures to a large extent. Thus, the fire resistance of the retrofitted structures should always be paid attention to. Compared to conventional RC beams, the influence of fire on BSP beams is far more complex, which might affect concrete, steel plates, steel bars, anchor bolts, and adhesive mortar for bolt anchorage. Therefore, the fire resistance and post-fire performance of BSP beams are highly dependent on the coupling effects of all the components. Arioz [
47] and Kodur [
48] studied the effect of fire on the mechanical properties of concrete, such as density, compressive strength, and modulus of elasticity. Kadhum [
49] found that with increasing temperature, the strength, ductility and stiffness of concrete were progressively reduced, and the crack width increased. Topcu and Unluoglu [
50,
51,
52,
53] found that the post-fire yield and ultimate strengths of rebars decreased when the temperature goes up. Ergun and Kurklu [
54] found that the residual bond strength between reinforcing bar and concrete decreased with increasing temperature. Since the steel plates in BSP beams are directly exposed to fire, the effect of fire on steel material and the possible fire protection measures are of the greatest importance. Li et al. [
55] conducted experimental studies on the high-temperature properties of two kinds of constructional steel widely used in China. Miamis [
56], Kwon and Shin [
57] found that the ductility of steel increased with elevated temperature, but the yield and tensile strengths, modulus of elasticity, and elongation decreased with the elevated temperature. Santiago et al. [
58] proposed that the yield strength of anchor bolts decreased and the ductility increased significantly after the temperature was higher than 500 °C. Kirby [
59] found that the behaviour of high-strength Grade 8.8 bolts highlighted a marked loss in the ultimate capacity at elevated temperatures between 300 and 700 °C.
Since the bolting connection has a dominant effect in the performance of BSP beams, the adhesive mortar used for bolt anchorage is of great importance for the BSP technique. Banea and Sousa [
60] found that the ultimate tensile stress of epoxy adhesive decreased linearly as the temperature keeps increasing. The glass transition temperature of the epoxy adhesive is approximately 155 °C, which reflects the poor fire resistance of epoxy adhesive. On the other hand, magnesium oxychloride cement (MOC) is widely used in fireproof materials and thermal insulation materials due to its advantageous characteristics such as high fire resistance, low thermal conductivity, flame retardant activity and good bonding performance [
61]. The mechanical properties, compositions, and manufacture of the MOC have also been studied comprehensively [
62,
63,
64]. In view of the admirable fire-resisting properties of the MOC, it can be selected as the anchor adhesive mortar of the BSP beams to prevent the premature anchorage failure during and after fire exposure.
Existing researches on the behaviour of BSP beams mainly concentrate on their flexural performance at ambient temperature, and very little work has been done on the shear performance, especially for BSP beams after fire. In light of this situation, an experimental study was conducted including fire tests and post-fire shear tests for several BSP beams in the present study, which aims to investigate the post-fire residual shear performance of BSP beams. Thus, the influence of elevated temperature, types of anchor adhesive, and the depth of steel plates on the failure mode, cracking load, shear capacity, stiffness, ductility, strain development, and relative slips on the plate-RC interface were investigated in detail.
4. Conclusions
The shear performance of BSP beams and their post-fire performance have been investigated based on four-point-bending shear tests after exposure to fires. The main findings are summarized as follows:
During the fire test, the temperature of concrete decreased with the distance from the beam bottom and side surfaces, as well as the embedment depth in the same hole. Three representative failure modes were observed in the post-fire test: the brittle shear-tension failure for specimen CTRL, the mid-span flexural failure for P3B1-MOCT, and the shear-compression failure for the rest specimens.
The load of the first flexural crack ranged from 0.06 Pu to 0.14 Pu for all the specimens, which indicates that the flexural crack load was independent of the depth of the bolted steel plate, the temperature, and the category of anchor adhesive mortar. The first diagonal crack in the BSP specimens appeared later than that in CTRL, which was controlled by the plate depth and bolt spacing, but irrelevant to the type of anchor adhesive and temperature environment.
The shear capacity, stiffness and ductility of the BSP beams were greater than CTRL. The anchor adhesive of MOC is more efficient than HIT-RE500 for enhancing the shear capacity and ductility, and a smaller improvement in stiffness. Moreover, although the stiffness of the post-fire BSP beams is lower than that of CTRL, BSP beams exhibit a higher ductility and shear capacities. Thus, increasing the depth of the steel plates could effectively improve the residual shear capacity of BSP beams after exposing to fire.
The tensile and compressive strains of the bolted steel plates increased gradually along the beam axis from the support to the mid-span, and the strains at the lower edge were higher than that at the upper edge. In addition, the longitudinal reinforcement strains of the post-fire BSP beams were greater than that in the unfired BSP beams. Thus, the failure modes changed from shear failure to mid-span flexural failure.
The longitudinal and transverse relative slips proved the uncoordinated deformation between the bolted steel plates and the RC beam, as well as the lagged deformation of the steel plates.