Investigation on concrete-PCM interface under elevated temperature: At material level and member level
Introduction
Large numbers of reinforced concrete (RC) structures deteriorate before the end of their intended service life due to various loading and environmental impacts. Repairing (or restoration) is a more suitable solution, economically and environmentally, than reconstructing such deteriorated RC structures. Overlaying is one of the efficient ways of repairing and even as strengthening (or upgrading). Various types of cementitious repair materials are available nowadays and polymer cement mortar (PCM) is one of the most durable among them and easily available in almost all parts of the world [1].
PCM-concrete interfacial bond plays an important role in achieving monolithic response of PCM repaired or strengthened members. PCM has good adhesion strength due to penetration of polymer film into pores of concrete [2], [3]. At material level, interfacial strengths in tension and shear can be evaluated by using split tensile strength test and Bi-surface shear strength test, respectively, which are commonly used methods by many researchers [4], [5]. At member level, performance of structural elements like RC beams strengthened by PCM on tension face was investigated [6], [7], [8]. Increases in flexural strength were observed with the increase in reinforcement area and can be evaluated by using conventional methods for analyzing RC beams. However, ductility and ultimate deflection reduced with the increase in strengthening reinforcement [9]. Debonding failure modes are commonly reported for overlay strengthened beams and the debonding mechanism was proved to be similar to those of steel plate or FRP strengthened beams [6], [8], [10]. Various models were presented in the literature to predict debonding in the flexural zone, shear zone and flexural shear zone. All types of debonding modes were investigated analytically and experimentally [6], [8], [11], [12], [13], [14], [15], [16]. Some models contain the function of peeling strength, which depends on both interfacial normal and shear strengths between concrete and steel plate/FRP [11], [15].
Many RC structures are exposed to daily high fluctuations of temperature, which may rises up to 60 °C in peak summer in the Middle East and some parts of North America. In some regions of the subcontinent, temperature rises annually due to rapid industrialization, increase in traffic volume and non-sustainable development and may reach 60 °C after few years [17], [18]. Such hot climates may accelerate the deterioration of RC structures [5], [17]. Temperature influences mechanical properties of concrete and PCM. Significant degradation in the compressive and tensile strengths of PCM was observed with the increase in temperature. Interfacial tensile strength of PCM-concrete composites was reduced due to degradation in the intrinsic properties of constituents and deformation of interface [5], [19]. To enhance the interfacial zone between concrete-PCM, primer, which can be affected by temperature as well, was used at interfaces to resist penetration of moisture from PCM to concrete and increase the adhesion strength. High temperature affects the bond strength of steel to concrete in RC beams and bond failure of rebar lap splices was observed by Khan et al. [20]. Degradation in the interfacial strengths at material level may cause debonding mode of failure at member level, at high temperature. Therefore, it must be ensured that repaired RC structures at high temperature would maintain the required performance over intended service life, both at material level and member level.
With such background, detailed experiments were designed to evaluate the behavior of RC beam with or without PCM overlay strengthening under exposure condition of 20, 40 and 60 °C, respectively. The selected temperature range may cover most regions in the world during summer. Along with experimentation, analytical works were also conducted to verify experimental observations and for applicability in practical design. Outcomes of this study will help to establish guidelines for designing of repair of RC structures in such conditions. Prediction formulas were proposed for interfacial strengths in shear and tension and verified with current experimental observations along with previous data under the selected temperature range. At member level, a debonding model was proposed by incorporating interfacial shear strength and was used in the truss analogy approach for the strengthened beams. Finally ultimate shear load and failure modes were predicted and found to be in good agreement with the experimental observations at all temperature levels.
Section snippets
Exposure conditions and testing procedures
Three temperature levels were selected to cover hot climates in the most regions of the world. Exposure and testing conditions were maintained at 20, 40 and 60 °C temperatures while relative humidity was maintained at 60%. All specimens were exposed to the target temperature level and relative humidity for more than 16 h until temperature gradient became insignificant and then tested under condition similar to the exposure condition.
Four types of testing were performed on concrete, PCM and
Compressive, tensile and shear strength
The compressive strengths of both specimens, concrete and PCM, were evaluated on cubical specimens and presented in Table 6, where compressive strength was expressed as the average of three specimens and standard deviation is shown in parenthesis under the corresponding data. The effect of temperature on the concrete compressive strength was observed to be insignificant, whereas large amount of reduction was observed in the compressive strength of PCM.
Similar trend was observed in the tensile
Failure modes and failure loads
The observed failure modes of strengthened beams were flexural, shear, flexural-shear failure and debonding mode of failure. All types of observed modes are presented in Fig. 9 and also summarized in Fig. 10. From Fig. 10 it is also clear that primer have insignificant effect on failure modes. At 20 °C temperature, the failure mode of strengthened beams up to reinforcement area of 112.20 mm2 (4∅6), was flexural failure like in the control “Con” specimen. With further increase in area of
Conclusions
In the present study, influence of temperature on interfacial bonding between concrete and PCM as overlay was investigated in both material and member levels. Selected temperature ranges cover the real warm climatic conditions for most of the regions having abundant RC structures in the world. Three sets of beams were prepared and tested in four-point bending at temperature of 20, 40 and 60 °C. In each set of beam, one unstrengthened RC beam and 10 strengthened beams (5 with primer case and 5
Acknowledgment
The financial supports from the Grant-in-Aid for Scientific Research (A) of Japan Society of Promotion of Science (No. 26249064), the Natural Science Foundation of China (Grant Nos. 51308494) and NEXCO Group Companies’ Support Fund to Disaster Prevention Measures on Expressways are greatly appreciated. The authors are also grateful to DENKA for providing polymer cement mortar and concerned information.
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