Structural performance of concrete sandwich panels under fire
Introduction
Precast concrete sandwich panels (PCSPs) are commonly used in building construction, as exterior or interior walls. They are composed of two reinforced concrete (RC) wythes, separated by an internal insulation layer [1]. The wythes are linked by shear connectors to provide a composite action [2]. This configuration is energy-efficient because of the excellent thermal insulation and structurally-efficient because of the relatively high bending stiffness and strength to weight ratios. PCSPs can be used as non-load bearing panels that are attached to the outside of the building and act as a façade or cladding. In these applications they are expected to carry wind load only. In other applications, PCSPs can be designed as load-bearing wall panels that carry gravity load (axial load) in combination with wind load [3]. In many cases, PCSPs that are made with fiber-reinforced-polymer (FRP) diagonal-bar connectors are considered because FRP has low thermal conductivity compared to steel and the diagonal shape of the connectors enforces a truss mechanism to develop within the panel to ensure a high degree of composite action. This study focuses on these types of panels when they are designed as load-bearing walls.
The structural performance of load-bearing sandwich panels under fire is critical for their safety. In fire situations, the elevated temperatures cause significant degradations of the strength and modulus of elasticity of both concrete and steel reinforcement [4], which can lead to a significant reduction of the load-bearing capacity of the panel. Due to the high thermal mass of concrete and the effect of the insulation layer, sharp temperature gradients can develop through the thickness of the panel and can result in a significant thermal bowing [5]. High temperature gradients also lead to non-uniform degradation of the material properties through the thickness, thereby shifting the neutral axes of the panel and of each wythe. As a result, the eccentricity of the axial loads can change under fire conditions and this can potentially lead to buckling failures at axial loads that are much smaller than the failure load under normal conditions. Such phenomenon was observed in solid RC walls under fire [4]. In addition, significant relative slip can potentially develop between the wythes due to the large deformation, resulting in splitting of the insulation layer and brittle failure of the FRP-bar connectors. The high temperatures also cause softening of the FRP-bar connectors [6], which can potentially lead to significant loss of the composite action with a high risk of buckling failure of the panel under fire.
Little research has been conducted on the structural performance of PCSPs under fire conditions. Hulin et al. [5,7] conducted an experimental and numerical investigation on the fire performance of concrete sandwich panels made with thin concrete wythes and steel sheet connectors. This is the only study reported in the literature regarding the structural performance of PCSPs under fire. The sandwich panels investigated by Hulin et al. [5,7] were subjected to a time-temperature heating regime following ISO-834 [8] on one side and the exposed wythe was subjected to axial compression. It was found that breakage of the insulation occurred at the interface with concrete due to differential thermal expansion, and the exposed wythe cracked under the combination of load and heat. However, these findings are limited to the 1-h fire exposure time implemented on this particular type of panel. The effects of the load level and load eccentricity, along with the influences of key parameters that can shed light on the structural performance were not investigated.
On the other hand, several studies were conducted in the literature regarding the performance of PCSPs under lateral and axial loading. Einea et al. [9] tested PCSPs made with diagonal FRP-bar connectors by flexural loading. The results showed an overall nonlinear and ductile behavior of the panels. Salmon et al. [10] tested four full-scale PCSPs in a vertical position under a uniformly distributed load. Two of the specimens were made with diagonal FRP-bar connectors and two contained steel truss connectors. It was found that the ultimate strength of the panels made with diagonal FRP-bar connectors were comparable to the strength expected of fully-composite panels. Benayoune et al. [11] presented an experimental investigation of the ultimate strength behavior of PCSPs made with steel truss connectors under eccentric loads. The results showed that the ultimate strength of the PCSPs decreased nonlinearly with the increase of the slenderness ratio. Hamed [12] developed a theoretical model to investigate the behavior of PCSPs made with truss shear connectors. The model was developed based on variational principles and equations of equilibrium. Account was taken of the axial and bending rigidities of the RC wythes and their potential cracking, the shear and normal rigidities of the insulation layer, and the elastic flexibility of the diagonal truss connectors. The parametric study showed that the diameter of the diagonal truss bars was a key parameter in determining the degree of composite action. The results also demonstrated that cracking can lead to a significant reduction in the overall stiffness of the panel. Tomlinson and Fam [13] developed a numerical model to predict the response of load-bearing PCSPs under eccentric axial loads. A bond-slip model was used to simulate the partial composite action between the two RC wythes resulting from various configurations of insulation and shear connectors. Various failure modes were detected in the analysis, including concrete crushing, steel yielding and progressive failure of the shear connectors.
In a previous study conducted by the authors regarding the fire performance of solid RC walls [4], it was found that slender load-bearing RC walls that are exposed to one-sided fire can undergo buckling failure under axial loads that are significantly smaller than the axial strength of the wall under normal ambient conditions. The buckling failures were characterized by unbounded increases in the axial shortening and the deflection of the wall with significant increase in the stresses. It was revealed that the load level, the eccentricity of load, and the geometrically nonlinear effects play key roles in the structural behavior and fire resistance of RC walls.
In this paper, a comprehensive study of the structural performance of load-bearing PCSPs made with FRP diagonal-bar connectors under fire is conducted. A theoretical model is developed for this purpose, which considers the composite action of RC wythes, transient creep of concrete, strain softening of concrete in compression, cracking and tension stiffening, yielding of steel reinforcement and geometric nonlinearity. The structural deformations, failure behavior and mechanism of load-bearing PCSPs made with FRP diagonal-bar connectors under one-sided fire are investigated and clarified. The mathematical formulation is presented first, followed by the validation of the model and numerical and parametric studies.
Section snippets
Modelling
The theoretical model consists of a heat transfer analysis and a structural model.
Model validation
In this section, the proposed model will first be validated by comparing with an experimental study and other models in the literature. The model is then used to demonstrate numerical examples and to undertake a parametric study.
Fig. 2 shows the comparison of the predicted and measured temperatures at various depths of the concrete sandwich panel S4 that was tested by Hulin et al. [5]. The panel was 1660 mm long and 710 mm wide, with 30 mm thick RC wythes and an overall thickness of 260 mm. The
Numerical study
The numerical study includes a numerical example that investigates the behavior of PCSPs made with FRP diagonal-bar connectors under fire and demonstrates the capacities of the proposed model. A parametric study is also presented.
Conclusions
The structural performance of load-bearing PCSPs that are made with diagonal-FRP bar connectors and subjected to one-sided fire has been investigated through the development of a theoretical model. The composite behavior between RC wythes is modelled through an equivalent shear stiffness of the middle layer, and the effect of rupture or crushing of the FRP bar connectors is considered. The model accounts for the transient creep of concrete, along with geometric and material nonlinearities,
CRediT authorship contribution statement
Jun Chen: Conceptualization, Methodology, Software, Investigation, Writing - original draft. Ehab Hamed: Conceptualization, Methodology, Resources, Writing - review & editing. R. Ian Gilbert: 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.
Acknowledgement
The work reported in this paper has been undertaken with the financial support of the Australian Research Council (ARC) through a Discovery Project (DP160102027). The support is greatly acknowledged.
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