Cristian Maluk

A contemporary review of large-scale non-standard structural fire testing

In recent years, large-scale structural fire testing has experienced something of a renaissance. After about a century with the standard fire resistance test being the predominant means to characterize the response of structural elements in fires, both research and regulatory communities are confronting the many inherent problems associated with using simplified single element tests, on isolated structural members subjected to unrealistic temperature-time curves, to demonstrate adequate structural performance in fires. As a consequence, a shift in testing philosophy to large-scale non-standard fire testing, using real rather than standard fires, is growing in momentum. A number of custom made, non-standard testing facilities have recently been constructed or are nearing completion. Non-standard fire tests performed around the world during the past three decades have identified numerous shortcomings in our understanding of real building behavior during real fires; in most cases these shortcomings could not have been observed through standard furnace tests. Supported by a grant from the Fire Protection Research Foundation, this paper presents a review of relevant non-standard structural fire engineering research done at the large-scale around the world during the past few decades. It identifies gaps and research needs based both on the conclusions of previous researchers and also on the authors’ own assessment of the information presented. A review of similar research needs assessments carried out or presented during the past ten years is included. The overarching objective is to highlight gaps in knowledge and to help steer future research in structural fire engineering, particularly experimental research at the large-scale.

Bond Strength of CFRP and Steel Bars in Concrete at Elevated Temperature

Novel concrete elements are emerging utilizing high performance self-consolidating concrete (HPSCC) reinforced with high-strength, lightweight, and non-corroding carbon fiber reinforced polymer (CFRP) prestressed reinforcement. The fire performance of these elements must be understood before they can be used with confidence. In particular, the bond performance of the novel CFRP reinforcement at elevated temperatures requires investigation. This paper examines the bond performance of a specific type of CFRP tendon as compared with steel prestressing wire. The results of transient elevated temperature bond pullout and tensile strength tests on CFRP tendons and steel prestressing wire are presented and discussed, and show that bond failure at elevated temperature is a complex phenomenon which is influenced by a number of interrelated factors, including the type of prestressing, degradation of the concrete, CFRP, and steel, differential thermal expansion, thermal gradients and stresses, release of moisture from the concrete, and loading. It is shown that CFRP tendons are more sensitive to bond strength reductions than to reductions in tensile strength at elevated temperature.

An investigation into temperature gradient effects on concrete performance at elevated temperatures

To assure adequate fire performance of concrete structures, appropriate knowledge and adequate, practical models of concrete at elevated temperatures are crucial yet current lacking, prompting further research. This paper first highlights the limitations of inconsistent thermal boundary conditions in conventional fire testing; and of using constitutive models developed based on empirical data developed testing concrete under minimised temperature gradients in modelling of concrete with significant temperature gradients. On that basis, the paper outlines key features of a test setup used for the accurate control of the thermal boundary conditions when testing concrete at elevated temperatures, using radiant panels to generate well-defined and reproducible heating regimes. The repeatability, consistency and uniformity of thermal boundary conditions are demonstrated using measurements of heat flux and in-depth temperature of test specimens. Compressive strength is also investigated. The initial data collected clearly suggested that the incident heat fluxes, and thus the associated temperature gradient, has potentially significant effects on concrete mechanical properties at elevated temperatures. Further research is thus ongoing to quantify such effects and also to develop constitutive models accounting for a wide range of heating conditions; from very slow to extremely rapid heating. The proposed models could be included into effective rational knowledge-based fire design and analysis of concrete structures.

Effects of temperature and temperature gradient on concrete performance at elevated temperatures

To assure adequate fire performance of concrete structures, appropriate knowledge of and models for performance of concrete at elevated temperatures are crucial yet currently lacking, prompting further research. This article first highlights the limitations of inconsistent thermal boundary conditions in conventional fire testing and of using constitutive models developed based on empirical data obtained through testing concrete under minimised temperature gradients in modelling of concrete structures with significant temperature gradients. On that basis, this article outlines key features of a new test setup using radiant panels to ensure well-defined and reproducible thermal and mechanical loadings on concrete specimens. The good repeatability, consistency and uniformity of the thermal boundary conditions are demonstrated using measurements of heat flux and in-depth temperature of test specimens. The initial collected data appear to indicate that the compressive strength and failure mode of test specimens are influenced by both temperature and temperature gradient. More research is thus required to further quantify such effect and also to effectively account for it in rational performance-based fire design and analysis of concrete structures. The new test setup reported in this article, which enables reliable thermal/mechanical loadings and deformation capturing of concrete surface at elevated temperatures using digital image correlation, would be highly beneficial for such further research.

Concrete Fire Testing - A Review of the Thermal Boundary Conditions

Experimental studies of concrete in fire or at elevated temperature have traditionally given relatively little scientific attention to quantifying the severity, and to some extent reproducibility, of the thermal boundary conditions imposed on specimens during testing. This paper examines the heat transfer fundamentals of fire testing when controlling the time-history of temperature inside a furnace (or oven), versus controlling the time-history of incident radiant heat flux at a specimen’s exposed surface. The thermal boundary conditions of a concrete specimen during fire testing are fundamentally based on conservation of energy, and thus typically formulated in terms of heat fluxes. While from the standpoint of concrete fire behaviour the aim is typically only to gauge the distribution of temperatures inside concrete; this is rarely explicitly acknowledged or quantified during concrete fire testing. This shows that continued unexamined use of varied heating techniques presents a serious threat to harmonization of the thermal boundary conditions imposed during concrete testing. The current work proposes adopting test control by in-depth temperature distributions or net heat fluxes for a rigorous comparison of the thermal boundary conditions imposed on test specimens when using different heating techniques.

Predictive testing for heat-induced spalling of concrete

This paper describes Phase II of a project being undertaken to develop a predictive test method to investigate heat-induced explosive spalling of concrete, with a specific focus on concrete used in tunneling applications (but obviously applicable to other applications). The test method seeks to allow careful control of the thermal and mechanical transient conditions influencing the occurrence of heat-induced concrete spalling, thus enabling convenient, representative, repeatable, and comparable testing to be carried out on various concrete mixes under various potentially relevant conditions. Phase I of the project focused on establishing suitable thermal exposures to use for testing based on the thermal exposures which a sample would be exposed to during a standard furnace test (cellulosic or modified hydrocarbon) in the Promethee testing facility at CERIB in France. The work described in this paper deals with establishing suitable mechanical loading conditions for a spalling test, the focus in the current work is to enable provision of a representative test for precast segmental concrete tunnel linings (as opposed to sprayed or cut-and-cover tunnel linings). With small adaptations the spalling test method could be adjusted to suit other applications. This paper focuses on the motivation for developing the testing method and outlines the testing to be carried out. Tests are currently underway, and the full suite of results will be presented at the conference.