Explosive spalling of concrete in fire: novel experiments under controlled thermal and mechanical conditions
Rickard, Ieuan Jack Clowes
Modern concretes are increasingly susceptible to a failure mechanism known as heat-induced explosive concrete spalling. This involves the sudden loss of material during severe thermal exposures such as might be experienced during a fire. This typically occurs violently, and could result in; severe loss of cross-section, direct thermal exposure of the internal steel reinforcement or prestressing, and – in some cases – structural collapse. As such, heat-induced explosive concrete spalling in fire poses a credible risk to reinforced and prestressed concrete structures and has received considerable research attention in recent decades. Certain types of structures such as tunnels have historically been more susceptible to spalling due to the concrete used, the particular mechanical conditions, and the potential fire loads and dynamics involved. However, with advancements in concrete technology being driven by sustainability and optimisation, spalling is becoming more prominent in conventional concrete structures such as buildings and bridges. Due to the complex nature of heat-induced explosive spalling there is currently no validated guidance to enable the design of concrete mixes to prevent spalling, nor any established, widely verified, repeatable test methods to confidently quantify or demonstrate spalling resistance for a particular mix in a given end-use application. As a result of this lack of validated testing and the complexity of the phenomenon, no theoretical or computational models yet exist that can predict spalling with sufficient confidence to be used in design. The aim of this thesis was to advance experimental spalling research through the development of an experimental apparatus which addresses deficiencies of others, most notably repeatability and control of thermal and mechanical boundary conditions under a range of relevant heating scenarios. Following this, the aim was to carry out carefully controlled, repeatable experiments with a precision and scale which has not been achieved before, in order to understand variability in spalling phenomena and improve knowledge of some of the main parameters that are known to influence spalling. In this thesis the key variables known to influence concrete’s spalling propensity are identified, initially based on the available research literature. Following this, an experimental method and framework to allow repeatable testing of concrete mixes, with careful control of these key variables, is developed, presented, and applied. This experimental method is based on the use of a radiant panel array impose an incident heat flux on samples. This approach to heating was developed previously at The University of Edinburgh, and has been referred to as H-TRIS within the spalling community. This name will be used throughout the thesis but it should be noted that since this work other authors have made use of the same apparatus and chosen not to use this name to describe the movable radiant panels. The experiments and development of the method are split into three stages within this thesis. Initially, four heavily instrumented furnace tests were carried out within the Prometheus testing facility at CERIB, France, on a total of thirty six concrete samples with a range of mix (i.e. polypropylene fibre content) and geometric parameters (i.e. plan dimensions and thickness). Importantly, this also allowed the thermal exposure received by the samples when tested under exposure to standard cellulosic, and French modified hydrocarbon temperature versus time curves to be monitored and characterised, as well as giving insight into the influences of different key sample parameters. Second, thirty three samples, all of which were cast in parallel with the samples used in the CERIB furnace tests, were transported to Edinburgh and tested using the H-TRIS experimental method and apparatus. This involved upgrading the existing H-TRIS apparatus with more powerful radiant panels to allow replication of the more severe thermal exposures used to assess concretes for use in tunnels. This test series allowed validation of the thermal exposures being used and a comparison of the differences between the two test methods, as well as further investigation of key parameters thought to influence spalling. For the final test series, a 3 Meganewton uniaxial loading frame was designed and constructed within the H-TRIS apparatus to allow application of simultaneous uniaxial external loading coincident with heating. Understanding the influence of mechanical loading and restraint on spalling performance is critical, since concrete is typically loaded in real end-use applications. The loading method and magnitudes were chosen so as to maintain relevance to concrete tunnelling construction within this thesis – however the versatility of the experimental method ensures the relevance of the resulting data to a range of possible structural concrete applications. Forty-five samples were tested in this test series, allowing the influence of loading and restraint to be carefully investigated, as well as permitting comparison of the effects of various other parameters believed to influence spalling. Taken together, the 114 concrete samples tested under various conditions for this thesis have provided a range of insights into the influence of different parameters on heat-induced explosive concrete spalling. These include – for the first time ever – careful quantification of the variability of spalling and specific knowledge of the parameters which may play the most important causal roles with respect to spalling in practice. Through the testing, a test apparatus able to accurately and repeatably control the necessary thermal and mechanical parameters was developed, and a guidance for carrying out spalling experiments has been developed and presented, making it available for future research studies and possibly also for compliance testing in practice. It has been shown existing experimental approaches have deficiencies and that this kind of carefully considered approach to experimental spalling research is required.