An Advanced Numerical Tool for the Prediction and Analysis of Spalling in Concrete Structures Exposed to Combined Thermal and Mechanical Loading

混凝土是世界上应用最广泛的建筑材料之一。在这种广泛的使用中，许多混凝土结构都会受到高温的影响，要么是由于其预期的功能，例如在核反应堆容器或飞机跑道中，要么是无意的，通常是由于意外或故意的火灾。在这两种情况下，混凝土在高温下的结构性能显然是非常重要的。众所周知，高温对混凝土有许多容易观察到的影响，包括强度损失、刚度损失和剥落，即混凝土构件表面的材料断裂和损失。剥落是本项目的重点，在发生、程度和严重程度上有很大差异，经常观察到从轻微和非暴力到严重和爆炸性的各种表现。总共发现了五种不同形式的剥落。按暴力程度增加的顺序是后冷却剥落、骨料剥落、角部剥落、表面剥落和爆炸性剥落。无论是哪种形式，剥落都可能对结构和安全产生重大影响，具体取决于受影响的结构。在柱和墙等结构构件暴露在高温下的情况下，剥落会导致承重截面的损失和钢筋的外露，这非常容易受到热损伤，最终可能导致整个结构的倒塌。即使在没有发生坍塌的情况下，也有与维修时间和费用相关的重大经济影响，这延伸到坍塌不是问题的情况，例如在飞机跑道上，即使是轻微的剥落也可能会对安全或(至少)适用性产生影响，从而产生相关的经济后果。许多研究的结果和对包括英吉利海峡隧道火灾在内的几起备受瞩目的事件的分析证明了这一点。在这些事件中，人们提出了一系列假设，但对于观察到的剥落行为背后的确切机制，还没有真正的共识。这项工作的目的是开发一种先进的数值工具，通过考虑暴露在高温下的混凝土内部的物理过程，最终能够预测任何特定结构在温度和机械荷载的任意组合下可能发生的剥落的发生、类型和程度。因此，该模型将在实际、历史或假想的热和机械加载情况下对现有或拟议的新结构进行结构和安全评估。此外，通过在广泛的一系列数值实验中应用该模型，可以探索潜在的和控制剥落的基本过程，并可以更好地理解这种现象。对剥落行为的更好的理解和模型的应用将允许对混凝土、混凝土结构和补救或保护技术的类型进行设计，以便能够具体地解决和优化它们在恶劣条件下的性能，从而可以最大限度地减少结构暴露在高温下的结构、经济和安全影响。

Concrete is one of the most widely used construction materials in the world. Throughout this extensive usage many concrete structures are subjected to high temperatures either by consequence of their intended function, for example as in nuclear reactor vessels or aircraft runways, or unintentionally, often as a result of accidental or deliberate fires. For both of these scenarios the structural behaviour of concrete under highly elevated temperatures is clearly very important.High temperature is known to have a number of easily observable effects on concrete including loss of strength, loss of stiffness and spalling, i.e. the fracturing and loss of material from the surface of concrete elements. Spalling, which is the focus of this project, varies considerably in occurrence, extent and severity, and manifestations, ranging from minor and non-violent, to severe and explosive, have been regularly observed. In all five distinct forms of spalling have been identified. In order of increasing violence these are Post Cooling spalling, Aggregate spalling, Corner spalling, Surface spalling and Explosive spalling.Whatever the form, spalling can have significant structural and safety implications depending on the structure that is affected. Where structural members, such as columns and walls, are exposed to elevated temperatures, spalling can result in a loss of load bearing cross-section and the exposure of steel reinforcement, which is highly susceptible to heat damage, and can ultimately lead to the collapse of the entire structure. Even where collapse does not occur there are significant economic implications associated with the time and cost of repair and this extends to situations where collapse is not an issue, for example in aircraft runways, where even minor spalling can have safety or (at the very least) serviceability implications and hence associated economic consequences.Despite its common occurence there is a fundamental lack of understanding of the phenomenon of spalling and the processes that control it. This is demonstrated by the results of numerous studies and the analyses of several high profile incidents including the Channel Tunnel fire, in which a range of hypotheses have been presented but no real consensus as to the exact mechanisms underlying the observed spalling behaviour has emerged.The aim of this work is to develop an advanced numerical tool that, by accounting for the physical processes at work within concrete exposed to elevated temperatures, will ultimately be capable of predicting the occurrence, type and extent of spalling that may be expected in any particular structure subjected to any combination of thermal and mechanical loading. The model will thus have significant applications in the structural and safety assessment of either existing or proposed new structures under actual, historical or hypothetical thermal and mechanical loading scenarios.Furthermore, by applying this model in an extensive series of numerical experiments the fundamental processes underlying and controlling spalling may be explored and a better understanding of the phenomenon can be achieved.The improved understanding of spalling behaviour and the application of the model will allow types of concrete, concrete structures and remedial or protective techniques to be designed such that their performance under severe conditions can be specifically addressed and optimised, and hence the structural, economic and safety implications of structural exposure to elevated temperatures can be minimised.