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，许多混凝土结构要么由于其预期功能而承受高温，例如在核反应堆容器或飞机跑道中，要么由于意外或故意的火灾而无意地承受高温。对于这两种情况，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，即混凝土元件表面的断裂和材料损失。剥落是本项目的重点，其发生、程度和严重程度各不相同，经常观察到从轻微和非暴力到严重和爆炸性的各种表现。在所有五种不同形式的剥落已被确定。根据暴力程度的增加顺序，这些是冷却后剥落，骨料剥落，角剥落，表面剥落和爆炸性剥落。无论何种形式，根据受影响的结构，剥落都可能对结构和安全产生重大影响。当结构构件（如柱和墙）暴露在高温下时，剥落会导致承载截面的损失和钢筋的暴露，这非常容易受到热损伤，并最终导致整个结构的倒塌。即使在没有发生坍塌的情况下，也存在与维修时间和成本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会对安全或（至少）可维护性产生影响，从而产生相关的经济后果。尽管它经常发生，但对剥落现象和控制剥落的过程缺乏基本的了解。许多研究结果和对包括英吉利海峡隧道火灾在内的几起引人注目的事件的分析证明了这一点，其中提出了一系列假设，但对于观察到的剥落行为背后的确切机制尚未出现真正的共识。这项工作的目的是开发一种先进的数值工具，通过计算混凝土暴露在高温下的物理过程，最终能够预测任何特定结构在任何热和机械载荷组合下可能出现的剥落的发生、类型和程度。因此，该模型将在实际、历史或假设的热和机械负荷情景下对现有或拟议的新结构进行结构和安全评估。此外，通过在一系列广泛的数值实验中应用该模型，可以探索剥落背后和控制剥落的基本过程，并可以更好地理解这一现象。对剥落行为的更好理解和模型的应用将使混凝土、混凝土结构和补救或保护技术的类型得以设计，从而使其在恶劣条件下的性能能够得到具体解决和优化，因此结构暴露于高温下的结构、经济和安全影响可以最小化。

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.