RAPID, CATASTROPHIC DECAY OF BUILDING LIMESTONES: IMPLICATIONS FOR MASONRY SELECTION AND LIFETIME BEHAVIOUR

石头被广泛认为是一种可持续的建筑材料，也是世界上许多有形文化遗产的宝库。随着这种认识的产生，人们开始认识到，石头的寿命是有限的，当它被放置在通常具有侵略性的城市环境中时，它的寿命可能会被急剧缩短。特别是，许多常见的建筑石灰石经历了似乎不可预测的、间歇性的、有时是灾难性的破裂，因为石头的强度超过了逐渐腐烂、缓慢积累的内应力和/或受到极端外部应力(如严重霜冻)的影响。快速腐烂的时期可能夹杂着相对稳定的时期，例如，由污染引起的硫酸钙结壳的形成是标志。因此，为了控制潜在的灾难性衰退，有必要了解为什么会触发快速撤退，是什么允许它继续下去，如何阻止它，以及如何从一开始就避免原因。在不适当的保护可能会加速腐烂，以及必须在可能的替换石材和与新结构相关的石材选择之间做出选择的情况下，情况尤其如此。为了实现这一理解，需要问四个问题1。是什么过程导致了快速退缩？2.什么物理、化学和矿物学特征决定了石头快速退缩的敏感性，这些特性在腐化过程中如何变化？3.随着石块退缩，石材表面及其下方的小气候条件如何变化，这些如何影响衰变机制？4.是什么允许风化材料在快速流失的情况下继续风化？例如，有害盐类集中在风化材料中？这个跨学科的项目将通过实地研究牛津大学和附近地区由橄榄石灰岩(如巴斯和科茨沃尔德石灰岩)建造的易于快速退缩的石头结构来研究这些问题。与此相关的是光纤传感器的开发，这种传感器将允许根据环境条件(包括温度、相对湿度和表面湿润)监测各个区块内的水分和盐分运动。这些数据以及相同的传感器技术将与对风化石材的分析相结合，以设计实验室实验，使用不同品种的浴石来模拟块体后退和逐渐得到保护时的击穿模式和盐分和水分运动的动态。将结合现场研究和受控实验室实验的结果来解释(模拟)决定快速后退或稳定性的总体敏感性的因素以及导致腐烂的过程的操作。特别是，结果将被用来确定是什么触发了正反馈和负反馈，分别加速和减速了石头腐烂系统中的变化。在与最终用户的讨论中，这一理解将被用于制定石灰石保护和选择与特定环境条件相匹配的新石材和替代石材的协议。

Stone is widely recognised as a sustainable construction material and as a store of much of the world's tangible cultural heritage. With this recognition has come an understanding that stone has a finite life that can be drastically curtailed when it is placed in the often-aggressive urban environments. In particular, many common building limestones experience seemingly unpredictable, episodic and sometimes catastrophic breakdown as stone strength is exceeded by gradual decay, the slow accumulation of internal stresses and/or subjection to extreme external stresses such as a severe frost. Episodes of rapid decay may be interspersed with periods of relative stability marked by, for example, the formation of pollution-derived calcium sulphate crusts. To control potential catastrophic decay it is therefore necessary to understand why rapid retreat is triggered, what allows it to continue, how it can be halted and how the causes can be avoided in the first place. This is particularly true where inappropriate conservation could accelerate decay, and where choices have to be made between possible replacement stone and stone selection in relation to new structures. To achieve this understanding four questions need to be asked.1. What processes are responsible for rapid retreat?2. What physical, chemical and mineralogical characteristics determine stone susceptibility to rapid retreat and how do these properties change during decay?3. How do microclimatic conditions at and beneath the stone surface change as stone retreats and how do these influence decay mechanisms?4. What permits continued weathering despite rapid loss, of weathered material in which, for example, damaging salts are concentrated?This interdisciplinary project will examine these questions through field studies of stone structures in Oxford and nearby areas built of oolitic limestone (e.g. Bath and Cotswold limestones) that is prone to rapid retreat. Linked to this will be the development of fibre optic sensors that will allow moisture and salt movement within individual blocks to be monitored in relation to environmental conditions, including temperature, relative humidity and surface wetting. These data, and the same sensor technology will be combined with analyses of weathered stone to design laboratory experiments using different varieties of Bath Stone to simulate breakdown patterns and the dynamics of salt and moisture movement as blocks retreat and are progressively sheltered. Results from field studies and controlled laboratory experiments will be combined to explain (model) the factors that determine overall susceptibility to either rapid retreat or stability and the operation of the processes responsible for decay. In particular, results will be used to determine what triggers positive and negative feedbacks that respectively accelerate and decelerate change within the stone decay system. This understanding will be used, in discussion with end-users, to develop protocols for limestone conservation and the selection of new and replacement stone matched to specific environmental conditions.