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，也是世界上许多物质文化遗产的储存材料。有了这种认识，人们认识到石头的寿命是有限的，当它被放置在经常充满侵略性的城市环境中时，它的寿命可能会大大缩短。特别是，许多普通的建筑石灰石经历了看似不可预测的，偶发性的，有时是灾难性的破坏，因为石头的强度被逐渐腐烂，内部应力的缓慢积累和/或遭受极端的外部应力，如严重的霜冻。快速衰变的时期可能穿插着相对稳定的时期，例如，以污染引起的硫酸钙结壳的形成为标志。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衰退，是什么让它继续下去，如何阻止它，以及如何从一开始就避免其原因。在不适当的保护可能加速腐朽的地方尤其如此，在必须在可能的替换石和与新结构相关的石头选择之间做出选择的地方尤其如此。要达到这种理解，需要问四个问题。什么过程导致了快速撤退？什么物理、化学和矿物学特征决定了石头对快速后退的敏感性，这些特性在衰变过程中如何变化？当石头后退时，石头表面和石头表面下的小气候条件是如何变化的？这些条件是如何影响衰变机制的？是什么使风化物质（例如，集中了破坏性的盐）迅速流失，但仍能继续风化？这个跨学科项目将通过实地研究牛津和附近地区的鲕状石灰岩（如巴斯和科茨沃尔德石灰石）建造的石头结构来研究这些问题，这些石头结构容易迅速退缩。与此相关的将是光纤传感器的发展，它将允许监测单个块内的水分和盐的运动，与环境条件有关，包括温度、相对湿度和表面润湿。这些数据和相同的传感器技术将结合对风化石的分析来设计实验室实验，使用不同种类的巴斯石来模拟分解模式和盐和水分运动的动态，因为块体撤退并逐渐被遮蔽。实地研究和受控实验室实验的结果将结合起来，以解释（模拟）决定对快速退缩或稳定的总体敏感性的因素，以及负责衰变的过程的操作。具体来说，研究结果将用于确定是什么触发了正反馈和负反馈，分别加速和减缓了石头衰变系统中的变化。在与最终用户讨论时，将利用这一了解来制定石灰石保护和选择符合具体环境条件的新石和替代石的协议。

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.