Understanding the cracking behaviour of reinforced concrete elements subjected to the restraint of imposed strains

混凝土是使用最广泛的建筑材料，对全球基础设施更新计划至关重要(全球估计约100万亿美元，包括英国国家基础设施计划4000亿GB)(www.oxfordeconomics.com/publication/open/283970).设计不良的钢筋混凝土(RC)结构由于约束而产生的过度裂缝是混凝土建筑行业中普遍存在的问题，并导致许多情况下昂贵的补救措施和延误。例如，最近在英国的一个项目被推迟了，原因是施加的应变(由于早期的热和收缩作用)的限制导致了过度裂缝。随后，申请人在施工过程中建议进行更改，以限制早期热应力和收缩应变的边缘约束，为客户节省了约1.75亿GB的实际成本。这项研究开发的设计指南将提高新的RC基础设施的性能和效率，并通过改善对裂缝的理解来延长现有基础设施的寿命。在许多情况下，由于所施加的变形的约束而产生的裂缝可能难以避免。事实上，早期热运动的约束下的开裂(通常被称为非结构性开裂)是约束引起的开裂的最常见形式。在设计中，裂缝是通过提供钢筋来管理的，钢筋旨在以控制裂缝模式和限制裂缝宽度的方式分布内部应变。目前英国/欧盟关于约束诱发裂缝的设计指南包含在EN1992-3：2006和CIRIA报告C660/766中。这些文件中的基本设计方法已经使用了30多年，存在缺陷。这反映在现场观察发现的裂缝模式与EN1992-3：2006预测的裂缝模式相反-并且裂缝宽度超过-。显然，这种“不符合”的情况是由错误的基本假设造成的；特别是，边界(约束)条件在确定裂缝模式方面发挥的作用比目前设计指南中假定的更重要。这项研究的结果将在三代英国/欧盟规范中首次为执业工程师提供正确设计RC构件的能力，以限制短期和长期施加的应变。计划的传播途径将大大有助于减少违规案例的频率和总数，这些案例目前是由于对约束导致的裂缝缺乏了解而导致的，并影响到英国混凝土建筑的方方面面。

Concrete is the most widely used construction material and is essential to the global programme of infrastructure updating (global estimate ~$100tn including UK National infrastructure plan £400bn) (www.oxfordeconomics.com/publication/open/283970). Excessive cracking due to restraint in poorly designed reinforced concrete (RC) structures is a widespread problem in the concrete construction industry and leads to many instances of costly remedial measures and delays. For example, a recent project in England was delayed due to excessive cracking caused by the restraint of imposed strains (from early thermal and shrinkage actions). Subsequent changes recommended by the applicants during the construction programme to limit the edge restraint of early thermal and shrinkage strain produced a real cost saving to the client of approximately £1.75M. The design guidance developed in this research will increase the performance and efficiency of new RC infrastructure as well as prolong the life of existing infrastructure through improved understanding of cracking. There are many situations when cracking due to the restraint of imposed deformations may be difficult to avoid. In fact, cracking from the restraint of early thermal movements (often referred to as 'non-structural' cracking) is the most common form of restraint induced cracking. In design, cracking is managed by the provision of reinforcement intended to distribute internal strains in such a way as to control the cracking pattern and limit crack widths. Current UK/EU design guidance on restraint induced cracking is encapsulated in EN1992-3:2006 and CIRIA report C660/766. The underlying design methodology in these documents has been used for over 30 years and is flawed. This is reflected in field observations identifying cracking patterns contrary to - and crack widths in excess of - those predicted by EN1992-3:2006. It is apparent that such 'non-compliance' cases result from erroneous basic assumptions; in particular; the boundary (restraint) conditions play a more significant role in determining the crack pattern than assumed in the current design guidance. The outcome of this research will provide practising engineers with the ability for the first time in three generations of UK/EU codes to correctly design RC elements for the restraint of short and long-term imposed strains. Planned dissemination routes will significantly aid the reduction in frequency and overall number of non-compliance cases, which currently result from the poor understanding of restraint induced cracking and affect all aspects of concrete construction in the UK.