A Novel Method for Assessing Cracking Tendency of Concrete at Early Ages

混凝土是世界上使用最广泛的建筑材料，地球上每个人每年生产的混凝土超过1立方米。然而，混凝土的抗拉强度远低于抗压强度，因此在正常使用条件下很容易开裂。混凝土中的裂缝会降低承载能力，并可能导致过早退化。就钢筋混凝土而言，裂缝会使水和腐蚀剂渗透，从而降低结构的整体耐久性，从而加速钢筋的劣化。根据建筑研究机构的数据，高速公路桥梁、建筑和海上设施等混凝土结构中的钢筋锈蚀每年给英国造成的损失估计为5.5亿英镑。这些结构中的大多数仍然需要广泛的维护或更换。多年来，工程师们一直在寻求开发简单的测试，以评估给定的混凝土混合物可能有多容易开裂。目前，评价混凝土混合料开裂能力的试验方法主要有三种：环法、梁法和板法。虽然圆环试验已经成为一种标准方法，但它的开裂敏感性很低，非常耗时且不划算。此外，圆形环形试件的圆周上任何地方都可能出现裂纹，这使得定位非常困难。另一方面，梁和板试验很难提供恒定的约束和终止条件，这使得它们的应用非常有限。在本项目中，我们将开发一种新的试验方法，使用椭圆环形试件来评估混凝土混合料的开裂倾向。与圆形环形试件相比，椭圆环形试件可以导致更高的应力集中和更高的开裂敏感性，使得椭圆环形试件测试方法耗时更少，效率更高。此外，由于应力集中，裂纹的位置将集中在椭圆环形试件圆周上的可预测位置，从而更容易被检测到。因此，椭圆环试验有助于确定混凝土混合料在短得多的时间内开裂的相对可能性，并有助于选择在用于建筑工程之前不太可能开裂的混凝土混合料。我们还将建立一个理论模型，为设计工程师提供一个工具，以预测各种混凝土混合料的开裂潜力。该理论模型将能够预测混凝土椭圆形环形试件中何时何地发生裂缝。我们还将探索试件尺寸/几何形状和约束程度对混凝土收缩裂缝的影响，以提供选择椭圆形混凝土环试件和约束钢环的几何形状的指导，以评估现场混凝土的裂缝倾向。研究结果将通过高质量的期刊论文和在著名会议上的演讲向学术界进行传授。本研究探讨的技术将通过项目伙伴通过其网络组织的技术研讨会、在混凝土行业杂志上发表的文章、项目网站和咨询服务转移到建筑行业。我们亦会向有关的标准组织，特别是欧洲标准化委员会(CEN)提交这项计划所得的研究结果，作为评估混凝土混合料裂缝倾向的标准测试方法。

Concrete is the most widely used construction materials in the world with more than 1 m3 of concrete being produced every year for every person on the planet. However, concrete has much lower tensile strength than compression strength and, therefore, is easy to crack under normal service conditions. Cracking in concrete can reduce load carrying capacity and may lead to premature deterioration. In the case of reinforced concrete, cracks reduce overall durability of structures by allowing the penetration of water and aggressive agents, thereby accelerating the deterioration of reinforcing steel. Corrosion of the reinforcing steel in concrete structures such as motorway bridges, buildings and marine installations costs the UK an estimated 550m per year according to Building Research Establishment. Most of these structures continue to require extensive maintenance or replacement. Over the years, engineers have sought to develop simple tests to assess how susceptible a given concrete mixture may be to cracking. Currently, there are three main test methods for assessing cracking potential of concrete mixtures: the ring, the beam and the plate tests. Though the circular ring test has become a standard method, it has very low cracking sensitivity, making it very time-consuming and not cost effective. Besides, cracking can appear anywhere along the circumference of a circular ring specimen, making it very difficult to be located. On the other hand, it has been found difficult to provide a constant restraint and end condition for the beam and plate tests, making their applications very limited.In this project, we will develop a new test method by using elliptical ring specimens to assess cracking tendency of concrete mixtures. Compared to circular ring specimens, the elliptical ring specimen can lead to a higher stress concentration and increased cracking sensitivity, making the elliptical ring test method less time-consuming and more efficient. Besides, due to stress concentration, the location of cracking will concentrate on predictable positions along the circumference of an elliptical ring specimen, thus making it much easier to be detected. The elliptical ring test is therefore helpful for determining the relative likelihood of cracking of concrete mixtures in a much shorter period and for aiding in the selection of concrete mixtures that are less likely to crack before they are used for construction projects. We will also establish a theoretical model to provide design engineers with a tool to predict cracking potential of various concrete mixtures. The theoretical model will be able to predict when and where crack will occur in a concrete elliptical ring specimen. We will also explore the effects of specimen size/geometry and the degree of restraint on shrinkage cracking of concrete to provide the guidance on choosing the geometries of both the elliptical concrete ring specimen and the restraining steel ring for estimating cracking tendency of concrete in the field.Dissemination of the findings to the academic community will be made by quality journal papers and presentations at prestigious conferences. The technique explored in this research will be transferred to the construction industry through technical seminars organized by the project partners through their network, articles published in magazines of the concrete industry, the project website and consultancy service. We will also present our findings obtained from this project to relevant standard organisations, particularly the European Committee for Standardization (CEN), for possible recommendation as a standard test method for estimating cracking tendency of concrete mixtures.