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

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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.