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

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.

The economic and societal beneficiaries of this project can be classified into three broad categories: (1) the construction sector including concrete producers and users; (2) the public sector bodies such as the UK Highways Agency (UKHA) and Department for Transport (DfT); and (3) the standards organisations such as the European Committee for Standardization (CEN). Impact on the construction sector: more efficient use of materials, reduced project cost and increased project sustainability credential. The premature deterioration of concrete structures caused by cracking is a major and growing world-wide problem. Cracking in concrete may be of significant concern with regard to durability, watertightness or hazardous waste containment. This project will provide the construction sector with a fast and reliable test method for determining the relative likelihood of cracking of concrete mixtures and help the construction industry to make the right decision in the selection of concrete mixtures that are less likely to crack in service. A consequence of these will be reduced project life costs, longer service life of structures, more efficient use of resources for construction, potentially shorter construction periods, less construction waste generated, reduced negative environment impact from construction and increased project sustainability credential. Impact on the public sector bodies: enhanced infrastructure safety and effectiveness of public transport services. Many of our infrastructures are made of concrete and cracking is one of the main reasons that cause deterioration of concrete infrastructures. Repairing cracking of concrete could bring a big burden to our economy. In most developed countries, including the UK, around 50% of the construction budget is devoted to repair and maintenance of structures with over 30% of this expenditure on concrete structures. Traffic delay costs due to inspection and maintenance programmes are already estimated to be between 15%-40% of the construction costs. Moreover, the efficiency of the public transport service will be reduced and the service may have to be temporarily shut down during the repairing and maintenance periods. In a sustainable society we want to use many of our infrastructures effectively for a long time. It is therefore evident that test procedures and predictive models are required such that cracking potential of concrete which ensures long-term durability can be determined before it is used for an infrastructure. The findings of this project will enable the public sector bodies, such as the UKHA and DfT, to be able to assess quality of concrete used for public projects, such as highway bridges, so that to enhance the safety of our infrastructures, to reduce project life cost and possible traffic delay caused by repairing and maintenance. Hence the findings of this research will help to improve infrastructure health and safety and to enhance the efficiency of public transport service to the general publics. Impact on standards organisation, i.e., CEN: filling in the gap in technical service to better meet industry needs. Currently there is no existing European or British standard for estimating cracking potential of concrete. Our economy is now knowledge-based in which technical standards are becoming very important components. The new test method has great advantages over those standard methods recommended by ASTM and AASHTO. Incorporating this method into European standards can fill in the technical gap so that to facilitate the CEN to provide better technical service to meet industry needs. Thus it will place the European and the UK consultant firms and contractors at the fore front of technology and help them to win contracts in global markets. Therefore, this research will help to foster the economic competitiveness of the Europe and the UK.