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

Lead Research Organisation: University of Leeds
Department Name: Civil Engineering

Abstract

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 particularthe 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.

Planned Impact

People and Knowledge

Research team and project partners
The project partners, investigators and PDRAs will benefit though enhancement to their knowledge and skills. This will provide a valuable pool of knowledge and people with skills highly relevant to the UK construction industry and wider economy.

Students
Students at the UoL and ICL will benefit through enhancement to their courses through the integration of the learning from this project into teaching. The research will also enhance the strong PhD programmes of study currently offered at Leeds and Imperial (whilst also helping to support postgraduate study at other universities). This collaboration will consolidate the industry - academic link present at these institutions, further benefiting the successful teaching programmes at Leeds and Imperial. The proposed project will serve as a perfect example of promoting the training and development of researchers (PDRAs) in a dual environment of academia and industry, significantly enhancing their employability (specifically by providing them with design experience).

The two universities offer several MSc and Undergraduate courses, which would benefit from the outcomes of this research. This research will contribute to the enhanced learning and skills development of future engineers making them more competitive in this international market.


Economy

UK Construction Industry
The construction output in the UK is more than £110billion per annum, contributing around 7% of GDP accounting for approximately 3 million jobs, 10% of total UK employment and including both manufacturing and services. UK industry will benefit through the creation of design tools and guidance in this project, which will be disseminated as described in the Pathways to Impact section, enabling more productive and timely construction. The UK's reputation for high quality construction expertise will be further established through the enhanced technical advice that the Principal Investigators an those related to the research team will be able to offer to industry, through their established consultancy routes, following this project. Specific benefits will include -

1. Design / Construction stage (1) - optimised efficient designs leading to savings in materials cost, construction time (e.g., due to better pour sequencing), potential elimination of movement joints (material and time saving).

2. Construction stage (2) - elimination of problems during construction and hence reduction in additional costs due to delays, elimination of litigation costs due to negation of construction problems.

3. In-use - better performing structure reducing maintenance and repair costs, thereby reducing the whole-life or 'total expenditure' (TOTEX).


Society

UK and European Policy
The knowledge gained during this project will help inform both UK and European building standards. In turn this will lead to the following societal benefits -
1. Lower construction costs for government who are often responsible for funding large construction projects (directly or indirectly) leading to a saving for the UK tax payer.
2. Reduced maintenance costs for publicly owned infrastructure and a subsequent reduction in disruption caused by closure of infrastructure for maintenance work.
3. Better quality publicly owned buildings - quality of buildings is known to have a psychological impact on residents, particularly in local authority housing.

Publications

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