Paul Burling obtained a Higher National Certificate in Production Engineering with Merits and Advanced Mathematics from Cambridge College of Art and Technology. He has extensive experience in project management of large commercial projects world wide, which demand in depth knowledge of composite material and adhesives for military and commercial applications.
Almost half of the UK's construction sector output consists of repair and maintenance of existing structures. Expected heavier rainfall, hotter summers and extreme weather events will impact more heavily on the built environment. The UK is already seeing signs of this happening with coastal territory being eroded and subsidence costing £400 million in maintenance. Higher winds are likely to damage up to one million buildings and cost £3.4 billion per year and contribute to additional flooding. Improving our existing environment by fully understanding repair and renovation practices will enhance living conditions and add value to existing facilities. As Paul Burling reports environmental issues are an increasing cause for concern. There are structures that have been attacked by corrosion from seawater and other harmful substances, such as acid rain. The extent of the damage will inevitably need to be assessed and, in the worst cases, repairs will be required or replacement. In other cases additional support will also be required. Many of the existing built structures for sea defences were never designed for the extreme conditions that are being seen today.
There are many structures that are in need of repair but have been left to decay because traditional materials do not have the ability to address the structural requirements imposed on them by the change in climate. The use of advanced materials can address a number of these issues, for example, using glass reinforced plastic and carbon fibre reinforced plastic to enhance the performance of concrete. It has the advantage of reducing corrosion and maintaining the integrity of the structure. This has a direct effect on environmental considerations of energy and pollution, as many of these types of repair are permanent.
The high specific properties of composites also allow repair materials to be placed directly and preferentially at the area where it is required. An example of this type of repair or reinforcement is found in CFRP plate bonded under bridges.
Advanced materials such as GRP and CFRP are difficult to dispose of both in the initial manufacturing cycle and at the end of life of a product.
TWI is committed to spreading best practice through all avenues, drawing on materials processing know-how not only within the UK but also from centres of excellence throughout the world. Practical knowledge on materials joining is spread through training, workshops, conferences and seminars, through publications and through interactive involvement with single companies or groups of companies.
Problems and issues with some concrete structures
Chemical attack such as chloride ion is one of the main causes of reinforcement bar corrosion. When chlorides penetrate concrete from external sources, such as de-icing salts and seawater, carbon steel re-bar corrodes. Rust forms, occupying a volume about five times that of the original steel, this causes the concrete to crack and spall, breaking away from the reinforcement. Once cracks become established the ingress of the corroding medium is accelerated.
Whenever there is chloride in concrete there is an increased risk of corrosion of the embedded metal. The chloride content can vary within the mix but as a general rule, the higher the chloride content the higher the curing temperature, or if subsequently exposed to warm moist conditions, the greater the risk of corrosion.
Also the mix of alkali silica reacting is the chemical result of the alkaline in the cement and the silicon in the aggregate reacting with one another and giving way to the concrete cracking open. When the concrete starts to break away, this leads to other problems, such as allowing water to penetrate the concrete to a deeper level and affecting the steel reinforcement. There are ways of preventing this during construction which are set out in standards for concrete structures, but even when using coated or stainless steel reinforcement bar there is always the possibility of corrosion at a later date.
It should be noted that alkali silica reaction causing the concrete to break up in the UK is small, but the corrosion of the re-bar caused by other external sources has been a problem since metallic reinforcement was first used. Placement of the reinforcement is critical to obtain the best structural performance from a reinforced concrete beam, but also in making sure that the structure has long-term durability.
Problems and issues with disposing of composite laminates
Composite laminates consisting of glass, kevlar and carbon are widely considered to be un-recyclable. Composite materials have high strength and stiffness and have been engineered to carry out a particular task, such as part of an aircraft wing or boat hull. They often include specific orientation of the fibres to allow loads to be transmitted, and are therefore tailored to the end product.
These composite laminates are strong and difficult to process. The machinery needs to be heavily engineered to be able to shred large sections into smaller manageable units. In addition to this, the wear on grinding equipment is costly. Composite materials are heavily engineered and are made for specific reasons. Additional design, labour, and energy has been incorporated into their overall costs. These materials are valuable even at the end of the original life for which they were intended. If these materials can be used to improve and possibly replace original materials, the environmental and economic benefits would be considerable.
What is a composite laminate?
The definition of a composite material can easily be expressed as two or more materials working together to exceed the performance of one. There are many forms of composite materials and a composite laminate is just one of those forms. These can be manufactured in a variety of ways. Wet lay-up using a simple fibre that has been wetted with a resin (matrix) is most common. Alternatively they are produced on a machine and impregnated with a resin (matrix) into a fabric that is laid in a single direction or one that has been woven with several directions of reinforcement. These are known as Pre-preg and are highly engineered see Fig.1.
Recommendations
The overall aim of this work is to substitute composite materials to replace re-bar where the finished structure is open to attack from the environment. Examples are sea defenses and kerbs for large car parks that are heavily treated in winter with rock salt. These comments follow on from the work carried out by TWI's Anita Buxton, and detailed in the November/December issue of Bulletin, on reinforcement of a drainage channel.
The technical issues yet to be addressed are to:
- Establish basic mechanical properties of the waste composite materials
- Engineer typical physical characteristics of the composites
- Address surface preparation required to enhance concrete structures
- Benchmark against other commercial reinforcement products
There are many applications and opportunities to re-use unwanted engineering composites for the construction industry. TWI will be investigating ways in which these materials can be used and welcomes industry's involvement. This work includes feasibility studies in pursuing these ideas with other engineering materials to enhance durability, reduce corrosion and so help the environment.
