Throughout their lifetimes, structures made from steel and concrete can experience extreme levels of wear and tear. Whether damaged by accidental impacts or natural disasters, or subjected to heavy loads, structures including bridges and overpasses will inevitably develop cracks and vulnerabilities over time. Such vulnerabilities can threaten the safety of the many people who depend on them.

To address some of these damages as quickly and thoroughly as possible, engineers have turned to a material named carbon fibre reinforced polymer – or ‘CFRP’ for short. This strong, lightweight material is made from strands of carbon fibres, which can be layered and embedded in epoxy adhesive to create laminate sheets.

By simply bonding CFRP laminates to the outside of damaged structures using a strong adhesive, engineers can improve their strength, stiffness, durability, and load-carrying capacity, providing a quick and highly effective fix for their structural issues. However, the technique isn’t without its drawbacks.

So far, joining two such CFRP laminates together has involved the use of a ‘splice plate’: usually a short piece of CFRP laminate that bonds by traversing over the ends of two adjacent laminates to maintain continuity. Research has found these splice plates to be ineffective due to premature debonding. To prevent the need for splicing of CFRP laminates, they need to be applied as one long continuous laminate. This is not only time-consuming and labour-intensive; it is also incredibly difficult in areas where access is limited – including bridges over waterways, and overpasses crossing multiple lanes of traffic.

In 2011, Dr Abheetha Peiris along with his PhD advisor Dr Issam Harik, envisaged a solution to these problems as part of Dr Peiris’s PhD Dissertation. They proposed that if panels of CFRP strips or rods could be seamlessly fitted together during the retrofitting process, the retrofit could become far more resilient to debonding, and would also drastically cut down on time and labour costs.

The now patented concept centred on creating a finger joint – a technique widely used in woodworking to create strong, unbroken joins between planks.

Using the same principles, Dr Peiris and Dr Harik proposed that panels made of CFRP rods could be joined together sturdily and seamlessly. The concept involved designing the panels so that the rods are evenly spaced, and a portion would poke out of the sides to serve as the fingers. To achieve this, small-diameter carbon fibre rods are first placed side by side, with a uniform spacing between them. Then, the rods are mounted onto a fibreglass backing to maintain the spacing between them, while leaving the ends of the rods exposed. Similar panels can be made using thin strips of CFRP instead of rods.

For a particular retrofitting project, CFRP rod panels can be manufactured in any size or shape, then slotted into each other by interlocking these exposed rods. With this modular approach to assembly, panels can be fitted together one at a time when bonded to the outside of a structure – both removing the need for splice plates and cutting down the costs of labour and equipment.

One of the key challenges Dr Peiris faced when designing his new strengthening method is the drastically different mechanical properties of steel and concrete. If the CFRP panels used to retrofit a structure don’t match or exceed the stiffness of the substrate, the project would fail to achieve the desired results.

Through his research, Dr Peiris has shown how the CFRP material’s stiffness – defined by a value named its ‘elastic modulus’ – can be adjusted to match the material being strengthened, be it steel or concrete.

In two of his latest studies, he demonstrated two different CFRP designs: one with an ultra-high elastic modulus, which deforms very little when put under mechanical stress, making the material better suited for bonding to stiffer steel beams. In contrast, his other design has a lower elastic modulus to match the properties of reinforced concrete.

Over a series of 8 journal studies carried out since his PhD thesis, Dr Peiris and his research collaborators Dr Issam Harik and Dr Akram Jawdhari have verified the effectiveness of the CFRP rod and strip panel strengthening through experimental studies, analytical evaluations and also field applications. In these studies, he compared the performance of his CFRP rod panels with that of more traditional laminates, joined together with splice plates.

In the experimental studies on concrete, the panels remained firmly bonded together by their finger joints, even when subjected to levels of tension and crushing which caused debonding failure in the more traditional CFRP laminates. For Dr Peiris, this success clearly demonstrates the superiority of his retrofitting approach to the techniques most widely used today.

Dr Peiris has now applied these designs across 10 bridge retrofit projects, mostly involving bridges that had been damaged by impacts from over-height trucks. Based on this initial success, Dr Peiris now hopes that his CFRP rod panels may soon become a popular retrofit method, especially for structures such as bridges over water and overpass bridges, where access is limited. If achieved, this could help engineers to maintain damaged, vulnerable, and hard-to-reach structures far more easily – all while keeping to tight constraints in time, equipment, and labour costs.