FRPs exhibit both lightness and strength

FRPs were developed in the early 20th century and used partly as a structural material in lifeboats and fighter jets during wartime. At present, they are indispensable in manufacturing automobiles and in other aspects of our lives. However, in the late 20th century, FRPs started gaining attention as a material for building and civil engineering materials. Professor Matsumoto offers the following explanation as his reason behind researching FRPs, which are an uncommon material for construction and civil engineering.

"All materials have advantages and disadvantages, but the biggest advantage of FRPs is their lightness and strength. Furthermore, the main materials of FRP are originally fibers; therefore, they resemble a cloth before being molded with resin, giving them a very high degree of freedom in shape. Another advantage is that they do not corrode as steel does. Although FRPs have not yet become widespread in construction, I believe their unique features can be utilized in many situations."

The biggest significant disadvantage of using FRPs is that we do not yet fully understand these materials or the best way to design and use them. Professor Matsumoto indicates that applications of these materials in construction are not suitable for mass production, unlike with automobiles for example. In addition, we need to learn how to use the material characteristics to achieve maximum strength, which is all important for construction.

"FRPs are materials with variable qualities, and their strength changes depending on the direction of the fibers. The structural property of FRPs has two aspects. The tensile strength, which indicates if the material will tear when pulled, is undoubtedly stronger than that of steel. However, the rigidity, which indicates the extent of deformity when force is applied, is sometimes less than half that of steel. Certain buildings in the world, even in Japan, are made entirely from FRPs. However, realistically, instead of using only FRPs, I think combining them with steel, concrete, and other materials and finding the appropriate material and place to use them will be wiser."

Utilization for seismic reinforcement of joints and steel materials

Professor Matsumoto discusses the reinforcement of steel materials and their joints with carbon-fiber-reinforced plastics (CFRPs) as an example of the combination of FRPs and other materials.

"As you may have seen the brace members in factories, L-shaped components, known as angle irons, are installed diagonally and fastened with bolts to form part of the structure. However, with buildings that were built per old structural design standards, the force and strain of an earthquake on the building can weaken the strength of the bolt holes which will likely cause structural failure. To address this issue, various solutions have been developed and put into practical use, such as reinforcement through welding and additional bolts. However, in reality, limited progress has been made in reinforcement work because of issues at some factories, such as a reluctance to weld due to the risk of sparks, or an inability to shut down for several days for reinforcement work. Therefore, we developed a simple method for affixing the carbon fiber over the bolt, injecting it with resin, and hardening it while it adheres."

According to Professor Matsumoto, the advantage of this method is that no large-scale preparation is required, and there is no need to build scaffolding or carry heavy steel plates. "When we conducted the experiment in several locations, students were able to complete the task in approximately two hours using only a four-legged scaffolding stepladder." As mentioned earlier, FRPs can be wrapped around the shape of an object; therefore, they can be flexibly adapted on-site.

Professor Matsumoto also believes that there is considerable potential for increasing the all important strength of the material.
"In the experiments, we deformed the structure horizontally to simulate the horizontal shaking of an earthquake, but the bolt joints did not break even after moving about 120 mm. Without reinforcement, it would have broken at approximately 30 mm and lost its supporting power immediately. However, if reinforced with FRPs, it will continue to maintain its strength up to an extent even after 120 mm, and will not suddenly collapse. This toughness is not due to the strength of the carbon fibers but rather an inherent property of steel. In other words, supplementing the structure with FRPs at its weak points allows us to make the most of the inherent qualities of steel."

Understanding how to get the best out of the variable qualities of FRPs

Owing to their low weight, GFRP bridges using glass fibers have already been developed as pedestrian bridges. Professor Matsumoto however, advises caution when using these "quirky materials".

"In regular FRPs, most of the fibers are in the vertical direction (longitudinal direction). However, if a hole is made perpendicular to the fiber and hooked to a bolt, then that part may act as a trigger and cause the bolt to slide out. In other words, the weakness of FRP is that if used incorrectly, it loses its strength. Therefore, we proposed a method for increasing the strength by tilting the fibers at a 45° angle."

However, if special FRPs with tilted fibers are used in all locations, then productivity will drop and the unit cost of materials will substantially increase. Therefore, to maximize performance, Professor Matsumoto proposed a method for attaching the diagonal-fiber-containing materials only around the bolts.

"Simply affixing FRPs with 0.5-mm-thick fibers diagonally on both sides of a 6-mm-thick regular FRP more than doubles the yield strength. Moreover, even if the structure is broken by an unexpected force, it will not suddenly lose its strength and the diagonal fibers will not fall apart. I think that FRPs can be used most advantageously in situations that make the most of their low weight and strength, such as in seaside pedestrian bridges, which are susceptible to corrosion, bridges in locations where temporary construction using heavy machinery is difficult, and temporary bridges erected during a disaster."

Developing a simple method that does not require adhesives

When reinforcing with FRPs, another drawback is affixing the fibers with adhesives. The performance of FRPs deteriorates if the adhesive peels off. Professor Matsumoto was working on the use of optical fiber sensors for detecting signs of adhesive peeling. However, he realized that there was no need to worry about the adhesives peeling off if no adheres were used. Therefore, he is currently investigating a method for maximizing the material performance without using adhesives.

"Roofs can be made of slender steel pipes, such as the steel three-dimensional truss structure (space frame) used for the festival plaza at the 1970 Osaka Expo. However, if an unexpected force greater than the design force is applied, such a roof might break. To reinforce it, we developed an extremely simple method of wrapping CFRPs around the center of the steel material."

The work is remarkably simple: similar to a plaster cast, the CFRP is fitted like a plaster cast to match the shape of the steel. Furthermore, Professor Matsumoto said that they succeeded in increasing the yield strength of the steel material by 20–30%. CFRPs are light; therefore, the weight increase can be minimized on the structure.

"Even if a compressive force is applied on the column, the CFRP-reinforced parts will not deform, as opposed to the unreinforced parts. This procedure involves covering the material with a semicircular pipe-shaped CFRP, which makes working on roofs very easy. Although the high material costs of the CFRPs themselves are unavoidable, the lightweight nature of these materials renders additional expenses such as a crane or scaffolding unnecessary, as well as not requiring advanced techniques. Therefore, overall, CFRPs can be considered to be more efficient and economical than other reinforcement materials. Moreover, Japanese manufacturers lead the CFRP market as they own the top three carbon fiber manufacturers. Furthermore, I think that the integration of construction technology with Japan’s material technology is also an advantage."

Seismic reinforcement of factories and buildings built as per old structural design standards is an urgent issue in Japan, a country prone to major earthquakes. We look forward to the widespread use of the simple seismic reinforcement using FRP developed by Professor Matsumoto and his team.

Reporter's Note

Professor Matsumoto says that he loved playing in the garden since he was a child, making all kinds of things with his hands using garden plants and soil. He wanted to work in building structures; thus, he studied at the Department of Architecture in the National Institute of Technology, Yonago College, and then progressed to the Toyohashi University of Technology.

"Perhaps because of the thriving automobile industry in the Tokai region, I had many opportunities to work with FRPs at the university, and I soon became an expert in FRPs." However, surprisingly, he said that this has not always been easy. "Experiments often fail, and a particularly large obstacle was the adhesive issue with FRPs." He experimented while brainstorming with his students, and after much struggle, he proposed a method that did not rely on adhesives. Failure is said to be the key to success, but perhaps the determination shown by Professor Matsumoto was also a key ingredient.