In the late 1980s, some researchers introduced the unorthodox concept of strengthening concrete structures by external bonding Fiber Reinforced Polymers (FRP) products to the tension face of beams. This resulted in the first paper published on this subject. Following the collapse of many bridges in the 1989 Loma Prieta earthquake, California Department of Transportation (Caltrans) was seeking ideas to confine bridge piers. The same researchers were the first team to propose an extension of their research to externally wrap those piers with FRP. Shortly thereafter, the author received the first U.S. National Science Foundation funded grants to study retrofit of bridge piers and unreinforced masonry walls with FRP products. What was considered to be an unusual approach by many skeptics at the time has since become a mainstream technique for repair and retrofit of structures worldwide.
The flurry of global research and development activities in FRP products has resulted in a number of international conferences and the publication of the American Society of Civil Engineers (ASCE)’s Journal of Composites for Construction since 1997. For many rehabilitation projects, the high tensile strength, light weight, durability and versatility of FRPs have made these products the material of choice. Numerous buildings, bridges, pipelines, etc. have been retrofitted with these products worldwide. With the publication of design guidelines it is fair to say that FRP is no longer an experimental product but rather a relatively well accepted construction material. New applications such as blast retrofit of buildings have been also been recently proposed. The forms of FRP products that have been used to date can be categorized into two categories: fabrics and Pre-cured products.
Fabrics offer the widest versatility in the field and are installed following a procedure commonly referred to as the wet lay-up method. This technique requires properly trained technicians to prepare the resin in the field, saturate the fabric with resin and apply it to the structural member. Care must be taken to remove all air bubbles before the fabric is cured. The strength of the finished FRP product is greatly influenced by the quality of the installation.
Pre-cured products are manufactured in plants with higher quality control and uniformity. Partly due to their higher fiber to resin ratios, these products usually offer higher strength and stiffness than their wet lay-up counterparts. Pre-cured products are available in the shape of reinforcing rods or tendons as well as narrow unidirectional laminate strips; the latter is typically produced in widths ranging from 3-4 inches (75–100 mm) and a thickness of approximately 0.05 inches (1.3 mm). In the field, these laminate strips are bonded to the exterior surface of the structural element using epoxy putty.
Although the laminate strips offer ease of installation and higher strength than the wet lay-up system, their use has been relatively limited for the following two reasons: the unidirectional reinforcement in these strips makes them primarily suitable only for flexural reinforcement of beams and slabs, with some applications for shear strengthening of beams; and the stiffness of the laminate strips does not allow them to be coiled into a circle smaller than approximately 30 inches (750mm) in diameter (as discussed later this is a major limitation for certain applications).
The current equipment and technique used to manufacture these laminate strips does not lend itself to making larger laminate sheets that are appreciably different from these products. Thus, the overcoming of the above shortcomings in laminate strips is not a trivial matter.
For over two decades the construction community has been using either fabrics in a wet lay-up application or laminate strips. These materials have by nature had limitations and they have prevented structural engineers from offering cost-effective high quality solutions to several applications.
Super laminates are a new generation of FRP products that have been recently developed, by the researchers discussed above, to overcome the shortcomings of the above mentioned laminate strips. As discussed below, these products make many applications that have challenged the engineering and construction professionals for decades possible. In some cases, the solutions would not have been possible without the development of super laminates.
Super laminates are constructed with specially designed equipment. Sheets of carbon or glass fabric up to 60 inches (1.5m) wide are saturated with resin and passed through a press that applies uniform heat and pressure to produce the laminate. Super laminates offer three major advantages over conventional laminates. First, by using unidirectional or biaxial fabrics, the laminate may provide strength in both longitudinal and transverse directions. This is a tremendous advantage that opens the door to many new applications. Secondly, they are much thinner than conventional laminate strips; with a typical thickness of 0.025 inches (0.66 mm), they can be easily coiled into a circle with a diameter of 12 inches (300 mm) or smaller. Lastly, the number and pattern of the layers of fabrics can be adjusted to produce an endless array of customized products that can significantly save construction time and money (following figure).
The flexibility of the super laminate is demonstrated by coiling it in smaller diameters and by easily folding a corner of the large carbon panel. Super laminates can be produced with highest quality control under ISO 9000 certification; this will provide a leapfrog advance in wider acceptance of FRP products in construction projects.