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12 chapter three Manufacturing techniques Several manufacturing techniques are available to produce FRP composites for highway infrastruc- ture, although a number of other methods exist for mechanical or aerospace industries (Figure 2). A brief discussion is provided to describe FRP manufacturing, and further details are available in other sources dedicated to composites manufacturing such as Mazumdar (2001) and Strong (2007). â¢ Pultrusion: the word pultrusion is derived from a combination of âpullâ and âextrusion,â which can represent the process of pulling continuous fibers impregnated in a polymeric resin to form a composite shape. Because of an automated manufacturing process, pultrusion is a suitable method for the mass production of FRP profiles having constant cross-sectional dimensions at minimal costs. The size of FRP sections or bars is determined by the configuration of a preheated pultrusion die, through which the resin-impregnated fibers are pulled and cured. By adjusting the shape of the die, virtually all types of structural sections can be produced from open to closed profiles (e.g., C-channels and hollow beams) without any restriction on their length. However, it can be noted that straight products are manufactured, and curved geometries in the pulling direction may not be achievable. â¢ Wet lay-up: the in situ impregnation of dry fiber fabrics with a resin matrix to form composite laminates or sheets is called the wet lay-up method. The fiber fabrics are mostly unidirectional (i.e., often called a 0Â° fiber angle), whereas multidirectional fabrics can also be employed. This approach, exclusively used to strengthen existing structural members, is a convenient means to manufacture FRP products without the use of special equipment or facilities. The wet FRP sheet is then bonded to the substrate of a structural member using an adhesive and cured at ambient temperature to increase the capacity of the member. It is important that the wet lay-up applica- tion be completed within a specific workable period prior to the hardening of the bonding agent. Either a single ply or multiple plies of FRP sheet may be used, depending on the amount of external reinforcement required to address the deficient structural capacity. Quality control is an important factor influencing the performance of strengthened members (e.g., proper bonding without entrapped air bubbles between the bonded FRP and substrate, surface preparation of the substrate, and curing conditions). â¢ Filament winding: this manufacturing technique produces FRP tubes for structural application (e.g., concrete-filled FRP tubes as load-bearing members). Continuous fiber strands are pulled from spools, impregnated with a liquid resin matrix, and transversed along a rotating mandrel. After completing the automated filament-winding processes, the shaped impregnated fibers are cured and removed from the mandrel. This method can conveniently tailor the thickness and fiber angle of FRP composites to satisfy the requirements of various end-users. â¢ Other techniques: although the foregoing three methods are predominantly utilized in manu- facturing FRP composites for structural applications, other approaches are available: vacuum- assisted resin transfer molding (VARTM), centrifugal casting, resin transfer molding, and compression molding. The VARTM technique can be used to manufacture FRP bridge decks. It requires open or closed molds with a vacuum bag where dry fibers are placed, and a liquid resin is gradually infused by a vacuum without air leakages. A visual inspection or vacuum pressure monitoring detects the presence of air bubbles or leakages. After squeezing out the residual resin, the impregnated fibers are cured to produce a structural composite.
13 FIGURE 2 Manufacturing process: (a) pultrusion (image courtesy of Strongwell); (b) wet lay-up [Yang et al. 2011 (used by permission from American Concrete Institute)]; (c) vacuum-assisted resin transfer molding (used by permission from Carbonbydesign). (a) (b) (c)