Constituent Materials and Properties - Strengthening Design of Reinforced Concrete with FRP - page 46

3.3.2 t hermoplaStiC r eSinS
Thermoplastic resins obtain their mechanical properties through entanglement of the
polymer chains; thus, they do not produce any cross-linking chemical process. The
entanglement process is reversible, so upon heating, the chains disentangle, causing
the polymer to turn to a viscous fluid. Upon cooling, the resin solidifies into a plastic
shape. Thermoplastic resins are very viscous at processing temperatures, easily causing
damage to long fibers. It is important to note that thermoplastic resins have no limits on
shelf and pot life, giving them an advantage in this respect (Barbero 2011). In addition,
composites made of these resins are easy to repair by heating individual parts to their
soft state, reshaping them, and then cooling them off (Hyer 1998). However, thermo-
plastic resins are more expensive than thermosetting resins and require higher energy to
form. Some of the widely used thermoplastic resins are listed in Table 3.4 (Hyer 1998).
3.4 FIBER AND COMPOSITE FORMS
For infrastructure-strengthening applications, fibers or composites are available in
various forms. The most widely used forms are
1. Uniaxial fiber sheets : These are sheets of dry continuous fibers in their sim-
plest unprocessed state. These sheets are the most commonly used fiber forms
with hand lay-up processing, in which the resin is manually added to make the
composite layers (Arduini, Tommaso, and Nanni 1997; Rasheed et al. 2010), as
illustrated in Figure 3.4.
2. 2-D fabrics : These are 2-D (two-dimensional) textile fabrics of continuous
uniaxial fibers along with woven fibers in the transverse direction. Because
of the weaving angle, the mechanical properties are slightly lower than
those of sheets with higher variability in the properties of the manufac-
tured fabric composites because of the existence of resin-rich regions at the
weave locations. It is also common to use the hand lay-up process with the
fabric (Bencardino, Spadea, and Swamy 2002), as seen in Figure 3.5.
3. Prefabricated plates : These are laminated plates manufactured from preim-
pregnated (prepreg) tapes or sheets. The prepreg lay-up process may include
autoclave oven curing with heat, pressure, and suction to improve the quality of
the final product (raise the T g and reduce the voids; Tamimi et al. 2011), as seen
in Figure 3.6.
4. Pultruded FRP bars and tape : These are pultruded FRP rounded bars or flat FRP
tape with a typical 60% fiber content by volume. They are used as resin-bonded
near-surface-mounted (NSM) bars or tape available as a final product (Alkhrdaji,
Nanni, and Mayo 2000; Rasheed et al. 2010), as seen in Figure 3.6-3.7.
3.5 ENGINEERING CONSTANTS OF A
UNIDIRECTIONAL COMPOSITE LAMINA
Unlike isotropic materials that have similar properties in all directions, a uniax-
ial lamina is orthotropic, with distinct properties along the fiber, transverse, and
through-the-thickness directions, as seen in Figure 3.8. Isotropic materials have