PART 2—MATERIALS
CHAPTER 4—CONSTITUENT
MATERIALS AND PROPERTIES
The physical and mechanical properties of FRP materials
presented in this chapter explain the behavior and properties
affecting their use in concrete structures. The effects of
factors such as loading history and duration, temperature,
and moisture on the properties of FRP are discussed.
FRP strengthening systems come in a variety of forms
(wet layup, prepreg, and precured). Factors such as fiber
volume, type of fiber, type of resin, fiber orientation,
dimensional effects, and quality control during manufacturing
all play a role in establishing the characteristics of an FRP
material. The material characteristics described in this
chapter are generic and do not apply to all commercially
available products. Standard test methods are being developed
by several organizations, including ASTM, ACI, and CSA,
to characterize certain FRP products. In the interim, however,
the licensed design professional is encouraged to consult
with the FRP system manufacturer to obtain the relevant
characteristics for a specific product and the applicability of
those characteristics.
4.1—Constituent materials
The constituent materials used in commercially available
FRP repair systems, including all resins, primers, putties,
saturants, adhesives, and fibers, have been developed for the
strengthening of structural concrete members based on
materials and structural testing.
4.1.1 Resins—A wide range of polymeric resins, including
primers, putty fillers, saturants, and adhesives, are used with
FRP systems. Commonly used resin types, including epoxy,
vinyl esters, and polyesters, have been formulated for use in
a wide range of environmental conditions. FRP system
manufacturers use resins that have:
• Compatibility with and adhesion to the concrete
substrate;
• Compatibility with and adhesion to the FRP composite
system;
• Resistance to environmental effects, including but not
limited to moisture, salt water, temperature extremes, and
chemicals normally associated with exposed concrete;
• Filling ability;
• Workability;
• Pot life consistent with the application; and
• Compatibility with and adhesion to the reinforcing
fiber; and
• Development of appropriate mechanical properties for
the FRP composite.
4.1.1.1 Primer—Primer is used to penetrate the surface
of the concrete, providing an improved adhesive bond for the
saturating resin or adhesive.
4.1.1.2 Putty fillers—Putty is used to fill small surface
voids in the substrate, such as bug holes, and to provide a
smooth surface to which the FRP system can bond. Filled
surface voids also prevent bubbles from forming during
curing of the saturating resin.
4.1.1.3 Saturating resin—Saturating resin is used to
impregnate the reinforcing fibers, fix them in place, and
provide a shear load path to effectively transfer load between
fibers. The saturating resin also serves as the adhesive for
wet layup systems, providing a shear load path between the
previously primed concrete substrate and the FRP system
440.2R-12 ACI COMMITTEE REPORT
4.1.1.4 Adhesives—Adhesives are used to bond precured
FRP laminate and NSM systems to the concrete substrate. The
adhesive provides a shear load path between the concrete
substrate and the FRP reinforcing system. Adhesives are also
used to bond together multiple layers of precured FRP laminates.
4.1.2 Fibers—Continuous glass, aramid, and carbon fibers
are common reinforcements used with FRP systems. The
fibers give the FRP system its strength and stiffness. Typical
ranges of the tensile properties of fibers are given in
Appendix A. A more detailed description of fibers is given
in ACI 440R.
4.1.3 Protective coatings—The protective coating protects
the bonded FRP reinforcement from potentially damaging
environmental and mechanical effects. Coatings are typically
applied to the exterior surface of the cured FRP system after
the adhesive or saturating resin has cured. The protection
systems are available in a variety of forms. These include:
• Polymer coatings that are generally epoxy or polyurethanes;
• Acrylic coatings that can be either straight acrylic
systems or acrylic cement-based systems. The acrylic
systems can also come in different textures;
• Cementitious systems that may require roughening of
the FRP surface (such as broadcasting sand into wet
resin) and can be installed in the same manner as they
would be installed on a concrete surface; and
• Intumescent coatings that are polymer-based coatings
used to control flame spread and smoke generation per
code requirements.
There are several reasons why protection systems are used
to protect FRP systems that have been installed on concrete
surfaces. These include:
• Ultraviolet light protection—The epoxy used as part of
the FRP strengthening system will be affected over
time by exposure to ultraviolet light. There are a
number of available methods used to protect the system
from ultraviolet light. These include: acrylic coatings,
cementitious surfacing, aliphatic polyurethane coatings,
and others. Certain types of vinylester resins have
higher ultraviolet light durability than epoxy resins;
• Fire protection—Fire protection systems are discussed
in Sections 1.3.2 and 9.2.1;
• Vandalism—Protective systems that are to resist
vandalism should be hard and durable. There are different
levels of vandalism protection from polyurethane coatings
that will resist cutting and scraping to cementitious
overlays that provide much more protection;
• Impact, abrasion, and wear—Protection systems for
impact, abrasion, and wear are similar to those used for
vandalism protection; however, abrasion and wear are
different than vandalism in that they result from
continuous exposure rather than a one-time event, and
their protection systems are usually chosen for their
hardness and durability;
• Aesthetics—Protective topcoats may be used to conceal
the FRP system. These may be acrylic latex coatings
that are gray in color to match bare concrete, or they
may be various other colors and textures to match the
existing structure;
• Chemical resistance—Exposure to harsh chemicals,
such as strong acids, may damage the FRP system. In
such environments, coatings with better chemical
resistance, such as urethanes and novolac epoxies, may
be used; and
• Submersion in potable water—In applications where
the FRP system is to be submerged in potable water, the
FRP system may leach compounds into the water
supply. Protective coatings that do not leach harmful
chemicals into the water may be used as a barrier
between the FRP system and the potable water supply.
4.2—Physical properties
4.2.1 Density—FRP materials have densities ranging from
75 to 130 lb/ft3 (1.2 to 2.1 g/cm3), which is four to six times
lower than that of steel (Table 4.1). The reduced density
leads to lower transportation costs, reduces added dead load
on the structure, and can ease handling of the materials on
the project site.
4.2.2 Coefficient of thermal expansion—The coefficients
of thermal expansion of unidirectional FRP materials differ
in the longitudinal and transverse directions, depending on
the types of fiber, resin, and volume fraction of fiber. Table 4.2
lists the longitudinal and transverse coefficients of thermal
expansion for typical unidirectional FRP materials. Note that
a negative coefficient of thermal expansion indicates that the
material contracts with increased temperature and expands
with decreased temperature. For reference, concrete has a
coefficient of thermal expansion that varies from 4 × 10–6 to
6 × 10–6/°F (7 × 10–6 to 11 × 10–6/°C), and is usually assumed
to be isotropic (Mindess and Young 1981). Steel has an
isotropic coefficient of thermal expansion of 6.5 × 10–6/°F
(11.7 × 10–6/°C). See Section 9.3.1 for design considerations
regarding thermal expansion.
4.2.3 Effects of high temperatures—Beyond the Tg, the
elastic modulus of a polymer is significantly reduced due to
changes in its molecular structure. The value of Tg depends
on the type of resin but is normally in the region of 140 to
180 °F (60 to 82 °C). In an FRP composite material, the
fibers, which exhibit better thermal properties than the resin,
can continue to support some load in the longitudinal direction
Table 4.1—Typical densities of FRP materials,
lb/ft3 (g/cm3)
Steel GFRP CFRP AFRP
490 (7.9) 75 to 130 (1.2 to 2.1) 90 to 100 (1.5 to 1.6) 75 to 90 (1.2 to 1.5)
Table 4.2—Typical coefficients of thermal
expansion for FRP materials*
Direction
Coefficient of thermal expansion, × 10–6/°F (× 10–6/°C)
GFRP CFRP AFRP
Longitudinal, αL
3.3 to 5.6
(6 to 10)
–0.6 to 0
(–1 to 0)
–3.3 to –1.1
(–6 to –2)
Longitudinal, αT
10.4 to 12.6
(19 to 23)
12 to 27
(22 to 50)
33 to 44
(60 to 80)
*Typical values for fiber-volume fractions ranging from 0.5 to 0.7.
DESIGN AND CONSTRUCTION OF EXTERNALLY BONDED FRP SYSTEMS 440.2R-13
until the temperature threshold of the fibers is reached. This
can occur at temperatures exceeding 1800 °F (1000 °C) for
carbon fibers, and 350 °F (175 °C) for aramid fibers. Glass
fibers are capable of resisting temperatures in excess of 530 °F
(275 °C). Due to a reduction in force transfer between fibers
through bond to the resin, however, the tensile properties of the
overall composite are reduced. Test results have indicated
that temperatures of 480 °F (250 °C), much higher than the
resin Tg, will reduce the tensile strength of GFRP and CFRP
materials in excess of 20% (Kumahara et al. 1993). Other
properties affected by the shear transfer through the resin,
such as bending strength, are reduced significantly at lower
temperatures (Wang and Evans 1995).
For bond-critical applications of FRP systems, the properties
of the polymer at the fiber-concrete interface are essential in
maintaining the bond between FRP and concrete. At a
temperature close to its Tg, however, the mechanical properties
of the polymer are significantly reduced, and the polymer
begins to lose its