fractions. CFRP composites that are applied using the wet layup method for field
applications to concrete will tend to have lower fiber volume fractions (more epoxy and
more voids) than CFRP composites constructed using vacuum bagging and other
advanced techniques used in a factory setting. The low thermal conductivity of epoxy
will lead to a higher thermal diffusivity and thermal effusivity for CFRP composites
applied to concrete than what would normally be expected for an aerospace quality
composite.
Table 6-18. Typical thermal properties for materials of interest (Maldague 2001)
k p c ax 106 e
Material (W/m-K) (kg/m3) (J/kg-K) (m2/sec) (J/m2-K) FRP
Concrete (moist) 1.8 2500 1280 0.56 2400 0.38
Concrete (dry) 1.0 2400 800 0.53 1683 0.22
Carbon FRP
( to fibers) 0.8 1600 1200 0.8 1067 0
Epoxy .2 1300 1700 0.14 795 -0.15
Air 0.024 1.2 700 28 4.5 -1.0
From the standpoint of step thermography analysis, the most important physical
property is thermal effusivity. Consider the one-dimensional model of a two-layer
sample provided in Figure 6-53. If the two materials have the same thermal effusivity,
the thermal mismatch factor, F, will be zero. In this case, Equation 6-5 reduces to
Equation 6-4 (homogeneous case). The surface temperature increase (AT) is plotted as a
function of timel2 in Figure 6-53. For F = 0, the resulting curve is linear. IfF is not
equal to zero, the surface temperature rise will diverge from the homogenous case at
some transit time, tT, which is the time required for the thermal front to travel from the
surface to the second layer. tT is proportional to the thickness of the top layer squared
divided by the thermal diffusivity (L2/a).