Uniform Applied Heat r < 0
Flux <3 = 0
Y _x AT measured
at surface
L L >0
Layer 1: p ki cl
Layer 2: P2 k2, c2
Note: Thickness of Layer 2
assumed to be infinite ____
tT Time1/2
Figure 6-53. Surface temperature increase due to uniform heat flux applied to multi-layer
specimen
Based on the values provided in Table 6-18, the thermal mismatch factor for carbon
FRP and concrete (assuming the FRP is on the surface) is 0.22. This indicates that once
the thermal front reaches the FRP concrete interface, the rate of temperature rise on the
surface will decrease (second derivative will become negative).
Since the thermal effusivity of air is significantly smaller than the effusivity of
concrete, the thermal mismatch factor for an air-filled defect will always approach -1.
This will result in a relative surface temperature increase with respect to the
homogeneous case. Epoxy-filled defects also represent a case where the thermal
mismatch factor is less than zero (F = -0.15).
It is interesting to compare the relative magnitudes of for a CFRP/concrete
interface and a CFRP/epoxy interface. According to the theoretical model (and assuming
the material properties provided in Table 6-18 are correct), the concrete beneath the
surface of the CFRP will have a larger effect on the surface temperature than the presence
of epoxy. Epoxy results in a relative surface temperature increase (F = -0.15) and the