“NDE of Reinforced Concrete Strengthened with
Fiber-Reinforced Polymer Composites using
Infrared Thermography”
Jeff R. Brown and H.R. Hamilton III
University of Florida – Department of Civil and Coastal Engineering
ABSTRACT
The objective of this research is to investigate various heating and infrared (IR) scanning methods to
evaluate the bond between FRP composites and concrete. Experiments were performed on small-scale
specimens using IR heating and halogen lamps. A special scanner cart was also designed to facilitate
rapid scanning of an FRP surface (approx. 3 ft
2
/ min).
Infrared inspections were also performed on four full-scale AASHTO Type II girders that were loaded to
failure in the Florida Department of Transportation Structural Testing Facility. Vehicle impact damage
was simulated in these girders by cutting several of the girder’s prestressing tendons. An FRP
strengthening system was then applied to each girder in order to restore its structural capacity. IR
thermographic scanning was performed at various stages of loading and after failure in order to monitor
the effect of loading on debonded areas.
Keywords: Reinforced Concrete, FRP Strengthening, NDE, Infrared Thermography
INTRODUCTION
The use of fiber-reinforced polymers (FRPs) to strengthen existing civil infrastructure is expanding rapidly.
While the short-term performance and strengthening capabilities of FRP composites have been well
documented, techniques for evaluating long-term performance (durability) and quality control during
installation are still needed. Infrared thermal imaging inspection has been used to evaluate
FRP/Concrete bond in both laboratory and field applications in recent years. This technique allows non-
contact sensing and a more global range than that of traditional mechanical sounding methods (coin-tap).
An earlier study by Levar and Hamilton (2003) demonstrated that IR thermography is capable of
identifying debonded areas between concrete and FRPs. These experiments involved the structural load
testing of reinforced concrete beams that had been strengthened in flexure using FRPs. 500 W halogen
lamps were used to heat the FRP surface and an IR camera (Raytheon Palm IR250) was used to monitor
the surface temperature profile during cooling. Debonded areas appeared as “hot-spots” in the thermal
images and patterns of debonding were monitored as each beam was loaded to failure (standard 4-point
bending). Important observations from these experiments included the following:
•
The total debonded area increased as the load was increased up to failure.
•
Certain debonded areas appeared to have different thermal signal strengths.
The above-mentioned infrared thermography experiments were qualitative in nature. The current
research objective is to incorporate quantitative IR theral imaging techniques into the inspection process
and attempt to draw more detailed conclusions about observed defects. In quantitative thermography,
actual surface temperatures are recorded in a thermal image and post-processing of a sequence of
images allows for the examination of surface heating and cooling over time. This can reveal important
information about defect depth. For multi-layered strengthening systems, it would be useful to
differentiate a gap between the concrete and FRP (debond) from a gap between adjacent layers of FRP
(delamination). Quantitative techniques can also be used to evaluate the heat-flow characteristics of a
known defect over time. Can the presence of moisture or excess resin be differentiated from typical air
pockets? Do small problems at the time of installation develop into bigger problems later? The purpose
InfraMation 2003
ITC 092 A 2003-08-15
