InfraMation 2002

ITC 035 A 2002-08-01

Detection of air blisters and crack propagation in FRP strengthened

concrete elements using infrared thermography

C W Hu*, J K C Shih+, R Delpak*, D B Tann*

cwhu@glam.ac.uk

* University of Glamorgan, Pontypridd, Mid Glamorgan, CF 37 1DL, UK

Email: cwhu@glam.ac.uk Tel: +01443 482159 Fax: +01443 482158

+ Ming Hsin Institute of Technology, No. 1, Hsin-Hsi Rd, Taiwan

ABSTRACT

This paper explores the possibility of (i) detecting the extents of air-voids between the bond-line of advanced

composite materials (FRPs) and concrete substrate and (ii) predicating crack initiation and propagation in a

reinforced concrete (RC) beam at early stage of failure. Both studies have been carried out successfully using

Infrared (IR) thermography.

The artificial blisters (air-voids) with controlled sizes, embedded between the interface of FRP and concrete, were

detected by the IR thermography remotely up to 20 metres away. The RC beam, which was initially at pristine

condition, was subject to continuously static or cyclic loading tests. The preliminary results show that the damaged

region of the RC beam, partially strengthened by glass fibre reinforced polymer (GFRP), which was covering the

cracks, was clearly identified using an IR thermal imaging system. The anticipated failure plane was proven to be

identical to the actual failure of the test beam.

Keywords:

Bond-line, Advanced Composite Materials, FRP, Crack Initiation/Propagation, Infrared (IR)

Thermography, Glass Fibre Reinforced Polymer (GFPR), Static/Cyclic Loading, Anticipated/Actual Failure Plane

1.

INTRODUCTION

1.1 Forward

It is generally accepted that care and maintenance of the infrastructure globally is a key issue of the new century and

a major challenge in the current decade. The construction materials used in infrastructure can often be regarded as at

different stages of integrity or distress, which could be due to: (i) prolonged period of use, (ii) over-loading at or

beyond serviceability limit, (iii) flawed initial design, (iv) poor workmanship and/or site supervision and (v)

aggressive environmental or chemical attacks. The recent report on the state of the Nation s Infrastructures in the

United States by the American Society of Civil Engineers estimates a remediation and retrofitting cost of US$ 1.3

trillion ($1.3 x 1012 or approximately £915 billion), ASCE (2001). Similar surveys elsewhere indicate comparable

costs of 5 billion per annum for the infrastructure in the United Kingdom, NCE (2001).

In many cases, demolition and reconstruction can be impractical, resource intensive, socially disruptive and

environmentally unacceptable. Hence, the alternative may remain to repair, strengthen and maintain the existing

structures and the built environment. Every retrofitting and upgrading activity should be preceded by an appropriate

material integrity/distress evaluation for a meaningful and cost-effective outcome. Therefore, with such an

increasing need in the construction industry, the non-destructive testing (NDT) techniques, such as IR

thermography, are being developed for use as predictive and preventive maintenance tools, Hu (2002).

1.2 Significance of Current Research

The research is intended to develop a robust and reliable inspection procedure to monitor the damage in building,

bridges and other infrastructures. This will help the practising engineers (i) assess the structural performance (ii)

verify quality assurance in final products and services, and (iii) make decision in whether to take preventative

strengthening and/or pre-emptive maintenance action, or allow the structure to become dysfunctional.

2.

EXPERIMENTAL PROGRAM

2.1 Case Study I – Detection of Blisters