International Journal of Adhesion & Adhesives 29 (2009) 56–66
Experimental and analytical investigations of creep of epoxy adhesive at
the concrete–FRP interfaces
Pania Meshgin
a
, Kyoung-Kyu Choi
b
, Mahmoud M. Reda Taha
c,
Ã
a
Department of Civil Engineering, University of New Mexico, 217 Tapy Hall, MSC01 1070, Albuquerque, NM 87131-0001, USA
b
Department of Architectural Engineering, Dankook University, Yongin-city, Gyeonggi-do 448-701, Republic of Korea
c
Department of Civil Engineering, University of New Mexico, 214 Tapy Hall, MSC01 1070, Albuquerque, NM 87131-0001, USA
Accepted 10 January 2008
Available online 3 February 2008
Abstract
This paper presents the results of experimental and analytical investigations on the long-term behavior of epoxy at the interface
between the concrete and the fiber-reinforced-polymer (FRP). Double shear experiments under sustained service load were performed on
nine specimens composed of two concrete blocks connected by FRP sheets bonded to concrete using epoxy. The primary investigation
parameters included the ratio of shear stress to ultimate shear strength, the epoxy thickness and the epoxy time-before-loading. Loading
was sustained for periods up to nine months. We show that the magnitude of shear stress to ultimate shear strength and the epoxy time-
before-loading could be the most critical parameters affecting creep of epoxy at the concrete–FRP interfaces. It was also found that the
creep of epoxy can result in failure at the interfaces due to the combined effect of relatively high shear stress to ultimate shear strength
and thick epoxy adhesive. This can have an adverse effect on the designed performance of reinforced concrete (RC) structures
strengthened with FRP. Based on the experimental observations, rheological models were developed to simulate the long-term behavior
of epoxy at the concrete–FRP interfaces. It is shown that the long-term behavior of epoxy at the interfaces can be properly modeled by
analytically for both loading and unloading stages.
r 2008 Elsevier Ltd. All rights reserved.
Keywords: Epoxy (A); Concrete (S); Rheology (M); Creep (P)
1. Introduction
As an infrastructure ages, rehabilitation of existing
reinforced concrete (RC) structures has emerged as a
primary issue to the civil engineering community. In the
last decade, the use of fiber-reinforced-polymer (FRP)
sheets and plates appeared as a promising alternative
to retrofitting materials (i.e. steel) due to advantages
of FRP such as high corrosion resistance, durability,
and high strength to weight ratio [1,2]. Because of its
ease of application, external FRP sheets are recognized as
an efficient strengthening alternative for existing RC
structures and are being widely used in buildings and
bridges [1,2].
However, the performance of RC structures externally
strengthened with FRP is highly dependent on the quality
of the bond provided by the epoxy adhesive at the
concrete–FRP interfaces. Recently, Roberts [3], Ziraba
et al. [4], Malek et al. [5] and Ahmed et al. [6] showed that
poor quality bonds at the concrete–FRP interfaces result in
unexpected stress distribution and significant stress con-
centration at FRP cut-off points. Such stress concentration
can initiate FRP debonding/delamination. Coronado and
Lopez [7] found that once the interfacial stresses exceed a
threshold value, numerous microcracks develop in the
concrete substrate beneath the concrete–epoxy interfaces.
Such microcracks can gradually grow to macrocracks and
cause premature failure at the interface. Moreover, Teng
et al. [8] and Hiroyuki and Wu [9] recognized that shear
strength does not increase with increases in the concrete–
FRP bond length beyond a minimum length that has been
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doi:10.1016/j.ijadhadh.2008.01.003
Ã
Corresponding author. Tel.: +1 505 277 1258.
E-mail address: mrtaha@unm.edu (M.M. Reda Taha).