Ageing of carbon/epoxy and carbon/vinylester composites used in the reinforcement and/or the repair of civil engineering structures

Abstract

This paper presents an experimental approach to study the durability of FRP composites used in the civil/structural engineering. Carbon epoxy (carbon/EP) and carbon vinylester (carbon/VE) composites were considered for accelerated ageing (thermal, hygrometric, chemical, thermochemical, hydrothermal, freezing–thawing cycles, etc.) over 18 months. The accelerated ageing tests were selected to reflect the real operating conditions on natural civil engineering environment. The same composites were also subjected to environmental ageing to assess the effectiveness and the relevance of the accelerated exposure. Changes in tensile strength and viscoelastic response were evaluated through mechanical testing and Kinemat analysis. The design factors prescribed by the existing principle guidelines were applied to experimental measurements. The results revealed the presence of drawbacks inherent in these regulations based on a determinist approach.

Introduction

The use of fibre reinforced polymer–matrix (FRP) composites in civil engineering applications has developed significantly developed in recent years. The applications of these materials include structural rehabilitation and the reinforcement of new infrastructures. The main aim in using the composites is, generally, to modify the properties in order to extend the life span of a structure [1], [2], [3]. The use of composites as replacements for conventional materials such as steel and timber is based on considerations including weight savings, tailored performance characteristics, and potential increases in overall durability and longevity. Carbon/vinylester and carbon/epoxy systems were often chosen due to easy processing and potential durability considerations. Yet, much remains to be described in terms of fundamental understanding of ageing and degradation mechanisms in the fields of the repair, strengthening and retrofit of concrete structures. The ageing process implies physicochemical and/or mechanical deterioration (Fig. 1) which leads to many changes in the properties of the composites to be characterised at different scales.

So far, the main short-term damage mechanisms of FRP composites are known better, while there are many gaps regarding the long-term projections for these mechanisms. The effects of coupling constraints are seldom taken into account especially in the field of civil engineering applications. The lack of data concerning the long-term performances of these materials and the lack of regulations regarding the prediction of their life span, constitute an obvious hindrance to further use and development.

The review of the scientific literature, dealing with the durability of composite materials used in the rehabilitation and reinforcement of civil engineering structures [4], [5], [6], [7], [8], [9], reveals a wide variety of ageing processes and many ambiguities between the recorded experimental results. This situation can be explained by the absence of standard characterisation procedures, the use of diverse fibre–matrix combinations and different ageing tests leading to contradictory results in the resulting large database.

Specific guidelines for assessment of the composites’ durability have already been proposed [10], [11], [12], [13], [14], [15]. These guidelines describe ways of identifying the most aggressive situations and then provide a series of reduction factors. These factors frame the variation of the deterioration potential of the investigated composite in a specific environment. Among these, the recommendations of the AFGC, proposed by Hamelin et al. [10], focus on the natures of the chosen matrix and reinforcement materials, the mechanical characteristics of the composite and the state of the concrete–composite interface (Fig. 2). The American Concrete Institute guidelines (ACI) [11] require the design of the ultimate strength to be determined by modifying the reported strength by an environmental reduction factor.

In this article, the changes in the mechanical and viscoelastic properties of composite materials, implemented under civil engineering, have been monitored. Composite specimens were subjected to different ageing constraints. Each of these constraints reflects the real conditions and the interaction with the surrounding environment. This study, specifically, focuses on estimating the residual characteristics of FRP composites, systems after ageing. The design/safety factors which are, generally, recommended in the existing guidelines were compared with coefficients experimentally determined; their validity and reliability were subsequently verified.