Degradation of fibre-reinforced cement composites Introduction

The durability of fibre-reinforced cements and concrete (frc) has been the subject of academic debate and commercial intrigue for around 40 years, which continues apace even as you read this. The level of interest is sustained by the considerable chemical and microstructural complexity of even the simplest frc. The time-dependent behaviour of the cementitious matrices is still not fully understood. This is particularly true of frc matrices, which are often of `ordinary’ Portland cement modified with various materials, or even of non- Portland cements. Residues of unhydrated cement continue to hydrate slowly for many years, and since many of the products of both this and primary hydration can be considered meta-stable, the hydrated phase assemblage will almost always be changing, albeit gradually, as time passes. Into this complexity we introduce fibres of various materials, compositions, morphologies and quality. The interactions between these fibres and the hydrating matrix are time- dependent over scales of months or years and occur on the microstructural level at the fibre-cement interface. The net result is that the engineering properties of some types of frc (in particular tensile/bending strength, toughness and strain to failure) can vary considerably with time; frequently, one or more will decline, giving rise to durability concerns.

Most fibres (with the exception of steel) are introduced as bundles of fibres rather than individual filaments and derive their reinforcing action from remain- ing as such. Initially, the matrix does not penetrate these bundles or completely surround the filaments (but may increasingly do so with time). In frc, the fibres tend to be more ductile than the matrix; frc is classed as a brittle matrix composite and the fibres are intended to imbue the composite with toughness. Analytical techniques derived for composites such as fibre-reinforced polymers (frp) ± where typically fibres are intended to act as individual filaments, are more brittle than the matrix and intended to provide stiffness and/or strength ± are, therefore, not appropriate for use with frc. Furthermore, frp is generally used in high specific value applications such as aerospace and sport, where a short-life/high-maintenance paradigm applies to durability studies. Frc is used in buildings and infrastructure where the exact opposite ± long-life/low-maintenance ± ethos applies. Thus we can see the investigation and modelling of frc durability must be unique and cannot be inferred from studies on other composite materials. Indeed, although the causes of degradation in all frc fall into a small number of broad categories, the details of how the properties of a given class of frc change with time are different for each fibre-matrix combination considered. As previously exotic fibres such as carbon become economically viable for use in concrete, and the variety of cementitious matrices for use therewith expands, one thing becomes increasingly true; frc durability is a law unto itself.

The remainder of this introduction will define some of the terminology used in frc studies, give some background on fibre types, configurations and produc- tion methods, and provide a primer on the mechanical behaviour of brittle matrix composites. The following section will discuss the time-dependent behaviour of frc, in particular how changes in properties are related to changes in the microstructure of the various composites. It also assesses how the behaviour can be modelled and predicted, in particular by using accelerated ageing techniques. Finally, future trends in frc, such as the use of textile reinforcement and the potential for load-bearing frc components will be briefly debated and sources of further information advanced. Overall, this chapter will attempt to review as much of the literature as possible and synthesise concise conclusions concerning the current state of frc durability paradigms.