Sulfate Attack on Concrete - Chapter 7 doc

7 Modeling of deterioration

processes

7.1 INTRODUCTION

Hydrated cement systems are used in the construction of a wide range of

structures. During their service life, many of these structures are exposed to

various types of chemical aggression involving sulfate ions. In most cases, the

deterioration mechanisms involve the transport of fluids and/or dissolved

chemical species within the pore structure of the material. This transport of

matter (in saturated or unsaturated media) can either be due to a concentra￾tion gradient (diffusion), a pressure gradient (permeation), or capillary suction.

In many cases, the durability of the material is controlled by its ability to act

as a tight barrier that can effectively impede, or at least slow down the trans￾port process.

Given their direct influence on durability, mass transport processes have

been the objects of a great deal of interest by researchers. Although the

existing knowledge of the parameters affecting the mass transport properties

of cement-based materials is far from being complete, the research done on

the subject has greatly contributed to improve the understanding of these

phenomena. A survey of the numerous technical and scientific reports

published on the subject over the past decades is beyond the scope of this

report, and comprehensive reviews can be found elsewhere (Nilsson et al.

1996; Marchand et al. 1999).

As will be discussed in the last chapter of this book, the assessment of the

resistance of concrete to sulfate attack by laboratory or in situ tests is often

difficult and generally time-consuming (Harboe 1982; Clifton et al. 1999;

Figg 1999). For this reason, a great deal of effort has been made towards

developing microstructure-based models that can reliably predict the behavior

of hydrated cement systems subjected to sulfate attack.

A critical review of the most pertinent models proposed in the literature is

presented in this chapter. Some of these models have been previously

reviewed by other authors (Clifton 1991; Clifton and Pommersheim 1994;

Reinhardt 1996; Walton et al. 1990). The purpose of this chapter is evidently

not to duplicate the works done by others, but rather to complement them.

© 2002 Jan Skalny, Jacques Marchand and Ivan Odler

In the present survey, emphasis is therefore placed on the most recent

developments on the subject. Empirical, mechanistic and numerical models

are reviewed in separate sections. Special attention is paid to the recent

innovations in the field of numerical modeling. Recent developments in

computer engineering have largely contributed to improve the ability of

scientists to model complex problems (Garboczi 2000). As will be seen in the

last section of this chapter, numerous authors have taken advantage of these

improvements to develop new models specifically devoted to the description

of the behavior of hydrated cement systems subjected to chemical attack.

It should be emphasized that this review is strictly limited to microstructure￾based models developed to predict the performance of concrete subjected to

sulfate attack. Over the years, some authors have elaborated various kinds of

empirical equations to describe, for instance, the relationship between sulfate￾induced expansion to variation in the dynamic modulus of elasticity of

concrete (Smith 1958; Biczok 1967). These models are not discussed in this

chapter.

It should also be mentioned that this chapter is exclusively restricted to

models devoted to the behavior of concrete subjected to external sulfate

attack. Despite the abundant scientific and technical literature published on

the topic over the past decade, the degradation of concrete by internal sulfate

attack has been the subject of very little modeling work.

7.2 MICROSTRUCTURE-BASED PERFORMANCE MODELS

Over the past decades, authors have followed various paths to develop micro￾structure-based models to predict the behavior of hydrated cement systems

subjected to sulfate attack. Models derived from these various approaches

may be divided into three categories: empirical models, mechanistic (or pheno￾menological) models, and computer-based models. Although the limits

between these categories are somewhat ambiguous, and the assignment of a

particular model in either of these classes is often arbitrary, such a classifica￾tion has proven to be extremely helpful in the elaboration of this chapter. It

is also believed that this classification will contribute to assist the reader in

evaluating the limitations and the advantages of each model.

Before reviewing the various models found in the literature, the characteris￾tics of a good model deserve to be defined. The main quality of such a model

lies in its ability to reliably predict the behavior of a wide range of materials.

As mentioned by Garboczi (1990), the ideal model should also be based on

direct measurements of the pore structure of a representative sample of the

material. These measurements should be of microstructural parameters that

have a direct bearing on the durability of the material, and the various char￾acteristics of the porous solid (e.g. the random connectivity and the tortuosity

of the pore structure, the distribution of the various chemical phases . . .)

should be treated realistically. As can be seen, the difficulties of developing

© 2002 Jan Skalny, Jacques Marchand and Ivan Odler