scope:

Introduction

Traditionally, hardened concrete has not been perceived to be a material which has contributed emissions adversely affecting the quality of the natural environment. Indeed, concrete construction in contact with the natural environment constitutes the bedrock of infrastructure and the built environment. Additionally, hardened concrete has never been shown to be responsible for any incidence of environmental pollution. Accordingly, within the range of traditional compositions used in the EU Member States, concrete's environmental service record can be taken to be unblemished.

Concrete, unlike most other construction materials, is an active material; its chemical and physical microstructure develops in a continuous process as it ages. These changes give rise to a densification of the matrix, with attendant reductions in porosity/permeability and a more efficient/effective binding of chemical species within the hydrate structures. It would be expected that concrete's leaching behaviour would also be subject to age-related changes and that this would be dissimilar to many other materials. Much research indicates that this is the case and so calls into question whether protocols, derived as in this study, from those developed for testing inert materials, are at all appropriate for concrete.

Concrete is, however, in common with other construction materials, subject to continual product development. Its compositional complexity is increasing, as constituent materials, formerly considered to be marginal, are either now in use or being considered for use. In the absence of quantitative information, some of the more marginal materials (e.g. where a total analysis reveals an apparently high heavy metal content) can give rise to concerns about their potential emission levels.

In addition, environmental regulatory activity, although at different points in the cycle in different EU Member States, is more and more subject to centralised direction via instruments such as EU Directives and mandates, and is generally increasing in its pace and scope.

Within this operational framework, standardised leaching tests, whether national or international, have taken a range of forms:

• characterisation;

• compliance;

• verification;

each of which can be used to evaluate the environmental performance/compatibility of hardened concrete, under different specified conditions using different assessment criteria. Characterisation leaching tests consist of an availability (granular or pulverized specimen) procedure and a sequential/periodic tank (monolithic specimen) procedure which together provide the means for discriminating between the several transport processes such as:

• dissolution;

• wash-off;

• diffusion;

and for predicting the rate of leaching and long term behaviour of a material.

In addition, physical characteristics such as tortuosity, which is a measure of the prolonged path along which leached components have to travel, can be calculated.

Compliance leaching tests consist of single extractions of short duration, generally without agitation, and which permit a direct comparison with regulatory limits for individual analytical components. Such tests use the prior output from characterisation tests to establish and optimise their parameters.

Verification leaching tests are essentially second order compliance tests, modified for operation in the field and used to identify/assess changes in established performance of batches of a material.

A final, and desirable, element in any authoritative procedure designed to evaluate environmental performance would be the preparation and maintenance of a certified reference material (CRM), for example, a certified reference concrete, preferably used within the context of a proficiency testing scheme (PTS), in order to monitor the performance of a laboratory and validate the accuracy of its procedures. In the case of concrete, the preparation and robust certification of a CRM is unlikely to be either attempted or to be feasible given the continuous changes in microstructure to be expected, with the likelihood of associated changes in its leaching characteristics.

Accepting that a concrete CRM is unlikely to be developed, then the preparation of a standard leachate, again for use within a PTS, would be the minimum expected for validation of laboratory performance.

It should be understood that the complete analysis of a concrete (or any of its constituents) in order to give a total elemental composition, is generally acknowledged to be of little environmental value and would be rarely undertaken in testing given that the greater proportion of most analytical components, whether environmentally significant or not, is known to be insoluble under naturally occurring exposure conditions.