Chemical testing to harderned concrete and allied techniques

Chemical testing of hardened concrete is mainly limited to the identification of the causes of deterioration, such as sulphates or chlorides, or to specification compliance, involving cement content, aggregate/cement ratio or alkali content determination. Water/cement ratio, and hence strength, is difficult to assess to any worthwhile degree of accuracy, and direct chemical methods are of limited value in this respect. Some chemical tests are expensive, and will often only be used in cases of uncertainty, or in resolving disputes, rather than as a means of quality control of concrete.

Specialist laboratory facilities are required for most forms of chemical testing. Basic procedures for the principal tests are outlined below, and emphasis has been placed on the interpretation and reliability of results. Techniques and procedures are generally complex, and extreme care must be taken both during sampling and testing if accuracies are to be achieved which are of practical value. One of the major problems of basic chemical testing is the lack of a suitable solvent which will dissolve hardened cement without affecting the aggregates, and if possiblesamples of the aggregates and cement should also be available for testing. Other instrumental techniques, such as differential thermal analysis, requires expensive and complex equipment together with a high degree of skill and experience, but growing in usage. The range of techniques available to the cement chemist is wide, and many are of such a highly specialized nature that they are outside of the scope of this chapter. Attention has therefore been concentrated on those methods which are most commonly used for in-situ investigations, whilst the more important of the other techniques are indicated together with their most commonly used applications.

ASTM standards are available for commonly used chemical tests, but BS 1881: Part 124 (302) provides more comprehensive guidance and procedural details for many tests. These include cement content, aggregate content and grading, aggregate type, cement type, original water content and bulk density, as well as chloride, sulphate and alkali contents. These procedures apply to calcareous cements, and to natural or inorganic artificial aggregates. Additional background information and details are given by Figg and Bowden , and a comprehensive Concrete Society Technical Report offers further detailed guidance. It is particularly important that an engineer requiring chemical analysis of concrete should be aware of the limitations of the methods available, and in particular the effect that some materials’ properties may have on the accuracy of analysis. The most likely causes of lack of accuracy are:

(i) Inadequate sampling or testing
(ii) Aggregates contributing to the analysis
(iii) Cements with unusual and unknown composition
(iv) Changes to the concrete from chemical attack or similar cause
(v) Presence of other materials.

It is also essential that an experienced concrete analyst should be employed. He must be given a clear brief of the information required from testing, and all relevant data concerning the constituents and history of the concrete must be made available.