Cement Content Testing
The cement content of concrete is important from the aspect of durability, impermeability and strength. Too low a cement content may cause inadequate structural capability or more frequently may not provide a durable protective environment for the steel reinforcement, permitting rapid carbonation and subsequent loss of the protective alkaline environment for the steel. Too high a cement content may cause excessive shrinkage, particularly if inadequately cured, thermal cracking from the heat of hydration if large pourings are involved, or the risk of alkali silica reaction if a susceptible aggregate has been used and the cement is not a low alkali type.
The test involves crushing a representative sample of the concrete usually a core, down to a fine powder and chemically analysing for insoluble residue, soluble silica and lime content. If control samples of the cement and aggregates are available then it is simply a matter of analysing these for the same parameters when the cement content can be determined by simple proportion. If control samples are not available then reasonable assumptions as to their composition must be made. It is normally possible to assess the cement content to within plus or minus 15 percent and often better.
Prior to carrying out the analysis the concrete sample is examined in order to assess the approximate cement content and to broadly classify the nature of both coarse and fine aggregates under the following categories:
Type I: Natural aggregates essentially insoluble in dilute hydrochloric acid.
Type S: Natural aggregates essentially soluble in dilute hydrochloric acid.
Type O: Other aggregates.
Details of the method of test are described in BS 1881:Part 124:1988 and Technotrade Technical Procedures 1 & 2.