Briefing Note following the Discovery of Misreported ‘High’ Alkali Cement From Lafarge’s Westbury Works

Introduction
Lafarge have recently issued a Press Release stating that a problem had occurred at their Westbury Plant which resulted in cement being produced with the incorrect alkali content specified. Lafarge have announced that due to their own internal problems, for a period from September 2002 to December 2004, some cement produced by their works was designated to be of a lower alkali level than was actually the case.

This briefing note is designed to assist those who may be concerned that their projects could be affected, by explaining the significance of alkali content in cement and what options are available if this circumstance has arisen.

Alkali-silica Reaction in Concrete
Alkali-silica reaction (ASR) is the most common form of alkali-aggregate reaction. It is a chemical process in which alkalis (mainly from the cement) combine, in the presence of moisture, with certain types of silica in the aggregate. This reaction produces a gel which can absorb water and swell.

When conditions are favourable for the production of sufficient gel and its subsequent expansion, cracking and disruption of the concrete can occur.

Three features need to be present for ASR to occur in hardened concrete:

• a sufficiently reactive alkaline pore solution in the concrete
• a critical amount of reactive silica in the aggregate
• sufficient moisture in the concrete

The presence of alkali-silica gel does not necessarily mean damaging expansion will occur; the gel may remain undetected and have no effect on the performance of the concrete.

From the mid-1980s concrete specification in the UK should have included appropriate limits on the total alkalis and the reactivity of aggregates in the concrete mix, thus minimising the risk of damage from ASR.

The news that cement containing higher than certified levels of soluble alkalis has been incorporated in concrete in the south west of England has however re-awakened concerns.

If there is a risk that your project contains this cement, in the first instance you need to establish whether the concrete constituents and mix proportions do pose a risk.

• What was the cement source?
• Was a reactive aggregate used?

If the cement is suspected to contain high levels of alkalis and a reactive aggregate was used then the mix needs to be examined more closely.

• What is the cement content?
• Were cement replacements used (ggbs or pfa)?
• What is the total alkali content of the mix?

If the answers to these questions suggest that there are sufficient alkalis in the mix and a reactive aggregate is confirmed to be present, then further action may be needed.

The diagnosis of ASR normally comprises a combination of techniques beginning with site observations, and, if deterioration is seen and ASR cannot be discounted as a possible cause, sampling and testing.

Visible signs of deterioration due to ASR are very unlikely to be seen in concrete cast since 2002 (when the suspect cement was first produced), since this deleterious expansion process is slow. Therefore sampling and laboratory testing is required. Coring will provide suitable specimens for laboratory work. The most appropriate test is an accelerated expansion test. Cores are stored at 38°C in high humidity, conditions aimed to promote ASR. Even this accelerated test takes time - the changes in length of the cores are monitored over a period of time (normally one year) and the results interpreted against published guidance.

It may also be prudent to support this testing by microscopical examination of the concrete, initially to identify any possible early reaction sites and after the expansion test to assist in evaluation of the expansion reaction.

There are also wider implications to the problems generated by Lafarge at their Westbury works. Production was based on internal production quality control procedures (i.e. self-certification of product) without the involvement of an independent check testing authority. The failure of such a scheme on a single regional works has broader implications for other manufacturing works nationally. Confidence in self-certified products has been reduced and, as a consequence, serious consideration should be given to the re-adoption of independent verification.

Sandberg have extensive experience of the diagnosis of ASR from pre-construction risk assessment to evaluation of existing structures.