The papers in this issue were presented at an Ordinary General Meeting of the Society in February 1979. They illustrate some of the aggregate problems that can arise in modern concrete and the value of petrological and geochemical techniques in the study of potentially deleterious systems. It is common to regard concrete aggregates as inert filler. Perhaps in Britain this is fair, although in recent years several examples of damage due to aggregate instability have been described. Internationally, the difficulties of finding good aggregate are more profound; British flint may make good concrete but Danish and German flint is often reactive and extremely deleterious. The problems are particularly acute in extreme climatic zones and where hard rock and good gravel is scarce or must be transported for great distances.

The Chairman of the first session, Mr A. D. Robertson of Sir William Halcrow and Partners, referred to the very great interest created by the problem of the behaviour of concrete in the Middle East and recalled that one of the first occasions on which this had been aired was in the Society's meeting room, as recently as 1975. Since the first signs of trouble had manifested themselves in Bahrain the scale of construction in the Middle East had increased dramatically and the earlier problems had been exacerbated by the need to find sources of aggregate in quantities far in excess of earlier demands for use in more elaborate forms of construction capable of sustaining higher stresses. Early considerations of concrete

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