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Mr. D.A.G. REID, in a few words supplementing Dr. Glanville’s presentation of the paper, said that he might try to express in another form what he felt to be the value of the wet mortar test. It amounted practically to this, that if one had a strength-age curve for the wet mortar, and then immediately one had also a strength-age curve for a 1:2:4 concrete with a water-cement ratio of 60 per cent., and, with the aid of
a few general relationships, strength-age curves could be constructed for any other concrete made with that cement; and the strengths taken off from such strength-age curves would be correct to within about 10 per cent. If, however, any such operation were carried out with a dry mortar test, inevitably it seemed errors of 20 per cent. or more crept into the estimates. Of course, that could be got over if the mortar- concrete strength relationship were available for the particular batch of cement, but that contingency was not very likely. Dr. Glanville had mentioned the carrying out of
preliminary tests before the commencement of the main programme. The results of these tests had proved very instructive, and had been of considerable value in assisting in the interpretation of the results of the main series. Mr. Reid felt that this portion of the work would be of great interest to anyone connected with the testing of cements and concretes.
Psychology. To appreciate the conditions of the problem which we are studying to-night it is necessary to visualise the circumstances which surround buildings at the present time, the conditions under which we live, and the constructional materials in common use. All these circumstances have very direct bearing, not only upon the amount of noise which exists, but upon the extent to which people using buildings and living in houses, are uncomfortably conscious of this noise; in fact, it may not be too much to say that consciousness of noise is very much more important than noise itself.
Revolutionary changes in structural design are so rare that particular interest attaches to occasional reports, principally from foreign countries, describing the manufacture in light aluminium alloys of structures which would normally be of steel. Clearly the choice of material is not dictated by some irresponsible whim, but by economic considerations, or other equally strong justification. For many years, the use of aluminium alloys has been standard practice for certain special types of structures, e.g., aircraft, and if proof were necessary of the suitability of aluminium alloys for withstanding stress, fatigue, corrosion, and other factors looked for in a structural material, it would be found in the behaviour of the delicate,
complicated girderwork of a rigid airship, or the less intricate but equally elegant structure forming the fuselage and wings of an aeroplane. For such structures, however, the paramount importance of the stress-weight ratio reduces to a secondary consideration those questions of comparative cost which in general engineering are so important. The fact that the cost of duralumin may be 2s. 6d. per lb., whereas structural steel costs less than 2d., is of less importance than the fact that l-lb. of
duralumin in the form of a bar of any given length will support nearly three times the
load carried by l-lb. of structural steel of the same length.