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Mr. A. R. WARNES (Associate) said:-"That the subject of stone decay is of importance to the structural engineer is obvious, and Mr. Power is to be congratulated in bringing this subject before the members.
During a summary of the paper the following additional points were mentioned by the
author :- In several places in the paper attention has been drawn to the possibility that large masses of concrete may not behave in precisely the same manner as small specimens, and, on page 60, in discussing the application of residual coefficients, and the experimental evidence in support of their use, this point is particularly emphasised since in this case the specimens were smaller than usual. This problem of the effect of the mass of the concrete on its behaviour is receiving considerable attention in various laboratories throughout the world, and I propose to show two interesting photographs illustrating ways in which large masses of concrete behave differently from small masses. It is well known that when cement sets a considerable amount of heat is evolved. In concrete this heat results in a rise in temperature which depends on the mass of the concrete, the mix proportions, the insulation afforded by the shuttering, and the external conditions. My colleague, Mr. Norman Davey, has been carrying out experiments on this heat evolution and its effect on the strength and other properties of the concrete, and the two photographs illustrate results he has obtained on a mass of concrete measuring 3 ft. by 4 ft. by 2 ft. 6 ins. high. The first photograph is of a model which was constructed to represent the temperatures at various points across a horizontal mid-section of the concrete. The height of the model at each point represents the temperature rise at that point above the external air temperature. You will notice that the temperature increase is higher at the centre than at the edges where the loss of heat through the shuttering occurred. The second photograph represents the effect of this temperature gradient on the strength of the concrete at various points across the same section of the concrete. The strengths were obtained from cubes of concrete which were cast in the mass of the concrete in specially prepared moulds. You will see that in the same way as the temperature was higher at the centre of the mass the strength is also higher, and is over 50 per cent. greater at the centre than at the corners. Since both shrinkage and creep vary with strength, it is reasonable to suppose that they also will vary throughout the mass of the concrete. In addition concrete in large masses hardens at a time when the temperature increase due to heat evolution is considerable and consequently the return of the concrete to normal air temperature, which may in some cases take a matter of months, must be accompanied by a heat contraction which is additional to the normal shrinkage. As a matter of interest it may be mentioned that as a result of these effects very great precautions are being taken in the construction of modern concrete dams. In America, in particular, every effort is being made to keep the temperature of the concrete down by special cooling devices, and by
The question of determining camber allowances, discussed in general terms would not convey much real information, as each different type of bridge requires special consideration. This applies particularly to the two-hinged spandrel-braced steel arch
which introduces problems peculiar to itself. Consequently this type is taken as an example and considered in det'ail, in the hope that in this way the various principles and processes will be clearly demonstrated, so that suitably modified they may be of use when considering similar complex structures.