Summary Experiments have been made on concretes with varying aggregate/cement and water/cement ratios, with the main object of ascertaining the effects of changes in age and water/cement ratio upon the relation between ultrasonic pulse velocity and the compressive strength of concrete. It was found that the ratio of changes in pulse velocity and compressive strength due to a change in water/cement ratio is not generally the same as that due to a change in age. Because of this, the relation between pulse velocity and compressive strength cannot be expected to be independent of age and water/cement ratio. It appears, however, that, for compressive strengths up to about 4,000 lb/in2, the relation may for practical purposes be regarded as being so. For concrete of the same aggregate)cement ratio, it was found that low pulse velocity at an early age predicts low strength at later ages.

Summary It is well known that different types of aggregate produce different degrees of workability when used in concrete of given mix proportions and water/cement ratio. A rounded aggregate will, for example, give concrete of higher workability than all angular aggregate. Thirteen coarse aggregates were investigated to determine the effects of their shape, surface texture and water absorptive capacity on the workability of concrete. An attempt has also been made to assess these effects quantitatively. The main conclusions were as follows. (1) Changes in the angularity of coarse aggregates have a greater effect on the workability of concrete than changes in the flakiness of the aggregates. Increased angularity and/or flakiness leads to a reduction in the workability of concrete. (2) Although there was a wide variation in the surface textures of the aggregates, no correlation was found between this property and the workability of concrete. (3) The differences in the capacities of the aggregates to absorb water were insufficient to produce significant changes in the compacting factor. No correlation was, therefore, found between this property and concrete workability. This does not rule out the possibility that highly porous aggregates when used in a dry condition will affect the workability of concrete.

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Many authors have studied the safety of disposal of highly radioactive wastes in excavated cavities beneath the earth. 'Work has been concentrated in three areas: prediction of future events and processes which could aA'ect waste containment, mathematical modeling of failure scenarios, and estimation of uncertainties in model predictions. The results of past safety assessments are reviewed and compared in this paper. Anything but a very small release of radioactivity from a repository would appear to be quite unlikely; a quantitative evaluation of the probabilities of small releases has not proved possible. CONTENTS III. IV. V. VIII.Refe similar to those encountered elsewhere in the mining and nuclear industries (GEIS, 1980); the principal safety issue is the possibility that radioactivity would leak from a repository and cause damage at some time in the future. have addressed the safety of disposal at particular sites.More commonly, one studies a hypothetical "generic" site. Studies of generic sites are of considerable value in identifying the role of different elements of a repository

Summary Experiments have been made to determine the flexural and compressive strengths of concretes containing thirteen different coarse aggregates. In the compression tests, observations were made by an ultrasonic pulse technique to detect the onset of cracking prior to failure. The main results were as follows. Concretes containing smooth gravels began to crack at lower compressive stresses than did concretes containing coarser-textured aggregate. The compressive strengths did not show such appreciable differences. Concretes which best resisted pre-cracking in compression also gave highest flexural strengths. The relation between the flexural strength of the concrete and the stress at which cracks first occurred in compression was independent of the type of coarse aggregate; no such unique relation existed between flexural strength and ultimate compressive strength. Tentative conclusions are drawn on the mode of failure of the concrete as follows. Pre-cracking in a cube when tested in compression probably arises from local breakdown in the adhesion between the coarse aggregate and the cement. The average compressive stress at which the first cracks occur is determined by those properties of the aggregate which influence the aggregate-cement bond. Pre-cracking in compression does not lead to the immediate fracture of the concrete specimen. A contribution to the ultimate strength is provided by the mechanical interlocking of the coarse aggregate after pre-cracking has occurred. Thus the compressive strength of concrete is usually greater than the compressive strength of its comparable mortar. In flexure, pre-cracking causes fracture of the concrete specimen and the only properties of the coarse aggregate which influence the flexural strength are those which appear to determine the aggregate-cement bond. In general the flexural strength of concrete is lower than the flexural strength of the corresponding mortar.