Summary: An experimental vibrator, operating on the top surface, has been used to compact freshly placed concrete (mix 1:2½:5; water-cement ratio 0·60) of low to very low workability. A study has been made of the effects on the compaction of slabs 18 in. thick of varying the frequency of vibration within the range 1,500-6,000 v.p.m., the acceleration within the range up to 12g, the amplitude within the range 0·004 to 0·064 in., and forward speeds of travel within the range 1-8ft per min. The density of the concrete was measured after test, while measurements were made during the experiments of the vibration occurring at various depths in the concrete. A small variation in the workability of the concrete was found to have a considerable efIect on the surcharge required and the depth of compaction achieved. Consequently the effect of the method of compaction has only been studied so far for concrete of one workability, only those tests where the compacting factor was within a narrow range around 0·80 being considered. When the vibrator operated at constant acceleration and constant number of vibrations per foot of travel, increase of amplitude of the beam increased the depth of compaction. At constant acceleration and amplitude of the beam, slower speeds of travel gave increased depth of compaction. When both the amplitude of the beam and number of vibrations per foot run were constant, the depth of compaction was unaffected by change of acceleration. The total work done on a particle at a given depth was found to be related to the density attained at that depth. A connexion has been found between equivalent depth of compaction and the product of the amplitude of the beam and the number of vibrations per foot run. The depth of compaction increases with this factor but attains a limiting value, probably dependent on the concrete mix and the characteristics of the vibrator. It seems desirable, with vibrators designed to operate at constant acceleration, to operate them with a large amplitude and low frequency.

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D.S.I.R.) referred to an informal international committee of engineers from several European countries which had been formed as a result of discussion at the Tenth International Congress of the Permanent International Association of Road Congresses held at Istanbul in 1956. Representatives of seven countries had got together with the idea of having a small body of people who could carry out a really detailed study of each other's practices and methods of design to compare and contrast the reasons for those designs.159. The members of the committee did not represent their countries in any way, but were all engineers of very wide experience in both practice and research on the subject of concrete roads. A report had been prepared as a result of the examination of these detailed methods of design and it was hoped to publish it in the near future. Some of the main conclusions had been very similar to those of the Authors.160. For instance, the thicknesses of concrete used for design purposes for given weights of traffic were similar in these seven European countries. For a particular set of conditions of soil and traflic, the design thicknesses ranged from 20 to 24 cm-a relatively small variation compared with some of the other variations in design which had become all too evident in the discussions. These thicknesses were expected to provide adequate service; but, since the members of the committee had not agreed on the expected life of the road, or on the exact nature of the traffic which was going to use the road, or, in fact, on what constituted failure, it had not been possible to arrive at a definite thickness which could be used in all conditions. 161. It had been agreed, however, that what was now wanted was information on the nature and volume of the traffic using the roads, its distribution across the road, and the nature of the permissible wheel loads used; that was where discussion had broken down in trying to reach precise agreement on the question of thickness. Mr Sparkes thought that the question of the thickness of the concrete had been about the only point on which the committee had reached any substantial measure of agreement. In alm?st all other respects there were large differences of opinion.162. There was wide variation in the adopted distances between expansion joints as the Authors had pointed out, from a distance of about 35 ft in Belgium up to almost t Proc. Instn civ. Engrs, vol. 9, p. 23 (Jan. 1958

AUTHORS' PRESENTATIONIn introducing a major new item of equipment such as a slip-form paver it was vitally important that its performance and operational characteristics should be evaluated and understood from the outset.139. The design of a satisfactory dowel bar support system had proved elusive for a number of years, although many arrangements had been tried. The Cromwell trials faced up to the problem, which was more acute with slip-form pavers, and the welded assembly with bent stalk feet in its final form had a substantial claim to general adoption as the standard British joint for use with conventional as well as slip-form construction. This would take much of the uncertainty out of present-day construction. 140.There had been a tendency to identify slip-form pavers with large-scale construction and very high outputs, but this overlooked their singular capacity to perform satisfactorily at lower output levels. The Authors believed that there was a much wider field of operation open to these pavers than had hitherto been contemplated, housing estate roads being one example.141. The pooling of resources of design; research and contracting interests at Cromwell in mutual consultation and co-operation throughout the planning and execution of the trials resulted in the development of designs and viable techniques in a remarkably short time, and this form of joint effort led to development in pavement construction which could not occur under normal construction processes.Sir William Glanville (Past-President) said the Authors had been responsible for one of the biggest and most important experiments in mechanized concrete construction which had ever been made in Britain. He had found the Paper particularly valuable because the Authors had described fully the difficulties they had encountered and how they had overcome them, and had pointed out quite frankly that some difficulties still remained.143. The Authors had shown very considerable ingenuity in overcoming difficulties and had demonstrated that with ski11 and intelligent supervision the dip-form paver could be made to work, and work well, both for plain concrete and for reinforced concrete. Sir William had no doubt at all that, given further opportunities for experiment, which he hoped that they would have, the Authors would overcome the remaining difficulties which they mentioned and develop still further the potentialities of the machine. The machine had, of course, a voracious appetite for concrete, and the team operating the machine needed to know not only how to operate it in a general sense but how to deal quickly with the ordinary run of problems, such as changing weather conditions and unexpected variations in aggregates. It seemed to him that it was one thing to operate the machine in the predictable weather of California and quite another to operate it in Britain. What was perhaps even more