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Mr. C.J. JACKAMAN (Member of Council) said that the Institution was very greatly indebted to the author of this Paper. He had listened with great appreciation to what the author had said, and it had given rise to a number of questiom in his mind, which he would refrain from asking because there would not be time to answer them. But the author had covered many if not all the most salient features relating to aerodromes. His paragraph headed "A Nationa1 Necessity" was most opportune, as were also his remarks on accessibility.
Sir,-In the discussion on Mr. Gueritte’s illuminating paper, a speaker suggested that quicker hydration of the cement was partly responsible for the increase in strength of vibrated concrete. I think there is a great deal in this suggestion. The formation of colloid round a cement grain hinders the further access of water, and delays hydration. If this colloid could be removed, and fresh water allowed to reach the cement, a more uniform and richer colloidal solution would result, and, finally, a better gel. Increasing the period of mixing does this to some extent, and results in stronger concrete. The extreme rapidity and small magnitude of the motion in vibrating concrete is an infinitely more efficient method of tearing the colloid skin from its parent cement grain, and enabling fresh water to carry on the process of hydration. The formation of a very rich colloidal solution is shown by the quickness with which the concrete jellifies and stiffens enough for forms to be removed. If samples of vibrated andunvibrated concrete were tested shortly after casting, the vibrated concrete would show a very much higher percentage of water held chemically and colloidally, which is one way of expressing a stronger concrete. The reduction of voids does, of course, play its part; but is not responsible for all the increase of strength; and the rich colloidal solution forms a lubricant which facilitates compaction.
This Panel of the Institution’s Concrete Sectional Committee was set up two years ago for the purpose of collecting and collating available information in regard to the sectional resistance of various strata, and the effect on existing formulae, and the carrying capacities of piles of varying design. The Panel has already submitted a Preliminary Report, which deals with the classification and investigation of strata, standardised driving test, and use of a rational pile-driving formula. In presenting this Report, however, it was explained that the Panel members desired to study further some systematic pile driving records, having found that previous records available hardly afforded the requisite information. To assist in the collection of the essential data, a Pile Record Sheet was prepared, which was published by the Institution in June last, a Journal notice being inserted inviting members to apply for copies for filling in particulars for furthering the Panel’s work.