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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.
The PRESIDENT said that the applause which had followed the reading of the paper indicated that the meeting as a whole had enjoyed it as much as he had. It had been made quite clear in the paper that the subject was a very difficult one, and it was a very pleasant departure from the usual procedure-for the Institution dealt with rather different subjects as a rule-to have a paper from so great an expert as Mr. Bennett, indicating how little we knew on the subject of sound.
1. These recommendations embody the views of the Institution of Structural Engineers on the use in structural work of steel as specified in British Standard Specification No. 548. They are based on investigations carried out by the Institution.