Concrete: Wet or Dry?

Progress, Volume IV, Issue 2, 1 December 1908, Page 62

Concrete: Wet or Dry?

(Contributed.) The proper amount of water to be used in mixing concrete has been a much discusser l question ever since the latter came into favour as a constructive material. Authorities have argued on one side and the other until it is hopeless to attempt to base a conclusion upon the current literature of the subject. The writer has carried out a simple experiment which strengthens his confidence in wet concrete. The idea of the experiment was derived from a similar one, a description of which was published in one of the cement journals some seven or eight years ago. Two boxes of equal size and shape were balanced, one on each end of a strong beam, which was supported under the centre like a "see-saw." When the adjustment was com-

pleted and the boxes exactly balanced, a support was placed under each and they were filled with concrete: one with a dry mixture and the other with a very wet mixture. The pioportions of cement, sand, and gravel used were the same in each case, the only difference being m the amount of water used. The dry concrete was tamped very thoroughly, but the other was too wet to admit of any tamping. As soon as the concrete had set the supports were taken from under the ends of the beam. The end containing the wet-mixed concrete sank at once, and the beam had to be shifted nearly two inches on its support in order to rebalance the boxes. This indicated that the concrete mixed wet was much denser than the dry. This was veiified when the boxes were broken away. The dry block was rough, and contained many unfilled spaces, while the wet was very smooth and contained no visible holes. When they came to be broken in half the wet block was much haider to split. From the lesults of the abore^ experiment, and from other personal observations and experiences, the writer concludes that a moderate excess of water above the amount absolutely necessary in mixing concrete is not injurious, but is, in fact, decidedly beneficial, as wetmixed concrete is: 1. Denser, stronger, and more impervious to water. 2. Cheaper to put in place, 3. Eas-ier to mix thoroughly, and 4. Gives a smoother finish on the surface next to the forms.