To obtain good water-resisting properties the first precaution is to make the concrete sufficiently wet. Dry-tamped backs, even from rich mixture, will always be porous and absorbent, while the same mixture in plastic condition will give blocks which are dense, strong, and water-tight. The difference in this respect is shown by the following tests of small concrete blocks, made by the writer. The concrete used was made of 1 part cement and 5 parts mixed fine and coarse sand, by weight.

No. 1. With 8 per cent water, rather dryer than ordinary block concrete, tamped in mold.

No. 2. With 10 per cent water, tamped in the mold, and the mold removed at once.

No. 3. With 25 per cent water, poured into a mold resting on a flat surface of dry sand; after 1 hour the surface was troweled smooth; mold not removed until set.

These blocks were allowed to harden a week in moist air, then dried. The weights, voids, and water absorption were as follows:

1 2 3

Damp Wet Poured

tamped tamped

Weight, per cubic foot, pounds..... 122.2 123.9 110.0

Voids, calculated, per cent of volume 25.9 24.9 33.3

Water required to fill voids, per cent of weight....... 9.8 9.4 12.5

Water absorbed, after 2 hours, per cent of weight. . . 8.8 6.4 10.5

The rate at which these blocks absorbed water was then determined by drying them thoroughly, then placing them in a tray containing water 1/4 inch in depth," and weighing them at intervals.

1 2 3

Damp Wet    Poured tamped tamped 

1/2 hour.......... 2.0 0.9 1.8

1    hour.......... 3.2 1.1 2.5

2    hours......... 4.1 1.6 3.2

4 hours......... 5.2 2.0 3.8

24    hours......... 6.1 3.4 7.0

48 hours......... 6.4 4.3 7.5

These figures show that concrete which is sufficiently wet to be thoroughly plastic absorbs water much more slowly than dryer concrete, and prove the importance of using as much water as possible in the damp-tamping process.