To secure a water-tight joint between old and new concrete,requires a great deal of care. Where the strain is chiefly compressive, as in foundations, the surface of the concrete laid on the previous day should be washed with clean water, no other precautions being necessary. In walls and floors, or where a tensile stress is apt to be applied, the joint should be thoroughly washed and soaked, and then painted with neat cement or a mixture of one part cement and one part sand, made into a very thin mortar.

Fig. 8. Rammer for Dry Concrete. (Shoe 6 inches square.)

Fig. 8. Rammer for Dry Concrete. (Shoe 6 inches square.).

Fig. 9. Rammer for Wet Concrete.

Fig. 9. Rammer for Wet Concrete.

In the construction of tanks or any other work that is to be water-tight, in which the concrete is not placed in one continuous operation, one or more square or L-shaped joints are necessary. These joints are formed by a piece of timber, say 4 inches by 6 inches, being imbedded in the surface of the last concrete laid each day. On the following morning, when the timber is removed, the joint is washed and coated with neat cement or 1:1 mortar. The joints may be either horizontal or vertical. The bond between old and new concrete may be aided by roughening the surface after ramming or before placing the new concrete.