It is necessary to exercise great care in the placing of the concrete. Where practicable, the structure or object should be poured in one operation to avoid leaky joints, but when this is not possible precautions should be taken to secure a tight joint between concrete of different ages. The surface of concrete which has set must be cleaned of dirt and scum down to the true concrete. This surface then should be well whetted and painted immediately with a creamy mixture of cement and water before placing the new concrete. A good plan, when discontinuing work on structures intended to hold liquids, is to embed a 6 or 8 inch strip of tin or thin sheet metal to half its width in the concrete so that the other half will project into the new concrete. A wall thick enough to resist the stresses put upon it will generally resist percolation of water, but 6 inches may be considered as a minimum. Contraction and expansion must be controlled to avoid the occurrence of leaks. To guard against cracks due to unequal settlement or other causes, most concrete designed for water-tightness should be reinforced. In some mass work, special contraction joints, as described on page 23 may be necessary. Rules for the use of reinforcement and contraction joints can not be given, as the requirements in each case vary with the conditions to be met. * * * * * ORGANIZATION OF THE UNITED STATES DEPARTMENT OF AGRICULTURE October 21, 1929