=95. Mixing and Placing Concrete.=—Careful workmanship is desirable in the mixing and placing of concrete in sewers since water-tight construction is desired. Because of the difficulty of inspecting concrete in wet, dark and crowded excavations, and the careless habits of workmen experienced in concrete sewer construction, the highest class of concrete work cannot be expected. The situation is met by designing thick walls as shown in the sections illustrated in Fig. 22 and 23. In the report of the Joint Committee on Concrete and Reinforced Concrete in Transactions of the American Society of Civil Engineers for 1917, on page 1101 the recommendation is made concerning the mixing and placing of concrete as follows:[61] The mixing of concrete should be thorough and should continue until the mass is uniform in color and is homogeneous. As the maximum density and greatest strength of a given mixture depends largely on thorough and complete mixing, it is essential that this part of the work should receive special attention and care. Inasmuch as it is difficult to determine by visual inspection whether the concrete is uniformly mixed, especially where aggregates having the color of cement are used, it is essential that the mixing should occupy a definite period of time. The minimum time will depend on whether the mixing is done by machine or hand. (_a_) Measuring Ingredients: Methods of measurement of the various ingredients should be used which will secure at all times separate and uniform measurements of cement, fine aggregate, coarse aggregate and water.