(See Vol. XV, Proceedings Am. Concrete Institute) ─────────────────┬─────────────────┬─────────────────┬───────────────── Diameter in │Minimum Thickness│ Number of Rings │ Cross Sectional Inches │ of Shell in │ │Area of Each Ring │ Inches │ │ ─────────────────┼─────────────────┼─────────────────┼───────────────── 24 │3 │1 │.058 27 │3 │1 │.068 30 │3½ │1 │.080 33 │4 │1 │.107 36 │4 │1 │.146 39 │4 │1 │.146 42 │4½ │1 │.153 48 │5 │2 │.107 54 │5½ │2 │.123 60 │6 │2 │.146 66 │6½ │2 │.168 72 │7 │2 │.180 84 │8 │2 │.208 96 │9 │2 │.245 ─────────────────┴─────────────────┴─────────────────┴───────────────── Three methods for the reinforcement of concrete sewers are shown in Fig. 76 B. =93. Proportioning of Concrete.=—In the proportioning of concrete questions of strength, of permeability, and of workability[54] may need consideration. All of these qualities are affected by the amount of cement, the nature and gradation and relative proportions of the fine and the coarse aggregate, and the amount of mixing water used. Other things being equal the strength varies with the amount of cement put into the concrete. For the same amount of cement and the same consistency of the mixture, the strength increases with increased density of concrete (that is, with decreased voids), and the effort should be made so to proportion the fine and coarse aggregates as to produce the densest concrete (least voids) with the aggregates available. For the same consistency, the strength then will vary with the ratio of the amount of cement to the amount of the voids. So far as the mixing water is concerned, the greatest strength in the concrete will be attained at a rather dry mix; that which produces the least volume of concrete. The addition of more water results in a concrete of less strength; 40 per cent more water may give a concrete of less than half the normal strength. The reduction in strength is then very marked for the wetter mixes, and the water content used is a feature of considerable importance in the design of concrete mixtures. Permeability is affected by the same elements as strength, but the size and discontinuity of the pores have a greater influence.