Sieves shall conform to the U. S. Bureau of Standards specifications for sieves.
3. The fine aggregate shall be tested in combination with the coarse aggregate and the cement with which it is to be used and in the proportions, including water, in which they are to be used on the work, in accordance with the requirements specified in Section 6....
7. Coarse aggregate shall consist of crushed stone, gravel or other approved inert materials with similar characteristics, or a combination thereof, having clean, hard, strong, durable, uncoated pieces free from injurious amounts of soft, friable, thin, elongated or laminated pieces, alkali, organic or other deleterious matter.
The following Table indicates desirable gradings, in percentages, for coarse aggregate for certain maximum sizes.
| Gradings of Coarse Aggregates | |||||||||
|---|---|---|---|---|---|---|---|---|---|
| Maximum Size of Aggregate Inches | Circular Openings, Inches | Passing Screen Having Circular Openings ¼ Inch in diameter, not more than | |||||||
| 3 | 2½ | 2 | 1½ | 1¼ | 1 | ¾ | ½ | ||
| 3 | 100 | 40–75 | 15 per cent | ||||||
| 2½ | 100 | 40–75 | 15 per cent | ||||||
| 2 | 100 | 40–75 | 15 per cent | ||||||
| 1½ | 100 | 40–75 | 15 per cent | ||||||
| 1¼ | 100 | 35–70 | 15 per cent | ||||||
| 1 | 100 | 40–75 | 15 per cent | ||||||
| ¾ | 100 | 15 per cent | |||||||
The manufacture of small size cement pipe requires relatively more skill than equipment. As a result great care must be observed in the inspection of cement pipe and in the enforcement of specifications. For large size concrete pipe and reinforced concrete pipe the difficulty of holding the pipe together during transportation and lowering into the trench aid in insuring a good product.
Cement pipe is made by ramming a mixture of cement, sand, and water into a cylindrical mold and allowing it to stand until set. The mold is then removed and the pipe stands for a further period of time to become cured. The selection and proportion of materials, the amount of water, the method of ramming, the period of setting, the length of time of curing, and the control of moisture and temperature during this period are of great importance in the resulting product. E. S. Hanson[[52]] states that the most conservative engineers recommend a mixture of one sack of cement to 2½ cubic feet of aggregate measured as loosely thrown into the measuring box. In making up the aggregate, clean gravel or broken stone up to ¼ inch in size is used. The American Concrete Institute recommends that 100 per cent pass a ½-inch screen, 70 per cent a ¼-inch screen, 50 per cent a No. 10, 40 per cent a No. 20, 30 per cent a No. 30, and 20 per cent a No. 40. The materials should be carefully graded by experiment and not guessed at, as the behavior of all aggregates is not the same. Too coarse an aggregate is difficult to handle in manufacturing. It causes loss of pipe when the jacket or mold is removed and results in rough pipe, stone pockets, and pin holes through which water spurts when pressure tests are applied. Too fine an aggregate causes loss of strength and with ordinary mixtures tends to produce a pipe which will show seepage under internal pressure tests. The amount of water in the mixture will vary, from 15 to 20 per cent. The mixture should appear dry but should ball in the hand under some pressure.
Fig. 74.—Details of 24–Inch Concrete Pipe Form.