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Oct.1968 CONCRETE RESERVOIR DESIGN 1193 concrete. To insure concrete density throughout the structure, especially next to waterstops, and a low water - cement ratio constant vigilance by the inspector during placing operations is necessary. Concrete mix must contain hard, durable, and well -graded aggregates free from excessive silt or substances reactive with the alkali in the cement, at least six sacks of cement per cubic yard, as little water as possible, and yet still be workable. In no case should the mix contain more than 6 gal of water per 94-lb sack of cement. This is equivalent to a water cement ratio of 0.53 on a weight basis. Al- though a mix leaner than six sacks of to segregation during transportation and placing and somewhat more dense and impervious. Good concrete construction demands constant attention to proper placing procedures. The inspector must make sure that every load of concrete does not exceed the allowable water -cement ratio, is placed in the forms in such a manner as to avoid segregation, is con- solidated with the specified type of vi- brator, and is vibrated for the proper length of time and in a manner that ef- fectively consolidates the concrete and works large air bubbles to the surface. The common so-called "worm holes" in a formed concrete surface are not the result of air introduced into the mix Multi -rib or Rubber No.4 ® 12-in. 64n. PVC Waterst stop Labyrinth WaterSealant S9-in. min. No. 4 4 Q 12-in. Bondbreaker (a) (b) Fig. 12. Typical Alternative Floor .Taints cement normally would produce con- crete having a compressive strength of at least 3,000 psi at 28 days, some excess cement is needed for concrete used in hydraulic structures to insure satisfactory autogenous healing of cracks. An air -entraining agent is helpful in improving workability and increases the resistance of concrete to freezing and thawing. An air -entraining agent does reduce the strength of the con- crete by 14-16 per cent, but the bene- ficial effects offset this undesirable characteristic. By substituting poz- zolanic cement for 10-12 per cent of the portland cement in the mix, the concrete can be made less susceptible C -Apply Bandbreaker 8-in. min` Alternate Bars at Joint by an air -entraining agent, but usu- ally are due to improper or insufficient vibration of the concrete. The sources of such air may be from the fine ag- gregate, cold mixing water, or me- chanical air entrainment in the mixing operations. Concrete should not be placed during excessively hot weather or during freezing or near -freezing weather un- less special precautions are taken to keep the temperature of the concrete within safe limits. The importance of adequate curing is often overlooked; inadequate curing can result in low strengths, excessive cracking, surface spalling, and gen- erally poor wearing qualities. A 14- RICHARD BRADY &ASSOCIATES, INC. F-62 25B-140