PLAIN CONCRETE 247 



such difficulties, the stone should be thoroughly wetted 

 with a hose, and the sand and stone should be kept under 

 cover, away from the direct rays of the sun. Likewise, the 

 mixing platform or machine should be roofed over. It is 

 well, also, to wet down the finished concrete work with a 

 hose several times a day in extremely hot weather, and less 

 frequently in moderate temperatures. 



Concreting in Freezing Weather. Although it is practi- 

 cable to mix and ,place concrete at a temperature as low as 

 27 P., it is not advisable to lay concrete work when the 

 temperature is below 32; neither should it be mixed and 

 placed even at this temperature, if there is a possibility that 

 the temperature will fall. If concrete is frozen, its setting 

 is retarded and it is liable to become worthless, never properly 

 setting and obtaining the requisite hardness and strength. 

 There is, however, no certainty of the action of frost on 

 concrete, as frozen concrete will frequently thaw out and 

 set, with apparently little loss of strength. 



To prevent the freezing of concrete when the temperature 

 has fallen below 32 P., salt is sometimes used in the mixture. 

 The addition of 1J Ib. of salt to the water used with 1 bag 

 of cement will not decrease the strength of the concrete; 

 or, a 10% solution of salt can be used in the water employed 

 in mixing the concrete. The addition of salt, however, is 

 never advisable if a surface finish is required, as it is liable 

 to cause efflorescence, or a white deposit, on the surface 

 causing the work to become very unsightly. 



Aggregates that are coated with ice or that have been 

 exposed to severe weather for a long time should be heated 

 or thawed out before being used. Concrete that is exposed 

 to freezing after it was set should always be protected by 

 placing over it a layer of boards and straw, or salt hay, or 

 cement bags; or, where the work is in the nature of a rein- 

 forced-concrete floor system, by heating the interior of the 

 structure by means of salamanders or fires. 



Joining of Old Concrete With New. New and old concrete 

 can be joined only with difficulty, and the strength of such 

 a connection is always uncertain. The joining of old and 

 new concrete work is best done by thoroughly chipping, or 



