CONCRETE 149 



freezing point of the mixture. Protecting coverings should 

 be left on the concrete for approximately five days to give 

 it an opportunity to harden. 



Removal of forms. Too early removal of forms may 

 seriously injure concrete work. For such work as vertical 

 walls where the stresses due to supporting its own weight 

 are not great, three days are a safe limit in clear, dry weather. 

 The other extreme may frequently be as long as five weeks 

 for reinforced work where the structure must support 

 itself, and, if the weather is extremely cold, longer periods 

 may be necessary. Beams, slabs, and arches are examples. 

 It is evident that it requires more lumber for form work 

 to keep a job moving when forms cannot be removed and 

 used over again; hence it is profitable to remove forms as 

 soon as it is safe. 



SPECIAL REQUIREMENTS AND STRENGTH 



Prevention of cracks. Any movement of the cement 

 mass after casting will cause cracks. Consequently the 

 soil on which cement walls or slabs are to be placed should 

 be of sufficient solidity not to settle or be moved by freezing 

 and thawing. For this purpose good drainage is necessary. 

 These conditions are secured by means of tamping the ground 

 thoroughly and then placing a sub-base of sand or cinders. 

 Sidewalks, floors, and smaller work should have from 4 

 to 6 inches of sand or cinders thoroughly rammed under- 

 neath. The walls should have a similar foundation unless 

 they extend deep into the ground. Flat cement work 

 similar to floors, sidewalks, etc., should be cut in squares 

 from 3 to 6 feet on a side with a groover. The object of 

 this is to weaken the slabs slightly at the grooves so that 

 if cracks occur they will follow the grooves. Slab work 

 exposed to changes of temperature should be cut through 

 the base course to permit contraction. A quarter-inch 

 joint is frequently left every 12 or 18 feet, the opening being 

 filled with tar or building felt. 



