FORMS FOR CONCRETE; CURING CONCRETE 133 



d) One that is true and properly aligned. 



e) One that is made of good material suited to its use. 

 Soft woods are better than hard because they (a) are cheaper, 



(b) do not crack so badly, (c) are an easier material to work. 

 Spruce and yellow pine make good forms; the boards used 

 should be sound and free from knot holes. Partly green lum- 

 ber is better than either green or kiln-dried, because it will 

 swell just enough to make tight joints without buckling. 

 Dressed lumber has several advantages over undressed: (a) 

 It makes truer work, (b) tighter joints, (c) smoother surfaces, 

 (d) forms are easier removed, and (e) forms are easier cleaned. 



131. Use of Old Lumber for Forms. Where old lum- 

 ber is to be used, it should be sorted and listed so that new 

 lumber can be ordered of proper sizes that will work in best. 

 Care must be observed in the use of old lumber to see that it 

 is strong enough to support the load put on it by the con- 

 crete. A great deal of expense can be avoided by taking ad- 

 vantage of old lumber. 



132. Sharp Corners in Forms. Sharp corners should 

 be avoided as much as possible in concrete work. It is best 

 to bevel the corners by setting strips in the forms, especially 

 on inside angles. This gives both greater strength and better 

 finish to the work. 



133. Removing Forms; Care of Forms. Forms should 

 not be removed until the concrete is thoroly set. The time of 

 setting varies with the wetness of the mixture, and with the 

 weather. Concrete sets much faster in warm, dry weather 

 than in cold or damp weather. On foundation walls or simi- 

 lar work, where the concrete is used in direct compression, the 

 forms may be removed in a few days. Under floors or beams, 



