FACING CONCEETE SUEFACES 133 



Contraction in a wall may also be provided for by vertical joints 

 at intervals, which divide the wall into separate sections. As each 

 section contracts these joints widen ; in effect we are confining the 

 cracking to straight lines. Such joints may be made by moulding the 

 wall in sections, and allowing one section to set before the adjoining 

 sections are filled in. This results in a weak junction between new 

 and old sections. Sometimes the sections are toothed into one another, 

 or tongued and grooved. That is to say, in the end of one section a 

 vertical groove is formed, into which a projection on the adjoining 

 section fits. 



If it be desired to make perfectly sure of localisation of the con- 

 traction we must insure complete separation of adjoining sections 

 by inserting some foreign material between them. This may be done 

 by leaving a half-inch space between the tongued and grooved ends of 

 adjoining sections, and filling it in with pure asphalte. 



Experience seems to teach that, in the case of thin walls, contraction 

 joints should be provided about every 30 feet. In thick walls the joints 

 may be further apart. 



In order to limit contraction-cracking to straight lines, in the case of 

 concrete floors, such floors are laid in squares of about 4 feet side, any one 

 square being allowed to set before adjoining squares are laid. In most 

 cases, however, it is considered sufficient to divide up only the plaster 

 or granolithic, with which the floor is finished, into squares of the above 

 size. 



Facing Concrete Surfaces. — In building work, walls are often plastered 

 or "rendered." The usual mortar employed for plastering consists of 

 1 part cement to 3 parts clean sand ; but for surfaces on which animals 

 have to walk, or where water-tightness is required, the proportions had 

 better be 1 cement to 2 sand. The sand should be screened through 

 a 12 mesh sieve, that which passes through the sieve being used. 



A good surface for a floor is formed by a granolithic finish, which is 

 applied in two layers, as follows : — 



(1) A layer about £ inch thick, consisting of 1 part cement, to 



1 part clean sand, to 3 parts granite chips capable of passing a 

 f-inch square mesh. 



(2) An upper layer £ inch thick, consisting of 1 part cement, to 



2 parts granite chips capable of passing a £-inch square 

 mesh. 



