42 



FAKM BUILDINGS IN SOUTH AFBICA 



be seen from the accompanying illustrations) which are wedge- 

 shaped. 



Arches should be built of carefully selected bricks, laid in cement 

 mortar consisting of 1 part cement to 3 parts clean, coarse sand. The 

 bricks should be soaked just before they are laid, in order to prevent 

 rapid absorption of water from the mortar and consequent weakening 

 thereof. The rise of an arch for an opening 3 feet wide may be about 

 3 inches, and proportionately larger for wider openings. 



Fig. 35 illustrates an outside door frame, the opening for which is 

 bridged by an arch consisting of two half-brick rings. These are used 



Fig. 35. 



in preference to one full-brick ring 9 inches thick, since the thickness 

 of the mortar joints, at the convex surface of each half-brick ring, will 

 be much less than that of the joints at the corresponding surface (extrados) 

 of the equivalent full-brick ring. In positions where great weight is to 

 be borne, as in a two-storey building, three half-brick rings would be 

 used. The space between the head of the door frame and the intrados 



is built in with pieces of brick, 

 this filling being called the core. 



While an arch is in process of 

 being built, a temporary support 

 usually of timber, known as a 

 centering, must be used. Such a 

 centering is shown in Fig. 36. It 

 is supported by planks P,P, the 

 lower ends of which rest on any convenient solid surface, such as a 



