136 FAEM BUILDINGS IN SOUTH AFRICA 



is sprinkled over the surface of the old concrete, just before the new layer 

 is deposited. 



In cases where the stress is mainly compressive as, for instance, in 

 foundations, all that is necessary is to thoroughly clean and wet the 

 surface of the old concrete before the new is superimposed. 



Hard-core. — Horizontal, and also not too sloping layers of concrete, 

 such as floors, etc., are generally founded on a layer of stones of about 

 the size of a quarter of a brick. This layer is called " hard-core." It is 

 usually 6 inches thick, and should be well rammed with a heavy rammer. 



Repairing old Concrete. — A broken place in a concrete surface may be 

 repaired as follows : — 



Break away the loose broken concrete ; hack the surface thus exposed 

 so as to make it very rough ; sweep it clean with a stiff brush ; wet it 

 with clean water, and paint it over with neat cement grout (cream of 

 cement and water). While the grout is still wet, fill the hole with 

 cement mortar consisting of one part cement to two parts clean sand, 

 ramming it well in, and smooth off the new surface with a trowel. Keep 

 the patch wet for several days after laying. 



In the case of a patch in a granolithic floor, use the same mixture 

 to fill the hole with, as was used in the first instance for the purpose 

 of surfacing the floor. This is usually one part cement to two parts 

 granite chips capable of passing a ^-inch square mesh. 



Reinforced Concrete. — In recent years reinforced concrete, i.e. concrete 

 strengthened by iron or steel imbedded in it, has come into extended use. 

 Selected from a multitude of applications, the following may be mentioned 

 as specially appertaining to farming : fencing-posts, drinking-troughs, 

 storage-bins, dipping-tanks, silos, concrete reservoirs, walls of buildings, 

 weirs and dams. 



Object of Reinforcement. — Concrete is strong in compression, but very 

 weak in tension ; on the other hand, wrought iron and steel are very 

 strong in tension. 



If, therefore, in any concrete structure in which tensile stresses are 

 induced, steel or iron be imbedded so as to effectively take up these 

 tensile stresses, we will have a very strong form of construction. 



Or, stated from another point of view : the effective use of reinforced 

 concrete, in structures subjected to tensile stresses, will result, without 

 sacrifice of strength, in a much lighter structure than would be possible 

 if plain concrete were the material employed. 



