128 FARM BUILDINGS IN SOUTH AFRICA 



coarse. It should also be free from all organic matter. The sand is a 

 most important constituent ; to use dirty, unsuitable sand is often tanta- 

 mount to wasting good cement. To test a sand roughly for cleanness, 

 rub some of it in the palm of one hand with the fingers of the other, and 

 note the extent to which the palm is discoloured. Eiver sand is generally 

 very suitable, as it is washed clean by the flow of the water in the river. 

 Dirty sand may be artificially washed on a fine-meshed screen. The 

 water carries the clay through the screen, while the clean sand is left 

 on top. 



Coarse sand makes a much stronger concrete than does fine sand. 

 Hence, if the use of fine sand cannot be avoided, the concrete should be 

 made richer, i.e. the proportion of cement should be increased. The best 

 sand is that which contains all sizes of grains, up to particles that will 

 just pass a J-inch square mesh. 



Stone. — The broken stone should be of a hard close-grained quality. 

 It may be used as it comes from the crusher (crusher-run), but it is 

 preferable to screen out the fine particles. The latter may, if desired, be 

 used as a portion of the sand. For mass work, stone which passes a 

 2^-inch ring, or even as high as a 3-inch ring, may be used ; for floors 

 the stone should pass a 1^-inch ring ; for reinforced concrete the stone 

 should be capable of passing a f -inch ring. 



In the body of a thick concrete wall, such as a dam wall, large, sound, 

 clean stones may be imbedded without detriment to the structure. Such 

 work is sometimes called " Cyclopean masonry." 



If gravel be used it should be thoroughly clean. A layer of clay or 

 dirt on the stones comprising the gravel prevents adhesion between the 

 cement and the stones, and weak concrete is the result. Concrete made 

 with gravel is, as a rule, slightly less strong than concrete made with 

 broken stone. 



Setting. — The hardening of cement is spoken of as " setting." Cement 

 concrete goes on hardening for an indefinite time after it has been made. 

 The setting or hardening is due to a process of crystallisation of the 

 cement. Since crystallisation proceeds more perfectly when the material 

 is undisturbed, it is important that concrete should not be shaken, jarred, 

 or interfered with while the initial setting is taking place. The rate at 

 which crystallisation proceeds depends on temperature, thus cement sets 

 much quicker in hot, than in cold weather. It is therefore advisable to 

 use a little extra water when mixing concrete during hot weather, as 



