380 Rural Architecture. 



a thin layer between the faces of the fragments to be joined 

 together. 



466. Materials for Concrete Floors. The materials used 

 for cement walks and floors should be (1) as large, clean 

 fragments of hard rock as can be readily mixed and worked 

 into the forms and thickness of layer desired; (2) a finer 

 grade of crushed rock or coarse clean gravel which will 

 readily pack into the voids between the larger fragments ; 

 (3) a clean, coarse, sharp sand to fill the pores between the 

 fragments of gravel or fine screenings ; (4) enough Port- 

 land cement to fill the space between the sand and bind the 

 whole together; (5) and finally, water enough to wet all 

 surfaces, fill the pore space of the cement and make the 

 mortar plastic, 



467. Presence of Earth, Loam or Dust. It is of the great- 

 est importance that all of th'e materials used be perfectly 

 clean and free from dirt or other fine grained material 

 having the texture of -the cement itself. If a fine dust is 

 present in the rock, gravel or sand it will tend to form a 

 layer over the surfaces of the fragments which prevents 

 the cement from coming in contact with the pieces which 

 are to be cemented together and a weak concrete results. 

 The fundamental is to have nothing but hard rock frag- 

 ments large enough to be cemented together and nothing 

 fine present but the cementing material itself. 



In the concrete pavements used on the streets of London, 

 and which have a much longer life than the best paving 

 blocks, great care is taken to wash out of the crushed 

 granite and its screenings all dust particles before using 

 them, although the dust may be from the granite itself. 



468. Wetting the Crushed Rock Before Use. There are 

 two important reasons why crushed rock or coarso screened 

 gravel, to be used as the body of concrete, should be wet be- 

 fore mixing with the cement. These are (1) to displace 

 as much adhering air as possible, and (2) so as not to draw 



