nent Floors and Walks. 381 



out from the cement 'the water needed to maintain its 

 plasticity and to assist in the setting. 



If the coarse materials are mixed with the cement dry 

 a large amount of air will be set free and entangled in the 

 concrete, which will prevent all spaces being filled, but the 

 chief difficulty comes from the air preventing the cement 

 from adhering to the surfaces. So strongly does air adhere 

 to coarse sand that it must be boiled some time under water 

 before it is all removed. 



469. Ratio of Ingredients fcr Concrete. The amounts of 

 each ingredient required to make a solid concrete with all 

 spaces filled depends upon the pore space in the different 

 materials. Trautwine assumes that for each ingredient the 

 voids are near enough to 50 per cent, so that as a safe work- 

 ing basis this should be taken. 



To make a cubic yard of concrete it would be necessary 

 to use, on Trautwine's basis,. 



Crushed rock. Gravel or screenings. Coarse sand. Cement. 



27cu.ft. 13.5cu. ft. 6.75cu.ft. 3.375cu.ft. 



This ratio for pore space is certainly larger than is likely 

 to occur and for farm purposes it will be safe enough to 

 take the ratios of 



Crushed rock. Gravel or screenings. Sand Cement. 



27 cu. ft. 12.69 cu. ft. 5.584 cu ft. 2. 122 cu. ft. 



These figures assume the pore space of the rock to be 47 

 per cent., of the gravel 44 per cent, and of the sand 38 per 

 cent. 



470. Ratio of Ingredients for Finishing. Where good 

 plastering sand is used for making the finishing surface the 

 pore space to be filled will be about 35 per cent, and this 

 would require a little more than one of cement to two of 

 sand, and unless there is some gravel or screenings to use 

 with the sand it will be safer to make the facing 2 of sand 

 to 1 of cement. 



