412 AGRICULTURAL ENGINEERING 



Material for one yard oj concrete of different proportions. 



ADDITIONAL DATA: 1 bbl. of Portland cement weighs 376 Ibs.; a 

 sack, 94 Ibs. A barrel contains 3.5 cu. ft. between heads. Concrete 

 weighs about 150 Ibs. per cu. ft. 



A medium mixture is used for thin foundation walls and 

 for floors and sidewalks. 



An ordinary mixture is used for heavy walls which are 

 not subject to heavy strains. 



A lean mixture is used for heavy work where the material 

 is subjected to only compressive stresses. 



Reinforcement. Concrete is a very good material to 

 carry compressive stresses. Concrete and steel have very 

 Gtnenfu ^^ nearly the same coefficient of ex- 

 pansion for changes in tempera- 

 This makes possible the use 



ture. 



i 



Fig. 262. sketch showing of a combination of j these mate- 



the proper location of steel in . 1/1 



a concrete slab to resist tensile rials to the very best advantage 



stresses due to bending. . ., ,. . , 



in building construction. The 



steel is placed in position to resist tensile stresses to the 

 best advantage, and the concrete is poured around it. 

 When used economically the cross-sectional area of the steel 

 is equal to J4 to 1 per cent of the cross-sectional area of the 

 beams. The steel is usually placed from % to 1 inch be- 

 neath the surface of the concrete, in order to be thor- 

 oughly protected from corrosion. 



