412 AGRICULTURAL ENGINEERING 



Material for one yard of concrete of different proportions. 



Mixture 



Proportions 



Cement, bbls. 



Sand, bbls. 



Gravel, bbls. 



Rich 



1:2:4 

 1:2^:5 

 1:3:6 

 1:4:8 



1.57 

 1.29 

 1.10 



.85 



3.14 

 3.23 

 3.30 

 3.40 



6.28 



Medium 



Ordinary 



Lean 



6.45 

 6.60 

 6.80 







Additional data: 1 bbl. of Portland cement weighs 376 lbs.; a 

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

 weighs about 150 lbs. 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 



nearly the same coefficient of ex- 

 pansion for changes in tempera- 

 ture. This makes possible the use 

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

 rials to the very best advantage 

 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 34 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. 



Concr ete &ee*m-y 

 .V > J V \ > 



1 



S 



t- Steel Qe,r>r ncem e nt Ew^ 



the proper location of steel in 

 a concrete slab to resist tensile 

 stresses due to bending. 



