240 



PLAIN CONCRETE 



inR the safe unit values of plain concrete, and there is no 

 uniformity in the building laws of the several cities regard- 

 ing the strength of this material. This is shown by the 

 following table, which gives the working values of concrete 

 allowed by the building laws of several cities: 



UNIT WORKING VALUES OF CONCRETE ALLOWED BY 

 VARIOUS CITIES 



An average ultimate unit compressive strength of 2,000 Ib. 

 would be conservative for a 1-2-4 concrete, from 1 to 3 mo. 

 old, and 1,600 Ib. for a 1-3-6 concrete of the same age. 

 Mixtures varying in richness between these limits would 

 have proportional values. 



The tensile strength of concrete is more affected by the 

 quality of the mixture than is its compressive stress. There- 

 fore, a conservative ultimate-tensile-strength value of a 

 1-2-4 mixture would be about 200 Ib., while for a 1-3-6 

 mixture, it would be about 125 Ib. 



The shearing strength of concrete is usually much less 

 than the compressive strength. For a 1-2-4 mixture an 

 average ultimate sheaiyng strength of 1,480 Ib. per sq. in. 

 has been determined* by tests, while a 1-3-5 mixture has 

 given 1,180 Ib., and a 1-3-6 mixture has given a value in 

 shear of 1,150 Ib. per sq. in. 



The average modulus of rupture for plain concrete that was 

 from 33 to 35 da. old has been found to be 439 Ib. per sq. in. 



