34 



THINGS TO KNOW. 



(1) The tensile value of concrete is about 1-10 the 

 compressive value. Tetmajir's formula: 



T= 



Compressive Strength . 

 8. 64+1.8 log. of age in mo. 



(2) Concrete in mass is 1-50 the cost of steel, and 

 reinforced concrete about 1-30, volume for volume. 



(3) Steel will support 10 times the load on concrete 

 when the same sectional area is considered. 



(4) The cost of concrete in compression Is 6-10 that 

 of steel 1, on the average. This is approximate. 



(5) By properly mixing the sizes of broken stone or 

 gravel the voids can be reduced 20%. 



(6) Compressive strength of concrete depends on the 

 proportion of cement to sand in the mortar. (See page 



These can be found by Thachers formulae: 



Volume of sand 



7 day old equals 1800-200 ( ) 



Volume of cement 

 1 mo. old equals 3100-350 

 3 mo. old equals 3820-460 

 6 mo. old equals 4900-600 



Ultimate Compressive Strength of Concrete as Determined 

 by Timelier' 8 Fornntlox. 



Mixture 



Age 



