22 REINFORCED CONCRETE CONSTRUCTION 



and observations that, when well placed, the concrete affords 

 complete protection of the steel against corrosion. 



16. Modulus of Elasticity. To gain clearness in this study, 

 modulus of elasticity will be treated in Art. 25. 



17. Weight of Concrete. A dense, well made, 1:2:4 concrete, 

 when dry, will weigh approximately 155, 152, 148, and 143 Ib. 

 per cubic foot, according as the aggregate is trap rock, gravel, 

 limestone, or sandstone. These figures represent average values 

 although the weight is affected but little by any ordinary varia- 

 tion of proportions. It is found that a wet concrete when dried 

 out will weigh less than a well compacted concrete containing 

 originally less water. The addition of reinforcing steel in the 

 usual proportions will add from 3 to 5 Ib. Cinder concrete has 

 an average weight of 112 Ib. per cubic foot. 



The assumed weight of reinforced concrete is usually 150 Ib. 

 per cubic foot. 



CHAPTER II 

 STEEL 



18. General Requirements. The reinforcing steel in reinforced 

 concrete construction is mostly in the form of rods, or bars, of 

 round or square cross-section. These vary in size from about 1/4 

 to 3/8 in. for light floor slabs, up to 1 1/2 in. as a maximum 

 size for heavy beams. Somewhat larger sizes are sometimes 

 used for columns. Rods can be procured varying by 1/16 in. 

 increments from 1/4 in. to 1 in., and then by 1/8 in. increments 

 to 2 in. Above 2 in., rods to the nearest 1/4 in. should be se- 

 lected. Stock lists of steel companies should be consulted as these 

 show the normal stock in tons of the different sizes. Various 

 bars of deformed cross-section are widely advertised, some of 

 the more common types being shown in Fig. 6. Woven wire 

 and thin punched plates are sometimes used in thin slabs with 

 good effect. 



Authorities differ as to the quality of steel to be used for re- 

 inforcement medium and high carbon steel being used by 

 different engineers. Medium steel is better fitted for most classes 

 of structures than high steel, while in some structures high steel 

 will answer just as well although probably with very little gain in 

 economy. No matter what kind of steel is used in beams, low 

 unit stresses are much to be preferred on account of the large 

 distortions involved. 



