REINFORCED CONCRETE &7 



360. Waterproofing. Waterproofing shall be effected by 

 using a concrete of maximum density in connection with special 

 reinforcement to prevent leakage cracks (Par. 355d), or by a 

 waterproof coating, or by addition of an approved waterproof 

 compound. 



The proportion of aggregates to be used in obtaining such 

 concrete shall be determined for each case by experiments. 



The waterproof coating shall generally consist of from 4 to 8 

 layers of a strong flexible felt or cloth fabric, so treated in manu- 

 facture that all pores are closed, cemented together with specially 

 prepared preparations of asphalt, coal tar or coal-tar pitch, 

 or of a thick layer of a similar bituminous compound applied as 

 a mastic, upon the surface after being troweled off with cement 

 mortar. (Par. 377.) 



361. Weatherproofing. Concrete may be rendered weather- 

 proof, even waterproof under moderate pressure, by applying 

 alternate washes of aluminum sulphate and soap, or by a wash 

 of cement grout. These washes shall be applied by a soft 

 brush after the surface of the concrete has been thoroughly 

 cleaned. 



362. Construction Joints. Construction joints in reinforced 

 concrete shall, whenever unavoidable, be located as follows: 



In slabs and beams, near the center of the span. 



In girders near the center of the span, except when another 



girder or beam joins at this point. 

 In such case the joint shall be offset a distance equal to 



the depth of the girder and shall be reinforced for shear 



if necessary. 

 In columns horizontal joints shall preferably be made flush 



with the lower side of the girders, or the upper side of the 



floor line. 

 All construction joints in slabs, beams, girders, or columns 



shall be perpendicular to the plane of their surfaces. 



363. Expansion Joints. In exposed work, when not fully 

 reinforced for temperature stresses, special expansion joints 

 shall be provided at intervals usually not exceeding 75 ft. Guides 

 for cracks shall be provided at distances to be approved by the 

 Engineer. 



Expansion joints shall interlock, wherever practicable, usu- 

 ally by tongue-and-groove joints in the concrete, reinforced by 



