WATERPROOFING 345 



coating. Sometimes this coating is simply a rich, dense 

 cement mortar; sometimes it contains a waterproofing com- 

 pound similar to those mentioned in connection with the 

 integral method; and sometimes a special waterproof cement 

 is used. 



The adhesion between the coating and the concrete 

 work requires the most careful consideration. The concrete 

 surface on which the waterproof coating is to be placed 

 must first be chipped and scraped to remove all glaze and 

 to obtain a rough texture. The surface must then be thor- 

 oughly washed with clean water to remove all dust, and all 

 cracks must be filled with mortar. While the surface is still 

 wet, it is painted with a mixture of cement and water of about 

 the consistency of thick cream. This mixture is applied 

 with a stiff brush and must be well rubbed into the surface. 

 Before this coat dries, the first coat of waterproof mortar is 

 put on, usually about i in. thick. This coat must be troweled 

 into the surface with great care. Then, before this coat sets, 

 another coat of waterproof mortar of about the same thick- 

 ness or a little thicker is applied. 



MEMBRANE METHOD 



Bituminous Membranes. The bituminous -membrane method 

 consists in the use of a felt saturated with bituminous 

 material and a bituminous binder. By a binder is meant a 

 material that binds two surfaces together; that is, an adhesive 

 material. By a bituminous material is meant any material 

 containing a large proportion of solid or semi-solid bitumen, 

 bitumen being that portion of pitch or asphalt that is soluble 

 in carbon bisulphide, benzol, petroleum, ether, or other simi- 

 lar solvent. 



The bituminous membrane is built up in place in suc- 

 cessive layers, and should form, when finished, a practically 

 homogeneous and continuous waterproof envelope. The 

 felt serves to hold the bitumen in place, while the latter is 

 the waterproofing material. 



Specifications for Coal-Tar Pitch and Felt Roof Over Con- 

 crete. The following specifications, known as Barrett specifi- 



