ASPHALT. 213' 



A still cheaper construction is asphalt macadam. This is made 

 with a foundation course of 4 inch to 1 inch stone, thoroughly 

 rolled, upon which is spread loosely a 3 inch course of 2h to 1 inch 

 stone, which is sometimes harrowed, though the benefit seems ques- 

 tionable. Upon this top course is poured from pots a soft asphalt 

 cement, at about 320° F., in an amount equal to 1 gallon per inch 

 of thickness, so as to completely cover the stones, and bind them 

 together. For this purpose Bermudez asphalt is used instead of 

 Trinidad, because the melting point is lower and a portable kettle 

 is used right on the work so that the saving in fuel is a considera- 

 tion ; besides this the presence of the colloidal clay in the Trinidad 

 asphalt would necessitate constant agitation to prevent a possible 

 sedimentation at the elevated temperature. As soon as the asphalt 

 cement has been applied the surface is covered with | to | inch 

 stone screenings (chippings), which are swept over the surface and 

 then rolled till smooth with an 8-ton tandem roller. If a smoother 

 surface is desired another application is made after rolling, or even 

 after having been opened to traffic for a short time. This time 

 the application is in a much smaller amount, I gallon per square 

 yard. It is spread with brooms, or squeegees, and then covered 1 

 with Jin. stone chippings free from dust, and all retained on a 

 10 mesh sieve. It is then again rolled, though this may be omitted 

 if desired, traffic being allowed to complete the surface. This make* 

 a very good and inexpensive pavement for country roads. 



Asphalt Macadam, Mixed Method. 



Where a paving plant is available a superior form of the 

 previous pavement can be made at a slightly increased cost by 

 means of the mixing process. The stone in use is from 1| to h inch, 

 and is mixed with 10 to 15 per cent, of sand. The mineral aggre- 

 gate must be dry, though not necessarily hot, but the asphalt 

 cement must be heated to 320° F., and in sufficient quantity to 

 completely cover each stone. It is then spread and rolled with an 

 8-ton tandem roller. When well rolled and quite smooth a seal 

 coat is given as described for the penetration method. 



Foundation-. 



To the last has been left what really comes first, but it is only 

 necessary to say that this should be adequate to sustain the traffic 

 as imparted to it through the thickness of the surface and binder. 

 No pavement is stronger than its foundation, and as stated by 

 Mr. Charles N. Forrest: — 



"It must always be assumed that the foundation of the pave- 

 ment is sufficient to support the surface. It necessarily follows 

 that failure of a perfect surface is due to improper foundation. 

 It also necessarily follows that if the foundation is sufficient, or 

 perfect, any failure of the pavement is due to certain defects in 

 the surface. It does not appear to be good policy to endeavour to 

 design surface mixture with a view to compensating for certain 

 defects in the foundation." 



Bearing the foregoing in mind if is well to consider the thick- 

 ness of foundation necessary to sustain the traffic it will be called 



