(16) Prefabricated Asphalt Panels . A layer of a very dense 

 mixture of asphalt and filler sandwiched between two layers of some 

 tough, asphalt-impregnated material and usually coated with water- 

 proofing asphalt." 



b. Properties of Asphalt Materials . Asphalt has many properties that 

 make it particularly suitable for use in hydraulic and costal zone structures, 

 It is versatile in form and application. Asphalt can be used alone (as in 



an asphalt membrane), or it can be mixed with other materials producing 

 mixes for a variety of purposes. It can be combined with graded aggregate 

 to form a voidless and impermeable mix. On the other hand, it can be 

 combined with an open-graded aggregate to form a porous mixture allowing 

 free passage of water. 



Asphalt is stable in the presence of nearly all chemically-laden sub- 

 stances. It is normally unaffected by the usual concentrations of acid, 

 salt, and other waste solutions. This important characteristic makes it 

 useful for waterproofing reservoirs. However, since asphalt is refined 

 from petroleum, other petroleum-based products (which are solvents of 

 asphalt) cannot be stored in asphalt-lined structures. 



An important property of asphalt is its flexibility. This allows 

 asphalt structures to conform to slight irregularities in the subgrade, and 

 to adjust to small differential settlements that inevitably occur after the 

 completion of a structure. 



The physical properties of asphalt mixes generally depend on stress 

 conditions and temperature. The ingredients that comprise asphalt mixes 

 have completely different characteristics. The mineral aggregate that 

 makes up the major part of the mix is mainly elastic. The asphalt part, on 

 the other hand, behaves as a viscous liquid at high temperature and under 

 impact load; consequently, asphalt mixtures have both plastic and elastic 

 properties . 



For many years asphalt cement has been graded on the basis of the pene- 

 tration test, an empirical measures of consistency. Recently, however, the 

 penetration grading of asphalt cements has been replaced by the more funda- 

 mental viscosity grading. Two systems of viscosity grading are currently 

 used. The AC system is based on the viscosity of the original asphalt 

 cement. The AR system, used mostly on the Pacific coast of the United 

 States, is based on the viscosity of the residue of the asphalt cement after 

 it has been subjected to hardening conditions approximating those occurring 

 in normal hot-mix plant operations. 



The relationships between the various grading systems are shown in 

 Figure 38. 



c. Asphalt Mixes . 



(1) Objectives of Asphalt Mix Design . The design of asphalt 

 mixes, as with other engineering materials designs, is largely a matter of 

 selecting and proportioning materials to obtain the desired properties in 

 the finished construction. The overall objective for the design of asphalt 

 mixes is to determine an economical blend and gradation of aggregates 



168 



