properties are not necessarily required in these instances, but quality- 

 asphalt concrete linings having low voids effectively resist the destructive 

 effects of wave and current action as well as their abrasive effects (Figs. 

 42 and 43). Impermeable asphalt mixes may be used for the entire lining of 

 the structure. They may also constitute a part of a more complex lining. 

 They can, for example, be placed as the surface of a composite section made 

 up of different asphalt layers. 



(3) Porous Hot-Mix Asphalt Linings . Porous asphalt mixes for 

 hydraulic structures are characterized by the absence or reduced amount of 

 fine aggregate or sand in the mix. As a consequence, the asphalt content 

 is also reduced. The mixes have interconnected pores that permit passage 

 of water. A harder, or more viscous, grade of asphalt cement is desirable 

 in these mixes to allow sufficient film thickness and to prevent drainage 

 from the aggregate. This choice of asphalt also provides additional cohesion 

 in the mix between the aggregate particles. 



There are two types of porous asphalt linings: permeable and open- 

 graded. Permeable hot-mix asphalt linings serve as a cover over an earth 

 embankment to protect it from erosion by wave action or surface runoff. 

 Open-graded asphalt linings, with higher void content than the permeable 

 lining, serve as drainage layers under an impermeable lining while at the 

 same time contributing to the structural strength of the lining. In either 

 case, the purpose is to provide free drainage to prevent hydrostatic 

 pressures from building up in the embankment or within the lining itself. 

 Asphalt, as a surface lining, allows water to flow to and from the embankment 

 through the lining. As a drainage layer, asphalt collects the subsurface 

 water, channeling it to drains for removal. 



(4) Asphalt Mastic Mixes . Asphalt mastic mixes for hydraulic 

 structures are essentially mixtures of mineral aggregate and filler where 

 the voids in the mineral matrix are overfilled with asphalt cement. The 

 result is an asphalt mix that can be applied by pouring or by hand-floating 

 into place. Asphalt mixes require little or no compaction after placing 

 because void spaces in the aggregate matrix are filled or slightly over- 

 filled with asphalt. Asphalt mastics may be made from a variety of aggre- 

 gate materials ranging from well-graded coarse and fine aggregates and 

 mineral filler to essentially mineral filler alone with or without an 

 additive such as asbestos fibers. The mastic is voidless except for air 

 bubbles that may be trapped during the manufacture and placing. 



Asphalt mastics can be used in several ways to waterproof, protect, or 

 reinforce a hydraulic structure. For waterproofing, asphalt mastics have 

 been used for cutoff walls for dams as well as for the central core of the 

 dam itself. They are also used as exposed watertight surface linings. 



Asphalt mastic mixes are erosion-resistant; therefore, they can be 

 exposed to waves and abrasive water action. They are also used to form 

 protective covers on embankments or over the floor of channels or estuaries 

 that are subject to erosion. Hot mastic mixes can be placed underwater 

 through tremies, chutes, or by simply dumping in masses. They are also 

 used for constructing flexible slabs or mattresses that are lifted into 

 place to form a protective blanket or cover. 



177 



