Road-Mix Pavements

an asphalt-paving project is the temperature at the time of  compaction. During rolling, the roller wheels must be kept moist with  only  enough  water  to  avoid  picking  up  material. Rollers move at a slow, but uniform, speed with the drive wheels nearest the paver. The speed should not exceed 3   mph   for   steel-wheeled   rollers   or   5   mph   for pneumatic-tired rollers. A roller must be maintained in good  condition,  capable  of  being  reversed  without backlash. The line of rolling should not be suddenly changed  or  the  direction  of  rolling  suddenly  reversed, because  these  actions  will  displace  the  mix.  Any pronounced change in direction should be made on stable  material. When rolling causes material displacement, the affected areas should be loosened at once with lutes or rakes and restored to their original grade with loose material  before  being  rerolled.  Heavy  equipment, including rollers, should not be permitted to stand on the finished surface before it has thoroughly cooled or set. Rolling freshly placed asphalt mix is done in the following  order: 1. 2. 3. 4. 5. Transverse  joints Longitudinal  joints Breakdown  or  initial  rolling Intermediate  or  second  rolling Finish rolling The five steps in rolling freshly placed bituminous or  asphalt  mix  are  covered  in  chapter  11  of  this TRAMAN. MIXED-IN-PLACE   CONSTRUCTION Emulsified  asphalt  and  many  cutback  asphalts (although the use of cutbacks is declining) are fluid enough to be sprayed onto and mixed into aggregate at moderate  to  warm  weather  temperatures.  When  this  is done on the area to be paved, it is called mixed-in-place construction. Although mixed-in-place is the more general term and is applicable whether the construction is on a roadway, parking area, or airfield, the term  road mix is often used when construction is on a roadway. Mixed-in-place   construction   can   be   used   for surface,  base,  or  subgrade  courses.  As  a  surface  or wearing course, it usually is satisfactory for light and medium traffic, rather than heavy traffic. However, mixed-in-place layers, covered by a high-quality asphalt plant-mix  surface  course,  make  a  pavement  suitable  for heavy  traffic  service.  The  advantages  of  mixed-in-place construction  include  the  following: 1. Utilization of aggregate already on the roadbed or available from nearby sources and usable without extensive  processing. 2. Elimination of the need for an asphalt mixing plant. Construction can be accomplished with a variety of  machinery  often  more  readily  available,  such  as motor graders, rotary mixer with revolving tines, or traveling  mixing  plants. ROAD-MIX  PAVEMENTS Road-mix pavements consist of mineral aggregate and mineral filler uniformly mixed in place with a bituminous material and compacted on a prepared base course or subgrade. A single layer, about 1 1/2 inches to  3  inches  thick,  is  generally  used.  This  type  of pavement is likely to become defective unless it has a sound,  well-drained  subgrade  and  is  well-mixed, uniformly  spread,  and  properly  compacted.  Road-mix pavements  may  be  used  as  a  wearing  surface  on temporary roads and airfields and as a bituminous base or binder course in construction of more permanent types of roads and airfields. Road mix is an economical method of surfacing small areas when aggregate can be used  from  the  existing  base  or  when  satisfactory aggregate  is  nearby. For road-mix pavements, the grade and type of bituminous  material  depend  upon  the  aggregate  and equipment available as well as weather conditions and time required to complete the project. Good weather is important to the success of a road-mix project. Where possible,  road-mixing  operations  should  be  scheduled when  weather  conditions  are  likely  to  be  hot  and  dry during,   and   for   some   time   after,   the   project. Recommended  types  of  bituminous  materials  suitable for road mix are asphalt cutbacks, asphalt emulsions, and  road  tars.  A  medium-curing  cutback  is  generally used in a moderate climate, and a rapid-curing cutback is  used  in  a  cold  climate. Viscosity  required  is determined  by  the  temperature,  aggregate  gradation, and method of mixing. The highest viscosity that will completely and uniformly coat the particles of aggregate should be used. In general, open-graded aggregate requires  a  high  viscosity;  a  gradation,  containing mineral filler, requires a less viscous grade. Aggregate, used in road mix, may be scarified from the existing subgrade or hauled in from a nearby source. A wide range of coarse and fine aggregate and mineral filler may be used. The ideal aggregate for road-mix 16-12


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