Performance-Related Tests of Recycled Aggregates for Use in Unbound Pavement Layers (2008) / Chapter Skim
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4Background Unbound pavement layers are used in flexible and rigid pavements to provide one or more of the following functions: • A working platform for construction, • A frost blanket (frost-free layers) , • A drainage layer, and/or • A structural layer for the pavement system.
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5Distress Description of distress Unbound layer failure mechanism Contributing factors Fa tig ue C ra ck in g Fatigue cracking first appears as fine, longitudinal hairline cracks running parallel to one another in the wheel path and in the direction of traffic; as the distress progresses the cracks interconnect, forming many-sided, sharp angled pieces; eventually cracks become wider and, in later stages, some spalling occurs with loose pieces prevalent. Fatigue cracking occurs only in areas subjected to repeated loadings.
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From page 6... ...
6Distress Description of distress Unbound layer failure mechanism Contributing factors C ra ck in g Cracks transverse to the pavement centerline, generally within the center one-third of the slab. Corner breaks and diagonal cracks appear as top down hairline cracks across slab corners where the crack intersectsthe joints less than 6 ft from the corner; cracking progresses to result in several broken pieces with spalling of crack and faulting at the crack or joint up to ½ in.
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the "drained" condition (pore pressures allowed to dissipate during testing)
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8subject to freezing and the material is even moderately frost susceptible, then the measure of shear strength must reflect strength loss due to freeze-thaw. Resistance to permanent deformation, which is nearly synonymous with shear strength, is an important characteristic for aggregates that are to be used as base course materials in pavements.
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particles provide higher internal friction and shear strength; thus, it is desired that a certain percentage of particles be crushed. The mineral and geological properties of the rock formation and the crushing process define the shape of the crushed particles.
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From page 10... ...
10 the fines consist of clay-sized particles. The use of more opengraded aggregates for pavement base courses can decrease moisture sensitivity of RAP material (less fines, more film thickness)
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