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Benefits
Applications
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Concrete Whitetopping Joint Design
To best control cracking, the maximum spacing between joints should be kept at the lower limits recommended by ACI 330. Table 3 suggests maximum joint spacings for white-topped parking areas. These spacings should be modified based on local experience.
In addition to joint spacing, the following suggestions for joint layout can improve whitetopping performance:
® Joints should be laid out to form approximate square panels. When this is not practical, rectangular panels can be used if the long dimension is no more than 1.25 times the short dimension.
® The maximum angle between two intersecting joints should be limited to 80 degrees.
® Joints must intersect pavement-free edges at a 90-degree angle and extend straight inward for a minimum of 1.5 feet from the pavement edge.
® The joints should be aligned in adjacent panels.
® Joints in attached curbs should be aligned with the pavement joints.
® Where there are manholes, catch basins, small foundations, and other built-in structures, the joints should be laid out so that they meet the corners of the structure.
Isolation (expansion) joints should be used only to isolate fixed objects abutting or within the pavement area. ACI 330 recommends constructing control joints to a depth of at least one-fourth the pavement thickness in order to control cracking. This is generally adequate for concrete parking areas built on shaped subgrades or subbases, where the final thickness of the slab can be well-controlled. For whitetopping, however, the concrete overlay is placed on an existing asphalt pavement, which may contain ruts, dips, and other surface distortions.
Though the minimum overlay thickness should be carefully controlled to ensure adequate pavement strength, the concrete overlay may be thicker in some areas than the minimum thickness specified by the designer. In addition, the asphalt base restrains the concrete pavement more than an untreated base material does. Control joints, therefore, should be deeper for whitetopped parking lots than for new construction. Control joints should be constructed to a minimum depth of one-third the overlay thickness.
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ConcreteAnswers for Architects, Engineers and Developers:
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The design
guidelines given in ACI 330 are generally appropriate for whitetopping. However,
concrete placed on existing asphalt experiences much more subgrade restraint
than concrete placed on an unstabilized subbase or subgrade.
