150_5320_6d_part5 - AC150/5320-6D 7l7J95 Increasing C...

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7l7J95 AC150/5320-6D (2) Increasing C,, Factor. A value of C, lower than 0.75 represents a severely cracked base slab, which would not be advisable to overlay without modification due to the likelihood of severe reflection cracking. See paragraph 406 f. In some instances it may be advantageous to replace several slabs and restore load transfer along inadequate joints to raise the Cb value. Increasing the Cb value will decrease the required overlay thickness. A detailed condition survey of the existing pavement which examines the subsurface drainage conditions, structural capacity of the slabs, foundation strength, flexural strength of the concrete, load transfer along joints and thickness of the component layers is strongly encouraged to properly design a hot mix asphalt overlay. C. Example. An example of the hot mix asphalt overlay design method is given below: (1) Assumptions. Assume an existing rigid pavement 12 inches (305 mm) thick is to be strengthened to accommodate 3000 departures of a dual wheel aircraft weighing 180,000 pounds (81,800 kg). The flexural strength of the existing concrete is 725 psi (5.00 MN/m’) and the foundation modulus is 300 pci (81.6 MN/m’). The condition factor of the existing pavement is 0.95. (2) Single Slab Thickness. Compute the single slab thickness required to satisfy the design conditions given in (1) above. Using Figure 3-17 the slab thickness is found to be 13.9 inches (353 mm). The F factor is determined from Figure 4-3 and equals 0.93. Applying the overlay formula given in paragraph 406 yields: t =2.5 (0.93 x13.9 - 0.95 x12) t = 3.82 inches(97 mm) This thickness would be rounded up to 4 inches (100 mm) for practicality of construction. - d. Previously Overlaid Rigid Pavement. The design of a hot mix asphalt overlay for a rigid pavement which already has an existing hot mix asphalt overlay is slightly different. The designer should treat the problem as if the existing hot mix asphalt overlay were not present, calculate the overlay thickness required, and then adjust the calculated thickness to compensate for the existing overlay. If this procedure is not used, inconsistent results will often be produced. e. Example. An example of a hot mix asphalt overlay design for a rigid pavement which already has an existing hot mix asphalt overlay is given below: (1) Assumptions. An example of the procedure follows. Assume an existing pavement consists of a lo-inch (255 mm) rigid pavement with a 3-inch (75 mm) hot mix asphalt overlay. The existing pavement is to be strengthened to be equivalent to a single rigid pavement thickness of 14 inches (355 mm). Assume an “F” factor of 0.9 and “C,” of 0.9 are appropriate for the existing conditions. (2) Ignore Existing Overlay. Calculate the required thickness of hot mix asphalt overlay as if the existing 3-inch (75 mm) overlay were not present.
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