Item 452 Non-Reinforced Concrete Pavement is recommended for all unbonded concrete overlays and the replacement areas. A base of at least 6 inches (150 mm) of Item 304 should be placed under the new concrete. The thickness required for the new pavement is the thickness calculated for new pavement when designing the unbonded concrete overlay, T N. New concrete pavement should be placed wherever pavement must be removed to meet the elevation of mainline bridges or to provide clearance at overhead bridges. Deteriorated joints and cracks do not need to be repaired prior to the overlay. To minimize the elevation increase of an unbonded concrete overlay, removal of any existing asphalt overlay prior to placing the bondbreaker is recommended. An example of an unbonded concrete overlay design is given in Figure 601-1. The effective thickness of the existing concrete can also be estimated using the AASHTO ’93 procedure when FWD measurements are not available. The design period used in deflection analysis does not matter as the Deff (PCC) does not change with different traffic inputs. Best practice dictates averaging all the readings for the entire project, both directions, rather than averaging each direction separately and using the smaller or larger number. The effective thickness of the existing concrete comes from the analyzed FWD measurements and a manual average of the effective thickness of the existing concrete, Deff (PCC). T E = Effective thickness of the existing concrete. T N = Required thickness for a new concrete pavement. T UCO = Required thickness of the unbonded concrete overlay. The equation for determining the thickness of an unbonded concrete overlay, developed by the U.S. Next an asphalt overlay is designed using deflection analysis described in Section 500. The design of an unbonded concrete overlay begins with the design of a new rigid pavement according to the procedures in Section 300. The thickness of an unbonded concrete overlay is derived from the required thickness for a new concrete pavement reduced by an amount based on the effective thickness of the existing concrete. 601 Unbonded Concrete OverlayĪn unbonded concrete overlay is a new concrete pavement placed on top of an old, deteriorated concrete pavement with a thin layer of asphalt in between to act as a bond-breaker. The processed FWD data provides the subgrade design modulus for each direction tested. If necessary for a preliminary design, subgrade strength can be estimated from historical subsurface investigations or by using the W(60) sensor readings from the FWD. Prior to requesting pavement design for a major rehabilitation, soil borings should be taken and analyzed according to guidance from the Office of Geotechnical Engineering. To design all major rehabilitations, including complete replacement, it is necessary to know the strength of the subgrade under the existing pavement. The design of new concrete and asphalt is given in Sections 300 and 400 respectively. Major rehabilitations include the techniques given here, as well as complete removal of the existing pavement and replacement with either concrete or asphalt. Most major rehabilitations require a life-cycle cost analysis using the procedures in Section 700 and a pavement selection according to Section 100. The design period for major rehabilitations is established in Section 100. However, funding is not always available to elevate a project to major rehabilitation. For example, project level analysis may reveal excessive repair quantities that make minor rehabilitation a poor choice economically. Sometimes minor rehabilitation projects need to be elevated to major rehabilitation because of specific project conditions. Major rehabilitations are performed when the pavement condition is such that minor rehabilitation is no longer feasible or when geometric or capacity improvements require major work. Published: Janu600 Major Rehabilitation Design 600.1 Introduction
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