Synthesis of design and construction practices
State-of-the-Practice Survey
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| State-of-the-Practice Survey
A web-based survey was distributed to all state pavement engineers to investigate the extent of the use of composite pavements in the U.S. The survey questions are presented in Appendix A and the results are shown in Appendix B. Responses, received from 34 state DOTs, suggested that several agencies have composite pavements that are the result of an HMA overlay of an in-service, and likely distressed, rigid pavement. In addition, three DOTs reported that they had some degree of experience in designing and constructing new composite pavements (i.e., composite pavements that did not result from an HMA overlay of a distressed concrete pavement).
The South Carolina Department of Transportation (SCDOT) uses the AASHTO 1993 method to design composite pavements. Approximately 2% of their road network consists of newly constructed composite pavements. These pavements typically consist of a dense-graded HMA placed on a CTB. SCDOT uses a structural coefficient of 0.34/in for the cement-stabilized aggregate base. In addition, a minimum base thickness of 150 mm (6 in) with a preferred 14 thickness of 200 mm (8 in) to 250 mm (10 in) is required due to the brittle nature of the material. The typical cement content for the base course is 2% to 5% (by weight) with a 4.14 MPa (600 psi) compressive strength requirement at 14 days.
The Texas Department of Transportation (TxDOT) uses their own FPS-19W software to design new composite pavements and their network includes an estimated 4% of newly constructed composite pavements. The composite structure that is used in Texas is HMA on cement stabilized base (CSB). TxDOT recommends that the modulus of the base, during the design input process, should not exceed 1725 MPa (250 ksi) to not “underdesign” the total structural thickness. Their recommendations for compressive strength are in the range of 2.07 MPa (300 psi) to 2.76 MPa (400 psi) in hopes to avoid thermal/shrinkage cracking. The typical cement content is 3% to 4% (by weight), resulting in a 7-day compressive strength of 2.41 MPa (350 psi) for the rigid base layer.
The Tennessee Department of Transportation (TnDOT) uses the AASHTO method to design their new composite pavements, which comprise approximately 2% of their network. The agency uses composite pavements consisting of HMA over CTB, and HMA over lime fly-ash treated bases. The designs are normally used on interstates, freeways, or multi-lane divided arterial highways.
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