Rigid pavement thickness design systems investigated during this study were the 1986 AASHTO, American Concrete Pavement Association (ACPA), Portland Cement Association (PCA), and Kentucky methods. The ACPA system is a computer program based upon the 1966 AASHTO design equation. It was difficult to evaluate and compare the Kentucky method to the PCA system because the input and analysis procedures differ greatly.

The Kentucky method is based upon the fatigue relationship involving the value of work at the bottom of the concrete pavement caused by the applied load and repetitions of an 18-kip single axleload. The AASHTO method was derived from data obtained at the AASHO Road Test where the rigid pavements failed primarily due to pumping of the subgrade from under the slab. In Kentucky, pumping is a minor problem compared to failures caused by compressive forces at joint openings. Compression occurs due to annual temperature fluctuations resulting in slab movement and subsequent intrusion of debris into the joint openings. Eventually, the slab cannot move and compressive forces increase until failure occurs. Failure criterion used in the Kentucky thickness design system is quite different from the mode of failure observed at the AASHO Road Test and makes direct comparisons between design methods somewhat questionable.

The expression of soil stiffness values is a major contributor to the confusion arising between design methods.

Using elastic theory to develop load equivalency relationships, the ratio of rigid pavement EALs to flexible pavement EALs is approximately 1.1. According to W-4 Tables, the ratio of AASHTO rigid pavement EALs to AASHTO flexible pavement EALs is approximately 1.6. Thus, the AASHTO combination of pavement structures used in W-4 tables are not equivalent for fatigue calculations. Another combination should be chosen.

Thickness designs using the 1966 AASHTO, ACPA, and Kentucky methods can be made to match provided the terminal serviceability varies with Kentucky CBR. To help understand the behavior at the AASHO Road Test, published data for the cracking index, pumping index, and serviceability index were investigated. All three data sets influenced one another and could be correlated fairly well for serviceability values greater than 1.5. A method was devised to normalize the data to account for tire load and pavement thickness variations.

Serviceability data proved to be revealing. Of the 76 rigid pavement sections at the AASHO Road Test, 43 were given a serviceability rating greater than 1.5 at the end of testing operations. Of the 43, 10 had ratings between 2.5 and 4.0. The remaining 33 sections had ratings of 4.0, or greater. The 1986 AASHTO Guide recommends a terminal serviceability of 2.5 for major highway pavements. A 2.5 rating is appropriate for flexible pavements. From this investigation, a rating of 2.5 appears to be low for rigid pavements and suggests that a comparable set of serviceability values should be derived.

It is recommended that Kentucky Department of Highways use the 1984 Kentucky Concrete Thickness Design Curves for design of rigid pavements.

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Revised August 1991.

The contents of this report reflect the views of the author who is responsible for the facts and accuracy of the data presented herein. The contents do not necessarily reflect the official views of policies of the University of Kentucky, of the Kentucky Transportation Cabinet, or of the Federal Highway Administration. This report does not constitute a standard, specification, or regulation. The inclusion of manufacturer names and trade names are for identification purposes and are not to be considered as endorsements.