Equations for Nonbonded Concrete Overlays


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Title: Equations for Nonbonded Concrete Overlays

Author(s): Y. Chou

Publication: Special Publication

Volume: 93


Appears on pages(s): 125-142

Keywords: concrete pavements; concrete slabs; elastic properties; modulus of elasticity; resurfacing; structural design; thickness

Date: 9/1/1986

The nature of the design equations for the nonbonded concrete overlays currently used by the US Army Corps of Engineers was examined, and the original source of the equation was also examined. Using simple mechanics, it was found that the existing equation is true when the condition Elhl = E2h2 is true, where E1 3 E2 9 hl 9 h2 are the elastic moduli of the concrete overlay and the base slab, and the thickness of the overlay and the base slab, respectively. When El is assumed to be equal to E2 ' the existing equation is valid when hl = h2 is true. Also using simple mechanics, new overlay equations were developed which are suitable for different thicknesses and elastic properties in the overlay and base concrete slabs. The difference in the computed overlay thickness between the new and existing equations is not large when the overlay thickness is equal to or greater than the base slab. The difference can become significant when the overlay thickness is much less than that of the base slab. The new equations were compared with the finite element computer program for concrete overlays with various combinations of slab thickness, elastic property, and subgrade modulus. The comparisons were very favorable, indicating that the overlay equations developed in this paper are analytically correct. The validity of the equations was also examined using the results of the field full-scale accelerated traffic tests on concrete over-lays. It was difficult to judge whether the new equations are superior to the existing equation. This conclusion was expected because for all the seven test sections analyzed, the overlay thicknesses used were either equal to or greater than those of the base slabs. It is recommended at this time that the existing design equations for nonbonded overlays not be replaced by the new equations because of lack of verification from field test data.