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EFFECT OF BONDING FORCE ON STRESSES IN CONCRETE SLABS (Full Text)

Authors: Samir N. Shoukry, Gergis W. William, Mourad Y. Riad and Sri vani Sirisha Motamarri

Abstract

Dowel-jointed concrete pavements often show serious cracks long before their design life expires. The most common forms of cracks are mid-slab transverse cracking and transverse joint damage. Steel dowel bars embedded in transverse joints (for efficient load-transfer from one slab to the next) contribute to both forms of damage. The interfacial friction forces between dowel bars and the surrounding concrete hinder the free axial movement of the slabs as the concrete expands or contracts due to ambient temperature changes. This induces excessive axial dowel-forces that may cause mid-slab transverse cracks. Strong dowel-concrete bonds induce large shear stresses at dowel-concrete interfaces that combine with the dowel-concrete contact stresses (due to traffic loading and slab curling) to induce tri-axial state of stress that often result in failure of the concrete that support the dowels. While dowel-concrete friction forces are the major parameter contributing to such forms of damage, current concrete pavement design procedures are based on the assumption that such forces do not exist. Design specification state that the steel dowels should be coated with bond-breaking fluid that is assumed to fully eliminate dowel-concrete frictional forces. However, laboratory studies as well as field-testing using instrumented dowel bars, indicate that bond-breaking agents such as Tectyl (605) or Silicone are incapable of achieving zero coefficient of friction at dowel concrete interfaces. Lack of measured values of dowel-concrete friction coefficient hinders accounting for dowel-concrete axial forces in the design of dowel jointed concrete pavements.

In this report, the dowel-pulling force and the dowel-concrete coefficient of friction were measured using a novel laboratory setup of vibrating wire strain gauges embedded in both the dowel and concrete. The gages are set to measure the shrinkage strain induced in the concrete that surrounds the dowel as it cures causing the solidified concrete to clamp on the steel dowel. The measurements reveal that radial strain in concrete around the dowel is not uniform along the dowel circumference. As the dowel is pulled out of concrete, both the dowel-pulling force and the elastic strain recovery in concrete are recorded versus the dowel displacement. A theoretical model is developed to enable calculation of the dowel-concrete friction coefficient. Three dimensional finite element analysis is used to estimate the stress field in the concrete surrounding dowel bars. Experiments are conducted to examine the effect of dowel bar diameter and type of bond-breaking agent on the friction coefficient and the magnitude of dowel-pulling force. The results indicate that the use of Silicone as bond-breaker produces lower dowel-concrete coefficient of friction than that obtained using Tectyl (506). The results from finite element analysis indicate that the magnitudes of stresses in concrete surrounding uncoated steel dowels are higher than those obtained if the dowels are coated using bond-breaker.

The full report will be available pending WVDOT's approval.