Most of the rigid pavements constructed before the mid-1980s did not include a tied concrete shoulder. Until 1986, the

AASHTO Guide for the Design of Pavement Structures

did not consider or give credit for the inclusion of a tied concrete shoulder. Rarely were tied concrete shoulders included. It is possible to infer what the performance may have been by examining the urban freeways where frequent entrance and exit ramps encouraged the heavy volumes of truck traffic to use an interior lane where the lane edges were supported by another PCC travel lane. The performance of these interior travel lanes with heavy volumes of truck traffic has greatly exceeded expectations. The reasons for the tremendous success of truck travel lanes supported at the edge by concrete pavement could be attributed to a combination of structural support from the tied concrete pavement reducing stresses in the travel lane, or from the tied concrete pavement helping to keep water from infiltrating into the base. Construction procedures for retro-fitting a pavement with concrete shoulders are identical to procedures for widening an existing concrete pavement. For more information refer to Chapter 9, Section 7.