Evaluation of Alignment Tolerances for Dowel Bars and their Effects on JointPerformance

Abstract

The Michigan Department of Transportation (MDOT) uses dowel bars to assure that adequate load transfer takes place across transverse joints in rigid pavements. Dowel bars are placed at pavement mid-depth, and care is taken to minimize the detrimental effects of misalignment. Dowel bars at contraction joints should be exactly parallel to both the surface and centerline of the hardened slab. If they deviate from the desired position, they are said to be misaligned. Misalignment may result from misplacement (initially placing the dowels in an incorrect position), displacement (movement during the paving operation), or both. Dowel bars are typically placed in the joint by positioning them in wire basket assemblies, which in turn are pinned to the subbase or subgrade prior to paving. This secures the bars during paving when the concrete is placed. During the last decade, MDOT has allowed the use of a dowel bar inserter (DBI) attachment to the paver, in lieu of using a fixed assembly. The specified placement tolerances for a dowel bar, whether placed by a DBI or as part of a load transfer assembly, are the same. The basis for these alignment tolerances needs to be evaluated. Several major pavement performance studies (Yu et al. 1997, Khazanovich et al. 1998, Hoerner et al. 2000) demonstrated that properly placed dowels significantly reduce transverse joint faulting and corner cracking. However, dowel misalignment may cause unwarranted restraint at joints (locked joints) and result in transverse cracking, corner breaks, and spalling at the concrete face around the dowel. Moreover, once spalling occurs around a dowel, load transfer effectiveness may decrease and pavement roughness may increase. To prevent these detrimental effects, transportation agencies limit allowable dowel misalignment. Currently, no clear consensus exists among state agencies regarding the level of practical limits on dowel placement tolerances. Normally, a maximum allowable alignment error of 0.25 inches per 18-inch length of dowel bar is specified. However, the Georgia Department of Transportation specifies an allowable tolerance of 3/8 inch/foot in both the horizontal and vertical direction, and several other agencies specify an allowable tolerance of 1/4 inch/foot in both the horizontal and vertical directions. These specifications were developed based on very limited data from field and laboratory performance studies.