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4 Table 2.2. Specification limits for position and alignment of dowel bars (MTO, 2007). Misalignment Lower Limit Upper Limit (mm) (mm) Horizontal skew (mm per 450 mm -15 15 dowel) Vertical tilt (mm per 450 mm dowel) -15 15 Longitudinal translation (mm) -50 50 Depth tolerance (for specified slab thickness): 200 mm (mid depth - 6 mm/+6 mm) 94 106 225 mm (mid depth -12 mm/+15 mm) 100 127 250 mm (mid depth -15 mm/+25 mm) 110 150 260 mm (mid depth -15mm/+25 mm) 115 155 1 in. = 2.54 mm portation of Ontario (MTO, 2007). The MTO tolerances are effect on pavement performance was longitudinal translation based on research performed in Ontario to determine the extent (causing low embedment length). An example of the effect and effect of dowel misalignment in pavement construction. of low embedment length was observed on I-35 near Fergus Recent guidelines developed by the FHWA also are based on Falls, Minnesota, where significant early faulting occurred the alignment and performance data (FHWA 2007). when dowel embedment lengths were less than 2.5 in. [63 mm] (Burnham, 1999). 2.2 Dowel Misalignment Assessment 2.2.2 Laboratory Testing This section summarizes available information on the state- of-the-art in field and laboratory testing, as well as analytical Previous laboratory studies generally have been limited modeling, for dowel misalignment. to dowel pullout tests that focused on dowel resistance to joint opening by measuring pullout force and by evaluating concrete distresses. These tests include standard pullout tests 2.2.1 Field Testing and slab pullout tests. In the former test, a dowel is pulled away There have been a limited number of field studies of dowel from an anchored concrete slab. In the latter, a moving or misalignment (Tayabji and Okamoto, 1987; Yu et al., 1998). "transient" slab is pulled away from an anchored or "stationary" Devices used for identifying dowel misalignment include MIT slab to open a dowelled joint to a specified width. Up to five Scan-2, the Profometer, and ground penetrating radar (GPR) dowels are tested in the joint. Slab tests have been used by (Khazanovich et al., 2003). In 2005, FHWA identified the MIT Tayabji (1986), Prabhu et al. (2006), and others to model Scan-2 as a tool that could potentially improve the assessment slab expansion. of concrete pavements (FHWA, 2005). An assessment con- The standard pullout test data typically are presented as a ducted by the Virginia DOT also identified MIT Scan-2 as a plot of pullout force versus dowel horizontal displacement. viable technology for construction quality control (Hossain The results of such tests have been used to calibrate a finite and Elfino, 2006). element model (Khazanovich et al., 2001). This well-controlled Inspection of pavements in several states revealed that the test provides valuable information related to dowel-PCC misalignment of dowels generally occurs regardless of the friction. More information on this test and modifications placement method. For example, significant dowel misalign- made to the test to better characterize the interaction between ment was identified in a pavement section constructed using a misaligned dowel and the surrounding concrete are presented dowel baskets on Highway 115 in Ontario (Leong, 2006) and in Section 2.3.3 and Appendix C. in a pavement constructed using a DBI on I-16 in Georgia The slab pullout test data can be used to model the effects (Fowler and Gulden, 1983). Field studies also have shown of several misaligned dowels on joint behavior during joint variability in dowel position and alignment from one project movement. The following trends have been observed (Prabhu to another (Yu, 2005). While a majority of dowel bars meet et al., 2006; Tayabji, 1986): state specifications for alignment on most projects, there are a number of dowel bars that do not meet specifications. · The force required to displace the dowel increased with the The performance of some of these sections indicates that increased misalignment. slab cracking and other forms of distress may not always · Nonuniform misalignment had a greater effect on pullout occur as a result of such misalignment. Field studies have force and distresses than uniform misalignment (non- shown that the only type of misalignment that clearly had an uniform misalignment refers to dowels oppositely misaligned
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5 and uniform misalignment refers to dowels all misaligned Type I models are suited for analyzing the effects of dowel in the same direction). misalignment on bearing stresses and joint stiffness, whereas · Slabs develop cracking only at significant misalignment Type II models are suited for multi-slab analysis to predict levels (over 3/4 in. [19 mm]) when the alignment of dowels mid-slab stresses. These models were evaluated based on the along the joint is nonuniform and when excessive levels of following criteria: joint opening (over 0.5 in. [13 mm]) are present. · Minor spalling around dowels was found in slabs with · Ability to model dowel-PCC slip. uniform and nonuniform significant misalignment (Prabhu · Ability to model stress distribution around misaligned dowel. et al., 2006). · Ability to model subgrade and base support. · Ability to model nonuniform misalignment. Because rotation of the beam in the direction of the mis- · Ability to model multiple joints. aligned dowel may occur during testing and affect test results, · Model flexibility. provisions must be made to ensure proper anchoring of the · Input requirements. concrete (Tayabji, 1986). Nevertheless, slab testing is not expected to provide detailed information about the inter- Based on these criteria and experience, the ABAQUS 3-D action between the dowel bar and the surrounding concrete models were selected for use and modification in this study. (Prabhu et al., 2006). 188.8.131.52 Performance Prediction Models 2.2.3 Analytical Models Models for predicting jointed plain concrete pavement The two main categories of analytical models that can be used (JPCP) cracking, joint faulting, spalling, and roughness were for assessing the effects of dowel misalignment are structural identified and evaluated. The evaluation revealed that none of response models and performance prediction models. the performance models consider dowel alignment as an input parameter. However, mechanistic-empirical (ME) pavement 184.108.40.206 Structural Response Models performance models can be adapted to account for the effects Several finite element and finite difference models have been of dowel misalignment. used to analyze the effects of dowel misalignment (Khazanovich Several faulting models relate concrete bearing stresses et al., 2001; Davids, 2003; Leong, 2006; Prabhu et al., 2006). under the critical dowel with the rate of load transfer efficiency Some of the major findings include: (LTE) deterioration and faulting development (Owusu-Antwi et al., 1997; Hoerner et al., 2000; Khazanovich et al., 2004). · Dowel misalignment increases PCC-dowel contact stresses. Higher bearing stress accelerates joint LTE deterioration and · When embedment length falls below some critical level, causes early faulting. Thus, if higher bearing stresses were bearing stresses increase. observed for reduced dowel diameters and also observed for · If several consecutive transverse joints are subject to lockup, joints with misaligned dowels, levels of dowel misalignment stresses increase away from the joint, with high stresses could be equated to reduced dowel diameters when bearing developing at the mid-slab location. stress is considered. Cracking models relate PCC pavement longitudinal bending The analytical models for predicting the effects of dowel stresses developed at mid-slab with the percentage of cracked misalignment on concrete pavement behavior can be classified slabs. If dowel misalignment causes joint lockup, it may cause according to the degree of detail used for modeling the dowels additional tensile stresses that should be accounted for in the and their interaction with concrete as Types I and II. cracking model. Type I models provide detailed modeling of dowels and Available performance prediction models that can be used dowel-PCC interaction. These models include: for development of guidelines for dowel alignment were eval- · ABAQUS 3-D model for a single dowel; uated based on the accuracy of predictions, simplicity of use, · ABAQUS 3-D model for several dowels; and and simplicity of integration with the dowel misalignment · FLAC-3-D model. analysis. The evaluations indicated the following: Type II models provide simplified modeling of dowel-PCC · The Mechanistic-Empirical Pavement Design Guide interaction. These models include: (MEPDG) (AASHTO, 2008) faulting model was appropriate for prediction of the long-term effects of dowel misalign- · ISLAB2000; ment on joint faulting. · EVERFE; and · The MEPDG cracking model was the most comprehensive · ABAQUS-2D multiple slab model. model available for cracking prediction.