Click for next page ( 16


The National Academies | 500 Fifth St. N.W. | Washington, D.C. 20001
Copyright © National Academy of Sciences. All rights reserved.
Terms of Use and Privacy Statement



Below are the first 10 and last 10 pages of uncorrected machine-read text (when available) of this chapter, followed by the top 30 algorithmically extracted key phrases from the chapter as a whole.
Intended to provide our own search engines and external engines with highly rich, chapter-representative searchable text on the opening pages of each chapter. Because it is UNCORRECTED material, please consider the following text as a useful but insufficient proxy for the authoritative book pages.

Do not use for reproduction, copying, pasting, or reading; exclusively for search engines.

OCR for page 15
15 CHAPTER 3 Findings and Applications This chapter describes the results of field studies, laboratory ered vertical translation. Negative vertical translation indi- testing, and analytical modeling conducted in this project. cates that the dowel bar is closer to the surface and positive These results were used to develop the design and construction vertical translation indicates that the dowel bar is closer to guidelines provided as Attachment A to this report. the base. Average measured vertical translation for individual projects 3.1 Field Testing ranged from -1.1 in. to +0.9 in. [-28 mm to 23 mm]; the dis- tribution of these averages is presented in Figure 3.1, which MIT Scan-2 data from 60 projects located in 17 states were shows that, although 63% of the projects are within the typical used to evaluate typical values of dowel alignment and position. DOT-specified vertical translation limits of 0.5 in. [13 mm], These data included measurements of over 2,300 one- or it is possible for entire projects to have an average vertical two-lane joints and more than 28,000 dowel bars. Data that translation level of more than 0.5 in [13 mm]. Over 95% of appeared to have been affected by nearby metallic objects, such the projects have an average vertical translation level within as tie bars or traffic loops, were not included in the analyses. 1.0 in. [ 25 mm] of the slab mid-depth. This can be a The relationship between dowel misalignment and pave- result of using dowel baskets of incorrect height, the use of an ment performance also was evaluated. Dowel misalignment improperly set DBI and/or concrete mix-related issues, or the was characterized by its type (e.g., embedment length, vertical placement of pavement that is thicker (or thinner) than spec- translation or concrete cover, and rotational tilt) and magni- ified. The vertical translation distribution of the dowel bars tude. Pavement performance was characterized by observed within each nominal thickness is shown in Table 3.1. distresses such as transverse joint faulting and transverse crack- When absolute values are considered, the average vertical ing. The performance of some joints also was evaluated using depth deviation for all projects is 0.46 in. [12 mm] and the FWD testing. The results of these analyses are presented below. standard deviation is 0.6 in. [15 mm]. As mentioned previously, some of the variability in vertical 3.1.1 Typical Misalignment translation or concrete cover is due to differences between The misalignment levels measured in this study give insight designed and as-built slab thicknesses, and the computation into the level of alignment that can be achieved using current is based on the nominal design thickness. For example, if the construction practices. The achievable level for which there is design thickness is 10 in. [254 mm], then the dowel bar is no observable effect on performance is used in the development expected to be located at a depth of 5 in. [127 mm]. However, of the guidelines. All rotational misalignments (i.e., horizon- if the as-built thickness is 10.24 in. [260 mm] and the dowel tal skew and vertical tilt) are expressed as a deviation from bar is located at mid-depth (5.12 in. [130 mm] from the pave- alignment over 18 in. [457 mm], which is the length of a ment surface), then a deviation of +0.12 in. [3 mm] would be typical dowel. assumed. The vertical translation manifests itself most notably in reduction of the concrete cover either from the top or the bottom surface. Table 3.2 shows the average concrete cover, 3.1.1.1 Vertical Translation dowel depths, and corresponding standard deviations for Dowel bars are assumed to be embedded at the mid-depth various projects with nominal concrete thickness ranging of a slab. A vertical deviation from this position is consid- from 8 to 12 in. [203 to 305 mm].

OCR for page 15
16 35% The standard deviation of longitudinal translation for all 30% of the individual dowels was 1.2 in. [30 mm]; Figure 3.2 shows the longitudinal translation distribution. Over 91 and 98% of 25% all bars are within 2 in. [50 mm] and 3 in. [75 mm] from Percent of Sections the transverse joint, respectively. 20% 15% 3.1.1.3 Horizontal Skew 10% The MIT Scan-2 unit determines the positions of the two 5% ends of a dowel bar and computes the horizontal skew as a deviation from the longitudinal axis over the length of the 0% 1.0 in. dowel. The horizontal skew can be positive or negative depend- Vertical Depth Deviation, in. ing on the horizontal angle of the dowel bar relative to the longitudinal joint. The absolute values were used for mean Figure 3.1. Measured vertical translation distribution. analysis and the actual values were used for standard deviation analysis. Average absolute horizontal skew for individual projects 3.1.1.2 Longitudinal Translation ranged from 0.13 in. to 0.41 in. [3.3 mm to 10.4 mm], and the average absolute horizontal skew for all projects was 0.23 in. The longitudinal center of a dowel bar is designed to be at [5.9 mm]. The horizontal skew standard deviations for indi- the location of the transverse joint saw cut. Therefore, any vidual projects ranged from 0.1 in. to 0.34 in. [2.6 mm to deviation in longitudinal dowel placement with respect to the 8.7 mm], and the average standard deviation was 0.19 in. joint axis is considered longitudinal translation (i.e., reduced [4.7 mm]. dowel embedment on one side of the joint). Key factors influ- The average horizontal skew for all bars from all projects encing longitudinal translation are joint marking and saw cut was 0.24 in. [6.1 mm] with a standard deviation of 0.21 in. operations. [5.3 mm]. Less than 80% of all bars were within 3/8 in. [9 mm]. The average longitudinal translation for all projects was Figure 3.3 shows the horizontal skew distribution for all bars 0.86 in. [22 mm], and the standard deviations within individ- from all projects. Almost 90, 98, and 99.5% of the dowel bars ual projects ranged from 0.4 in. to 1.9 in. [9 mm to 49 mm]. have horizontal skew less than 0.50 in. [13 mm], 0.75 in. This suggests that the dowel bars were placed in their accu- [19 mm], and 1.0 in. [25 mm], respectively. rate longitudinal positions in some projects and in varying longitudinal positions in other projects. The average standard 3.1.1.4 Vertical Tilt deviation for all projects was 0.9 in. [23 mm]. The maximum average longitudinal translation was 1.9 in. [49 mm], resulting The MIT Scan-2 unit pinpoints the vertical positions of the in a lowest average embedment length of 7.1 in. [180 mm] for two ends of a dowel bar and determines the vertical tilt as the the 18-in. [457-mm] long dowels. vertical deviation from the longitudinal axis with respect to Table 3.1. Vertical dowel translation for sections with different thicknesses. Concrete Thickness (in.) 8 9 10 11 12 # of Projects 3 7 11 26 5 # of Dowel Bars 1036 4847 7321 9529 2388 Dowel Bar Dowel Diameter, in. 3 X 1.25 4 X 1.25; 3 X 1.5 3 X 1.25; 8 X 1.5 2 X 1.25; 24 X 1.5 5 X 1.5 Statistics Construction 3 Basket 2 Basket; 5 DBI 1 Basket; 10 DBI 20 Basket; 6 DBI 3 Basket; 2 DBI Average Depth, in. 3.76 4.56 5.13 5.47 5.94 Standard Deviation 0.38 0.57 0.49 0.69 0.46 < -1 in. 0.0% 0.0% 9.1% 3.8% 0.0% -1.0 to -0.5 in. 25.0% 10.0% 0.0% 23.1% 0.0% Dowel Bar -0.5 to 0 in. 50.0% 40.0% 18.2% 23.1% 40.0% Deviation 0 to 0.5 in. 25.0% 30.0% 54.5% 30.8% 40.0% Distribution 0.5 to 1.0 in. 0.0% 20.0% 18.2% 19.2% 20.0% >1 in. 0.0% 0.0% 0.0% 0.0% 0.0%

OCR for page 15
17 Table 3.2. Measured depth of dowel bars from slab surface for various concrete thicknesses. Concrete Thickness (in.) 8 9 10 11 12 Projects 3 7 11 26 5 Dowel Bars 1036 4847 7321 9529 2388 Dowel Bar Dowel Diameter, in. 3 X 1.25 4 X 1.25; 3 X 1.5 3 X 1.25; 8 X 1.5 2 X 1.25; 24 X 1.5 5 X 1.5 Statistics Construction 3 Basket 2 Basket; 5 DBI 1 Basket; 10 DBI 20 Basket; 6 DBI 3 Basket; 2 DBI Average Depth, in. 3.76 4.56 5.13 5.47 5.94 Standard Deviation 0.38 0.57 0.49 0.69 0.46 < 2.0 in. 0.0% 0.0% 0.0% 0.0% 0.0% 2.0 to 2.5 in. 0.1% 0.1% 0.0% 0.0% 0.0% 2.5 to 3.0 in. 1.0% 2.8% 0.0% 0.0% 0.0% 3.0 to 3.5 in. 26.4% 3.3% 0.8% 0.1% 0.0% 3.5 to 4.0 in. 42.7% 5.4% 2.9% 1.4% 0.0% 4.0 to 4.5 in. 27.2% 29.1% 3.9% 8.1% 0.4% Dowel Bar 4.5 to 5.0 in. 2.7% 38.3% 27.7% 16.3% 1.2% Depth 5.0 to 5.5 in. 0.0% 19.0% 46.1% 23.8% 13.1% Distribution 5.5 to 6.0 in. 0.0% 1.6% 16.1% 28.5% 44.7% 6.0 to 6.5 in. 0.0% 0.2% 2.2% 15.2% 27.9% 6.5 to 7.0 in. 0.0% 0.1% 0.2% 5.8% 11.4% 7.0 to 7.5 in. 0.0% 0.0% 0.1% 0.8% 1.3% 7.5 to 8.0 in. 0.0% 0.0% 0.0% 0.0% 0.1% > 8.0 in. 0.0% 0.0% 0.0% 0.0% 0.0% the length of the dowel. Although the vertical tilt can be pos- These values are nearly identical to the horizontal skew values. itive or negative depending on the vertical angle of the dowel Approximately 80% of all bars were within 3/8 in. [9 mm]. bar relative to the surface of the slab, the absolute values were Figure 3.4 shows the vertical skew distribution for all bars used for mean analysis, and the actual values were used for from all projects. About 91, 98, and 99% of dowel bars had standard deviation analysis. vertical tilt less than 0.50 in. [13 mm], 0.75 in. [19 mm], 1.0 in. Average absolute vertical tilt for individual projects ranged [25 mm], respectively. from 0.11 in. to 0.51 in. [2.9 mm to 13.1 mm], and the average vertical tilt for all projects was 0.24 in. [6.1 mm]. The stan- 3.1.2 Effect on Pavement Performance dard deviation for individual projects ranged from 0.1 in. to 0.53 in. [2.7 mm to 13.5 mm], and the average standard devi- Distress data were collected for 37 pavement sections, many ation for all projects was 0.19 in. [4.9 mm]. of which had almost no distresses. Some projects exhibited The average vertical tilt for all bars from all projects is minor shallow surface spalling (less than 0.5 in. [13 mm] deep) 0.23 in. [6 mm] with a standard deviation of 0.21 in. [5.4 mm]. that apparently did not result from dowel misalignment but 25% 70% 60% 20% 50% Percent of Bars Percent of Bars 15% 40% 30% 10% 20% 5% 10% 0% 0% 0.0 to 0.25 to 0.5 to 0.75 to 1.0 to 1.25 to 1.5 to 1.75 to 2.0 to 2.25 to 2.5 to 2.75 to 3.0 to < 0.25 in. 0.25 to 0.50 to 0.75 to 1.00 to 1.25 to 1.50 to 0.25 in. 0.5 in. 0.75 in. 1.0 in. 1.25 in. 1.5 in. 1.75 in. 2.0 in. 2.25 in. 2.5 in. 2.75 in. 3.0 in. 6.5 in. 0.50 in. 0.75 in. 1.00 in. 1.25 in. 1.50 in. 3.50 in. Longitudinal Translation, in. Horizontal Skew, in. Figure 3.2. Distribution of longitudinal translation. Figure 3.3. Distribution of horizontal skew.

OCR for page 15
18 70% all four categories (1-OH3) had the highest amount of trans- 60% verse cracking (48% of the slabs). Because of the differences between the projects in factors 50% such as design, traffic, age, climate, and materials, project-level analyses were conducted to examine the development of dis- Percent of Bars 40% tresses in the different slabs of a specific project containing 30% joints with varying degrees of dowel misalignment. 20% 10% 3.1.2.2 Project-Level Analysis 0% Dowel alignment levels are not uniform within each project, < 0.25 in. 0.25 to 0.50 to 0.75 to 1.00 to 1.25 to 1.50 to so the effects of dowel misalignment on the distribution of 0.50 in. 0.75 in. 1.00 in. 1.25 in. 1.50 in. 4.00 in. Vertical Tilt, in. distresses within the sections were analyzed. Two types of project-level analyses were conducted. In one analysis, joints Figure 3.4. Distribution of vertical tilt. or slabs with high levels of distresses and joints or slabs with no significant distresses were identified, and the dowel mis- alignments for the two groups of joints were then compared. was more likely due to saw cut timing. To evaluate the effect The second analysis involved sorting the joints with respect of dowel misalignment, the performance of sections with high to misalignment level from highest accuracy to lowest accuracy and low levels of misalignments were compared. and comparing the distresses of those joints or adjacent slabs. Examples of these analyses for transverse cracking and joint 3.1.2.1 Comparison of Section Performance faulting are presented here. Appendix B provides more detail on the analysis for these distresses and joint opening. Dowel alignment of the 37 projects was classified in Groups A, B, and C with regards to the four misalignment Transverse Cracking and Joint Spalling. Of the 37 sec- categories: (1) vertical depth deviation, (2) longitudinal trans- tions surveyed, 26 projects did not exhibit any transverse lation, (3) horizontal skew, and (4) vertical tilt. For analysis cracking, and 33 projects did not exhibit any high-severity purposes of each misalignment category, projects with place- spalling at the joints. Only six projects (1-AZ3, 1-AZ9, 1-CA3, ment accuracy in the bottom third were placed in group C, 1-IL2, 1-OH3, and 1-WI2) had a considerable amount of trans- projects with placement accuracy in the top third were placed verse cracking or high-severity joint spalling (greater than 10% in Group A, and the rest were placed in Group B. of slabs or joints); they could be considered for project-level Only two sections were included in Group C in all four analysis. All other projects could not be used for project-level categories, eight sections were included in Group C in three of analysis because they did not have significant amounts of the four categories, only one section was included in Group A such distresses. in all four categories, and seven sections were included in 1-OH3 was not used for project-level analysis because it Group A under three of the four categories. had an unusually high percentage of slabs with transverse Tables 3.3 and 3.4 give the percentage of slabs with differ- cracking (48%), such that no difference in joint and slab ent distresses for the different projects of Groups A and C, performance could be attributed to the varying levels of mis- respectively. No clear trend was observed with respect to any alignment (nearly all joints had a cracked adjacent slab). 1-CA3 of the distresses between the Group A and Group C sections. was excluded from the analysis because it included dowel In fact, the only project that was included in Group A under retrofitted joints, and most of the cracking probably occurred Table 3.3. Slabs with distresses in Group A sections. Slabs with distress, percent Project 1 - AZ 8 1 - AZ 6 1 - WI 1 1 - MN 2 1 - MN 1A 1 - OH 1 1 - AZ 4 1 - WI 3 1 - IL 2 1 - MN 1B Slabs Cracked (Transverse) 0 0 6 0 0 0 3 7 14 0 Spalling (Major) 0 0 0 0 0 0 0 15 3 0 Corner Breaks 0 0 0 0 0 0 0 0 0 0 Slabs Cracked (Longitudinal) 0 3 0 6 0 0 3 0 3 0 Joint FDR 0 0 12 0 0 0 0 0 0 0 Midpanel FDR 0 0 0 0 0 0 0 0 0 0

OCR for page 15
19 Table 3.4. Slabs with distresses in Group C sections. Slabs with distress, percent Project 1 - NC 1 1 - NC 4 1 - OH 4 1 - WI 2 1 - NC 3 1 - IN 2 1 - CA 3 1 - OH 3 Slabs Cracked (Transverse) 0 0 0 0 0 0 24 48 Spalling(Major) 0 3 0 40 0 0 0 0 Corner Breaks 0 0 0 0 0 0 6 0 Slabs Cracked (Longitudinal) 0 0 0 0 0 0 0 0 Joint FDR 0 0 0 0 0 0 0 0 Midpanel FDR 0 0 0 0 0 0 0 6 before the joints were retrofitted. Therefore, the analysis was Faulting and LTE Analyses. Many of the evaluated proj- performed on the other four projects (1-AZ3, 1-AZ9, 1-IL2, ects had only small levels of faulting ranging from 0 to 0.1 in. and 1-WI2). [0 to 3 mm] at most of the joints. The low faulting could be Thirty percent of the slabs in project 1-AZ3 exhibited trans- attributed to the use of relatively large dowel bars (1.25- and verse cracking, and none of the joints had any major spalling. 1.5-in. [35- and 38-mm] diameter) and the young age of the A statistical analysis was conducted to compare the dowel pavement sections. For the faulting analyses that follow, only alignment of joints adjacent to slabs that exhibited transverse older pavements (> 10 years) that exhibited some significant cracks with that of joints adjacent to slabs that did not exhibit amount of faulting (mean faulting > 1 mm) were considered. any transverse cracking. The test section had 33 joints, 16 of Faulting measurements were taken in the wheel path and at which were adjacent to slabs with transverse cracking (Group A) the slab edge, and the maximum of the two values was used and 17 of which were adjacent to slabs without any transverse for analysis. cracking (Group B). Student's t-tests were conducted to deter- Vertical Translation. Analysis was conducted to compare mine whether there were any statistically significant differences faulting and LTE at joints with dowels that were centered within between the two sets of joints with regard to the average absolute 0.25 in. [6 mm] (on average) of mid-depth with those that values of vertical and longitudinal translation, vertical skew, had dowels centered more than 1.0 in. [25 mm] closer (on and horizontal tilt at the individual joints. average) to the pavement surface. The average vertical transla- There was no statistical difference in average vertical trans- tion at each joint in each project was computed with respect to lation, average longitudinal translation, or average vertical tilt the mid-depth of the pavement. For a given project, the average between joints that are adjacent to slabs exhibiting transverse faulting of all joints with the smaller level of vertical translation cracking and joints adjacent to intact slabs. However, there was (Group 1) was paired with the average faulting of all joints with a statistically significant difference between the two groups the higher level of vertical translation (Group 2). Ten projects with respect to horizontal skew. Contrary to expectations, the were considered in this analysis. The same analysis procedure joints adjacent to the intact slabs had higher levels of average was used for joint LTE; five projects were considered. horizontal skew than the joints adjacent to cracked slabs. This The relatively high P-values suggest that there are no suggests that factors other than misalignment contributed to statistically significant differences in faulting or LTE between cracking. Moreover, the actual levels of misalignment of the two groups (i.e., joints with average vertical translation both groups are less than 0.32 in. [8 mm], which is well within < 0.25 in. [6 mm] and joints with average vertical trans- typical specification tolerances, and should not cause joint lation > 1.0 in. [25 mm] closer to the slab surface). Note that lockup. the faulting levels considered in this study were extremely low, The analyses for test sections 1-AZ9, 1-IL2, and 1-WI2, and the vertical translations greater than 1.0 in. [25 mm] closer presented in Appendix B, also showed that there is no statisti- to the surface were observed on thick slabs with sufficient cover cally significant difference between the alignments of dowels (i.e., the dowels were still 4 to 5 in. [102 to 127 mm] from the in joints adjacent to cracked slabs and alignments of dowels surface of the slab). adjacent to uncracked slabs for these sections. Therefore, the results of the project-level analyses suggest Longitudinal Translation. Analysis was conducted to that, within the nonextreme limits of dowel translations compare faulting and LTE at joints with dowels that were (vertical and horizontal) and rotations (vertical tilt and hor- centered within 0.5 in. [13 mm] (on average) of the transverse izontal skew) measured in this study, there appear to be no joints with those that had dowels that were centered greater differences in the amounts of transverse cracking and joint than 2.0 in. [51 mm] (on average) from the transverse joints. spalling as a result of dowel misalignment. Although longitudinal translations over 3 in. [76 mm] were