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20 observed on some newly constructed pavements, these proj- skews of less than ±0.25 in. per 18 in. [± 6 mm per 457 mm] ects were not included in the performance analysis because of (on average) with those that had dowels with horizontal their age. skews greater than ±0.75 in. per 18 in. [±19 mm per 457 mm] For a given project, the average faulting of all joints in (on average). Group 1 (joints with dowels that were centered within ±0.5 in. The average horizontal skew at each joint at each project [±13 mm], on average, of the transverse joints) was paired with was computed. For a given project, the average faulting of all the average faulting of all joints in Group 2 (joints that had joints in Group 1 (joints with dowels that had horizontal skews dowels that were centered greater than 2.0 in. [51 mm], on of less than ±0.25 in. per 18 in. [± 6 mm per 457 mm]) was paired average, from the transverse joints). The same analysis was with the average faulting of all joints in Group 2 (those that conducted for joint LTE. Fourteen projects were considered had dowels with horizontal skews greater than ±0.75 in. per in this analysis, but only four projects provided sufficient data 18 in. [± 19 mm per 457 mm]). The same analysis was con- points for this paired t-test. ducted for joint LTE. The same procedure was followed for joint The high P-values suggest that there is no statistically sig- LTE, where the average LTE of all joints in Group 1 was paired nificant difference in faulting or LTE between the two groups with the average LTE of all joints in Group 2 for each project. (i.e., joints with average longitudinal translation < ±0.5 in. Fourteen projects were considered in this analysis, but only four [±13 mm] of the transverse joint and average longitudinal projects provided sufficient data points for this paired t-test. translation > ±2.0 in. [±51 mm] of the transverse joint). Note The P-value of 0.45 calculated for faulting suggests that that the faulting levels measured in this study were extremely there is no statistically significant difference in faulting between low, and none of the joints considered in this study had sig- the two groups (joints with average horizontal skew < ±0.25 in. nificant levels of average longitudinal translation (> 3 in. [± 6 mm] and joints with average horizontal skew > ±0.75 in. [76 mm]); the average minimum embedment length was 7 in. [± 19 mm]). The P-value of 0.11 calculated for LTE, however, [178 mm] or more. Therefore, the effects of higher longitudinal suggests that there is moderate statistical significance in the translation on faulting and LTE cannot be determined on the differences in LTE between groups of joints with these differ- basis of this data set. However, other studies (Burnham, 1999) ent levels of horizontal skew. The joints with higher average showed that embedment lengths of 2.5 in. [64 mm] or less horizontal skews had slightly lower joint LTE. It should be resulted in higher levels of faulting at these joints. noted that (1) the faulting levels are extremely low, (2) only a small number of joints at each section had average skew Vertical Tilt. Analysis was conducted to compare faulting > ±0.75 in. [±19 mm], and (3) a small number of sections and LTE at joints with dowels that had vertical tilts less than provided data for LTE comparisons. ±0.25 in. [±6 mm] (on average) with those that had dowels with vertical tilts greater than ±0.75 in. [±19 mm] (on average). 3.1.3 Summary of Field Study Analyses The average vertical tilt at each joint at each project was computed. For a given project, the average faulting of all Review of the field data from 60 projects indicated the joints in Group 1 (joints with dowels that had vertical tilts less following ranges for dowel misalignments in the majority than ±0.25 in. [±6 mm]) was paired with the average faulting of joints: of all joints in Group 2 (joints that had dowels with vertical tilts greater than ±0.75 in. [±19 mm]). The same analysis was · Vertical translation: ± 0.5 in. [± 13 mm] for pavement that conducted for joint LTE. Fourteen projects were considered is 12-in. [305-mm] thick or less; in the analysis, but only four projects provided sufficient data · Horizontal skew: ± 0.5 in. per 18 in. [± 13 mm per 457 mm]; points for this paired t-test. · Vertical tilt: ± 0.5 in. per 18 in. [± 13 mm per 457 mm]; and The P-value of 0.024 calculated for faulting suggests that · Longitudinal translation: ± 2 in. for 18-inch dowels [± 51 mm there is a statistically significant difference in faulting between per 457 mm]. the two groups (joints with average vertical tilt < ±0.25 in. [±6 mm] and joints with average vertical tilt > ±0.75 in. These ranges of misalignment represent tolerances that are [±19 mm]). The joints with higher average vertical tilts had easily achieved in the field. Furthermore, dowel misalignment higher levels of average faulting. Note that the faulting levels within these ranges on slightly higher levels does not appear are extremely low, and only a small number of joints at each to affect pavement performance significantly. section had average tilt > ±0.75 in. [±19 mm]. However, there was no statistically significant difference in LTE between the 3.2 Laboratory Testing two groups as indicated by the relatively high P-value of 0.474. This section summarizes the results of dowel pullout and Horizontal Skew. Analysis was conducted to compare shear tests conducted to evaluate the effects of dowel mis- faulting and LTE at joints with dowels that had horizontal alignment on joint lockup and dowel efficiency.
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21 3.2.1 Modified Pullout Testing 10000 9000 18.104.22.168 Results Overview 8000 It was observed that greasing or not greasing the dowels 7000 Aligned Pullout Force, in. greatly influences pullout force as shown in Figure 3.5 for 6000 Dowel with dowels embedded 6 in. in the same beam. Ungreased dowel 5000 3 in. embedment requires a significantly higher force to cause pullout failure. 4000 Embedment length also had a significant effect on pullout 3000 force. Figure 3.6 shows the pullout force versus relative dowel 2000 and displacement for two aligned dowels (to illustrate the 1000 variability in pullout force) and for a dowel with 3 in. [76 mm] 0 of embedment. The figure shows that the dowel with lower 0 0.05 0.1 0.15 0.2 0.25 embedment length required a lower pullout force than either Relative Dowel Displacement, in. of the aligned dowels and illustrates the large variability in Figure 3.6. Effect of embedment length on pullout pullout force. force versus displacement. Inspection of the interface between the dowel and concrete surface after each pullout test indicated slight surface paste chipping for dowels embedded with 1 in. [25 mm] of tilt and spalling damage for dowels embedded with 2 in. [51 mm] of 22.214.171.124 Trends rotation. To test for statistically significant differences in pullout Figure 3.7 presents the distribution of the maximum forces forces between the various groups of dowels, Student t-tests required to pull out dowels embedded with different types and were conducted (details of all these tests are provided in levels of misalignment in 4 groups. One group includes prop- Appendix C). The analysis confirmed that greased dowels with erly aligned, ungreased dowels with 6 in. [152 mm] of embed- 6 in. [152 mm] of embedment require significantly lower ment. Another group includes properly aligned dowels, dowels pullout forces than similarly embedded ungreased dowels, with 2 in. [51 mm] of rotation, and dowels with 4 in. [102 mm] and even greased dowels with 9 in. [229 mm] of embedment of rotation, all greased with 9 in. [229 mm] embedment. The require a lower mean pullout force than that of ungreased third group is similar to the second group, except the dowels dowels with 6 in. [152 mm] of embedment. had 6 in. [152 mm] of embedment. The fourth group includes It can be observed in Figure 3.7 that rotational misalignment unrotated greased dowels with 2 and 4 in. [51 and 102 mm] up to 2 in. per 18 in. [51 mm per 457 mm] dowel length did embedment length and dowels with 3 in. [76 mm] of embed- not have a significant effect on pullout force, but rotational ment and 2 in. [51 mm] of rotation. 12,000 10,000 8,000 Pullout Force, lbs. 6 in. embedment 6,000 with no grease 4,000 6 in. embedment 2,000 with grease 0 0 0.05 0.1 0.15 0.2 0.25 0.3 Relative Dowel Displacement, in. Figure 3.7. Distribution of maximum pullout forces Figure 3.5. The effect of greasing dowels on pullout for greased and ungreased dowels with varying force versus displacement. degrees of misalignment.
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22 misalignments of 4 in. per 18 in. [102 mm per 457 mm] dowel 12000 length had a significant effect. The analysis further illustrated 9 in. embedment that there is no statistically significant difference between 10000 4 in. embedment the means of pullout forces for the aligned and 2 in. [51 mm] 3 in. embedment rotated dowels, while the 4 in. [102 mm] rotated dowels 8000 Shear Force, lbs. 2 in. embedment required significantly larger pullout forces. This suggests that dowels that are not properly greased or dowels that experi- 6000 ence extreme rotation would increase longitudinal restraint at the joints. 4000 Also, because of the reduced dowel-concrete contact area, a lower pullout force is required for a reduced embedment 2000 length. For example, the 9-in. [229-mm] embedded dowels require a significantly larger pullout force than the 3-in. 0 0 0.02 0.04 0.06 0.08 0.1 0.12 [76-mm] embedded dowels. Relative Displacement, in. It has been reported that some distresses may develop pre- maturely due to joint lockup caused by dowel misalignment Figure 3.8. Illustration of reduced ultimate shear (Tayabji, 1986). However, analysis of the pullout data obtained capacity and loss of stiffness with increased dowel misalignment. in this study has shown that moderate misalignment of indi- vidual dowels did not have a significant effect on the maximum required pullout force, and greasing the dowels prior to embed- ment reduced the required pullout force significantly. lower ultimate shear capacity (about 5 kips [22 kN]) but there is also a loss in shear stiffness from almost the beginning of load application. 3.2.2 Shear-Pull Testing Although dowels transfer load through shear and moment 126.96.36.199 Trends mechanisms, numerous studies have shown that the shear mechanism dominates and the moment transfer mechanism Shear stiffness and ultimate shear capacity can be used to can be neglected (Guo et al., 1996). The MEPDG structural compare the effects of different types and levels of misalign- analysis model assumes that dowels transfer the load in ments. For example, Figure 3.9 shows that vertical tilt of up shear only. to 2 in. [51 mm] did not have a significant effect on shear The shear-pull test was used to evaluate the ability of stiffness or ultimate shear capacity while 4 in. [102 mm] of dowels with various misalignments to resist a shear load after vertical tilt greatly reduced the shear stiffness and ultimate being subjected to the pullout test, and it simulates the ability shear capacity. This suggests that the shear capacity decreases of a dowel to transfer a wheel load (in shear) after the joint has as vertical tilt increases above 2 in. [51 mm]. been opened due to slab contractions caused by temperature Because the shear test was performed after the pullout test, change or shrinkage. Shear performance measures (such as the extreme loss in stiffness experienced by the 4-in. [102-mm] shear stiffness and shear capacity) were used to evaluate the vertically-tilted dowel was probably caused in part by the effectiveness of each dowel-concrete system in resisting applied shear loads. Shear capacity is defined as the load at which the concrete around the dowel experiences shear failure. Shear 12000 stiffness is defined as the relationship between changes in shear 10000 force in relation to changes in relative dowel displacement. An example of shear force versus relative dowel displacement 8000 Shear Force, lbs. Aligned for dowels with 2, 3, 4, and 9 in. [51, 76, 102, and 229 mm] of 1 in. vertical tilt 6000 embedment is shown in Figure 3.8, which illustrates how the 2 in. vertical tilt ultimate shear can be affected by dowel misalignment. The 4000 4 in. vertical tilt figure shows that a 9-in. [229-mm] embedded dowel has a 2000 higher ultimate shear force than any of the dowels with a lower embedment length. Also, there is no loss of shear stiff- 0 ness in the dowel with 4 in. [102 mm] of embedment until the 0 0.02 0.04 0.06 0.08 Relative Dowel Displacement, in. system fails at a load of about 7 kips [31 kN]. For the 2 and 3 in. [51 and 76 mm] embedment cases, the dowel not only has a Figure 3.9. Vertical tilt shear pull shear capacity.