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Publication Number:  FHWA-HRT-14-093    Date:  December 2014
Publication Number: FHWA-HRT-14-093
Date: December 2014

 

Interlaboratory Variability of Slip Coefficient Testing for Bridge Coatings

APPENDIX C. AGING STUDY

The Specimen Preparation section of this report indicated that each lab participating in the variability study received 20 test plates for each coating series. The intent was to test five specimens, and an extra five plates were available if the situation arose where one of the primary plates was untestable for any reason. At the end of the variability study, each participating lab sent all their extra plates back to lab 1. Lab 1 then matched the extra plates from each coating series together based on DFT similarities to form additional specimens. The intent of the extra specimens was two-fold: to see if additional curing time would change the slip coefficient and to assess an alternate way to measure slip displacements.

The question regarding aging effects was posed midway through the project and not addressed as part of developing the main variability study. In bridge fabrication, sometimes it is the case that the fabricated steel will sit in the primed condition for many months before slip-critical connections are ever assembled. Since the slip coefficient is based on recommended manufacturer cure time, it was thought that organic zinc-rich primers may continue to cure and harden beyond this period. If the primer continues to harden, then it is possible the slip coefficient could decrease (i.e., harder surfaces would tend to not stick together as easily).

The alternate slip displacement measuring technique was conceived in the middle of the variability study, but it was not explored to avoid introducing additional variables during the variability study. Since each lab was effectively measuring the slip as the displacement between the loading platens, all labs observed a soft slip response as the loading systems settled into each specimen. The notion was to get the LVDTs to measure slip displacement within the specimen, not between the loading platens as illustrated in figure 53. Close-up views of individual pieces are shown in figure 54 and figure 55.

The number of specimens that were tested for aging effects varied between one and six specimens depending on the coating series. This occurred because for some coating series, either the labs used the extra plates as part of the variability study or the DFTs on them could not be matched with others available to make a viable specimen.

The aging study began after RCSC had begun to digest the preliminary results from the interlaboratory variability study. As such, RCSC had formed a task group to make recommendations to modify their testing specification, and the aging study was used to test some of the preliminary ideas. Mostly, the aging specimens were tested according to the existing RCSC specification except for two modifications. First, after clamping, the specimens were preloaded with 5 kip of vertical load and then unloaded to 0 kip of vertical load to set the specimen against the platens. Second, the test began from a state of 0 kip of vertical load, whereas in the variability study, it was implied that testing should begin from 1 kip of vertical load. The preload step was ignored when testing the five specimens of coating A2 to test if that step was necessary with the modified displacement measuring device.

This illustration shows a general view of the alternative slip monitoring device, which consists of two main parts: an upper and lower bracket. The upper bracket slides over the middle plate of the specimen arrangement and is clamped in place with two thumbscrews to the middle plate. The specimen assembly slides between the lower bracket and is held in place with two thumbscrews. The linear variable differential transformers (LVDTs) are held in place within the upper bracket and react off vertically oriented thumbscrews in the lower bracket. The vertically oriented thumbscrews offer the ability to quickly adjust the LVDTs within the wanted range.
Figure 53. Illustration. Overall view of modified slip measuring device mounted to a specimen.

This illustration shows a detailed drawing of the upper bracket of a modified slip measuring device. The top left shows a plan view of the bracket with a specially shaped rectangular hole that slides over the middle specimen plate. The lower left shows an elevation view of the bracket, and the right side shows an end view of the bracket.
Note: Units are shown in inches.
Figure 54. Illustration. Upper bracket detailing of modified slip measuring device.

This illustration shows a detailed drawing of the upper bracket of a modified slip measuring device. The top left shows a plan view of the bracket with a specially shaped rectangular hole that the specimen assembly slides within. The lower left shows an elevation view of the bracket, and the right side shows an end view of the bracket.
Note: Units are shown in inches.
Figure 55. Illustration. Lower bracket detailing of modified slip measuring device.

The results of the aging study are presented in table 11 in terms of the slip coefficient for each specimen. The total cure time for each coating is also shown. The load versus slip displacement plots comparing the aged (i.e., extended cure) specimens to those tested at lab 1 as part of the interlaboratory study are shown in figure 56 through figure 65 for each of the 10 coating series. The graphs clearly indicate a stark contrast in the initial loading behavior between the 10-day and extended cure specimens. That is, the 10-day cure specimens with LVDT referencing platen motion demonstrate a soft initial nonlinear response indicative of shakedown of the specimens into the loading platens, whereas the extended cure specimens with modified LVDT holder all showed initially linear behavior with no shakedown response. This was observed for all 10 coating series, including A2 that did not preload the specimen to 5 kip before testing.

Table 11 . Slip coefficient results of aging study.

Specimen

Coating

A1

A2

B1

B2

C1

C2

D1

D2

E1

E2

1

0.54

0.56

0.50

0.54

0.63

0.65

0.52

0.51

0.47

0.46

2

0.49

0.56

0.53

0.52

0.62

0.64

 

0.50

0.47

0.48

3

0.54

0.55

0.46

 

0.62

0.63

 

0.48

0.49

0.48

4

0.53

0.56

0.48

 

0.63

0.63

 

 

0.47

0.45

5

 

0.57

 

 

0.63

 

 

 

 

0.47

6

 

 

 

 

 

 

 

 

 

0.46

Cure (days)

176

78

149

163

108

94

232

204

135

122

Note: Blank cells indicate that there were not enough spare plates to make more specimens.

This graph shows a comparison of slip behavior with cure time for coating A1. Load is on the y-axis from 0 to 60 kip. The x-axis shows the slip from -0.005 to 0.050 inch. Two distinct groups of lines are plotted: black represent the 10-day cure, and red represent the 176-day cure. The black lines have an initially upward curved shakedown response followed by a linearly increasing portion to a plateau in load. They peak at loads ranging between 50 and 53 kip and at displacements between 0.012 and 0.015 inch. The red lines have an initially linear response ultimately rounding over to a load plateau. They peak at loads ranging from 47 to 52 kip and at displacements between 0.010 and 0.013 inch. All tests were terminated at displacements in excess of 0.050 inch.
Figure 56. Graph. Comparison of slip behavior with cure time for coating A1.

This graph shows a comparison of slip behavior with cure time for coating A2. Load is on the y-axis from 0 to 60 kip. The x-axis shows the slip from -0.005 to 0.050 inch. Two distinct groups of lines are plotted: black represent the 10-day cure, and red represent the 78-day cure. The black lines have an initially upward curved shakedown response followed by a linearly increasing portion to a plateau in load. They peak at loads ranging between 49 and 53 kip and at displacements between 0.016 and 0.025 inch. The red lines have initially linear response ultimately rounding over to a load plateau. They peak at loads ranging between 53 and 56 kip at displacements between 0.013 and 0.015 inch. All tests were terminated at displacements in excess of 0.050 inch.
Figure 57. Graph. Comparison of slip behavior with cure time for coating A2.

This graph shows a comparison of slip behavior with cure time for coating B1. Load is on the y-axis from 0 to 60 kip. The x-axis shows the slip from -0.005 to 0.050 inch. Two distinct groups of lines are plotted: black represent the 10-day cure, and red represent the 149-day cure. The black lines have an initially upward curved shakedown response followed by a linearly increasing portion to a shallowly increasing load plateau. They peak at loads ranging between 51 and 55 kip and at displacements between 0.012 and 0.017 inch. The red lines have an initially linear response ultimately rounding over to a shallowly increasing load plateau. They peak at loads ranging between 45 and 52 kip and at displacements between 0.006 and 0.009 inch. All tests were terminated at displacements in excess of 0.050 inch.
Figure 58. Graph. Comparison of slip behavior with cure time for coating B1.

This graph shows a comparison of slip behavior with cure time for coating B2. Load is on the y-axis from 0 to 60 kip. The x-axis shows the slip from -0.005 to 0.050 inch. Two distinct groups of lines are plotted: black represent the 10-day cure, and red represent the 163-day cure. The black lines have an initially upward curved shakedown response followed by a linearly increasing portion to a plateau in load. They peak at loads ranging between 54 and 57 kip and at displacements between 0.020 and 0.025 inch. The red lines have an initially linear response ultimately rounding over to a load plateau. They peak at loads ranging from 50 to 52 kip and at displacements ranging from 0.007 to 0.010 inch. All tests were terminated at displacements in excess of 0.050 inch.
Figure 59. Graph. Comparison of slip behavior with cure time for coating B2.

This graph shows a comparison of slip behavior with cure time for coating C1. Load is on the y-axis from 0 to 60 kip. The x-axis shows the slip from -0.005 to 0.050 inch. Two distinct groups of lines are plotted: black represent the 10-day cure, and red represent the 108-day cure. The black lines have an initially upward curved shakedown response followed by a linearly increasing portion to a plateau in load. They peak at loads between 58 and 61 kip and at displacements between 0.017 and 0.020 inch. They red lines have initially linear response ultimately rounding over to a load plateau. They peak at loads ranging from 60 to 62 kip at a displacement of about 0.010 inch. All tests were terminated at displacements in excess of 0.050 inch.
Figure 60. Graph. Comparison of slip behavior with cure time for coating C1.

This graph shows a comparison of slip behavior with cure time for coating C2. Load is on the y-axis from 0 to 60 kip. The x-axis shows the slip from -0.005 to 0.050 inch. Two distinct groups of lines are plotted: black represent the 10-day cure, and red represent the 94-day cure. The black lines have an initially upward curved shakedown response followed by a linearly increasing portion to a plateau in load. They peak at loads ranging between 58 and 60 kip and at displacements between 0.022 and 0.025 inch. The red lines have an initially linear response ultimately rounding over to a load plateau. They peak at loads ranging from 60 to 62 kip all at a displacements of about 0.012 inch. All tests were terminated at displacements in excess of 0.050 inch.
Figure 61. Graph. Comparison of slip behavior with cure time for coating C2.

This graph shows a comparison of slip behavior with cure time for coating D1. Load is on the y-axis from 0 to 60 kip. The x-axis shows the slip from -0.005 to 0.050 inch. Two distinct groups of lines are plotted: black represent the 10-day cure, and red represent the 232-day cure. The black lines have an initially upward curved shakedown response followed by a linearly increasing portion to a plateau in load. They peak at loads ranging between 43 and 46 kip and at displacements between 0.013 and 0.025 inch. The one red line has an initially linear response ultimately rounding over to a load plateau. It peaks at a load of about 50 kip and at a displacement of 0.007 inch. All tests were terminated at displacements in excess of 0.050 inch.
Figure 62. Graph. Comparison of slip behavior with cure time for coating D1.

This graph shows a comparison of slip behavior with cure time for coating D2. Load is on the y-axis from 0 to 60 kip. The x-axis shows the slip from -0.005 to 0.050 inch. Two distinct groups of lines are plotted: black represent the 10-day cure, and red represent the 204-day cure. The black lines have an initially upward curved shakedown response followed by a linearly increasing portion to a plateau in load. They peak at loads ranging between 43 and 46 kip and at displacements between 0.010 and 0.035 inch. The three red lines have an initially linear response ultimately rounding over to a load plateau. They peak at loads ranging from 46 to 50 kip and at displacements between 0.009 and 0.011 inch. All tests were terminated at displacements in excess of 0.050 inch.
Figure 63. Graph. Comparison of slip behavior with cure time for coating D2.

This graph shows a comparison of slip behavior with cure time for coating E1. Load is on the y-axis from 0 to 60 kip. The x-axis shows the slip from -0.005 to 0.050 inch. Two distinct groups of lines are plotted: black represent the 10-day cure, and red represent the 135-day cure. The black lines have an initially upward curved shakedown response followed by a linearly increasing portion to a distinct peak in load followed by a descending branch in loading to a lower plateau in load. They peak at loads ranging between 42 and 45 kip and at displacements between 0.011 and 0.020 inch. The red lines have an initially linear response ultimately rounding over to a peak load followed by a descending branch of loading to a lower load plateau. They peak at loads ranging from 45 to 47 kip and at a displacement of about 0.010 inch. All tests were terminated at displacements in excess of 0.050 inch.
Figure 64. Graph. Comparison of slip behavior with cure time for coating E1.

This graph shows a comparison of slip behavior with cure time for coating E2. Load is on the y-axis from 0 to 60 kip. The x-axis shows the slip from -0.005 to 0.050 inch. Two distinct groups of lines are plotted: black represent the 10-day cure, and red represent the 122-day cure. The black lines have an initially upward curved shakedown response followed by a linearly increasing portion to a distinct peak in load followed by a descending branch in loading to a lower plateau in load. They peak at loads ranging between 43 and 45 kip and at displacements between 0.010 and 0.020 inch. The red lines have initially linear response ultimately rounding over to a peak load followed by a descending branch of loading to a lower load plateau. They peak at loads ranging from 43 to 46 kip and at a displacement of about 0.009 inch. All tests were terminated at displacements in excess of 0.050 inch.
Figure 65. Graph. Comparison of slip behavior with cure time for coating E2.

Table 12 compares the average slip coefficients attained from the aging study to those from the interlaboratory variability study using the peak load ignoring the 0.02-inch criteria. When using the modified measuring device, none of the specimens ever came close to being controlled by the 0.02-inch displacement criteria. COV is not presented in table 12 since each coating series had different numbers of specimens, many not even having enough specimens to make a viable COV calculation. The final row in the table presents the difference in slip coefficients between the aging and variability portions of the overall study. Mostly, the average slip coefficient increased with the extended cure time. The one noted exception is coating B, which exhibited more variability and overall reduced average slip coefficient with an extended cure at both the +1- and +2-mil thicknesses.

Table 12 . Comparison of slip coefficients between 10-day and extended cure.

Coating

Extended Cure Average

10-Day Cure Average

Difference

A1

0.53

0.53

0.00

A2

0.56

0.53

0.03

B1

0.50

0.55

-0.05

B2

0.53

0.56

-0.03

C1

0.63

0.62

0.01

C2

0.64

0.62

0.02

D1

 

0.47

 

D2

0.50

0.45

0.05

E1

0.47

0.45

0.02

E2

0.47

0.45

0.02

Note: Blank cells indicate that only one specimen could be tested for coating D1 with the extended cure, and an average could not be calculated. Because an average could not be calculated for the extended cure, the difference relative to the 10-day cure could not be calculated either.

 

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