Interlaboratory Variability of Slip Coefficient Testing for Bridge Coatings

CONCLUSIONS

The following list represents the major findings from this study:

• All labs participating in this study followed procedures that differed slightly in terms
  of equipment, test process, specimen alignment, and data acquisition. However, all lab procedures fell within the mandates of the current RCSC specification. Data generated by all labs in this study were deemed acceptable for comparison.
  
  
• The variability seen between the labs was likely related to subtle but important differences in the construction of their test apparatus and the method used to measure the slip displacement during the test. More variation was deemed possible using only one displacement measuring device versus two. The current RCSC specification allows for one displacement measuring device provided it measures "...movement of the loading head relative to the base."(pg. 71)⁽²⁾ The current RCSC language is unclear whether the loading head is above or below the spherical bearing.
  
  
• The slip coefficients measured for the representative group of five organic zinc-rich primers fell close to the specification limit of 0.5 for class B. Some coatings fell under class B, while others fell just short to be classified as class A. The data clearly indicate that the specific choice of primer material can be important in terms of meeting class B for slip, and at times the result could change based on the testing agency.
  
  
• Differences in results observed for coatings applied at manufacturers’ recommended thickness of +2 mil versus the same coatings applied at +1 mil did not exist.
  
  
• The impact of the 0.02-inch slip offset requirement to define failure during the test (as required in the specification) can be an important factor in limiting the calculated slip load for a particular coating. For the vast majority of the data from most of the labs, the 0.02-inch criterion did not play a factor in the result, but for selected replicates, this factor was important. It was thought the method used to measure the displacement was more of an influence than the 0.02-inch value itself.