The GripTester braked-wheel fixed slip device has been used extensively on the Scottish trunk road network since the publication of Transport Scotland’s TS2010 surface course specification in 2010. Currently GripTesters are used to test the early-life skid resistance of both TS2010 and standard Clause 942 surface courses at a nominal period of four weeks and six months.

The aim of this study is to determine whether any relations exist between early-life GripTester measurements and other factors such as material characteristics, Site Categories and traffic levels. The data was also examined to see whether any relations existed between the six month GripTester result and subsequent SCRIM surveys.

Measurements taken by GripTesters between 2014 and 2016 were compared to other information held in Transport Scotland’s Integrated Road Information System, including material type, traffic data, Site Category and subsequent SCRIM surveys. Preliminary analysis indicated that the study should focus on examining the six month data. Based on the analysis of data the following findings were made:

  • GripTester results were observed to be higher during winter months (November – March).
  • Six month GripTester results taken on TS2010 surface course show a broad relation to SCRIM results taken when the material is between one and three years old.
  • Six month GripTester results show no relation to SCRIM results on Clause 942 materials.
  • No TS2010 sites fell below the TS2010 contractual levels and only one Clause 942 site fell below the TS2010 contractual levels.
  • No relations were established between six month GripTester results and PSV.
  • Smaller aggregate sizes provided higher levels of skid resistance, although this was based on a limited data set.
  • No relations could be established between six month GripTester results and traffic levels and Site Class Categories.

The report includes recommendations including restricting GripTester surveys to the summer months and conducting additional analysis to include the 2018 SCRIM survey to provide confidence in establishing a relation between six month GripNumbers and future SCRIM surveys and its potential to be used as a predictive tool. Reviewing the benefit of using GripTesters to test Clause 942 materials is also recommended.

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