Analysis of TxDOT Thickness Measurement Procedures for Thermoplastic Pavement Markings (4150-1)

Research has shown that thickness plays a major role in the retroreflective performance of thermoplastic pavement markings, especially on newly sealcoated roadway surfaces. Therefore, accurate thickness measurement by inspectors on striping jobsites is necessary to ensure adequate thickness is being achieved. This report presents the results of a statistical analysis comparing two thickness measurement methods for thermoplastic pavement markings that are commonly used by Texas Department of Transportation (TxDOT) inspectors. These two methods are: 1.) measure thickness of thermoplastic pavement marking sample with a standard laboratory caliper and 2.) measure thickness of thermoplastic pavement marking sample with a needlepoint micrometer. The main difference between the two measurement methods is the accuracy of the instrument. The needlepoint micrometer allows for accurate measurement to the top of the binder material, while it is very difficult to get a measurement to the top of the binder material with a caliper. Therefore, caliper measurements are often made to the top of the drop-on beads. Current TxDOT policy states that thermoplastic thickness be specified and measured to the top of the binder material. However, many TxDOT field inspectors still use calipers to make thickness measurements. Therefore, it was deemed important to quantify the differences in the two measurement methods. Forty-seven thermoplastic pavement marking samples of varying thickness were taken from striping jobsites statewide and used in the analysis. The results show that the caliper measured an average of 20.5 mils (0.0205 inch) and 16.7 mils (0.0167 inch) thicker than the needlepoint micrometer for large and small bead samples, respectively. As such, the researchers recommend that needlepoint micrometers be used for all thermoplastic thickness inspection performed by TxDOT. Further findings and recommendations are made in the report.