Measurements of asphalt layer densities concrete in place by nuclear methods - interlaboratory comparison

European Standard EN 12697-7:2014 [1] specifies a method for measuring the bulk density of pavement mixtures using a transmission type gamma radiation test bench. The standard contains approximate description of measurement procedure. Procedure is not described in details and there are several unknowns during measuring procedure if only European Standard EN 12697-7:2014 is used. Surprisingly precision data at the end of this standard is giving unrealistic good precision (Repeatability r = 0.007 Mg/m3, Reproducibility R = 0.020 Mg/m3). According to this data it seems that laboratory determination of bulk density of bituminous specimens according to EN 12697-6:2020 [2] is less precise (Repeatability r ≥ 0.017 Mg/m3, Reproducibility R ≥ 0.022 Mg/m3). From laboratory experience it can be claimed that precision data in European Standard EN 12697-7:2014 is unrealistic and not applicable for measurement of density of asphalt pavements. In American standard ASTM D2950/D2950M-14 [3] procedure of measurement is explained more in details and precision data seems realistic (Repeatability r = 0.070 Mg/m3, Reproducibility R = 0.078 Mg/m3). In order to get better insight in these problem two interlaboratory comparisons were performed in Slovenia. First interlaboratory comparison was performed in 2005, where 6 laboratories equipped with Troxler type nuclear devices were included. Calibration of all 6 devices was performed on the same 10 places with known densities. On test section each laboratory performed 30 measurements. When average of all was compared we obtained good precision data with averages variation from 2,323 Mg/m3 to 2,336 Mg/m3. But we must be aware that during real in place measurements each laboratory would select different places for calibration and also for measurement. That would lead to bigger differences between laboratories. Second interlaboratory comparison was performed in 2021 and results are still evaluated.