rolled-asphalt wearing course is generally used wherever flexible construction has been specified for a road carrying heavy traffic. although most bitumens are capable of providing a non-skid surfacing material, some are known which can give a smooth surface which is slippery when wet. because no laboratory test had been found which was able to predict performance of a binder, bitumens on their own have not until recently been permitted for roads carrying >0.5 msa; mixtures of bitumen with pitch or with lake asphalt have been required on these heavily trafficked roads. this report describes work carried out to develop a test for selecting suitable bitumens. a technique in which cores of different wearing-course materials could be inserted in road surfaces was developed to evaluate the performance of a range of bitumens under traffic. at the same time a number of laboratory tests were carried out on the bitumens and it was found that permittivity (dielectric constant) correlated best with development of texture in rolled asphalt, and that the order of merit was maintained irrespective of the composition of the asphalt and of the traffic carried. it was concluded that petroleum bitumen having a permittivity of 2.650 (25 degrees c, 1.59 khz) or more will provide a satisfactory texture under any conditions. the test is applicable only to 100 per cent petroleum bitumen. a method has been devised for measuring permittivity accurately. (a)

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