Description of a volumetric type testing and FPC procedure for SMA to prevent premature permanent deformation

One of the main reasons for using SMA as a surface layer is its excellent resistance to permanent deformation under heavy traffic loadings. This is caused by the fact that SMA is a stone skeleton mix. This implies that the traffic loading is carried by the large mineral aggregate parts in the mix which are in mutual contact. This is only possible in case the air voids in the mineral aggregate part of the SMA are not completely filled with mortar (sand, filler and bitumen). In normal situations an air void content of about 4%V/V should be available in the SMA. However, due to compaction during construction or due to traffic loading these air voids can reduce to zero. In this case SMA becomes instable and permanent deformation occurs. In the Netherlands this problem occurs regularly. One of the main causes is the fact that the characteristics of the constituent materials vary through the year. This implies that it is possible that the materials, which are used during production, are slightly different from the materials which are used in the type testing procedure. In case e.g. more weak parts are present in the coarse mineral aggregates during production than in type testing it is possible that these weaker parts are crushed during production, compaction and traffic loading and the SMA will be overfilled. To tackle this problem an adjusted type testing and factory production control procedure is developed in the Netherlands. In this procedure the characteristics of the coarse mineral aggregate, which are used in the type testing procedure, are established in an extra test using gyratory compaction. With this test the FRS-value (Filling Ratio Stone Skeleton) is determined. During production of the SMA the FRS-value of the coarse mineral aggregate is constantly compared with the VRS-value in the type testing procedure. In case there is a discrepancy between the two values, the amount of coarse aggregates in the SMA is adjusted. In the paper the background of this new type testing and factory production control procedure will be presented. Also the extra tests, which will be part of this procedure, will be discussed. Finally, the field experiences with the new procedure will be evaluated.