Cement-bound gravel (CG) is used as a rigid roadbase and is included in a pavement type in the Swedish road construction directives (BYA-84). BYA-84 stipulates, in addition to the CG layer, a relatively thick hot mix base layer (AG), and an asphalt wearing course. This means that in most cases CG is not an economically viable alternative to an asphalt pavement. The AG layer is intended to prevent any cracks in the CG layer from propagating upwards as reflection cracks through the overlying asphalt layer. Former tests were carried out in order to find out whether reflection cracks vary with thickness of the hot mix base layer. In 1989 new tests were performed on an urban main road. The principal aim has been to study how road. The principal aim has been to study how different design of the asphalt layers can eliminate or further reduce the reflection cracks. The intention is to improve the tensile strength and strain properties of the asphalt layer. Eight test sections have been laid in succession. Three of the sections have been laid with standard mixes and serve as control sections. Asphalt strengthening has been applied on two sections, using partly a grid (Armapal) and partly a geotextile (Fibertex). On two sections modified asphalt mixes have been used as wearing course, one being RUBIT, which contains granulated rubber, and the other containing a polymer-modified binder, PMB 20 (Bitulastic). On one of the test sections, fibres have been mixed in the CG layer instead of strengthening the overlying asphalt layer. On the sections which were constructed in warm weather in May, there were early occurrences of transverse shrinkage cracks in the CG layer with a spacing of 10-15 m. On the section where plastic fibres had been mixed in the CG material, the cracks were somewhat further apart, 15-20 m. On the test section laid under more favourable weather conditions during September, no early shrinkage cracks were observed. The test sections will be followed up primarily in regard to reflection cracks in the asphalt wearing course. One year after construction no reflection cracks were observed in any of the test sections. The follow-up will also include bearing capacity measurements with a falling weight deflectometer and measurement of evenness and rutting. It will continue for a period of at least three years. (A)
Samenvatting