Reconstruction of tasman highway sections

This report discusses laboratory and field studies carried out in 1976/77 on sections of the tasman highway in the north east district of the Department of Main Roads, Tasmania. The highway was constructed of a granitic gravel pavement material, surfaced with a bituminous seal, and became due for early reconstruction after log haulage trucks started using it. The objective of the studies, which comprised moisture and density determinations on the subgrade and pavement, field CBR tests, and measurements of pavement deflection and deflection ratio, was to determine quality of the existing gravel base and thicknesses of different types of material - gravel and bituminous concrete - required for overlaying after widening of sections. Thicknesses were calculated on the basis of reduction of existing deflections to tolerable values. Three methods of reconstruction, one with a quartzite gravel overlay and the other two with a bituminous concrete overlay of different cross sections were costed and work proceeded in 1978 with a bituminous concrete overlay some 90 mm thick at the centre and 125 mm at the edge. Prior to the placing of the overlay, granitic gravel was placed in 450 x 300 mm widening trenches and different methods of compaction tried. The most successful method was using the wheels of a grader for the first lift and a vibrating plate compactor for the second. The bituminous concrete overlay was first placed in two lifts of 40-60 mm and then a change made to placement in three lifts of 30-40 mm. To achieve a target density of 96 per cent marshall a rolling procedure of 12 passes of a 10 t smooth drum roller was adopted, after experimenting with different roller combinations. Measurements of roughness made on the reconstructed pavement showed that an adequate standard of quality had been achieved. Measurements of deflection indicated values within the tolerable value selected of 0.9 Mm and that the overlay had reduced existing deflections to the predicted region.