ASSESSING BINDER/AGGREGATE BOND IN FUNDAMENTAL UNITS

The Spin Test was devised in Sweden to determine in fundamental units the strength of the bond between binder and aggregate in the surface dressing of roads. This paper presents some results of a study aiming to broaden the scope of the Spin Test to include a variety of surface dressing binder types. To be usable, a binder in surface dressings must: (1) adhere to the road; (2) adhere to the aggregate chippings; and (3) have sufficient strength or stiffness ('cohesive strength') to resist traffic forces. The strength of the retentive forces, holding a chipping in place, depends on which of these factors is weakest, and sometimes on a combination of these factors. In the Spin Test, a simulated surface dressing is placed on a rotating metal disk, and spun at given speeds for 30sec. The binder's retentive strength is related to the speed at which about half the chippings are lost. Some results are given showing how aggregate binder bond strength increases with curing time, and that, even after two weeks laboratory curing, only a fraction of the binders' ultimate potential performance may have been achieved. Bond strengths were also found to be much higher at 25 degrees Celsius than at about 40 degrees. Controlled breaking agents such as Nybreak can minimise the vulnerability of binder emulsions early in their life.