Question

Review Technical Note 87 Bituminous Surfacing –Stripped Seals from VicRoads).

Describe what is meant by a stripped seal and name 4 precautions should you take to avoid the stripping of aggregate from a fresh seal.

Answer 1

Stripping or loss of aggregate, particularly in newly completed sprayed seal work, is usually caused due to the following major factors:

1. Insufficient binding material to hold the aggregate in place.
2. Incorrect and skill-less application rate of aggregate to the binder.
3. Inadequate adhesion between binder and aggregates.

Stripping phenomena can also occur in aged seals due due to hardening of binder as a seal approaches the end of its service life. When aggregate particles are applied to a binder placed at a designed application rate it is intended that they form an interlocking mosaic that will squeeze the binder an appropriate height up the aggregate particle thus enabling the seal to withstand the stresses of traffic.

If insufficient aggregate is applied, the mosaic does not form and less binder is 'squeezed' up the sides of the aggregate and individual aggregate particles are left unsupported
leading to stripping.

Precautions to be taken to prevent stripping of fresh seals:

1. The amount of cutter oil should be appropriate to binder type, aggregate size, pavement temperature and traffi c volume. It should be no more than that necessary for effective adhesion, as excess cutter oil can cause bleeding in subsequent hot weather or excessive fl uidity and variation of binder application rate through fl ow from high to low areas of pavement.
2. Prompt covering of binder by aggregate is essential in cool conditions. In cold weather, shortening of sprayer runs may be necessary to ensure that aggregate is applied without delay.
3. Rain on newly completed work creates the greatest risk of substantial stripping by disrupting the bond between the binder and aggregate. In extreme situations, emulsifi cation of binder can occur by the combined action of moisture and traffi c. Binder/aggregate adhesion may, however, be re-established when moisture is no longer present, provided that the aggregate is not disturbed in the meantime. It is possible, therefore, to avoid severe stripping by excluding or strictly controlling traffi c on the wet surface. Careful rolling and controlled slow moving traffi c may be used to establish adhesion as the surface dries out. If binder emulsifi cation occurs, total loss of aggregate can only be avoided by exclusion of all traffi c until suffi cient drying has taken place to allow rolling to recommence.
4. Bitumen emulsion binders are particularly susceptible to rain damage if the emulsion is not fully set up and bitumen emulsion and should not be used where there is any risk of rain on the new work.