Road Works

Bituminous surfacing over concrete structures

The use of bituminous surfacing over concrete structures (e.g. existing concrete roads) is widespread to improve the skid-resistance and the general appearance of roads on one hand, and to avoid the pre-mature failure of concrete surface by frost spalling in cold countries on the other hand.

In designing the bituminous surfacing over concrete, there are several areas to which engineers should pay attention. Firstly, the laying of thin bituminous material over the joints or existing cracks of concrete structure would lead to reflective cracking because the thermal movement of concrete induces swift formation of cracks in bituminous surfacing. Past research demonstrated that with the adoption of minimum thickness of 100mm bituminous surfacing the occurrence of reflective cracks would be delayed when compared with the use of thinner surfacing. Secondly, sufficient adhesion between concrete and bituminous surfacing has to be achieved. Therefore, it is recommended to apply a layer of tack coat on the concrete surface to promote bonding.

Corner reinforcement for concrete pavement

Consider a panel of concrete slab without any load transferring devices at it edges. When the concrete panel is subjected to traffic loads, the maximum stress induced in the concrete panel is at its four corners. Other than panel corners, the next significant stress induced in concrete slab is its four edges.

To avoid the structural failure of concrete pavement, one can locally increase the thickness of corners and edges to reduce the induced stresses. However, such local thickenings also increase temperature stresses. Moreover, the construction of non-uniform concrete pavement is not convenient from practical point of view. The other way out is to use load transfer devices like dowel bars at the edges of concrete panels. However, in situation where the designed thickness of concrete pavement is small which renders the provision of dowel bars not practical, special design of corner reinforcement has to be considered.

Compacted thickness of bituminous pavement

The choice of compacted thickness is closely related to the nominal maximum size of aggregates of bituminous materials. Based on the recommendation by Dr. Robert N. Hunter , the rule of thumb is that the compacted layer thickness should exceed 2.5 times the maximum size of aggregate. If the layer thickness is less than 1.5 times the nominal maximum size of aggregates, the mechanical properties of bituminous material is impaired by the possible crushing of larger

sizes of aggregates. Hence, controlled thickness of compaction of bituminous material should be clearly stated in works specification.