Road Works

Skid resistance of wearing course

The skid resistance of wearing course in a bituminous pavement is contributed by the macrotexture (i.e. the general surface roughness) and the microtexture (i.e. the protruding from chippings) of the wearing course. These two factors affect the skid resistance of flexible carriage in different situations. For instance, when the carriageway is designed as a high-speed road, the tiny channels among the macrotexture help to drain rainwater to the side of the road and avoid the occurrence of aquaplaning. In low speed roads the microtexture has particular significance in providing skid resistance by gripping the car tyres to the road surface.

Tack coat – emulsified asphalts vs cutback asphalts

Emulsified asphalt is a suspension of asphalt in water by using an emulsifying agent which imposes an electric charge on asphalt particles so that they will join and cement together. Cutback asphalt is simply asphalt dissolved in petroleum. The purpose of adding emulsifying agent in water or petroleum is to reduce viscosity of asphalt in low temperatures.

The colour of emulsion for tack coat is brown initially during the time of application. Later, the colour is changed to black when the asphalt starts to stick to the surrounding and it is described as “break”. For emulsified asphalts, when water has all evaporated, the emulsion is said to have “set”. Cutback emulsion is described to have been “cured” when the solvent has evaporated. There are several problems associated with cutback asphalts:

(i) Emulsified asphalt can be diluted with water so that a low application rate could be achieved.

(ii) The evaporation of petroleum into atmosphere for cutback asphalt poses environmental problem.

(iii)The cost of production of petroleum is higher than that of emulsifying agent and water.

Unsealed contraction joints in concrete pavement

For unreinforced concrete pavement, the contraction joint is an approximately 3mm wide groove with a depth of about one-third to one-fourth of slab thickness and a regular spacing of normally 5m. The grooves are designed such that they are too narrow for stones to fall into when the cracks are open due to the contraction of concrete. The groove location is l a plane of weakness and the groove acts as a potential crack-inducing device where any potential cracks due to shrinkage and thermal contraction may form will be confined to the base of the groove. It will not cause any unpleasant visual appearance on the exposed surface of unreinforced concrete pavement. The above-mentioned contraction joints can be designed as unsealed.

These grooves are very narrow so that stones can hardly get into these grooves even when the joint undergoes contraction. The fine particles or grit entering into the groove are likely to be sucked out by the passing vehicles. The joints can be self-cleansing and it may not be necessary to seal the joints for fear of attracting the accumulation of rubbish and dirt.