Road Works

“Mortar mechanism” vs “stone contact mechanism” in bituminous materials

“Stone contact mechanism” applies to well graded aggregates coated with bitumen (e.g. dense bitumen macadam) where the traffic loads on bituminous roads are resisted by stone-to-stone contact and by interlocking and frictional forces between the aggregates. It is essential to adopt aggregates with a high crushing strength. The bitumen coatings on the surface of aggregates merely serve to cement the aggregates together.

“Mortar mechanism” involves the distribution of loads within the mortar for gap-graded aggregates (e.g. hot rolled asphalt). The mortar has to possess high stiffness to prevent excessive deformation under severe traffic loads. It is common practice to introduce some filler to stiffen the bitumen.

Noise absorptive materials – how it works

The basic mechanism of noise absorptive material is to change the acoustic energy into heat energy. The amount of heat generated is normally very small due to the limited energy in sound waves (e.g. less than 0.01watts). The two common ways for energy transformation are: 

(i) Viscous flow loss

The absorptive material contains interconnected voids and pores into which the sound energy will propagate. As sound waves pass through the material, the wave energy causes relative motion between the air particles and the absorbing material and consequently energy losses are incurred.

(ii) Internal fiction

The absorptive materials have some elastic fibrous or porous structures which would be extended and compressed during sound wave propagation. Other than energy loss due to viscous flow loss, dissipation of energy also results from the internal friction during its flex and squeezing movement.

Necessity of air voids in bituminous pavement

If the presence of air voids is too high, it leads to an increase of permeability of bituminous pavement. This allows the frequent circulation of air and water within the pavement structure and results in premature hardening and weathering of asphalt. Therefore, too high an air void content poses detrimental effect to the durability of the bituminous pavement.

If the presence of air voids is too low, flushing, bleeding and loss of stability may result under the effect of prolonged traffic loads because of the rearrangement of particles by compaction. Aggregates may become degraded by traffic loads leading to instability and flushing for such a low air void content. The air void space can be increased by adding more course or fine aggregates to the asphalt mix. Alternatively, if asphalt content is above normal level, it can be reduced to increase air voids.

Oil interceptors

Grease and oils are commonly found in stormwater runoff from catchments. They come from the leakage and spillage of lubricants, fuels, vehicle coolants etc. Since oils and grease are hydrocarbons which are lighter than water, they form films and emulsions on water and generate odorous smell. In particular, these hydrocarbons tend to stick to the particulates in water and settle with them. Hence, they should be trapped prior to discharging into stormwater system. Oil interceptors are installed to trap these oil loads coming from stormwater. In commercial areas, car parks and areas where construction works are likely. It is recommended to establish oil-trapping systems in these locations. Typical oil interceptors usually contain three compartments:

(i) The first inlet compartment serves mainly for the settlement of grits and for the trapping of floatable debris and rubbish. 

(ii) The second middle compartment is used for separating oils from runoff.

Optimum binder content in bituminous pavement

The amount of binder to be added to a bituminous mixture cannot be too excessive or too little. The principle of designing the optimum amount of binder content is to include sufficient amount of binder so that the aggregates are fully coated with bitumen and the voids within the bituminous material are sealed up. As such, the durability of the bituminous pavement can be enhanced by the impermeability achieved. Moreover, a minimum amount of binder is essential to prevent the aggregates from being pulled out by the abrasive actions of moving

vehicles on the carriageway.

However, the binder content cannot be too high because it would result in the instability of the bituminous pavement. In essence, the resistance to deformation of bituminous pavement under traffic load is reduced by the inclusion of excessive binder content.

Purpose of reinforcement in concrete roads

The main purposes of reinforcement in concrete roads are:

(i) to control the development and pattern of cracks in concrete pavement.

(ii) to reduce the spacing of joints. In general, joints and reinforcement in concrete structures are common design measures to cater for thermal and shrinkage movement. 

Hence, the inclusion of reinforcement allows the formation of tiny cracks in concrete pavement and this allows wider spacing of joints.

In fact, the amount of reinforcement in concrete slab is not substantial and its contribution to the structural strength of roads is not significant.