Road Works

“Pumping” at joints in concrete carriageway

Pumping at joints in concrete carriageway occurs in the presence of the following factors:

(i) Fine-grained subgrade;

(ii) Seepage of water into subgrade due to improper or inadequate drainage design;

(iii)The presence of heavy vehicular loads.

It involves the pumping out of water-borne particles of the subgradeowing to the deflections at the end of concrete slab. The first mechanism of pumping involves the softening of subgrade by water and the reduction in bearing capacity. It causes a larger instantaneous deflection at the slab ends under heavy traffic loads. During deflection, water containing fine soil particles is pumped out at the joints. Consequently, voids are formed in subgrade region and the void size grows by repeating the above sequence.

Purpose of using capping layers in pavement construction

When the California Bearing Ratio of subgrade is checked to be below a certain percentage (e.g. 5%), a capping layer is normally provided to reduce the effect of weak subgrade on the structural performance of the road. It also provides a working platform for sub-base to be constructed on top in wet weather condition because the compaction of wet subgrade is difficult on site. The effect of interruption by wet weather can be reduced significantly and the progress of construction works would not be hindered. Most importantly, the cost of capping layers is low because the material can be readily obtained locally.

Purpose of tar in bituminous materials

Tar is commonly incorporated in bituminous materials because of the following reasons:

(i) Blending of tar with bitumen possesses better binding performance with roadstone than bitumen.

(ii) Resistance to fuel oil erosion is high. Tar is used in roads where there is frequent spillage of fuel from vehicles.

Roadbase vs basecourse in flexible carriageway

Roadbase is the most important structural layer in bituminous pavement. It is designed to take up the function of distributing the traffic loads so as not to exceed the bearing capacity of subgrade. In addition, it helps to provide sufficient resistance to fatigue under cyclic loads and to offer a higher stiffness for the pavement structure. However, the basecourse is normally provided to give a well-prepared and even surface for the laying on wearing course. Regarding the load distribution function, it also helps to spread traffic loads to roadbase but this is not the major function of basecourse.

Sand layer vs cement sand used as bedding of precast concrete paving units

Cement sand is a mixture of cement and sand and it acts as a cohesive mass once mixed. Normally, a 20mm to 30mm sand layer is laid underneath precast paving block units. However, in locations of steep gradients where it stands a high possibility that rain runoff will wash out infilling sand and sand layers, cement sand should be sued instead. Similarly, when high pressure jetting is anticipated to be employed frequently in routine maintenance, sand layers beneath precast paving block units is not preferable owing to the reason of potential washing out of sand.

Sub-base for concrete carriageway – non-strength provider

Basically, sub-base for a concrete carriageway is provided for the following reasons:

(i) It provides a smooth and even surface between the subgrade and concrete slab. This avoids the problem of uneven frictional stresses arising from the uneven interface under thermal and shrinkage movement. It also improves the uniformity of support provided to concrete slab to enhance even distribution of wheel load to the subgrade.

(ii) For heavily trafficked carriageways with frequent occurrence of a high water table, it serves to prevent the occurrence of mud pumping on clayey and silty subgrade. The loss of these clayey soils through carriageway joints such as contraction and expansion joints will cause structural failure of concrete slab under heavy traffic load.

The stiffness of concrete slab accounts for the strength of rigid road structure. It is normally uneconomical to employ sub-base as part of the strength provider because a much thicker layer of sub-base has to be adopted to reduce the thickness of concrete slab by a small amount.

Hence, it is more cost-effective to increase the depth of concrete slab rather than to enhance foundation strength in order to achieve a higher load-carrying capacity of the concrete pavement.