Road Infrastructure projects are time constrained; early completion of project entitles rewards while delays attract penalties. The plants and equipment being used determines the quality of the road construction, and the output of the plants and equipment is dependent on the skill of the operator and the condition of the plants/equipment including other factors.
Dr Anjaneyappa, Department of Civil Engineering, RV College of Engineering, Bengaluru
One of the most important quality parameters for unbound aggregate pavement layers like GSB (Granular Sub-Base) and WMM (Wet Mix Macadam) is gradation, which is maintaining appropriate size, shape and proportion of aggregates in mix. Similarly, for bituminous layers like DBM (Dense Bituminous Macadam) and BC (Bituminous Concrete), the most important quality parameters are gradation and temperature of mix during production, spreading and rolling. Segregation is the separation of coarser particles from finer particles. It affects the gradation and results in poor quality pavement layer. However, segregation cannot be avoided but is minimized through all possible means during construction. This article discusses the influence of crushers, tippers, hot mix bituminous plants, pavers, and rollers on the quality of pavement construction.
Crushers: Gradation is influenced by the crushing plant and its processes
- Continuous feeding of mix sizes of aggregates into crushers results in better shape of aggregates with less flakiness (flaky aggregates are not desirable in the mix).
- A combination of two curved jaws for crushing in the jaw crusher will enable aggregates with lesser flakiness. This combination is also helpful when relatively continuous feeding of aggregate input is not possible. It is also better than the combination of two straight jaws or one curved and one straight jaw.
- Impact crushers at secondary or tertiary stage result in good shape of aggregates without any hidden cracks, hence, gradation does not change during rolling.
- Height of fall of crushed aggregates from the conveyor belts to the heap (stockpile) needs to be reduced to the extent possible or use of ladders and conveyors for vertical dropping of aggregates need to be implemented. This will reduce segregation and help in maintaining the gradation.
- Loading the hopper bins of HMP (hot mix plants) to be carried out carefully either using tippers or loaders. The spillage or mixing of aggregates of one bin with another bin during loading is to be avoided as it influences the job mix formula, based on which openings of hopper bins are adjusted. Uniform gradation will be difficult to obtain. Separator plates between the bins are a useful attachment to control the mixing or spillage of aggregates.
- Pug mill of HMP to be neither overloaded nor under loaded as both will result in poor gradation.
- Worn out paddles / plates of pug mill to be replaced for getting a better mix
- Heating of bitumen at higher temperature than specified or heating for longer durations either in bitumen tank or in pug mill reduces the quality of bitumen and mix. This, in turn, reduces the sticking property of bitumen with the aggregates and also the life of bitumen.
- Holding the bituminous mix in bins or storage silos of plants for longer durations due to non-availability of tippers reduces the quality of mix by reducing stickiness and life of bitumen.
- Loading to tippers is to be done in the pattern shown in figure 2 to minimize segregation.
- The height of fall of mix from silo to tipper to be reduced.
Tippers: Situations leading to segregation during transportation of aggregates and mixes
- Not to carry the smaller loads in tippers. Load in the tipper to be full. Smaller loads cause segregation of aggregates, coarser aggregates roll down during movement of tippers leaving finer particles in the top. The segregation is more severe in case of unbound aggregate mixes like GSB WMM compared to bituminous mixes like DBM or BC.
- Tipper should not come and touch the paver while unloading. It should be about one foot away; the paver should move and touch the tipper then unloading should start. If tipper hits the paver, new mat / layer under construction gets undulated.
- With end dump trucks, the proper procedure for dumping the HMA mix into the hopper is to raise the truck bed slightly and allow the bituminous mix to slide against the tailgate before it is released. This procedure will allow the HMA mix to flood the hopper – not allowing the HMA mix to dribble from the truck into the hopper. This method reduces the segregation of mix.
Pavers: Since paving is a continuous operation, a paver should not be stopped unless necessary. Frequent stop and go paving operations produce weak planes, leading to premature failures. Paving speed needs to be adjusted with respect to production capacity of HMP, lead and thickness of layer, so that the paver moves continuously. It may be noted that majority of the compaction is done by the paver.
Some good paving practices:
- Maintaining uniform head to about mid height of auger results in uniform thickness. The force will begin to lift the screed when head of material is high. If the head of material is too low, the lack of force will allow the screed to lower.
- No frequent changes to be done for thickness of pavement layer. Any change of thickness/angle of attack done takes few meters by the time changes to be effective in the layer.
- Heating the screed of the paver before starting paving and not to allow hot bituminous mix to heat the screed.
- Paver hopper should never be made empty during paving as this results in segregation.
- Paving need to be continuous, production of the bituminous mix, transportation, thickness of pavement layer all to be synchronised to paver speed so that paver does not stop.
- Only a few skilled workers to be involved during paving.
- Paver with additional attachment for visualising the temperature and thickness of the layer paving is desirable.
Rolling: Compaction of bituminous mixes using rollers is an important operation to
- Improve mechanical stability
- Improve resistance to permanent deformation
- Reduce moisture penetration
- Improve fatigue resistance
- Reduce low-temperature cracking potential
- Thin bituminous layers
- Urban areas, near structures
- Joints with less damage to existing pavement
- Uniform speed to be maintained so that uniform density is achieved.
- Compactor not to stand on hot mix during rolling as it will result in undulated surface
- Avoid jerky starts, sudden braking, and sharp changes to get a uniform surface profile
- Change of lane to be made only on an already compacted surface
- Small amount of detergent / vegetable oil to be used instead of water during rolling
- Rolling in vibratory mode on thin layers (less than 20 mm) not to be carried out.
- First pass on the paved layer to be carried carefully, later changing the profile after first pass is not desirable.
- Additional attachment for visualizing density of layer compacted is desirable.
The plant and equipment inducted in projects many times do not match with each other with respect to their output capacities as shown in Figure 5. The present utilization of plants and equipment in a typical road project is not more than 50%. Hence, balancing the production by synchronising the output of all the plants and equipment must be carefully planned for higher production and return on investment for each plant/equipment used.
All the plant and equipment are handled by operators. Quality of product / road not only depends on equipment but also on skill and mood of the operator. Attracting and retention of the expertise of operators is the one of the important aspect for better quality roads. This may be possible with better benefits and by instilling the confidence of sustainable/ permanent job for operators in the industry.