Anjaneyappa, PhD, MIE, MASCE, Associate Professor, Department of Civil Engineering, RV College of Engineering, Bengaluru
Massive road development work for huge road network of the country’s national assets is essential for overall development of the country. However, major road development projects consume a huge quantity of soil and crushed aggregates. The consumption of these materials will continue for years to come as more road networks need to be designed and developed. Hence, sustainable road construction practices need to be adopted in reconstructing and rehabilitating the pavements to meet the functional and structural requirements of pavements.
The induction of modern plants and equipment is an essential requisite for the production of uniform and high-quality pavements, as utilisation of appropriate plants and equipment for sustainable construction enhances the quality and production.
The stabilization of subgrade, granular sub-base, and base layers using conventional or commercial binders decreases the thickness of pavement layers compared to conventional pavement composition, thereby reducing consumption of soil and crushed aggregates. Similarly, use of reclaimed bituminous materials in bituminous binder layers reduces the consumption of virgin aggregates and binder.
Given below are a few important modern equipment used for the construction of stabilised pavement layers and reclaimed bituminous layers, including equipment used for pavement maintenance:
Stabilization of Subgrade
A Spreader is used to spread the defined quantity of lime or cement binder uniformly over a graded, loose layer of subgrade. The quantity of lime/cement binder depends on the degree of stabilization/strength desired. The Stabiliser machine mixes the soil and binder at the optimum moisture content of the soil. The stabiliser machine helps in uniform mixing and controls the depth of mixing. The pad-foot roller is used to compact the soil layer mixed with binder. The pad-foot roller helps in achieving higher density across the depth of the layer.
A Motor Grader is used to level the layer compacted by the pad-foot roller. The layer is allowed to cure for few days while light sprinkling of water is carried out. Pulverization, uniform spreading of binder, mixing, and compaction are important factors influencing the quality, which are better controlled by the spreader, stabiliser, and pad-foot roller. The same procedure is followed for the stabilisation of the subbase and base layers. Process of stabilisation of subgrade is presented in Figure1.
Picture courtesy: Wirtgen Group
Rehabilitation of the bituminous layer
Sustainable construction practices can be adopted for rehabilitation of distressed bituminous layers. A spreader is used to spread the virgin aggregate mix of known gradation and quantity on the distressed bituminous layer. A Stabilizer machine is used to mix the virgin aggregates, foamed bitumen/emulsion and reclaimed bituminous material in situ. A grader is used to get the required cross and longitudinal profile. A vibratory roller is used for compacting the graded layer. A grader is again used to level the compacted surface and is required only when pad-foot roller is used (Smooth drum vibratory can also be used to compact the graded layer instead of pad-foot roller). A smooth wheel dual drum wheel roller is used for final rolling and also for removing roller marks.
Uniform spreading of aggregates, mixing of aggregates, bitumen and reclaimed bituminous material, and compaction greatly influence the quality of pavement, which is better controlled using these equipment. Rehabilitation of typical distressed bituminous pavement is shown in Figure 2.
Hot mix plant with a RAP bin
Construction of bituminous pavement layers using reclaimed asphalt materials (RAP) is yet to start in the country. Making use of milled bituminous material is important towards sustainable pavement construction practices. A RAP bin attachment can be made compulsory to all the hot mix plants used for major projects in the country. RAP bins are not purchased while procuring hot mix plants as they are not used. Use of RAP in bituminous layers must start and should become a conventional practice in the future. RAP bin attachment to hot mix plant is shown in figure 3.
Reclaiming of bituminous materials from the bituminous pavement through milling operations is an important activity in sustainable bituminous pavement construction practice. A milling machine mills the existing pavement to the required depth without damaging the bottom layer. The milled materials are further stockpiled for use in bituminous mixes for binder pavement layers. Milling is a necessary and a better option for any cracked pavement layer which does not warrant rehabilitation or reconstruction. Typical milling of bituminous pavement is shown in Figure 4.
Micro surfacing machine
Maintenance is important for pavement to serve the road users at desired level during its design life. Micro surfacing is slowly gaining importance for preventive and maintenance of bituminous pavements in the country. Single setup is where preparation of bituminous mix using emulsion and its spreading is carried out. No rolling is required and traffic is allowed after layer sets which is generally about an hour. Micro surfacing is shown in Figure 5.
Aggregate spreader can be used effectively for maintenance activities such as surface dressing. Graded aggregates are spread evenly across the width of the pavement on an emulsion coated surface. These machines could be more useful in maintenance activities and yet to be seen in the country. A typical aggregate spreader is shown in Figure 6.
Why are sustainable practices not being followed?
Except for R&D and demonstration projects, sustainable practices like stabilisation techniques and use of RAP in bituminous layers are not being practiced. The reasons for these may include resistance to change, fear of failure, intensive supervision during construction for these types of projects, additional approvals and consequential delays, and lack of needed support from clients and consultants.