Emerging Methodologies & Technologies for Construction of Flexible Pavement

M.N.Nagabhushana, Senior Principal Scientist, Flexible Pavements, Co-ordinator (APTF), Pavement Engineering Area, CSIR-CRRI, New Delhi

Introduction

While an efficient transport system is needed for growth of economy, roads play an important role as most sought mode of transport. India has an extensive road network of 4.6 million kms and number of vehicles are growing at almost 10% per annum in last 5 years with overloading and growing traffic stressed road infrastructure. To cope up with increasing transportation demand, in the immediate next following decades, newer and different transportation systems will be greatly encouraged; however, road transport will continue to play a major role. As this new regime approaches and the earlier methods tend to become unacceptable, labour and labour-intensive techniques will experience shortages. In addition, the world is facing acute shortage of naturally available ingredient materials and the cost of construction materials increasing every year; the situation is forcing the agencies to find alternate sources and materials. Also, design, materials, and workmanship, will have to provide a long-lasting product to avoid the need for major traffic disruptions for maintenance and rehabilitation or reconstruction, keeping in mind the health and safety issues as well. This scenario demands new techniques, construction-automation and equipment, so that equivalent work can be performed with fewer crew. The automation is likely to prove more reliable, efficient, and cost-effective as well. Road pavements and allied structures, thus, are expected to undergo significant changes in terms of materials and methods of construction operations.

A pavement structure, understandably, is interposed between the wheel and ground (soil), in order to support and sustain the repeated applications of wheel loads without undergoing undue deformation. The roadway structure is, thus, expected to be stable and non yielding so as to generate only least possible rolling resistance for the heavy wheel loads. The pavement is made of higher quality materials, though, the strength of materials of different layers differ with a more specific functional attribute. Based on the composition, pavement structure is categorised as either flexible or rigid or composite. A typical flexible pavement is composed of layers as in the figure below.


Though, flexible pavements conventionally mean just granular beds, with or without; thick or thin bitumen treated layers, new technologies have been allowing materials which are otherwise considered as non-conventional, and hence, it is pertinent to review these technologies which offer specific roles.

Need for Advanced /High-Performance Materials and Technologies

With natural materials getting depleted, efforts were on to find succor in other methods and materials, but till recently, the development of improved materials was mainly focused at improving specific properties of locally available materials by using additives (admixtures, extenders, modifiers). There had been no strong impetus to seriously consider replacing conventional construction materials with new materials. However, by realising that the age of natural construction materials and the use of conventional materials in their present form is coming to an end at a faster pace. The new technologies need to be developed fast to continue to support the construction activities including rehabilitation and reconstruction of pavements along the road network. Thus, concerns about limited availability and sustainability are driving the search for new and advanced materials for roadway construction.

The currently used materials for pavement construction can be classified as follows:

1. Natural (Raw) Materials: Stone or brick aggregates, bitumen and natural resins.
2. Manufactured (Processed) Materials. Metallic materials (steel, aluminum, zinc), ceramic-based materials (portland cement, natural pozzolans), industrial by-product materials (fly ash, slag, silica fume), other waste products (crumb rubber), polymers, fibers and fiber-reinforced polymers, synthetic aggregates—typically, lightweight and slag aggregates.
3. Composite Manufactured Materials like. PCC and clad steels.

Further, there is an array of identified materials under the above classes of materials that show potential applications and these advanced materials identified include the following:

1. Aggregate Materials: Synthetic Aggregates, Manufactured Aggregate Using Captured CO2
2. Bituminous Binder Materials: Sulfur-Extended Bitumen/Asphalt, Bio-Derived Bituminous/Asphalt Binders, High Modified Asphalt Binders (HIMA)
3. Bituminous Mixes: Warm Asphalt Mixtures, Perpetual Asphalt Pavement Systems, Porous Asphalt Pavement, Recycled Asphalt Pavements (RAP).
4. Cementitious Materials: Performance-Specified Cements, Next-Generation Sustainable Cements, Eco-Friendly Cements, Energetically Modified Cement.
5. Concrete Materials: Engineered Cement Composites (ECCs), Titanium Dioxide–Modified Concrete, Pervious Concrete, Self-Consolidating Concrete, Sulphur Concrete, Autoclaved Aerated Concrete, Geopolymer Concrete, Hydrophobic Concrete, Ductile Concrete.
6. Metallic and Polymer Materials: Vitreous Ceramic Coatings for Reinforcing Steel, Fiber-Reinforced Polymer Bars for CRCPs and Dowel Bars, Zinc-Clad Dowel Bars, Microcomposite Steel for Dowels/Tie Bars.

However, the article is focused on the emerging and potential materials and techniques that are more relevant to flexible pavement only.

Factors Responsible for The Paradigm Shift

There is a strong need worldwide to optimise the use of materials currently used for pavement construction and to seek advanced materials that are cheaper, better performing, and less damaging to the environment. The factors seeking advanced highway construction materials include:

• Conservation of resources—support national efforts to create sustainable solutions to minimise the impact of construction on the environment
• Reduced ecological footprint
• Reduced costs—get more lane-kilometers constructed or rehabilitated for a given constrained budget
• Extended service life
• Optimised use of locally available materials
• Achieving environmental benefits—reduced carbon footprint, reduced congestion-related emissions
• Reduced work zone–related traffic delays and safety concerns—use materials that reduce the potential for early failures

Emerging Technology Specifications for Flexible Pavements

Presently, there is a necessity of extending our road network to all terrains and environment and flexible pavement being preferred pavement type, there is an immediate need of switching over to high performance/new/alternate materials and technologies that are applicable to flexible pavements and are able to provide sustainable solutions. To exemplify, bitumen bound layers are normally used in wearing, surfacing, base and binder courses. They may be thick or thin, hot or cold, plant-mixed or site-mixed and so on, but the binding constituent, bitumen has many variants at this time and may include neat or straight run bitumen, cutback bitumen, fluxed bitumen, bitumen emulsion, to name. Likewise, there are other options with different materials to be used in different layers of flexible pavements. The promising new technologies are:

• High performance materials (modified bitumen & multi-grade bitumen)
• Stone matrix asphalt (SMA)
• Warm mix technology
• Cold bituminous mix and half-warm mix technologies
• Waste plastic in road construction
• Self repairing roads
• Micro surfacing
• Recycling (RAP in bituminous and granular layers)
• Sulphur extended bituminous base courses
• Geo-cells and geo-textiles in pavement construction
• New generation additives for soil stabilisation
• Waste materials including industrial slags and fly ash

Some of these are discussed further.

Modified Bitumen

Though modified bitumen is not a very new concept, the regular usage has been into practice in recent times only. Certain additives or blend of additives called as bitumen modifiers can improve properties of bitumen and bituminous mixes. Bitumen treated with these modifiers is known as modified bitumen. Modifier can be of the following categories:

• Elastomers
  • Natural Latex Rubber
  • Synthetic Latex
    • Styrene-butadiene (SB)
  • Block Copolymer
    • Styrene-butadiene-styrene (SBS)
    • Reclaimed Rubber
• Plastomers
  • Polyethylene
  • Polypropylene
  • Ethyl-vinyl-acetate (EVA)
  • Polyvinyl-chloride (PVC)
• Combinations

Polymer modified bitumen (PMB)/ crumb rubber modified bitumen (CRMB) should be used according to the specification requirements of the job. The detailed specifications for modified bitumen have been issued through IRC: SP: 53. It must be noted that the performance of PMB and CRMB is dependent on strict quality control, especially on temperature during construction. The advantages of using modified bitumen are as follows:

• Lower susceptibility to daily and seasonal temperature variations
• Higher resistance to deformation at high pavement temperature
• Better age resistance properties
• Higher fatigue life for mixes
• Better adhesion between aggregates and binder
• Prevention of cracking and reflective cracking

Stone Matrix Asphalt (SMA)

SMA is a gap graded aggregate bitumen mix that maximize the bitumen content and coarse aggregate fraction. SMA provides a stable stone on stone skeleton that is held together by a rich mixture of bitumen, filler and stabilizing additive. Cellulose fibers prevent draining of excess bitumen and help in forming a matrix to hold the high bitumen in SMA. SMA is high performance mix for heavy traffic roads prone to rutting.


Warm Mix Asphalt

Warm mix technology is emerging technology, which allows the mixing, lay down, and compaction of bituminous mixes at lower temperatures compared to hot mix. Further, low temperature mixes are classified as Warm Mix Asphalt(WMA) which is produced in the temperature range of 120°C to 135°C and Half Warm Mix Asphalt(HWMA) which is produced in the temperature range of 80°C to 100°C. These technologies have the advantages like:

• Energy saving
• Decreased emissions
• Reduced fuel costs
• Reduced aging of binder
• Lower fumes and odor emissions
• Cool weather paving
• Compaction aid for stiff mix
• Extend paving window
• Generation of carbon credit
• Increase use of RAP

Several process have been developed to improve mixture workability allowing lower range temperatures for production and lay down. WMA technologies can be classified broadly as (a) those that use water, (b) those that use organic additive or wax (c) those that use chemical additives or surfactants.

Cold Bituminous Mixes

In India, hot mixed bituminous materials and mixes are generally used for the construction of base course, binder course and wearing course of a flexible pavement. The paving bitumen (VG-10, VG-20, VG-30 and VG-40; as per IS:73), is used as a binder. It is either solid or semi solid at ambient temperature and converted into fluid state by either heating or by the addition of petroleum solvent or by emulsifying bitumen in water. High amount of energy is consumed for heating of aggregates and bitumen for construction of roads using traditional hot mix technology.


The following are some of the disadvantages of hot mix technologies:

• High level of noise and air pollution
• Emission of green house gases
• Compromise with the durability of bitumen due to aging during heating
• High energy consumption
• Unsafe for maintenance crew

Lately, Bitumen emulsion has also been used in half warm mixes, micro surfacing and in cold recycling; more especially in full depth reclamation works. Several factors that have lead to the increasing use of bitumen emulsion in road construction and maintenance are listed below:

• The energy scarcity and crisis.
• Bitumen emulsions do not require petroleum solvent to make it liquid.
• Bitumen emulsions can generally be used without additional heating.
• Concerns about reducing atmospheric pollution.
• The ability of certain types of bitumen emulsions to coat a damp aggregate surface which eventually reduces the fuel requirements for heating and drying aggregates.
• Availability of a variety of emulsion types which have led to development of new formulations and improved laboratory procedures in order to satisfy the design and construction requirements.
• The ability to use cold bituminous materials at remote sites.
• The applicability of emulsions for use in preventive maintenance so as to increase the service life of existing distressed pavements.
• Health of construction workers

Bitumen emulsion based mixes include:

(A) Cold Mixes (CM): These mixes are normally produced with unheated aggregates and bitumen emulsion.

(B) Half Warm Asphalt Mixes (HWAM): These mixes are produced by mixing bitumen emulsion, or foamed bitumen, with warm aggregates (110+10ºC), laid and compacted at a temperature between 80-90ºC.

The cold emulsions based mixes, thus, may be conveniently used in the specified conditions and as per the guidelines given in IRC:SP 100 -2014.

The uses of emulsion in major construction and maintenance applications covered in the guidelines are given below:

Surface Treatment Including Preventive and Corrective Maintenance	Maintenance Including Periodic Treatments		Other Applications
	Cold Mixes	Warm Mixes
Fog Seal Sand Seal Slurry Seal Microsurfacing Cape Seal Chip Seal	Patching Pothole Repair Cold Recycling Bituminous Macadam Premix Carpet Mix Seal Surfacing Semi-Dense Bituminous Concrete	Semi-Dense Mixes Dense Mixes	Prime Coat Tack Coat Crack Sealing Soil Stabilization

Waste Plastic in Road Construction

Plastic, a toxic material is extensively used in different forms in present day’s life and constitutes to almost 5% in municipal solid waste. It is a common sight in India to find empty plastic bags and other type of plastic packing material littering the roads and choking the drains. Due to its impermeability clubbed with non-biodegradability, it creates stagnation of water and associated hygiene problems besides reducing the fertility of the land. In order to reduce the accumulation of waste plastic and also use it for betterment of structures like roads, research carried out has indicated that the waste plastic, when added to hot aggregate will form a fine coat of plastic over the aggregate and when mix is produced with the binder, is found to give higher strength, higher resistance to water and better performance over a period of time. Therefore, it is pertinent to use waste plastic in the construction of roads.

It has been observed that modification of bituminous mix with shredded waste plastic may marginally increases the cost by about Rs.3000 per tonne. However this marginal increase in the cost is compensated by increase in the volume of the total mix, thereby resulting in less overall bitumen content, better performance and environmental conservation with usage of waste plastic.

Self Repairing Roads

Since long the need has been felt to develop ‘Self Repairing Roads’through suitable technology inventions. This requires a research that clubs the materials science and structural engineering to create self-repairing roads that are cost effective, have greater longevity and are sustainable, though no such success stories have been heard till now. Of late, it is reported that researchers have developed this distinct technology by formulating stabilised road structures. The regular cement concrete roads have been replaced by this unique formulation wherein about 60% of the cement is replaced with flyash, thus curbing the usual carbon footprint, especially as cement production releases greenhouse gases. It comes with built-in crack healing, as high strength concrete is supplemented with fibre reinforcement with nano-coating that makes it absorb water and keeps the road hydrated. The mechanism, the researcher claims have fibres which have a hydrophilic nano-coating on them. By hydrophilia (means attract water) this water then becomes available for crack healing. Every time a crack appears, there is always this unhydrated cement supplemented by this water the hydration capability, producing further silicates which actually closes the crack in time.

Microsurfacing

Microsurfacing is mainly a cold mix technique which uses modified bitumen based emulsion to produce a mix of relatively small sized stone aggregates alongwith cement filler to produce and lay a thinner surface coat. The technique, however, is useful to refresh the surface of an otherwise structurally sound pavement since it cannot provide any additional strength to the structure. It helps to cover very minor undulations and improve skid resistance properties. It is mostly used in urban roads where adding thick layers leads to other problems like need to increase the footpaths and drainage structures. It is, therefore, regarded as a cosmetic bituminous treatment only.

The Microsurfacing shall consist of mixture of modified (Polymer or Rubber Latex) bitumen emulsion, well graded mineral aggregate, water, filler and additive (if needed) proportioned, mixed and uniformly spread over a properly prepared urface.

There are two types of microsurfacing, by aggregate size and layer thickness thereby, as per IRC:SP81-2008.

The finally laid microsurfacing shall have a homogeneous mat, adhere firmly to the prepared surface and provide friction resistant surface texture throughout its service life. The mix is to be a quick setting system i.e. it should be able to receive traffic after a short period of time preferably within about one hours of its laying depending upon weather conditions. It is applied on an existing pavement surface which is structurally sound but is showing the signs of functional distress such as loss of riding quality, cracking and polishing. Generally, microsurfacing is laid in single layer, but when the existing surface is highly polished and/ or cracked, it may be applied in two or more layers.

Ingredients	Percent by weight of aggregate
Aggregate	100 %
Emulsion	13 %
Portland Cement	1.5 %
Additive	0.5%
Water	14 %

The uses and benefits of microsurfacing may be listed as following.

• Preventive and periodic maintenance and renewals on structurally sound pavement
• Surface defects like cracks and polished surface of various types may be treated
• Used for delay of reflection cracking
• Improvement of skid resistance and rejuvenation of hungry surface
• Noise reduction on concrete pavement
• Fastest construction technology

Pavement Recycling

The process involves:

• Cold Milling and
• Cold Recycling (CR)
  • In-plant
  • In-place (CIPR)
  • Full Depth Reclamation (FDR)
    or
    Hot Recycling
  • Central Plant Hot Mix Recycling
  • Hot In-place Recycling (HIPR)
Bitumen emulsions are most frequently used for Cold Recycling and Full Depth Reclamation (FDR)

Fly Ash in Pavements

Fly ash is a by-product of burning pulverized coal in an thermal power generating station

• Considering the economic and environmental aspects, industrial by-products like fly ash can be investigated for their suitability as full or partial replacement of materials
• As most of such industrial products are non-plastic, with the use of stabilizing agents, these low-quality materials can be economically upgraded

Geocells

Geocells are believed to provide solutions where the ultimate bearing capacity of foundation soil is not adequate to withstand the applied load, wherein notwithstanding the possibility of shear failure, the anticipated long-term settlement is also high. The simple technology is reported to provide a cost-effective ground improvement system with following features:

• Geocells are normally filled-in with non-plastic granular material, the conventional gravel-sand mix for flexible pavements
• Geocells, used as above improve the bearing capacity of soft strata and construction of the upper layers is possible proceed immediately.
• Geocell laying and filling is fast and an all-weather installation solution. No skilled labour is required. Installation is rapid and local non-plastic material could be used as in-fill.

These features, when successfully achieved, result in substantial savings in money and resources

Jute Geotextiles

• Jute geotextiles are indigenously made and eco-friendly
• JGT – Different strength characteristics & life span
• Successfully used in geotechnical and highways engineering projects, such as:
  • Road construction
  • Erosion control applications
  • Slope stabilisation and drainage
• Usage is economical

Antistripping Agents

Stone aggregates are either hydrophobic (water repelling) or hydrophilic (water loving) in nature. Bitumen coating gets peeled off in presence of water more easily in case of hydrophilic aggregates. Also, bitumen-aggregate interactions in pavements directly affect the adhesion & bond strength. Chemistry of asphalt & surface chemistry of aggregates particularly affects the interactions. In order to minimise the water related damage to bituminous pavements, chemical additives called anti-stripping agents are used in mix production. The additive, by its specific mechanism, alleviates the problem of stripping or minimises it. There are a variety of antistripping additives, like the following.

• Organic Small Molecules
  • Amines, Silanes
  • Polyamines, Amidoamines
• Inorganic Aggregate Treatments
  • Hydrated Lime
  • Portland Cement
• Polymeric Agents
  • Functionalized Butadiene
  • Styrene-Butadiene

Related other Technologies

Inaddition to the pavement technologies, there are technologies for monitoring, guiding and coordinating construction equipment and robots, which include the following to name afew.

• Inertial Navigation Systems
• Active Beacon Systems
• Global Positioning System
• Ground-Based Radio Frequency Systems
• Ultrasonic and Optical Systems
• Radio Frequency Identification Systems

In addition, smart structures and modular construction and advanced materials are also expected to be helpful.

Concluding Remarks

Large amounts of stone aggregate and manufactured materials are being used to support highway construction and rehabilitation in the country and worldwide. However, the poor availability of good quality aggregates in many parts of the country and thereby increasing financial and societal costs are creating a concern for the policy makers, planners and engineers. It is, therefore, imperative that new and improved sources of highway construction materials be developed that will result in improved performance of the highway system, be cost effective, and incorporate sustainable technologies.

These advanced/new/Innovative con- struction materials can be categorised as:

• Materials that replace currently used/conventional materials.
• Materials that are less expensive.
• Materials that result in longer service life.
• Materials that result in sustainable solutions.
• Materials that improve the properties of marginal materials.
• Waste and recycled materials that are optimised for use.

The characteristics of the advanced construction materials, however, need to be ascertained by field performance evaluations.

The road pavements are to be designed and constructed to standards prescribed in the Indian Specifications, published mainly by bodies like Indian Roads Congress(IRC), Ministry of Road Transport(MoRT&H), Ministry of Rural Development(MoRD) etc. In order to ensure the compliances, the agencies have published design standards, construction specifications, manuals, guidelines, quality control handbook etc. to encompass following aspects to be taken up at various stages of the bitumen incorporated pavement construction process.

Proper evaluation/ testing and recording is an essential feature of pavement construction execution which believes that relentless testing and meticulous quality control is the only way to build good roads. Some mandatory tests are to be conducted to assure the quality of the road works in general. More specific tests and guidelines for the new technologies are available through published specifications and standards.