Dr. Manamohan R Kalgal, Technical Advisor, UltraTech Cement Ltd.

Preamble

In India, the emphasis in recent years has rightly been on the housing and infrastructure, including, amongst other things, roads, power and irrigation sectors. The Global Infrastructure Outlook reflects that rising income levels and economic prosperity is likely to further drive demand for infrastructure investment in India over the next 25 years. Around USD 4.5 trillion worth of investments are required by India till 2040 to develop infrastructure to improve economic growth and community well-being, says the Economic Survey tabled by the Finance Minister in the Parliament in January 2018.

Road Infrastructure

India has one of the largest road network across the world, spanning over 5.5 million kms. It transports 64.5 percent of all goods in the country and 90 percent of India’s total passenger traffic uses the road network to commute. Road transportation has gradually increased over the years with the improvement in connectivity between cities, towns and villages. The GOI has set a target for construction of 10,000 km national highways in FY19. In 2017-18, the total length of roads constructed under Prime Minister’s Gram Sadak Yojana (PMGSY) was 47,447 km1.

It is imperative to look at the materials and the technologies that bring about speed in construction, ensure long life of the structures, with minimal maintenance and meets the expectations of sustainable development. It is in this context that this article recommends concrete as an appropriate material and looks at the technologies of construction associated with the use of concrete in the Road Sector.

Concrete as a Material of Choice

Concrete is one of the most versatile and durable construction materials that has been in use for centuries, in one form or the other. Present day concrete provides innumerable applications with very few limitations - be it in buildings, roads, bridges, railways, or dams. The world's concrete consumption is estimated to be more than 1 ton per capita (present population being above 7.5 billion!).

Concrete is a sustainable material for construction in comparison to the available alternatives of similar virtues. Although the embodied energy associated with concrete is already low, it can be further reduced through use of Supplementary Cementing Materials (SCMs) as well as substitutes for coarse and fine aggregate. As a responsible country, India is also putting in efforts in this direction. The construction industry is mandated to use mineral admixtures as cementitious component in concrete. Emphasis should be laid on using Recycled Aggregates derived from reprocessing materials previously used in construction.

Concrete Roads

Concrete has been used extensively for paving highways and airports as well as city roads. Concrete pavements for new construction or reconstruction are typically of 3 types:
  • Jointed Plain Concrete Pavement (JPCP) – with non-reinforced pavement slabs having dowelled transverse joints and tie rods across longitudinal joints. Sometimes, even dowels are omitted, assuming aggregate interlock to transfer the stresses across the transverse joints.
  • Jointed Reinforced Concrete Pavement (JRCP) – with dowelled transverse joints, tie rods across longitudinal joint and mesh reinforcement in the pavement slab
  • Continuously Reinforced Concrete Pavement (CRCP) – with no transverse joints and having continuous longitudinal reinforcement in the pavement slab
The concrete pavement thicknesses typically range from 200mm to 400mm for roadways and highways, depending on traffic, environment, soils, as well as the subbase construction. Curling stresses due to temperature variations and stresses due to loading are the primary factors in deciding the thickness of pavements.

To prepare for paving, the subgrade — the native soil on which the pavement is built — must be graded and compacted. Preparation of the subgrade is often followed by the placing of a subbase — a layer of material that lies immediately below the concrete. The essential function of the subbase is to prevent the displacement of soil from underneath the pavement. Subbases may be constructed of granular materials, cement-treated materials, lean concrete, or open-graded, highly permeable materials, stabilized or un-stabilized. Once the subbase has hardened sufficiently to resist marring or distortion by construction traffic, dowels, tie bars, or reinforcing steel are placed and properly aligned in preparation for paving2. It is often observed that it is lack of attention to the construction of sub-grade and sub-base that leads to premature failure of the pavement.

Custom-made concrete pavements are suitable for any application one has in mind. Pervious concrete for parking lots filters rainwater through to the ground below, helping solve storm water runoff problems. Pavements topped with interlocking concrete pavers also provide a drainable surface that can recharge groundwater systems and still carry heavy loads, such as container handling equipment at port operations and heavy trucks on streets and intersections. For industrial applications, Roller-Compacted Concrete pavement can cover large areas very economically with a tough surface that stands up to heavy traffic, massive loads and adverse weather conditions.

Rehabilitation of Roads
White Topping

Rehabilitation of Roads

Concrete overlay over bituminous/Hot Mix Asphalt (HMA) surface is known as white topping. White topping overlays have proven to be a successful pavement rehabilitation method. The concrete thickness for an Ultra-Thin White topping (UTW) is equal to or less than 100 mm (4 in.). A Thin White Topping (TWT) is greater than 100 mm (4 in.) but less than 200 mm (8 in.). Conventional White Topping (WT) is an overlay of 200 mm (8 in.) or more. In most cases, a bond between the new concrete and existing HMA layers is not only assumed during design, but specific measures are taken to ensure such a bond during construction. The success of this bond, leading to composite action, has been found to be critical to the successful performance of this pavement-resurfacing alternative.

White Topping

There are some key factors to be considered when selecting, designing, and executing a TWT or WT project, including
  • Distress mode and severity of the existing HMA pavement,
  • Stiffness of the existing HMA pavement,
  • Proper thickness and joint design,
  • Surface preparation of the HMA before overlay (commonly milled and cleaned),
  • Fiber reinforcement for UTW and possibly TWT concrete,
  • Proper joint sawing depth and timing, and
  • Proper curing practices.
IRC guidelines do not recommend Ultra-Thin White Topping for Indian Roads. This can be considered only for interior roads of a colony, where heavy traffic is not expected.

Several municipalities and corporations have taken to Thin White Topping. Mumbai Municipal Corporation leads with several hundred kms of TWT. In Bangalore, one of the longest stretches (11.5kms) of ring road carrying highway traffic was white-topped with more than 63000 cum of concrete in about 3 months’ time in 2012 and has been operational without any problems since then.

Precast Concrete Pavement

A promising alternative for rehabilitation of roads is to use modular pavement technologies, especially Precast Concrete Pavement (PCP) systems, which provide for rapid repair and rehabilitation of pavements and result in longer-lasting pavements. Rapid construction techniques can significantly minimize the impact on the driving public, as lane closures and traffic congestion are kept to a minimum. Road user and worker safety is also improved by reducing their exposure to construction2

PCI and Federal Highway Administration (FHWA) have worked through the consensus process to build guidance for owner agencies for determining the appropriate applications for precast concrete pavement systems (PCPS)4. These documents describe the benefits of PCPS for the traveling public realized through reduced traffic disruption due to speed of construction; improved durability; improved safety, and all-weather construction.

White Topping

Prestressed Precast Concrete Panels (PPCP) are generally provided in sizes to match the width of one, two, or three lanes of the pavement permitting one or multiple lanes of an existing pavement to be reconstructed at one time, depending on site clearance constraints. The precast panels are commonly oriented perpendicular to the roadway centerline and may also include one or both shoulders. In general, the panels are pretensioned in the longer direction during fabrication, and post-tensioned together in groups longitudinally, in the direction of traffic, to act as continuous slabs after installation. In some applications, the panels can be post-tensioned together in both directions during construction in addition to plant pre-tensioning. Regardless of the configuration, it is important that the pre-stressing is provided in both directions if the maximum benefits of pre-stressing are to be realized. The panels are installed on a prepared base, post-tensioned together, and opened to traffic. The image gives a schematic view of a typical PPCP.

As per TRB report3 PCP technology is ready for implementation, and many of the proprietary and nonproprietary PCP systems available in the United States are capable of meeting the four key attributes of PCP systems:
  1. Constructability: availability of techniques and equipment to ensure acceptable production rates for installation of the PCP systems.
  2. Concrete durability: confidence that plant fabrication of the precast panels results in excellent concrete quality with respect to strength and durability.
  3. Load transfer at joints: availability of reliable and economical techniques to incorporate effective load transfer at transverse joints of PCP systems.
  4. Panel support condition: availability of techniques to provide adequate and uniform support conditions.
The cost of PCP systems is expected to be routinely competitive with CIP concrete pavement repair and rehabilitation in the near future as the market size increases and more fabricators and contractors enter the market.

Sustainability Bonus with Concrete Pavements

Environmental Benefits: Concrete pavements are an excellent choice when considering the lifetime environmental impact. From mining of the locally produced raw materials through construction and long-term maintenance-free performance, concrete stands out as the most cost-effective, sustainable paving material.

Fuel consumption is a major factor in the economics of roads, with the rolling resistance of the pavement being an important contributor to the fuel consumption and the corresponding CO2 production. Rolling resistance can be attributed in part to a lack of pavement rigidity. In the case of a heavily loaded truck, energy is consumed in deflecting a non-rigid pavement and sub-grade. Using rigid concrete pavement will result in less fuel consumption, and a decrease in associated emissions5.

Another benefit of using concrete as opposed to alternative flexible pavements is a reduced need for street lighting, due to higher surface reflectivity after dark. Better light reflection on the brighter surface could potentially result in electricity savings of about 30% for lamps, lampposts and signs. However, the largest savings from higher surface reflectivity are to be gained from a reduction in accidents, and the associated loss of life and serious injury2.

Urban Heat Island: As urban areas develop and vegetation is replaced with buildings, roads, and other heat-absorbing infrastructure, the ambient urban air temperatures increase. Essentially, cities become “heat islands” surrounded by significantly cooler, rural areas. Concrete has a higher solar reflectance or albedo that minimizes the urban heat island effect.

Recycled Concrete: Eventually, all pavements must be replaced. After a long and reliable service life, concrete pavements can be crushed and reused. In fact, concrete is 100% recyclable (and the steel rebar if provided within, is recyclable as well).

Concluding Remarks

From runways to highways, from subways to transit-ways, concrete helps develop and maintain a sustainable, environment-friendly transportation infrastructure. Regardless of the type of roadway or current pavement conditions, there is a concrete solution. Concrete can be used for new pavements, reconstruction, resurfacing, restoration or rehabilitation. Concrete pavements generally provide the longest life, least maintenance, and lowest life-cycle cost of all alternatives. Plus, due to higher bitumen prices, concrete has become a competitive alternative for even new construction on a first-cost basis.

References
  1. IBEF, Report on Road Infrastructure https://www.ibef.org/industry/roads-india.aspx
  2. Portland Cement Association website https://www.cement.org/cement-concrete-applications/products/concrete-pavement
  3. TRANSPORTATION RESEARCH BOARD, SHRP 2 Report S2-R05-RR-1 ISBN: 978-0-309-12944-2, Library of Congress Control Number: 2013935791 © 2013 National Academy of Sciences.
  4. PRECAST/PRESTRESSED CONCRETE INSTITUTE, Precast Concrete Pavement, https://www.pci.org/PCI/Design_Resources/Transportation_Engineering_Resources/Precast_Concrete_Pavement.aspx
  5. JAMIESON, N.J. & CENEK, P.D., Effects of pavement construction on the fuel consumption of trucks. Options for a post millennium pavements symposium. New Plymouth. New Zealand Institute of Highway Technology. 1999 Quoted in Concrete3 of New Zealand http://www.sustainableconcrete.org.nz/page/concrete-roads.aspx
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