Dr. V. Ramachandra, Zonal Head (Tech), UltraTech Cement Ltd., Bangalore
IntroductionConcrete Roads were first built by Romans (300 BC – 476 AD). They were quite innovative in the construction with the use of innovative materials viz., use of ‘Pozzolana' cement from the village Pozzouli near Italy, horse hairs as fibres in concrete, admixtures in their primitive form (like animal fat, milk & blood). These roads, scientifically designed and constructed had a long life and thus lead to the adage ‘all (concrete) roads lead to Romel.'
Portland Cement Concrete (PCC) overlay on an existing bituminous pavement is commonly known as White topping. The principal purpose of an overlay is either to restore or to increase the load carrying capacity or both, of the existing pavement. In achieving this objective, overlays also restore the ride-ability of the existing pavements which have suffered rutting and deformations, in addition to rectifying other defects such as loss of texture. In our country, bituminous overlays have been popularly constructed in the past mainly due to abundant supply of bitumen, its amenability to stage construction and manageable traffic conditions, in terms of volume and axle loads in addition to the comfort levels of construction methods among engineers. It was also making economic sense to make bituminous pavements as it was relatively cheaper. In recent times all these advantages are reversed viz., petroleum industry is using refined processing technology leading to reduction in the production of bitumen leading to increased imports, favourable cost economics of cement concrete and rapidly changing traffic scenario (in terms of volume as well as axle loads). In addition, rapid developments in concrete material technology and mechanization (both in concrete production & its laying) are favouring concrete overlays as a sustainable option. In recent times PPP (Public-Private Partnership) models are becoming popular in road construction shifting the focus on selection of overlays based on life-cycle costs rather than initial costs. India is currently producing about 240 million tonnes of cement and cement industry is quite matured and equipped to meet the challenges in terms of various grades of cements as well as high quality blended cements suitable for making Pavement Quality Concrete (PQC).
|Dr. V. Ramachandra|
Overlay Solutions for RehabilitationType of overlays is chosen depending upon the condition of the existing pavement. Bonded overlays are preferred when the condition of existing pavement is good and the design will ensure that the thickness of overlay is relatively less. This is due to the fact that the pavement system behaves as a composite structure, utilizing the flexural strength of flexible pavement as well. In case the pavement is already deteriorated, it can only be regarded as a base; to separate it from the concrete overlay (in order to avoid reflective cracks), a separation layer is provided.
Concrete overlays have been used to rehabilitate bituminous pavements since 1918 in USA. There has been a renewed interest in whitetopping, particularly on Thin White Topping (TWT) and Ultra-Thin White Topping (UTWT) over Conventional White Topping. Based on the types of interface provided and the thickness of overlay, classification is as follows:
- Conventional White topping – which consists of PCC overlay of thickness 200 mm or more, which is designed & constructed without consideration of any bond between existing overlay & underlying bituminous layer (without assuming any composite action).
- Thin White topping (TWT) – which has PCC overlay between 100 – 200 mm. It is designed either considering bond between overlay & underlying bituminous layer or without consideration of bond. High strength concrete (M 40 or higher) is normally used to take care of flexure requirement. Joints are at shorter spacing of 0.6 to 1.25 m.
- Ultra-Thin White topping (UTWT) – which has PCC overlay of less than 100 mm. Bonding between overlay & underlying bituminous layer is mandatory. To ensure this, the existing layer of bitumen is either milled (to a depth of 25 mm) or surface scrapped (with a non-impact scrapper) or gently chiseled. Joints are provided at a spacing of 0.6 to 1.25 m.
Advantages of White topping
- Reduced thickness – due to thickness of overlay remaining constant for over 2 decades.
- Fast-Track construction – making use of innovations in concrete technology & batch mixing, concretes can be designed to have 3 –days' compressive (& flexural) strength, so as to open the road for traffic within 5 days of construction.
- Reduced maintenance – as the concrete overlays live for over 2 decades, with least maintenance.
- Cost-effective compared to asphalt overlays – when Life Cycle Cost is taken into consideration.
- Improved service life – with better riding quality, improved fuel efficiency of vehicles.
- Little pre-overlay repairs
- Improvement in safety in view of the increased reflection of light – particularly on city roads, it would save 24% less electricity compared to flexible pavements.
- Reduction in operational costs and lower absorption of solar energy
|Concrete roads require lesser illumination during night|
Technology Demonstration ProjectIn India, concrete overlays on existing flexible pavement (white topping) have been constructed in recent years in Mumbai, Nagpur, Indore among other places. In South India, several new concrete roads were constructed during the last century which have given excellent service for over four decades, the examples being the Bangalore-Mysore road constructed by the then Maharaja of Mysore, the coastal roads in Kerala etc. Due to lack of experience and expertise in white topping, these roads were converted to flexible overlays in recent times.
With a view to familiarize with the advantages of white topping, a Technology Demonstration Project was undertaken in Bangalore jointly by the Bruhat Bangalore Mahanagara Palike (BBMP), Task Force for Quality Assurance, Govt. of Karnataka and Cement Manufacturers' Association. The objective of this project was to demonstrate the advantages of white topping by making use of the advances in construction equipments and methods.
Characterisation of the existing flexible pavement on the trial stretch on Hosur Road in Bangalore yielded the following data.
|1||CBR||8 to 10|
|2||Commercial vehicles per day||1000|
|3||Temp differential at Bangalore||17.3 deg C|
|4||Thickness of bituminous layer||4 to 7 inches|
|5||Base (40 mm metal) thickness||4 to 9 inches|
|6||Road width||100 feet|
|7||Length of road||350 m|
|8||Concrete grade||M 45|
|9||Axle load||16 T|
Design of Pavement and Concrete MixDesign of the overlay was carried out using Westergaard's Equation and warping stress as per IRC:58 -2002 and IRC: SP: 76 – 2008. Total stress (including temperature stress) was obtained as 30.83 Kg/cm2 and corresponding flexural strength requirement was 4.7 MPa. Design was done by M/s L.R. Kadiyali & Associates, New Delhi. Thickness of white topping was 150 mm.
Concrete mix design was arrived at by evaluating trial mixes and the design mix was arrived at with a cement content of 430 kgs, fly ash – 30 kgs, with a w/c ratio of 0.283, achieving a slump of 40 – 60 mm at site.
Details of ConstructionIn order to achieve the desired advantages of concrete roads, three essential conditions need to be satisfied.
- Production of concrete in a RMC plant or in a dedicated batching plant.
- Using either fixed form or slip form mechanical pavers
- Strict quality control at site including testing of fresh, hardened and extracted specimens of concrete and tests on pavement quality.
|Surface preparation and alignment of rails for paver||Paver machine in operation|
|Surface texturing on green concrete||Spraying of curing compound on fresh concrete|
Salient features of Construction
- Surface preparation: In case of TWT, bond between PCC overlay & existing bituminous pavement is atleast partly desirable; in case of UTWT, effective bond is essential. To ensure this any of the following method can be adopted.
- Milling the existing bituminous surface to obtain a uniform surface. Milling can be used to remove surface distortion like cracks in the top portion and adjust cross slopes. Thickness of milling usually is in the range of 25 to 50 mm.
- Surface scrapping is carried out on bituminous surfaces which are quite hard. This can be for a depth of 10 mm and carried out with tools which have vertical impact control, so that the sub-grade is not damaged.
- Chiselling of the surface at regular intervals, if the existing surface is hard.
- Profile correction is carried out with the objective of filling existing potholes, ruts and wide cracks and also to ensure a level surface for resting the pavement. Profile correction and correction of camber can be carried out together with a thin bituminous leveling course or with dry lean concrete (DLC).
- If the existing road surface is good & only a few localized potholes / cracks exist, they can be repaired with a bituminous mix before concreting is done.
- If potholes/ cracks are wider than 3 mm, they have to be treated with bituminous emulsion, slurry seal after trimming them to shape and cleaning out loose fragments with compressed air. Milling of the existing surface also addresses this problem.
- Laying of PQC is quite similar to the construction of new concrete pavement. As mentioned earlier, concrete should be made either in an RMC plant or in a weigh batching plant. Use of either fixed form pavers or slip form paver machines is an essential ingredient for getting a good quality pavement. In the current project, a fixed form paver was used with a fixed side formwork (steel channel box section) with 16mm diameter steel rods of 1 m length as tie rods @ 500mm c/c) and the paver had gang mounted vibrators equally spaced with variable rpm and three integral steel tubes with 8 Tonnes vibratory rollers for screeding, levelling, compaction and finishing.
- Finishing of the surface is mostly achieved by the paver itself. But to achieve uniform finish, a simple hand operated bull float is used when concrete is still in its fresh state. After the bull float operation, uniform surface texture is provided by using steel wire brush.
- To avoid evaporation of surface water from concrete surface (which leads to plastic shrinkage cracks), wax based curing compound is sprayed. As an additional measure, plastic sheets are spread over the pavement surface till normal curing process starts.
- Contraction joints are provided by cutting groves (for a depth of one-third of the depth of white topping, 150 mm in this case) at a spacing of 1.2 m in longitudinal as well as transverse directions. The joints are cut using electrical grove machines within about 8 to 10 hours of pouring concrete. These joints are sealed with high quality sealant (either bitumen or poly sulphides) to prevent moisture and incompressible infiltration into the overlay system.
- To ensure effective load transfer across the longitudinal segments as well as transverse construction joints, tie bars and dowel bars are provided.
|Groove cutting in progress|
Test ResultsStrict quality control not only during mix design and production of concrete, but also testing for quality at regular intervals (for every 50 cum of concrete) was carried out. These tests included:
- Tests conducted on fresh concrete (slump test at site)
- Tests on hardened state (compressive strength of cube & cylinder specimens for 1, 3, 7 & 28 days), split tensile strength, flexural strength of concrete beams
- Fatigue and abrasion tests on pavement quality (results awaited).
- Test on extracted specimens of concrete (core tests) would be conducted to assess the long term performance of concrete.
|Compressive Strength of Concrete Cubes (Total no. of samples 936)|
|No.||Sample size||One-day strength (MPa)||3-day strength (MPa)||7-day strength (MPa)||28 – day strength (MPa)|
|60 cubes per test||Max: 24.1|
Due to advances in the area of mechanization and fast track construction, concrete roads and white topping provide a sustainable as well as cost effective option for pavement construction. This technology demonstration project in Bangalore has evoked positive response from people across the spectrum viz., technical consultants, construction industry, academic & research institutions. The Government of Karnataka, BBMP and other civic agencies have come forward to adopt this technology. On behalf of the cement industry, CMA (Cement Manufacturers' Association) has come forward to assist all those involved in the construction on concrete roads/ white topping by way of several useful publications, user-friendly software for analysis, design and estimation of quantities (and comparative cost), conducting training programs for engineers and providing the necessary technical assistance.
|Compressive Strength of Concrete Cylinders (Total no. of samples 60)|
|No.||Sample size||28 – day strength (MPa)|
|60 per test||Max: 61.4|
|Flexural strength of Concrete |
(Total no. of samples 60)
|No.||Sample size||28 – day strength (MPa)|
|60 per test||Max: 7.35|
|Split Tensile strength of Concrete (Total no. of samples 60)|
|No.||Sample size||28– day strength (MPa)|
|60 per test||Max: 4.01|
- Concrete Overlays – White topping of roads, Cement Manufacturers Association, 2010
- Tentative guidelines for conventional, thin and Ultra-thin white topping, IRC: SP: 76 – 2008, The Indian Roads Congress, New Delhi, 2008
- Handbook on Cement Concrete Roads, Cement Manufacturers Association, 2010
- Michael E. Ayers & Dale Harrington, Selection and use of concrete overlays, The Indian Concrete Journal, May 2010
- Guide to Concrete Overlays – Sustainable solutions for resurfacing and rehabilitating existing pavements, National Concrete Pavement Technology Centre, USA, Sept. 2008.