Whitetopping is the covering of an existing asphalt pavement with a layer of Portland cement concrete. Whitetopping is divided into types depending on the thickness of the concrete layer and whether the layer is bonded to the asphalt substrate. 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.
Whitetopping is suitable for asphalt pavement with little deterioration, although repairs can be made to the asphalt if necessary. If the pavement is badly damaged, it should be completely removed and a new concrete pavement should be installed. The pavement should be relatively hard, as well. Deterioration of overlays is significantly increased on asphalt bases with high viscosity. If a grade or a distance between the pavement and a bridge needs to be preserved, the asphalt can be milled so that the height of the pavement does not change. However, whitetopping requires the asphalt layer to be at least three inches thick. If necessary, a section of new concrete roadway can be placed under a bridge with gentle slopes on either side that meet up with the whitetopped portions of the road.
Types of White Topping
It is seen that PCC overlay of white topping can be of two types, namely, bounded type and un-bonded type. These are similar to un-bonded type except that in such cases PCC overlays are laid directly on the existing bituminous pavement without much of surface preparation
Bonded Type- Ultra ThinWhitetopping is an example of bonded type. These are used at locations like intersections, round-abouts, parking lots etc. Bonding of PCC overlay to the underlying bituminous pavement surface is done by mechanical process known as milling, which achieves the monolithic behavior of the two layers. It is a very economical and efficient means of enhancing the structural capacity of the bituminous pavement. The only disadvantage to bonded type is that, if it is laid on a badly cracked surface the cracks will reappear. Thus the removal of such cracks on bituminous road is compulsory. One of the pre-requisite for the construction of PCC overlay is the uniform support condition of the PCC Overlay on the existing surface. In the absence of uniform support condition, satisfactory performance of any concrete pavement including overlay like White topping cannot take place. Most of the premature PCC overlay failures are observed due to violation of this single requirement i.e. lack of uniform support.
Un-bonded type – Thicker PCC overlays of un-bonded type commonly known as Conventional white topping can be used on deteriorated bituminous pavements. This type of white topping is suitable for strengthening of distressed bituminous pavement on moderate to heavy trafficked roads. In India stage construction has been adopted for bituminous pavements. All these pavements, which have now become badly due for strengthening, are the potential candidates for White topping. Conventional White topping is known to extend the life of bituminous pavement by 20 to 25 years. Technically they are similar to a newly laid concrete pavement except that bituminous surface is the subbase instead of Dry Lean Concrete (DLC) and no separation membrane or bond breaking film is provided
Advantages of White topping road :
It is advantageous were there is high volume of traffic and where it is more prone to damage , and maintenance will be more. Some of the advantages, which are given below.
- Prevents rutting, structural cracks and potholes, which provides a safer and faster commute.
- Improves the structural capacity of existing pavements.
- Non- rutting surface that doesn’t trap water
- Initial budget is slightly more than bitumen roads but the life-cycle cost is far lower than both asphalt and concrete roads.
- With a turnaround time of just 14 days, it’s much faster than the turnaround time for concrete roads.
- Improves visibility and commuter safety at night by enhancing light reflectance. This reduces the illumination load of any road, thus saving energy (20-30%).
- Reduces pavement deflection, resulting in less vehicular fuel consumption (10-15%) and thus, reduced emissions.
- Lowers vehicular braking distance, making it safer in both dry and wet surface conditions.
- Reduces the urban heat island effect by absorbing less heat, in turn lowering the energy consumption for air conditioning in urban buildings.
- Excellent surface drainage
- Better light reflectance
- Emission Reduction due to White topping;
- White Topped pavement is 100% recyclable and can be crushed and reused at the end of life.
Construction Stages of white topping :
The subgrade must be prepared to lay a concrete pavement and the cross slope will be gentle then bituminous pavement,and in some cases reinforcement are also provided and curing will take the usual time stipulated. The stages of white topping are-
- Milling & Profile Correction
- Surface Preparation
- Concrete Overlay
- Surface Finishing
- Groove Cutting
- Curing & Testing
- Curb Laying & Lane Marking
The cases of white topping
There have been few cases of white topping and further development is being made on the technology, some of the notable ones are given below.
The Greater Hyderabad Municipal Corporation (GHMC) is probably the first corporation in India to use white topping technology for road construction. White topping is a new technology of providing cement concrete overlay on the existing damaged and distressed bitumen roads with a purpose either to restore or to increase the load carrying capacity, or both. The newly-inducted technology by GHMC also works on the existing pavement and prevents frequent damage of roads.
GHMC has adopted this technology for two pilot projects – Road No 10 in Banjara Hills and PG Road in Secunderabad. After successful implementation of laying these roads stretches, the Government of Telangana and GHMC have formulated a proposal to carry this work over bitumen roads in several stretches in the premises. Accordingly, as many as 64 road stretches have been identified for the deployment of white topping overlay over bitumen roads in a phased manner.
Stretch of Hosur Road between Ayyappa Temple and Madiwala stretch was white-topped (concrete) in 2010 and has a lifespan of around 30 years. The project to white-top major radial roads never took off even though the pilot was successful. Essentially, white-topping is covering the existing bitumen layer with a concrete slab. The technology has been adopted on a section of NICE Road.
BBMP has pushed for white-topping, stating that is the ultimate cure for the pothole menace, since concrete roads have a lifespan of around 30 to 50 years, unlike asphalt roads, which need to be tarred every three to four years. TenderSure roads are roads designed by Jana Urban Space, a non-profit that has also designed spaces for utilities such as power, water, sewage, drainage, telecom and gas lines. According to the BBMP website, there were 12 roads allocated to TenderSure works under two packages and about 50 other roads, a total of 103.60 km, that were to be upgraded to TenderSure standards.The TenderSure project was designed to address issues that have made Indian roads so notorious for their chaotic traffic, potholes, broken footpaths, overflowing drainage, poorly placed power transformers and their hanging, tangles of electrical wirings and telecom fixtures. Till date, several roads in Bengaluru’s Central Business District have been renovated under TenderSure – Church Street, Richmond Road (General Thimmaiah Road), Residency Road (Field Marshal Cariappa Road), Commissariat Road, Museum Road, Nrupatunga Road and Kempegowda Road.
Nagpur- Development of Ultra-Thin Precast Concrete (UTPC) white-topping method
A team of officials from BBMP will be visiting Nagpur to study the efficacy of the Ultra-Thin Precast Concrete (UTPC) white-topping method for increasing the speed of Bangalore white topping road projects. The new method will not cost more than the existing method.The same concrete mix that we use on the road will be mixed somewhere else, and fixed when it’s ready to be laid. In the UTPC method, one metre-long precast concrete slabs will be placed on the road and then the edges will be placed on the road and then the edges will be attached to each other. Much like the precast magic boxes which were used to make underpasses, this method will save time and cause less inconvenience to commuters. With the UTPC, the white topping will be completed in three to four days.
In addition, rapid developments in concrete material technology and mechanization 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. White topping is gaining momentum in road construction across the country. It can act as a long-term alternative for the rehabilitation or structural strengthening of roads.