The asphalt pavement mix has a visco-plastic behaviour. Therefore, it is common to experience functional distresses such as rutting of the pavement surfaces and general deterioration, which reduce step by step the comfort for the road user. In our country, it is very common to see placement of an overlay of bituminous layer after each monsoon or at a period of 3-5 years as a measure of restoring the functional characteristics of the bituminous pavement. The general deterioration due to common distresses occurring in a bituminous road is shown in Figure 1.
|Figure 1: General deterioration in a bituminous pavement|
The bituminous overlays have remained popular mainly due to its amenability to stage construction and comfort levels of construction methods among engineers. This type of periodical overlay of the damaged bituminous roads not only consume scarce materials at a regular period but also increases the pavement surface levels from time-to-time leading to several problems especially in moderately to thickly populated areas. In this current era of sustainable development, total transportation cost, rapidly changing traffic scenario (in terms of volume as well as axle loads), rapid developments in concrete technology and mechanization both in production and its laying etc., whitetopping i.e. concrete overlay is becoming a choice for the repairs and rehabilitation along with capacity augmentation of damage roads in India, particularly in cities.
Whitetopping, which is a portland cement concrete overlay on existing distressed bituminous pavement was developed as an effective solution for the repair and strengthening of damaged bituminous roads. It is used to improve the structural and functional capacity of roads. Very little stresses are transmitted by the concrete layer to the pavement bases and sub-grades in contrary to flexible pavement, which avoid permanent deformation on such layers resulting in a durable solution for the resurfacing. Now-a-days, concrete overlays both thin and conventional are being used to rehabilitate damaged bituminous roads at a large scale in New Delhi. Figures 2 & 3 show thin overlay used for the colonies roads.
|Figure 2a||Figure 2b|
|Figure 2: Existing road surfaces before whitetopping|
A properly designed concrete overlay has an expected life cycle of at least 20 years. Many projects have lasted for as long as 30-40 years. Today, there is an ever increasing interest in whitetopping. Whitetopping is replacing bituminous overlays because of better performance against rutting and cracking besides being competitive at cost. Due to much higher stiffness compared to bituminous overlay, reflective cracking are eliminated with white topping over a bituminous road. A properly designed and constructed whitetopping offers:
- Improved performance with respect to structural and functional distresses
- Ability to maintain surface grade
- Competitive price with other resurfacing methods by using proper concrete technology
Based on the concrete slab thickness used in the overlay and interface with old pavement whitetoppings or concrete overlays are broadly classified in two categories.
1. Bonded overlays and
2. Un-bonded overlay or conven- tional whitetopping
|Figure 3a||Figure 3b|
|Figure 3: Roads surface after whitetopping|
Bonded overlays are normally used for resurfacing and minor rehabilitation of the distressed pavements. The thicknesses of these overlays vary from 50mm (two inches) to 150 mm (six inches). Based on thickness, these overlays are further divided into two groups, thin and ultrathin. Thin whitetopping (TWT) uses a bonded layer of concrete that is 100 mm to 150 mm while an ultrathin whitetopping (UTWT) layer is of 50 mm to 100 mm thick. When utilizing bonded overlays the existing pavement (concrete or asphalt) must be in good condition without structural deficiencies and significant stresses, as it becomes an integral part of the pavement structure. The bond is made by texturing the bituminous layer. Milling the bituminous surface also helps to improve the bond between the concrete overlay and asphalt interface. The bond between the two pavements plays a critical role for the performance of the new pavement as it ensures the concrete overlay and existing bituminous pavement to act as one structure. These overlays eliminate surface distresses and also provide added str
uctural capacity to the pavements. In these overlays, no dowel bars are used for the load transfer. Load is transferred by means of aggregate interlocking. Saw cuts joints must be made as soon as possible, without delay at a prescribed spacing for the success of the overlay.
Un-bonded Overlay or Conventional Whitetopping
These overlays are generally much thicker than bonded overlays. The thickness of unbonded whitetopping varies between 200 mm and 275 mm and is not bonded to the pavement. Basically, these overlays are constructed as a new pavement on an existing pavement which acts as a stable base. Pre-overlay repairs are generally not required except for special cases where significant distress such as the sub-grade/sub-base is unstable. A typical conventional whitetopping under constructed over half width of the existing bituminous road in Delhi has been shown in Figure 4.
|Figure 4: A typical surface of constructed portion of conventional whitetopping over existing bituminous road in Delhi||Figure 5: A typical Joint spacing used in thin whitetopping|
Whitetopping with thickness varying from 100 mm to 250 mm can be used as one of the sustainable solution options for resurfacing and rehabilitating a large network of existing bituminous roads in India. There are several additional sustainable benefits beyond longevity of such overlays.
Joint in Whitetopping
Commonly observed distresses in whitetopping pavements are:
- Corner cracking
- Mid-slab cracking
- Joint faulting
- Joint spalling
Joint spacing plays a significant effect on the rate of corner cracking. Short joint spacing is used on ultrathin and thin whitetopping to reduce load-related stresses. The commonly used joint spacing is 1 to 1.2 meter. Load transfer designs for thicker and conventional whitetopping are identical to those for new concrete pavements. Dowels are recommended in 150 mm thick thin whitetopping for high truck traffic. In general, load transfer by means of dowel bars is not required for thin whitetopping. Load transfer at joints is by aggregate interlock across, which is enhanced by short joint spacing and by the stiff support of the underlying pavement. A typical joint spacing of 1.2 m used in Delhi on a thin whitetopping is shown in Figure 5.
Advantages and Disadvantages of Different Types of Whitetoppings
The major advantage of the thin whitetopping is its thinness in comparison with the conventional white topping because of composite layer action due to bonding with underlying pavement. Phasing in construction is also easier. It has a wide range of economical applications. Thin whitetopping is very suitable for the situations where overhead clearance is restricted.
The major disadvantages include accurate evaluation of the existing bituminous road as the most common distresses of bituminous road are related to the choice of ultrathin and thin overlays. Ensuring an adequate bonding also is complicated. It requires substantial thickness of bituminous layer to ensure adequate bonding. Increased caused may incurred due to shorter joint spacing.
The greatest advantages of this whitetopping include minimal pre-overlay surface preparation as it takes advantages of existing pavement as a strong base course with insignificant potential for pumping, faulting, and loss of support.
The major disadvantages of this overlay include its thicknesses which tend to be in the range of 150 mm to 250 mm and some- times unsuitable in situations where a specific elevation must be maintained.
Advantages of Whitetopping
The following important advantages are commonly experienced by adopting whitetopping as a measure of repairs and rehabilitation of distressed pavements.
- Whitetopping proves an economical long-lasting solution for distressed roads.
- Properly designed and well constructed whitetopping has 20-30 years maintenance free life. Therefore, it eliminates periodic repairs & overlays resulting in huge saving in scares materials such as aggregates, which ultimately results in reduction of CO2 emissions.
- The level of the pavement remains at least the same for the next two to three decades.
- It provides improved safety due to elimination of distresses such as rutting and potholes etc.
- Visibility is increased because of its better light-reflective surface.
- Street lighting costs may be reduced because of better light-reflective surface.
- Fast track construction is possible.
- Reduction in fuel consumption for heavy trucks compared with bituminous surfaces also results in CO2 emissions which helpful in global warming reduction.
- It provides a cooler surface resulting in environmental benefits. In urban areas, combined with the effects of trees whitetopping can reduce temperature by 10ºF, thereby conserving energy needed for air- cooling/ or conditioning.
- Eliminate the use of petroleum-based asphalt products.
- Simplicity in construction, abundant availability binding materials, possibilities for by-product utilization, and use of available concrete paving equipment etc. are some of the driving forces in favour of whitetoping.
Whitetopping as a measure of repair and rehabilitation of distressed road is gaining popularity and acceptance at a large scale in India due to its sustainable characteristic and freedom from the requirement of subsequent periodical overlays. It is a cost effective option to overcome ever deteriorating road infrastructure at a large portion of the road networks. Several sustainable benefits such as less emission of environmental pollutants due to the reduced fuel consumption, saving in electricity for lightening the street, better night time visibility, use of industrial by-product materials, longevity of the overlays etc. whitetopping provides a sustainable solution for repairs and rehabilitation of a vast network of roads. Proper application of whitetopping overlays requires attention at all stages, which includes selection of the candidate project for a proper overlay type i.e. bonded or un-bonded, good design, materials selection, construction, maintenance, and eventual rehabilitation or replacement. The selection of a particular type of white topping as a possible rehabilitation alternative for an existing pavement is very important for the success of whitetopping.
The authors are grateful to the Director, Central Road Research Institute for his permission to publish the paper. The help provided by Ms S. Juneja (RI) is thankfully acknowledged.
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