The’ White Topping’ procedure has been in news quiet frequently in recent times. It is not surprising given the fact that roads in India are, more often than not, in need of repairs. The procedure offers multiple benefits to the problem of road rehabilitation, especially when it is a bituminous road and where it has been identified that repeated overlays have little or unviable lasting effects.
White Topping can be categorized into three main heads
- WT- White Topping: which is usually a conventional overlay having a particular mix design for that specific project and usually about 200mm thick.
- TWT- Thin White Topping: This is usually about 100-200 mm thick.
- UTWT- Ultra Thin White Topping: which is sensitive and from 50 to 10mm thick.
Several factors need to be taken into consideration before a particular type of equipment is chosen for a White Topping project. These include, mix design, grading of aggregates, water-cement ratio, additives, admixtures, size of the project, and calculated adversities, among others. It would be difficult to prescribe a one quick-fix solution for all White Topping requirements. This fact has been corroborated by numerous feedbacks and mentioned in the Guide to Concrete Overlays- National Pavement Technology Center, USA, September 2008.
The basic selection process for the right equipment begins with the listing of all the above parameters. The project team then needs to visit the designated site at several instances to assess the impact of their suggestion and the subsequent work flow. Since White Toppings is typically executed on busy city streets, team needs to look at the traffic flow, barricade impact and then monitor the actual available width and length, per work shift. This volume should be proportional to the actual availability of ready-mix concrete. Often unavailability or inadequate availability of constant and consistent concrete leads to failure of White Topping projects.
Once these factors have been taken care off, then comes the decision to choose the right equipment. A basic design with side edges to match seamlessly, fastened easily with nut-bolts and which allows for easy alignment and removal should be preferred.
UTWT – As a thumb rule the basic equipment for UTWT (less than 100 mm) would be a ‘Vibratory Truss Screed’. For correct output, the equipment should be well balanced and offer vibrations distributed over the entire width with eccentric vibrators driven by engine, electric motors and in some specific applications with pneumatic air driven pistons which offer vibrations upto 75-100 mm at best. The truss screed needs to be well balanced and offer crowning arrangements to offset any sagging, which could lead to damaged surfaces.
A word of caution here- the truss screed should not be mistaken for a double beam screed or locally made equipment which have very basic function and cannot have the same effect as a truss screed.
TWT For such applications it becomes essential to ensure that the ‘Fixed Form Paver’ such as the Allen Engg Corp (USA) make’ Triple Tube Roller Paver’ (TRTP) which has its own set of vibrators, is used. In this there is added advantage since the vibrators have equidistant space with dedicated engines, along with options that allow them to be immersed to desired depth and vary the internal vibrations, as per individual batch poured out from mixer trucks. This feature has been of immense utility as in most sites supplies could be delayed due to traffic congestion. The paver could also have options for heavy auger or steel tubes for the entire paving widths, supplemented with additional force for even compaction with heavy frame and ability to extend or retract when required. Reversing ability with high pressure sprayer nozzles that offer retarders have a major advantage when the supplies are abrupt. Most TWT applications do not exceed 300- 550 meters of work per day and usually the widths are between3.5 to 5.5 meters. With over 4 successful projects in different states the TRTP has offered unmatched results.
WT- White Topping is usually carried out over wider roads and those that carry heavy multi-axle traffic. Hence the need for thicker slabs. It is therefore essential to assess the available space if the option of a ‘Slipform’ is exercised. As the best option low & narrow profile slipforms could be given preferences. This is to assist low center of gravity and narrow design feature allows minimal widths to offset the space required for setting up of string lines on each side and barricading the sides yet further. This fact along with the basic need to have at least 60- 90 cub.m concrete flow in front of the slipform paver to ensure correct and uninterrupted working must be factored. In many cases abrupt supply has led to cold joints. If all such parameters are considered then for job sizes of 6 meter and more, for an average output over 400 cub.m, the narrow & low profile slipform pavers are the ideal choice.For further details:
Bharat Road Development Combines Private Limited
14, Viral Apartments, Opp. Shopper’s Stop, S.V. Road, Andheri (West), Mumbai-400058.
E-mail: email@example.com, Web: www.brdc.co