BMC Mumbai: Road Repair with Thin White Topping & Ultra-Thin White Topping

Thin White Topping/Ultra-Thin White Topping overlays are innovative pavement repair and strengthening solutions and are also cost-effective and less maintenance intensive, having demonstrated performance comparable to traditional concrete pavements.

Dr. Vishal Ramesh Thombare - Executive Engineer Roads, BMC, Mumbai

What proven methods of constructing Rigid Pavements would you like to create awareness about?

Many metropolitan cities in the country including Mumbai face the problem of addressing severe pavement distresses in the old flexible (bituminous) pavements after each monsoon, incurring huge expenditure. Though cement concrete pavements, because of their long life and superior performance, greatly reduce the maintenance expenditure, but due to their initial investment constraints, this option is usually not chosen.

There is a need for the development of new and innovative pavement repair and strengthening strategies that are cost-effective and less maintenance intensive. One such solution is the Thin White Topping (TWT) or the Ultra-Thin White Topping (UTWT) overlay. The performance of these overlays is comparable to traditional concrete pavements, but at a competitive price.

Ultra-Thin White topping is a rehabilitation technique in which a 50 to 100 mm thick layer of high strength, fiber-reinforced, cement concrete is placed on a milled surface of rutted and/or cracked bituminous concrete pavement, while Thin White topping can be used for thicknesses of more than 100 mm.

Please give us a case study of their application of TWT and UTWT Toppings.

A recent study in Greater Mumbai demonstrates that with high early strength concrete, roads can be rehabilitated and reopened within a week, showing promise for broader application in urban infrastructure.

Thin and UTWT overlays were successfully utilized as pavements for roads with low to medium traffic. This study provides the details of investigation, structural design, mix design, construction, and instrumentation carried out for an actual thin white topping pavement overlay constructed over an existing flexible pavement for a road in Greater Mumbai.

Through thermocouples installed in the pavement, the temperature differential between the top and bottom of the TWT was measured and interpreted. With the use of a specifically designed high early strength concrete, it was possible to open the rehabilitated road within a week’s time. Following a thorough analysis of the temperature data, it was found that the temperature differential between top and bottom of the thin white topping layer is much less than the one specified in IRC guidelines.

Please elaborate on the construction of UTWT/TWT.

Pre-Overlay Repair: Before the overlay is laid, repair of the existing bituminous pavement is essential to provide necessary uniformity to the support system. The techniques to be adopted for repair would depend on the condition of the existing pavement, which includes pothole patching, crack sealing, milling, shoving, leveling, etc. Out of these, the technique of milling is most commonly used and strongly recommended, as it helps in establishing a better bond between the bituminous layer and the concrete TWT.

For TWT, milling is not mandatory, unless the concrete-HMA bond is assumed in the design. If the rutting thickness is more than 50mm, milling becomes a necessity. Incidentally, excessive milling should be done carefully as it is considered essential to have at least a 75-mm thick bituminous layer below the concrete overlay.

Concreting Operations: Concrete of required characteristics as per the design is produced in a commercial ready-mixed concrete plant. The concrete has to be procured through an approved ready mix plant. The grade of concrete used for UTWT/TWT is M60. This grade achieves early flexural strength due to which traffic can be opened within seven days instead of 28 days, after curing.


Before undertaking this, it is highly essential to optimize the concrete mix after detailed initial trails. All other operations such as transportation, placement, vibrations, finishing and curing are done in a conventional manner. If a large stretch of the overlay is available for construction, it may be advisable to use an automated concrete paver with the necessary attachments, like the Slip Form Paver.

Saw Cutting of Joints: Both UTWT and TWT are characterized by their shorter joint spacing, which is usually 12 to 18 times the thickness of the slab. Such joint spacing reduces the curling stress and when combined with the adequate concrete-bituminous layer bond, also reduces the flexural stresses in the concrete panels. However, one needs to ensure timely joint cutting, as sawing too early may lead to raveling of the joints, while late sawing may allow stress built up, resulting in random cracking in slabs.

For both the UTWT and TWT works, it is reported that “early entry" saws are commonly used. These saws have been specially developed for early-age sawing, minimizing raveling of concrete at the saw cuts.

The large number of joints will obviously increase the cost; however, it has to be weighed against the cost of thicker slab. Incidentally, the total cost of the overlay would be the most significant decision-making criterion. Here, it would be advisable to adopt the life cycle cost analysis approach. For this work, the joint cutting is done within 8 -12 hours of placing concrete to form 1 m × 1 m panels.

Please share the experiences and insights of the thin white-topping pavement overlay constructed on Vithalbhai Patel Road in Mulund, Mumbai.

The investigations carried out for obtaining the design parameters were detailed and the design data was used for the design of the TWT overlay for the road under reference. The laboratory attempts for the design of an appropriate concrete mix for the TWT that can achieve early high strength were used.

The performance of the pavement in the first 15 years has been found to be excellent without any distresses. The pavement is expected to serve without much maintenance interventions for 25 years.

The following conclusions were drawn from this study:

• Thin White-Topping pavement overlay can be advantageously carried out over existing bituminous pavements, resulting in road pavements with prolonged service life with negligible maintenance interventions for even medium to moderately heavy traffic.
• During the investigation for obtaining the design parameters for the design of TWT, it was found that the effective k value found based on deflection criteria was found to be much less than that found based on the k value of the subgrade and the thickness of pavement layers. Therefore, while designing the thin and ultra-thin white topping overlays it is more appropriate to find the effective k value based on Benkelman beam deflection.
• The concrete mix with 20% ASTM type F fly ash and appropriate quantity of polypropylene fibers and superplasticizer achieved a flexural strength of 4 MPa at the end of three days. Such mixes can be efficiently used for opening the TWT overlaid road within a week’s time to traffic.
• The maximum TWT pavement temperature has been found to be only 10% more than the maximum ambient temperature. This prompts separate guidelines relating to temperature stresses for concrete roads in built-up areas with high rises and trees on both sides.
• The temperature measurements revealed that the temperature gradient across the thin white topping is only one half the value suggested by IRC:58 (2011). In view of this observation, it may be inferred that the temperature differentials that are used in the design are on the highly conservative side.