With more than 75% of the concrete road network in Mumbai being older than its 20 year design life, major investments into structure strengthening and drainage system of the roads are required. Every municipality and state is facing the same pressures: do more with less of tax-payers? money. So the keyword among today?s material specifiers is value, value that delivers more performance for every rupee spent. In road maintenance and repair, everyone recognises the long life and superior performance characteristics of concrete paving. But budget constraints have often stopped specifiers from choosing concrete. Hence, new innovative pavement repair strategies need to be developed that will be more cost effective.
Although concrete pavements staged a comeback in India in the 1990s and are now considered a viable option to the bituminous pavements for a certain set of conditions, the share of concrete in the total paved roads in the country is abysmally low. A number of factors are responsible for this, main being the non-availability of cement for road works, the practice of stage construction prevalent in the respective states and the central highway departments which favour the bituminous alternative, the higher initial cost of the concrete pavements and the reluctance of stakeholders to adopt the lifecycle cost approach. It is no wonder then that an overwhelming majority of paved roads in the country ? more than 99% ? are of the bituminous variety.
Bituminous pavements have many disadvantages. Unlike concrete pavements, they require repeated repairs and maintenance. Further, the design life of these pavements is assumed to be around 10 years after which they need upgrading and strengthening to cater to higher traffic. In case serious pavement deterioration in the form of excessive rutting, cracking, stripping, settlements, etc., happens earlier, as often is the case in India, their rehabilitation needs to be taken up prior to the expiry of the design life. For both repairs and strengthening of the bituminous pavements, the current practice in India and many other countries is to go in for the same basic materials of construction again, that is, bitumen. Recent experience in the USA and some other countries, however, indicates that thin concrete overlays in place of hot mix asphalt may prove to be a viable and cost effective solution for pavement rehabilitation.
One such possible innovative pavement repair strategy is Ultra-Thin White Topping (UTWT). With the development of Ultra-Thin Concrete White topping, material specifiers in municipal and state governments have been given the opportunity to choose a no-compromise material that delivers the looks and performance characteristics of concrete at a competitive price. Invented in the USA, UTWT has been utilised as pavement for low to medium traffic, parking lots, apron at airports, etc., in the USA, Europe and recently in the rehabilitation of steel bridge decks in the Netherlands. This study provides a brief overview on the topic, based on the latest literature. Some laboratory work carried out to develop concrete mixes for UTWT is also included.
What is UTWT?
White topping or concrete overlay is a plain concrete layer constructed on the top of the existing hot mix asphalt (HMA) pavement. Conventional white topping has a thickness of 200mm or more, thickness of Thin White Topping (TWT) is between 100-200mm and that of UTWT is 50-100mm or less.
The laying of UTWT is a paving process in which a thin layer of high-strength concrete, usually fibre-reinforced, is placed over a prepared existing surface of distressed asphalt. A low water binder ratio of 0.26 to 0.29 enables the mix to develop sufficient strength allowing the pavement to be opened to traffic within 72 hours after placement. It is less porous than normal concrete and develops a high strength in initial 48 hours. UTWT, first invented in the USA, has also been extensively used in Europe, Canada, Japan, South Korea and even in the developing countries like Mexico and Brazil. UTWT has many advantages over bituminous pavements such as durable surface, reduced thickness, little pre-overlay repairs, quick construction, reduced maintenance, cost-effectiveness when compared with asphalt overlays, improved service life, improvement in safety due to the increased reflection of light, reduction in operational cost and lower absorption of solar energy. UTWT techniques have shown to be viable for roads of light to moderate traffic. The basic material performance properties as against the normal concrete are shown in Table 1.
A recent publication of the Transportation Research Board, USA documents the experience of hundreds of worldwide projects using the TWT and UTWT techniques and concludes that these types of overlays have proved to be successful when compared with the rehabilitation done with hot-mix asphalt.
The use of UTWT for the structural strengthening of roads has been explored further in Mumbai by constructing experimental sections with 100mm to 150mm UTWT. The sections were subsequently tested by embedding instruments in concrete (during construction) and the design was refined using a laboratory test.
Design Considerations in laying UTWT
Necessity of sound bond between asphalt and concrete
The pavement structure of UTW and TWT overlays is fundamentally different from that of the conventional plain concrete or the HMA pavement. UTWT and, in most cases, TWT overlays are designed and constructed with the assumption of a sound bond between the plain concrete and HMA materials. Such bond ensures composite action between the two, and as a result, the stresses in the top concrete layer are significantly lower, enabling the adoption of a thinner concrete slab. Thus, sound bond is the key to achieving successful performance of the overlays.
Since the performance of UTW and TWT are closely related to the properties of the support layers, especially those of the HMA, it is essential to adequately characterise the same. Both, a thorough visual inspection and laboratory tests of HMA samples, would be important for this. The laboratory tests include the Hamburg wheel track test to determine the rutting and moisture susceptibility of
the HMA sample and the resilient or dynamic modulus measurement. Further, the susceptibility of HMA to permanent deformation significantly affects the UTW and TWT performance and hence, it would be important to assess this factor.
Design of overlays
As with any pavement design, the thickness of the new concrete pavement is determined by the type and volume of expected traffic, the strength and the condition of the existing pavement and the material properties of the concrete to be used.