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Concrete Mixes
The concrete mix selected for a particular project is matched to the traffic conditions and requirements for opening it for traffic. A normal mix design includes cementitious materials (cement, micro silica and fly ash), coarse aggregate, fine aggregate, air entraining agent, admixtures (water reducers and/or plasticisers), synthetic fibres (as specified), and a low water cement ratio.
Since the UTWT and TWT overlays are intended for HMA rehabilitation and need to be constructed when roads need to be kept open for traffic, use of ?fast-track? construction, requiring high-early strength concrete is desirable. It is today possible to produce such mixes using a low water-binder ratio with the incorporation of a superplasticiser and a variety of mineral admixtures such as silica fume, fly ash and high reactivity metakaolin. Such mixtures have greater potential for shrinkage and hence, proper care is essential to avoid shrinkage cracking.
The co-author of this article, Vishal Thombare, has done some experimental work on this subject as a part of his Ph.D dissertation. The work was done at V P Road, Mulund, Mumbai using the design of concrete mixes of UTWT. The aim was to arrive at concrete mixes that can be used for fast-track construction. The governing criterion was achievement of flexural strength of around 4MPa (mega pascals) at the age of at least three days. Thombare found that it was essential to use both mineral and chemical admixtures in the mixes for getting the desired results. After many initial trials, a total of six mixes were finally selected (The details, being many, are not being produced here).
It is seen that the flexural strength of 4 MPa can be achieved with a use of silica fume and superplasticiser. As an alternative, ASTM type F fly ash was also tried and it was found that the flexural strength of 4 MPa can be achieved with 20% addition of fly ash and superplasticiser and polypropylene fibres. For this mix, the 28-day flexural strength was 6.2MPa. Figures 2 & 3 show the actual compressive and flexural strength observed during construction of experimental stretches at Ganesh Gawade Road and in Mumbai.
Construction Considerations
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 the repair would depend upon the condition of the existing pavement and include pothole and crack repairs, milling, shoving and levelling. Out of these, the technique of milling is most commonly used for UTWT and is strongly recommended as it helps in establishing better bond between the HMA and the concrete. 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 eschewed as it is considered essential to have at least a 75mm thick bitumen layer below the concrete overlay. Figure 5 shows a milled surface at Ganesh Gawade Road in Mumbai.
Concreting operations
Concrete required characteristics can be produced in a commercial ready-mixed concrete plant. Before undertaking this, it is highly essential to optimise the concrete mix after detailed initial trials. All other operations such as transportation, placement, vibrations, finishing and curing can be done in a conventional manner. If a large stretch of the overlay is available for construction, it may be advisable to use the automated concrete paver having necessary attachments.
Joint Spacing
Both UTW and TWT are characterised 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 combined with the adequate concrete-HMA 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 ravelling of the joints while late sawing may allow stress build up resulting in random cracking in slabs. For both the UTW and TWT works, ?early entry? saws are commonly used. These saws have been specially developed for early-age sawing, minimising ravelling of the concrete at the saw cuts. The large number of joints obviously increases the cost. However, this 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.
Experimental studies
Figure 5 shows the arrangement of thermocouples, strain gauges, connecting cables, Datalogger and signal receiving end during the co-author?s experiments.
From the study, it was clear that values suggested in the IRC-58; 2002 are higher than the temperature difference in high performance concrete (HPC) slabs such as those built using TWT and UTWT. The lesser temperature difference in HPC shows that the warping stress in HPC pavements will be less than the normal Portland cement concrete pavements. Due to a reduction in warping stresses caused by a lower temperature differential the total stresses in the pavements are reduced, hence the thickness required for these pavements will be less than the conventional concrete pavements.
The studies show that TWT and UTWT which are well established techniques in the West can be considered as alternatives to upgrading bituminous pavements with HMA. During construction and repair of pavements in Mumbai, it is difficult to keep the traffic lanes closed for long due to the pressure from the heavy traffic. In such cases the fast construction time by use of UTWT, which makes it possible to open pavements to traffic under 24 hours, will be of great advantage. In fact some experimental stretches have been laid using UTWT/TWT in the city of Mumbai and the results have been encouraging.
The Future
Future research on Ultrathin White Topping by research bodies and the concrete industry can focus on arriving at a better understanding of joint spacing requirements for thickness, traffic and environmental conditions. The specific requirements such as synthetic fibres in the mix for various traffic conditions or the absence of fibres under certain design conditions should be determined. The temperature variations and stress in UTWT can be found out by instrumentation transducers such as strain gauges during the construction process. This will help in further improved design. A composite pavement design with UTWT as the surface course for improved light reflectivity and coolness in urban area can also be considered. UTWT needs to be considered as the preferred choice for pavement rehabilitation and maintenance overlay.
At the current juncture when India has embarked upon a massive programme to upgrade its road infrastructure, we suggest that the adoption of TWT and UTWT may be considered as an alternative to the rehabilitation with hot-mix asphalt. To begin with, some more experimental stretches using these techniques may be laid immediately for gaining experience and confidence in the technique.
Prof K V Krishna Rao
Head of Department-
Civil Engineering, IIT Bombay
(Prof. Rao?s research interests include Travel Demand Modeling and Forecasting, Behavioural Travel Modelling, Urban Land Use Transport Modeling and Public Transit Network Design. He has also completed several sponsored research and consultancy studies in these areas.)
Vishal R Thombare Asst Engineer (Roads)
Municipal Corporation of Greater Mumbai & Research Scholar, IIT Bombay
(Thombre is presently pursuing a doctorate in roads from Indian Institute of Technology ? Bombay. He has presented several papers on pavements and roads all over India.)
