The UTWT-24, ultra-thin white topping, is a very viable solution for roads which need faster pace in addition to low maintenance cost and economy. Avijit Chaubey and Arun Prakash, of ACC present a case study of two locations where UTWT-24 was put to use.
Roads have been constructed with bitumen binders for a long time now but they face a lot of issues, when compared to concrete roads. The biggest issue with bitumen roads is their service life, which usually is not more than two years if it is a road seeing heavy traffic or prone to heavy rainfall.Conventional concrete roads are much durable and have a life of more than 35 years, like ACC's venture on Marine Drive, which was constructed fully with concrete. However, repairing an existing road with concrete in highly trafficked areas is a big challenge and is almost impossible at times, since it requires traffic control for almost 14 days after construction.
A solution has now been found and incorporated across many places. This is the UTWT, the ultra-thin white topping. . The concept is based on high strength concrete, of 35 MPa compressive strength and 3.95 MPa flexural strength, which allows traffic to ply within a period of four days. With this method, joints are spaced at reduced intervals of one metre in both directions, reducing the probability of cracking due to flexure. To achieve this performance in concrete at an early stage is a feat in itself, as normally this kind of strength is conventionally achieved at 28 days.
However, repair of roads in very highly trafficked areas with UTWT poses a big challenge both to the traffic plying on the road as well as to the contractor, given the four days repair period.
ACC has now come up with a solution wherein a road can be repaired and opened to traffic within 24 hours from the time of casting. The concrete has been so designed that the required performance of concrete is achieved within 24 hours. The main challenge in developing this product was that any concrete which is designed for high early strength does not have the required retention period, say above half an hour. Due to a very low water-cement ratio, the heat liberated through hydration, accelarates the hydration process, thus reducing the slump of concrete at a very rapid rate. Also, concrete which has high strength usually has high stickiness in its fresh state, which creates a lot of issues regarding placement and finish.
Then, apart from the strength and workability issues, the biggest concern for an engineer is always the cost of the product, and this is an expensive product. The solution developed, thus, had to take into consideration all these issues. UTWT-24 was an outcome of a series laboratory trials, more than a hundred of them, and more than 30 site trials, on a total of almost 150 cubic metres of concrete. The product achieves a compressive strength of 45 MPa+ and flexural strength of 4 MPa+ in 24 hours; it not only concentrates on compressive strength but also on the toughness of concrete.
Toughness is the property which measures the strain before the concrete fails, unlike strength, which measures the stress required for concrete to fail. For example: in Fig 1, the concrete fails almost at 0 mm deflection, whereas in Fig 2 the concrete fails at much higher deflection, thus acting as an medium of tranferring the load effectively to the sub-base.
Case 1: Tembhi Naka (Thane west)
A stretch of 40 m was taken up at Tembhi Naka, to be repaired by overlay. The existing road was first milled down to get a rough surface so that bond between the existing subgrade and the concrete overlay was achieved. UTWT-24 was laid on the road which had properties as detailed in Table 1. The concrete was sampled from three different vehicles to check the uniformity of the product.
Sampling for hardened properties was done from four transit mixers. The hardened properties were measured from the time of batching and are illustrated in Table 2
Laying of UTWT-24 and related operations
Based on test results of the concrete in a fresh state, it is clear that the concrete had self-levelling properties. The average slump slow was 600 mm. Based on this result, it may seem that maintaining the required camber with this concrete may not be possible, but that was not the case. The concrete was laid and a camber of 2 cm in a horizontal distance of 4 m was maintained. (The camber can be made even steeper if required with the same concrete.) The curing practice adopted earlier was by using chemical membrane but later it was learnt that the best form of curing, specially to avoid plastic shrinkage cracks, was by water spraying.
As discussed earlier, concretes which give high strength at early stages show signs of a fast decline in workability. Hence, the workmen were trained to syncronise their activities so that all the surface finishing was done before the concrete sets. Brooming of the surface started at five hours from the time of batching, and groove cutting started at 16 hours. The area is situated in a locality where groove cutting would have created a nuisance for those in the vicinity. So, groove cutting was done in the morning, due to which the age of concrete went up to 16 hours.
Opening road to traffic within 22 hours from the time of laying
The road was opened to traffic which is usually a mix of heavy and light vehicles.
Case 2: VECL service road (Luna- Vadodara)
VECL (Vadodara Enviro Channel) is a quasi-government organisation led by eminent leader Satish Panchal. The organisation is engaged in the treatment of industrial effluents from various industries in the industrial belt. The industries dispose of the effluents into the VECL channeland VECL then treats them and finally disposes of them into the sea. Due to the presence of harmful chemicals in the effluents, the channel itself made of concrete and constantly facing deterioration, requires frequent repairs.. A parallel service road to the channel gives access to VECL engineers to repair and maintain it.The road also is used by heavy industrial vehicles and villagers; in fact, though built for servicing the channel, the road is used by everyone as there is no other option. The entire service road is 55 kms long and has been in service since the 1970s. The organisation anually spends more than Rs 1 crore on repair of the road.
After the launch of UTWT-24, VECL came forward to implement the technology.
The road has random stretches which are damaged, and many potholes. It was decided that instead of going for patchwork and repairing only the potholes, a stretch of the worst affected portion would be repaired.
The road was constructed for a stretch of 500 m and a stretch of 50 m was taken up at a time. The fresh and hardened properties of concrete sampled from a transit mixer are illustrated in Table 3.
The construction of bitumen roads does not last the entire designed life of the project, and leads to a high maintenance cost. Moreover, the roads give way during the monsoons, resulting in slow-moving traffic and poor ride quality.
Concrete roads, though, have a better service life and show the least signs of damage during its life, but may not be suitable for repair work because of its long curing period.
UTWT-24 is a concrete overlay solution for the repair and construction of roads which need to be thrown open for traffic soon.
The case studies presented here portray the performance and speed of implementation of UTWT-24 for construction of roads. It may be concluded that UTWT-24 is a viable solution for roads which need faster pace in addition to low maintenance costs.