Satander Kumar, Scientist (Retd.), Central Road Research Institute, emphasises on the importance of rigid pavements.The main concrete roadwork for city roads was started in Mumbai in 1924. Marine Drive was constructed in 1938-1939 using semi mechanised methods and was laid on reclaimed soil in coastal area and the road is still performing well in certain locations after nearly 63 years of service despite unfavourable environmental conditions. Later, concrete roads were constructed in Shivaji Park Area in 1943.The use of paver requires huge quantity of concrete, as the speed of the paver is about 1m/minute. The quantity required is very difficult to reach because of congestion and heavy traffic and also more space is required for keeping the machineries on the congested roads. Therefore, it is difficult to use paver for construction of concrete roads mainly in the cities. Further, concrete road work is in progress using semi mechanised methods and fixed form pavers at many places like Mumbai, Pune, Thane, Delhi, Bangalore, Goa, Nagpur, Chanderpur, Noida, Dadri, Rohtak, Jhajar, Chandigarh, Shimla. At present cost of rigid pavement and flexible pavement is at par for very heavy traffic roads for National Highways. Most of the National Highway pavement are being constructed with slipform pavers/mechanised pavers. For toll roads, rigid pavement is preferred and is being adopted. At certain location, semi-mechanised in place of slipform pavers is also being used in Highways near structures or for full depth repair etc.Many of our airfield pavements have been made with as rigid pavement especially at the apron and runway ends. In India, first mechanised road was for two additional lanes for 56 km long stretch of Delhi Mathura road -major concrete pavement project sponsored by MOSRTH during 1993-1994 executed by IRCON/UP PWD and Haryana PWD. Rigid pavement under NHDP programme is approximately 1680 km (equivalent two lanes) sponsored by NHAI in NH-2 and NH-60 in 2004-2005. The Mumbai Pune Expressway is the first international-standard six lane expressway constructed in India and that too in concrete. 8 lakh cu m of Pavement Quality Concrete (PQC) and three lakh cu m of DLC was produced for making rigid pavements. Next rigid pavement in the expressway was, Yamuna Expressway. Flexible pavement was adopted only in marshy regions.On deck slab, 15 mm mastic with 50 mm BC on the top in two layers was laid. Presently white topping (ie. Rigid pavement over flexible pavement) and paver block technology is being adopted in many states.Need and advantagesAdoption of cement concrete pavement is satisfactory in certain locations such as parking areas, heavy traffic roads, National Highways, urban roads, aprons, ends of the runways (Airfield), under passes, steep terrains, low lying areas, because of its low life cycle cost, and long term durability/serviceability. Cement concrete pavement had been considered as good potential and cost-effective as these hav; longer design life, lower maintenance cost, good performance and provides reduced vehicle operational cost.Reinforced cement concrete (RCC) is suitable at steep terrains, on very low bearing capacity soils, where length/breadth ratio of PQC panel is more than 1.5, in case of full depth repair, for better riding quality with lesser no. of joints, where there is likely chances of cracking, to minimise reflection or sympathetic cracking. Recent activities in this field are white topping and ultra thin white topping, self Compacted Concrete pavement, high performance concrete pavement, recycling of rigid pavements/flexible pavement as base, composite pavements, pre-stressed rigid pavement, early opening to traffic rigid pavement (traffic can be opened in 3-6 hours.Innovation in flyash One of the most problematic solid industrial wastes posing serious problem is the fly ash from thermal power stations. Coal based thermal power stations have been set up in large numbers to meet the increasing energy demand. In a thermal power station, two types of ashes are produced from burning a coal. Lighter fraction goes up the chimney and is caught by electrostatic precipitator (ESP). This is known as fly ash (Passing 45 micron). This is used for making blended cement. The other fraction containing coarser material collected at the bottom of the furnace is called bottom ash (of size 1 mm or less). This non plastic fly ash had been used in one of the layers of high embankments. About 80 per cent of the total ash produced is fly ash. In the wet disposal system, which is being followed by most of the power stations in our country, both fly ash and bottom ash are mixed with water and pumped into artificial lagoons. This combined ash is known as pond ash. Combined ash or bottom ash has also been used as soil replacement in high embankments in India on heavy traffic roads and a minimum cover of good earth of 1 m had been provided as the fly ash used is non plastic. Processing of fly ash and High volume fly ash concrete, Self compacted concrete, use of fly ash with silica fume in concrete are best innovation in fly ash.Innovation in flyash in concreteFlyash in the blended cement improves characteristics of plastic concrete. Briefly, flyash in blended cement usage also results in reduced mixing water, improved workability, pumpability and finishing,reduced bleeding, better appearance during night and better in high strength FRC for making ultra thin whitetopping.It has been observed that flexural strength of the fly ash admixed concrete (which is basic requirement of rigid pavement on which its design depends) is improved much as comparative to compressive strength. 90 days or 365 days strength is much more than OPC concrete without fly ash.Recently, Portland Pozzolana Cement (PPC) Gr.I, (Fly ash Based) has been now used for making concrete pavement. "Code of Practice for Plain and Reinforced Concrete". Use of blended cement is to minimise environment pollution. M40 Grade concrete or higher required for PQC, either 43 or 53 grade of cement is considered more feasible containing fly ash up to maximum 25 per cent as per IRC 15. Innovation superplasticiser High Range Water Reducing Admixtures/poly carboxylic based super plasticizers further influence the strength of cement-aggregate bond. For making concrete using fly ash/silica fume very low water/cement ratio is required but at the same time concrete should be workable. This workability is achieved by the use of high range water reducing admixtures (superplasticizers).Normally sulphonated melamine formaldehyde condensate and sulphonated naphthalene formaldehyde condensate are used as high range water reducing admixtures. Especially where very high fineness materials such as fly ash or silica fume collected from electrostatic precipitator (ESP), are used it is essential to use super plasticizers ( poly carboxylic ether based) in the concrete as per IS 9103-1999. Use of plasticizers along with slight content of air entraining agent also produces very good finish of concrete and gives very uniform cohesive mix.Innovation-fibre reinforced Fibres are now being used in paving concrete. However, polymeric fibres also have been used in concrete at toll plaza. Fibre not only increases the flexural strength but also post cracking ductility of concrete is increased. Fibres may be used to improve the toughness of concrete also and also improves the impact strength and inherent flexural strength. Fibre, silica fume and fly ash admixed concrete behaves better in poor shaped panels where some time specified L/B ratio is difficult to maintain and to minimise plastic shrinkage cracks and where tie/bars are also not used. Innovation using blended cement Blended cements have now been recommended by IRC in not only roads, but also in bridges. Following are some of the innovation and special properties of blended cements:• PPC contains less per cent age of Ordinary Portland Cement as compared to Pure OPC. It may contain 10-35 per cent fly ash.• In PPC therefore there is likely delay in setting and hardening and may take more time in winters, and also gives better results in the tropical environment.• In case of OPC, joint cutting may not have any problem in winter, however in summer due to rapid setting it is difficult to manage joint cutting in the stipulated time, joints are cut in stages/alternate initially.• Reverse is the case with PPC, as in winter it is difficult to cut joint even with in 24 hours and this may cause transverse cracking.• Because of slow setting and with light movement of traffic; texture depth in localised areas may be affected early. Use of silica fume/ 53 grade OPC along with polymeric or steel fibres may resolve this problem. Curing time is increased by two or more days as compared to OPC concrete.Rigid pavement at toll plazaVehicles are required to decelerate while entering the toll lane, stop for payment and then accelerate and merge in the main line traffic of the highways/expressways. All these operation of vehicles at toll Ppaza are prone to oil/POL spillage on the surface, which may have cut-back action on bituminous surface. Therefore, concrete pavement or rigid pavement or white topping payment would be preferred in the toll plaza area including tapering zone from durability and long term serviceability consideration. The minimum grade of concrete shall be M 10 at 7 days for dry lean concrete (DLC) as per IRC: SP 49. The minimum grade of concrete for PQC shall be M 40 and minimum flexural strength or modulus of rupture shall be 4.5 mpa. Use of stabilised sub grade, sub base, bases with cementitious soil stabiliser such as innovative and patented RBI Grade 81 or equivalent will produce long lasting pavement like in China, mostly stabilised layers are being used for making roads. Crusher dust or stone dust conforming to fine aggregate shall not be more than 20 per cent in case of pavement quality concrete (PQC) and in dry lean concrete (DLC) to the extent of 100 per cent crusher dust as an alternate to river sand or natural may be used. To make the concrete more ductile, fibres in high strength concrete i.e M40 and above were added in PQC at toll plaza up to 25 m length. Use of fibres- Irc 15-2011Fibers were used as provision in the design to reduce the shrinkage cracking and post-cracking. The fibers may be steel fiber as per IRC: SP: 46 or polymeric Synthetic Fibers (e.g. polypropylene, polyester, polyethylene, nylon).Innovative solutions in design of the rigid pavementThe stage construction shall not be permitted in case of rigid pavement. The new rigid pavement shall be designed in accordance with IRC 58. The existing rigid pavement may be rehabilitated either by rigid or flexible over layers. In place of alone sub grade CBR, is considered taking effect of embankment CBR. Minimum CBR is 8 per cent and in flexible pavement minimum CBR is 5 per cent. For widening and extending rigid pavement with flexible and vice versa, rigid with rigid, proper key/stepping of the existing with rigid may be carried out as per IRC: 15-2010. The DLC shall be extending beyond the PQC (including that in paved shoulder, if any) by 1 m on either side with a camber of 1.6 to 2 per cent. A properly designed drainage layer (GSB) of thickness 150- 200 mm shall be provided using motor grader and rollers with permeability of 20 m/day.The density of sub grade shall not be less than 1.75 kg/cubic cm. The thickness of subgrade shall be minimum 500 mm with a camber of 3 per cent. Each layer to be compacted shall not be more than 200 mm. Reasons of crackingHeat development in cement-rich concretes can result in high internal temperature stresses. Thus, consideration is given to minimising thermal gradients which result in cracking which will reduce the life and performance of concrete pavement by adding fly ash or making high volume fly ash concrete for pavements. When high-strength concrete is used in floor slabs, plastic shrinkage cracks may result when just after construction there is a loss of water 1 kg/sq m/ hour. The primary cause for these plastic shrinkage cracks for freshly placed slabs is very rapid loss of moisture from the concrete caused by low relative humidity, low water content per cubic meter of concrete, high wind and high temperature differential in concrete. Pavement performance indicators The roughness as measured in each lane with 5th wheel bump indicator (BI) shall not be less than 2000 mm/km. No cracks of thickness more than 0.3 mm other than shrinkage cracks are permitted. Other distress shall not be more than as specified in IRCSP:83. Total length of cracks shall not be more than 2000 mm and individual crack shall not be more than 1000mm in a normal panel (4.5 mX 3.5 m) of concrete road. Also,Rigid pavements shall be provided with surface and subsurface drainage system so that all the storm water is drained off efficiently and stagnation of water takes place at any areas. Adequate water supply during construction shall be provided. Maintenance of WhitetoppingBroken edges shall be shaped neatly with a vertical cut with chisels/joint cutting machine into the shape of rectangle. Small pneumatic chisels also may be used, provided the cutting depth can be controlled. The depth of the cut shall be the minimum to affect repair. After shaping the spalled area, it shall be cleaned and primed all sides vertical and bottom. The epoxy mortar/concrete is then applied using hand tools like trowels, straight edges, brushes etc. The repaired edge shall be in line with the joint groove and shall be flush with the concrete slabs. During the repair work, any damage noticed to the joint sealant shall be made good by raking out the affected portion and resealing. Low viscosity epoxy resins shall be used for sealing fine cracks with injection/pouring and also sealing the sides of repaired patch to make it monolithic.Economical rigid pavementThe cost of rigid pavement can be reduced/controlled in certain cases by adopting design of rigid pavement for 90 days, closer joint spacing in PQC, use of local material – flyash, slag, marginal materials, roller compacted concrete pavement (RCCP) where riding quality upto 3000 mm/km may be accepted like in parking bay, use of Hot sealant, Use of tie bars at butt type only, Undowelled joints/untied joints, making of joints with T beam Conclusion Based on the current practices being adopted, conclusions that can be derived are; the grade of PQC M40 or more than is essential for abrasion resistance and structural consideratio. Use of polypropylene fibre as per IRC 44 and foaming agent for initial curing, may be used to minimise plastic shrinkage cracks which are due to high wind speed or high ambient temperature very low relative humidity. These cracks will occur, when rate of evaporation of water is more than 1 kg/sq m /hour from top PQC surface as per IRC 15. The ability to control the size and amounts of cracks will also lead to improved durability by the use of fibre reinforced concrete or by stabilisation of subgrade or sub base with cementitious soil stabilisers accredited by IRC. In PQC panel, where ratio of length/width of the panel is more than 1.5, nominal reinforcement of 12 mm deformed bars of Fe 500 as per IS 1786 at 150 mm c/c, with 50 mm cover placed 50 mm down from top or 50 mm up from the bottom of slab.• There is innovation in slip form paver in which DLC and PQC can be laid simultaneously besides intelligent rollers which can measure density also. • Cone crusher/vertical shaft impactor/ aggregate washing plant in aggregate field and 240 cu/hr batching plant with inbuilt ice plant.