Note: This highly informative and elaborate paper was written by the Author many years back and holds valuable information. Readers should update themselves with the latest developments that might have occurred after the presentation of this paper.
National Highway Development Plan drawn by the central government a few years back to develop highways of international class in India leased a new life to infrastructure development in India. Everyone visiting the West, especially USA, was found talking about the class highways and freeways there. The plan aimed to grant India an equal status. A huge amount of Rs. 54000 crores was earmarked under the plan for development of world class highways having uninterruptedly flowing traffic. Further under the plan, Golden Quadrilateral Project was developed to lay 5846 kms of highways connecting the four big metros, Delhi, Mumbai, Kolkata and Chennai. The stage was all set for putting highway development in India on a fast track. This one action also ensured arrival and use of latest equipment for highway and expressway construction in the country.
Equipment for concrete pavements: Under this article, we are talking of concrete pavements only. Laying of concrete pavements for highways and expressways demands fast speed of construction without any compromise on quality. Global standards on quality and speed are achievable only through sophisticated machinery that may lay kilometers of length in a day and control construction parameters to the fraction of millimeters. This equipment is none other than pavers. Only the use of pavers can put our meticulous planning into practice and achieve desired standards of quality and speed.
Pavers for pavements: Pavers for concrete pavement construction are of two types—
1. Fixed form pavers.
2. Slip form pavers.
Fixed form pavers run on rail forms. The width and thickness of the pavement is fixed. The pavers keep laying the concrete within the rail forms and compact and finish it. As the width and thickness of pavement is pre-fixed, these are called fixed form pavers.
Slip-form pavers don’t run on rail forms. In their case, the forms too keep slipping further. That’s why these are called slip-form pavers. While using these pavers, it is possible to alter the thickness of concrete pavement also. These pavers are therefore much more advantageous than fixed form pavers. Some grade to the concrete pavement itself can be provided while using these pavers.
Preparations for use of pavers: For the construction of concrete pavements, pavers do the work of only laying, compacting and finishing of concrete. Brushing of concrete to create anti-skidding effect on pavement surface can also be done by them. However concrete production and supply has to be separately managed. The following preparations are therefore required before use of pavers for laying of pavements:
1. Concrete production arrangements: For this, a nearby ready-mixed concrete (RMC) plant shall have to be tapped for production of required quantity of concrete of desired mix and water-cement ratio. In case a RMC plant is not available, a concrete batching plant shall have to be installed at a suitable location keeping in view the length of the highway or expressway to be constructed. The location of batching plant shall be governed by the fact that distance of farthest point of pavement to be laid is kept as minimum possible. Special care has to be given to the slump of concrete at the placement point if best performance is to be achieved from the pavers.
2. Concrete supply arrangements: Next shall be the availability of transit mixers for supply of required quantity of concrete from the batching plant or RMC plant to the pavers. Number of transit mixers must be sufficient to ensure uninterrupted working of pavers. Slump of concrete must be controlled at the production point and in transit by calculating the loss of slump that is likely to occur during transportation of concrete. As low slump concrete is used in pavements laid by slip forms, rear opening transit mixers are used.
3. Preparation of sub-base: The sub-base has to be laid, compacted well, checked to be in required grade and ready to receive the concrete pavement. It has to be seen that it doesn’t suffer from unnecessary pits or undulations. In addition to the laying of sub-base to the required width of pavement, it has to be kept in mind that the slipping forms of the paver have to abut the pavement and move along its outside edge. The area outside the pavement width at sub-base level should therefore be clear and well compacted for the movement of slip forms.
Slip form Pavers: The slip form pavers rest on the slip forms that are enclosing the concrete pavement and moving forward, leaving behind the pavement unsupported at edges. It is a phenomenon similar to the vertical movement of slip forms in high rise structures where the forms keep moving upwards, leaving behind the freshly laid concrete unsupported. The forms bear the load of the paver. Major components of slip-form pavers are:
1. Main frame and slip forms
2. Concrete receiving hopper
3. Leveling cylinder
6. Leveling sensor
7. High frequency vibrator
8. Slope controllers
The operation: The concrete produced at the batching plant or RMC plant is transported through transit mixers and unloaded into the concrete receiving hopper of the pavers. When the concrete is to be laid in front of the paver, a side conveyor system is established to transfer it from the transit mixers to the pavement area. When loaded into hoppers, it flows to the space between the slipping forms and the leveling cylinder. The spreader spreads the concrete to the required depth. The high frequency vibrators compact the concrete under completely controlled conditions. No over-vibration or penetrating the sub-base is permitted. The compacted concrete is finished smooth by the smoother. The finished surface is given a few hours till concrete gets set partially. Now, the brush or texture-provider is applied to the surface to roughen it up. This is necessary to reduce skidding effect and is done through an attachment with the slip-form pavers as otherwise it is very difficult to texture the whole length and width of highways. Thereafter, either curing compound is applied to the concrete, again through a spray attachment with the paver or the pavement portion completed with gunny sheets for periodic sprinkling of water on them to keep them wet and cure the concrete.
Additional vibrators should be kept at site during use of slipform pavers. While the laid concrete in pavement is well vibrated by the slip-form vibrator, there are left certain areas at the end of day’s work which need compaction through normal concrete vibrator.
Laying capacity: Slip-form pavers are known for their high productivity. Their output ranges between 50 to 300 cum of concrete per hour. A 7 metre wide pavement having a thickness of 350 mm can be laid at a speed of 40 to 50 metres per hour. A 12 feet wide concrete road with a pavement thickness of 250 mm can be laid at a progress of 100 metres per hour. Thus, even a 1 kilometer length can be laid in a day. If the working hours are 8.00 am to 8.00 pm i.e. 12 hours, the speed of the pavers has to be about 1.5 metre per minute under a continuous operation to lay 1 km length.
Width capacity: Being crawler track type, slip form pavers are highly suitable for larger road width. The pavers being produced in India can lay road width varying from 2 metre to 17 metre in a single pass. During expressway construction, general preference is to use full width slip form pavers. However it should be seen whether the batching plant installed is able to produce the large quantity of concrete demanded by full width paver and further, if adequate transportation arrangements are available. Otherwise, it is preferable to deploy partial width laying pavers.
Concrete characteristics: Cement concrete pavements are being preferred these days over asphalt pavements because of their longer life, little maintenance requirements and zero permeability. During rains, cement concrete pavements remain unaffected while asphalt pavements disintegrate quickly. For best results from concrete pavements, it is necessary that these make an impervious surface. The concrete mix design should be one with low slump. Use of super-plasticizers should be made where required, especially during transportation of concrete from the production point to the pavement point. While choosing a plasticizer, its compatibility with cement should always be examined through tests. Normally, the admixtures or super plasticizers add to the strength of concrete. M40 or above mix should be used. Full attention should be paid to the laying and compaction of lean concrete provided below the main pavement concrete. As the thickness of concrete pavement is around 300 mm to 400 mm, coarse aggregate of 25 mm maximum size should be preferred over usual 40 mm size. The lean concrete too can be laid by use of slip form paver itself.
Cement Content of concrete: While using slip-form paver for expressway construction, effort is to be made to optimize cement content of concrete instead of specifying high minimum cement contents. High cement content may result in cracks in pavement as higher heat of hydration is released. Minimum cement content for M40 concrete is 350 kg per cum. However keeping in view the excellent quality and grades of cement available these days, required strength is achievable with lesser content of cement. The minimum cement content can then be reviewed on case to case basis by keeping the durability and abrasion resistance requirements in view.
Temperature of concrete: In most cases, it is specified that temperature of concrete being laid in pavement should not exceed 35 degree Celsius. To maintain this requirement, chilled water should be used to produce concrete at the RMC plant or batching plant. Temperature of concrete produced should be around 22 to 25 degree Celsius so that by the time the concrete is laid in position, its temperature doesn’t exceed 35 degree Celsius. It is always better to make these observations in actual as the weather and ambient temperature will play important part here.
Slump Control: It is very important to control slump of concrete while using slipform pavers. Slump can neither be more nor less. More than designed slump will not allow the pavement to stay intact and it will tend to spread at ends while lesser slump will obstruct slipform paver’s working and will not allow its smooth slipping forward. During the use of slip forms to build structures, the slump of concrete at concrete production point and concrete placement point is worked out and specified. Same procedure is applicable to slip-form pavers also. These two slumps should be worked out by working backwards. First, the slump at placement point, suitable for slipform paver, should be decided. Next, the time taken in transportation of concrete and likely loss of slump during this period should be noted. Accordingly, the slump required at concrete production point can be worked out.
Precise controls: Slip-form Pavers are proving to be wonderful machines that can give utmost accuracy in road construction. Automatic controllers have been developed and are provided on slip form pavers to control longitudinal level and transversal slope of roads. Electronic level sensors are mounted for longitudinal level checking and these continue to check the material thickness with respect to a fixed datum. Transversal slope controllers are mounted on the cross beams and send signals as and when there is a change in slope. In addition to these electronic controllers, manual controls are kept stand-by. Now-a-days, microwave technology and laser systems are being inducted in road-construction-equipment-control. During the laying of concrete, the thickness of finished portion of pavement should be continuously checked for accuracy.
Action against cracks: A close watch should be kept on the laid concrete pavement during the initial few days of laying of concrete against development of any cracks. If any sort of cracks appear in the pavement, these should be fully analyzed. The mix design should be re-examined, the extent, width and depth of cracks should be checked. Excess of water in concrete, non-curing of concrete pavement, effect of movement of local machinery or the paver itself may be behind these cracks. To safeguard against sudden downpours, polythene sheets should be stocked at site and arrangement for their immediate spread over laid pavement should be kept ready. Pavement edges should be carefully examined. If these are not found intact and given way after the slipping of forms, cracks could transmit to the pavement. Necessary corrective steps should be taken to prevent further appearance of cracks. However, any cracks wherever noted should be well grouted with cement grout or epoxy materials.
Slip form pavers in India: Gomaco, USA has been distributing its slip form pavers in India through Aquarius Engineers, Pune. A number of models are supplied by Gomaco. Its pavers can lay 15.2 metres or 50 feet wide pavements. Maximum pavement thickness can be 483 mm or 19 inches. Gomaco’s Commander III slip form paver is becoming popular all over the world because of its multi-application capability. It can lay the pavement, curb and gutter, monolithic sidewalk, barrier wall, bridge safety parapet also. It is a four track paver that can lay 6 metre wide pavement. Wirtgen GmbH has opened its offices in India and this famous group is producing more than 10 models of slip form pavers. Its SP 1600 model can lay a paving width anywhere between 5 m to 16 m with maximum pavement thickness of 450 mm. It can lay even a two-layer pavement in a single pass. It is also a four tracked paver. It is operated by a 6 cylinder CAT engine and produces a pavement speed between 0 to 5 metres per minute.
Power Pavers Inc. models SF 2700 and SF 3000 are much faster in construction of pavements than fixed form pavers. Automatic dowel bar inserter is also available in them. Mid sized S850 slipform paver of Guntert and Zimmerman can lay pavements of 3.5 metres to 9 metres width. It is equipped with an oscillating correcting beam and a finishing pan thus avoiding a separator smoother to finish the concrete. This paver also has the Compact Dowel Bar Insertion (CBDI) facility. G&Z has devised a complete package including mix design consultation, choosing of aggregates, suggesting cost saving techniques etc for the buyers of its pavers.
Uses in India: India has fast realized the advantages of slip form pavers. Precise accuracy provided in thickness, grade and width by slip form pavers has attracted all major departments. Today, the majors like NHAI, MES, IRCON specify use of slip form pavers for construction of all national highways and expressways. The contract documents have been devised accordingly. These need to be made more specific by specifying the width and thickness production capacity of pavers to be deployed by the construction firm. In addition, environmental aspects of less pollution and less noise also need to be prescribed. Versatility of the paver in changing the pavement width through hydraulically operated telescopic modular frames, cooling capacity of the paver to maintain low temperature of hydraulic oil etc also needs to be checked by the parent departments before showing green signal to the paver model proposed by the construction firms. Such steps result in smooth and quicker completion of projects.