|
|
|
Cold Recycling for Roads |
|
|
An engineer can not think of constructing a road without the availability of a Hot remix plant with him. And the process that he looks forward to is the one that may allow him to save bitumen, lay strong and durable roads at a lesser cost, re-use the aggregate scrapped off the worn out road, open the road built by him to traffic immediately on its laying and continue laying of a road even if a sudden down pour begins ! This certainly seems to be a figment of imagination, a far-fetched idea but has become possible in actual—With the advent of Foamed Bitumen technology for roads. Many countries around the world including Australia, Brazil, Canada, Finland, Mexico, Norway, South Africa, UK and USA are trying to use the Foamed Bitumen technology in an enthusiastic manner. The results are encouraging. Foamed bitumen has been successfully used in new road construction, in strengthening of existing roads, as a base course material and to correct the subsidence caused in roads due to heavy traffic. Its area of application may further widen as it is showing full stability even under extreme climatic conditions with successful use in countries like Saudi Arabia, Iran, Nigeria and South Africa. Next is the turn of India and whole of Asia. Foamed Bitumen: Foamed Bitumen is essentially the material that is produced when small amounts of cold water are added to very hot bitumen thereby increasing the surface area and reducing the viscosity of bitumen thus raising a lot of foam. The quantity of water added is only 2 to 3% of weight of bitumen. The bitumen to be used need not be of special grade but with normal penetration. 80/100 grade bitumen generally used in roads in India is fine for use. As the bitumen is hot, water coming in contact with it evaporates in an abrupt manner thereby causing foaming of bitumen along-with. The bitumen expands up to 15 to 20 times its original volume, its extent depending upon the quantity of water added and the temperature of hot bitumen. The water is added by the method of injection by use of a specially designed spray-bar and under controlled physical conditions. The Foamed Bitumen produced is well suited for mixing with cold and even damp aggregate. So it is mixed with the cold aggregate to produce the ready-mix for laying on roads. Recycled Aggregate: The aggregate used for mixing with foamed bitumen may be the worn out material milled off the already laid road. In the conventional road building process, this aggregate is removed and disposed off and new aggregate is used. But while using foamed bitumen, this aggregate can be put to re-use as there is no reduction in strength or durability of roads on this account. Thus a lot of saving on account of cost of aggregate can be made. It is however to be checked that the milled aggregate is well graded. Its sieve analysis may be done to check the fine material content in it. In foamed bitumen technology, availability of fine material in aggregate carries importance. Therefore, if required, crushed stone may be needed to be added to the aggregate at its optimum moisture content. This fine material in fact gets mixed with foamed bitumen to form a mortar which coats and binds the coarse aggregates. The Technique: While the recycled aggregate is the base material for relaying of roads, foamed bitumen acts as the binding material for the aggregate. For successful production of foamed bitumen, the physical conditions of temperature and pressure need to be controlled. The two most important characteristics of foamed bitumen to be controlled to produce quality material are its Expansion and Half-life. Ratio of new volume to original volume of bitumen is its Expansion. As already explained, it ranges from 15 to 20. Half-life is the time in which foamed bitumen loses 50% of its expansion. Generally half-life is of about 10-12 seconds. Best foamed bitumen is the one having more expansion and more half-life. The temperature of bitumen, the amount of water added to the bitumen and the pressure under which the bitumen is injected into the expansion chamber control these two characteristics of foamed bitumen. After laying the road with foamed bitumen, it is sealed with a thin layer of asphalt. The result is availability of a high grade stabilized pavement. Great advantages: A highly interesting aspect of foamed bitumen road construction is that the mix of aggregate and foamed bitumen need not be heated in a hot mix plant but can be laid in the cold condition itself. Not only this, if the produced mix does not get laid by the closure of working hours in a day, it may continue to lie on the road side and can be laid in position next day. The mixed material can even be stock- piled on the road side and remains soft till the time it is laid and compacted. After compaction, the road can immediately be opened to traffic. Thus the road-closing hours with ‘road work in progress’ sign boards troubling the vehicle drivers get minimized when foamed bitumen is used. And what may come as a big relief to a road builder is that the laying of foamed bitumen mix can be kept continued even if it starts raining. Moreover the cold process also helps in lesser consumption of fuel thus bringing savings. Saving in bitumen quantity: As the volume of foamed bitumen increases to many times the original volume of bitumen, its viscosity decreases. This enables a much less quantity of foamed bitumen to coat the aggregate. The foamed bitumen mixes with the finer particles of the aggregate thus producing a mastic to bind the large particles. As a result, the consumption of bitumen decreases thus bringing savings in cost on this front also. However there is no loss on strength front as a foamed bitumen road gives stiffness values equal or greater than those of a hot mix bitumen road. Cold Milling and Recycling Machines: With so many advantages in view including savings in bitumen, fuel and aggregate; recycling of aggregate; cold processing; working in extreme climatic conditions and quicker opening of roads, engineers are working out complete design process of foamed-bitumen-pavements by studying the test results and analyzing the behavior of such laid roads. Wirtgen GmbH, Germany has developed advanced equipment that mills out old and worn out roads, adds milled aggregate to the mixer drum, adds micro-processor controlled bitumen to the drum, injects micro-processor controlled water to the drum for the foaming of bitumen, mixes all the ingredients and then lays the mixed material along the road—all in one go. The quantities of various ingredients can be controlled with respect to the thickness of layer to be laid and the speed of construction of a road. The foaming process takes place in individual expansion chambers when temperature of bitumen is about 180oC and pressure is about 5 bar. Essential features of Recyclers: Essential features of a good recycler can be listed as below: - It should be able to do milling in any direction while moving in forward direction.
- The power output control should be automatic.
- The milling and mixing drum should cater to all applications.
- Volume of mixing chamber should get adjusted automatically according to quantity of material to be processed.
- The injection system for foamed bitumen and water should be a combined one.
- The recycler should be able to use all binders—bitumen emulsion or cement or both or foamed bitumen.
Spraying technology: To ensure that the foamed bitumen gets homogeneously distributed in the material to be recycled, the machines carry high quality spray bars with 16 nozzles for spraying foamed bitumen over full working width. The spray bars can be controlled by the operator with respect to the road width under consideration for spraying. The foamed bitumen’s properties of Expansion and Half-life can be checked and corrected by taking foam samples from an external test nozzle provided in the machines and then comparing their parameters with desired results. Projects accomplished: Some of the projects accomplished around the world using Cold recyclers and foamed bitumen are tabulated below: | Sr. | Country | Job | Machinery Used | Remarks | | 1. | Saudi Arabia | Heavy traffic desert road of 380 km length | - Wirtgen Cold Recyclers WR 2500
- Mobile Slurry Mixing Plants WM 400
- Vibratory Rollers
- Pneumatic Tire rollers.
| 1. Total length of 380 km was finished in 180 days only.2. 5% foamed bitumen and 2% cement slurry were used as binders to do 35000 sq. m area daily. | | 2. | USA | Rehabilitation of canal road network | - Wirtgen Cold Recyclers WR 2500
- Vibratory Rollers
- Pneumatic Tire rollers
| 1. 1.5% Cement and 3% foamed bitumen were used as binders. | | 3. | Iran | Rehabilitation of Motorway | 1. Wirtgen Cold Recyclers WR 25002. Cement Slurry Mixing Plant WM 10003. Vibratory Rollers4. Pneumatic Tire rollers | 1. 3.5% foamed bitumen and 1.0% cement slurry were used.2.Total area of 8 lac sq.m was laid. |
Advantages: The following advantages of foamed bitumen roads over those with cement or bitumen emulsion as binding agents can be listed: - Earlier laid aggregate gets reused without compromise on strength or quality. This is a big advantage. Wastage as well as disposal problems are avoided.
- Standard penetration grade bitumen is used. So there is no problem on account of non- availability of bitumen nor any special quality or grade of bitumen is required.
- Traffic obstructions are minimum as the roads can be opened immediately after their treatment with cold recycled material.
- Very small quantities of foamed bitumen as binder are required. Thus the process is highly cost effective.
- Foamed bitumen can be produced directly in the recycler. Additional machinery is thus avoided.
- The foamed bitumen treated material can even be stored.
- The work can proceed even on rain-wet roads.
Future Prospects: Cold recycling process for rehabilitation of repairable roads is therefore quite cost effective and may find great scope in India. There has been an unprecedented growth in road traffic in India. Cargo transport has grown by 60 times during the last fifty years. Passenger traffic has risen by 65 times and the number of vehicles by 80 times. The vehicular and passenger traffic is bound to rise further. The roads therefore need better care and frequent repairs through some economical methods. Under the circumstances, making use of cost effective technique of Foamed Bitumen Construction may prove quite useful for the country. |
|
Tip of the Moment
PPC has no grading and its bags carry IS1489 mark.
|
IMP : Due to His Pre-Occupations, The Author Is Unable To Find Time To Compile The Tips In Book Format. As And When, The Book Is Completed And Near Publication, It Will Be Notified On The Website. The Author Expresses His Gratitude To The Readers And Site-Visitors For The Huge Demand Made By Them For The Book.
|
|
