Bituminous roads are the most common type of roads which are found almost in every country whether the country is developed or developing country. Bituminous road is most widely being used due to its various advantages that it has in contrast to the other types of roads. Bituminous pavements are flexible pavements. Some of the advantages of the bituminous roads are as follows
Advantages of Bituminous road
- These roads are much cheaper to construct as compared with any other stable form of road.
- Maintenance cost is lesser than rigid pavements.
- Maintenance procedure is much easier than rigid pavements.
- Construction cost is less as compared to rigid pavement.
- Earlier opening of roads under operation is possible which is a main drawback in case of rigid pavements
Tendency of Present Day Engineers
Being an Engineer in Government sector, I had the chance to monitor and supervise many roads works directly in field and also from the desk. I have observed that people, no matter how much experienced they are and how long they have been working in construction of bituminous roads tends to neglect in field quality controlling regarding the procedure of construction of bituminous roads.
It can also be seen that, though many Engineers are strict in controlling various parameters of quality controlling in bituminous road construction, but they simply ignore the importance of temperature controlling while undertaking any project involving the construction of bituminous road. In most of the cases this type of lacuna in adhering to the prescribed standards of temperature controlling arises out of nil awareness regarding the out comings of what will happen if temperature controlling of bituminous road is not done.
Field Controls adopted by Engineers for Bituminous Road Construction
In most of the cases it is seen and feasible that the following field controlling are done during the execution of any bituminous road work
- Quality of the Coarse Aggregates, Fine Aggregates and Fillers like their shape, size, crushing strength, impact strength etc.
- Existing bed condition and preparation of bed prior to receiving of the bituminous layers on it.
- Mixing plant, Tar Boilers conditions and their proper working, specially firing capabilities.
- Mix proportioning as per the specifications and production of the mix.
- Temperature controlling at different stages
- Laying operations of the mix.
- Rolling operation of the laid mix.
- Synchronizing between all operations of the bituminous road construction in such a way that maximizes the work completion rate, efficiently uses resources and most importantly gives proper space for maintaining the specification.
Importance of Temperature Controlling in Bituminous Road Construction
In bituminous road construction mainly two components are used as ingredients, these are Aggregates which forms the bulk of the mix and gives the medium to provide road/ road surface and secondly Bitumen which acts as the binding material of aggregates.
Now, it is known well to all that in the normal atmospheric temperature bitumen is a stiff that is highly viscous material without any flow-ability. Hence, for using bitumen and mixing it in road construction we need to melt it sufficiently so that it can easily flow like liquid and get well mixed with aggregate. Turning the stiff bitumen to behave like a liquid is done by heating it sufficiently.
Bitumen behaves just like a regular paper glue. We use glue to stick paper in liquid form and let it become dry and once it dries it becomes stiff and glues the required article. In case of bitumen also we have to heat it to make it flow like liquid and then we intimately mix it with aggregates, now as continuously the heat is being released from the bitumen by cooling in air during the normal construction procedure stages after mixing is done, then gradually the bitumen loses its flow-ability and becomes viscous and stiffer gradually becoming highly sticky with aggregates it being adhered to hence binding the aggregates together.
Now, we want the mix to become stiff or act as a solid mass only after laying it over road properly to the required shape and thickness and being well compacted. From mixing to finalization of the laying operation including properly compacting by roiling takes some time obviously. During this time the mix should have sufficient malleability that is the ability of the mix to take any shape, for this thing the bitumen which is the main binding agent must remain sufficiently soft or less stiff. This can be achieved only if the bitumen is still hot enough to remain fluid. For this reason, we have to heat bitumen sufficiently so that it can remain fluid until the finishing operation is completed.
It will not be out of place to also mention that, if bitumen becomes stiff very early due to improper heating then as the whole mix will become stiff and losing its movability the whole layer of road laid with such stiff and cold mix will not be compacted properly as due to the wheel load exerted by the roller on the laid mix will not be able to press the aggregates in to voids of one another forming the mechanical interlocking and compaction as the binder being stiff will act as a strong glue and not allow the aggregates in the mix to rearrange themselves under the wheel load.
Stages of Temperature Control in Bituminous Road Construction
As by now I have no doubt that why temperature controlling is important for bituminous road construction have been understood by you I will now go onto one step further in advance mode. Though in the last point I have stressed only on the temperature relevance of the bitumen, but the fact is that bitumen is only a component of the whole mix. In order to achieve our objective of obtaining well compacted and well glued bituminous road we are actually always dealing finally with the mix as a whole not with the single component. As such, there are stages of temperature controlling of bituminous road works from the preparation of the mix till its rolling being finished. These standard temperatures controlling values are given as below in stage wise for Penetration Resistance Grade of 60-70 corresponding to VG-30 Grade of Bitumen as per Indian Road Congress, IRC: 14-2004 Code.
- During preparation of the mix – Separately Bitumen must be heated in the range of temperature of 150°C to 165°C (shall never exceed 170°C) and Aggregates must be heated in the range of temperature of 150°C to 170°C. While the Bitumen and Aggregate will be mixed their individual temperature difference should not be more than 10°C.
- After Mix being discharged from the plant – The mix being discharged from the mixing plant should have maximum of 165°C and laying at site should be done at minimum of 125°C temperature.
- During Rolling Operation – After mix being discharged and laid in the surface where the road is being constructed the temperature while rolling and before the rolling operation completed shall not fall below 90°C.
The whole process of mixing, transporting and rolling of the mix should be well synchronized in such a way that the temperature controlling in all three stages as mentioned above are satisfied.
For other grades of bitumen please refer to Indian Road Congress, IRC: 14-2004, Recommended Practice for Open Graded Premix Carpet, Table No. – 7 Page No. 9. You can find this code just by googling (I will post the whole table here within this week when I will just get some time, it is already 02:20 in mid night now!!!)
What have been discussed over here till now is very important, or else it may be assured that the bituminous road so constructed will not survive its standard life.
How to measure the Temperature of Bituminous Road Work
It is the most sizzling question perhaps coming into mind right now, that how can we measure the temperature of bituminous road work, especially when the temperatures are above normal range that may burn out skin.
Temperature of bituminous road construction may be measured in various temperature measuring gauge or instruments. Most common being the following two
- Metal Clad Thermometer – This one conventional instruments being used since ages for bituminous road construction. Metal Clad Thermometer is just like a normal thermometer but much bigger in size. The process is simple just dip the thermometer in the mix or bitumen or aggregate as required, hold it for awhile so that the indicator may rise up and become stable, and then taking the reading from the graduated scale on the body of the Metal Clad Thermometer. For using the thermometer each time after it is used for taking temperature of bitumen, its body is to be cleaned with oil or other suitable solvent of bitumen because as in each reading will be taken the bitumen will get stick to it and afterward it will become stiff and later on reduce the heat conduction acting as a membrane over the thermometer.
- Digital Infrared Based Thermometer Gun – This is the modern type of instrument which I also use during my works at site. This is just like a playing kid’s pistol. This instrument reads the temperature very precisely, instantly and most importantly you don’t need to be too very close to the object of which the temperature you have to measure. This instrument is running on 3V battery and its price is also not very high only about $35 you can get a pretty good Infrared based Thermometer. You can measure the temperature of the bitumen just by targeting the gun on the surface of the bitumen and pulling the trigger. It will instantly show you the temperature both in Centigrade and in Fahrenheit scale on the LCD display it has. It also has a red targeting laser beam so that you can easily and precisely spot your target.
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