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Asphalt pavements can be referred to as any pavement surface in which the top layer (wearing coat) is made of Asphalt. Asphalt is basically a heterogeneous mixture formed by specific elements like aggregates, binders, and certain amounts of filler. In most cases, Bitumen is used as a binder for asphalt and dust or fly ash can be used as a filler.
Asphalt pavements are made up of five different layers and are explained below.
The foundation for the pavement is a natural subgrade and it is also called an embankment. It is simply a natural ground surface containing all kind of materials only but compacted to some extent so that the remaining layers can be levelled on top of it.
The top 300mm of natural subgrade can be considered as the compacted subgrade. It is mostly filled with soil and boulders of different types and is compacted to a greater extent for supporting the remaining layers. The entire load coming onto the pavement is distributed to this layer, hence needing higher compaction.
The subbase layer is generally between 100 to 300mm in range. 40 mm grade aggregate chips are laid and rolled to the required compaction in the dry state.
In this layer, a material called “WET MIX MACADAM” is laid in 2 layers. The thickness of the layer ranges from 100 to 300mm. It consists of clean, crushed, graded aggregates in the proportions mentioned below:
All these materials are mixed with the required amount of water and rolled to a dense mass on a prepared surface. Bitumen Emulsion mixed with water to form a layer called prime coat is applied onto this surface after compaction. This will improve the bonding between the base course and the binder course.
In this layer, a material called “DENSE GRADED BITUMEN” is laid in 2 layers. The thickness of the layer ranges from 50 to 100mm. It consists of clean, crushed, graded aggregates in the proportions mentioned below:
A material similar to prime coat but with less viscosity is applied between binder course and surface course to have better bonding between these two layers.
In this layer Bituminous concrete material is laid and rolled to the required compaction. The thickness of the layer varies from 25 to 50mm depending on the traffic volumes and load it is carrying. It consists of clean, crushed, graded aggregates in the proportions mentioned below:
Bitumen mixed with higher quantities of water to form a seal coat and is applied on the finished or compacted surface of the wearing coat. This will close all the micropores on the top of the surface and will provide a smooth ridable surface for the users. Various materials used for the asphalt pavement with their shape and size are mentioned in the below table.
|Granular sub-base||40 mm||Irregular|
|WWM (Base Course)||10mm,20mm,40mm||Angular|
|DBM (Binder Course)||10mm,20mm,40mm||Angular|
|Surface course/wearing coat||10mm to 12mm||Angular|
Polished stone value (PSV) is a measure of skid resistance of a surface. Polished stone value is the resistance value of an aggregate against the polishing act of the automobile tyres under the conditions like road surface. The PSV of an aggregate can be evaluated by applying a certain polishing process to the aggregate and then testing it with a portable skid resistance tester to determine the PSV.
The PSV of a good aggregate depends mainly on two factors like Microtexture and Macro texture.
Microtexture Polished value of each aggregate is different and is generally constant over time. The average Microtexture polished values for different aggregates are mentioned below. (1)
|Average Micro Texture PSV||52.74||41.35||35.16||29.59|
Aggregates with high hardness are generally having less macro texture polished value and the aggregates with low hardness are having higher macro texture polished value. The average Macro texture polished values for different aggregates are mentioned below.
|Average Macro Texture PSV||24.56||28.95||31.24||38.11|
Hot rolled asphalt is one that is produced in the manufacturing plants under high-temperature conditions. The average temperature of the mix in the production stage is generally between 150 - 1800 C.
Stone mastic asphalt is generally stone-on-stone like the structure of gap graded aggregate which are bonded together by mastic with higher binder content, filler and fibre to reduce the draining of binder. Because of the typical structure of the SMA, they have better performance than other asphalt grades like open-graded and dense-graded asphalt mixtures. (2)
The thickness of asphalt pavements mainly depends on the incoming loads coming onto the pavement, the strength of subgrade soil and the quality of materials used for all the other layers. Based on the resistance value(R) of the subgrade soil, the thickness of asphalt pavement for various conditions are mentioned below. (3)
|Subgrade soil||Car lots & Driveways||mall truck lots (<100 trucks/Day)||Large truck lots (>100 trucks/Day)|
|AP wear||AP base||Total||AP wear||AP base||Total||AP wear||AP base||Total|
The cost of asphalt pavement depends on various factors like excavation cost, Material cost, transportation cost etc. In general, the cost of construction for Asphalt pavement is ranging between $3 to $4 per square foot. The average total cost for asphalt pavement ranges between $4500 to $6100. This total cost includes all the lead & lift charges along with the cost of materials used. (4)
Based on the requirement various kinds of innovative asphalt pavement solutions came into the picture. Some of them are explained below.
Based on the recent research if the air voids within the asphalt blend are around 2.3% - 28.5% they can be considered as Low Noise generating asphalt (Less than 100 dB). If the air voids are more than 8% then the asphalt pavements will produce less than 98.5 DB. (5)
When high-quality aggregates are mixed with a polymer resin binder to form High friction surfacing asphalt which is used to maintain the pavement friction for the distressed pavements.
In conventional asphalt mix if the reused or recycled materials are used as a fractional replacement for aggregates or filler they can be considered as Eco friendly, renewable, or reusable asphalt. Some of the replacement products are fly ash, recycled aggregate etc.
Long-life pavements are generally referred to as the pavements in which there is no significant deterioration developed in the base layers (Subgrade/Embankment) over the long service life, provided that the top asphalt layer is maintained to a minimum extent. (6)
Porous asphalt pavements are mainly designed to manage stormwater. Porous asphalt pavements are mainly used for parking lots in which they permit water to pass through the pavements base layers and will infiltrate into the soil below it. (7) I hope the blog provides you with adequate knowledge of Hot Mixed Asphalt and its associated features.
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(1)https://www.mdpi.com. (2)https://www.engineeringcivil.com/stone-mastic-asphalt.html (3) https://cdn.ymaws.com/www.asphaltisbest.com/resource/resmgr/MAPA-Asphalt-Paving-Design-G.pdf (4) https://www.morganpavement.com/blog/how-much-does-asphalt-pavement-cost/ (5)https://www.mdpi.com. (6) https://www.asfaltblij.nl/media/1315/2007-long-life-asphalt-pavements.pdf (7) https://www.sciencedirect.com/topics/engineering/porous-asphalt
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