Compaction of, asphalts

The compaction of asphalt layers is possibly the most critical stage of asphalt works. It is needed to achieve proper and uniform compaction, which in turn ensures a better long-lasting performance. 

During compaction, the coated aggregates are compressed, are re-oriented and take such positions that the distance between them becomes the smallest possible. As a consequence, the air voids decrease and the mixture density increases. 

Because of aggregate re-orientation, the stability of the mix and the strength of the asphalt and of the pavement increase. 

The reduction of air voids results in a mixture that is less pervious to air and water and increases the stiffness of the asphalt. The reduction of permeability is mainly due to the fact that the number of interconnected voids decreases. In case of under-compaction, the number of interconnected voids is relatively large. However, the number of interconnected voids may also increase in case of over-compaction. This is more predominant in dense-graded asphalts owing to the development of internal hydraulic pressures related to the excessive reduction of available space (voids). Considering the above, it is obvious that an optimum void content, not the minimum possible, is required to be achieved during compaction. 

The reduction of permeability attributed to proper compaction has the benefit of not only permitting smaller water quantity to pass through the layer to the underlying layers, bearing the well-known negative results, but also allowing less air to move through the asphalt. As a consequence, the rate of bitumen oxidation decreases. 

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As a general rule, compaction of asphalt with grade bitumen should never start when the mix temperature is less than 85 C to 90°C. In most cases, insufficient compaction is attributed to the low mix temperature starting the compaction. However, there may be the case that the temperature of the asphalt before compaction is high and the mix cannot sustain the weight of the roller. This situation is solved by simply waiting the temperature of the mix to drop, which only delays compaction procedure. 

To achieve proper and effective compaction of asphalt layers, the following points are recommended ... 

Shell Bitumen. 2003. The Shell Bitumen Handbook, 5th Edition. London Thomas Telford Publishing. Tunnicliff D.G. 1977. Symposium on vibratory compaction of asphalt pavements. Journal of the Association of Asphalt Laving Technologists, Vol. 46, p. 259. 

The common form of asphalt is a black, compact, amorphous, brittle mass of dull luster, which breaks showing a polished surface and fuses at 188-90°F (ca 87°), d 1.40-1.42 at 77°F (25°C) and Moh s hardness 1 to 2... 

Figure 20. Placement and compaction of direct-substituted sulfur-asphalt...

Mixing of the asphalt mixture materials was conducted for 3 minutes at 152 to 157°C in a mixer. Compaction of the asphalt mixture into a mold was conducted at 140 to 144°C with the Marshall test rammer or a roller compactor. 

It is mentioned that nuclear devices may also determine the compacted density of asphalt layers. In this particular case, the method of determination is specified by ASTM D 2950 (2011). 

West C.R., D.E. Watson, P.A. Turner, and J.R. Casola. 2010. Mixing and Compaction Temperatures of Asphalt Binders in Hot-Mix Asphalt. NCHRP Report 648. Transportation Research Board. Washington, DC National Academy of Sciences. 

Additionally, unlike the European specification CEN EN 13108-1 (2008), the compaction of the specimens under the Superpave design methodology is restricted to gyratory compaction only. This is a positive step since it simulates better the compaction carried out on site and eliminates variations on the mechanical properties of asphalt mixtures attributed to type of compaction. 

However, the variety of WMA additives/systems certainly needs to be addressed in a future specification regarding the batching, mixing, conditioning and compaction of the WMA mixture. The required additions/modifications could easily be implemented in Appendices or Annexes to the current HMA specifications. It is believed that the expected revision of the European specification for hot asphalts in 2015 will include WMA or HWMA mixtures. Such a proposal has already been made and carried out by NCHRP Report 691 (2011) for the AASHTO R 35 (2012) specification. 

CEN EN 12697-32 2003-tAl. 2007. Bituminous mixtures - Test methods for hot mix asphalt - Part 32 Laboratory compaction of bituminous mixtures by vibratory compactor. Brussels CEN. 

LAYING AND COMPACTION OF COLD ASPHALT MIXTURES 6.6.1 Laying... 

Compaction of the cold asphalts begins when the emulsion starts to break (the colour of the mixture changes from brown to dark-brown/black) provided the mixture can support the roller without excessive displacement. 

Finally, the fracture energy, Gf, of asphalt-aggregate mixtures using the disc-shaped compact tension geometry may also be used to describe the fracture resistance of asphalt concrete. The test is performed in accordance to ASTM D 7313 (2013) specifications. 

Determination of fracture energy of asphalt-aggregate mixtures using the disc-shaped compact tension geometry... 

Production, transportation, laying and compaction of hot mix asphalts 409 8.5 TRANSPORTATION OF HOT MIX ASPHALTS... 

The screed is perhaps the most important part of the asphalt paver. It is essentially a wide flat piece of heated steel that levels and pre-compacts the asphalt placed in front of it by the augers to a specific width, grade and cross slope or crown profile. 

The advantages of high-compaction screeds are as follows (a) they minimise the compaction effort on subsequent compaction equipment to achieve the required densities, (b) they open up the possibility of single-layer construction of thicker layer and (c) they minimise the effect of environmental factors on the cooling of asphalt mat. 

Figure 8.14 The effect of air voids obtained during compaction on the durability of asphalt concrete layer.

The grade or type of bitumen and its quantity in the mixture are the major factors affecting asphalt compaction. Each grade bitumen, at a specific temperature, has its respective hardness and viscosity. As a consequence, when hard bitumen is incorporated into the asphalt, the compaction at a given temperature is more difficult than the one of asphalt containing soft bitumen. 

Finally, the increase of bitumen content has, up to a point, a beneficial effect on the asphalt workability/compaction. This derives from the fact that the bitumen operates as a lubricant medium between aggregates and decreases aggregate-to-aggregate friction during compaction. However, asphalts rich in bitumen, if they also contain harder bitumen, are more difficult to compact. 

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