Technical Article. Pre-compaction/ Speed of Paver/ Paving Angle.
Asphalt pavement compaction is one of the most vital phases during road construction, which can determine the service quality as well as durability of the pavement. During the process of pavement construction, the compaction phase consists of two main stages. The first stage is compaction by a paver; it can be called the pre-compaction or paving compaction. The other part is the finishing by a different combination of rollers (still, vibrating, pneumatic, etc). Therefore, the required density of the pavement compaction can be attributed to the initial density compacted by the paver and the final density behind the rollers.
According to construction guidance from different countries, the evaluation of the quality of pavement compaction normally relies on the required density, and is always assessed after the final compaction finished by the rollers. However, during the compaction the dominant density and strength of asphalt pavement has already been reached after the paver, the preliminary frame structure of mixture has been formed as well.
The mechanical behaviour of the pavement is directly determined by the skeleton, and its durability will be consequently affected by the quality of the paver compaction. On the other hand, if the pavement was constructed with inadequate paving compaction, the evenness of the road surface would be influenced, as there can be wave asymmetry deformation caused by rolling compaction. Adequate pre-compaction from a paver can guarantee the overall quality of compaction, as well as being very helpful for rolling compaction (e.g. reducing the passes of roller compaction).
One of the core parts of a paver is the screed, which works as a vibrating/tamping compactor, paving the material using its own weight. Meanwhile, the screed is connected to the main body of paver through the screed tow point, this articulated point makes it capable for the screed to pave and compact material with a floating function. The floating function of the screed can diminish the irregularities on the ground when the paver passes over. In addition, the thickness of the paved material can also be liberated adjusted, altered by the screed tow point when adjusting the paving angle.
Generally, a screed of the paver consists of tamper and screed plates, which accommodates the compacting systems to provide high density, a smooth surface, and durable results. The tamper, working with a higher vibration is supposed to provide pre-compaction of the laid material, while a relatively lower vibration was imposed to the screed plate to vibrate the paving layer for achieving an optimal even surface. Another function of the tamper is to improve the bearing capacity of the laid material so that the screed can float well on the paved material. If the paving speed was increased without considering matching the frequency of tamper, there would be a change in uplift of the screed, and thus the paving thickness would decrease as well. On the other hand, increasing the tamper vibration without matching the paving speed would result in a rise of the pre-compaction, the paving thickness would be altered as well.
As for the paver, the paving angle is also very important during paving compaction, the optimized angle of paving compaction indicates the optimised position of the tamper. Besides the asphalt mixture, other materials such as the subbase of pavement, concrete, or railway ballast can also be paved by the paver with special tamper profiles. Theoretically the operation of the screed vibration and paving speed are also determined by the type of material, the mixture gradation, and paving temperature, as these factors have direct influence on the quality of paving compaction.
Examples of Screed Angle (the good, bad, and ugly)
1/Screed working within its parameters, laying 400mm depth with good level of pre-compaction. (note screed angle – parallel to ground).
2/Too much Screed Angle - extensions trailing their rear edges. Paver is trying to lay depth and not floating or not adjusted correctly (Seen when paver is operating outside of its normal range).
3/Side arms at their maximum travel, because of screed unstable due to stability of mix – not floating efficiently, resulting in angle of screed to severe with tamper angle not optimise and limited depth achieved.
Extensions leaving lines (Rear edges on extensions trailing).
Greater the depth being laid - greater the importance of all the factors previously mentioned
Screed angle = Tamper angle = Maximum Pre-compaction
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