Basic knowledge Asphalt paving 06/2010

Material flow The asphalt mix is delivered into the receiving hopper (1) of the paver as it pushes the rear tyres of the haul truck (not shown). The mix is carried from the hopper to the rear of the machine by twin or single conveyors (2), and then on to the auger (3) which distributes the mix laterally over the entire working width of the screed (4). 1 2 3 4

Material flow The material flow is regulated by the speed of the conveyors (2), and the auger (3). The conveyor speed is automatically correlated to the forward speed of the paver and the height of the material which is spread out ahead of the screed. This height has to be kept as constant as possible. 1 2 3

The screed The screed levels and pre-compacts the asphalt mix to a specified thickness. To produce an even surface - even if the underlying base is somewhat irregular - the screed is constructed as a “floating” working tool. Therefore the screed (1) is attached to the paver by side arms (2) at tow points (3) located on either side of the paver. Via the tow points the angle of the screed can be varied and thus cause a slow rise or fall of the screed. 2 1 3

Angel of attack The angle between the bottom plate of the screed and the surface being paved is known as the angle of attack. The angle of attack may be increased or decreased by raising or lowering the tow point level. Any movement of the tow points results in a rise or fall of the screed by getting more or less material under the screed. lower angle flow down higher angle flow up

Effects on the level of the screed Effects which cause a height-change of the screed Screed flow up by “Raising” of the tow point Increasing material feed Increasing driving speed Decreasing material temperature Screed flow down by “Lowering” the tow point Decreasing material feed Reducing driving speed Increasing material temperature Screed stays level when all factors remain constant

Characteristics of screed levelling affected by material (& temperature)! no direct reaction – slow acting! no regulation without machine forward movement! affected by driving speed high, constant ,vibration

Use of automation To ensure all influences that can affect the screed level are kept as constant as possible, different sub-ranges can be automated. Thus easing the number of tasks the operator has to manage and therefore creating good conditions for a continuous, high quality, finish. Automation is available for the following functions : Material (feeder) control The amount of material - transported by the conveyor and auger – is observed by sensors. All detected changes cause a reaction to the valve outputs, thus slowing down or speeding up the feed of material. Screed levelling The height and cross inclination of the screed are adjusted by grade- and slope-controllers following a default reference (string line, ground, curb, laser, . . .). Thus irregularities - in the sub-base material or caused by movements of the tractor - can be compensated for. Ignition-/Temperature control Sensors detect the actual temperature of the screed and, when required, activate heating (gas ignition) to the set value.

Why construct using several layers ? The aim is to produce an even, homogeneous surface. To minimise irregularities the asphalt is usually deposited in several layers.

Why construct using several layers ? Using a good reference to control the height: The irregularities minimise layer by layer 5 cm Foundation: Height difference: 5 cm 5 cm 10 cm Base layer: 5 (10) cm after additional roller compaction of 20% Height difference: 1 cm 1 cm 3 cm 4 cm Second layer: 3 (4) cm after additional roller compaction of 20% % Height difference: 2 mm 2 mm 2 cm Top layer: 2 cm after additional roller compaction of 20% Height difference: 0.4 mm 0.4 mm

Why construct using several layers ? Overcoming problems during street maintenance - With only one layer to remove the irregularities 5 cm Surface Height difference: 5 cm If constructed with a constant thickness: can only copy a bad profile 4 cm 4 cm Layer: 4 cm after additional roller compaction of 20% Height difference (remains): 5 cm 5 cm Using a good reference to control the height : all but 20% of the irregularities can be removed 4 cm 9 cm Layer: 4 (9) cm after additional roller compaction of 20% Height difference (now only): 1 cm 1 cm