John Wainwright - Surfacing

Surfacing

After decades of being a supplier of materials, the company made a decision to start a surfacing division. Offices were put in place, skilled labour and staff were employed and the department opened for business on 8th January 2001.

THE LAYING OPERATION – MACHINE LAY

A considerable process goes into manufacturing, high quality material and then delivering it to site, then an equal amount of effort needs to go into producing a finished road surface with the material. The potential for problems before finishing the final surface is great and includes weather, traffic, and mechanical breakdown. The best way to counteract these problems is to have a skilled workforce and well-maintained equipment.

Preparation prior to surfacing can be part of a new construction on top of sub-base, overlaying of an existing surface or the old material removed by milling.

Milling

Milling machines can cut widths from 0.3m to 2.5m and to a depth of 330mm in one pass. They consist of a rotating drum fitted with renewable tungsten carbide teeth. The material once removed travels up a conveyor and is then deposited in the back of a lorry.

The Paving Machine

The Paving machine will receive the material from the lorry and place the material at the correct thickness and width onto the prepared surface forming a “mat”. This mat is then compacted by various types of roller depending on the nature of the work.

The Paver consists of a tractor unit and a screed. The tractor unit moves forward pushing a lorry containing the material and towing the screed.

The screed floats on the material delivered to it by conveyors from the hopper, through augers distributing the material along the width of the screed. The angle of the screed is adjusted by hydraulically moving the side arm towing the screed up or down, the angle of the screed determines the thickness of the mat. The width of the screed can also be adjusted hydraulically from its normal 2.5m. The screed must be heated otherwise the material sticks to the cold metal; this causes the material to be dragged producing a very poor finish to the mat. The screed is also fitted with a tamping bar this oscillates in front of the screed plate tucking the material under the leading edge and giving the mat a degree of initial compaction. Without the tamper the mat would tear a producing holes and a poor finish.

The paving machines are very much a variation on the same theme and can be fitted with additional levels of sophistication. This may include automatic level systems, integral spray system to supply a tackcoat between layers and be wheeled with one or two axles, or have tracks.

Compaction

The life of the new mat will depend on the correct amount of compaction during construction being achieved. The binder on the hot material acts as a lubricant allowing the particles of aggregate in a given mix to move and finally lock together. This gives the material its full strength and in the case of the Surface Course produces an imperious layer protecting the underlying construction from the elements.

Typically the compaction is achieved from two types of roller, the dead weight and the vibrating roller. There are pneumatic tyre rollers, but these are used in more specialised circumstances.

Normally 3 points, which relate to the 3 smooth barrels operating between 9.5 to 15 tonnes. The weight can be adjusted by ballast either water or sand in the barrel wheels.

Vibrating rollers range from a small, single drum pedestrian roller to large twin drum rollers. The compaction is achieved through a combination of their static weight with dynamic loading from the vibration mechanism housed within the drum.

What ever type of roller is used, the success depends on good material at the right temperature to receive full compaction and the skill of the roller driver. The roller will need to make a number of passes over the mat, in the correct pattern also closing up joints between laid sections. Finally with the Surface Courses removing marks and blemishes.

Hand-lay

For the smaller or inaccessible areas too small for larger machines, hand-laying material is the answer. This involves depositing the material in the correct area from the back of the delivery vehicle or taking the material by barrow to it destination. The material is then raked to its correct level before it is compacted by a roller. All the above rules for compaction apply and it is especially important with a small amount of material laid thin to keep the material temperature sufficiently high. Hand-laying material is a highly skilled job, consideration must be given to the final level producing a finish, which will allow water run off and not leaving puddles.