Tar spray and chip path

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What is it?

Two layers of bitumen emulsion and stone chippings on top of a Type 1 granular sub base on geotextile in a formation tray. This bound surface is commonly referred to as 'tar spray and chip', but its correct surfacing term is 'surface dressing'. It can also be directly applied to an existing worn bitmac or asphalt surface, to add surface texture and grip.

tar spray and chip path oatridge college

How does it work?

Surface dressing creates a stable mosaic of stone chippings, securely attached to the surface by a bitumen emulsion. Quite simply, the bitumen emulsion glues the stone chippings to the surface. The chippings may look loose, but are firmly held in place by the bitumen.

What are the benefits?

Surface dressing is generally applied on worn road surfaces to reduce the risk of skidding. For paths, it can provide a number of benefits:

  • It is a more durable surface than whin dust in places that get heavy wear and tear, or that might be affected by fast moving surface water.

  • It changes the colour of a 'black' bitmac path and helps to lessen its visual impact. The finished look depends on the colour of the stone chippings used. Light grey chippings are much the same colour as a grey whin dust path surface.

  • It creates a non-slip surface for most path users, improving grip for horses and skid resistance for cyclists.

  • It restores surface texture and seals minor cracks in a worn bitmac or asphalt surface. This protects the path's base against the ingress of water, slows down the deterioration of the path and prolongs its life.

  • It keeps maintenance costs reasonably low.

Is it suitable?

Surface dressing only works on well-compacted sub base surfaces that are smooth and even, because the finished surface treatment is thin. It is liable to crack if the sub base surface is loose and uneven. The time period available for laying surface dressing is very narrow, only the summer months are really suitable, as there can be a risk of surface failure when temperatures are low. It also needs to be laid by experienced surfacing contractors, who are proficient in its application. Good access to the path for specialist plant and machinery, such as spraying equipment, is also important.

How much will it cost?

Surfacing dressing may cost between £15 - £35/ m² in addition to the cost of building the path's sub base.

How do you install it?

Surface dressing involves either single surface dressing or double surface dressing over a prepared surface, such as a sub base, worn bitmac or asphalt. If applied directly to a sub base, two layers of surface dressing are required to make a durable surface. On top of an existing worn bitmac or asphalt surface, one layer of surface dressing is adequate.

Before laying surface dressing, treat the new sub base or existing worn bitmac or asphalt surface with weed killer – weeds may break through the thin surface and crack it. Use a residual weed killer subject to environmental considerations.

Surface dressing quality depends on the evenness of the surface that it will be applied to. It is important to specify a surface regularity of a maximum gap of 10mm under a 3m straight edge with no high or low points or hollows. If possible, use a drag box to lay the sub base to achieve an even surface. If the sub base lacks sufficient fines to provide an even surface, blind it with a dust. If 'as dug' or recycled aggregates are to be used to build the sub base, regulate with 50mm depth of Type 1 to provide a strong, even surface that is suitable for laying the surface dressing on.

Before opening the path for use, loose stone chippings must be swept from the finished surface. Loose stone can cause a slip hazard.

Detailed drawing - Tar spray and chip path

If you like the look of the tar spray and chip path at Oatridge College, as a potential path surface for your project, download the detail drawing here icon Tar Spray and Chip Path - Detail Drawing

For a generic standard detail drawing and specification details, download them here icon Surface Dressed Path - Standard Detail Drawing & Specification Details

What variations are available?

'Fibredec' is a proprietary variation on the double surface dressing method. It has a layer of chopped glass fibres between the two layers of proprietary bitumen emulsion, and a top layer of stone chippings to provide a high tensile strength and crack resistant thin surface layer. It can be laid on top of a new sub base or an existing worn bitmac or asphalt surface.

Other types of surface treatment, similar to 'tar spray and chip', that can be laid on top of new sub base or worn bitmac or asphalt surface are called resin bonded surfacing or resin bound surfacing.

© 2014 Paths for All - Registered Scottish Charity No: SC025535, Company Limited by Guarantee No: 168554 inc. 19 Sept 1996 at Companies House, Edinburgh