Bound surfaces

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The surface laying methods used for most bound surface applications fall into three categories:

Where the surface layer is to be machine laid, the base layer must be wide enough and capable of supporting the weight of plant and tar lorries. Path widths less than 2m can be difficult to lay using paving machines. Path construction sites where the ground adjacent to the path may be soft can be problematic - a machine or lorry can get stuck if it moves onto a soft verge.

Always specify the tolerance of surface eveness for bound surfaces - less than 5mm gap under a 3m straight edge placed along the path surface provides a smooth and even surface which will prevent puddles forming, and pleasant surface for all path users to enjoy safely.

Materials

Surface dressing (tar spray and chip)

surface dressing

There are two methods for laying surface dressing:

Single layer

The simplest and cheapest method is a single layer of bitumen emulsion spray and one layer of washed stone chippings rolled in. This is a particularly useful way of upgrading an existing unbound or semi-bound surfaced path or even worn bound surface at a relatively low cost. Alternatively, a new path which has both level and sloping sections can be an unbound surface throughout its length, and then the sloping sections which are susceptible to surface water scouring can be sealed with a single surface dressing layer.

Double layer

A more substantial surface layer is a two layer application i.e. bitumen emulsion applied to the base layer with washed stone chippings rolled in and then a second layer of bitumen emulsion and washed stone chippings are laid and rolled in. The double layer will be a strong, homogenous surface which will be harder wearing and less susceptible to washout than a single layer. There are fewer loose stone chips on a two layer surface than on a one layer, if properly laid.

 

Specification
icon Surface Dressed Path - Standard Detail Drawing & Specification Details

 

Fibredec

‘Fibredec’ is a proprietary variation on the two layer method. It has a layer of chopped glass fibres between the two layers of proprietary bitumen emulsion binder, and a top layer of washed stone chippings to provide a high tensile strength, crack resistant thin surface layer on top of new base layer or existing surface layer.

The new formation layer or worn surface layer must be treated with weedkiller prior to laying surface dressing or Fibredec. Weeds may break through causing the thin surface to crack. Use a residual weedkiller subject to environmental considerations.

Surface dressing and Fibredec quality is dependant on how well a new base layer is laid or how smooth and even the existing worn surface is. Specify a tolerance of surface evenness - maximum gap of 10mm under a 3m straight edge placed along the surface with no high or low points or hollows. Ideally lay the new base layer or regulate the worn surface with a drag box to achieve a smoooth and even surface. Likewise, if as dug or recycled secondary aggregate materials are used to build the base layer, regulate with 100mm depth of Type 1 granular sub base to provide a strong even surface layer suitable for surface dressing or Fibredec. If the regulation layer lacks sufficient fines to provide a smooth surface, blind with quarry dust before laying the thin surface layer.

Bituminous macadam (bitmac)

bitmac

This is a proprietary surfacing technique that requires an asphalt paving machine for high quality laying and finishing. The mixture of bitumen and graded aggregate is laid on a prepared base layer and rolled to provide a durable surface course layer. Bitmac surfaces are best laid with a cross fall rather than camber, to avoid a reduction in surface depth at the edges, where cracking can occur. This type of surface should be laid to defined tolerance margins to provide a high quality finish that justifies the investment.

The bitumen content in making bitmac can vary slightly in hardness. This is measured in terms of the bitumen “penetration grade” (measurement of how deep a standard needle penetrates the hardened bitumen at a standard temperature under a standard pressure). Higher penetration grades (where the needle penetrates deeper) are softer and generate a smoother and more forgiving surface. Typical penetration grades for shared use paths are 160/220 pen (softer) or 100/150 pen (harder). The other main variation in bitmac surfaces is the graded aggregate size that is used. Larger aggregate sizes make a porous surface, which allows surface water to drain through rather than flowing across the surface. Smaller aggregate size gives a smoother surface, which is usually preferred by wheeled users, particularly cyclists. Standard maximum aggregate sizes are 20mm, 14mm, 10mm and 6mm.

Brushing whin dust into the surface, immediately after laying, will reduce the initial visual impact of a bitmac surface. The dust will eventually wash off, by which time the bitmac will have faded to a grey colour. Alternatively, dress bitmac with 3mm grit, rolled in before the bitumen binder hardens or lay coloured stone chippings rolled into a bitumen emulsion layer (as described in ‘Surface Dressing (tar spray and chippings)’ above). Coloured bitmacs are available but they are extremely expensive and the colour will fade quite quickly leaving the exposed grey aggregate.

 

Specification
icon Dense Bitumen Macadam (DBM) Path - Standard Detail Drawing & Specification Details

 

Hot rolled asphalt

hot rolled asphalt

Hot rolled asphalt is a surface course layer used occasionally for path surfacing but is commonly used for road or pavement surfaces. It is a bitumen and sand mix that relies on the presence of hard stone chippings being embedded into the surface to provide durability and traction.

Hot rolled asphalt surface course layer is usually paver laid on top of a binder course, with bitumen coated stone chippings scattered over the newly laid surface at a specified rate, and compacted into the asphalt with roller when still hot.

 

Specification
icon Hot Rolled Asphalt Path - Standard Detail Drawing & Specification Details

 

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