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| Tarmac - Construction Layers |
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| This page looks at the main layers within a tarmac pavement. A more general overview of the use of tarmac/bitmac is given on the Tarmac Basics page and a case study of the actual construction methods for a footpath/driveway can be found on the Laying Tarmac Paths page. |
IntroductionThe construction methods considered in this section refer to three typical small projects - a footpath, a residential driveway and an access road for small-medium vehicular loads. For heavier applications, you should consult a reputable local contractor, or email us for a more detailed construction specification.The construction of a typical bitmac footpath is covered in more detail on the Laying Tarmac - Drives and Paths page. There should be at least 4 layers to all tarmac pavements: |
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The sub-base and base course layers may be constructed from more than one individual layer of the specified material, but for our purposes, we will consider each layer as being a single entity.
Sometimes, levels permitting, an existing tarmac or concrete surface can be overlaid with a new wearing course. This is acceptable provided that the new surfacing is properly bonded to the old surface. It is not good practice to overlay existing flags/slabs. Overlays are considered in more detail on the Tarmac Basics page. |
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Construction MethodMark out the site with sand guide-lines, if required. It is assumed that the surfacing is to be flush i.e. level, with the existing ground. It is a good idea to dig wider than the planned width, allowing an extra 300-450mm at the edges makes handling much easier, especially if kerbs are to be used.See Setting Out page. In most cases, a well-laid tarmac surface is impermeable to water, and should be constructed in such a manner that surface water will be directed to a gully or similar drainage point. There are pervious or permeable bitmacs available from the batch plants, and while they do have their uses, they are not considered any further in this section.
ExcavationThe surface needs to be dug off to a depth of at least 175mm. The depth of dig can be roughly calculated as....(wearing course + base course + sub-base) |
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Specification Table |
So, for the three projects under consideration, the dig depth is given in the table opposite. All figures are in mm and are typical values we have used for tarmac work in the past 20 years. Conditions in other parts of the country vary and a local contractor may suggest differently, based on their experience. |
| All weeds and other unwanted organic matter, along with topsoil must be removed and any soft spots excavated and filled with compacted sub-base material. If the area is troubled with weeds, it should be treated the excavated sub-grade with a general weedkiller such as Sodium Chlorate, but it is unlikely any weed will be able to penetrate the upper layers. A Geo-membrane can be used if deep-rooted, pernicious weeds are a problem, of if there is any concern regarding the competence of the sub-grade. | |
| Edgings or kerbs MUST be used on free edges, i.e. those parts of the perimeter of the surfaced area not bounded by walls or other solid structures that will act as a retainer for the bitmac. This is to prevent the tarmac crumbling at the edges, as shown in the photograph opposite. Brick edgings, plain or decorative edging kerbs, or setts laid lengthways are all suitable, and should be constructed at this stage. |
No edging/kerb = crumbling edges |
Sub-baseThis is essential if the pavement is to last longer than a couple of years. Do not employ any contractor who tells you that a sub-base is not required, unless there is a suitable existing sub-base or base layer. The sub-base should be a minimum 100mm thick. Refer to the sub-base page for full details on installation of a sub-base. |
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Prior to final surfacing with the wearing course, which may be several weeks or months later, the base course needs to be painted with a bonding emulsion to ensure proper adhesion.
For more information, see the notes on overlays on the Tarmac Basics page. |
Wearing course |
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| This is the top layer of the tarmac, the layer that is seen and trafficked. It needs to be fairly regular to provide a smooth ride for wheeled vehicles, although this is much more important on higher speed pavements than on residential driveways. A macadam wearing course should consist of a small, hard aggregate, usually 6mm or 10mm, in a tar or bitumen binder. Alternatively, an asphalt may be used; this material is also known as a sand carpet or asphalt carpet, and is prepared in a batch plant to a specified recipe, consisting of selected sands and grits mixed into an asphalt matrix, with coated chippings sprinkled over the surface and rolled into the asphalt as part of the compaction process. Again, it may be machine or hand laid, depending on area and access. |
Hardstone 6mm Dense Wearing Course |
| The wearing course should be at least 20-25mm thick when rolled, and should not deviate from the correct level by more than ± 6mm. There should be no roller marks in the finished surface. We recommend a minimum fall of 1:80 for tarmac areas, although a fall in the range 1:40-1:60 is preferred. | |
Limestone Wearing Course |
Limestone based wearing courses will wear over time to give a greyish appearance to the surface, as the tar binder is gradually eroded by traffic and exposure to the elements, to reveal the grey-white limestone aggregate.
Hardstone based wearing courses, such as that shown above, are generally much darker, relying on a basalt or other hard, dark aggregate, and so when they wear and weather, they tend to remain dark in appearance. |
A Touch of ColourColoured macadam wearing courses are available in two basic types. In the cheaper type, the binder (tar/bitumen) is coloured (e.g. red), but the aggregate is the same as for a normal black wearing course, e.g. a limestone or hardstone. This type of wearing course may well lose its colour over time when the aggregate becomes more and more exposed as the binder is gradually worn away. |
Weathered red wc next to plain limestone wc - not very red, is it? |
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| The best quality coloured wearing course material will use a coloured aggregate that matches the colour of the binder. For example, a good red wearing course will comprise a red binder with a red aggregate, such as quartzite. Then, as the binder weathers, the aggregate will ensure the surface doesn't lose the colour for which you have paid a lot of money! |
Red quartzite wc |
Red binder wc |
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The latest developments in binder technology have now made available highly coloured wearing course materials, such as the stunning red, gold and green surfacing supplied by Bardon Aggregates to the Millennium Dome and Buckingham Palace in London.
Although this ability to create custom colours for individual jobs has not yet filtered down to the 'average' residential driveway market, it is predicted that these new colours could reclaim some of the civic paving market that the bitmac manufacturers have lost to block paving over recent years. |
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| The chippings may be artificially coloured by coating them with a pigmented binder, or they may rely on the natural rock colour, eg a red granite, to introduce colour to the surface. In all cases, the chippings are pre-coated to ensure proper adhesion to the surface layer. Uncoated chippings are NOT suitable. |
HRA with red-coated chippings |
Chippings in the Wearing CourseOn the subject of chippings, these are sometimes added to residential driveways for decorative purposes. However, there are a couple of drawbacks with this practice. Un-coated chippings will not adhere properly to the wearing course, and, with macadam (rather than HRA) wearing courses, there is a problem ensuring the chippings are properly rolled into the surfacing material and thereby held firmly in place. For this reason, we prefer not to use chippings on this type of work, but, if required to do so, a 3-6mm fine-grained dense macadam wearing course would be used and lightly sprinkled with the chosen chipping which will generally be 6-10mm in size. The practice of scattering 18-20mm unwashed limestone chippings over a 6mm or 10mm macadam wearing course is not recommended and is often a trademark of the cowboy element. |
Finishing-offAll bitmac and asphalts are consolidated by rolling. In most cases, a mechanical roller suitable to the size of the job will be used. This may be a single-drum walk-behind roller, a ride-on twin-drum roller or, on the larger jobs, a full-sized, triple-drum road roller (still fondly referred to as a "Steam Roller" by some members of the public). All modern rollers offer a vibratory option, which can be useful to ensure thorough consolidation but needs to be used with care on wearing courses at it can result in causing the drums to skid which may mark the surface. |
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Only in very constricted areas will a hand-roller (pulled or pushed by the operatives) or vibrating plate compactors (wacker plates) be used. However, an asphalt punnel may be used around edges, gullies, manholes etc. to hammer down the surfacing to the required level.
Tarmac should be sufficiently cool and hard enough to be trafficked 1-6 hours after being laid. However, some hand-laid tarmacs contain a 'cutback' agent that retards the setting and may need to be left overnight to thoroughly cool and harden. The construction of a typical bitmac footpath is covered in more detail on the Laying Tarmac - Drives and Paths page |
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Jointing TarmacA wearing course on a footpath or driveway is usually laid as one expanse of tarmac, however, there may be situations where the new wearing course is to abut an existing surface, or where the final surfacing can not be completed in one operation. On wider areas, the surfacing may be laid as a number of parallel strips, each 2m to 6m wide. |
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| In such situations, the 'joint' between adjacent surfaces should be neatly cut, primed and sealed with a proprietary compound, referred to as 'cold pour' or similar. This is applied to the joint to prevent water penetrating beneath the wearing course, where a freeze/thaw action can result in the wearing course being 'lifted' from the base beneath. Where a multi-layer pavement needs to be jointed, the joints need to be staggered by 300mm, not created as a single full-depth joint through the pavement. |
Jointing |
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Coverage RatesCoverage rates are variable according to the thickness of each layer. Tarmac is sold by weight and the standard coverage rates we work with are...
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Construction Diagram |
British Standards covering tarmac, bitmac, HRA, etc. |
Other Tarmac resources on this site... |