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| Laying Tarmac - Drives & Paths |
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IntroductionThis page looks at the methods used in constructing a tarmac footpath. A more general overview of the use of tarmac/bitmac is given on the Tarmac Basics page and an examination of the main layers within a pavement can be found on the Tarmac Construction page.Although this sequence of photographs depicts the construction of a public footpath, the same procedures, in the same order would be use to construct a bitmac driveway at a private residence. Pavements for heavier usage, such as carriageways or car parks will probably include extra layers (such as a Roadbase) and each layer will be thicker than that used in this example. As a general rule, all bitmac surfacing should be machine laid by a paving machine unless there is good reason not to do so or if the pavement is intended for foot traffic only. On this particular project, machinery including a JCB-type excavator and a narrow-width paver machine have been used on this project; hand-working and/or smaller kit may be used on a driveway job. |
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Excavation |
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After the setting-out work is completed, the ground is cleared and excavated to Formation Level. Assuming there is no bad ground or other complications, Formation Level for a typical footpath or driveway would be....
(sub-base + base course + wearing course) =
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| The excavated material is carted away to elsewhere on the site or to a licensed off-site tip. Due to the ever-increasing cost of landfill, and the recently introduced landfill tax, contractors and developers try to keep all material onsite to minimise costs. Excavated material may be spread out over an open area, or mounded into a low hill and then landscaped. | |
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In wet weather or in winter, the sub-base material, in this case, a crushed limestone [DTp1], may be placed before the edgings are laid. This helps keep ensure the kerb-layers are not bogged down in the mud and, more importantly, help ensure the laying and haunching concrete do not become contaminated with mud or earth.
Some projects may use a geo-membrane at Formation Level between the sub-grade and sub-base, but it is not essential unless bad ground or pernicious deep-rooted weeds are a problem. The sub-base material is levelled out roughly. It will be levelled out more accurately once the edgings are in place. |
Lay Edging Kerbs |
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The edging kerbs are laid on a bed of suitable concrete with at least 75mm depth beneath the base. See Laying Edgings for fuller details.
Once the kerbs are in place and have been checked for alignment, they are haunched front and rear. The rear haunching should be at least 100mm thick and is always more substantial than any haunching at the front. With some types of paving, front haunching is avoided, as it can lead to problems with differential settlement, but, for bitmac paths that aren't expected to take regular vehicular traffic, it is usually allowed to pass. |
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Sub-baseOnce the concrete haunching to the edgings has set (usually at least 24 hours) the sub-base material will be placed (if not already in plcae), levelled out with shovels and/or rakes, topped-up as and where required, and then be compacted with a mechanical roller. Some smaller jobs may use a vibrating plate compactor to consolidate the sub-base, but, as a roller is essential for later compaction of the bitmac layers, there is normally one available on site and besides compacting the sub-base much more thoroughly than many plate compactors, they are considerably faster. |
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The accuracy of the sub-base layer will be checked at regular intervals by placing a straight-edge timber from the road kerb to the edging kerb and 'dipping' - measuring the distance between base of straight-edge and the consolidated surface of the sub-base. For most jobs, the sub-base is expected to be accurate to ± 10mm. Although a tape measure can be used to check the 'dip', we find that requires far too much bending to read the tape. |
| A noggin of timber cut to the required depth (in this example, 75mm) can be fastened to the base of the straight-edge and used to judge the dip - if the sub-base is too high, the ends of the straight-edge won't sit on the kerbs; if too low, the noggin will be carried above the surface of the sub-base. |
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Base CourseThe base course is the first of the two bitmac layers used in a typical footpath or residential driveway. The aggregate in this type of tarmac is chunkier than that used in the wearing course, a typical size being 20mm. The base course material is delivered to the site in insulated wagons which keep it hot, and it is then laid as quickly as possible, before it gets too cool and becomes unworkable.The type of job normally dictates how the bitmac will be laid. If being machine-laid, the delivery wagon empties the bitmac into the paver machine as it progresses along the run. The paver machine re-heats the bitmac before spreading and screeding it level in one continuous operation. |
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| One hand-lay jobs, the bitmac is normally treated with an oil-based compound known as "cut back" that helps to keep the bitmac workable for longer at lower temperatures. The bitmac will be tipped off the delivery wagon onto a clean surface, such as the existing roadway, and then sheeted over with large tarpaulins that help retain the heat. A JCB or other Loading Shovel may then feed the bitmac to the laying gang, tipping it out as needed to each section of path, where it is raked level by the rake-hands who use their skill and experience to judge the accuracy of level. |
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On smaller jobs, it may not be feasible to use a Loading Shovel and so the bitmac may be manually shovelled from the sheeted pile into barrows and then wheeled to the laying edge by the labourers who tip it out where it is needed as directed by the Rake-hands.
The carrying bucket of the Loading Shovel or the skip of a wheelbarrow will be 'painted' with red diesel (fuel-tax exempt and so relatively cheap) as this helps prevent the hot bitmac sticking to the cold metal and building up over time into a solid, immovable mass. Similarly, the rakes and shovels will be occasionally dipped in diesel to help prevent the bitmac sticking and the Rake-hands like to keep their tools hot, heating them over a flame or fire-bucket, as this too helps keep them from becoming clogged and unmanageable as the bitmac cools. The base course is rolled as soon as it is laid. It will take at least half-a-dozen passes with the roller to consolidate the still-warm bitmac. To prevent the bitmac sticking to the roller, water is continually dripped over the drums from an on-board storage tank. This is the cause of the steam often seen rising from freshly-laid bitmac and asphalt surfaces. For the base course, the roller driver will continue rolling continually until there are no roller marks in the surface. The compacted level of the base course is required to be accurate to around ± 10mm. |
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Wearing Course |
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Finally, the wearing course is laid. This contains much smaller aggregate than the base course, intended to give a finer, smoother finish. The wearing course is laid in much the same way as the base course, except even greater care is taken with levelling as this is the final layer and will need to be accurate to around ± 6mm.
Machine-laid wearing course material is accurately levelled as a matter of course, but with hand-lay jobs, the Rake-hands will often use a 'Lute' (has no teeth, gives smoother finish to dense wearing courses) to spread and level the wearing course material, especially when working with Dense material. The newly laid bitmac can be walked upon immediately after rolling. For residential driveways, it is often wise to alolow and hour or so for the wearing course to completely cool before trafficking with vehicles. |
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Also, it should be noted that with driveways, extra care should be taken with cars and other vehicles for the first week or so, especially if a "cut-back" wearing course has been used. Wheel spins, sudden acceleration or deceleration, and sharp turning at speed or "on the spot" can scuff or "tear" the wearing course. It can be impossible to invisibly repair such scuffing, and no contractor is likely to accept any liability for damage caused in this way.
Finally, the site should be cleaned and cleared of all detritus, dregs, bits of spilled or surplus bitmac, and any other construction materials. The haunching to edging kerbs should be covered over with topsoil, turf or other suitable material, taking care not to contaminate the new surface. |
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Other Tarmac resources on this site...
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