TARMAC DRIVEWAYS

Tarmacadam or “Bitmac” for short, is often believed to be the cheapest method of surfacing a driveway or forecourt, but for smaller areas it can work out quite expensive, and the red tarmacadam can be more expensive than block tiling. Tarmacadam has become a popular, although technically incorrect, term for both bitmac and for asphalt used to surface pavements, highways and even interior.

Precisely the term “Tarmac” ™ © ® is the name of a public known company and they become irritated with lawyers regarding those who take their name in vain and use it to refer to the material “tarmacadam”, even though the name has entered the dialect in the same was as, for example, “Hoover”, “Biro” and “Sellotape”. Throughout these pages, we have sought to use the contentious term as little as possible, preferring to refer to ‘bitmac’ or ‘macadam’ instead.

Surfacing contractors usually charge by the weight or area of material placed, but for smaller areas, they may charge on a day basis that will include conveying the required equipment to the place. A typical surfacing gang of 4 men (1 x Rake-hand, 1 x Roller Driver and 2 x Laborers) will cost approximately £60-120 per hour + VAT.

Bitmac is suitable for areas to be dealt in by vehicles, such as drives and forecourts, and can be used for paths, although it is not suggested to use bitmac for garden paths. The wearing course (the uppermost surface) can be customized into ‘colored’ by using a colored binder, often with colored collections, or by joining colored, coated chippings into a HRA wearing course.

In general, all bitmac is to be machine-laid by a paver machine except where it would be difficult to use a paver. Such exclusions include small areas (such as some private driveways), narrowed spaces and footpaths. On private works, the contractor will determine the most appropriate laying method to use. In most cases, the finish achieved with machine-laid bitmac is far superior to hand-laid material.

All bitmac should be laid by a professional company as the tools and techniques are not in reach of most DIY enthusiasts. Accordingly, the methods outlined on this site are intended to give the casual reader a guide to good practice, relatively than a step-by-step guide to laying bitmac.

Armed with a thoughtful of how a bitmac pavement or highway should be constructed, it helps you so that you can check that the work being undertaken on your behalf is being done correctly. This is particularly important given the huge number of ‘tinkers’ who roam around, offering to lay bitmac at a discount price – they usually just throw down a thin layer of wearing/surface course, good to know that they will be long gone in a couple of weeks as your new drive starts to fall apart!

Beware of anyone knocking at the door proposing bitmac ‘left over from a big job down the road’ – reliable contractors do not cold call to swish up work!

To try and clear up the confusion and get the terminology correct, here’s a brief explanation….

Bitumen is a by-product of the oil-refining and petro-chemical industries. Very basically, it is the thick, sticky, remaining sludge that’s left over when all the useful fuel oils, enlisted as kerosene, petrol, diesel, etc., have been removed by fractional distillation of a crude oil.

Macadam is a process of binding together smaller combinations, as prepared by the legendary John Mac Adam in the 19th century (although there are tales of the Chinese having used a form of “macadam” as far back as 3000BC). His work resulted in the development of tar-based macadams, which became abbreviated to Tar-mac. Nowadays, we use bitumen from the oil industry relatively than naturally occurring tar, and therefore we now have bitumen macadam or, in the trade, bitmac.

Asphalt, according to British and European (CEN) definitions, is a mixture of bitumen and minerals. However, in the US, they use the term ‘asphalt’ for what we in Britain and the rest of Europe is referred to as a Bitumen. This is the foundation of much of the confusion amongst non-trade professionals as to what’s what and what’s not.

So, when Americans talk of asphalt roads they do not mean roads surfaced with the hard, shiny, black floor covering found beneath the carpets in many newer houses, and, for American readers, when you hear about British and Irish homes having asphalt floors, this is not the same stuff as used to surface your roads and sidewalks. Confusion it creates? Well, it’s good!

Bitumen, the binder used in Bitmac, comes in a variety of “penetration grades”. There are specific trials to determine only the “penetration grade” of a sample of bitumen, but, rather than go into a lot of very technical and insomnia curing detail, it is best to think of ‘pen’ as being a measure of “bitumen softness”, of how long a point load will drop into a block of pure bitumen at normal temperatures.

The pen grade of the bitumen binder has a direct bearing on the ‘hardness’ of the bitmac produced using that specific bitumen, and so we have quite an array of penetration grades, from 15pen, which is pretty damned hard, to 450pen, which is comparatively soft. A ‘softer’, more flexible binder produces a softer, more flexible bitmac. Imagine trying to shove the blade of a Phillips screwdriver into a bitmac surface; some bitmacs will offer far more fight than others, and a sufficient difference is dependent on the ‘pen grade’ of the bitumen binder. So, some bitmacs are harder, or softer, than others, and the degree of flexibility can be personalized to suit the necessities of the job in hand.

As a comprehensive generalization, the harder grades (70pen or less), are used as ‘asphalts’, while the softer grades (which start from 100 pen or more, are used as binders for bitumen macadams (bitmac). Bitumens of different pen grades can be blended to create binders with a specific pen grade.

Under normal circumstances, a bitumen requires high temperatures (150°-180°C) to make it ‘workable’ on site. However, there are multiple reasons why such high temperatures may not be attainable or even desirable, the most common reason of which is that the mixed material will lose heat as it is transported from the production plant to the site. And so, the bitumen technologists have developed what is known as “Cutback”, a technique that condenses the bitmac hence making it workable at lower temperatures for longer periods. This effect is achieved by ‘doping’ the bitumen binder with a lighter oil (often kerosene, creosote or similar), which acts as a solvent, delaying the setting process unless it is evaporated.

There are different grades of cutback ranging from slow-curing, through medium to fast-curing. Medium-curing cutback is a common choice for hand-laid work, as it gives a good degree of workability for a reasonable amount of time, but still gives a useable surface in a matter of hours.

Slow curing bitmac (aka Deferred Set Bitmac) is mostly used by Local Authorities and the Utility Companies to effect minor or temporary repairs to roads and footpaths, because it can be stored in their depot or on the back of a wagon for several days, and then used as and when required. Once it has been trampled, it will achieve a degree of stability and be firm enough to carry traffic. This type of bitmac is the stuff found in the 25kg packs of DIY/repair macadams being sold in many Builder’s Merchants.

Fast-cure cutback bitmacs would usually be used in machine-laid projects, where a discharge of additional heat from the Paver Machine will solidify the bitmac that bit more workable and easier to level precisely with the screed bar at the rear of the Paver.

Types of Materials

There are dozens of different types of ‘tarmacadam’. Different cumulatives, different cumulative sizes, different binders, different binder colors, the list goes on and on and there is a collection for construction industry devoted to the specification of the various materials and the development of new products.

There are generic names for the swarm of surfacing materials, e.g. a 6mm dense hardstone wearing course, and there are Proprietary Brands, e.g. “ProDrive”, a surfacing course product manufactured exclusively by Bardon – Cumulative Industries. While “ProDrive” may be a 6mm wearing course material, it is to be clearly understood that not all 6mm wearing course materials will be “ProDrive”.

Macadams are all based on the code of a cumulative coated with a binder, usually bitumen, hence “bituminous macadam”. Asphalts are a mixture of asphaltic cement or mortar (often a bitumen with fine cumulative such as sands and grits) and some thicker total, such as gravel or crushed rock.

The major difference between macadams, other than cumulative size and pen grade, is whether they are classed as Open Graded or Close Graded. Open graded macadam is composed of cumulative with very little fines and may be penetrable. It is a popular choice for hand-laid base courses as it remains workable for longer at lower temperatures. Dense macadam contains a significant amount of fines (material of 3mm or less). This means it is often classed as an impermeable material, and the tighter-looking finish make it a popular choice for wearing/surface courses.

The choice of cumulative will affect the presence of the surfacing in the medium-to-long term. All macadam (unless intentionally tinted) tends to look jet black when first laid, which is largely because the bitumen binder coats all of the cumulative, making it appear black.

As the surface is traded, the bitumen on the surface is rubbed and weathered, exposing more and more of the bare cumulative content, enlightening what may be a new color.

Where a darker appearance is required over the longer term, a dark cumulative is required. This is often a basalt but other ‘hardstones’ are used, depending on what is available locally. In some parts of Britain, a granite cumulative, just as hard as basalt, may be used, but its lighter color will result in a much paler, light grey look once the surface has been trafficked. In much or Ireland and parts of northern England, a limestone cumulative is a popular choice, and while not considered a ‘hardstone’ it is perfectly adequate for residential driveways, footpaths, car parks and other areas with slow vehicular traffic, but it wears to a very pale grey, almost white color over time.

The thing to keenly remember is that the color of macadam when first laid may well not remain what it will be in 12 months’ time.

The basic principle of a bitmac or asphalt surface is that the substantial is laid while hot and glutinous, levelled and compacted as quickly as possible, and then allowed to cool so that it ‘sets’ and each particle is inevitable to its neighbors.

Bitmacs with large cumulative (up to around 70mm) are used as road bases or base/binder courses, and the smaller cumulative are used for the surface layer, more commonly known as the surface or wearing course. The overall opinion is that progressively smaller cumulative are used from the bottom of the pavement up towards the top, as this helps to spread the loads imposed upon the finished pavement by vehicles or other road users.

During the recent review of British Standard 4987: Covered macadam for roads and other paved areas: Parts 1 and 2, the “Powers That Be”, who wouldn’t know which end of a tarmac degenerate to hold, decided that the terminology used for the layers of a tarmac surface are to be replaced as part of the ongoing ‘harmonization’ with Europe. Road base will be known as Base, while the base course evolves to Binder Course and the Wearing course will become Surface Course. In the long tradition of the UK and Irish Building Trade, we are totally confident that the older terminologies will remain persevered for many years to come.

On the left, the carriageway comprises a scarcely observable Sub-Base, covered by a Base, then a Binder Course and finally a Surface Course which brings the level up to that of the watermark on the kerbline.

On the right, the footway is incomplete, and includes (at this stage) only the Sub-Base and the Binder Course. The Surface Course was laid at a later.

There are three main types of ‘bituminous’ surface;

Hand Laid Bitmac, for inland driveways, footpaths and other small areas
Machine Laid Bitmac for larger drives, courtyards and roads
Asphalt (SMA/HRA) machine-laid as a wearing course on public roads.
HRA on high speed roads is increasingly being replaced with SMA which is Stone Mastic Asphalt. This is a thicker, rougher, more tough asphaltic material strengthened with fibers and has been shown to produce less traffic noise, much to the pleasure of the Highways Agency (HA). As with HRA, it must be machine-laid.

As mentioned elsewhere, a wearing/surface course may be set over an existing bitmac or concrete surface. This most commonly called as an “Overlay”. The overlay surfacing should be a minimum 25mm thick and MUST be fused with the prevailing surface with a cationic or anionic bonding suspension, commonly denoted to as a ‘Tack Coat’.

Tack coat is a brownish-black liquid of bitumen that is applied to the prepared, existing surface before setting the new surface/wearing course. The suspension bonds to the current surface, and then the new wearing course will bond to the emulsion, ensuring full bonding. It also helps safeguard that water does not find its way between the new and existing surfaces where it can freeze and lift off the new surface. The tack coat is unconditionally essential in almost all instances of overlay work, the only distinguished exception being where a wearing course is laid over a recently laid (ie, within the preceding 7-14 days) base course.

Tack coat is naturally spray-applied to the existing surface at the rate of 0.4 to 0.6 liters/m². Some tack coats need to be heated preceding to application; some are cold applied. On smaller jobs where a spray unit isn’t feasible, the tack coat might also be applied by a brush. The keys points are that the exposure should be complete, i.e., no ‘bald’ spots, and there should be no tarns of liquid on the treated surface. Tack coat ‘breaks’ (dries/sets/cures) in 10-15 minutes and is extraordinarily sticky when it does, so safeguards may be needed to prevent operatives or vehicles crossing cured areas and picking up the bitumen emulsion as they pass.

The surface to be covered must be clean and sound and free of organic material or other debris. If the new material is laid over a badly cracked concrete base, old blocks, or any other flawed base, any joints or cracks will soon be transmitted to the surface of the new bitmac, and the surfacing will fail.

This phenomenon is known as “Reflective Cracking” and is a well-documented problem for all types of overlay surfacing, not only with bitmac, but with all other surfacing materials, such as concrete and resin-bound cumulatives. Any ‘contractor’ recommending an overlay of badly cracked concrete or old flagstones should be told to get back on his horse and take a ride back home!

It is important that any proposed overlay to a residential driveway will not raise the level of the surface to an unreasonable height, such as at garage door thresholds, and that it the new surface is at least 150mm below any damp-proof courses in the brickwork of the house or garage. It may be necessary to cut-out ‘keys’ at the thresholds to lodge the new surfacing.

Also, the level of any utility covers within the pavement being overlaid will need to be adjusted. These include water Stop Tap boxes, manhole and inspection chamber covers, cable tv boxes, etc. Unless there is a significant “lift” in level of 50mm or more, ravines, linear pipes and the like are best left at their original level and keys cut around them, much as demonstrated opposite for a threshold, and then the new surfacing is feathered-in to meet the original level.

As mentioned earlier, bitmac can be a cheap, reliable surface over larger areas. Small areas can often prove quite costly as the ‘set-up costs’ of transporting a roller and other plant, an protected bitmac wagon, and a laying gang account for a larger proportion of the overall cost. For residential driveways, it is often wise to embolden a few neighbors to have their surfacing work done at the same time, thereby reducing the impact of the fixed, set-up costs.

All bituminous materials will slowly degrade in UV light, which results in the binder becoming ‘brittle’. This is the primary reason why older bitmac drives often have a crumbly appearance and the cumulative tends to be scraped free of the surface quite easily. While there are some products that can ‘rejuvenate’ old, tired bituminous surfacing, they may not work too well on badly weakened surfacing.

Modern binders are much more light-stable than those of just a few years ago and we estimate that a newly-laid bitmac driveway or pathway should give at least 10 years trouble-free service. We know of bitmac driveways that have survived in excess of 30 years, and while they may be thought satisfactory for a private householder who requires only a firm surface on which to park the car, their condition is generally poor and, in the public area, they would be thought unsafe and warrant immediate replacement.

As bitmac is an oil-based product, discharges of light oils, petrol, diesel, paraffin etc. will dissolve the binder, and this dissolution can penetrate to a significant depth if left unchecked. The only remedy is to cut out the contaminated material and patch-in with new.

Bitmac, like concrete and other non-elemental surfacing, cannot be ‘invisibly’ repaired. Patching is the only solution, but will be obvious to the eye. If a sizable area is to be ‘patched’, it may be better to resurface the entire area, to maintain the pleasing advent of a uniform surface.

Un-Trafficked areas, paths and drives in shady and/or damp locations, and older bitmac surfaces are disposed to moss and algae, which can become dangerously sloppy. This is the main reason why we NEVER recommend bitmac as a surface for garden paths.

Surface vegetation is best removed by scrubbing with water, a general weed killer and a stiff brush. Power washers can remove individual stones within the cumulative of an older bitmac surface and so should be used with care. Test out on a subtle corner before attempting to power-wash a larger area.

Bitmac must be properly levelled and laid to acceptable falls that will drain the surface water to a gulch or other suitable drainage point. Whereas a machine-laid bitmac surface can be laid to falls as tight as 1:80, hand-laid surfacing is particularly disposed to small low-spots, and, in order to avoid the puddles that seem to bother so many bitmac drives, a fall of not less than 1:50 is recommended.

The price of bitmac or asphalt surfacing is somewhat unstable, to say the least. As the key materials are all oil-based, their price is largely resolute by the price of oil, which has a habit of rising and falling in an unpredictable manner.

Previously, guide prices for various “typical” installations have been given, but in the wake of the roller-coaster oil price white-knuckle-ride of recent years, it has been decided to withdraw price information as it became impossible to ensure accurate pricing from one day to the next. However, surfacing contractors are usually happy to provide prices for any work and so it is recommended that you enquire locally.