Most jobs involving tarmacadam installation fall into one of three main categories.
This is where a brand new driveway or patio is installed onto virgin land which requires excavation.
Excavation
Typically, for tarmacadam installation on a domestic driveway, the ground should be excavated to a depth of around 180 mm below the proposed finished surface level, which itself should be 150 mm below the damp proof membrane of the property.
Some companies will do it the hard way with the existing surface hand dug, transferring spoil into wheelbarrows, then skips. The more professional tarmacadam installation companies will have mini-diggers and their own large trucks to take spoil away from the site. The latter group are also more likely to have the proper machinery required to compact the sub-base of stone and the layers of tarmacadam which will result in a more professional finish.
Drainage
Once the ground has been excavated, the planned underground drainage systems need to be installed.
This may involve digging out a dry-well and back-filling it with crushed stone to create a soak-away, the pipework leading from proposed aco drainage to existing drains or soak-aways or preparing French drains. All new drainage should satisfy current Building Regulations.
Geo-Textile Membrane
Before the sub-base is installed and compacted, some companies will install a geo-textile membrane but there is much debate about the job it is supposed to do.
It is often referred to as a weed membrane but few installers will guarantee against weed damage to new tarmacadam driveways and patios, as their seeds are more likely to settle in small joints and sprout from above. Most weeds will struggle to grow through a new, properly constructed tarmacadam installation but where there is a known threat from particularly pernicious weeds such as Japanese Knotweed or Horse Tail, a geo-textile membrane can sometimes help.
Other claims for the benefit of a membrane include the reduced thickness of sub-base material required (possibly, but no excuse for too little) or as a separation membrane to prevent sand settling into the sub-base below (this only applies to block paving but if the layers are compacted well enough a membrane becomes irrelevant). In any instance, if a membrane is installed, it should never by polythene or any other non-permeable product as this cause all manner of problems when it starts to hold water.
Sub-Base Layer
The sub-base layer in a tarmacadam installation is normally the main load-bearing layer of a driveway or patio and works by distributing a specific load (such as that provided by car tyres), over a larger area.
This should prevent or at least reduce rutting over a period of time when a car makes the journey from the driveway entrance to the garage on the same length of tarmacadam, over a period of years. The thickness of sub-base material should be greater for a driveway which provides vehicular access than for a patio which is for pedestrians only.
Typically, 150 mm for a driveway and 100 mm for a patio, with both of them compacted to within a 10 mm tolerance. Dropping all of the sub-base material onto an area and then compacting it can lead to the lower section remaining too loose, especially if no more than a vibrating plate compactor is used. For a driveway at least, a double drum Bomag™ type, ride-on roller / compactor is recommended.
Crushed stone (but not old broken bricks) can be used for the sub-base but often Type 1 MOT is used – MOT refers to a range of Ministry of Transport recognised materials and Type 1 is a particular specification, there being other types of sub-base material.
On a normal tarmacadam installation for a driveway or patio area, the sub-base material is the least expensive material used per ton, but skimping on the type of material, its thickness and method of compaction, is one of the best ways of shortening the life of the newly finished surface.
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Kerbs or Edging
Whilst the sub-base is being prepared, a string line is positioned following the route of any proposed edging or kerbs. Edging is required for new tarmacadam surfaces to prevent it from breaking up where it abuts a soft area of the garden such as a border or grass area.
Various types of either purely functional or decorative edging can be installed according to taste and budget. The simplest form is a basic concrete edging (900 mm long x 150 mm deep x 50 mm wide) which is concreted into place and once the area has been completed, leaves just the 50 mm wide top exposed, level with the surface of the new driveway. Traditional kerbs tend only to be used in commercial areas and for public footpaths.
Very often, a decorative block paving edge in a contrasting colour to the new tarmacadam driveway or patio is installed and although it is not required against a boundary wall, it can be included as a feature. Also, where the garden or border area is higher than the finished surface level of the driveway or patio area, a raised kerb edge restraint (known as a K.L.) is usually installed to prevent soil or loose gravel spilling over onto the tarmacadam. Both K.L.’s and block paving edging should be concreted into place and haunched so that they do not move thus threatening the edge of the tarmacadam surface.
Before the two layers of bitmac are installed surface drainage equipment such as new sections of aco and surface gullies are positioned and connected up to the underground pipework. The position and height of these is critical to ensure that surface water runs off the new driveway into them.
Base Course of Tarmacadam
A standard tarmacadam installation for a driveway or patio area will have two different layers of bitmac and once the concrete supporting the edging has hardened, the first one to be compacted into place is the dense base course, sometimes referred to as the binder course.
The bitmac binder course is delivered to site in hot condition and once placed onto the driveway it is raked level and rolled as soon as possible. The vast majority of new domestic tarmacadam surfaces are hand-laid and machine rolled as opposed to machine-laid as in a large commercial job.
Typically a binder course for a domestic drive will be compacted down to a thickness of 50 mm and will contain aggregate with a maximum size of 20 mm. The double drum Bomag™ type, ride-on roller / compactor is quite noisy and will take five or six passes over the binder course to level and compact it.
Surface or Wearing Course of Tarmacadam
The surface course of a new tarmacadam installation for a driveway or patio area should be laid immediately after the binder course is finished and again is delivered to site hot.
It would generally contain a finer aggregate than the binder course and a maximum size of 6 mm would be normal, though if the homeowner uses a large van or has a heavy 4 x 4 vehicle, 10 mm aggregate is sometimes recommended by the installation company. The larger the aggregate size the higher proportion of it there is exposed on the newly finished surface compared to solidified bitumen.
Since aggregate is more hard wearing than solid bitumen a tarmacadam driveway with a larger aggregate will cope with heavier vehicles. The surface course is raked and compacted in a similar way to the binder course but more attention is given to achieving a flat and level surface with no roller marks.
Making Good
After tarmacadam installation, the site area should be cleaned and all surplus bitmac, other construction materials and tools removed from the site. The new surface should be suitable for pedestrian traffic almost immediately (though stiletto heels on the new tarmacadam should be avoided for a while).
The weather and temperature on the day the driveway is completed will generally dictate how long vehicles should be left off it to enable the surface to cure properly. The warmer it is the longer it should be left and in very hot weather the contractor may request that you hose it down to speed up the curing process.
One of the common complaints about new tarmacadam driveways relates to scuff marks created during early vehicle usage. These can be reduced by avoiding wheel spins, turning the steering wheels prior to moving off, and sudden acceleration and deceleration. Though scuffs caused in this way can rarely be repaired, they will form part of the normal wear and tear marks in time and no contractor will accept liability for such surface damage.
Often, tarmacadam installation companies are asked to install the binder course and surface course to a new driveway when other contractors have dealt with the excavation, drainage, kerbs, edging and stone sub-base.
This usually applies in respect of new properties or when a builder has extended an existing property and a driveway is to be widened. The building contractor may be quite capable of preparing the ground works but he rightfully recognises that laying bitmac is a specialist job requiring experience and the correct equipment.
Having two different contractors taking responsibility for one job can give rise to later problems when each can blame the other in respect of a fault. In many cases, the tarmac contractor will arrive to lay the two layers of bitmac to find that the levels are wrong, there is insufficient stone sub-base or it has not been properly compacted.
The tarmacadam installation contractor has a valid case to charge for completing the stone sub-base but in all other respects, compacting the two layers of bitmac are as referred to in the section on a Full Installation above.
Pot Holes
This applies to existing tarmacadam driveways or patios which have become worn or have even developed pot holes. Repair of any pot holes would be the first job to tackle and a rectangular area should be cut out of the two layers of the old bitmac.
Crushed stone and dense binder should be compacted into the hole to repair it and bring it up to the level of the existing surface. It is just as important to properly compact this small area as it would be for a new driveway to avoid later sinkage.
Any man holes and other iron works should be raised to the new level and any weeds and other organic material should be removed from the area to be re-surfaced.
Tack Coat
Once the pot holes have been repaired, and the area brushed clean of debris, a tack coat should be applied before tarmacadam installation. This is a dark emulsion of bitumen which bonds the existing old surface to the new surface course of bitmac.
It also ensures that no water can penetrate between the two layers which could freeze and cause the new surface layer to peel off. Tack coat is supplied in drums and is either sprayed onto the old surface or applied with a brush. There should be no part of the old surface left dry of tack coat. Once applied, it dries in around 15 minutes and as you can imagine, is very sticky.
New Surface Layer
The new surface or wearing course layer should be at least 30 mm thick once compacted and should include the correct size and type of aggregate to deal with proposed traffic.
It is normally necessary to cut out keys at the entrance and exits to driveways to avoid a lip down to an adjoining surface (public pavement or garage floor). This helps to maintain the required thickness of the new surface layer and avoid ‘feathering’ which would eventually form a weak area.
Care should be taken when re-surfacing an existing tarmacadam installation to ensure that garage doors and gates still open. Also, raising the level of a driveway should not cause the new surface to be less than 150 mm from the damp proof membrane of the house.
We have seen many situations where a homeowner has requested a quotation for a driveway re-surface when, because it has been already done so many times in the past, the surface has already breached the d.p.c. In such cases a full installation should be recommended to the homeowner.
Light oils such as petrol, diesel and even brake fluid can melt or dissolve the bitumen in both the surface course and binder course of a tarmacadam driveway and once damaged it cannot be repaired. The only solution is to cut out the affected area, including approximately 150 mm around the damaged surface. If left, the fluid will continue to seep into surrounding areas causing even more contamination and damage.
The damaged area should be repaired in the same way as a pot hole and the new surface layer should be sealed to the surrounding driveway. Unfortunately this is not an invisible repair and the patch will remain a potential eyesore. The only solution is to re-surface or overlay the entire driveway once the repair has been completed and you are happy that all the contaminated bitmac has been removed.
Reference has been made earlier in this website to tarmacadam, or bitmac being delivered to sites in a hot box or insulated truck in order to keep it hot and workable for proper tarmacadam installation. Temperatures as high as 150º to 175º are quite normal, but there are sometimes reasons why such heat cannot be maintained or when the cooling process needs to be slowed down to keep the product workable for a longer period of time.
Cut-back is a process which, by adding a light oil such as kerosene or creosote to act as a solvent, the bitmac retains its workable properties for longer periods at lower temperatures. The amount of solvent added determines the grade of cut-back from slow curing to medium or fast curing without affecting the ability of the bitmac to take traffic within a reasonable time.
Slow curing bitmac is used for minor repairs to pavements, as once produced it can be stored for a number of days, often on the back of a truck before being used. It is also the type of bitmac sold in packs in builders’ merchants and DIY stores to be used for minor domestic repairs.
Although there are other brands of instant tarmacadam repair compound, Quick Mac™ comes ready to use for remedial works on driveways, footpaths and patio areas. It can be used to repair holes between 20 mm and 60 mm deep and the surface can be used immediately, though joints with the existing retained surface should be sealed.
Tarmacadam, which is properly known as bituminous macadam, and is abbreviated to bitmac, consists mainly of bitumen and aggregate. Different types and sizes of aggregate are used to produce a particular grade of bitmac, making it more suitable for one application or another.
Unsurprisingly then, there are also different grades of bitumen, the binder used in bitmac. The method used to measure the hardness of bitumen is extremely complicated but in simple terms, refers to how easily it is penetrated – hence the penetration, or pen grade.
A needle is used to penetrate the hardened bitumen and a penetration of 0.5 mm indicates a penetration grade for the bitumen of 50. Pen grades range from 15 pen to 450 pen and the lower figure signifies a lower penetration of the needle’and therefore the harder the bitumen is.
Generally, the harder types of bitumen are used in asphalts, for stone mastic asphalt (70 pen or less) and the softer ones used in tarmacadam, or bitmac.
For tarmacadam installation to be carried out effectively, pen grades must be taken into account.
