Asphalt work

Introduction 

It is perhaps unfair to refer to any particular trade as special, for every operative in the industry demonstrates skills which are unique to the craft he follows. However, asphalt work is generally referred to as a specialist trade because it encompasses some unique features and is practiced by a relatively small number of craftsmen employed by a relatively small number of companies. To further emphasize this specialism, contracts for this type of work are normally let as a subcontract and most commonly to a nominated subcontractor. 

The most unique aspect of the trade is the material used. Unlike every other trade, the material, mastic asphalt, is handled by the operatives while it is hot, generally between 200 and 220 °C. Pitch mastic is laid at the lower temperature of 160 °C. 

This has two consequences. The first is that the work is dangerous; protective clothing is essential and safety procedures must be adhered to. 

The second is that a relatively rapid pace is needed in the application of the material, as it sets by cooling to ambient temperature. As soon as the material is drawn from the cauldron or mixer it starts to cool. When it is applied to a surface the rate of cooling accelerates for two reasons. First, the base surface is at ambient temperature and a base of masonry presents an enormous heat sink. Second, as the material is spread out,' more of it comes into contact with this cold base and with the cool air over it. 

Perhaps as a consequence of the specialist nature of the work there seems to be no well-defined procedure used by estimators for building up rates for measured items of work. Therefore the data and examples which follow are given only as a guide to one possible approach to achieve comprehensive financial cover for all expenditure. 

Before commencing examples it is proper to consider the logistics and practice in the trade. 

Plant 

Plant can be divided between melting equipment and small tools. 

Melting equipment comes in a range of capacities, that is the weight of asphalt or pitch with which it is possible to deal in one operation. The smaller capacity items are called cauldrons or pots. Pots comprise a cylindrical lidded container with a heat source under-neath. These pots are generally fired from cylinders of propane gas. While mastic asphalt is being melted and kept molten it is occasionally stirred to keep the temperature even. Pitch mastic has to be continuously stirred, and pots for use with that material have mechanically driven paddles built in for that purpose. Pots generally have a capacity of 500 kg of asphalt; this will take approximately 25 kg of propane to melt down. 

Larger capacity plants are called mixers. The melting chamber incorporates paddles for mechanical mixing. The fuel used was once coke or coal but this has been replaced with oil or propane. Mixers may be mounted permanently on either a lorry or a trailer chassis. Usually mixers have a capacity of 1500 kg of asphalt, and naturally take proportionately more fuel to melt down all the asphalt. 

Small tools and accessories include the following: 

• Various trowels for spreading asphalt, working fillets, turning into grooves, etc.
• Lidded buckets for transporting the melted asphalt from pot to site of laying 
• Rules, straight edges and gauges
• Tin snips, pliers, hammers, knives, etc. to deal with underlays and reinforcing meshes. 

Materials 

Rock and mastic asphalt and pitch mastic are purchased by weight and supplied in blocks or cakes, usually weighing 25 kg. 

Typically 1 tonne (1000 kg) of asphalt will cover 35 m2 to a thickness of 12 mm, this being the thinnest single coat usually laid in one-coat work. Coats as thin as 10 mm are laid but only in combination with other coats. This cover rate takes into account a certain degree of unevenness in the substrate. The figure of 12 mm therefore would be used for a first coat. Subsequent coats will have an increased cover rate as there is no need to take any unevenness into account; the increase in cover rate is approximately 5 per cent. 

So for a layer 20 mm thick, whatever the number of coats, the quantity of asphalt used per m2 would be as follows: 

First 12 mm thickness: 1000/35                                               28.57 kg

Further 8 mm thickness: (1000/35) x (8/12) x 1.05                 20.00 kg 

                                                                                                 48.57 kg 

This is equivalent to 1000/48.57 = 20.59 m2 per tonne. Waste is not included in the calculation; it is normally added to the cost of the material when working out the rate (see later). Bituminous felt and glass fibre cloth or tissue are used where an underlay is required; the bituminous felt should be type 4A(i) complying with BS 747. Bitumen coated expanded metal lath is used as a reinforcement for vertical work and in up stands, where adhesion to the substrate is not possible; this is supplied in sheets of approximately 2500 x 700 mm. Galvanized clout nails or wire staples are used to secure underlay and reinforcement to timber substrates. 

Rubber/bitumen primer is used to coat metallic surfaces prior to the application of asphalt. This ensures a watertight bond. Coverage is approximately 10 m2 per liter. Surfaces treated might include lead flashings and drips, cast iron rainwater outlets and gullies in roofs and pavings, pipes passing through a surface, balusters and other metal work. The primer has a solvent base — usually petroleum spirit - and is highly flammable in its liquid state. 

Sundry materials used include crushed limestone dust to line buckets, thus preventing the asphalt or pitch mastic sticking to the bucket, and sharp sand to obtain a sand rubbed finish.  

Labour 

The size and nature of the job, the timetable and the accessibility of the works will determine whether pots or mixers are to be used. There may be a combination of a mixer for the large areas and a pot for the ancillary work such as "skirtings, upstands, kerbs and drips.