Coating Process And Coated Products
In North American and European terminology, bitumen is referred to as "asphalt".
Bitumen's natural waterproofing and adhesive qualities have been known for a long time. Natural bitumen from seepages was used by the ancient Egyptians to preserve bodies - the well-known 'mummies'.
Today, bitumen, as a binder or 'glue', is a key component of the majority of road surfacing materials in use today. It is a residue produced from the distillation of crude oil derived from heavy crudes produced in the Middle East or South America. In contrast, some oilfields, such as the United Kingdom's, yield only light crudes that do not contain the heavy fractions required to produce bitumen.
Bitumen is composed of long chain hydrocarbon molecules. As the molecules are long, they only move with difficulty, hence bitumen appears to be solid at ambient temperatures. However, bitumen does 'flow', though only very slowly (like ice in a glacier), as it is a highly viscous liquid. Heating bitumen increases the molecular activity and the long hydrocarbon chains start to move and slide over each other - the bitumen softens and then becomes a runny liquid. This property of bitumen, near solid at ambient temperature, but liquid at higher temperature, together with its adhesive and waterproofing qualities, is the reason why bitumen is so useful in road construction. Most bituminous coated materials are workable when mixed and applied hot, but on cooling become solid and stiff enough to bear the weight of heavy trucks or planes.
There is no single grade of bitumen that can be used for all purposes; therefore different grades of bitumen are made.
The main type of bitumen used in road construction is called penetration grade. At the refinery, the bitumen has oxygen blown through it. The oxygen displaces some hydrogen and links in with the carbon, making the bond stiffer. This treatment makes the base bitumen harder. The hardness of bitumen is measured in two distinct ways as follows:
Penetration test - in this test, a 50ml metal cup is filled with hot liquid bitumen to be tested. After cooling, the filled cup is placed in a heated water bath maintained at an even temperature of 25ºC. The cup is kept in the water bath until it has reached 25ºC.The cup is then placed in a metal carrier beaker filled with water at the same temperature and placed on the penetrometer. The penetrometer has a shaft which holds a calibrated needle in an assembly weighing 100g. The needle is then racked down until its tip just no more than touches the surface of the bitumen. A button is pressed which frees the needle assembly so that it presses the needle into the bitumen. A built-in electronic timer times five seconds and then stops the needle assembly moving. The depth that the needle has penetrated into the bitumen is then measured using the instrument's measuring gauge. The needle depth is reported in decimillimetres (1/10th of a millimetre) and the figure is the penetration of the bitumen or 'pen' for short. A reading of 50 dmm is called 50 pen (0.5cm), a reading of 190 dmm is called 190 pen (1.9cm).
Ring and ball softening point - for this test, hot liquid bitumen is poured into two brass rings placed on a silicone proofed glass plate. The rings are filled to the brim and the excess bitumen struck off with a metal edge. After cooling the bitumen filled rings are freed from the glass plate and mounted over holes on a twin holder. A steel ball bearing weighing 3.5g is placed on each bitumen ring. The holder, with a thermometer clipped onto it, is then placed in a water filled glass beaker. The beaker is then placed on a controlled heating ring and the water temperature is raised at a defined rate of 5oC per minute. The temperature of the water in the beaker is kept evenly distributed using a magnetically driven stirrer. At the bitumen's softening point, the bitumen in the ring deforms under the weight of the ball and both flow down through the ring. The temperature is recorded when the bitumen touches a metal plate mounted 25 mm below the ring. This is the softening point.
A 160/220 (190) pen bitumen has a softening point between 35-43oC, and is called a soft bitumen. In contrast, the 40/60 (50) pen bitumen with a softening point between 48-56oC, is called a hard bitumen. The softening point defines a bitumen's temperature susceptibility. Soft bitumens are mixed at temperatures lower than harder bitumens. The harder 40/60 pen grade is used for mixing hot rolled asphalt, the softer 160/220 pen grade is used for mixing bitmacs.
Bitumen is generally supplied and used in the hot liquid state. To allow mixing with aggregate, the aggregate must also be heated. Very often mixing is done in batch mixers. The general principle for batch mixing plants is illustrated below.
The collecting belt transfers the aggregate into a rotary drum dryer. The dryer is a large, slightly inclined, rotating cylinder with a large burner at the end opposite the aggregate feed. Inside the drum, shelf-like lifters are fitted along its length. When the drum rotates, these continually lift up and drop the aggregate through the burner flame.
The heated and dried aggregate leaves the dryer via the collecting ring at the burner end. The hot aggregate is then lifted in a chain bucket elevator to a screen mounted in the next section of the mixer. This screen sorts the aggregate into its component sizes and directs it into corresponding hot bins below. Coating plants can have up to seven hot bins.
Situated below the hot bins is the weigh bin. This is suspended on load cells which measure the weight of aggregate fed into it from the hot bins. The hot bins are opened separately in sequence and closed when the required weight is reached.
At the same time that all this is going on, hot liquid bitumen is pumped from the storage tank to a small weigh hopper, or 'kettle', mounted directly over the mixer. Load cells on the kettle shut off the bitumen when the correct amount has been weighed. If hot rolled asphalt, or a mix requiring filler to be added, is being made, limestone filler is drawn from a tower silo by an auger screw feeder to a filler weighbin.
When the aggregate has been weighed up to make a batch, it is dropped from the aggregate weighbin into the mixer. At the same time that the aggregate arrives at the mixer, the bitumen (and filler if required) is injected and all constituents are mixed together. A typical batch plant mixer is a horizontal shaped double trough with two horizontal rotating shafts with paddles attached. The paddles are interleaved. At the end of the mix cycle the mixed batch is dropped out of a hatch in the bottom of the mixer into a winch hauled skip which then transfers it to heated storage bins for subsequent discharge into waiting lorries below.
Another type of mixer is the drum or continuous mixer. As with a batch mixer, cold feed hoppers are loaded with aggregate. The aggregate is drawn from each hopper by calibrated feeder belts onto a collecting conveyor. The collecting conveyor transfers the material to a dryer drum. Bitumen is injected halfway down the dryer drum onto the heated aggregate. Mixing of bitumen through the aggregate is done in the second half of the drum and the mixed material collected in a dump skip before being dropped into a travelling skip for transfer to heated storage silos. Unlike the batch mixer, drum mixers mix material continuously. Drum mixers are very efficient to run, but they require careful control to ensure that the aggregates drawn from the cold feeder bins are correctly proportioned with respect to each other and that the bitumen is injected at the right rate. As bitumen is injected downstream of the dryer flame, there is also an increased risk of fire. As some of the bitumen can be volatilised by direct heat from the burner flame, careful control is also exercised on the dust filter system to ensure that not only is dust extracted but bitumen fume as well. Larger continuous mixers may also have an inlet ring next to the bitumen injector to allow recovered asphalt planings (RAP) to be fed into the mix.
Note 1: Size designations are now aligned to BS EN 13043 which was implemented in the UK from 1 January 2004. Click here to read about Euro aggregate sizes etc. For example, "20mm" precoated chippings are now designated as "14/20 mm" precoated chippings. The first number refers to the smaller limiting sieve on which a specified percentage of chippings must be retained, the second number refers to the larger limiting sieve through which a specified percentage of chippings must pass. In the case of graded materials, e.g., crushed rock fines, now designated "0/4", the zero indicates that there is no small sieve specified, the second number refers to the upper limiting sieve through which a specified percentage of particles must pass, therefore the "0/4" designation is inclusive of all particles from the finest micron-sized ones, right up to 4mm particles.
Note 2: Material descriptions and designations for hot rolled asphalt and asphalt concrete are as per BSEN 13108:2010 and PD6691.
Asphalt Concrete (Bitmac)
Asphalt Concrete is a layer of crushed rock, which relies on the mechanical interlock and internal friction of the aggregate to provide load-bearing strength. As the aggregate fraction is composed of a mix of various particle sizes, coarse medium and fine. Binder, typically bitumen, is used to make the layer cohesive and prevent it from falling apart.
From the 1800s to the mid 1970s, tar was produced as a by-product at gas works. Like bitumen, tar is a waterproof adhesive and was used early-on as a binder in road construction, hence the word tarmac (tar macadam). The term "Tarmac" is still ubiquitously used to describe any non-cementitious road surface, even though tar has not been used for many years as it is no longer readily available.
Dense Base (Roadbase) - is a layer made up of a graded mix of 0/32 mm crushed rock aggregate. Dense Base has 1% less bitumen compared to binder course. Dense Base is used as the first bound base layer in major road construction where high strength and stiffness in the total construction is required. Layer thickness is between 100 to 230mm, depending on the volume of traffic, particularly heavy goods vehicles, that will use the road.
Binder course - is a layer made up of a graded mix of 0/32 mm or 0/20 mm crushed rock aggregate, but with 1% more bitumen than dense base. This more cohesive material is used as the supporting or base layer to the surface course. It also serves to even-out any undulations. Layer thickness is between 50 to 70mm.
Surface course - is a layer made up of a graded mix of the smaller sizes (0/14 mm, 0/10 mm, 0/6 mm) crushed rock aggregate. Aggregate size / layer thickness varies according to end-use. Due to the larger surface area of the aggregate to be coated, bitumen content of surface course is 1% higher than in the binder course. Footpaths and driveways will be surfaced with 0/6 mm or 0/10 mm size asphalt concrete. Car parks and minor roads will be surfaced with 0/10 mm or 0/14 mm asphalt concrete.
Asphalt concrete surface courses are generally not used to surface high stress sites, such as main roads with high volumes of heavy goods vehicles, as it is not strong enough.
Open textured asphalt concrete is made with less fines and therefore have more void spaces between the coarse aggregate particles. Both open-graded binder and surface courses are available. Being porous (15-22% void space), they allow water to pass readily through their structure, hence their use as a base layer for flat sports surfaces, such as artificial turf pitches; and as free-draining surface courses. Due to the lower amount of fines requiring less bitumen, open asphalt concrete are cheaper to make than close graded bitmacs
Modern equivalents are the Porous Asphalts. These are designed to have 22-30% void space. Similar proprietary (designed by the manufacturer) mixes with 18-20% voids are categorised as Thin Layer materials. These are bituminous bound hot-mix materials that do not exceed 30mm in thickness. Polymer modified bitumen may be used either in the hot mix or in the tack or bond coat, but generally not in both, due to cost. As these materials allow water to flow through, they must be laid on an impervious base such as a hot rolled asphalt or a thick bond coat, to prevent the ingress of water into the base layers of the road.
Hot Rolled Asphalt (HRA) (as used in Britain and Ireland)
HRA was first laid in Britain in 1895 on the Kings Road, Chelsea, and Pelham Street, Kensington. The first British Standard dealing with it was published in 1935. HRA is a mortar made up of a mix of fine sand, ground limestone filler and bitumen. The strength and resilience of hot rolled asphalt comes from the mortar. Depending on end-use, single sized aggregate is generally added to the asphalt mortar to extend it. HRA, is made of a coarse aggregate fraction bound by a fine mortar fraction and is therefore gap graded. Being rich in bitumen and containing filler and fine sand brought from outside sources to the coating plant, HRA has always been more expensive to produce than bitmac, though this is compensated for by its greater durability.
Base (roadbase), binder course and regulating mixtures have a high stone content (50% or 60%). If thick layers of base or binder course are required, the added stone is often single-size 10/20 or 20/32 mm. In regulating courses, 4/10 mm or 6.3/14 mm stone is used for levelling prior to final surfacing. Being filled with stone, these HRAs require lesser quantities of bitumen, filler and sand in their manufacture.
Surface course Hot Rolled Asphalt - has a lower stone content (30% or 15%) than base course asphalt. HRA can be made with either 6.3/14 mm or 4/10 mm stone. When manufactured and laid correctly, it is flexible and durable. However, having a high percentage of mortar, it can permanently deform (rut) under certain conditions, such as hot weather and high traffic volume and loading. To counteract this, surface course asphalt can be made stiffer by varying the binder amount, or its properties, or by changing the type of sand used, using a special design process and test equipment. Such an asphalt is called a "design mix".
On its own, HRA, being mortar rich, has a smooth surface which would be unsafe to drive and brake on, particularly in wet weather. To give grip to the road, pre-coated chippings (PCCs) are distributed evenly over the hot asphalt immediately it is laid using a specially designed chipping spreader. The chippings are then rolled into the surface of the asphalt. Chipping size is typically 14/20 mm, though 8/14 mm chips can also be used. Only certain types of aggregate resistant to polishing from scrubbing by vehicle tyres can be used to make pre-coats. An aggregate's skid resistance is measured by the Polished Stone Value (PSV) test performed in the laboratory. The most common polish or skid resistant type of aggregate naturally available in Britain and Ireland is the sedimentary rock geologically termed greywacke, colloquially called gritstone. The laboratory measured PSV of gritstones is in the range 59 to 68. Generally a PSV of 68 is the highest value for naturally occurring aggregates. The minimum PSV specified for high-speed roads without any hazards (sharp bends, junctions) is 60. A PSV of 65+ is specified for roads with more hazards. For the most highly stressed sites, a minimum PSV of 68 is required. This can only be reliably achieved by using calcined bauxite, an artificially manufactured aggregate.
30% 0/14 HRA with pre-coated chips, laid 35 mm to 40 mm thick, has been traditionally used to surface many main roads, particularly high speed roads such as motorways and dual carriageways. However, it has fallen into some disfavour in mainland United Kingdom due to its propensity to rut. Even if the HRA does not rut, the high PSV stone chips can polish too quickly under the highest traffic volumes and become slippery. It has largely been overtaken by Stone Mastic Asphalt. HRA is still popular with road authorities in both the Republic of Ireland and Northern Ireland.
15% 0/10 HRA surface course is fine enough to be laid up to 25 mm deep and is extensively used to surface footpaths and driveways. It is always chipped, often with decorative 6.3/10 mm white granite or red porphyry chippings.
Very few other countries use HRA.
Sand carpet is similar to mastic asphalt in that it is bitumen rich, but contains less filler, the remainder of the mix being made up with asphalt sand. Unlike mastic asphalt, it contains no stone. It is used in several specialised applications such as a bridge deck warning layer and as a tank base. Most bridge decks are nowadays waterproofed with an expensive epoxy polymer sealant. To protect the polymer membrane, a thin layer of red "ox-blood" sand carpet is laid as an warning indicator on top of it before the road surface is laid. This method of construction is designed to prevent damage to the membrane if the road surface subsequently requires repair or replacement.
Mastic asphalt is an asphalt rich in bitumen and filler. The filler used is coarser than that used in road asphalt. The mastic may be extended with grit or stone depending on end-use. Being filled with bitumen and with no internal void spaces, it is therefore impervious to water. Due to its impermeability, it is used to surface large flat roofs, multi-storey car park decks and bridge decks. It is supplied in ready-mixed blocks. These are put into a large mobile boiler on site and melted down to give hot molten mastic. The mastic is then laid by hand and levelled off using wooden floats. Some types of mastic asphalt can be laid through conventional pavers. Manufacture and installation of Mastic Asphalt per se is covered by its own British Standards BS 1446, BS 1447, BS 6692, not BS 594.
Stone Mastic Asphalt (SMA) was developed by Dr Zichner in Germany in the late 1960s, SMA was originally designed to resist the severe wear and tear of studded snow tyres used in winter in Germany. Wear resistance was imparted by employing a high stone content matrix, the void space of which was filled and bound by a rich mastic of bitumen, crushed rock fines (with or without sand) and filler. Binder volume and the thickness of the binder coating around particles was also originally increased by the addition of cellulose fibres (derived from paper), making a durable binder-rich mixture. The fibres also inhibited binder drainage. SMA is gap graded, as it is composed of a coarse aggregate fraction and a fine mastic fraction. Almost as soon as it was invented, studded snow tyres were banned. However, the new material was thought to perform better than asphaltic concrete and so its future was assured.
The void structure between the stone skeleton was almost completely filled to the surface in the original German version, leaving only 2-4% voids. The amount of mastic had to be carefully calculated to avoid overfilling, which gives a bitumen-rich or fatty surface.
Due to the rich binder layer at the surface, SMA can have significantly reduced early life skid resistance, to counteract this; a light 3mm or 6mm grit was applied and rolled into the original SMA in Germany. In its migration to the UK and Ireland, where skid resistance is taken seriously (and high PSV aggregates are readily available), SMA was re-developed with a greater void content (4-6%), which, when the hot mastic in the laid mat had settled, gave an inverted texture to the surface. Due to the inverted texture, gritting was rendered ineffective, as the grit rapidly fell into the voids, negating the texture, hence, in the UK and Ireland, gritting immediately after laying was discontinued.
At an early stage in the development of SMA in the UK, a major bitumen supplier developed a polymer modified bitumen which admirably thickened the binder film while increasing both stiffness and fatigue life. With the modifier in the actual bitumen, with none of the drawbacks associated with handling fibres, SMA made with polymer modified bitumen is understandably very popular.
Some concerns about SMA's early life skid resistance have been raised in both the UK and Ireland.
As mentioned previously, concerns have been raised over some incidences of SMA fatting up. Although simple in theory, SMAs have to be carefully designed to match the quantity of mastic with the size, shape, surface area and void space of the aggregate combination used, so that the void space is not under or over-filled. High mastic content, combined with flaky stone that is more through-graded, rather than gap graded, can result in over-filled voids, giving a smooth fatty surface. The opposite effect can result from a low mastic content combined with stone that is too single sized producing a very open textured finish reminiscent of "popcorn".
With the popularity of SMA in the UK mainland, with many producers and contractors offering their own version, the Highways Authority stepped in and now demands that all SMA is manufactured and laid under an independently assessed quality scheme. Assessment and certification of SMA producers and installers is by the British Board of Agrément Highways Authority Product Approval Scheme (or BBA HAPAS for short).
Thin layer materials are thin surface overlays that are between 20 mm and 30 mm thick. They add little structural strength to the road, but are designed primarily to restore surface texture and grip. They also improve ride quality, and, due to their inverted surface texture, both reduce noise (increasingly important in urban areas) and surface water, reducing water spray and increasing visibility in wet weather. Thin layer design can be thought to originate through two distinct routes. In the late 1980s in France, the Laboratoire Centrale des Ponts et Chausees (LCPC), in collaboration with Screg Routes, designed a hot mix composed of basically 10 mm aggregate with sufficient fines to increase the binder film. This was laid on a thick polymer modified binder layer sprayed immediately ahead of the hot mix by an emulsion tank and spray bar built-in to a purpose built machine. Laid through a screed, and only 25 mm thick, the integral sprayer/laying machine could cover large areas in a day. It crossed the channel to the UK in the early 90s as the "Safepave" process. It can be considered to be a highly specialised version of surface dressing. Good texture, noise level reduction, spray reduction and ride improvement were claimed.
In competition to this, another French company, John Lefebvre, developed another version called "Ultra Mince" (that's French for ultra thin).
Other companies developed their own particular varieties. Some types are modified versions of pervious bitmacs or friction courses. All employ polymer modified bitumen for added strength and cohesiveness. Depending on the system, the polymer is either added to the bond coat or to the binder, but generally not to both.
After a period of rapid development of various proprietary systems by producers and contractors, the Highways Authority demanded that all such systems be independently certified as to their quality. As with SMA, this is done under the auspices of BBA HAPAS.
Tennis court materials
AC6mm open textured surface course laid on a 0/20 mm open graded binder course is very suitable for tennis court surfacing. These surfaces are laid to accurate surface tolerances using levelled screeds. The hot mix is polymer modified to aid workability and laying by hand. The finished surface is made very attractive by the spray application of coloured acrylic paint, traditionally red or green. The paint finish is not only aesthetically pleasing, it also imparts good even bounce to the ball.