Macadam is a combination of quarried, crushed stone and a binder,
usually a hydrocarbon bitumen. It may be produced by raising the temperature of
both components to 150°C-180°C, and mixing so that the binder coats the surface
of the stones. Alternatively, an emulsion of bitumen in water may be used instead
of a hot binder, in which case the resulting macadam is referred to as a cold
macadam or as emulsion macadam.
The production units for the use of hot or cold binder are normally
static, and sited at a quarry. Transportable production units do exist, but these
are always static during the actual production of Macadam.
Macadams are of a granular nature and employ predominately a single
or nominal sized stone, the size varying according to the requirement. For instance
a macadam may be available with nominal stone sizes of 3mm, 6mm, 10mm, 14mm or
20 mm. The nominal sized stone may be combined with smaller sizes in varying proportions
to provide an open textured, medium textured or dense macadam. The material is
capable of being heaped, and the technique of placing is either by hand raking
or paving machine. Such a paving machine is filled with pre-mixed macadam, which
it then deposits in a layer on the surface to be covered. Normally a very thin
layer of bitumen (perhaps 0.1-0.2 litres of 40% bitumen emulsion per M2)
would be applied to the recipient surface to augment the adhesion of the new macadam
to the surface. This is called a "tack coat" and a very low level of adhesive bonding
Macadams require mechanical compaction by rolling with metal wheels
to achieve both structural stability and a smooth finished surface. The surface
displays voids and a negative or concave texture. Macadams have good regulating
capacity, improve ride quality and contribute to the structural strength of the
road. Open or medium textured macadams have a high texture depth, and are pervious.
They provide good skid resistance, surface water shedding and drainage and low
perceived levels of tyre noise.
A drawback of macadams is that they do not achieve structural stability
in layers of less than 20 mm, and the normal lower limit for thickness on roads
is 30 mm. To allow for this thickness it is necessary either to remove an equivalent
depth of the substrate, or to raise kerbs and other street furniture. Both of these
alternatives are costly and inconvenient.
Another example of a product made by combining stone with a bitumen
emulsion is Slurry Seal or Micro Asphalt. This material employs stones of gradually
reducing size, smoothly graded from the maximum size employed to less than 75
microns. The stones are combined with binder emulsion, water, and other additives,
so as to produce a flowable liquid slurry with no capacity for heaping. The material
is placed in layers either by brush, rubber bladed squeegee, or a towed applicator
with rubber sealing gaskets at the leading edge and elsewhere to reduce material
leakage. The layers are normally relatively thin, i.e. around 3 mm to 20 mm, and
the use of slurry seal therefore avoids the problems of macadams described above.
Slurry seals are very dense impervious materials which are not normally
rolled to achieve compaction. If rolling is specified, it is with a smooth pneumatic
tyred machine. Slurry seals display a relatively low texture depth. Texture is
achieve by designing the material so that a proportion of the largest stones therein
are exposed so as to provide a positive, or convex texture. Drawbacks of slurry
seals are that:
- a) They do not contribute significantly to the structural strength of the road.
- b) They have limited regulating capacity, tending to follow and reproduce,
albeit to a reduced extent, profile deficiencies in the substrate. In consequence,
they will not significantly improve the riding quality of a road.
- c) They are not pervious, and water will be shed across the surface rather
than through the material.
- d) They do not achieve high skid resistance, and are not usually recommended
for high speed roads.
- e) Thinner slurry seals, e.g. nominal thickness of 3-6mm would not be expected
to provide long service life.
Document EP-A-0409700 discusses a method and apparatus for producing
a surface coating on a surface such as a road. The surface coating comprises a
layer of binding material into which loose aggregates are compacted. These aggregates,
which consist of loose chips covered in bitumen mixed with a pulverulant material,
have been described as dressed aggregates so as to distinguish them from macadam
which is a bituminous coated product formed as an amorphous mass.
It is therefore desirable to provide an apparatus for laying a surface
for roads and the like which avoids the drawbacks of a traditional macadam surface,
without the loss of performance that results from using a slurry seal.
The invention provides a mobile apparatus for producing and laying
a cold or emulsion macadam on a surface to be covered, comprising:
The apparatus preferably further includes means for providing primer and macadam
for hand application in inaccessible areas.
- means for metering stones and a binder into a mixing chamber;
- means for mixing the stones and the binder together in the mixing chamber to
produce a cold or emulsion macadam;
- means for laying a primer on the surface to be covered;
- means for laying said macadam on the surface to be covered, including means
for distributing said macadam laterally and means for controlling the depth of
said macadam laid;
- means for moving the apparatus along the surface to be covered such that any
part of the surface is first laid with said primer and then with said macadam.
The invention also provides a method for producing and laying a cold
or emulsion macadam on a surface to be covered, the method comprising the steps
- providing a supply of stones and binder within a mobile apparatus;
- metering the stones and binder into a mixing chamber within the mobile apparatus;
- mixing the stones and binder in the mixing chamber to produce a cold or emulsion
- laying a primer on the surface to be covered;
- thereafter laying said macadam on the surface to be covered, distributing said
macadam laterally and controlling the depth of said macadam laid on the surface.
The step of laying the primer and the macadam on the surface to be
covered may be carried out by hand in inaccessible areas, with the primer and the
macadam being provided from the mobile apparatus.
In easily accessible areas, these steps are preferably carried out
fully automatically, and the method further includes the step of moving the mobile
apparatus along the surface to be covered so that any part of the surface is first
laid with said primer and then with said macadam.
Cold or emulsion macadam is used and therefore no heating is required
in the mixing chamber. Cold macadam is non-hazardous in terms of high temperature
and fumes, and is therefore preferable to hot macadam from an environmental and
a Health and Safety point of view. In addition, hot macadam becomes progressively
less satisfactory for laying as its temperature falls, and viscosity of the binder
increases. The consequence of delays is that either the material is rejected or
defective work is carried out.
The binder may be an emulsion of bitumen, polymer modified bitumen
or other hydrocarbon binding agent, and additives may be included to improve its
Solid components may be conveyed to the mixing chamber by use of
a moving belt, a screw, by air blowing or by gravity. The rate at which these materials
are conveyed may be controlled by adjustment of the linear or rotational velocity
of the belt, air flow, screw or rotary valve, or alternatively by adjustment of
a gate or aperture.
Liquid components may be conveyed by creating elevated pressure in
the storage tanks, pumping or any other means of liquid propulsion.
The mixing of the stones and binder in the mixing chamber may be
carried out by any design of blending apparatus, provided that thorough and continuous
blending is achieved in the macadam discharged therefrom. An effective design
is a horizontal twin shafted pug mill with contra rotating shafts, and interlocking
blades. The length of the pug mill may be 1 - 2.0 metres long designed such that
the components enter at one end, and are blended en route to the discharge or
The application of the adhesive or priming layer of binder to the
road surface may be effected by a series of spray nozzles placed within a spray
bar, such that an even transverse distribution of binder is made over the operational
width. The binder is conveyed to the spray nozzles under pressure. Other methods
of distribution may include brushing, screeding, extrusion and the like. The distribution
rate may be controlled or varied by effecting a constant flow in the priming binder,
and varying the forward speed of the apparatus, or integrating the flow rate with
a variable forward speed.
The macadam discharging from the blending apparatus is preferably
distributed across the pre-primed operational width required by screw augers which
may be rotated in either direction. As the apparatus moves forward, macadam passes
under a screed bar whose height and inclination may be adjustable as required to
provide depth control, and conform to other features such as camber.
Screed adjustment may be effected mechanically, hydraulically or
electrically by manual or automatic means. Screed adjustment control may be provided
remotely by cabling or radio transmission, and a platform may be provided at the
rear of the screed for use by the operator. Heating of the screed may be required,
and this may be provided by means such as compressed combustible gas.
Following the laying of the macadam, the new surface is preferably
compacted by rolling, as previously described.
By using a priming layer of high performance binder or adhesive,
the invention enables the adhesive strength of the bond between the macadam and
the substrate to be greatly increased. The amount of binder used in the priming
layer is preferably between 0.6 and 1.8 litres of binder per M2, i.e.
significantly more than is used in the traditional "tack coat" for macadam. The
greatly increased bond strength enables the independent strength of the macadam
layer to be reduced below its normal lower limit for independent structural stability,
without adversely affecting the structural stability of the road. The loss in
strength in the macadam is made up for by the increase in bond strength.
By carefully controlling the application of the priming layer and
the macadam, the invention allows the layer thickness of the macadam to be as low
as 15 mm to 25 mm, dependent on the nominal stone size used.
In addition, the invention allows macadam to be mixed and laid continuously
which is more convenient than the traditional method of pre-mixing the macadam
at a quarry. It provides independence of any external manufactured material supply
source, and eliminates the normal requirement for manufactured material supply
vehicles to operate in train. The total size and weight of machinery is therefore
reduced and access and manoeuviability are improved. Only sufficient macadam required
for laying is manufactured and wastage is therefore negligible.
Figure 1 is a schematic representation of a side view of apparatus
according to the invention.
Figure 2 is a top view of the apparatus of Figure 1.
Description of Preferred Embodiment
Referring to Figures 1 and 2, apparatus according to the invention
includes a stone aggregate hopper 4, which is filled with stones of an approximately
uniform size. Their size depends on the macadam specification and may be, for
example, 6, 10, 14 or 20 mm.
A binder tank 2 is filled with binder material to be used in making
the macadam, and a water tank 3 stores the water that is also required in this
process. Additional liquid components such as chemical additives are stored in
tanks 11 and 12. Each tank includes means for metering its contents into a blender
chamber 6, where the binder, water and stone and any additives are mixed together
to make a macadam. Additional fines or other solids may be fed into the mixture
from a solids hopper 5. The rate of delivery for each component is displayed on
an operator's instrument panel (not shown) and the systems are routinely monitored
for consistency and reliability.
A priming material tank 1 contains binder to be used as the adhesive
or priming coat, applied to the road surface before application of the macadam.
This binder is conveyed to a primer material spray bar 7 under pressure for application
to the road. The distribution rate may be controlled or varied by effecting a constant
flow in the priming binder and varying the forward speed of the apparatus, by
adjusting the flow speed, or by a combination of the two.
As the apparatus moves forward, macadam is discharged from the blender
chamber 4 onto the pre-primed road surface. The macadam is distributed across the
operational width required by screw augers 8 which may be rotated in either direction.
Behind the distribution auger 8 is a screed bar 9 whose height and inclination
may be adjusted by a screed height control 13 as required, to provide depth control
and ensure that the macadam layer conforms to road features such as camber. A platform
10 is provided at the rear of the screed for use by a screed control operator.