Technical information

Roadspan Asphalt products are available from 10 premix Plants in Mpumalanga, North West, Free State and the Eastern Cape. We are a high performance asphalt producing organisation, committed to making good quality asphalt and giving customer satisfaction. The company´s members of staff have many years of experience to enable us to achieve these aims.

Our hot mix asphalt is produced at a hot-mix asphalt facility. A hot mix facility is an assembly of electric equipment where aggregates are blended, heated, dried and mixed with bitumen to produce a hot asphalt mix, meeting specified requirements. The facility may be stationary or mobile enabling it to move from contract to contract. For hot mix asphalt we use the drum-mixing facility. Hot mix asphalt is produced using a cold feed system, a bitumen proportioning system, a rotating drum mixer, and a storage silo. In this process aggregate is both heated and blended with bitumen in the same unit, the drum mixer. However, for our cold mix asphalt we use a continuous mixer with a drum drying facility.

 

THE DESIGN OF ASPHALT MIXES

Our objective when designing asphalt mixes is to achieve a durable mix of high stability. The mixes must have sufficient voids between the aggregate particles. This ensures that sufficient bitumen is added to achieve flexibility, durability and workability, without sacrificing resistance to permanent deformation. An XY recorder device is used to determine the Stability and Flow characteristics accurately in our design laboratory.

Our mixes have to fulfil a wide range of requirements for today´s traffic. In particular they must have the ability to:

• Resist permanent deformation

• Resist fatigue cracking

• Be workable during laying, enabling the material to be satisfactorily compacted with the available equipment

• Be impermeable, to protect the lower layers of the road from water

• Be durable, resisting abrasion by traffic and the effects of air and water

• Contribute to the strength of pavement structure

• Be easily maintained and, most importantly, this must be cost effective

In addition to the above, wearing course materials must also fulfil the following tyre/pavement interaction requirements:

• Provide skid resistance surface under all weather conditions

• Have an acceptable level of rolling resistance

• Provide a surface which, under trafficking, produces an acceptable level of tyre/road noise

• Provide a surface of acceptable riding quality

HIGH QUALITY PAVING

Mixes with large differences in aggregate size, plus a low binder content are very prone to segregation. Steps which can be taken if segregation should occur while paving, are as follows:

FORMATION OF TRANSVERSE STRIPS

Strips of segregated mix appear in the pavement at right angles to the direction of travel when feed trucks change.

CAUSE

Segregation is promoted by frequently folding the hopper sides in or out when the material hopper is almost empty.

REMEDY

Ensure that the material hopper is always well filled. Watch the mix level of asphalt in the screed of the paver. See to a sufficient supply of mix at all times, so that the need to fold hopper sides in or out, is reduced to a minimum.

STRIPS IN THE MIDDLE OF THE PAVEMENT

A porous or rough strip of segregated material appears in the middle of the pavement.

CAUSE

The head of mix in front of the screed is too low.

REMEDY

Increase the clearance between the centre auger box and the tamper shield. Adjust the augers in height. Generally the auger blades should be set some 4 cm higher than the lower edge of the screed. Turn one or two auger blades in the area of the centre auger box round, to convey mix inwards.

STRIPS IN THE LATERAL AREAS

When paving large widths, strips of segregated mix appear in the lateral areas of the pavement.

CAUSE

No uniform head of mix in front of the screed across the pave width.

REMEDY

Fit limiting plates for the auger tunnel and deflector plates across the full pave width. Fit mix level sensors for the augers to the end plates of the screed. Check the mix level sensors for correct set-up and optimize settings, if necessary.

PATCHES OF MIX IN THE SURFACE TEXTURE

Patches sporadically appear in the surface texture while paving. The surface of these patches is smoother or smeared with bitumen.

CAUSE

Mainly fine grains with a high bitumen content, such as residues from the trucks, are delivered to the job site with the mix. Alternatively, the screed is not sufficiently heated.

REMEDY

Clean conveyer and screed and check the tamper shield for correct set-up. Reduce the tamper speed. Check screed heating for proper function.

 

AN OPTIMAL MATERIAL FLOW PREVENTS SEGREGATION
The paving material must be kept in a highly homogenous state on its way through the paver, from the material hopper to the screed.