Asphalt Laying & Quality

All underlying surfaces must be inspected and be structurally sound before any laying takes place. If the underlying surface is inadequate to support the new asphalt then laying should not commence until the condition of the surface is brought to the attention of the Client.

The levels of pavement courses should be determined from the true finished level of the road surface and calculated back from the vertical profile and the crossfall as shown in the relevant drawings.

Depth levels are checked and recorded on dip sheets to establish material requirements and conformity with the specified thickness.

If no kerbs or existing adjacent material is present then an imaginary edge should be blocked up to establish a means of level control.

A string or gut line should be set usually at 100mm above the applicable finished level. If the width of the carriageway is more than about 8m or is cambered, the block up height will have to be increased.

The receiving layer for each course shall be dipped to ensure that it has been finished to the correct level and adequate crossfalls are maintained in accordance with the contract specification before any new material is laid.

The spacing of the dips should normally be 10m longitudinally and 2m transversely or as specified in the contract specific Appendix 7/1, Schedule.

Following a visual inspection, the existing layer surfaces must be cleaned by mechanical road brush to remove all debris, dust and organic material prior to any fresh material being laid.

Paving on a contaminated surface will adversely affect the bond interface between layers. All existing channels should be cleaned and existing topsoil/vegetation encroaching on the surface to be overlaid should be removed prior to sweeping of the surface.

Prior to milling, Statutory Undertaker plans should be reviewed and the site appropriately surveyed for services. Where possible a Ground Penetrating Radar survey should be undertaken to detect any services within the pavement layers. The position of all existing ironwork should be recorded.

When milling (traditionally described as planning) existing asphalt, it is important to ensure that no thin veneer remains. All delaminations should be removed to prevent potential defects and lack of bonding. Where necessary, delaminated areas can be removed by increasing the depth of milling to establish a sound laying surface.

All drainage and ironwork should be cleaned and loose materials removed. Ironwork should be lifted or installed at its final level before the laying of surface course is undertaken, except where the installation method statement specifically states otherwise.

Gullies must be covered before surfacing with a suitably sized metal plate.

Vertical faces or ironwork should be painted with any of the following;

• 40/60 or 70/100 paving grade bitumen
• Cold applied thixotropic bitumen emulsion of similar grade
• Polymer modified bitumen emulsion bond coat

Unless otherwise specified in Appendix 7/1 (or other specification documents as applicable to the works) the faces of all cold upstanding edges, including previously laid asphalt, against which hot asphalt are to be laid to form joints shall be treated with one of the following:

1. Hot bituminous binder with a penetration of not less than 40 pen;
2. Hot elastomeric polymer-modified bituminous binder complying with BS EN 14023 with a penetration of not less than 40 pen;
3. Cold applied thixotropic bituminous compound of similar bitumen or polymer-modified bitumen grade;
4. Polymer-modified adhesive bitumen strip with a minimum thickness of 2mm.

A bond coat should be applied to the surface of the receiving layer prior to the laying of a new layer in the construction in accordance with SHW Clause 920.

The bond coat is applied at a uniform rate of spread using calibrated metered mechanical spraying equipment or spray tanker.

Any emulsions which have accumulated in hollows and the like should be dispersed by brushing before it is overlaid, to prevent blistering.

The bond coat must be allowed to break (turn from brown to black) before surfacing starts; weather conditions will dictate the break time.

Bond coat rates of spread are in accordance with BS 594987 or as detailed in proprietary system manuals.

Tack coats should only be used when small quantities of asphalt are being laid on lightly trafficked sites such as footways and driveways. The use of tack coat is for small and inaccessible areas, application of tack coats may be hand applied.

Levels for Minimum Compacted Thicknesses

The nominal size of the aggregate in the material should be appropriate for the nominal thickness of the layer and must be within the range specified in Tables 1A, 1B, 1C and 1D from BS 594987 (Tables produced by kind permission of BSI).

Regulating course should be used when levels require adjustment to achieve specified profile using appropriate materials from BS EN 13108-1, BS EN 13108-4 or BS EN 13108-5.

The thicknesses in the Tables are subject to any provisions detailed in Appendix 7/1, Schedule 3 or the Clients written instructions.

Table 1A – Nominal target and minimum compacted layer thickness for asphalt concrete (AC) mixtures conforming to BS EN 13108-1 (see PD 6691:2015, Annex B)

1. In favourable conditions, HRA surface course Type C 35/14 and Type F 35/14 may also be laid at a nominal target thickness of 45mm with a minimum compacted thickness at any point of 40mm.

Further guidance can be found in Transport and Road Research Laboratory Research Report 4 [3].

Table 1C

Nominal target and minimum compacted layer thickness for SMA surface course mixtures conforming to BS ENB 13108-5 (see PD 6691:2015, Annex D)

Table 1D

Nominal target and minimum compacted layer thickness for other SMA mixtures conforming to BS EN 13108-5 (see PD 6691:2015, Annex D)

The Approximate Rates of Spread of Asphalt

The rate of spread of material is influenced by the grading and density of the aggregate, condition of substrate and degree of compaction. Table B. 1 from BS 594987 is for guidance only and provide a range of spread rates that are most likely to be achieved taking into account the different asphalt mixtures and the range of compacted thicknesses. A heavier rate of spread may be required when the substrate is irregular.

Table B1

Approximate rate of spread  

Quarries have their own procedures for ordering material. Notice of imminent work may be required by the quarry in order to acquire the necessary components for the asphalts required e.g. specialist binder, fibres, etc.

Clear instructions of required material specification, tonnage, location, of site, time to be delivered to site, rate of delivery of the material to site, contact details and name of site supervisor, any special information required for site e.g. site inductions for delivery drivers, ingress and egress to site and any other information relevant to the job is required.

Weather and Surface Temperature

Asphalt should not be laid on surfaces that are frozen or covered in ice or snow, or under standing water. Laying should be suspended during heavy rainfall.

The cooling effect of wind acting on a newly laid causes the layer to cool more quickly than a low ambient temperature. Laying and compaction should be completed without delay and consideration given to increasing the depth of material and/or changing the binder type to suit conditions. Refer to SHW Clause 945 for further details.

Laying Conditions (Extract for BS 594987)

Asphalt must be delivered in insulated and sheeted lorries that are free from contamination inside the lorry body. Sealing Grit, water or a proprietary release agent can be used sparingly to assist the discharge of material.

Gas oil or diesel must not be used.

Delivery times should be planned in conjunction with commencement of laying to minimise interruption to the laying process and to minimise temperature loss.

A trained banksman must be present and must be readily identified. The banksman directs drivers reversing, discharging the load into the paver and leaving the paver and watches for the presence of overhead lines and other hazards or obstructions. Supply vehicles must not be allowed to bump into the paver.

The hopper should be filled to desired level before supply vehicles move off and no asphalt dragged by the lorry tailgate from the hopper onto the surface being covered.

Paver

To lay asphalt continuously at the required width, depth, profile, and camber, the paver used should be appropriate for the work to be undertaken. Some pavers, particularly older machines may require screed extensions.

Prior to the commencement of laying, the air and road surface temperatures must be checked, the screed must be pre-heated, the augers loaded with material, and the screed and auger set to the required laying depth.

Ideally, asphalts should be laid as soon as possible after delivery, with a continuous supply to the paver to minimise the number of joints.

Diesel must not be used for cleaning the augers and the hopper; cleaning solvents when used should not come into contact with the material to be laid. Personnel must not walk on a newly laid mat to avoid leaving footprints!

Delivery and rolling temperatures must be recorded for each load and included in site laying records by suitably qualified personnel. Calibrated thermometers should be used.

Material Delivery and Compaction Temperatures

Minimum Recommended Delivery and Rolling Temperatures

(Extract from Table A.1 of BS 594987)

Note: Different temperature may apply when using PMB or additives.

For the full range of materials and binder grades, refer to BS 594987 Table A.1

Important: Delivery temperature is the load measured within 30 minutes of arrival on site. Problems may occur with cooling in the wagon sides and ends of loads, forming lumps that can cause dragging.

The temperatures above are absolute minimums, material below this temperature should be sent back to the coating plant.

Joints

Longitudinal joints (sometimes called rip joints) should be staggered by 300mm parallel to the layer below, or as required by the contract Series 700 Appendices. Transverse joints should also be stepped to allow appropriate vehicular access to milled areas.

Transverse joint should be made flush by cutting the edge to give a vertical face, removing loose material and painting the vertical face with hot bitumen; joints can be sweated/heated using a hot tamper or hot shoe.

Longitudinal joints can either be cut vertically as above, but it is recommended that a chamfered edge is created during the compaction process.

Pavers working in echelon eliminate rip joints and are therefore highly desireable where practical.

Fully compacted and properly made joints are absolutely vital to ensure a durable asphalt.

Hand Laying

During hand laying excessive raking may cause segregation which results in poor compaction and possible failure of the surface. Hand laying should be restricted to small localised areas or locations such as footways.

Asphalts laid by machine benefit from being compacted initially by the action of the screed. In contrast hand laid material relies solely on compaction by the roller, therefore mechanised compaction and any hand tamping should commence as soon as possible once raking has been completed.

Thicknesses of Layers

If not stipulated in the site contract the permitted deviation of the level of the finished surface, at any point on the pavement layer from the true surface level, should not be greater than the following values (BS 594987):

1. subbase to receive base: -30 + 10 mm;
2. base to receive surface course ±8 mm,
3. base to receive binder course ±15 mm,
4. binder course to receive surface course on roads: ±6mm; and
5. binder course to receive surface course on areas other than roads, e.g. car parks and playgrounds: ±10mm.

Note that the provisions of SHW Table 7/1 requirements may take precedence over the values stated.

Table 7/1 (02/16) Tolerances in Surface Levels of Pavement Courses

*Where a surface water channel is laid before the adjacent road pavement layer the top of that layer, measured from the top of the adjacent edge of the surface water channel, shall be to the tolerances given in Table 7/1.

Chipping Machine

The chipping machine must be free from contamination and the rate of spread checked.

In the case of chipped hot rolled asphalt surface courses, prior to the first pass of the roller, a uniform layer of coated chippings should be applied to the surface which shall then be rolled to compact the mixture and embed the chippings.

Unchipped channels not less than 150 mm wide are normally left in order to encourage surface water to flow into the gullies.

Compaction

A hot tamper should be used to seal asphalt around ironwork and also to take out any anomalies in the surface if required.

There should be as many rollers as are necessary to compact the asphalt laid to specification. Equally, the capabilities of individual rollers need to be appropriate in the prevailing circumstances.

Cutting discs can be attached to rollers in order to cut the edge of rip joints.

Compaction should commence as soon as possible without causing undue displacement or surface cracking of the asphalt. It is recommended that rolling is undertaken as close as possible to the rear of the paver. Cold weather and incident wind in particular will reduce the time available for compaction; refer to Clause 945 of the SHW for further guidance. Rolling should be undertaken in such a way as to achieve the correct surface profile and finish. The first pass of the roller should be over the joint. For unconfined edges chamfered edge compaction, no distinct roller marks are to be left on the surface.

Rollers must not be allowed to stand on compacted asphalt that is still warm enough to result in indentation.

A suitable type and number of rollers will be required to match the tonnages laid as detailed in section 9.2.3 of BS 5949897.

Material Testing

Testing should be carried out as required by particular contract provisions, which will often require tests as set out in BS 594987 and Appendix 1/5. Such testing needs to be undertaken by suitable trained, qualified and experienced members of staff and laboratories accredited by UKAS for the relevant test should be employed. All test reports should refer to method(s) used.

Compositional Analysis

Compositional analysis normally means assessing the grading and binder content of an asphalt. Samples are taken of the specified material from which the binder content can be determined by difference or binder recovery using solvent or by ignition method.

The recovered aggregate is graded using a series of sieves. The aggregate grading result is calculated as the percentage passing each of the applicable sieves as referred to in BS EN 12627 parts 1, 2, 28 + 39. Sampling should be to BS EN 12697-27.

Rate of Spread of Precoated Chips

If there is no texture depth requirement, then the rate of spread of chipping is often specified as being not less than 60% of the shoulder-to-shoulder cover (as measured on a tray of minimum size 600 mm x 600 mm), This testing is often carried out by the contractor.

To check that this rate is complied with during the laying process, 300 mm square trays are laid out between the paver and the chipper as depicted in diagram. The trays are weighed using a spring balance and the weights recorded. Where chippings are added to the surface course of a footway, the rate of spread is not specified as the reason for doing so is purely aesthetic as described in BS 598-1.

Rate of Spread of Chipping Applied by Hand

The rate of spread is determined from the mass of chippings used divided by the area of the road covered by these chippings.

Compaction Testing Methods

Cores to be used in tests for compaction must not be taken from freshly laid asphalt until it has cooled. These cores are tested for their in-situ maximum and refusal densities and for their in-situ air void content.

Indirect density measuring devices can be employed to monitor the level of compaction being achieved during rolling. They are also used as an alternative to coring and/or to add to the data obtained from core locations. The advantage of these devices is that they are non-destructive test methods. Importantly, nuclear and other types of indirect density gauges require calibration for the specific mixture being tested. See Annex 1 pf BS 594987.

Measurement of Surface Texture

The volumetric patch test is undertaken after the asphalt has been laid, compacted and cooled. Testing is undertaken before the surface is trafficked. The surface must be completely dry.

The test is carried out over 50 m lengths evenly spaced, representative of the section of carriageway under construction. 10 individuals’ measurements, 5 metres apart are made over 50 m lengths diagonally across the lane width.

The average of the 10 results is calculated, the result of which shall be not less than the required value as shown in SHW Series 900 as outlined below:

• Table 9/3 for materials other than Clause 942 materials
• Tables 9/12 or 9/13 for Clause 942 materials

Where textures specified in the contract this takes precedence.

Rolling Straight Edge

The rolling straight edge is a 3 m long rigid frame consisting of two parallel rows of rubber tyred wheels which has, at its mid-point, a sensing wheel mounted between the two parallel rows of supporting wheels.

The central section is free to move such that it detects depressions, but not bumps, in the road surface. The operator pushes the straight edge along the length of road to be tested as a slow walking pace. A bell or buzzer is set to operate when a pre-selected size of irregularity is exceeded.

Vertical movements of this wheel are transmitted to a dial with graduated scales on the instrument head as well as the distance travelled, results of which are recorded.

Other tests and testing equipment

• Hot sand test for coated chipping to identify coking of the bitumen coating on precoated chippings.
• Light weight deflectometer is used to measure the surface modulus of unbounded materials and the test method is given in BS 1924.
• Falling weight deflectometer for measuring the deflection and structural properties of the pavement under loading.
• Coring for measurement of course thickness and examining the structure of failing pavement.
• Griptester for measuring and monitoring the potential skid resistance of the surface.
• ITSM (indirect Tensile Stiffness Modulus) a measurement of the resistance of the asphalt base and binder course to vertical loading stresses.
• Wheel tracking for measuring the asphalt’s resistance to deformation/rutting.

Falling Weight Deflactometer

For load/deflection measurement generated data combined with layer thickness can be confidently used to obtain the in-situ resilient E-moduli of a pavement structure.

Grip Tester

Skid resistance measuring device for roads. It can provide instant friction results helping to keep the network safe and moving.

Before opening the site to traffic a visual inspection should be carried out and the following points should be taken into consideration.

Priority:-

• Allow material to cool sufficiently before permitting trafficking/pedestrians to prevent deformation or scuffing and refer to BS 594987 section 10.
• Tidy site, sweep and collect loose chippings, clear site of debris, spoil, cutting, tools etc.
• All joints are sealed with appropriate material and open joints are ramped and signed.
• All Ironwork is sealed, adjusted to the correct level, is uncovered, lid closed, gullies cleaned and there is no standing water.
• New road markings to be as per Traffic Signs Manual Chapter 3.
• New signing to be as per Traffic Signs Manual Chapter 3.
• Any temporary traffic control measures still to be left in place to be as per Traffic Signs Manual Chapter 8.
• Remove all signs and cones by appropriate designated operatives when instructed by an authorised senior site representative.
• Make sure site is safe before opening to traffic/pedestrians.
• Inform NRSWA Officer, Client, Local Authority, Emergency Services (as appropriate) when works are complete.

Advisable: –

• Within 1 week – check site for abnormalities, surface damage, loose chippings, ironwork, deformation etc. and take any necessary corrective measures.
• 3 to 6 months – as above.