Dudley is an old industrial area where much of the Principal Route Network (PRN) has been identified as being in need of reconstruction. In view of the high quality subgrade/sub-base (usually Steel slag) on these roads, only the total flexible pavement (that is the bituminous layers) is being replaced during reconstruction, to a maximum depth of 200 mm. The bituminous pavement layers were originally constructed to a maximum depth of 200 mm due to the services and other restrictions at shallow depths making deeper flexible pavements impracticable.
The existing bituminous layers largely contain high quality aggregates, mainly sourced from the local basalt quarry (within 4 miles) at Rowley Regis. Using conventional pavement design, these layers are being replaced with 100 mm and 70 mm Heavy Duty Macadam (HDM) 50 base (roadbase) and binder (basecourse) courses respectively, using aggregates from the same basalt quarry. The surface course is 30 mm thick, 10 mm -Stone Mastic Asphalt (SMA). As an alternative to this conventional maintenance procedure, Dudley Metropolitan Borough Council (DMBC) propose to substitute the base and binder courses with cold recycled bitumen bound material produced using the Foamix process.
Since it is DMBC’s policy to encourage recycling across the board, in combination with the onset of the Aggregates Levy, it became apparent that the PRN reconstruction project provided an opportunity to increase recycling, to limit the movement of materials and to reduce energy input in the process. Some 30 km of the PRN has been identified as in need of reconstruction. Using the experience from three successful in situ recycling contracts during the last 4 years on relatively lightly trafficked roads, proposals are presently in hand to substitute the base course and eventually the binder course with ex situ HDM 50-compliant foamed material. To this effect a limited trial of 300 m length was carried out in July 2003 to test the foamed material in a heavily trafficked part of the Dudley Southern By-Pass.
In July 2003, DMBC initiated trials to test the use of ex situ recycled asphalt to heavy duty specification road products for use in heavily trafficked roads. If successful, the council intend to extend this recycling process across the Borough to reduce the environmental impact and the costs of the reconstruction of the Borough’s Principal Route Network. The trial was carried out as an integral part of a small contract to provide an acceleration lane on the busy A461 Dudley Southern By-Pass. This contract was awarded to Wrekin Construction under the Five Year Partnership Framework Agreement for the PRN. The recycled material was mixed on 14th July 2003 at the temporary Roads Recycling Centre and laid as the base course to the additional lane on the Dudley Southern By-Pass. This lane now carries an average of 12000 vehicles per 12 hour day of which 11% is HGV traffic. WS Atkins advised on the process prior to the trial and their laboratory was engaged to carry out independent testing which is still being analysed.
A nuclear density meter was used to monitor the density which averaged 95% of refusal density, as required in Table 9/13 of the Specification for Highway Works and cores were taken for accurate refusal density calculation. Samples of the mix were compacted at the site laboratory to a density less than refusal. These samples were oven cured at 60°C for 72 hours and then tested for Indirect Tensile Stiffness Modulus (ITSM) as required by Clause 948 of the Specification for Highway Works. These produced averages of 4418 MPa (dry test) and 3216 MPa (soaked test). This compares with minimum requirements of 2500 MPa (dry test) and 2000 MPa (soaked test) given in the Specification for Highway Works, Clause 948, Table 9/11. These results provide the confidence for implementation of the foamed bitumen process in future schemes. In general the material has shown it is an excellent alternative for conventional HDM material. This ex situ process produces a strong engineering material. The fundamental properties are derived from optimizing density and dynamic stiffness, achieved by adjusting the grading, moisture and foamed bitumen contents of the mix. Further contracts/trials are being considered before the process is fully implemented on the next phase of the PRN reconstruction project, beginning in March 2004. The foamed bitumen process should provide for material cost savings of approx 20% and major environmental benefits both within and outside the Borough boundaries through a major reduction in overall energy input and a significant reduction of journeys by construction lorries employed on the works.
All the recycled asphalt used on the trial was procured from recent Highway Contracts in the Borough; 70% of materials were generated from deep road planing of the existing PRN as part of the reconstruction program. The remaining recycled asphalt came from crushed road materials produced by similar reconstruction contracts. All the materials were screened to produce two stockpiles, 28-0 mm and 8-0 mm respectively. The cold recycling process known as Foamix was carried out by Colas and Roadstone Recycling for Wrekin Construction. The foamed bitumen product requires a minimum of 5% mass of aggregate passing the 75 micron sieve. As recycled asphalt generated by the planing processes usually has a low fines content, it was decided to add stockpiled pulverized-fuel ash (PFA) to boost the fines content. After gradings were measured on the three stockpiles, a blend of 80% 28-0 mm, 17% 8-0 and 3% PFA was used to meet the Zone A specification for aggregates used in cold recycled bitumen bound material.
The bitumen was 150 penetration produced by Nynas UK and heated to the correct 160 – 185o C prior to use. The 3% bitumen was injected into the Foamix plant and 2% Portland cement added. The feedstock had a moisture content of 4.5%, so 2.3% water was added to reach the optimum moisture content of 6.8%. The processed material was loaded into the lorries as a cold mix with a predicted curing time of 2 hours. This was laid on the site about 2 miles away and rolled using standard Bomag 161 vibrating roller and finished off with the 120 vibrating roller.
The use of cold recycled bitumen bound materials is permitted under Clause 948 of the Specification for Highway Works, May 2001 amendments. A design guide and specification is given in TRL Report 386.
Dudley Metropolitan Borough Council (from other road works in the Borough)
revised: 19 Mar 2004
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