The use of 0/16 mm and 0/20 mm of Incinerator Bottom Ash Aggregate (IBAA) as unbound subbase has environmental benefits and reduces waste. Advancements in the efficiency of the incineration process and recovery of energy from waste, has meant that 0/16 mm and 0/20 mm IBAA is more readily produced than the larger aggregate fractions 0/31.5 mm).
0/31.5 mm IBAA has track record of use in unbound subbases and is in compliance with Clause 803 of the Manual of Contract Documents for Highways Works (MCHW). 0/16 mm and 0/20 mm IBAA meets the same requirements except for having a different particle size distribution.
Blue Phoenix Group have developed 0/16 mm and 0/20 mm IBAA and demonstrate equivalent performance to conventional Type 1 in restricted Foundation Class 2.
Evidence of Performance
Works to demonstrate equivalence in mechanical performance between 0/16 mm and 0/20 mm IBAA and Type 1 unbound mixtures (0/31.5 mm) have been carried out by Blue Phoenix Limited and evaluated by AECOM Limited.
As part of this assessment, the key mechanical performance requirements of an unbound pavement foundation in accordance with Design Manual for Roads and Bridges (DMRB) CD225 ‘Designs for new pavement foundations’, were identified as the following:
1. Short term performance, including installation and compaction to provide a rut resistant unbound surface for direct trafficking; and,
2. Long-term confined foundation surface modulus linked to the foundation class.
The above two criteria are important to protect the subgrade from damaging strains while providing an adequately stiff base on which overlying layers can be compacted and supported in the long term during in-service conditions.
Performance requirements for 0/16 mm IBAA were assessed against the restricted design given in Design Manual for Roads and Bridges (DMRB) CD225 ‘Designs for new pavement foundations’.
Literature on the performance of unbound mixtures reports that maximum aggregate size has an influence on unconfined performance, including resistance to rutting during direct trafficking and has minimal influence on the mixtures confined stiffness.
Therefore, direct trafficking trials (as per CD225 performance designs) and measurement of confined stiffness of the foundation were included within the site assessments.
The information from the site trials are detailed in AECOM report ‘Technical report on the mechanical performance of 0/16 mm Incinerator Bottom Ash Aggregate for use as an unbound subbase’ (September 2020) and show through assessment and monitoring of two construction projects that 0/16 mm IBAA meets the requirements for an equivalent MCHW Clause 803 Type 1 mixture as detailed for Restricted Foundation Designs in CD225.
• Installation and compaction (as per MCHW Series 803)
• Suitability for direct trafficking as an unbound subbase (as per CD 225)
• Confined foundation stiffness (long term) for a restricted Foundation Class 2 (as per CD225)
This site comprised two trial areas; 300 mm compacted 0/16 mm IBAA subbase layer along the Near Side lane and a 300 mm compacted 0/31.5 mm IBAA mixture along the Off-Side lane.
The Canvey Island site included relatively “worse case” conditions for the construction of an unbound subbase layer over a relatively low construction subgrade surface modulus.
Both sections were installed in compliance with the standard end product requirements of the MCHW, with no observations of any difference in behaviour between them.
The Ferrybridge site comprised numerous trial areas that were monitored for a 12 month period. There was no measurable or observable difference between the 0/16 mm IBAA and control sections during the construction and end product testing.
Back-analysed foundation stiffnesses tested in a confined condition under different pavement configurations using the Falling Weight Deflectometer (FWD) deflection data demonstrate that at both sites, the design requirements of >100 MPa restricted Foundation Class 2 are met.
It was noted that the 0/16 mm IBAA achieved slightly higher foundation stiffness modulus than limestone Type 1.
In all scenarios the 0/16 mm IBAA behaved as a granular material with visibly high interlock, did not segregate during placement and could be readily compacted.
Rut measurements were taken in the vehicle wheel tracks to ensure the design requirements of CD 225 were met.
The trafficking trials demonstrated the required resistance to rutting versus the design requirement using the method detailed in MCHW Cl. 802