Trial proved environmental benefits of 0/16MM IBAA

Britpave member Blue Phoenix UK (BPUK) has undertaken site trials that underline not only the environmental benefits, in particular the lower carbon footprint, of Incinerator Bottom Ash Aggregate (IBAA) but also how the use 0/16mm IBAA can fully meet the performance benefits of specifications CD225 and the MCHW Series 800. National Highways accept the report provides adequate justification for a ‘departure’ from the grading requirements of Cl 803 on their schemes.

IBAA is a sustainable alternative to natural aggregates. It is derived from the Energy-from-waste (EfW) processing of residual municipal solid waste. The main residue of the process, approximately 25% of the initial volume of waste, is Incinerator Bottom Ash (IBA) which consists of materials such as metals, brick, stone, glass and ceramics. IBA can then be processed to remove the metals and screened to the required particle sizes. The resultant Incinerator Bottom Ash Aggregate (IBAA) can be used as a manufactured aggregate. IBAA is a prime example of the circular economy, where waste material is turned into a sustainable cost-effective construction material, with a lower carbon footprint than natural aggregates.

Typically, IBAA does not routinely meet the 0/31.5mm grading requirements for SHW Cl 803 Type 1, it needs to have an addition of coarse natural aggregate. This is both expensive in financial and carbon terms and technically unnecessary, as grading and performance as subbase are not directly related.

Encouraged by National Highways’ focus on innovative products and processes to meet the sustainability targets of the Road Investment Strategy, BPUK was determined to examine the potential of finer grade 0/16mm IBAA as an unbound subbase to achieve a Foundation Class 2 as specified within MCHW CD225. The company believed that the use of 0/16mm grade offers further environmental and construction benefits including lower carbon footprint, less segregation issues, reduced density compared to primary aggregates and improved compaction.

To prove these benefits BPUK commissioned fellow Britpave member AECOM to undertake an independent technical assessment of the mechanical performance of IBAA when used for an unbound subbase within standard pavement construction. A range of trial pavement areas were constructed and then monitored for a year at the Ferrybridge Multifuel 2 (FM2), West Yorkshire, site. FM2 is a new £300 million thermal power plant for Enfinium. The trial areas included both 0/16mm IBAA, 0/31.5mm IBAA and a local natural aggregate Type 1 for comparison.

The trial found no measurable or observable difference between the 0/16mm IBAA sections and the 0/31.5 control sections. The 0/16mm IBAA behaved as a granular material with visibly high interlock, it did not segregate during placement, could be readily compacted and fully met the design requirements of >100MPa restricted Foundation Class 2. Indeed, it was found that the 0/16 mm IBAA achieved slightly higher foundation stiffness modulus than the limestone Type 1, that was used for comparison. Rut measurements taken in the vehicle wheel tracks proved that the design requirements of CD225 were met whilst the trafficking trials demonstrated that the requirements of MCHW Clauses 802 and 803 were met.

To download a copy of the case study ‘The benefits of using 0/16mm incinerator bottom ash aggregate’ visit www.britpave.org.uk/publications