A few more of our industry, who attended CIRIA’s half day Geotechnical Conference on Tuesday 19th April, probably are, following Professor Stephan Jefferis’ talk on “Recycled materials, so sustainable are they always perfect?”
Recycled aggregates (RA) account for one fifth or more of the total volume of aggregate use in the UK. Increasingly RCAs are used in a range of applications within the permanent works for new construction, encouraged by the availability of end of waste protocols, specifications, and schemes such as BREEAM, where points can be scored if the RA meets the scoring criteria. On the face of it, what could be more sustainable than using unbound RCAs generated from previous buildings and infrastructure on site; reducing demands for primary aggregate, reducing waste and the cost/environmental/social impact of transporting these materials around the countryside. It is considered such a sustainability “no brainer” that this approach is the norm for much of the UK.
So what’s the problem? In some cases, there is so much focus on the requirements to ensure this material is no longer a waste, that the site specific assessment of suitability for reuse and subsequent specification are often overlooked, with catastrophic results, as illustrated during Professor Stephan Jefferis’ presentation. Heaving due to sulphate attack is costly enough to remediate beneath exterior works such as a carpark, but beneath a building the complexities of rectifying such a problem would be significant.
Such incidences of structural failure are occurring round the world including the UK. One such example in the public domain is the forensic investigation at Holloman Air Force Base (AFB) New Mexico, into heave of newly constructed airbase structures and exterior works which revealed an abundance of ettringite and thaumasite in the sub-base; evidence of sulphate attack of the RCA layers (Rollins 2003).
The key message is to check your specifications, undertake RCA testing and to think about suitability for use. In fact in a “Technical Submission to the Department of the Environment Expert Panel for Pyrites by The Association of Consulting Engineers of Ireland and Engineers Ireland” due to a particular problem with some of Ireland’s primary aggregates, went onto say “In light of the additional unknown risks that may be associated with manufactured or recycled aggregates and the difficulty and cost of rectifying problems ………….we recommend that…………only crushed rock aggregate be allowed under floor slabs of buildings.”http://www.environ.ie/sites/default/files/migrated-files/en/PyriteReport/FileDownLoad,30809,en.pdf