COMHAIRLE CONTAE ÁTHA CLIATH THEAS
SOUTH DUBLIN COUNTY COUNCIL
MEETING OF SOUTH DUBLIN COUNTY COUNCIL
Monday, March 10, 2008
QUESTION NO. 7
QUESTION: Councillor T. McDermott
To ask the Manger to take all reasonable steps to ensure that the materials used in the countywide footpath renewal programme contain the lowest level of embodied CO2 and energy, such as Ground Granulated Blastfurnace Slag (GGBS) and kerbing made from recycled material such as polymers and would he make a statement on the matter?
REPLY:
Advice was sought from a cement manufacturer regarding the use of GGBS in concrete footpaths.
In the case of concrete in roads and particularly in footpaths, the following technical points were made :
- GGBS concrete is a much slower setting mix than that with Portland cement. Thus, the more GGBS used in the concrete, the slower setting will be the concrete, requiring protection of the fresh concrete surface for longer. This could expose the surface to greater risk of cracking unless a more efficient curing regime is put in place.
- GGBS concrete is also slower to gain early strength and this may not be beneficial for publicly exposed elements such as footpaths. The use of the footpath could well be delayed and it is exposed to damage by vandals in the early hours after casting.
As regards the embodied energy issue, the following points are relevant:
- GGBS is a secondary or byproduct of the production of iron in a blastfurnace. Significant quantities of CO2 are generated in production of both of these products (approx 5 Tonnes of CO2 for every Tonne of GGBS, plus about 4 tonnes of pig-iron). At present, there is no formally agreed allocation of CO2 to each product - thus GGBS manufacturers claim that it is largely CO2 free. The fact that the product is produced abroad does not effect the Irish position regarding CO2.
Transport to Ireland of the raw slag also gives rise to additional CO2.
Roads Department have used polymer kerbs made from recycled material as part of a trial. The material was found not to be as durable as concrete kerbing and were also susceptible to fire damage.