Is there a way to put these two needs together for a solution – using crushed concrete in some construction applications, instead of pulling more rock from quarries? 

A landfill operator in Ireland asked WSP Golder to find a way they could produce recycled concrete aggregates, in a way that satisfied Ireland’s Environmental Protection Agency (EPA) that European Union (EU) regulations had been met. 

Why concrete waste matters 

The EU’s Waste Framework Directive says that four conditions must be met if a waste material is to become a “product” that can be used for other purposes. The material must be commonly used for specific purposes, it must have a market value, it must fulfil the technical requirements for the specific purpose, and its use cannot lead to adverse environmental or human health effects. 

WSP Golder soon learned that many entities had tried to satisfy the EPA that the EU’s four conditions had been met. The first three conditions were not hard to meet given that recycled aggregates are commonly used across the world, but the EPA had not been satisfied that crushed concrete would not have environmental or human impacts for the proposed applications. 

That is partly because while a lump of broken concrete might look benign, it can have impacts on the environment. Due to its high alkaline content, concrete has the potential to cause high pH drainage into groundwater. It is potentially high in metals content partly due to the binders used in making the cement. It may also include fly ash from coal-fired power generation. The result can be drainage that is high in sulfates or chloride briny waters. 

However, unlocking this puzzle – finding a use for broken concrete that could be shown to meet the EU standards – would have huge benefits for companies in the Irish construction sector. They would be able to save on landfill fees plus the cost of transportation and reduce what they spent on new aggregate material. They might even be able to dispose of the crushed concrete right on their own building site. 

Taking a risk assessment approach 

To solve the problem, WSP Golder took a hydrogeological risk assessment approach based on European Commission Guidance. 

This began by considering the kinds of uses for which crushed concrete would not be suitable. This involved any use that would place the concrete in contact with water – so this ruled out using it to fill a hole in the ground, where it would interact with groundwater. 

But uses above grade looked more promising, such as an embankment or berm, or as the base of a raised roadway that is above the water table, particularly if the road was paved with an impervious surface that would keep out precipitation. 

Our team investigated the potential impact that using recycled concrete aggregate for road construction might have on groundwater. We did this through probabilistic modelling of the risk posed to groundwater by the leaching of contaminants using WSP Golder’s groundwater risk assessment software package ConSim ®.  

We collected samples of recycled concrete and carried out leach testing to assess potential risks. The modelling exercise incorporated characteristics of Irish aquifers that control contaminant transport and fate, embedded into several scenarios that correspond to different aquifer vulnerabilities, to estimate the potential impact and risk. 

As part of this work, we also established leachate limits for the final product that are considered protective of the environment. We set out restrictions on the end use of the material, including that it cannot be placed in contact with water and was for use only in roadways as modelled in the risk assessment. 

This was based on the premise that restrictions and control measures including leaching limits are required to ensure that the end of waste process does not compromise the safety of the environment. 

Our materials engineering team also prepared a quality protocol, which sets out what the EU calls “end of waste” criteria for the production and use of the products. This means showing that the use of the material is lawful in that it fulfils the technical requirements for the specific purposes and meets the applicable legislation and standards, and that its use will not lead to overall adverse environmental or human health impacts. 

When the quality protocol is complied with, this ensures that the waste material may be used without undermining the effectiveness of the EU’s Waste Framework Directive, so there is no need for waste management controls. 

The way forward for the construction sector 

This is the first end-of-waste decision in Ireland in the construction and demolition sector, and only the second in total. Currently it is specific to our client, so they are the only organisation in Ireland licensed to recycle concrete to produce aggregates. 

Further works will aim to establish beneficial re-use of other types of waste including broken brick, glass and rock that has been isolated from excess fill excavated from the building site. 

This project has great sustainability benefits and is keeping with the principles of the European waste hierarchy which prioritises the reduction, recycling, and elimination of waste where possible. In Ireland, the circular economy also dictates that where materials can be renewed and reused towards a goal of designing out waste, this should be prioritised through policy actions and guidance documents at a local and national level.