CAC Crowdsourced Climate Implications
Public Works, Transportation
Crowdsourced Effort - Protected Bike Lanes - February 2023
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Executive summary
Background Information
- Aggregates are inert granular materials such as sand, gravel, or crushed stone that, along with water and cement, is an essential ingredient in concrete.
- How is Concrete made?
- Concrete is constructed using cement mixed with an aggregate – a grainy mix of materials such as stone and sand. After mixing, the concrete is poured into a mold and left to harden. The aggregates are sourced from a local body of water and crushed in a natural procedure. That process is not as carbon intensive as cement production.
- Cement is made by firing limestone, clay, and other materials in a kiln. CO2 is emitted from the energy used to fire the material and the chemical reaction produced from the mixture when it is exposed to heat.
- For a concrete mix, aggregates need to be clean, hard, strong particles free of absorbed chemicals or coatings of clay and other fine materials that could cause the deterioration of concrete. Aggregates, which account for 60 to 75 percent of the total volume of concrete, are divided into two distinct categories–fine and coarse. Fine aggregates generally consist of natural sand or crushed stone with most particles passing through a 3/8-inch sieve. Coarse aggregates are any particles greater than 0.19 inch, but generally range between 3/8 and 1.5 inches in diameter. Gravels constitute the majority of coarse aggregate used in concrete with crushed stone making up most of the remainder. The ratio of aggregate to concrete will affect the embodied carbon of the concrete.
- Recycled aggregates and regular aggregates (also known as virgin aggregates) are both used in construction, but there are several key differences between them:
- Source: Regular aggregates are sourced from quarries or natural sources such as rivers, lakes, or mountains. Recycled aggregates, on the other hand, are derived from recycled construction and demolition waste or industrial by-products. Concrete is the highest consumed product on earth besides water.
- Production process: Regular aggregates are produced by mining, crushing, and processing raw materials, which can have significant environmental impacts. Recycled aggregates are produced by crushing and processing recycled materials, which can help reduce the amount of waste sent to landfills and the need for new raw materials.
- Quality and consistency: Regular aggregates are produced from a consistent source, which ensures a uniform quality and performance. Recycled aggregates, on the other hand, can have variable quality and consistency depending on the source and processing method.
- Cost: Regular aggregates are often more expensive than recycled aggregates because of the high cost of mining and transportation. Recycled aggregates are generally less expensive because they are sourced locally and do not require mining or processing of new materials.
- Concrete releases over 4 billion tonnes of carbon dioxide annually from this industry. Cement is responsible for about 7% of global greenhouse gas emissions.
- The increasing material weight of the world’s economies is putting a more dangerous level of stress on the climate and natural life support systems than previously thought
- Resources are being extracted from the planet three times faster than in 1970, even though the population has only doubled in that time, according to the Global Resources Outlook, which was released in Nairobi on March 12, 2019 (Watts. 2019. “Resource extraction responsible for half world’s carbon emissions”. The Guardian. https://www.theguardian.com/environment/2019/mar/12/resource-extraction-carbon-emissions-biodiversity-loss ).
- Disruption to local ecosystems and habitats during the construction process is a significant concern regarding aggregate use. (Ontario Ministry of Natural Resources and Forestry. 2018. “Sidewalk Construction: Protecting Trees and Natural Areas.”)
Climate implications
Climate Implications
High-Level Questions
- Is a new building necessary or is it possible to use existing buildings for the project?
- Have all material options been considered (i.e., alternative materials to recycled aggregate, including mass timber)?
- If there is no other option for materials, can existing material be refurbished/reused (adaptive reuse)?
- Can the lifespan of the material be expanded to reduce maintenance needs?
- Have additives to the concrete mix (such as nano coil silicones) been considered?
- How can we reduce the cost of tearing down and building new?
- Is it possible to reuse “in place” instead of transporting recycled aggregate to new location, to reduce GHG emissions?
- What incentives exist to encourage recycling aggregate?
- Are there any guidelines around demolition to encourage recycling aggregate?
- Equity in the vendor selection process. Are testing and startup costs keeping smaller innovators out of the bidding process?
- Is the cost of recycling worth the environmental and financial implications? Is it cheaper? More costly? What are the benefits?
Climate Opportunities
- Regular aggregates have significant environmental impacts, including land disturbance, habitat destruction, and energy consumption.
- Recycled aggregates can reduce the amount of waste sent to landfills, conserve natural resources, and reduce greenhouse gas emissions.
- While cement causes the majority of emissions, recycled aggregate can help reduce emissions as a whole
- Link to use of recycled aggregate in Ontario municipalities.
- Reducing greenhouse gas emissions by decreasing the need for virgin materials and the energy required to extract, process, and transport them. (U.S. Environmental Protection Agency. “Advancing Sustainable Materials Management: Facts and Figures 2018.” 2018.)
- Concrete has a high embodied carbon; recycled aggregate has a lower carbon footprint. Most of the GHG implications come from the concrete component, most of the land use and water hazard issues come from the aggregate. Increasing use of recycled aggregate can significantly advance waste diversion goals.
- Where recycled aggregate can’t be used in construction projects there is low carbon virgin concrete that can be used. Resource on low carbon concrete.
- Conserves natural resources, such as gravel and sand. (Ontario Ministry of Transportation. “Recycled Aggregates.” 2019.)
- In addition to recycled aggregate, can introduce concrete rehab projects, plus increase the lifecycle of roads/sidewalks through additives – get this mandated (will be more costly, but cost savings in long term)
- Expand the lifespan of the concrete to reduce maintenance needs, such as through additives like nano coil silicones to strengthen the concrete mix
Climate Risks
- Challenges related to recycled aggregate use are highlighted here and here. It is from 2009.
- Inconsistent supply – having recycled options when you need it is a challenge
- Contamination of existing material – ex. Was looking for giant pieces of concrete, and there was supply next-door but it had to be extracted and there was more rebar in there, so chunks came out smaller
- Challenge of trying to move towards a circular economy
- Incentives and disincentives not really aligned; waste needs to be more expensive, so it’s more valuable to use recycled content (take-make-waste model is more economically viable) – systems change, guidelines around waste removal
- Tear down and new build is the most intensive carbon-wise and using recycled aggregate is not the most carbon-intensive part of concrete, and therefore is not enough to significantly lower the carbon footprint
- Using recycled aggregates changes the required volumes of other components, so may complicate the process, but will depend on the product
- Processing to reuse ‘in place’ must be prioritized because transportation costs for recycled aggregate from one location to another is very expensive and results in high GHG emissions
- It is still a part of a concrete mix so there will be significant GHG emissions – use alternative materials instead when possible such as asphalt and mass timber