Concrete is one of the most widely used building materials in the world. It’s strong, durable, and relatively inexpensive. However, the production of concrete requires a tremendous amount of raw materials like sand, gravel and cement. This results in a large carbon footprint.
An alternative is using recycled materials to make concrete. This reduces waste, saves money, and lowers the environmental impact. With some basic knowledge and techniques, it’s possible to reuse many discarded items to create recycled concrete.
Gathering Recycled Materials for Concrete
The first step is collecting suitable recycled materials that can be substituted for traditional concrete ingredients. Here are some of the most common recycled items used:
Crushed bricks or masonry: Bricks, concrete blocks, broken tiles and other masonry can be crushed into aggregates. Ensure there are no contaminants like plaster or mortar attached.
Recycled concrete rubble: Leftover concrete from demolition projects can be crushed into different sizes to replace gravel. Remove any steel reinforcement bars first.
Crushed glass: Clean glass bottles and containers can substitute for sand. Use safety glasses when crushing to avoid injury.
Slag: This byproduct from steel mills has a similar texture to sand. Iron slag gives concrete a blue-green color.
Supplementary Cementitious Materials
Fly ash: A byproduct of coal combustion in power plants, fly ash contains silica that can replace some cement.
Slag: Ground granulated blast furnace slag is another substitute for Portland cement.
Silica fume: This super-fine powder contains reactive silica produced when silicon metal is made.
Creating the Concrete Mix Design
The next step is developing a mix design using the recycled materials. Some key factors to consider:
Aggregate size and gradation – Use a blend of different sizes from fine to coarse to improve density.
Aggregate strength – Stronger aggregates like crushed concrete require less cement paste.
Aggregate porosity – Materials like slag absorb more water so adjust the water content.
Reactivity – Fly ash and slag react differently than cement and require testing.
Fresh concrete properties – Check for the desired workability and set time.
Compressive strength – Evaluate if the concrete reaches the needed hardened strength.
Testing trial batches will help finalize an optimal recycled concrete mix. Getting the proportions right takes some trial and error.
Mixing and Placing Recycled Concrete
The mixing process for recycled concrete is the same as conventional concrete:
Combine the dry ingredients – aggregates, cementitious materials, and additives.
Mix in water to reach the required consistency. Don’t add too much water.
Allow time for the concrete to become fully cohesive.
Transport or pump the concrete to the forms or molds.
Compact the concrete as it’s placed.
Screed off the surface.
Apply a finish if needed.
Pay special attention to ensure the recycled concrete properly consolidates without segregation. Some best practices:
- Use smaller maximum aggregate size
- Increase fine materials
- Add viscosity modifiers
Let the concrete cure adequately before removing any forms. Keep it wet for 3-7 days.
Pros and Cons of Recycled Concrete
Using recycled materials to make concrete offers many benefits:
- Reduces need for landfill disposal
- Lowers carbon footprint by reusing materials
- Can be cheaper than concrete made from virgin materials
- Helps conserve natural resources like gravel and sand
- More variable properties than conventional concrete
- May have higher shrinkage
- Lower strength potential in structural applications
- Requires more testing to optimize the concrete mix
With careful planning and testing, recycled concrete can often be a viable and eco-friendly building material alternative. Just take the time to understand the characteristics of the recycled ingredients used in order to produce quality concrete.