Concrete is a fundamental building material used in construction projects of all kinds. It is made by mixing cement, aggregate (such as sand and gravel), water, and sometimes additives or admixtures. Fresh concrete must be placed and cured properly to achieve the desired strength and durability. However, leftover concrete that has started to cure or “set up” is not necessarily useless.
What happens during concrete curing
Concrete curing is a process where the concrete hardens over time after it has been placed. Chemical reactions occur between the cement and water that cause the concrete to gain strength and become rigid. Hydration starts immediately after the cement and water are mixed, but most concrete continues to get stronger for days and even weeks after placement. Full curing can take over 28 days in normal conditions.
So when old concrete has been leftover after a pour, the curing process has already begun. The cement paste has likely hardened considerably compared to fresh wet concrete. Once concrete begins curing, it can no longer be reused as is because it will have lost its workability and ability to be placed and finished properly.
Evaluating old concrete mix
There are a few considerations when deciding whether old concrete can still be used or not:
- Age – Concrete that is weeks or months old is likely a lost cause, but concrete unused from the day before may still have potential.
- Storage conditions – Was the extra concrete protected from the elements or extreme temperatures? Concrete stored outside unprotected is prone to damage.
- Appearance – Check the concrete for visual defects like cracks,aggregate separation, excessive drying or bleed water.
- Consistency – Does the concrete still feel workable and flowable when agitated?
Testing the slump of the concrete can quickly indicate workability. Slump is measured by filling an upside-down cone with concrete, lifting it up, and measuring how much the concrete slumps or spreads out. Old concrete will have little to no slump compared to a freshly mixed batch.
Options for using old concrete
If the leftover concrete passes visual and slump evaluations, consider these options:
Remix and reuse
Adding water and remixing can restore old concrete to a usable state. This is most effective with concrete leftover from the day before that only had an initial set. Use caution not to add too much water which will weaken the concrete. Retesting the slump after water is added will confirm if the appropriate consistency has been reached.
Repurpose
Old concrete that has hardened can be crushed and repurposed as aggregate for new concrete mixes. The old material can be safely reused as up to 10-30% of the total aggregate in a new batch. The used concrete must be clean and free of debris before crushing.
Non-structural applications
Completely cured leftover concrete can still be used for non-structural projects, where strength is not a major factor. Some examples include:
- Gravel or fill material
- Temporary roadway base
- Landscaping elements like planter boxes
- Art, statuary or decorative objects
These applications take advantage of the available concrete without needing to meet any specification or performance measures.
Disposal
If old concrete is past the point of reuse, disposal is the last resort. Check local regulations for proper disposal procedures if the concrete cannot be recycled or repurposed. Letting it harden completely before disposal will prevent any environmental impact.
Key factors for reusing leftover concrete
Here are some key points to keep in mind when evaluating old concrete:
- The sooner leftover concrete can be reused, the better
- Water can restore workability if mixed back in promptly
- Old concrete may be used for non-structural projects
- Hardened concrete can be recycled as aggregate
- Test slump to check concrete workability
- Discard concrete that is damaged or extensively cured
Storing concrete properly for later use
Sometimes extra mixed concrete is intended for use later on. Follow these tips to store concrete properly so it stays usable as long as possible:
- Keep the concrete mix cool and dampened with water – cover with plastic sheeting or wet burlap
- Minimize air exposure while protecting from rain, wind, sun, and freezing
- Store concrete in bulk form, not spread out thin
- Seal storage containers like wheelbarrows to minimize moisture loss
- Discard concrete that smells bad, is cracked extensively, or is dried out
With good storage practices, unused concrete may remain workable for 1-2 days, allowing it to be remixed and poured later. Regularly check stored concrete and remix with water as needed.
Preparing old concrete for reuse
If old concrete is deemed suitable for reuse, it will need proper preparation before mixing into new batches:
- Break up any large chunks and sections – the aggregate should be uniformly sized
- Remove any contaminated pieces or debris like rebar, wood, or trash
- Separate out aggregate and sand from the hardened cement paste
- Crush or grind concrete into small pieces if it will be repurposed as aggregate
- Screen and clean concrete aggregate as needed for purity
- Make sure old concrete is properly graded for reuse as aggregate
Proper preparation helps ensure old concrete will perform as expected when reused and not undermine the overall strength and quality of a new mix.
Is reusing concrete worth the effort?
Reusing leftover concrete provides some advantages but also requires additional labor and processing:
| Potential benefits | Drawbacks |
|---|---|
| Reduces raw material costs | Increases labor for preparation |
| Less concrete waste | May compromise final strength/quality |
| Lowers environmental impact | More variables to control |
| Sustainable construction practice | Testing required to ensure suitability |
In many cases, the extra effort to reuse concrete is justifiable, especially considering environmental factors. However, reusing concrete that is excessively old or in poor condition may not produce a worthwhile payoff compared to purchasing fresh ingredients.
Testing old concrete before reuse
Various tests can help determine if old concrete is suitable for reuse in new mixes:
- Slump test – measures workability and consistency
- Compressive strength – indicates how strong the concrete is
- Petrographic examination – analyzes the cement paste microstructure
- Chemical analysis – identifies compositional properties
- Visual inspection – checks for physical defects and contamination
While most tests require laboratory equipment and expertise, the slump test can be performed on site using simple tools. Slump within an acceptable range indicates the concrete still has sufficient workability for reuse.
Acceptance criteria
When evaluating test results, here are some strength and slump criteria to determine if old concrete is acceptable:
- Compressive strength no more than 20% below original specified level
- Slump between 2-5 inches for reused concrete aggregate
- Slump between 1-3 inches for remixing old concrete
- No significant carbonation or unsound aggregate
- No major physical defects visible
Concrete meeting the above criteria stands a good chance of performing as needed for the intended reuse application.
Effects of reusing concrete
Recycling old concrete can alter the properties and behavior of new concrete batches in several ways:
- Workability – Old concrete aggregate tends to need more water to reach adequate slump.
- Strength – A portion of the overall strength capacity is lost by reusing concrete.
- Shrinkage – Old aggregate that is already cured can slightly reduce drying shrinkage.
- Durability – Carbonated recycled concrete aggregate may be more porous and less durable.
- Color – Variations may occur if the old concrete color differs from the new mix.
Testing of trial batches containing reused concrete is advised to ensure the final properties meet requirements. The mix design and water content may need adjustments to compensate when recycling concrete.
Reuse constraints and cautions
There are some constraints and cautions to keep in mind when reusing concrete:
- Higher strength concrete is not well suited for reuse.
- Old concrete should not exceed about 30% of the total aggregate volume.
- Contaminants and debris in old concrete must be removed.
- Higher water demand typically requires an adjusted mix design.
- Workability time will be shortened compared to normal concrete.
- Strength and durability may decrease slightly.
Within reasonable limits and with careful quality control, reusing concrete is possible in many non-critical applications. However, structural elements relying strictly on strength or longevity criteria may require use of only fresh conventional concrete ingredients.
Quality control
Maintaining quality control when reusing concrete is essential:
- Inspect and test old concrete thoroughly to verify suitability
- Adapt mix designs and proportions as needed
- Closely monitor slump, workability, and setting time
- Ensure contaminants are eliminated from recycled concrete
- Check strength and visual appearance as concrete cures
- Only reuse concrete that meets strength, durability, and aesthetic requirements
With careful control and oversight, concrete containing recycled ingredients can perform adequately. Documentation of quality control measures is recommended if results are challenged.
Environmental advantages
Reusing concrete offers several environmental benefits:
- Reduces demolition and construction waste needing disposal
- Lowers demand for virgin raw materials like gravel and sand
- Decreases mining impacts and natural resource depletion
- Cuts back on manufacturing-related fossil fuel use and emissions
- Can contribute to LEED credits in green building projects
Incorporating reuse and recycling practices into concrete work aligns with sustainability goals and responsible environmental stewardship.
Economic impact
The economic effects of reusing concrete include:
- Saves money by getting more use out of previously paid-for materials
- Avoids purchase costs for a portion of new aggregate and cement
- Additional labor needed to prepare and incorporate recycled concrete
- Testing expenses to validate concrete reuse
- Potentially shorter life span or greater maintenance
- Value gained by diverting waste from landfills
Performing a cost-benefit analysis can determine if concrete recycling pencils out financially for a given project. The upfront savings on raw materials are weighed against long-term factors like reduced lifespan.
Case Studies
Here are some real world examples that demonstrate the feasibility of reusing concrete:
Office Building Parking Garage
After demolishing an old 2-story parking garage, contractors were able to recycle 85% of the concrete rubble as aggregate for new construction. The material that could not be reused was sent to local ready-mix plants for use in lower grade concrete products. Saving on disposal fees and purchase of all new aggregate led to a 10% reduction in overall project costs.
Residential Housing Development
A residential project ground up old concrete foundations on site to create aggregate for driveway base material and drainage fill. Besides the direct savings on basic fill purchases, the reuse initiative created goodwill with the community and helped the contractor qualify for an environmental certification.
Commercial Building Renovation
During renovation of a commercial building, the contractor saved leftover dry concrete mix that was going to be discarded after a pour. The concrete was kept damp and sealed overnight. The next day it was remixed with a small amount of water and successfully used to patch some damaged exterior steps. No new materials needed to be ordered for the minor repair work.
Bridge Demolition Project
After demolishing a small vehicular bridge, the concrete deck sections were transported to a processing facility to be crushed. Approximately 2000 tons of reusable concrete aggregate was reclaimed. The material was used to construct temporary haul roads and equipment pads needed for the new bridge construction. This avoided purchasing virgin gravel and transporting material to the remote site location.
Recap
To summarize key points:
- Old concrete may potentially be reused if it meets certain criteria
- Test slump and strength to evaluate suitability
- Remixing with water can return workability
- Crush fully cured concrete for use as aggregate
- Recycled concrete lowers waste and raw material use
- Up to 30% recycled concrete may be used in mixes
- Adjust mix designs and control quality if reusing
- Environmental and economic benefits should be weighed
Conclusion
While reusing old concrete requires some extra precautions, with prudent testing and control it can be incorporated into new concrete mixes at moderate percentages. This recycling practice helps reduce construction waste, saves on raw materials, and decreases the environmental impact of concrete work. Reusing concrete also provides direct economic benefits in many cases. Contractors should evaluate their leftovers and structure processes to take advantage of concrete recycling opportunities when feasible.
