Following the launch of Master Builders Solutions’ revolutionary MasterSuna RCT 323 product in Australia in November 2021, numerous trials have been conducted across Australia to demonstrate the product's performance in treating returned concrete. Many of these trials have focused on reusing treated concrete back into a concrete mix as a sustainable option for the industry to ‘close the loop’ on returned concrete and prevent waste-to-landfill.
In May 2022, Master Builders Solutions, together with one of its valued customers in Western Australia, conducted a full-scale trial encompassing the entire process from start to finish, including treating the returned concrete and introducing the treated material back into a fresh concrete mix as a partial coarse and fine aggregate replacement at various percentages of total aggregate. The trial demonstrated how treated material was reintroduced and highlighted the cost savings associated with purchasing aggregates, haulage, and dumping.
Importantly, even though it is difficult to accurately calculate and extrapolate reductions in CO2(e) emissions in a trial of this size at one location, it is clear from the savings in materials, disposal, and transport that using MasterSuna RCT 323 recycled aggregate will not only deliver significant cost savings but also result in a substantial reduction in CO2(e) emissions for most, if not all, users.
Treating the Returned Concrete
On 3 May 2022 at 2 pm, the team treated approximately 2.4m³ of unwanted ready-mixed material returned from a concrete pour. Three bags of the MasterSuna RCT 323 product were added to the agitator drum of the concrete truck to complete the treatment. The process took approximately 3 minutes of mixing from the addition of MasterSuna RCT 323 product. Once the material had granulated, it was discharged from the agitator onto a suitable hardstand area to avoid cross-contamination with other aggregates while also leaving sufficient space for the materials to be accessed with a front-end loader.
Once the treated load had been fully discharged from the agitator, the piles of treated material were back-bladed to a height of approximately 150mm and left to dry out overnight, ready for remix and break up the material the following morning. The agitator bowl was later inspected at the slump stand to see if any material had remained in the bowl. As all material had successfully discharged, it was determined that the agitator would not have to be washed out after the material treatment. This not only leads to quicker turnaround times for reloading agitators but also helps minimise the risk of additional water in the bowl after wash-out. At 7 am the following morning, the material was re-mixed with the front loader to free up the material and prevent balling. If balling occurs, the loader bucket can be used to flatten the balls by back-blading. The operator can also use the wheels of the loader to roll over and break up the material if required. Once this has been completed, the material can be stockpiled and should remain free from binding together or balling as the hydration process has completed and the material will not rehydrate.
Lab Trials – Gathering Data
Before any full-scale trials are carried out, initial lab trials should be conducted to provide initial data to create a mix design replicable to the control mix. Always carry out PSD, Particle Density, and Water Absorption testing on the treated materials. This will provide data to create a suitable mix design to optimize the aggregate replacement and will allow you to calculate the theoretical yield and free water cement ratio.
Figure 1 – Strength Development for 20MPa footing using a mix with 20% RCT 323 aggregate replacement
Slump | Air Content | 7Day (MPa) | 28Day (MPa) | |
Control | 100 | 3.5 | 18.0 | 23.0 |
20% RCT 323 | 100 | 3.8 | 15.5 | 20.0 |
30% RCT 323 | 100 | 3.8 | 17.0 | 22.0 |
50% RCT 323 | 120 | 3.6 | 15.5 | 20.0 |
Table 1 – Mix Characteristics Using 20, 30 and 50 Per Cent RCT 323 Aggregate Replacement
Field Trials
On 4 May 2022, full-scale plant trials were conducted using a mix incorporating 15% of treated MasterSuna RCT 323 material. Before dosing the material, moisture tests were carried out to ensure that any water in the material was captured and taken into consideration before batching. The MasterSuna RCT 323 material was also re-mixed before use to ensure that it was free-flowing and completely free of any significant lumps.
Slump | Air Content | 7Day (MPa) | 28Day (MPa) | |
Control | 130 | 3.2 | 17.5 | 26.5 & 27.0 |
15% Replacement | 140 | 3.6% | 16.0 | 27.0 & 26.5 |
Table 2 – Control vs. 15% RCT 323 Aggregate Replacement Mix
A 20MPa footing mix was chosen for the trial. Both fine and coarse aggregates were replaced based on past PSD information gathered. Due to the cementitious material present, there was an increase in fines passing the 75-micron sieve, which allowed for the reduction of fine aggregate. The pour consisted of 25m³ for a residential footing. This enabled the team to sample an initial 5m³ from the control mix for comparative data. Midway through the pour, the team batched 5m³ of the MasterSuna RCT 323 mix. A visual inspection of the mix from the slump stand showed no noticeable difference in workability between the MasterSuna RCT 323 mix and the control mix.
A sample of the MasterSuna RCT 323 mix was taken at the plant to conduct initial type testing for slump, air content, and compressive strength, with subsequent testing at 7 and 28 days. Both mixes left the site at a 130mm slump due to the long transit. The control and RCT mix both looked identical in terms of workability and finishability. At the same time, the rheology within the MasterSuna RCT 323 was improved due to the addition of cementitious particles in the matrix. The pour was located 45 minutes away from the plant, which provided a good test for slump retention. No water was added on-site, and no negative feedback was recorded from contractors. The contractors did not notice any difference between the two mixes.
Commercial Benefits
Using mixes incorporating MasterSuna RCT 323 recycled aggregate delivers several significant commercial and environmental benefits, including:
Reduction in Aggregate Purchases: Treated concrete has already been paid for and is being reused.
Reduced Transportation Costs: Trucks are not diverted to landfills or quarries to dump returned concrete, saving on fuel and truck turnaround. Treated concrete is processed in the concrete yard and reused as an aggregate.
Water Savings: No washout is required after treatment, leading to less build-up of waste in washout pits that end up in landfills.
Efficient Use of Equipment: The front-end loader is fully utilised for concrete production and not tied up reloading pit waste into tippers.
Maintenance Savings: Reduced wear and tear on machinery.
Optimised Tipper Use: Tippers can be used for other sales rather than carting waste, reducing the need for washouts and increasing availability.
Quarry Efficiency: Less concrete waste in quarries, leading to more productive time for crushers processing virgin aggregates.
Reduced Waste-to-Landfill: Overall reduction in concrete waste being sent to landfills.
Reduction in CO2(e) Emissions
As noted previously, while it is difficult to accurately calculate and extrapolate reductions in CO2(e) emissions in a trial of this size at a specific location, it is clear from the savings in materials and disposal costs, as well as the reduction in material transport (kg/km), that using MasterSuna RCT 323 recycled aggregate will deliver a significant reduction in CO2(e) emissions for most, if not all, users when compared to traditional mixes.