How to Batch and Mix Roller Compacted Concrete with a CON-E-CO Lo-Pro Central Mix Concrete Batch Plant Utilizing a CON-E-CO 12 Cubic Yard Tilt Mixer
Roller Compacted Concrete (RCC) is a method of concrete production and construction that produces a “dry/lean/stiff” material which looks more like a traditional aggregate base than a traditional concrete “mud”. In a sense, it is not much different than Cement Treated Base (CTB), and in another sense it is not much different than Conventional Concrete (CVC). Where it differs from the other materials are in methods of engineering, production and placement. It is not a new material, with a history of use (civil and non-civil) dating back decades. It is, however, still somewhat a niche method, which warrants caution on any project. Fortunately, there are a plethora of available guides, specifications, and best practices available that can be utilized to mitigate many of the potential issues that can arise. This writing frequently references the American Concrete Paving Association’s ACPA Roller Compacted Concrete Guide Specification V1.2 (www.wikipave.org).

With an uptick in local projects, we have received multiple requests for experience-based knowledge of RCC as it relates to our more common ready-mixed concrete production.

Here is "DHE’s Guide to Utilizing a CON-E-CO Portable Central Mix Concrete Batch Plant for Production of High-Quality Roller Compacted Concrete"

Scope - There are two distinct ways to categorize an RCC mix, and this classification depends on the scope of the project among other requirements. The first is under the umbrella of paving type projects, to be used for transport or heavy load bearing surfaces; the second being other RCC products that are used surfaces that bear less compressive load, such as dam spillways, weirs, and other water utilities. Whether it falls under one category or the other will be controlled by the final intended application of the product. There are important considerations for both types of RCC. For the purposes of this writing, the principles apply generally to both categories. For RCC that is to be used for non-pavement applications, there is a high degree of comfort throughout the industry in producing these types of RCC mixtures with a tilting type of mixer. It is worth noting that extra caution is required for RCC mixtures for paving applications that are produced with a tilt mixer. Proper production and placement are vital in any paving environment. 

Mix Design - The mix design is a critical factor in any concrete production, and especially so for a less forgiving mixture like RCC. According to the ACPA “Aggregate quality and gradation is critical to RCC performance, because aggregate constitutes 85% of the volume of RCC.” Typically, the larger the top-sized aggregate, the higher the compressive strength, at the cost of increasing the mixtures sensitivity to segregation. The application tends to have a degree of control over the top-size of the aggregate gradation in the RCC mix design, (ACPA RCC Guide, p.10). Some RCC mix designs may specify a 1-1/2” size aggregate. To best utilize a tilting style mixer, one might reduce as much as possible the larger nominal sized aggregate in the gradation of the mix.

Batching - With regards to batching RCC mixes, the critical factors are consistency, and production throughput. Common practice when using a tilting mixer is to reduce the batch sizes to around one half of the volume of the mixer and increase the mixing time as needed. As stated by the ACPA “A key requirement for the mixing plant is that its production rate meets the requirements of the paving operations… to maintain uninterrupted placement. For high-production projects (~150YPH) … Tilt-drum mixers may be able to produce similar volumes… operated at less than full drum capacity (50-75% of the drum capacity), and with longer mixing cycles (90-120 seconds). The contractor’s quality management plan should indicate the target volume and mixing time, which will provide and indication that the contractor is aware of what is necessary.” (ACPA RCC Guide, p.15)

Mix Sequencing - It is important to pay attention to the sequencing of materials into the mixer for any concrete production. For RCC, the sequencing is not far off from conventional central mix concrete batching. Once all material target weights have been met per the batch controller scale indicators, the first material to discharge should be aggregate. Cement should begin to discharge once ~10-25% of the aggregate has left the aggregate scale. The weighed or metered water should begin to discharge once ~10-25% of the cement has left the cement scale. Water should be the first to finish discharging, then cement, and aggregate the last to enter the tilt mixer. 

Summary - There are many additional topics to cover when it comes to RCC production methods, this brief overview should serve as a starting point for research. Build With Strength organizations across the country work to promote the use of RCC, and the wider the spectrum of production methods (i.e. varying equipment arrangements), the greater the number of opportunities for new applications off RCC mixtures. For more reading, we urge all interested to visit the resources below.     

Happy Batching,
Dave Humphrey Enterprises, Inc. | 209.835.2222 | | Established 1979 | #PourtheWorld

References:
https://wikipave.org/index.php/Roller-compacted_concrete_(RCC)_pavement
https://rccpavementcouncil.org/wp-content/uploads/2016/06/ACPA-RCC-Guide-Spec-Ver1.2-9-4-2014.pdf
www.cptechcenter.org/publications
www.dhenoble.com