RCC PRODUCTION: WHICH PLANT IS BEST?

Twin-Shaft Continuous Pug Mill

By: Matthew W. Singel, PE
​Roller-Compacted Concrete (RCC) production for pavements raises many questions for the novice, from mix design, to admixtures, to plant equipment.  A common question is, “What equipment is best for producing/mixing RCC for pavements?”

Central Mix Concrete Plant

The circumstances of each project dictate how best to produce and place RCC pavement.  There are many opinions on which concrete production equipment offers the best quality control, uniformity or flexibility.  Well maintained equipment is arguably more important than any other factor.  RCC has been successfully (and profitably) produced in the continuous pugmill, central mix concrete plant (drum mixed), dry batch plant (truck mixed), horizontal shaft mixer, and volumetric mix truck.  To complicate matters further, there are inserts, attachments, etc., that work with existing plants to optimize production rates, minimize segregation, maximize water (and admixture) distribution and more.  Confusing?  A common-sense approach is best in this situation.
A continuous pugmill or horizontal shaft mixer makes sense for larger projects that require high production rates (100+ cubic yards/hour) and relatively uninterrupted production.  Even a central mix plant can tout high production rates if dedicated to a large project. 
For moderate production rates, any ready-mix plant (central mix or dry batch) can produce RCC.  The dry batch plant measures and transfers materials for mixing in the ready-mix truck.  Once mixed, the RCC must be off-loaded into dump trucks for delivery.  Central mix and other plants discharge directly into the dump truck for transport to the paver.
Volumetric mix truck production rates vary significantly.  This type of truck can be described as a small truck-mounted pugmill.  For small quantities, these trucks can arrive loaded with all materials, mix/discharge a limited quantity and drive away.  For increased production, several trucks can operate side-by-side with a bucket loader feeding them from aggregate stockpiles.
Ultimately, the deciding factors include project size and schedule, available equipment and desired results.  A 20,000 or 30,000 square yard project can justify mobilizing a large, high-production plant that is dedicated to that project.
A local ready-mix producer, if capable of meeting productions rates, might not want to turn away long-term customers and dedicate all production to one large RCC project.  However, operating a second shift into the evening can be a very attractive option.
Smaller projects would often be too expensive if a large production plant was transported and erected.  However, the local ready-mix producer could dedicate three or four mixer trucks for an RCC project of this size without interrupting service to its other customers.  The dedicated trucks stay on the property mixing RCC and offloading into dump trucks for delivery.
RCC pavement can often be the cost-effective option with adequate communication and common sense throughout the project.
"This blog was previously posted in the Cement Council of Texas' "Texas Cement and Concrete Blog" (now inactive) and was carried forward to the current blog ("Cementx Pavement Blog") as it contains content that may be of interest to the reader".

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