By: Jan R. Prusinski, PE
On a crisp fall day, four engineers from Laredo ventured up to the Port of Houston to view the recently-started roller-compacted concrete* (RCC) paving at the Bayport Terminal entrance gate. They were particularly interested in the possibilities of RCC for applications such as fast-track intersections, mill and replacements of worn asphalt roads, and RCC overlays. Also, they were aware that TxDOT recently let its first RCC pavement project: A safety rest area in the Brownwood District on I-20.
What they saw was possibly a first in the RCC world: RCC pavement that did not need the roller to consistently achieve specification density.
RCC, when placed from high-density "asphalt-style" pavers with vibrating screeds and dual tamping bars, normally achieves fairly high density right out of the paver--between 90 and 95 percent of modified proctor density. Sometimes, full spec density (usually ~96%) is achieved right out of the paver, but usually not consistently. That is why one to several passes with a 10-ton vibratory roller is necessary to fully compact the concrete. Strength and durability of RCC is highly dependent on its density, so this is a critical factor.
However, Rollcon has, from the get-go, achieved consistent densities between 98-101 percent, right out of the paver. This both is surprising, and enlightening. How did they do it? Well, certainly the equipment is key; placing RCC out of a "standard" asphalt paver would never approach these densities. Regular asphalt pavers would have density more in the 80-90 percent range. This requires more rolling, and often results in a wavier surface. The high-density pavers are a necessity.
However, even experienced RCC contractors do not normally achieve such high compaction results right out of the paver. What's the difference? The difference, it turns out, is in the mix design.
Rollcon spent time seeking and acquiring fine and coarse aggregate materials from the region that not only fit the specified aggregate gradation curves, but were particularly optimized for RCC use. Instead of a pre-blended one-part aggregate that is sometimes considered (and is subject to segregation), or a 2-part aggregate, Rollcon chose a multi-part aggregate that was conducive to "tweaking" so that they achieved spec requirements, and facilitated placement with minimal--actually no--further compaction beyond the paver screed and tamping bars. This is a truly optimized mixture.
To help with this optimization, Rollcon is using two pug mills: the first for pre-blending some of the aggregates/fillers, and the second to mix the aggregates and cement. Fly ash was actually pre-blended in the first pug mill.
The benefit of eliminating the compaction, is not only cost saving, but it also provides a smoother, more uniform surface. Additionally, in this project, with pavement thicknesses varying from 14 to 18 inches, the RCC is placed in two lifts. With near 100% density coming out of the bottom paver lift, the second paver can travel right on the heels of the first, providing for the greatest possible bond between the two lifts.
Certainly, not all RCC projects can be placed in this manner. Much depends on local materials. In this case, the aggregate-poor Houston area was a benefit, since Rollcon had to seek more distant aggregates. In its effort to minimize cost through a multi-part blend, they also found that mix optimization was more controllable and more easily achieved, with benefits accruing to both Rollcon and the Port.
I should add that some rolling is performed on the top lift, not for density, but to close up/finish the surface (though not much of this is needed either, as the surface looks quite good directly out of the paver). A combination of steel-wheeled and pneumatic-tired rollers are generally used here.
Kevin Wentland of Rollcon can be contacted about this project, particularly if you would like to view the RCC placement, which should continue at least through the end of December 2012.
*RCC paving is a type of concrete that is placed as a very stiff, zero-slump mixture, from an asphalt style paver. It is then compacted by vibratory roller to high density. It needs no reinforcing steel or dowels, and saw cutting the finished pavement is optional, depending on user requirements.
"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".
The Cementx Pavement Blog seeks to make pavement owners, engineers and contractors smarter about selecting, designing, constructing and maintaining pavements. New blog postings began February 1, 2017; however, we carried over pavement-related blog postings from our older blog, the "Texas Cement and Concrete Blog," which ran until December 2016.