Road Science Tutorial
“Many states fight potential recurrent D-cracking by crushing the RCA down to a minus 3/4-in. top size,” Snyder said. “They may also choose to design the pavement with drains or other features that keep the pavement relatively dry. Minnesota has had tremendous success doing this. They had a number of pavements that were badly D-cracked, including one on U.S. 59 that was crushed down to a 3/4-in. top size for the concrete paving mixture, and where the pavement structure included a drained foundation so the pavement would not become critically saturated. That pavement has now been in place for nearly 30 years with no evidence of recurrent D-cracking.”
The solution is to treat RCA as an engineered material. “Again, to use RCA successfully,” Snyder said, “it’s a combination of understanding what you’ve got, understanding the mechanisms involved, and designing so that those undesired mechanisms don’t take place, either chemically in the case of ASR, or mechanically via freeze-thaw in the case of D-cracking.”
Transverse cracking is a not uncommon problem with pavements containing RCA, but it’s not confined to RCA pavements.
In the 2009 presentation at the Transportation Research Board meeting, Performance of Rigid Pavements Containing Recycled Concrete Aggregates, 2006 Update, by Snyder, David L. Gress, Ph. D., P.E., Recycled Materials Resource Center at the University of New Hampshire, and Jeffrey R. Sturtevant, traffic engineer, Whitney Bailey Cox & Magnani, the authors describe excessive transverse cracking in pavements built prior to 1988 and containing RCA, such as I-94 near Brandon, Minn., where the recycled section developed more deteriorated transverse cracks than did the control section (31 percent vs. 0 percent).
“The transverse cracking was not necessarily related to the use of RCA,” Snyder said. “The jointed reinforced concrete pavement [JRCP] had longer 27-ft. panels, so they were longer than usual and one would expect them to crack anyway. However, the use of RCA in these long panels may have contributed to the deterioration of those cracks because of the higher shrinkage potential and thermal responses of the RCA concrete mixtures. Where RCA was used in jointed plain concrete pavements, we generally did not find excessive cracking in the RCA pavements we revisited.”
Typically, an owner will see a higher coefficient of thermal expansion with RCA, because less natural aggregate is present, and more mortar. “Natural aggregate is a stabilizing influence,” Snyder said. “Mortar has a much higher volumetric expansion and contraction, due to moisture shrinkage or thermal properties. When you increase your mortar content by including both new and reclaimed mortar, you tend to get a little more shrinkage and thermal responsiveness in the pavement. And when you have a long-panel jointed pavement like I-94, your curling and warping stresses will go up as well. In hindsight, some of the problems found on I-94 and other pavements could have been avoided by using shorter joint spacings and/or higher amounts of reinforcing.”
RCA and CRCP
Much of this is moot anyway, as JRCP designs are rarely seen these days, having been replaced by continuous reinforced concrete pavements (CRCP, see Road Science: The ABCs of Continuously Reinforced Concrete, Better Roads, May 2007).
“No one builds much JRCP anymore,” Snyder said. “It’s either continuously reinforced, or short-jointed plain with no reinforcing steel, mostly 15-ft. panels.”
Texas has done major work in the field in evaluating the use of RCA with CRCP, and is confident that it works, thanks to the largest application to-date of RCA in CRCP in 1995, a very heavily traveled section of I-10 in Houston between Loop 610 and I-45 involving 10 lanes, including HOV lanes.
Reconstruction of Houston’s I-10 (from Loop 610 to I-45) was the first project in the state in which all recycled aggregate was used for pavement concrete, according to TxDOT. Today, crushed concrete is used extensively in state projects in the Houston area and is fairly common in Dallas as well.
“Concrete from existing roadways, pavements, airfields, and buildings can be reused,” said Dr. Moon Won, P.E., now with Texas Tech University, who oversaw the I-10 work when with TxDOT.
“The project recycled everything, with nothing but recycled concrete aggregate, both coarse and fine, into the surface layer, and coarse and fine aggregates in the base layer,” Snyder said. “It was a huge project, now in place 15 years, and it looks great.”
The DOT’s objectives of the I-10 study were to evaluate the engineering properties of RCA and portland cement concrete made with RCA, investigate the effect of RCA and PCC properties on CRCP performance, and develop guidelines for the effective use of RCA for CRCP.
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