There are a number of factors affecting CRCP performance with RCA, TxDOT said. They include adequacy of the pavement structure, material properties, environmental conditions during concrete placement, and construction practices. The study included laboratory evaluation of RCA and PCC material properties, performance evaluation of CRCP with RCA sections in the Houston District, and analysis of information to develop guidelines for the use of RCA in CRCP.

TxDOT found that the CRCP sections utilizing 100-percent recycled coarse and fine aggregates have performed well. No distresses, including spalling, wide cracks, punchouts, or meandering cracks, have taken place. Transverse crack spacing distributions are comparable to those in concrete with natural siliceous river gravel.

The large amount of old mortar in recycled coarse aggregate did not appear to have an adverse effect on CRCP performance, TxDOT said. Moisture control of recycled aggregate was critical in producing consistent and workable concrete. No significant adjustment in paving operations was necessary due to the use of 100 percent recycled coarse and fine aggregate in concrete.

The agency found that RCA in this project did not have a pronounced effect on compressive strength, but that recycled fine aggregates had an adverse effect on flexural strength. The use of both recycled coarse and fine aggregates reduces modulus elasticity significantly, TxDOT said. For the same water/cement ratio, replacing virgin sand with recycled sand did not result in changes in tensile strength.

Echoing Snyder, the state reported that the thermal coefficient of concrete containing 100 percent recycled aggregate is much higher than that of virgin aggregate concrete, and that recycled coarse aggregate has a much higher thermal coefficient than virgin aggregate due to the attached old mortar.

Constructing the I-10 segment was not without complications, TxDOT said. In the beginning of the project, there was a problem producing concrete with consistent workability that met the minimum strength requirement. “The primary reason for inconsistent workability was due to the lack of moisture control of recycled aggregate,” TxDOT said “A better sprinkler system was installed later for aggregate stockpiles, and moisture of the recycled aggregate was better controlled. This system mitigated the inconsistent workability problem.”

Paving operations were closely monitored to identify any variations that might result from using the recycled aggregate. Not much difference was observed.

RCA and TriState Tollway

More recently, in 2009, RCA from the existing pavement was used in the complete reconstruction and widening of the TriState Tollway (I-294) in suburban Chicago, from the Wisconsin to the Indiana border.

The Illinois State Toll Highway Authority (ISTHA) said all existing concrete pavement was crushed on site and reused as base stone under new roadways.

“Recycling the existing road materials not only saved the cost of purchasing new materials for the roadway beds, but also eliminated the cost of hauling the old materials from the work site and disposal in landfills,” the authority said.

It resulted in 3.2 million tons of concrete recycled, which is enough concrete to build 4,000 miles of sidewalk, which would equal the distance from Chicago to San Diego and back, ISTHA said.

On the project, excavated concrete was broken up and crushed into smaller pieces – right on the roadway construction site with the use of mobile crushers – to create a high-quality aggregate base for new pavement. Up to 90 percent of each new roadway base consisted of recycled concrete.

But “Bein’ Green” means that you need to get recognition of it as well. Thus beginning in October 2009, roadway signs were being installed along the newly reconstructed roadways throughout the 286-mile tollway system to inform drivers that improvements have been made using “green” construction methods and materials.

“We will see much more of this in urban areas,” Snyder told Better Roads. “The hauling required to remove demolition concrete, and return with virgin aggregate, constitutes an unnecessary added cost. What the Illinois State Toll Highway Authority did was a model for the way highway agencies ought to be doing this in the future.”