Recycled Concrete: First Rate in Second Life

Where can you get your hands on some of the best aggregate around for road building — and usually at a bargain price? According to a report titled Transportation Applications of Recycled Concrete Aggregate, published by the Federal Highway Administration (FHWA), you need not look any further than the nation's aging concrete roadways and bridges. The report investigates potential road-building applications for recycled concrete aggregate, or RCA, and concludes that the material is a "valuable resource, and by proper engineering, it can be used for portland cement concrete and aggregate base."

Time was, of course, when most waste concrete was trucked to the landfill, and more than we'd like to think still probably ends up there. But, that said, the benefits of recycling old concrete continue to diminish the volume that's buried.

According to Jim Miller, manager of Southern Crushed Concrete, a flatwork demolition contractor with a dozen crushing facilities in the Houston area, landfill disposal costs around Houston can run to $20 per ton for concrete rubble (usually two tons per cubic yard). So, paying recyclers to haul the old material away can be a huge saving for those responsible for its disposal. And in the process, an immense amount of landfill space is preserved. Also preserved by recycling is the reserve of virgin aggregate in the nation's quarries, and because recycled-concrete products typically sell for somewhat less than natural stone, end-users may get a break.

In addition, recycled concrete rubble often is processed and reused close to its original placement, especially in urban areas, thus considerably reducing haul distances. Fuel savings and, consequently, reduced pollution are the benefits. Miller's firm, in fact, enlisted the assistance of researchers at Rice University to quantify potential reductions in emissions that might result from these reduced transport requirements. The study focused on nitrogen oxides (NOx) produced in the diesel exhaust of trucks hauling old concrete to disposal sites, and in the exhausts of trucks, barges and trains involved in supplying new aggregate.

Using three actual jobs that Southern Crushed Concrete had underway at the time, the study compared the theoretical quantity of regional NOx produced by trucking old concrete to the company's nearest crusher and then returning it to the site as RCA, versus hauling old material to the nearest landfill and then supplying new aggregate from the nearest practical sources. On the disposal side, the study concluded that recycling reduced the quantity of NOx produced per ton of material by about half, compared with landfill disposal. On the supply side, NOx produced per ton of recycled material was only about one-third of that produced by obtaining a ton of natural stone via the best (lowest-emissions) route.

Yet another advantage of RCA is its lighter weight (10 to 15 percent less for a given volume), compared to virgin aggregate. For example, if you used 5,000 tons of virgin aggregate to place 5,000 linear feet of road base, says Jason Buesing, president of ReCrete Materials, Arvada, Colo., you could buy 5,000 tons of recycled-aggregate base and perhaps go 5,500 feet at the same depth and width. You get more volume per ton.

According to the Construction Materials Recycling Association (CMRA), more than 140 million tons of waste concrete are crushed and reused each year in the United States. Although a portion of that volume goes back to work in such forms as riprap along eroding shorelines and large-stone fill on building-site access roads, an increasing amount appears to be going into roadway construction.

Indications are that a fair number of road-building agencies are substituting RCA for natural stone in road-base applications, and that a handful are employing it in a limited way (or at least experimenting with its use) as aggregate in new concrete paving mixes. But that said, progress in recycling concrete and using it in some aspect of the road-building/paving process has lagged behind that of some areas of the world where virgin aggregate — and landfill space — are scarce commodities.

In Austria, for example, RCA is highly valued and is used as aggregate in new concrete for the bottom layer of the country's two-lift paving technique. Typically, the first (bottom) lift is placed using about 8-1/2 inches of concrete made with the course fraction of RCA. Then, while still wet, the bottom layer is topped with a 1-1/2-inch, exposed-aggregate surface lift, which contains an expensive, relatively small natural stone that provides exceptional wear-resistance and skid-resistance, while also helping to mitigate tire-to-pavement noise.

"The use of RCA for making new-pavement concrete has been a routine process for reconstructing old concrete pavements in my country since 1991," says industry consultant Dr. Hermann Sommer. "According to our experience, RCA is at least as good as high-quality virgin aggregate: RCA bonds very readily with the new cement stone and gives very good flexural strengths. However, this is true only for the coarse fraction of the RCA. The fine material — smaller than 4 millimeters — may impair the frost/salt resistance of the concrete and would increase the alkali content of the mix, and thus increase the risk of ASR [alkali-silica reactivity]."

The aggregate in Austria's bottom-layer concrete consists of about 65 percent RCA, ranging in size from 4 to 32 millimeters (or slightly larger than 1/8 inch to 1-1/4 inches), and 35 percent natural sand. But, so as not to waste any of the RCA, the fines are added to the old granular subbase, which is then stabilized with cement via a mix-in-place process.

"In the future," says Sommer, "much — if not most — of our construction will be reconstruction, and a recycling concept should be applied, not only to the concrete pavement, but to all the layers of the road. I would always use high-quality RCA for concrete, and think it a waste to use it for subbase, which I would do only if the old pavement is of very poor quality or ASR-afflicted."

Of the 3 million tons of waste concrete that Southern Crushed Concrete processes annually, more than 90 percent is turned into road-base material, which, says Miller, finds a ready market in the Houston area. He makes the point that — in his operation and locality, at least — further processing the bulk of RCA to create a clean, large-fraction aggregate suitable for concrete paving mixes would add considerable cost to the product and would leave behind fines that have little market value.

In Miller's view, crushed concrete (he prefers the term to "recycled concrete aggregate") actually is a superior material for road base, compared to crushed limestone. Because crushed concrete is well-graded, he says, it can be compacted effectively, and because the material is generally more angular than virgin aggregate, it tends to establish a strong stone-on-stone matrix that provides strength, yet a degree of flexibility.

Probably safe to say is that the majority of concrete recyclers at present, as does Southern Crushed Concrete, find that converting concrete rubble to road-base material makes the best economic sense for their operations. But some concrete recyclers are pushing to expand markets.

For example, Recycled Materials Co. in Arvada (near Denver), Colo., and its subsidiary operation, ReCrete Materials, mentioned above, have for the past 8 years or so been processing 6.5 million tons of concrete and asphalt from Denver's old Stapleton Airport. A more recent project is that of processing 3.5 million tons of material from the El Toro Marine Corps Air Station near Irvine, Calif.

Around 70 percent of the concrete from Stapleton, says ReCrete Materials' Buesing, becomes structural fill and road-base, and another 15 percent becomes specialty products. The remaining 15 percent, however, is used as aggregate in new mixes produced by ReCrete Materials, which was formed by the parent firm after partnering with the Colorado School of Mines to develop methods for using RCA in ready-mix concrete.

Customers use RCA mixes for varied applications, says Buesing, including some urban-street and alley paving. So far, he says, the mixes have been approved by the City and County of Denver, as well as two nearby municipalities, Golden and Aurora.

According to Buesing, the aggregate in a typical mix includes 30 to 50 percent RCA, and the remainder is virgin aggregate and a varying amount of flyash. The RCA, he says, can be either #57 rock (with a gradation from 1 inch to 3/8 inch), or #67 rock (3/4 inch to 1/4 inch). The top size, he says, varies according to the mix and its use. One caution ReCrete Materials exercises in mix design, however, is avoiding the use of excessive recycled fines:

"If you try to use a high percentage of recycled fines," says Buesing, "water demand goes up and strength is lowered. You have to be careful. Most of our mixes have no more than 5 to 7 percent recycled fines, and the rest is virgin sand."

A concern expressed by some paving-materials specialists is the wisdom of creating new mixes from recycled concrete pavement that in its first life exhibited materials-related distress (MRD), such as alkali-silica reactivity (ASR) or D-cracking. According to FHWA Technical Advisory T-5040.37, "Use of Recycled Pavement as Aggregate in Hydraulic-Cement Concrete Pavement" (published July 2007), if old concrete exhibiting MRD is being considered for RCA in new mixes, then an engineer should visit the site, assess the type and extent of the distress, and collect samples for laboratory evaluation.

ASR occurs, says the Advisory, when certain siliceous aggregates react with alkali in the concrete paste to form a gel, which can expand after absorbing water and cause cracking. But that said, the FHWA report does suggest that using "supplementary cementitious materials" in the new mix — such as fly ash, granulated blast-furnace slag or lithium admixture — may help mitigate ASR recurrence. D-cracking is the result of water freezing in certain porous aggregates, and the Advisory suggests that when recycling pavement exhibiting this type of distress, the material may require additional crushing to reduce the maximum size of aggregate likely to cause the problem.

Austria's Hermann Sommer, however, seems to take a more conservative view:

"The old concrete," he says, "must be of good quality without any evidence of durability problems."

With that requirement established, Sommer details how, in his opinion, the material should be processed for use in a new mix.

"Joint sealants must be removed before the old pavement is loaded, as it is impossible to remove them once they're in the crusher. We use impact-type crushers and operate them at less than maximum output in order to obtain a high amount — at least two-thirds — of coarse aggregate. The RCA is sieved into the fractions of 0/4, 4/8, 8/16 and 16/32 [millimeters]. I always recommend wet sieving and storing the material in a way that it will not dry out. Otherwise, the concrete would stiffen quickly and consistency would not be uniform on the site."

ReCrete's Buesing also takes a cautious approach to the raw material used to create RCA for new mixes:

"The runways we're processing now are excellent feedstocks, with concrete averaging 8,000 to 10,000 psi and having very hard aggregate. But eventually, when we might not have an ample supply of quality material, we'll have to pick and choose the concrete coming in for disposal."

In general, the Technical Advisory concludes that recycled concrete used as aggregate in paving mixes must meet the same quality requirements normally established for virgin aggregate, and it cites pertinent industry codes, including those from the American Society of Testing and Materials (C 33), and from the American Society of State Highway and Transportation Officials (M 80 and M 6). The Advisory discourages the use of recycled fines, but if used, then suggests that finer natural sand be blended in to improve performance.

The Advisory also cautions that RCA made from old concrete exhibiting poor resistance to freeze/thaw damage may carry this weakness into new mixes. Higher doses of an air-entraining agent may be required, and the freeze/thaw resistance of new mixes should be evaluated in the laboratory before even thinking about placing them on a roadway.

Also, RCA containing more than 0.06 of a pound of chloride ion per cubic yard should not be used in new mixes in order to avoid the prospect of the chloride corroding reinforcing steel and joint dowels. In addition, shrinkage and thermal expansion characteristics of RCA-concrete mixes should be determined prior to pavement design, in order that the design can anticipate and accommodate these properties, which may require, for example, increased pavement thickness or changes in joint spacing.

At the Recycled Materials Resource Center, a partnership created between the University of New Hampshire and the FHWA "to promote the wise use of recycled materials in the highway environment," Asphalt & Concrete Coordinator and professor David Gress, Ph.D., P.E., has strong views about RCA paving mixes — both past and present.

"Many of the failures that occurred in the past were design errors — for instance, improper joint spacing and lack of load transfer — and had nothing to do with the RCA concrete. [Designers] should have known better, but didn't, and shame on them for not admitting [their failure] when it was found out, because this helped develop the negative aspect of using RCA in new concrete. [Waste concrete] can be recycled into better concrete than it was before."

ReCrete's Buesing agrees: "People have the mindset that if you're recycling something, it can't be as good. We're striving to make RCA mixes equal to or superior to virgin mixes."