Like modern chefs tweaking classic techniques, concrete industry insiders are experimenting anew with ingredients—additives and aggregates such as fly ash, slag and quarry waste—and “plating,” which includes methods such as two-lift paving and real-time smoothness monitoring. And like many chefs that emphasize food’s health aspects, concrete professionals are refocusing their goals from the strength of the material to its longevity and durability.
“In essence, we’re trying to change the fundamental basis on which we design concrete pavement,” says Gerald Voigt, president and CEO of the American Concrete Pavement Association. “We’re trying to design new mixtures and run qualifying tests based on chemical durability, resistance to freezing, et cetera, rather than on strength.”
The push to create standards for concrete to set quicker and handle extreme weather and chemicals partly comes out of resiliency considerations, says Michelle Wilson, director of concrete knowledge for the Portland Cement Association. “Current standards give you specific codes, but one size doesn’t fit all,” she notes.
Nationwide research efforts, including at the Federal Highway Administration (FHWA) and at Iowa State University’s National Concrete Technology Center (CP Tech Center), are working on new standards for “identifying what’s critical, how to measure it and how to specify it,” Voigt adds. “It’s really gaining momentum, and we’ve been strategizing how we go about it over the past two years.”
Peter Taylor, director of the CP Tech Center, says critical parameters have been developed. “The next stage is to find test methods to measure these properties, and much of this work is complete, including field evaluation in seven states last summer,” he says. “Next comes selection of pass-fail limits for regions and climates.” The team hopes this fall to develop a spec for review by state transportation officials, he adds.
The industry now realizes that good mixes don’t need as much cementitious material as previously thought, says Tom Van Dam, principal with engineering consultant NCE. “Fly ash and slag, which have pozzolanic properties, can make concrete more durable when used properly and dramatically reduces the carbon footprint.”
In turn, due to the lessened need for cementitious material, the process cuts costs, says Steve Gillen, materials manager for the Illinois State Toll Highway Authority. “We are increasing use of fly ash and slag in our mixes. Thirty-five percent is our minimum for portland-cement replacement, but it can go as high as 50%.”
The tollway authority’s 15-year, $12-billion capital program serves as an ongoing, field-testing showcase for a variety of concrete research in conjunction with academia and industry, says Gillen. The agency has tested two-lift paving; quarry waste aggregates, such as superfine sands in mixes; and crack-resistant concrete and precast approach slabs, which reduce joints in decks. Other efforts include re-engineering continuously reinforced-concrete pavement (CRCP), such as optimizing its quantities and applications. The authority also is using an automated plateload test system that is capable of measuring, in real time, the geotechnical properties of pavement subgrade (see p. 27).
The authority’s annual capital program is currently about $1 billion, with research investments at about $5 million, says chief engineer Paul Kovak. “The savings have been in the tens of millions [of dollars],” says Gillen.·
The authority also has used two-lift paving that includes, on the bottom slab, recycled materials such as reclaimed asphalt aggregate, and, on the top slab, higher-quality aggregates, Gillen says. Other states also are experimenting with two-lift paving, he adds.
Tyler Ley, associate professor of civil engineering at Oklahoma State University, cites concerns such as whether contractors will want to embrace the extra cost associated with two paving machines. Further, he says, “The idea is to put less-desirable material in the bottom layer, but how low do you go? How much gristle do you put in the burger?”
Ley led development of the Super Air Meter (SAM), a portable tool that, in about 10 minutes, measures air-void spacing and the air volume in fresh concrete, allowing for troubleshooting in real time. “The old way was to cut up the concrete after it hardened,” he says. In the past 18 months, in conjunction with FHWA, his team has demonstrated SAM throughout the country. Twenty-nine states and two Canadian provinces are using it, Ley says.
Testing concrete faster and more accurately is pushing a trend toward more resistivity testing, says Ley. “You take concrete and send a charge through it to measure conductivity,” he says, predicting that such tests will be widespread in the next few years.
Real-time smoothness testing, in conjunction with computerized stringless paving, also will become widespread, says Voigt. “You hang sensors off the back of a paver that can trace the surface with lasers,” he says. “Crews paving two to three feet behind the machine can treat a rough spot right then and there.”
Gary Fick, vice president of Trinity Construction Management Services, says the technology, studied in a Strategic Highway Research Program 2 effort, is in the implementation phase. “What we’re trying to do is loan equipment to contractors so they can try it,” he says. “One contractor, who bought the equipment last year, commented that it paid for itself in one season.” Loans, feedback and workshops on real-time smoothness testing will continue through 2017, he adds.
Though a net-zero carbon footprint for concrete may not yet be economically viable, Solidia Technologies has taken a big step in that direction, simply by moving some molecules around. Research by the Piscataway, N.J., company into the processes that affect calcium-silicate minerals that are chemically similar to portland cement has led to the development of Solidia Cement, which gains strength through carbonization, rather than hydration.
Solidia Chief Technology Officer Nick DeCristofaro says the patented process for making Solidia Cement is no different than for portland cement, which means one plant can make both. Due to its molecular makeup, Solidia Cement forms at lower kiln temperatures, cutting CO2 emissions by as much as 30%, according to the company. Curing concrete made with Solidia Cement shrinks the manufacturing chain’s carbon footprint even further, sequestering CO2 equal to 5% of its weight. The total carbon footprint associated with manufacturing is reduced by up to 70%. The process also makes it possible to recover between 60% and 100% of the mix water used to manufacture concrete products. Tests by the consultant CTLGroup have verified the material meets industry specifications for strength and durability. The most significant capital investments involve the CO2 curing chamber and having sufficient quantities of the gas on hand.
For Ley’s team at Oklahoma University, a major focus is on how to optimize aggregate sizes and volumes for concrete, not only for paving but also for bridge decks. “The difference in a bridge deck is that you want [the concrete] to be more flowable,” Ley says. With partners including the FHWA, Georgia Tech and the Army Corps of Engineers, the team is using a full-scale system of pipes and pumps to study portland-cement alternatives, such as calcium silica and aluminates.
The team also has developed an alternative to bulky burlap for protecting a newly poured concrete bridge deck while it is curing, Ley says. “We developed a biodegradable material from recycled newspapers, herbs, spices and water,” he says. “We can make it flow like water or sticky, for shotcrete curing. You put it down at a quarter-inch thickness with a plastic sheet on top.” The product recently was tested on three Oklahoma bridges, with more testing to come. “Applying it quickly without damaging the surface took some effort,” he says.
Concrete bridge experts concur with pavers on the trend regarding tweaks to mixes. The new I-35W bridge in Minneapolis used a 6,500-psi superstructure mix with portland cement comprising 77% of cementitious material, along with fly ash and silica fume, says Alan Phipps, senior vice president with Figg Bridge Group. To date, “the bridge has displayed very low permeability and strength results that exceeded expectations, with an average 28-day strength of about 8,000 psi, 23% above design requirements,” he says.
The l-90 Dresbach Bridge and the Winona Bridge, both currently under construction over the Mississippi River between Minnesota and Wisconsin, use superstructure concrete mixes designed for 7,000-psi and 8,000-psi strength, with approximately 70% of the cementitious material being portland cement and the rest comprising fly ash and slag, Phipps adds.
Emphasizing service-life design through anti-corrosion and durability more than rigid codes is as true for concrete bridges as for road paving, says David Goodyear, North American chief bridge engineer for T.Y. Lin International. “One of the things happening in the U.S, is the focus on a rational service-life design. Hopefully, in the coming years, we can achieve 150-plus-year life spans.”
Europeans Test ‘Cleaner’ Concrete
It could be possible to halve the air pollution in Milan by incorporating “smog-eating” products in just 15% of the city’s hard-surface areas, according to Italy’s leading concrete manufacturer. Such wide-scale developments are just a vision for now, but advocates of photocatalytic concrete (PC) point to its potential.
Each year, up to 2 million sq meters of photocatalytic surfaces are fanned with additives from Bergamobased ltalcementi S.p.A., says the product-line project manager, Gian Luca Guerrini. Projects consume about half the firm’s PC output, but high costs limit its use in highways, he adds.
Smog-consuming concrete uses a solid catalyst, usually anatase titanium dioxide. Under ultraviolet light, the chemicals oxidize pollutants in car emissions, explains Jos Brouwers, professor of building materials at Eindhoven University of Technology, The Netherlands. Oxides of nitrogen (NOx), for example, are converted to less harmful nitrates. A team led by Brouwers recorded significant NOx reductions on a Dutch street paved with concrete blocks comprising photocatalytic material, he says. The trial at Hengelo, completed in 2012, followed a decade of laboratory tests, he adds.
For the trials, Brouwers says researchers paved 150 m of a residential street with concrete blocks and monitored at Eindhoven University of Technology, The Netherlands. Oxides of nitrogen (NOx), for example, are converted to less harmful nitrates. A team led by Brouwers recorded significant NOx reductions on a Dutch street paved with concrete blocks comprising photocatalytic material, he says. The trial at Hengelo, completed in 2012, followed a decade of laboratory tests, he adds.
For the trials, Brouwers says researchers paved 150 m of a residential street with concrete blocks and monitored them for more than a year. Only the top 0.6 to 0.8 centimeters of the 10-cm-deep, brick-size blocks contained, at around 5% of the mix, the active ingredients.
Over a 24-hour period, NOx levels were typically 20% below those on an untreated control street, says Brouwers. However, public authorities remain skeptical about using the technology because of other tests that failed, he adds. But those failures occurred because other investigators selected poor trial sites, he says. Interest also is stifled because of PC’s cost penalty, which is 5% to 10% above conventional construction, he adds.
Guerrini agrees that the costs need to go down before Italcementi can boost sales of its patented PC additive, TX Active, which the company launched 10 years ago in Italy and France and a year later in the U.S. It processes the material in those countries and has licensing agreements in another 12. Protected by some 20 patents, the product has little competition, says Guerrini.
In the highway sector, “we are working with engineers to design the complete stratification of the pavement to evaluate what could be the best solution,” he adds. Guerrini hopes to cut costs by reducing the proportion of cast-in-place concrete from the 10 cm used in a U.S. trial.
The trial four years ago on 460 meters of a St. Louis highway used “wet-on-wet,” two-stage slipforming to demonstrate improved efficiency of PC construction, says Guerrini. Sponsored by Italcementi’s U.S. subsidiary, the Missouri Dept. ofTransportation, the Federal Highway Administration and others, that work involved casting a 25-cm-thick cement subbase topped by a thinner wearing course containing TX Active.
The architect of a bridge renovation project in Barcelona, Spain, is not only exploiting air-cleaning properties but also making it glow. And a $550,000 renovation in 2007 of the Umberto One tunnel in Rome included coating the 9,000-sq-m lining with TX Active paint. In a $1.6-million project due to end soon, the 60-m-long Sarajevo bridge over Avinguda Meridiana is being widened, landscaped and paved with ceramic tiles, asphalt and concrete, covering about 40% of the deck, says Manel Peribanez, a principal with Baena Casamor Arquitectes BCQ S.L.P. While wet, the cast-in-place concrete surface was infused with a photocatalytic material and a photoluminescent aggregate, intended to glow at night.
After a decade’s research into self-healing concrete, Dutch engineers plan to start this year commercial production of the necessary bacteria agents. But with uncertain economics and still-to-be-done development, the scale of production and sales remain unclear.
“We are building a factory that can produce a large amount of the self-healing agent,” says Erik Schlangen, professor of experimental micromechanics at Delft Technical University (DUT). The university began the research 10 years ago, after the emergence in the U.S. of self-healing polymers, he says, adding, “One of the first things I did was to hire a microbiologist.”
Self-healing concrete contains alkali-resistant bacteria, which lies dormant like seeds. Vital nutrients, such as calcium lactate, also are included. If protected by a coating, bacteria can live for 200 years after being mixed into concrete. When cracks occur, the bacteria are activated by exposure to air and moisture. They consume the lactate and oxygen, producing silicates that fill the cracks. DUT researchers observed roughly 0.5-millimeter cracks all but disappear in less than two months.
Two years ago, the DUT team went to full-scale trials, using a small irrigation channel in Ecuador. The engineers have yet to report on the performance of the 3-m-long, 1-m-deep Ecuadorian channel.
In the U.K. this summer, the Cambridge, Cardiff and Bath universities will complete a three-year, $2.9-million government-funded research program, says Robert Lark, a professor at Cardiff University. “We are the first people to have put the technique into a full-scale structure and in the open,” he observes.
The trials are probing three options of applying bacteria, including direct mixing into concrete; enclosing bacterial agents in microcapsules, which break when needed, and loading capsules with nutrients for the bacteria; and, in the structure, using a vascular tube system through which bacteria can travel. This system includes shape-memory plastic tendons in the concrete to close the cracks sufficiently.
These ideas are being tested on a set of six 2-m-tall, 1-m-wide and 25-cm-thick concrete earthwalls cast recently by the Maidenhead-based contractor Costain Group plc. at one of its highway contracts, near Abergevenny, 50 km from the university. Waiting to observe how the various options perform, the team has yet to crack the walls under simulated earth loadings.