A solution now available to concrete producers to fight alkali-silica reactivity (ASR) is attaining wider exposure nationwide. A lithium nitrate-based ASR inhibitor can keep concrete from failing when reactivity-prone aggregates are used, and has the added benefit of helping protect producers from legal exposure that may result from failed structures in the future.

But the low-bid, commodity pricing of concrete precludes some producers from adding nonspecified admixtures, like ASR inhibitors. As a result, many commercial mixes in ASR-prone areas are produced with only the most general protection against ASR, such as multipurpose Class F fly ash. In the meantime, government agencies in those areas are taking a longer-term view in specifying lithium nitrate-based liquid admixtures - like Boral LiNX, an ASR inhibitor from Texas-based Boral Material Technologies Inc. (BMT).

What is ASR? ASR is a chemical reaction that occurs between alkalis contributed primarily by cement and a reactive form of silica from reactive aggregate, which yield an alkali/silica gel. Under the right conditions - particularly enough available moisture - the gel will expand and produce stresses, damaging the concrete. Over time, this expanding ASR gel exerts internal pressure that can lead to cracking of the concrete. Cracks can provide pathways for potentially deleterious materials like water, sulfates and chlorides to the interior of the concrete matrix, which in turn can lead to serious durability issues such as freeze/thaw damage, sulfate attack or steel corrosion.

Practitioners acknowledge that ASR doesn't destroy concrete per se. Rather, ASR-compromised concrete is weakened so that day-in, day-out wear-and-tear becomes prematurely destructive. Clues to ASR's destructive chemical reactions include map and longitudinal cracking in bridge decks and pavements, and longitudinal cracking in structural columns.

This cause-and-effect pattern that ASR initiates - where reactivity leads to other destruction - prompts some to dub ASR the "AIDS" of concrete. The condition has long been thought to afflict mainly Western states, yet Strategic Highway Research Program publication C-343 Eliminating or Minimizing Alkali-Silica Reactivity underscores that "the potential for deleterious ASR in highway concrete exists in every state."

Quelling ASR The best way to avoid ASR in new concrete is to take precautions in the mix design. These include testing aggregates for reactivity; use of low-alkali cements, suitable pozzolans like ASTM C-618 Class F fly ash, and lithium-based admixtures; and knowledge of all materials' historical performance.

Use of nonreactive ("innocuous") aggregates can be specified, but these may not always be available locally. Aggregates can change in composition from one end of the pit or quarry to the other, making positive identification difficult. And established tests for reactivity may allow too much variability in the results. Lithium nitrate-based ASR inhibitors eliminate these uncertainties, the testing and associated labor.

Low-alkali cement, used alone, will not eliminate ASR. Many agencies that specify low-alkali cement for ASR control also specify other preventive measures. Similarly, control of the alkali content of cement can be problematic, owing to diverse sourcing and new regulations restricting fugitive dust from cement plants. Now, captured high-alkali dust often is returned to the finished product, thus increasing ASR potential.

While lithium compounds such as lithium hydroxide have been indicated for ASR inhibition for years, their caustic nature and product handling issues limited their market acceptance. However, the commercially available lithium nitrate solution, Boral LiNX, is a clear, odorless solution that is noncaustic and needs no special handling. When added to concrete at the plant, it is evenly distributed through the mix.

Boral markets its lithium-based alkali-silica inhibitor through an agreement with FMC Corp. Boral LiNX is based on Lifetime admixture technology as developed by FMC Lithium Division. Both Boral and FMC have committed to a long-term testing program that encompasses a variety of cements, fly ashes and aggregates from across the country, which will be developed into a database to benefit end users.

How lithium inhibits ASR Lithium nitrate reacts with reactive silica and moisture in a manner similar to other alkalies such as sodium and potassium. Unlike ASR, however, the gel formed by this product with alkalis does not absorb an excess amount of moisture, so harmful expansion is avoided.

Use of lithium nitrate lets engineers specify locally available aggregates at much reduced shipping costs, regardless of their ASR potential. Use of local aggregates is one of the main reasons the price of concrete can remain competitive against other materials; distant sourcing of aggregates runs the risk of negating concrete's cost-effectiveness.

Combinations of Class F fly ash and lithium nitrate ASR inhibitor can work synergistically in suppressing ASR expansion. Because Class F fly ash is used for cement replacement, the available alkalis are reduced by dilution. And because fly ash has a strong binding capacity for alkalies, and reduces permeability to water needed for ASR to form - with the chemical activity of the lithium compound itself in forming the gel - Class F fly ash and lithium nitrate can work together in controlling ASR.

ASR damage "I'm seeing ASR cracking all over the area," says Jeff Pecenka, P.E., supervising staff engineer, Board of Public Utilities (BOPU) in Cheyenne, Wyo. "We have reactive aggregate around Cheyenne and ASR is something we need to look at." He is supervising construction of new waste water infrastructure using various ASR mitigation techniques.

In Cheyenne, ASR was found to have damaged a treatment plant which was constructed in the 1970s. BOPU is working with consulting structural engineers and testing labs in making sure ASR doesn't affect new concrete projects under construction. The damaged facility is approximately 25 years old. "It showed signs of classic ASR deterioration," Pecenka notes, "including subparallel cracking in the walls of the filter base. We found cracking, efflorescence and leakage that shouldn't be there."

Subparallel cracking refers to internal cracking on a plane parallel to the faces of the wall, so that a core removed horizontally will come out in two pieces. Determined to keep new concrete from ASR development, BOPU and staff from consulting engineer Black & Veatch have specified a lithium nitrate ASR inhibitor and Class F fly ash for the Crow Creek Waste Water Treatment Plant expansion. Boral LiNX will be dosed to the manufacturer-prescribed amount and used in combination with 15 percent Class F ash.

"Our research shows [lithium] does seem to work best in abating ASR," Pecenka says. "Research by the FHWA indicates lithium and fly ash are the state-of-the-art methods of abating ASR." This is affirmed by other studies from Canada and Europe.

"There's no guarantee that anything will work forever," Pecenka notes. "Fly ash was thought to be the solution, but has turned out to be short-term. Research now points to lithium as a long-term solution when used with Class F fly ash, and I want to try it because it's my job to construct projects with the longest possible service life. Maybe we all need to rethink our methods so we can build things that last 90 years as opposed to 20 years."

"Lithium-based ASR inhibitors can have a huge impact on the durability and performance of a concrete structure," adds BMT's Russell Hill, Ph.D., vice president of technical development group. "When you look at service life or life-cycle cost analysis, this kind of product can have a dramatic effect.

"In principle, structural engineers have a general awareness of ASR, but usually are not sufficiently familiar with it to be fully aware of all the potential mitigating measures that are available or the ramifications that are associated with each mitigating method," Hill contends. "The awareness level in that regard still is pretty low."

Producer connects with LiNX Cheyenne's Tilton Ready Mix learned to use lithium-based ASR inhibitor as the result of the Crow Creek project's mix design. "We've seen an emphasis on ASR in this area in the last five years," says General Manager Dave Miller. "It began with customers demanding quarried aggregate, to get away from the local aggregate. On some projects they have searched for completely nonreactive aggregate." For Crow Creek mixes, he adds, nonreactive granite aggregate with potentially reactive local sand will be used.

"Here, we've always used nonreactive coarse aggregate and fly ash, and the use of lithium nitrate liquid for this project is something new," Miller notes. "We put it in like any other liquid admixture, dosing it through our dispenser bottles."

Pre-empting liability From the industry's point-of-view, use of an ASR inhibitor not only better serves clients, but also can protect from potential legal action that could arise if ASR is not mitigated when state-of-the-art remedies are available.

"Engineers are obligated to take good practice measures," BMT's Hill says. "There has been class-action litigation surrounding the issue of sulfate attack in residential concrete slabs. Exactly what constitutes appropriate and good practice is often very difficult to clearly define. Those issues have reached the forefront in lawsuits involving failed concrete.

"The ready-mix industry certainly was impacted by this litigation. Unfortunately, this is the climate we're dealing with." Nevertheless, he adds, good practice - like use of ASR inhibitors - makes for a good defense.

Low-bid lowers value "People need to become aware of ASR," says BOPU's Jeff Pecenka. "Most engineers don't abate it. We need to know that the problem exists and that there are remedies out there. And we need to spend some money on those remedies."

The utility's concern about ASR mitigation is not widely shared by ready-mix producers, Pecenka suggests. "There are a lot of Doubting Thomases [about ASR] among suppliers," he adds.

Use of lithium nitrate ASR inhibitor is being driven largely by government specifiers, with little demand from the private sector, due to its low-bid commodity pricing. But marketing of a value-added concrete containing lithium nitrate in ASR-prone markets can result in a durable, long-lasting product that can also enhance producers' margins.

Because the amount of lithium nitrate used varies according to the amount of alkali in the concrete, its price per cubic yard varies. Typically, use of lithium nitrate ASR inhibitor will add $10 to $20 to a cubic yard of concrete.

"In a low-bid market, the cost can discourage," says BMT's Ron Farris, speciality products manager. "But in many applications we've found it's a wash, because of the reduced cost of materials in freight. And the real long-term payoff is in enhanced durability."

Dave Miller and Tilton Ready Mix have heightened their awareness of ASR in recent years and taken steps to combat the condition. "It's one reason we've been using quarried rock," Miller says. But use of admixtures to fight ASR is discouraged by the low-bid mentality that drives bulk sales of ready-mix concrete, especially to the private sector, he adds.

"If nobody else in town is using it, and you are, the local homebuilders are going to go with the lowest price," Miller notes. "We do see ASR in concrete structures around town, but there is no strong demand for the admixture at this time. Many of the local ready-mix companies want to continue using the local aggregates, and customers allow it."

"We certainly will suggest use of lithium nitrate liquid admixture to customers to forestall potential ASR," he says. "But it's an affordability issue and the customer has to perceive value. If it's spec'd on a job, it's going to get used. If it's not spec'd on a job, the affordability becomes an issue because you're bidding against other suppliers and you won't get the job. But it is something we will bring to their attention.

"We're ready, willing and able to use the admixtures upon customer demand. But it has not gone that far in the residential area and some of the private or commercial work." However, he adds, city and state public works officials recognize ASR potential and have been requiring Class F fly ash in concrete.

"We base our policy on the research," Miller concludes. "It gives us satisfaction to know that we've provided concrete that will not crack because of ASR. We intend to be here for quite a few years and take pride in our projects."

The synergy between lithium nitrate liquid admixture and Class F fly ash in fighting alkali-silica reactivity (ASR) was spotlighted in a high-performance concrete mix specified this summer for a Cheyenne, Wyo., waste water treatment plant expansion. Cheyenne Board of Public Utilities officials followed consulting engineer Black & Veatch's mix design recommendations that would quell the ASR damage they were observing in some concrete.

The Crow Creek Waste Water Treatment Plant expansion incorporates a mix developed by Cheyenne's Tilton Ready Mix and Terracon, a materials consulting engineer. "We had to meet very tight shrinkage specs," says Tilton General Manager Dave Miller. It took three mix designs to get the successful result, he adds. "The third one we finally got right. It was all controlled in Terracon's lab. We had to reach 4,000 psi strength in the field, and the lab mix had to reach 1,200 psi over design."

The three mixes were formulated in accordance with American Concrete Institute procedures using the absolute volume method. The 6.5-sack mix was designed using Mountain Cement's Type I-II LA portland, supplemented with 15 percent Class F fly ash from Boral Material Technologies Inc., used at a 1 to 1.5 cement-to-fly ash replacement ratio, and Boral LiNX lithium nitrate liquid admixture. The mix had a maximum slump of 8 inches with 6 percent ñ 1 percent air, and a lab strength design of 5,200 psi, with 4,000 psi required in the field. Coarse and fine aggregate were local.

Lab tests determined that there were no detrimental effects to either plastic or hardened concrete properties, such as entrained air, slump, set time or strengths. Drying shrinkage tests were performed by Terracon in accordance with ASTM C 157 as modified by project specifications. The 4 x 4 x 11-in. specimens were molded using the rodding procedure. Seven days after the cure the specimen had shrunk 0.021 percent; after 14 days, 0.030 percent; and after 21 days, 0.035 percent.