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February 15th, 2009

NIST’s ‘VERDiCT’ may double concrete’s lifespan

Published on February 15th, 2009 | By: pwray@ceramics.org
The barely visible blue-green area at the top of this X-ray image of NIST-treated concrete shows that very few green chloride ions have penetrated the concrete’s structure.

The barely visible blue-green area at the top of this X-ray image of NIST-treated concrete shows little penetration by green chloride ions.

(corrected, courtesy of the comments from Dale Bentz, below)

The “verdict” is in – engineers at the National Institute of Standards and Technology say they have developed a technique that promises to double the service life of concrete. The soon-to-be patented method is the result of a project called VERDiCT – Viscosity Enhancers Reducing Diffusion in Concrete Technology. The technique entails mixing a nanoadditive into concrete to boost the viscosity of water molecules in its micropores. This, in turn, thickens the concrete solution at a microscopic level, slowing the diffusion of chloride and sulfate ions from road salt and other destructive agents into the concrete’s structure.

By delaying the entry of ions known to cause cracks and internal damage to concrete over time, NIST’s technique forestalls deterioration, says Project Manager Dale Bentz. According to Bentz, previous efforts to boost concrete’s lifespan have focused on increasing its density and making it less porous. While resulting products have proved stronger than conventional concrete, they’ve also demonstrated a propensity for cracking, he says. NIST’s approach is different, he contends, because its focus is microscopic viscosity. “Swimming through a pool of honey takes longer than making it through a pool of water,” he stresses. The key to NIST’s technique is the size of the additive’s molecules, he reports in a paper submitted to Concrete International magazine.

He says his team learned that – while additives with large molecules, such as cellulose ether and xanthum gum, can increase concrete’s viscosity – they aren’t effective at slowing ion diffusion. “When additive molecules are large but present in a low concentration, it is easy for the chloride ions to go around them, but when you have a higher concentration of smaller molecules increasing the solution viscosity, it is more effective in impeding diffusion of the ions,” he explains. Numerous experiments proved that additives with nanoscale molecules worked best at slowing ion diffusion. Another lesson learned was that, while additives could be mixed directly into concrete, better performance could be achieved by saturating sand and, then, mixing the sand into concrete.

Looking ahead, Bentz sees expense as a potential obstacle to commercialization. In an online Feb. 9, 2009,  Technology Review article, he reveals that additives totaled as much as 10 percent of the mixing water. his concrete’s composition. “The industry is comfortable with one percent, so there’s a cost factor, in that it’ll cost 10 percent more,” he admits. “Conventional admixtures are generally added at a concentration of 2 % of the mixing water or less. Thus, this additive could cost 5 times more than the cost of a conventional admixture and hence the desire to find better additives with equivalent activity at lower concentrations,” says Bentz Still, Bentz is optimistic. “We’ve demonstrated proof of concept, and now we’d like to find an additive that works at three or five percent concentration,” he says, noting that research is ongoing. Certainly the need for NIST’s technique exists. As the primary ingredient in millions of miles of roads and more than 600,000 bridges in the United States alone, concrete is an essential infrastructure component. Yet, according to a 2007 Federal Highway Administration report, more than 25 percent of U.S. bridges are rated as “structurally deficient” or “functionally obsolete.” In a separate infrastructure report issued this year, the American Society of Civil Engineers, gave U.S. bridges a “C” and roads a shameful grade of “D-.”

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3 Responses to NIST’s ‘VERDiCT’ may double concrete’s lifespan

  1. Dale Bentz says:

    As the previous comments have pointed out, an effective air void system will certainly be required to prevent freeze/thaw and salt scaling damage. The intent of the present additive is to reduce diffusion (and sorption) rates for deleterious species attacking the concrete from the environment. This would include chloride ions from deicing salts and sea spray that may accelerate corrosion of the steel reinforcement, and sulfate ions from sulfate-rich soils that may cause sulfate attack.
    To correct a few statements in the article above, the additive is added at a concentration of 10 % of the mixing water, not of the total concrete composition. Conventional admixtures are generally added at a concentration of 2 % of the mixing water or less. Thus, this additive could cost 5 times more than the cost of a conventional admixture and hence the desire to find better additives with equivalent activity at lower concentrations.
    Also, the best performance has been obtained using a solution of the admixture to pre-saturate lightweight fine aggregates (not generally nonporous ordinary sand).

    As the first comment pointed out, pozzolans such as silica fume and fly ash can be a very effective strategy for decreasing diffusion rates, particularly if (early-age) cracking can be avoided. These pozzolans are commonly employed throughout the industry at the present time.

  2. John J. Valenza says:

    It sounds to me that this treatment will prevent in-situ formation of new phases (Friedel’s salt, sulfate damage) that can damage the medium from crystallization pressure, and corrosion of reinforcing steel. However, the addiditve will do nothing to prevent salt scaling, which is superficial damage that occurs irrespective of the thermodynamic conditions in the concrete matrix (degree of saturation, and composition). If salt scaling is not prevented, this additive will be useless because the ingress of aggressive species will accelerate as the surface is damaged. Salt scaling can be prevented with adequate air entrainment. The protective mechanism being the suction imposed on the pore solution by the ice that forms in the voids. So in conjunction with adequate and proper air entrainment, this additive may be extremely effective.

  3. JP Storer-Folt says:

    Sirs; 2 major factors in concrete deterioration are freeze-thaw and Cl ions. For freeze-thaw resistance ~ 6 % air used to be specified, and for Cl resistance, pozzolon was added – the Roman concrete.

    Could the treatment be a surface one only, to maintain F-T resistance, with fly ash/micro-silica additive for Cl resistance. The Olympic stadium in Montreal is a prime example of lack of air entrainment with water reducing agents, and no F-T resistance – started to come apart soon after construction; no salt up there in the roof.

    Will your additive produce a F-T resistant concrete, and a lower % additive, by changing the particle size distribution? Your % addititive is more than the amount of cement currently used in mass concrete. Too many fine particles make too many small pores wiich resist the ability of the ice front to extrude thru the mass, thus building up destructive pressure.

    JP Storer-Folt, PE
    TO, ON, CA

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