Gulf Construction Online - New inhibitors fight corrosion better

Chemicals in Construction

One of the bridges where Grace has carried out corrosion mitigation.

New inhibitors fight corrosion better

With corrosion remaining one of the most serious and expensive problems facing concrete structures, Neal Berke, Antonio Aldykiewicz Jr and Lianfang Li of Grace Chemicals suggest ways of mitigating the effect of corrosive agents using new concrete admixtures.

01 September 2005

Corrosion mitigation practices have come a long way since they were first used a few decades ago.

Today, these practices include: increasing the concrete cover, reducing the concrete's permeability, using reinforcement that is more resistant to corrosion, and using corrosion-inhibiting admixtures.
Several such products are now commercially available, and new modeling programs can be used to compare the benefits of different corrosion inhibitors and other concrete additives. Recent advances in reducing concrete cracking have made models more applicable. Most significantly, these models show that inhibitors with good, long-term track records lower lifecycle costs.

New admixtures
Commercial corrosion inhibitors have been added to concrete since the mid-1970s. Calcium nitrite, for example, was first used in Japan to offset chlorides in sea sands, which were part of the concrete mix. It was first introduced in the US in 1978(1); however, because calcium nitrite acts as an accelerator in concrete, a more neutral-setting version of the product was introduced in 1988. Today, more than 10 million cu m of concrete throughout the world contain calcium nitrite admixtures and several admixture companies are now selling it as well.
The 1990s saw the introduction of new admixtures, including amino alcohols, morpheline derivatives, phosphates, benzoate, and a fatty ester of oleic acid amine(2). When added to the concrete mix, these admixtures can stop or slow corrosion. Moreover, many of these admixtures can be used in concrete repair, where they are applied to concrete surfaces and migrate to the embedded steel. The moot question is, what effects these admixtures actually have on corrosion. Scant data is available. Although many independent studies have documented the effectiveness of calcium nitrite(3), few have examined these new admixtures.
As a result, the ASTM has been hard at work developing a new test for corrosion inhibitors, which supplements the existing protocol, ASTM G109. The new test under development focuses on identifying the chloride threshold for the initiation of corrosion. It involves a combination of macrocell and polarization resistance measurements on a standard sample of mortar (cement paste and sand) and embedded steel bars which are subjected to alternate ponding of sodium chloride.
The macrocell measures the current between the top corroding steel bar and the bottom bar, simulating field conditions where steel bars exposed to chloride would corrode, much like the way a battery works. Polarisation resistance is a sophisticated electrochemical test that measures corrosion.
At the initiation of corrosion, the chloride content is measured at the surface of the reinforcing bar. The measurements can be used in a deterministic or statistical based model to predict the length of time before corrosion will begin. At least 10 samples are used for each test.
The ASTM is also developing a corrosion inhibitor specification, based on ASTM G109. In effect, any product that meets this specification will be classified as a corrosion inhibitor.
In Germany, a new passivating corrosion inhibitor specification, which relies on several tests, has been developed. It is worth noting that the neutral-set version of calcium nitrite has passed the requirements of this specification.

Are Inhibitors Effective?
Since cracks in concrete provide conduits for chloride to reach the steel reinforcements, engineers have been concerned with the effectiveness of inhibitors when cracks are present.
Calcium nitrite, for example, is effective in good quality concrete provided that the concrete cover is 40 mm or more, and crack sizes are less than 0.3 mm. Figure 1 shows the benefits of calcium nitrite in cracked concrete.
In addition, the admixture containing the ester of oleic acid amine has been shown to improve performance in cracked concrete, and other research on concrete without inhibitors has demonstrated that crack size, concrete quality, and concrete cover all play key roles in performance. Thus, if concrete meets the minimum requirements of ACI 318 of 40 mm of cover and water-cement ratio (w/cm) of less than 0.4 for chloride exposure, controlling crack sizes becomes critical to ensure good performance.
Recent advances in concrete technology have resulted in shrinkage reducing admixtures (SRAs) and non-corrosive, nonconductive structural synthetic fibers. These products can significantly reduce cracks that result from drying shrinkage and mechanical loading. Table 2 shows reductions in drying shrinkage for some mixtures containing calcium nitrite and an SRA.

Effective modeling
Fortunately, two excellent models are available to quantify the advantages of using corrosion inhibiting admixtures. To use these models, one needs to be able to predict three parameters: the chloride ingress over time, the critical threshold value for the initiation of corrosion, and the time to repairs after cracking starts.
The Concrete Corrosion Inhibitor Association's model, known as Life 365, and the DuraModel(4) have been developed and refined in recent years. Both models have incorporated chloride threshold values for calcium nitrite and butyl oleate plus amine (OCI), and assume a five-year window from the initiation of corrosion to first repair based on the government's Strategic Highway Research Program (SHRP). When modeling the use of OCI, the Life 365 model reduces chloride diffusivity by 10 percent.
A marine pile with a 75-year design life in a tropical environment was used to illustrate how low-permeability concrete with corrosion inhibitors can enhance durability. For this example, the DuraModel was used, but similar results have been obtained with the Life 365 model using the same parameters. The base concrete has a w/cm of 0.38, and diffusion coefficients are based on two-year ponding data adjusted to temperature. Because this example models a marine structure in a saltwater environment, a further reduction in the apparent diffusion coefficient over time is accounted for using a value (m in the Life 365 model) of 0.2. Note that the Life 365 model starts with a 28-day apparent diffusion coefficient and uses a higher value for pozzolans, but research in our laboratory indicates that after two years becomes insignificant.
Clearly, the reduction in permeability with silica fume or fly ash is beneficial, as is the addition of corrosion inhibitors. Based on these models, one can conclude that the best and most cost-effective solution combines low permeability concrete and corrosion inhibitors. This finding is highlighted by the cost rankings in Figure 2.
If structural synthetic fibres and an SRA are added, costs will increase, even when this additional cost is added to the cost of the corrosion inhibitor, the overall expense is more than offset by the savings in delayed or avoided repairs. Moreover, synthetic fibers and SRAs improve the concrete by reducing cracks and increasing toughness.

Conclusions
One of the best ways to protect bridges and other vulnerable structures from corrosion is to use the new corrosion-inhibiting admixtures in the original concrete mix. Moreover, treating existing structures with the same admixtures can be an effective way to address the ongoing problem of corroded bridges and marine structures. Recent advances in SRAs, which reduce cracking, and structural synthetic fibers further enhance structural performance and make the use of chloride prediction models more useful. These models have shown that corrosion-inhibiting admixtures can be a cost-effective solution to very expensive repairs. In either case, these new high-performing admixtures can help reinforced concrete structures reach their long-term design life.

References:
1. Berke, N S and Weil, T G. "Worldwide Review of Corrosion Inhibitors in Concrete," in Advances in Concrete Technology (2nd Ed.), ed. V M Malhotra, CANMET, Ottawa, Canada, pp. 891-914, 1994.
2. Elsener, B. "Corrosion Inhibitors for Steel in Concrete," European Federation of Corrosion Publications, No. 35, W S Manney and Son, Ltd. (2001), 68 pp.
3. Bentur, A, Diamond, S, and Berke, N S, Steel Corrosion in Concrete-Fundamental and Civil Engineering Practice, E & FN Spon, London (1998), 208 pp.
4. Berke, N S, Macdonald, A C, and Hicks, M C, "Use of the DuraModel for the Design of Cost-Effective Concrete Structures," ICCRRCS, Orlando, Florida, December 1998.
All three writers are with Grace Construction Products. Neal S Berke is a research and development fellow and manager of all durability-related research; Antonio J Aldykiewicz, Jr is a research manager for fire protection; and Lianfang Li is a research engineer in durability research. Information about the DuraModel can be found on www.graceconstruction.com.