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Publisher’s version / Version de l'éditeur:

Construction Canada, 39, Nov-Dec 6, pp. 36-39, 1997-12-01

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Research on cost-effective solutions for corrosion prevention and repair in concrete structures

Gu, P.; Beaudoin, J. J.

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Research on Cost-Effective Solutions for

Corrosion Prevention and Repair in

Concrete Structures

Author(s): Dr. Gordon Ping Gu and Dr. J. J. Beaudoin

Date: May 4, 2006

Report #: NRCC-41939

Version of: Construction Canada, 39, no. 6, Nov/Dec 1997, pp. 36-39

Abstract: The corrosion of reinforcing steel within concrete structures such as bridges and parking garages costs North American infrastructure managers billions of dollars every year. This Update describes ongoing IRC research into this problem, and provides the latest results.

The corrosion of reinforcing steel within concrete structures such as bridges and parking garages costs North American infrastructure managers billions of dollars every year. This Update describes ongoing IRC research into this problem, and provides the latest results.

Concrete that fails when its reinforcing steel, or rebar, corrodes is one of the primary causes of premature deterioration in North American bridges, parking garages and other concrete structures. Particularly prevalent in areas where road salt is applied to bridge decks, highways and other road surfaces during winter weather conditions, this corrosion leads to costly repairs: the annual repair bill for over 600 problem garages in Canada alone amounts to more than $200 million.

Addressing rebar corrosion is a high priority for both the Transport Association of Canada and the National Cooperative Highway Research Program in the U.S., who - while understanding that environmental conditions and structural use influence rebar longevity - are pushing for a service life of 100 years. In Europe, the aim is to achieve 125 years.

IRC and its research partners are approaching the problem of rebar corrosion holistically, researching facets such as effective diagnosis and repair, as well as underlying causes and prevention. Two key concerns of performance and price characterize the research, which should result in solutions that not only work, but are also practical in the context of repair and construction budgets.

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Rebar Corrosion Diagnosis

Identifying the degree of rebar corrosion is, of course, essential for choosing the correct repair alternative. IRC, Public Works and Government Services Canada and the Alberta Transportation Department worked together to establish the validity and

cost-effectiveness of a linear polarization technique. Used previously to diagnose the existence of rebar corrosion in bridge decks, the technique also proved capable of determining the rate of corrosion, an important predictor of future deck life.

The researchers selected test bridges and, after assessing maintenance histories together with visual and delamination inspections, took measurements at strategic locations. The work aimed to determine how precise the linear polarization technique was in mapping active corrosion areas in high-density concrete decks, and in decks with various

protective overlays. It also sought to validate the half-cell potential data (a simple, inexpensive way to determine field corrosion) obtained in previous research.

The results showed that the technique revealed realistic corrosion rates in most cases, with the exceptions being decks with chip-asphalt coatings or thick coatings of silane sealer.

Measuring the corrosion rate provides not only existing damage levels, but also predicts potential for future activity of the reinforced concrete. It thereby assists in estimating the concrete's remaining service life and determining the correct repair or replacement option.

Sacrificial Cathodic Protection

Repairing corrosion-damaged concrete includes options (used either alone or in

combination) such as removing unsound concrete, patching, and applying waterproofing membranes. When salt has caused the initial rebar and concrete damage, however, that salt contamination remains in the concrete, continuing to corrode the rebar.

Consequently, the repair only addresses the symptoms of the problem, rather than the cause.

Sacrificial cathodic protection can, if applied before severe damage occurs, significantly reduce repair costs. It works via a zinc film sprayed onto the concrete surface: the zinc, not the rebar, becomes the site of corrosion activity.

The technique has shown promise in preventing corrosion for Florida's coastal bridges: now, IRC and another NRC group, the Industrial Materials Institute (IMI) have teamed up to investigate its potential in Canada's challenging climate. In one IRC/IMI field study, in partnership with the Ministry of Transportation of Quebec, researchers flame-sprayed zinc onto seven reinforced concrete columns of a Montreal bridge: 20 months later, the zinc was still protecting the columns.

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Another field study linked IRC researchers with the International Lead Zinc Research Organization in metallizing driving surfaces in an Ottawa parking garage with zinc. Results showed high levels of protection, particularly in dry areas, and indicated that in wet areas, where the zinc sacrificed itself more quickly, a thicker level of zinc is needed.

Further research into this area is investigating the protection potential of metallizing alloys, which combine zinc with other materials such as magnesium or aluminum.

Corrosion Inhibitor Evaluation

Research into diagnosing and repairing corrosion deals with the present; research into protecting carbon steel from corrosion deals with the future. Previous studies have shown that the rate of corrosion is affected by such factors as the wetting and drying cycle (which affects the ingress of carbon dioxide), the porosity of the concrete and the presence of chloride ions.

One solution is to use corrosion inhibitors as admixtures to the concrete in which the steel bars are embedded. These admixtures must not affect the concrete adversely; they must effectively prevent corrosion, and they must be affordable.

To investigate this further, IRC has selected a bridge, owned by the Ministry of Transport of Quebec and located in the municipalities of Laval and Boisbriand, in order to assist the field evaluation of eight different commercial rebar corrosion-inhibiting products. One hundred and thirty-four reinforced concrete samples have been fabricated, with half the specimens containing four steel rebars, and the other half containing two carbon-steel and two epoxy-coated bars.

The sixteen corrosion-inhibiting admixtures were applied including rebar primers,

corrosion-inhibiting admixtures and spray-applied migrating corrosion inhibitors. Testing techniques included linear polarization, AC impedance and macro-cell current

measurements.

To date, no noticeable degree of corrosion has appeared. Indications are that, at this early research stage, the chloride penetration is either insufficient, or has not yet reached the steel bars in order to initiate corrosion, an idea that is supported by the fact that the chloride-contaminated and chloride-free specimens show similar results so far. Research is continuing, with testing every six months.

High-Volume Fly Ash Concrete

In addition, the Canada Center for Mineral and Energy Technology (CANMET) and IRC are working together in a joint research project to investigate rebar corrosion in high-volume fly ash (HVFA) concrete.

In this research, eight carbon-steel reinforced concrete specimens have been fabricated that include Portland cement concrete, concrete incorporating high-volume fly ash and

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concrete containing an HVFA blended cement. The specimens were subjected to a chloride solution and monitored after two and a half months of exposure. Several electrochemical techniques were used to assess corrosion, including half-cell potential measurements, linear polarization and A.C. impedance.

Results so far reveal that the less the concrete cover over the rebar, the more likely the rebar is prone to suffer corrosion. Among the rebar not showing a probability of

corrosion, even at low concrete covers, is that which is encased in HVFA concrete with Class F fly ash and Class C fly ash, both with water/cement (w/c) ratios of 0.32. In the Portland cement concrete samples, corrosion probability appears to increase with an increasing w/c ratio. Concrete incorporating 10% silica fume is showing a low

probability of corrosion, while that incorporating 55% ground granulated blast-furnace slag (w/c ratio 0.32) is showing a high probability. This research is continuing.

The Stainless Steel Alternative

Corrosion-inhibiting admixtures in concrete, application of waterproofing membranes, penetrants and sealers on concrete surfaces, electrochemical removal of chlorides, and cathodic protection have their own advantages and limitations, but they all represent secondary efforts at corrosion mitigation. To address the problem at its source, we must focus attention on the steel reinforcing bar before it becomes encased in the concrete.

Solid stainless steel and stainless steel cladded reinforcing bars have been used in Europe for many years. This is because its tolerance to chloride content is higher than the amount necessary to start corrosion in carbon steel. Stainless steel's corrosion rate is two to three times less than that of carbon steel. In North America, however, builders rarely use stainless steel because it is more expensive than uncoated or epoxy-coated carbon steel.

Life-cycle cost comparisons suggest that stainless steel, while initially more expensive, may offer significant maintenance and repair savings through reduced corrosion and increased service life. IRC, in cooperation with the Nickel Development Institute, is investigating the chloride-induced corrosion behaviour of four kinds of stainless steel rebar in chloride contaminated concrete.

The research is unique in that the rebar is pre-stressed. In order to more closely mirror industry use, the rebar was bent into "L" and "U" shapes, rather than left straight as in previous studies. The stainless steel rebar samples, along with a carbon steel sample for comparison, were placed in concrete samples which have three different chloride levels: 0%, 0.5% and 2%. The samples were then stored in a high-humidity, high-heat

environment designed to accelerate corrosion, and corrosion data were recorded monthly using advanced electrochemical techniques.

By determining the threshold level of chloride needed to induce corrosion in stainless steel rebar, monitoring the corrosion progression and comparing it to carbon steel rebar corrosion, IRC will be able to assess whether stainless steel can provide superior corrosion and cost performance in North America. This research is expected to be

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completed in 1998; however, interim results indicate that stainless steel rebar, even in the presence of chloride and elevated temperatures, is much less likely to corrode than carbon-steel rebar.

In fact, all the concrete specimens reinforced with carbon rebar and containing 2% chloride were severely cracked after four months, and showed severe rebar corrosion which left a corrosion layer on the outer surface that exceeded 1 mm. The stainless steel rebar concrete samples showed no sign of rebar corrosion at all.

Epoxy-Coated Steel Reinforcement

Another option for dealing with corrosion at its source is to coat carbon steel rebar with an epoxy coating known to provide good protection. Since 1975, millions of linear feet of straight fusion-bonded epoxy-coated reinforcing bars have been manufactured; however, shipping and careless construction practice can damage the coating, leading to pitting corrosion.

To address this, IRC and IMI have teamed with the 3M company to work on a six-month project that will evaluate the corrosion resistance of epoxy rebar and duplex galvanic metal/epoxy rebars. The objective is to develop innovative duplex galvanic metal/epoxy rebars to minimize pitting corrosion damage. It is expected that a combination of galvanic coatings such as zinc, magnesium or aluminum and epoxy should provide a high

corrosion resistance.

Industry Implications

IRC's research (with partners such as IMI) into rebar corrosion is both broad in scope and practical in application, addressing reinforcing materials, corrosion protection techniques and the concrete itself. Since the research began two years ago, more than 30 different coatings for carbon steel - including combinations of stainless steel, epoxy and galvanic metals - have been evaluated, as have different thermal spraying techniques for applying the coatings, such as high velocity oxy-fuel, electric arc and flame. Stainless steel itself is coming under close scrutiny, as is the chemical content of the concrete, while corrosion protection measures like corrosion inhibitors, cathodic protection and concrete coatings are under investigation.

Looking at all of these factors will allow IRC to determine the service life of reinforced concrete in various environments, along with price and performance factors of both materials and corrosion protection measures. In turn, IRC will be in a better position to provide the construction industry and infrastructure owners and managers with cost-effective solutions to the billion-dollar problem of corrosion in reinforced concrete structures.

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