Corrosion of reinforcing steel bar (rebar) in concrete structures is the primary cause of deterioration that limits the structures’ functional life. To control and remediate rebar corrosion in new and existing concrete structures, multiple methods are available. These range from changing the concrete’s cement chemistry to using corrosion inhibiting techniques such as pozzolanic cement, surface sealers, corrosion inhibitor admixtures, cathodic protection, and surface-applied corrosion inhibitors. Reducing the rate of rebar corrosion in a reinforced concrete structure provides value by extending the time it takes to reach structural insufficiency caused by concrete spalling and loss of rebar thickness.
Specifications for most of these methods are published by various organizations such as NACE International (Houston, Texas, USA), ASTM International (West Conshohocken, Pennsylvania, USA), American Association of State Highway and Transportation Officials (AASHTO) (Washington, DC, USA), and state transportation departments. More recently, Unified Facilities Guide Specification (UFGS) 09 97 23.171 was developed to provide guidelines for corrosion inhibiting coatings for concrete surfaces.
How Rebar Corrodes
Due to concrete’s high alkalinity (a pH level of 11 or higher), a protective, passive oxide layer on the steel rebar is formed by hydroxyl ions in the surrounding concrete cover, which reduces the steel corrosion rate to a negligible level. According to NACE member Robert A. Walde,2 chief technical officer with Surtreat Holding (Pittsburgh, Pennsylvania, USA), new concrete has a high alkalinity, typically a pH level of 13, and uncoated rebar will remain in a passive state as long as the pH stays above 11.
For rebar to corrode, the steel’s passive oxide layer must fail. If the protective concrete cover is damaged and the bond between the concrete and rebar is broken, the passive layer will break down and the steel will start corroding. Additionally, the presence of chloride ions and carbonation will damage the steel’s passive layer, even if the concrete cover is not damaged.
Walde explains that chloride ions from deicing salts and marine environments, as well as atmospheric carbon dioxide (CO2), will penetrate cement’s micropores. When chloride ions reach the rebar, they are able to penetrate the steel’s passive oxide layer and cause a defect. A build-up of chloride ions can cause the passive layer to break down, which allows corrosion to initiate. Carbonation results when CO2, an acidic gas, neutralizes the concrete’s alkalinity and lowers its pH. The more acidic concrete renders the rebar unable to form a protective passive layer and more susceptible to corrosion.
Surface-applied corrosion inhibitors, which penetrate the concrete and form a protective film on the steel rebar, have successfully reduced the rebar corrosion rate and extended the service life of many types of reinforced concrete structures, Walde notes. UFGS 09 97 23.17 describes the use of an organic vapor phase inhibitor, an ionic inhibitor, and a combination of both inhibitors along with a reactive silicone surface sealant or surface protection coating. The standard includes guidelines for structural repairs of damaged or delaminated concrete, preparation of the concrete surface, pre-application testing, application of the inhibitor and surface sealant, post-application testing, minimum performance requirements, environmental conditions, equipment, sequencing and scheduling, and maintenance test procedures.
A penetrating vapor phase corrosion inhibitor is defined in the specification as a solution of organic amine carboxylate compounds that migrate in the gas phase through the cement pores to form a corrosion inhibiting film on the reinforcing steel surface. A penetrating ionic corrosion inhibitor is a solution containing chemically reactive water-soluble inorganic silicates designed to act as an anodic inhibitor on the surface of the reinforcing steel. A surface sealant is a chemically reactive water dispersion of a silane/siloxane mixture that forms an insoluble cross-linked silicone membrane within the concrete matrix.
New Inhibitor Experiments
This standard is based on technical work performed by the NASA Kennedy Space Center (NASA/KSC), U.S. Army Corps of Engineers (USACE) Engineering Research and Development Center—Construction Engineering Research Laboratory (ERDC-CERL), and Naval Facilities Engineering Command (NAVFAC) in cooperation with a supplier of migratory corrosion inhibitors. Walde notes that the UFGS specification uses data from two benchmark experiments with surface-applied concrete corrosion inhibitors. He describes the experiments in a paper2 presented at CORROSION 2018.
The first experiment was initiated in 1998 at the NASA/KSC Corrosion Technology Laboratory, which is located in a corrosive marine environment in East Central Florida, USA. Two inhibitors—one identified as an inorganic ionic phase migratory inhibitor, and the other as an organic vapor phase migratory corrosion inhibitor—were provided by an inhibitor supplier, which also supplied corrosion test cells in accordance with a NASA/KSC design based on a modified version of one described in ASTM G109.3 Rebar was put into a corrosion active state by placing a 10% sodium chloride (NaCl) solution in the cell reservoir and allowing it to penetrate and create an active corrosion state as observed by the current flow (µA) between rebar at a depth of 1 and 2 in ( 25 and 51 mm) and rebar at a depth of 4 in (102 mm). Side ports on the cells were used to measure half-cell potentials on the rebar.
Two weeks after placing the salt solution in the reservoir, corrosion currents had reached 50 to 90 µA and half-cell potentials were –300 to –390 mV. The salt solution was removed, and the corrosion inhibitor solution was added to the reservoir and allowed to penetrate and migrate to the rebar levels. The corrosion current and half-cell potentials were measured over a one-year period. One year after application of the corrosion inhibitor solutions, the corrosion currents for both test cells had dropped to 2 to 3 µA and the half-cell potentials dropped to –150 to –200 mV.
Each of the two corrosion inhibitor types evaluated by NASA/KSC provided the same degree of corrosion rate reduction when applied to the test cells at a rate equivalent to 50 ft2/gal (1.2 m2/L), although each used different inhibiting mechanisms.
The two migratory corrosion inhibitors evaluated by NASA/KSC were also evaluated in 2007 in Okinawa, Japan by ERDC-CERL. Two reinforced concrete structures—ring girders in a warehouse at the military port in Naha and a patrol bridge at the U.S. Air Force Kadena Air Base fuel storage depot—were selected for the evaluation.
Surface repairs were made on the structures and ground wire connections made to the rebar. The concrete surfaces were kept wet for one hour to obtain reproducible corrosion current measurements. Multipoint baseline corrosion rates for the rebar (15 to 25) were measured in January 2007 on both structures by galvanostatic polarizing. Corrosion current measurements were converted into corrosion rates in terms of µm/y of steel loss.
The two migratory corrosion inhibitors were applied to the warehouse ring girders and the bridge wing wall supports at an average application rate of 100 ft2/gal (2.45 m2/L). The organic vapor phase inhibitor was applied first, followed by the inorganic ionic phase inhibitor. The structures were then lightly sprayed with water to drive the active ingredients into the concrete cement pores. A final application of a silane/siloxane water repellent was made at the rate of 200 ft2/gal (4.8 m2/L). According to Walde, six months after applying the two inhibitors to the two structures, the average corrosion rates were reduced: from 61.3 to 24.3 µm/y (with the median rate reduced from 41.1 to 5.7 µm/y) on the warehouse ring girders, and from 37.4 to 13.1 µm/y (with the median rate reduced from 28.9 to 7.8 µm/y) on the bridge wing wall supports.
Independent Verification
In July 2010, EDRC-CERL commissioned an independent validator to return to Okinawa and repeat the rebar corrosion rate measurements. The validator made measurements on both structures using the same rebar ground wire connections and measurement point matrix. Three years after the initial application of the corrosion inhibitor system, the median corrosion rate was reduced from 41.1 to 4.9 µm/y on the warehouse ring girders and from 28.9 to 6.9 µm/y on the bridge wing wall supports, Walde reports.
Based on the corrosion rate reductions described in the 2010 reports from ERDC-CERL and the independent validator, NAVFAC was commissioned to form a tri-service team tasked with reviewing the Okinawa project results and preparing a UFGS for the application of surface-applied corrosion inhibitors for rebar in concrete. In March 2016, NAVFAC contracted with an independent validator to return to Okinawa and perform corrosion rate measurements over the same points on the same warehouse ring girder and bridge wing wall supports. The validator removed a layer of concrete surface from part of the ring girder test area. The 2016 median corrosion rate on the ring girder was 6.6 µm/y, an 84% reduction compared to the baseline measurement taken in January 2007. The median rate on the bridge wing walls was 7.7 µm/y, a 73% reduction compared to the January 2007 baseline measurement.
A UFGS is used for construction specifications for the military services. Development of these guide specifications is a joint effort of USACE, NAVFAC, the Air Force Civil Engineer Center (AFCEC), and NASA.
References
1 UFS-09 97 23.17, “Corrosion Inhibitor Coating of Concrete Surfaces” (Washington, DC: NAVFAC, 2016).
2 R.A. Walde, “Examination of Methods for Reinforced Concrete Steel Corrosion Remediation and Structure Life Extension,” CORROSION 2018, paper no. 10859 (Houston, TX: NACE International, 2018).
3 ASTM G109-07(2013), “Standard Test Method for Determining Effects of Chemical Admixtures on Corrosion of Embedded Steel Reinforcement in Concrete Exposed to Chloride Environments” (West Conshohocken, PA: ASTM, 2013).