Influence of Adhesion at Steel/Mortar Interface on Corrosion Characteristics of Reinforcing Steel

The mechanism of corrosion of reinforcing steel in concrete is discussed based on electrochemical and electron microscopy observations. The importance of calcium hydroxide precipitation on the steel surf ace in the steel/mortar interface is evaluated by placing filter paper around reinforcing steel bar specimens prior to casting in mortar, thus preventing direct contact between steel and mortar. The voids created presumably prevent calcium hydroxide crystals from forming on the steel surface. Specimens with filter paper are compared to specimens with good steel/mortar adhesion using rapid macrocell and corrosion potential tests and a scanning electron microscope (SEM) equipped with an energy dispersive spectrometer (EDS). The study included 21 macrocell and 16 corrosion potential tests run for periods of 25 to 89 days. Seven specimens were subjected to SEM/EDS analysis. Electrochemical results are mixed due to the influence of crevice corrosion. but it is generally shown that better protection is provided for steel with good steel/mortar adhesion than with filter paper. The filter paper, indeed, prevents calcium hydroxide crystals from forming on the steel surface. Corrosion products on active specimens with good mortar cover are shown to grow preferentially in voids created by air bubbles trapped in the mortar. The protective mechanism of calcium hydroxide crystals is proposed to be due to pH buffering by the hydroxyl ions released when the crystals are dissolved, a fact that cannot be proven easily, since many other factors may contribute to the protection of steel in concrete. This report is based on research by Henrik J. Axelsson in partial fulfillment of the requirements of the M.Sc. degree in Engineering Physics at Chalmers University of Technology, Gothenburg. Sweden. The research was sponsored by the Department of Civil and Envirorunental Engineering at the University of Kansas and by Structural Metals, Inc

Evaluation of Corrosion-Resistant Steel Reinforcing Bars

The corrosion performance of a new reinforcing steel is compared with that of conventional steeL The effects of both microalloying and a special heat treatment are evaluated. The microalloying includes small increases in the percentages of copper, phosphorus, and chromium compared to conventional reinforcing steel (less than 1.5 percent total), and the heat treatment involves quenching and tempering after hot rolling. The increase in the phosphorus content exceeds the amount allowed in the ASTM specifications for reinforcing steeL The steels are evaluated using the Southern Exposure and Cracked Beam tests, which are generally accepted in United States practice, plus rapid corrosion potential and macrocell tests developed at the University of Kansas. Corrosion potential, macrocell corrosion rate, and macrocell mat-to-mat resistance are measured. Mechanical properties are compared with the requirements of ASTM A 615 to measure the affects of microalloying and heat treatment on the ductility and strength of the steel. The results indicate that the corrosion resistant steel has a macrocell corrosion rate equal to half that of conventional steel. The corrosion resisting mechanisms exhibited by the microalloying appear to involve the deposition of protective corrosion products at both the anode and the cathode. The epoxy coated corrosion resistant steel had a greater time-tocorrosion than epoxy-coated conventional steel. The microalloyed steel met the mechanical requirements of ASTM A 615 for reinforcement

Rapid Evaluation of Corrosion-Resistant Concrete Reinforcing Steel in the Presence of Deicers

Research to evaluate the properties of a corrosion resistant concrete reinforcing steel is reported. The steel is microalloyed (using copper, chromium, and phosphorous), and subjected to a special heat treatment, to provide corrosion resistance superior to conventional reinforcing steel. Rapid tests, developed at the University of Kansas, are modified and used to evaluate the corrosion properties of four types of steel representing combinations of alloying elements and heat treatment. The steels include two conventional steels, one hot-rolled and one subjected to a quenching and tempering heat treatment, and two corrosion-resistant steels, one hot-rolled and one heat-treated. The steels are evaluated in the presence of three deicing chemicals: sodium chloride, calcium chloride, and calcium magnesium acetate. The results indicate that the corrosion resistant steels exhibit a corrosion rate equal to about one half that of the conventional steels. At low concentrations, calcium chloride and calcium magnesium acetate appear to have a corrosive effect similar to sodium chloride. Calcium magnesium acetate appears to be less corrosive than calcium chloride and sodium chloride at intermediate concentrations. High concentrations of CaCl, and CMA appear to cause instability in the macrocell test, causing alternating patterns of active corrosion and passivation of the anode specimens

Evaluation of Corrosion Protection Systems and Corrosion Testing Methods for Reinforcing Steel in Concrete

Corrosion protection systems for reinforcing steel in concrete and the laboratory methods used to compare these systems are evaluated. The systems evaluated include concrete with a low water-cement ratio, two corrosion inhibitors (Rheocrete 222+ and DCI-S), three microalloyed Thermex-treated steels, one conventional Thermex-treated steel, MMFX microcomposite steel, epoxy-coated steel, two duplex steels (2101 and 2205), and three heats of uncoated normalized steel, used as control specimens. The duplex steels were tested in both “as-rolled” and pickled conditions. The corrosion protection systems are evaluated using the rapid macrocell, Southern Exposure, and cracked beam tests. Some corrosion protection systems are also evaluated using the ASTM G 109 test. The corrosion rate, corrosion potential, and mat-to-mat resistance are used to compare the systems. An economic analysis is performed to determine the most cost-effective corrosion protection systems. The degree of correlation between the Southern Exposure, cracked beam, and rapid macrocell tests is determined. The coefficient of variation is used to compare the variability in the corrosion rates and the total corrosion losses obtained using the different test methods. Impedance spectroscopy analysis is performed to obtain equivalent electrical circuits to represent the rapid macrocell and Southern Exposure tests. Results show that microalloyed steel and conventional Thermex-treated steel show no improvement in corrosion resistance when compared to conventional normalized steel. In mortar or concrete with a low water-cement ratio, corrosion losses are lower than observed at higher water-cement ratios for either cracked or uncracked mortar or concrete. In uncracked mortar or concrete (rapid macrocell and Southern Exposure test) containing corrosion inhibitors, corrosion losses are lower than observed at the same water-cement ratio but with no inhibitor. For concrete containing inhibitors, with cracks above and parallel to the reinforcing steel (cracked beam test), Rheocrete 222+ improves the corrosion protection of the steel, while DCI-S does not. iii MMFX microcomposite steel exhibits corrosion losses between 26 and 60% of the losses of conventional steel. Based on corrosion potentials, the two steels have a similar tendency to corrode. MMFX steel has a higher chloride corrosion threshold than conventional steel. Epoxy-coated steel, intentionally damaged by drilling four 3.2-mm (1/8-in.) diameter holes in the coating, exhibits low corrosion losses based on the total area of the bar, between 6 and 19% of that of uncoated conventional steel. Pickled 2101 and 2205 duplex steels exhibit very good corrosion performance. The average corrosion losses for these steels ranged from 0.3 to 1.8% of the corrosion loss for conventional steel, and in most cases, the corrosion potentials indicated a very low tendency to corrode, even when exposed to high chloride concentrations. 2205 steel performs better than 2101 steel when tested in the same condition (pickled or nonpickled). For bars of the same type of steel, pickled bars exhibit lower corrosion rates than the bars that are not pickled. Based on present cost, decks containing pickled 2101 or 2205 steel are more cost effective than decks containing epoxy-coated or uncoated conventional steel. Results from the rapid macrocell, Southern Exposure, and cracked beam tests show good correlation in most cases, and have similar variability in corrosion rates and losses. In general, total corrosion losses have less variability than corrosion rates

Evaluation of Corrosion Resistance of Microalloyed Reinforcing Steel

The corrosion resistance of three microalloyed steels and two conventional reinforcing steels in concrete is evaluated. The microalloyed steels contain concentrations of chromium, copper, and phosphorus that, while low, are significantly higher than used in conventional reinforcing steel. Two of the microalloyed steels contain amounts of phosphorus that exceed the amounts allowed in ASTM specifications (ASTM A 615), while the other microalloyed steel has normal amounts of phosphorus. One of the conventional steels and the three microalloyed steels are heat treated by the Thermex process, which includes quenching and tempering of the steel immediately after rolling, while the other conventional steel is hot-rolled. The study was undertaken because earlier tests on similar steels indicated that the Thermex-treated, microalloyed steel corrodes at only one-half the rate of conventional reinforcing steel. The relative corrosion rate dropped to one-tenth if both steels were epoxy-coated. In the current study, the reinforcing steels were tested using two rapid evaluation tests, the corrosion potential and corrosion macrocell tests, and three bench-scale tests, the Southern Exposure, cracked beam, and ASTM G 109 tests. The corrosion potential, corrosion rate, and mat-to-mat resistance are used to evaluate the steel. Tension and bending tests were performed to evaluate the effect of the microalloying and heat treatment on the mechanical properties of the reinforcing steel. Results show that the corrosion potential of the five steels is approximately the same, indicating that they have a similar tendency to corrode. The results from the rapid macrocell test showed that the five steels had similar corrosion rates, with no improved behavior for the microalloyed steels. The microalloyed steel with regular phosphorus content (CRT) exhibited consistently lower corrosion losses than conventional steel in the bench-scale tests. Although CRT appears to be much more corrosion resistant than conventional steel in the G 109 tests (64% less total corrosion loss after 70 weeks), its overall performance does not show such an advantage. In the cracked beam test after 70 weeks, it had only 4% less corrosion loss than conventional steel, which indicates that in cracked concrete the two steels behave in a similar manner. In the Southern Exposure test, CRT steel had a 11% lower corrosion loss than conventional steel after the same period. This improved behavior is not enough to use the steel without an epoxy coating or to justify continued research on the steel as a superior epoxy-coated material. The mechanical properties of the microalloyed steels were similar to those of conventional steel, indicating that the increased phosphorus content did not affect the mechanical properties

Evaluation of Multiple Corrosion Protection Systems and Stainless Steel Clad Reinforcement for Reinforced Concrete

The corrosion performance of multiple corrosion protection systems and stainless steel clad reinforcement is compared and evaluated in this study. Conventional steel and conventional epoxy-coated steel coated with 3M™ Scotchkote™ 413 Fusion Bonded Epoxy are used as “control” systems. The corrosion protection systems, which are compared to the control systems based on macrocell and bench-scale tests, include stainless steel clad reinforcement, conventional epoxycoated reinforcement cast in concrete containing one of three corrosion inhibitors (DCI-S, Rheocrete 222+, or Hycrete), epoxy-coated steel with the epoxy applied over a primer coat that contains microencapsulated calcium nitrite, epoxy-coated steel with the epoxy applied after pretreatment of the steel with zinc chromate to improve adhesion between the epoxy and the steel, epoxy-coated steel using improved adhesion epoxies developed by DuPont and Valspar, and multiple coated steel with a zinc layer underlying the DuPont 8-2739 Flex West Blue epoxy layer. Macrocell tests are conducted on bare bars and bars symmetrically embedded in a mortar cylinder. Bench-scale tests include the Southern Exposure, cracked beam, and ASTM G 109 tests. The results indicate stainless steel clad reinforcement exhibits very good corrosion performance when the cladding is intact. In uncracked mortar or concrete containing corrosion inhibitors, corrosion rates and losses are lower than observed using the same mortar and concrete with no inhibitor. For concrete with cracks above and parallel to the reinforcing steel, the presence of corrosion inhibitors does not provide an advantage in protecting the reinforcing steel. In uncracked concrete, a lower water-cement ratio results in corrosion rates and losses that are lower than observed at the higher water-cement ratio. In cracked concrete, a lower-water cement ratio provides only limited additional corrosion protection when cracks provide a direct path for the chlorides to the steel. iii When adhesion loss between epoxy and steel is not considered, a 230-mm (9 in.) deck reinforced with conventional epoxy-coated steel or one of the three high adhesion epoxy-coated steels is the most cost-effective. When the potential effects of adhesion loss are considered, at a discount rate of 2%, the most cost-effective option is a 216-mm deck containing stainless steel clad reinforcement