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Effects of Corrosion on Infrastructure and How to Reduce It

Research Summary

INTRODUCTION

KEY TAKEAWAYS

Epoxy-coated rebar (ASTM A775) showed 95x less corrosion current than carbon-steel bars in cracked concrete
Visual corrosion on epoxy-coated bars was only 2.5% of that observed on carbon-steel bars
Galvanized and zinc products actually performed worse than carbon-steel in macrocell corrosion tests
Low-carbon chromium steel (ASTM A1035) underperformed, corroding at 57% the rate of carbon-steel
Mixing epoxy-coated bars in the top mat with carbon-steel in the bottom mat creates a galvanic couple that accelerates corrosion
Epoxy-coated rebar delivered performance comparable to several stainless steel grades at a significantly lower cost
All visible coating damage must be repaired before concrete placement to maximize field performance

Corrosion is one of the leading causes of concrete bridge deck deterioration, frequently triggered by deicing salts that penetrate cracks and attack reinforcing steel. While reducing concrete permeability can slow chloride ingress, full-depth cracks allow deicing salts to contact reinforcing bars directly, accelerating the effects of corrosion on infrastructure. Frequently these cracks are full-depth, a result of restrained thermal and drying shrinkage. Corrosion-resistant reinforcing steel has been used to mitigate corrosion-related deterioration. The Epoxy Interest Group has compiled extensive research on this topic. The objective of the research program was to evaluate both the structural and corrosion performance of concrete bridge decks reinforced with corrosion-resistant reinforcement.

A three-phase experimental investigation was conducted using a wide range of corrosion-resistant reinforcing materials. In the first phase, structural tests were conducted on forty-five beam specimens with tension lap splices to evaluate the bond between corrosion-resistant reinforcement and concrete. Twelve slab specimens were tested in the second phase to evaluate the cracking behavior of slabs reinforced with corrosion-resistant reinforcement. Finally, 112 modified macrocells were constructed in the third phase to evaluate corrosion resistance under uncracked and cracked conditions, which are summarized below.

In 2014, a Ph.D. thesis was presented by Sim (2014) from Purdue University. This thesis contains data on corrosion tests conducted on several types of reinforcing steel. The work considered structural design relating to bond development and four different corrosion scenarios. This selected summary will focus on the corrosion results from cracked concrete test specimens with the same bars in the top and bottom mats, as the bars in the uncracked concrete specimens did not provide sufficient data to separate the performance of the various reinforcing bar systems. The results clearly show that epoxy-coated reinforcing steel in cracked concrete will provide significant life extension compared with uncoated bars and these compared favorably with some of the stainless steels.

Epoxy-coated rebar has been used across hundreds of real-world structures. View Project Examples to see where it has been successfully applied.

SPECIMENS

The studies used 112 concrete specimens. These specimens were from four different categories:

Uncracked
Cracked
Cracked with carbon-steel bar cathode
Cracked with different types of ties

Specimens measured 8 x 8 x 24 inches with each layer of the reinforcing consisted of two longitudinal reinforcing bars spaced at 6 in. and three transverse bars spaced at 5 in. The bar size chosen for the testing was #4 (1/2 in.) with a clear cover of 2 in.

Cracks were obtained in the concrete specimens by applying tensile loads to the longitudinal bars. These cracks penetrated the thickness of the slabs. Target crack widths were 0.020 in., designed to represent field conditions with full-depth cracks. The obtained widths of cracks from all specimens ranged from 0.005 in. to 0.079 in. and 86% of the crack widths fell within the range of 0.015 in. to 0.035 in.

After cracks were introduced, the specimens were ponded using a 3% sodium chloride by weight salt solution. This solution was added following the ASTM G109 test procedure with a two week wet-cycle followed by a two week dry-cycle.

MEASUREMENTS

The top reinforcing bars were electrically connected with a 14 gauge copper wire wrapped tightly around the bars. These copper wires were sealed with electrical tape and additionally attached to the reinforcing steel with plastic ties. The two bottom reinforcing bars were connected in an identical fashion. The corrosion circuit was completed by connecting a 100-ohm resistor across the two mats. The voltage drop across the resistor of each macrocell specimen was collected through an 18 gauge copper wire connected between the specimens and multiplexers. The voltage data of all specimens were automatically recorded every six hours.

CORROSION CURRENT RESULTS

The data showed very little corrosion occurred in the uncracked specimens during the test period. This result may have been expected due to the use of 2 in. of concrete cover and the high quality concrete. For this reason, only data from the cracked specimens with similar top and bottom bars are discussed within this review.

The total measured current flow in coulomb is indicative of the total amount of metal loss due to corrosion that has occurred in the steel specimens. For the carbon-steel bars in cracked concrete, the total current was 947 coulomb (Table 1). The three zinc or galvanized products exhibited significantly greater values than the carbon-steel bars and the LLC (A1035) bars exhibited a corrosion of 56% that of the carbon-steel bars (Figure 1). The epoxy-coated, dual-coated and stainless bars exhibited total corrosion amounts 95 times less than that of the carbon-steel bars.

The epoxy-coated, dual-coated and stainless bars exhibited total corrosion currents 95 times less than that of the carbon-steel bars.

AUTOPSY

At the end of the 503 days, a single replicate for each bar type and specimen configuration was carefully autopsied and the bars extracted.

Uncracked Specimens, Same Cathode

For the uncracked specimens, light corrosion was observed on the top longitudinal bars for the carbon-steel and LCC (A1035). None of the stainless steel reinforcement showed visual corrosion; however, discoloration was observed on XM-28 stainless steel. Epoxy-coated reinforcement had a small spot showing corrosion of base metal, located where the coating was damaged prior to the testing.

The galvanized (A767), zinc-clad, and tin-plated zinc-clad reinforcing steel did not have any evidence of white corrosion rust (from zinc oxides) or underlying base metal corrosion (from iron oxides), but the outer layer of the shiny pure zinc had been consumed. This zinc consumption was assumed to be mainly from the reaction of the zinc coating with the concrete during hydration rather than entirely from zinc reacting as a sacrificial anode because electrical currents demonstrated less corrosion activities. The dual-coated (A1055) bars showed no visual corrosion.

Cracked Specimens, Same Cathode

Both the carbon-steel and LCC (A1035) specimens had corrosion products in both the top and bottom mats. However, more corrosion products from iron oxides were located on the top mats, which is consistent with the electrical measurements that indicated negative currents.

Autopsy results of galvanized (A767), zinc-clad, and tin-plated zinc-clad steel reinforcement exhibited white corrosion products from zinc oxide. A greater amount of the white product was noted at the location of the cracks and especially on the transverse steel that was located parallel to the cracks.

The 316LN and 2304 stainless steel did not display any evidence of corrosion. However, the 2205 stainless steel was discolored. The XM-28 stainless steel exhibited more discolored locations and a reddish corrosion spot was observed on the transverse steel.

Corrosion was limited to a local area on the epoxy-coated (A775) reinforcing; however, corrosion of the underlying base metal was observed at the ends of the transverse steel (where they had been patched and repaired prior to casting) and where the coating peeled. In addition, due to corrosion of the base metal underneath the epoxy coating, a spot where the coating was bulging was observed.

Cracked specimens with dual-coated (A1055) bars did not show significant corrosion, but corrosion was observed at the location where damage was introduced prior to testing. The zinc under the epoxy layer was consumed and red iron oxide products were observed.

The surface area of all longitudinal and transverse steel was 103.7 in.² From the visual observations, the amount of corrosion was documented (Table 2). Only reddish-brown (from red iron oxide) and blackish (from black iron oxide) corrosion were considered in this evaluation and any white corrosion products from zinc were ignored.

The amount of corrosion of the top carbon-steel bars was 5.5% of the total bar area. Using this as the baseline, the LCC (A1035) materials exhibited almost the same amount of corrosion as the carbon-steel bars (Figure 2). The dual-coated (A1055) bars exhibited 10 times less corrosion than the carbon-steel bars. The epoxy-coated (A775) bars exhibited 40 times less corrosion than the carbon-steel bars.

The XM-28 bars exhibited corrosion approximately 80 times less than that of the carbon-steel (A615) bars. The other stainless steel bars did not exhibit corrosion.

The performance of the epoxy-coated (ASTM A775) reinforcing steel compared favorably with some of the stainless steels.

SUMMARY AND RESULTS

Based on the results of this study, Sim reached the following conclusions:

While all uncracked specimens showed relatively low currents at 503 days of exposure, cracked specimens demonstrated high corrosion activity that was electronically measured by the macrocell test and confirmed by visual examination through an autopsy of the specimen.
Autopsy results demonstrated that most of the longitudinal steel corroded at the intersection with the transverse steel while the transverse reinforcement corroded over its entire length. The transverse steel, typically located parallel to the cracks, was under direct chloride exposure over its entire length, while the longitudinal steel had direct exposure only at the location of the cracks.
When corrosion-resistant chromium-based reinforcing steel is used in the top mat and carbon-steel bars are used in bottom mats, a galvanic couple resulted where the bottom carbon-steel corroded to protect the top corrosion-resistant reinforcement. This galvanic couple occurred because the cracks in the macrocells were formed full-depth where chlorides can easily reach the bottom carbon-steel bars from the first day of testing.
Both the electrical current measurements and autopsy results demonstrated that mixing reinforcement where carbon-steel bars are provided in the bottom mat is detrimental to corrosion resistance.

Additional conclusions based upon review of the data from the thesis are shown below:

The length of testing was found to be insufficient for the uncracked specimens and few conclusions about the performance of the various products in uncracked concrete may be obtained.
The data presented for galvanized and zinc results are difficult to interpret due to the fact that the corrosion on these products are not the same as those derived from the carbon-steel bars. These products showed evidence of high macrocurrents and white zinc-based corrosion products.
The low-carbon chromium steel bars (ASTM A1035) did poorly in these tests with corrosion rates of 57% that of the carbon-steel bars. Observed corrosion that was 93% that of the carbon-steel bars.
The dual-coated (ASTM A1055) bars exhibited significantly lower corrosion rates than the carbon-steel bars and observed corrosion was also substantially reduced.
Two of the stainless steel bar types (2205 and XM-28) exhibited discoloration and one exhibited corrosion in the cracked beam test. (XM-28).
The epoxy-coated (ASTM A775) reinforcing steel showed significantly lower visual corrosion in the cracked concrete specimens than that of the carbon-steel bars. The amount of corrosion was approximately 2.5% that of the amount observed in the carbon-steel bars. These bars also exhibited measured corrosion currents 95 times less than that of carbon-steel bars.
The performance of the epoxy-coated (ASTM A775) reinforcing steel compared favorably with some of the stainless steels.

Learn More about epoxy-coated reinforcing steel research and specifications.

The amount of corrosion for the epoxy-coated was approximately 2.5% that of the amount observed in the carbon-steel bars.

What are the effects of corrosion on infrastructure?
Corrosion of reinforcing steel in concrete structures causes cracking, spalling, and structural deterioration that shortens the service life of bridges, parking garages, and other concrete infrastructure. Deicing salts accelerate this process by penetrating cracks and making direct contact with reinforcing bars, leading to costly repairs and premature replacement.

How does epoxy-coated rebar protect against corrosion?
Epoxy-coated rebar uses a fusion-bonded epoxy coating that acts as a barrier between the steel and corrosive elements like chloride ions from deicing salts. In this study, epoxy-coated bars (ASTM A775) showed corrosion levels 95 times lower than uncoated carbon-steel bars, even under cracked concrete conditions.

Is epoxy-coated rebar better than stainless steel?
Epoxy-coated rebar (ASTM A775) performed comparably to several stainless steel grades in this study, including outperforming XM-28 stainless steel in some measures. Given that epoxy-coated rebar costs significantly less than stainless steel, it is often the more cost-effective choice for corrosion protection in concrete structures.

Does galvanized rebar perform better than epoxy-coated rebar?
No. In this study, galvanized rebar actually showed higher macrocell corrosion current values than carbon-steel bars. While galvanized bars produced white zinc-based corrosion products rather than red rust, their overall corrosion performance in cracked concrete was significantly worse than epoxy-coated bars.

Does cracking in concrete affect rebar corrosion?
Yes, significantly. Uncracked specimens in this study showed very little corrosion regardless of bar type. However, once full-depth cracks were present, corrosion activity increased dramatically, particularly for carbon-steel and galvanized bars. This makes the choice of corrosion-resistant reinforcement especially important in structures prone to cracking.

Can epoxy-coated rebar be used if it gets damaged during installation?
Minor damage can occur during field installation, but industry specifications require all visible coating damage to be repaired before concrete is placed. Even with 1% damage per linear foot applied in this study, which is more severe than typical field conditions, epoxy-coated bars still dramatically outperformed all other coated and uncoated options tested.

REFERENCES

Sim, C., “Structural and Corrosion Performance of Concrete Bridge Decks Reinforced with Corrosion-Resistant Reinforcing Steel,” Ph.D. Thesis, Purdue University 2014.

McDonald, D.B., D.W. Pfeifer, and M.R. Sherman, Corrosion Evaluation of Epoxy-coated, Metallic-Clad and Solid Metallic Reinforcing Bars in Concrete. 1998, Federal Highway Administration: McLean VA. p. 137.

O’Reilly, M., D. Darwin, J. Browning and C.E. Locke, Jr., Evaluation of Multiple Corrosion Protection Systems for Reinforced Concrete Bridge Decks. 2011, The University of Kansas Research Inc.: Lawrence KS.