ORGANOFUNCTIONAL SILANE CORROSION INHIBITOR SURFACE TREATMENT OF CONCRETE TO MITIGATE CORROSION DUE TO CHLORIDES OR CARBONATION ABSTRACT

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1 Paper No ORGANOFUNCTIONAL SILANE CORROSION INHIBITOR SURFACE TREATMENT OF CONCRETE TO MITIGATE CORROSION DUE TO CHLORIDES OR CARBONATION Neal S. Berke, Ph.D., FNACE Tourney Consulting Group, LLC 3401 Midlink Drive Kalamazoo, MI USA Kristin M. Ade, P.E., CP II Tourney Consulting Group, LLC 3401 Midlink Drive Kalamazoo, MI USA Peter K. DeNicola Evonik Corporation 2 Turner Place Piscataway, NJ USA Atri Rungta, Ph.D. Evonik Corporation 2 Turner Place Piscataway, NJ USA ABSTRACT Organofunctional silanes (OS) are increasingly being used to protect concrete bridge decks against chloride ingress and resulting corrosion. This paper describes the evaluation of the OS in mitigating ongoing corrosion due to chlorides or carbonation. The procedure used for determining the mitigation of chloride-induced corrosion is a new Test Protocol M-82 developed by the U.S. Bureau of Reclamation in conjunction with the Strategic Development Council of the American Concrete Institute. The mitigation of carbonation induced corrosion was determined using a modified version of ASTM G109 in which the concrete was carbonated and then cyclically ponded with water. The OS inhibitor was effective in mitigating both chloride induced and carbonation corrosion. In the case of the chloride induced corrosion there was a reduction in additional chloride ingress. Implications on the extended service life of treated structures is discussed. Key words: Concrete, repair, inhibitors, silane, post-treatment 1

2 INTRODUCTION Surface applied organofunctional silane corrosion inhibitor are increasingly being used to prevent corrosion of rebar in concrete that has high amount of chlorides. 1-4 This paper builds upon earlier work done by the authors in this field. 2-4 Two organofunctional silane (OS) topical treatments were evaluated to determine their feasibility as a post-treatment to concrete that has been subjected to chloride ingress and in which the steel reinforcement is corroding. The term post-treatment is used to emphasize that the treatment occurs after corrosion has initiated. OS-1 has a corrosion inhibitor as part of its composition, whereas OS-2 was an organofunctional silane to which an amine based inhibitor was added. Research was conducted by the authors, according to a protocol developed by the U. S. Bureau of Reclamation with a 0.50 w/c and 1.0 inches (25 mm) of cover over the rebar. 5, 6 Carbonation induced corrosion is a common problem and occurs when carbon dioxide or carbon monoxide in the air permeates into concrete resulting in a lowering of the ph due to the conversion of calcium hydroxide into calcium carbonate. Steel is not passive at the lower ph (<10) and corrosion can initiate even in the absence of chlorides. This form of corrosion usually occurs in higher water-to-cement ratio (w/c) concrete and at lower concrete covers as the rate of carbon dioxide ingress decreases as concrete permeability decreases (decreased w/c), and the depth of penetration is proportion to square root of time. The paper describes the effectiveness in reducing carbonation-induced corrosion by using OS1. EXPERIMENTAL PROCEDURE Data from three different test series are presented in this paper. The first two test series looked at OS-1 and OS-2, according to the protocol in reference 5. The third test series was performed to determine if the OS-1 treatment can mitigate carbonation induced corrosion when applied soon after carbonation initiated, or after carbonation induced corrosion was already proceeding at a high rate. Chloride-Induced Corrosion Specimens Corrosion Test Slabs For each OS product tested, ten concrete test slabs were fabricated with dimensions of 40- inches wide by 40-inches long by 5.5-inches thick. The test slabs were constructed with six longitudinal #4 reinforcing bars spanning the entire slab with 5-inch spacing. Concrete cover was 1 inch over the top bars. The group of bars was offset in the slab to create a designated area for measuring the concrete internal relative humidity and temperature, concrete electrical resistivity, and extracting core samples for chloride profiles. Each slab contains W4/W4 6 x 6 welded-wire fabric (WWF) for the bottom layer of steel. The WWF represents a bottom mat of steel with a large surface area that can provide cathodic current to the top bars and thereby accelerating the corrosion process, as would occur with a bottom mat of reinforcing bars. All bars were electrically connected through individual 1-ohm resistors to the bottom mat, inside a junction box mounted on the front face of the slab to facilitate measurements of individual rebar. The macrocell current is equal to the voltage drop across the resistor divided by 1-2

3 ohm.each test slab had a 2-inch tall by 1-inch wide lip located around the perimeter of the test slab to facilitate ponding of a 5% NaCl solution. The test slab configuration is provided in Figure 1. 6 x6 7-Switch Box 2 Ponding Dam RH, r, Core Sample area W4/W4 6x6 WWF No. 4 Steel Reinforcing Test Bars Figure 1: Corrosion test slab configuration. The concrete was designed with a 0.50 w/c ratio, with an ASTM C Type I/II ordinary portland cement and coarse and fine aggregate meeting ASTM C The concrete mixture proportions, test methods, plastic and hardened properties are provided in Tables 1 and 2. Table 1 Concrete Mixture Design Materials Series 1 (OS-1) Series 2 (OS-2) lbs./yd 3 (kg/m 3 ) Cement, ASTM C Type I/II 564 (335) 564 (335) Limestone Coarse Aggregate, ASTM C (#7) 1750 (1038) 1750 (1038) Sand, ASTM C (748) 1309 (775) Air-Entraining Admixture (AEA), ASTM C , oz./cwt 0.98 (0.57 ml/kg) 1.10 (0.64 ml/kg) Mid-Range Water-Reducing Admixture (MRWR), ASTM C (Type A), oz./cwt 3.0 (1.75 ml/kg) 3.0 (1.75 ml/kg) Designed Air Content 5.0% 5.0% Designed Slump 4 inches (10.16 cm) 4 inches (10.16 cm) After casting, the slabs were cured under wet burlap and plastic for one week and then allowed to dry in a temperature-controlled warehouse environment for an additional 21 days. The slabs were then cyclically ponded with a 5% NaCl solution for 14 days, then allowed to dry for 14 3

4 days. Corrosion measurements were performed at the end of each wet cycle (1 cycle = 28 days). Measurements consisted of corrosion potentials to produce a potential map, and of the macrocell current between each top bar, the bottom mat, and other bars. Surface resistivity measurements were also performed. Table 2 Concrete Properties for the Concretes Used for OS-1 and OS-2 Evaluations Property Value OS-1 OS-2 Plastic Properties Slump (ASTM C ) 3.5 inches (8.89 cm) 3.75 inches (9.53 cm) Unit Weight (ASTM C ) Air Content (ASTM 6.0 % 4.6 % C ) Setting Time (ASTM C ) 4.9 h 6.3 h 4.3 h 5.6 h Hardened Properties Background Chloride 248 ppm 218 ppm Content (ASTM C1152) Mat-to-Mat Resistance 10.4 ohms 11.8 ohms Electrical Resistivity 4453 ohm-cm 3670 ohm-cm Compressive Strength (ASTM C39 22 ) 1-day/3-day 1803 psi (12.43 MPa) 3910 psi (26.96 MPa) 7-days 4320 psi (29.79 MPa) 4370 psi (30.13 MPa) 28-days 5150 psi (35.51 MPa) 5360 psi (36.96 MPa) 56-days* 6080 psi (41.92 MPa) 5670 psi (39.09 MPa) (calculated/actual) 90-days* (calculated/actual) 6560 psi (45.23 MPa) 6080 psi (41.92 MPa) Chloride Ion Resistance ASTM C1202 ASTM C1270 (Equivalent C1202) 28-days/36 days 5549 Coulombs 25.5 ms/m (4640 C) 90-days 5300 Coulombs 22.4 ms/m (4070 C) 180-days 3690 Coulombs 17.1 ms/m (3110 C) 365-days 3590 Coulombs 14.1 ms/m (2620 C) Application of OS1 and OS2 OS1 and OS2 were applied to the concrete by spray application at a net coverage rate of 100 square feet per gallon. This was achieved by applying two coats at 200 square feet per gallon for each coat. 4

5 Transport Properties Cylindrical (4-inch diameter by 8-inches long) cast specimens were used for determining the rapid chloride permeability (ASTM C ) or conductivity (ASTM C ), and bulk chloride diffusion (ASTM C ). Chloride Profile Chloride penetration was determined using ASTM C on cores extracted from each of the slabs in the designated area. Powdered samples were obtained by using a drill machine to drill a hole at the depth at which chloride content was being measured. The power obtained during the drilling operation was captured and digested in nitric acid. Profiles were measured at 1) corrosion initiation 2) just prior to application of OS and 3) at the end of testing. Internal Relative Humidity Relative humidity and temperature sensors were installed at the level of the top reinforcing steel within PVC tubes cast in the side of the slabs. The internal RH and temperature were automatically measured every 6 hours throughout the duration of testing. This is a nondestructive test method that provides an indication of the moisture content in the capillary pores. Over time, the relative humidity around the probes will equilibrate to that of the concrete capillary pores. Macrocell Current Measurements The macrocell current between each top reinforcing bar and the bottom welded wire fabric was determined by reading the voltage drop across a 1 Ω resistor connecting each component to a common bus bar at the end of each ponding cycle. The area of one top reinforcing bar is approximately 57 in 2 (#4 bar, 36 length exposed) or 301 cm 2. Degree of corrosion Degree of corrosion was assessed using the guidelines mentioned in the M82 protocol. The corrosion was classified into light, medium and heavy. Carbonation Induced Corrosion The carbonation corrosion testing was conducted using beams with the same dimensions as in ASTM G109, but with a reduced concrete cover over the top reinforcing bar of 0.5-in (12.7 mm) and the standard 1-in cover for the bottom bars. A w/c ratio of 0.6 was used to facilitate carbonation. An exact description of the concrete mix can be found in Reference 8. Specimens were left in the covered molds for one day after casting, stripped, and then moist cured through 7 days. Upon the completion of moist curing, the specimens were put in a room at 100F (38 C) for one week to accelerate drying. Carbonation was achieved by placing the beams in a carbon dioxide chamber. The total volume of the chamber was 1.36 m 3 and the total volume of the beams was 0.18 m 3. A mild vacuum was applied and then CO2 gas was used to fill the chamber. Then CO2 gas was metered into the chamber at 900 cc/min. The chamber was heated to 111F (44 C), and the 5

6 average RH was 50%. Ideal carbonation conditions are close to 60% to 70% RH, which were achieved, but due to the addition of dry CO2, not maintained over weekends. After three weeks in this environment the specimens were carbonated to the 1-in (25 mm) level as measured with phenolphthalein and a rainbow ph indicator on unreinforced concrete specimens, so the top bars were in carbonated concrete. Corrosion testing was according to ASTM G109 except that potable tap water was used instead of chloride solution, and the 100-ohm resistor was changed to a 10 ohm resistor. One cycle consisted of two weeks of ponding and two weeks of drying. Corrosion potentials and macrocell currents were measured as in accordance with G109. M-82 Protocol Test Results OS-1 Results RESULTS The time to corrosion in M-82 is defined as the time to 0.03 ma, and corrosion potentials Ecorr more negative than -300 mv versus saturated Cu/CuSO4 (CSE). For the slabs used in our study, we observed that the average time for corrosion initiation was 5 cycles. One cycle is 4 weeks long (2 weeks ponding, and 2 weeks of drying) with a standard deviation of 1.5 cycles. The relative humidity (RH) and temperature data for the control and treated slabs are shown in Figure 2. The control slabs essentially remained saturated from time of treatment whereas, the RH was lowered initially in the OS-1 treated slabs and gradually increased. Though the RH was lower than that in the control slabs, it was still in a region that would support corrosion. Slight up and down fluctuations occurred due to the ponding. The temperature profiles reflect the winter to summer shift in the storage facility. Figure 2: Relative Humidity (RH) and Temperature Vs. Time (OS-1 Series) 6

7 Coulumbs (C) The average chloride contents at the 1-inch level of the reinforcing bars at the time at which OS1 was applied and at the end of testing are shown in Table 3. At time of application of OS-1, the chloride values were not significantly different in the control and treated slabs. However, over time, as additional chlorides are being introduced into the chloride by ponding with NaCl, the increase in chloride ion penetration is much higher for untreated slabs compared to treated slabs. This indicates that OS1 is efficient at keeping majority of chlorides from penetrating into the concrete. Table 3 Concrete Average Values of Chloride (ppm on Concrete) at the Bar Level (1-in.) (OS-1 series) Control/Untreated Time at which OS1 was applied to treated slabs End of Test Time at OS1 application Treated End of Test Average SD Marcocell current was constantly being monitored for each slab. Just prior to application of OS- 1, the average macrocell current measured was 9097 coulombs. According to the M82 protocol, it is recommended that treatment should be applied well beyond the minimum value of 5000 C. Hence, considerable rust and corrosion on the bars is to be expected for all the slabs used in this study. The effectiveness of the treatment can be determined by comparing the macrocell current after the treatment was applied. The chart shows that since the time the slabs were treated with OS- 1, the integrated current increases at a much faster rate compared to the treated slabs. The 95% confidence limits are the dashed lines Average Integrated Current (After Treatment) Control Treated Cycles Compleated Figure 3: Average Integrated Current after treatment (OS-1 series). 7

8 Table 4 gives a comparison of the cracking behavior, which was significantly reduced in the treated slabs. Table 4 Crack length and area by treatment (OS-1 series). Length (mm) Area (mm2) Average Average Control Treated Table 5 shows the average degree of corrosion on the reinforcing bars for the slabs. The corroded area for the bars in the treated slabs is significantly less than the control slabs, indicating that corrosion was mitigated by the application of OS-1. Table 5 Visual appearance of the reinforcing bars at the end of testing (OS-1 series). Average Visually Examined Treatment Corrosion Top of Bars Light Moderate Heavy Total Control 0.64% 4.48% 41.25% 46.36% Treated 0.22% 2.64% 21.72% 24.65% OS-2 Results For the OS-2 study, 10 slabs were cast and allowed to cure for 28 days. After fully curing, all the slabs were ponded with 5% NaCl solution for 2 weeks and allowed to dry for 2 weeks to facilitate corrosion activity. This represents one cycle. At the end of cycle 5, it was observed that macrocell current was approximately 7000 Coulombs in the slabs indicating corrosion activity was ongoing in all the slabs. At this point in the testing phase, OS-2 was applied to the surface of five of the the corroding slabs which were labelled as treated. The study continued for an additional 13 cycles (for a total of 18 cycles). The study was stopped at 18 cycles at which point all five control slabs showed severe cracking and none of the treated slabs showed cracking. Figure 4 shows the relative humidity and temperature conditions that were present during the OS-2 study. It can been seen in the chart that even though the relative humidity fluctuates, it was constantly above 80%. Therefore, it is safe to assume that the effect of relative humidity on the corrosion between treated and untreated slabs is minimal. It must be noted that OS-1 testing and OS-2 testing was not conducted in the same location and neither run at the same time and thus the RH values measured in Figure 4 are different from RH values measured in Figure 2. 8

9 Figure 4: Relative Humidity (RH) and Temperature Vs. Time (OS-2 series) The chloride values after cycle 5, just prior to application of OS-2 and at the end of testing are shown in Table 6. It can be seen from the table that the increase of chlorides was much greater for the control slabs compared to the treated slabs. Table 6 Chloride (ppm on mass of concrete) just before treatment and at test end (OS-2 Series). Control Treated End of End of Cycle 5 Test (Cycle 18) Cycle 5 Test (cycle 18) Average SD Figure 5 shows the integrated macrocell currents after treatment. Cycle 0 in the graph represents the point at which five slabs were treated with OS-2. The chart shows that since treatment, the integrated macrocell current increases at a much faster rate in the control slabs as compared to treated slabs. 9

10 Figure 5 Average Integrated Current after OS-2 treatment. Table 7 shows the average crack length and corrosion area that was observed at the end of 18 cycles for control and treated slabs. The control slabs show higher crack length and crack area as compared to treated slabs. Table 7 Crack length and area by treatment (OS-2 series). Length (mm) Area (mm 2 ) Average Average Control Treated Table 8 shows the corrosion of the rebar that was observed when the rebar was removed after cutting open the concrete at the end of 18 cycles. After just 12 cycles after treatment, it can be seen that visual corrosion is 20% higher for the control rebar compared to treated rebar. It must be noted that since the slabs were already showing signs of corrosion before treatment; it can be assumed that most of the corrosion observed on the rebar from the treated slabs occurred prior to treatment. Table 8 Visual appearance of the reinforcing bars at the end of testing (OS-2 Series). Average Visually Examined Treatment Corrosion Top of Bars Light Moderate Heavy Total Control 1.52% 3.21% 17.46% 22.18% Treated 1.60% 4.12% 11.98% 17.71% 10

11 Carbonation Test Results for OS-1 Figure 6 and 7 shows the corrosion behavior for reinforced concrete beams that were carbonated. As can be seen in Figure 8 the OS-1 treatment was effective in mitigating carbonation corrosion in both cases. Figure 6: Corrosion potentials versus time for carbonated minibeams for OS1 treated concrete (1-cycle is 28 days). 11

12 Figure 7: Mitigation of carbonation corrosion by OS-1 CONCLUSIONS The results of the test program show that both OS-1 and OS-2 were effective topical surface treatments to mitigate severe chloride induced corrosion in reinforced concrete decks. The corrosion activity significantly decreased for the OS-1 and OS-2 topically treated slabs as measured by: Reduced macrocell corrosion More positive corrosion potentials with reduction in the steepness of the contour maps Reduced cracking Reduced corrosion area OS-1 as a topical treatment has shown to reduce corrosion activity in carbonated concrete. The reduction in corrosion was rapid, whether the carbonation corrosion had just initiated or was in an advanced state. ACKNOWLEDGEMENTS The authors wish to thank Evonik Corporation 1 for sponsoring the work and the laboratory personnel at Tourney Consulting Group 2 for their assistance in producing the specimens and in testing. 1 Evonik Corporation, 2 Turner Place, Piscataway, NJ Tourney Consulting Group LLC, 3401 Midlink Drive, Kalamazoo, MI

13 REFERENCES 1. Facilities and Infrastructure Corrosion Evaluation Study Final Report, Response to House Report Accompanying H.R. 1540, National Defense Authorization Act for Fiscal Year 2012, Department of Defense, July N. S. Berke, B. E. Bucher, M. A. Miltenberger, and P. K. DeNicola, Organofunctional Silane Inhibitor Surface Treatment for Mitigation Corrosion of Steel in Concrete, Corrosion 2012 Paper No N. S. Berke, B. E. Bucher, P. K. DeNicola and Christopher Studte, Organofunctional Silane Inhibitor Surface Treatment for Mitigation Corrosion of Steel in Concrete Autopsy Results, Corrosion 2014 Paper No N. S. Berke, B. E. Bucher, and P. K. DeNicola, Organofunctional Silane Inhibitor Surface Treatment of Concrete for Corrosion and ASR Mitigation, Transportation Research Board Paper , Washington DC, US Bureau of Reclamation, M-82 (M ): Standard Protocol to Evaluate the Performance of Corrosion Mitigation Technologies in Concrete Repairs, July P.K. DeNicola, N.S. and B. E. Bucher, Sustainable Concrete Repairs Using Surface Applied Corrosion Inhibitors. SSPC 2015, ASTM (1) G109 (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). 8. N. S. Berke, B. E. Bucher, K. M. Ade, and P. K. DeNicola, Protective Surface Treatments for New and Existing Steel Reinforced Concrete to Mitigate Corrosion, Department of Defense Allied Nations Technical Corrosion Conference 2015 Paper No ASTMC150 (2009), Standard Specification for Portland Cement, (West Conshohocken, PA: ASTM). 10. ASTM C33 (2013), Standard Specification for Concrete Aggregates, (West Conshohocken, PA: ASTM). 11. ASTM C1202 (2012), Standard Test Method for Electrical Induction of Concrete s Ability to Resist Chloride Ion Penetration, (West Conshohocken, PA: ASTM). 12. ASTM C1760 (2012), Standard Test Method for Bulk Electrical Conductivity of Hardened Concrete, (West Conshohocken, PA: ASTM). 13. ASTM C1556 (2011), Standard Test Method for Determining the Apparent Chloride Diffusion Coefficient of Cementitious Mixtures by Bulk Diffusion, (West Conshohocken, PA: ASTM). 14. ASTM C1152 (2012), Standard Test Method for Acid-Soluble Chloride in Mortar and Concrete, (West Conshohocken, PA: ASTM). 15. ASTM C876 (2009), Standard Test Method for Corrosion Potentials of Uncoated Reinforcing Steel in Concrete, (West Conshohocken, PA: ASTM). 16. ASTM C260 (2010), Standard Specification for Air-Entraining Admixtures for Concrete, (West Conshohocken, PA: ASTM). 17. ASTM C494 (2015), Standard Specification for Chemical Admixtures of Concrete, (West Conshohocken, PA: ASTM). 18. ASTM C143 (2015), Standard Test Method for Slump of Hydraulic-Cement Concrete, (West Conshohocken, PA: ASTM). 19. ASTM C138 (2014), Standard Test Method for Density (Unit Weight), Yield and Air Content (Gravimetric) of Concrete, (West Conshohocken, PA: ASTM). 13

14 20. ASTM C231 (2010), Standard Test Method for Air Content of Freshly Mixed Concrete by the Pressure Method, (West Conshohocken, PA: ASTM). 21. ASTM C403 (2008), Standard Test Method for Time of Setting Concrete Mixtures by Penetration Resistance, (West Conshohocken, PA: ASTM). 22. ASTM C39 (2015), Standard Test Method for Compressive Strength of Cylindrical Concrete Specimens, (West Conshohocken, PA: ASTM). 14