THE INVESTIGATION OF THE MCI EFFECTIVENESS ON RANDOLPH AVENUE BRIDGE

Prof. Dubravka Bjegović, Ph. D., University of Zagreb, Civil Engineering Faculty, Zagreb, Croatia

Jessica Jackson, Cortec Corporation, St. Paul, MN, USA

Richard D. Stehly Ph.D., American Engineering Testing, St. Paul, MN, USA

ABSTRACT

The bridge deck of the Randolph Avenue Bridge in St. Paul, Minnesota, was rehabilitated in 1986. The rehabilitation process included the application of an LSDC overlay to the two eastbound traffic lanes and the application of an LSDC overlay that included the corrosion-inhibiting admixture, MCI, to the two westbound traffic lanes.

In this paper the results of corrosion rates, electrochemical potential, chloride contamination level, concrete resistivity, ambient temperature and relative humidity of both the eastbound and westbound lanes on Randolph Avenue Bridge are given. The purpose of this investigation was to determine if MCI does indeed have a positive effect on the rate of corrosion, fourteen years after the rehabilitation.

The westbound lanes, treated with corrosion inhibitor, have lower half — cell potentials, lower corrosion rates readings, lower chloride levels and higher concrete resistivity than the eastbound lanes. This investigation shows positive effect of migration corrosion inhibitor as the method for reinforced concrete corrosion protection.

KEYWORDS: concrete, corrosion, corrosion-inhibiting admixture, effectiveness, reinforcing steel

INTRODUCTION

One of the main causes of reinforced concrete deterioration is the steel corrosion. The deterioration of reinforced concrete structures is connected with enormous costs and therefore the development of accurate corrosion protection methods is of the greatest importance. One of the possible methods is the application of migrating corrosion inhibitors (MCI).

MCI inhibitors are secondary electrolyte layer mixed inhibitors. Substances dispersed or dissolved in the electrolyte-layer cause electrolyte-layer inhibition. MCI possess appreciable saturated vapor pressures under atmospheric conditions, thus allowing significant vapor phase transport of the inhibitive substance (1). MCIs are a mixed (anodic /cathodic) amino-carboxylate based inhibitor system. The inhibition of cathodic process is achieved by incorporation of one or more oxidizing radicals in an organic molecule. Inhibitor molecules are hydrolyzed in the electrolyte and then adsorbed on the metal surface. The nitrogen of the amine group is capable of entering into a coordinate bond with metal thus enhancing the adsorption process. Adsorption of cations increases the over potential of metal ionization and slows down the corrosion. The monomolecular film serves as a buffer to hold the pH at the interface in the optimum range for corrosion resistance. MCI migrate or transport through concrete to the rebar depending on the MCI type and application method: MCI admixture type migrates to the reinforcement by diffusion (both liquid and vapor) and MCI topical application type migrates to the reinforcement first by capillary suction and than by diffusion (both liquid and vapor) (2).

Bjegović et all (3) suggest an approach to the migration corrosion inhibitor testing as the procedure in four steps: step I - the electrochemical methods, as a preliminary testing, step II - methods according the Japanese standard JIS A 6205, as an accelerated test, step III - long-term laboratory testing according to ASTM G 109 and step IV — field monitoring. A lot of laboratory testing has been still done worldwide (4). Only some field testing are in progress now.

One of such field testing was done on Randolph Avenue Bridge. The corrosion rates, electrochemical potential, chloride contamination level, concrete resistivity, ambient temperature and relative humidity of both the eastbound and westbound lanes on Randolph Avenue Bridgewere tested. The purpose of the investigation was to determine if MCI does indeed have a positive effect on the rate of corrosion, fourteen years after the rehabilitation.

In 1986, the deck of the bridge that carries Randolph Avenue over I-35E in St. Paul, Minnesota, was rehabilitated. The rehabilitation process included the application of a Low Slump Dense Concrete (LSDC) overlay. Migrating corrosion inhibitor was added to the LSDC at 1 lb/yd³ (0.6 kg/m³) for the two westbound traffic lanes. Prior to overlay, the deck was milled to a depth of 0.5 in (13 mm) and the areas of unsound concrete were removed. The cavities from the removal of the unsound concrete were filled with the overlay concrete.

Corrosion assessments were conducted on the eastbound (control) and westbound (with MCI applied) travel lanes of the structure on two occasions, June 1991 and August 1992. The assessments included visual inspection, delamination survey, cover-depth survey, chloride contents as a function of depth, corrosion potentials, and estimates of corrosion current densities (i_corr). SHRP-S-658 contains all information from the 1991 and 1992 study. In November 2000 new measurements has been done on the bridge. These included Gecor 6 readings and copper/copper sulfate half-cell potentials. A new chloride analysis was also taken at various depths. This paper contain data obtained from 1985 to 2000.

EXPERIMENTAL PROCEDURE

Half-Cell Potentials

ASTM C 876 corrosion half-cell potentials were measured for both the eastbound and westbound travel lanes with a copper/copper sulfate electrode (CSE) before reconstruction in 1985 and after from 1986 to 2000, five, six and fourteen years, respectively after reconstruction; and with Gecor 6 in November of 2000.and are given in Table 1.The results of the measurements in 2000 are shown in Fig.1. According to ASTM: C876, the results can be interpreted as follows in Table 2.

Table 1. East and Westbound Randolph St. Bridge Deck Half-Cell Potential and Gecor 6 Readings

East and Westbound Randolph St. Bridge Deck Half-Cell Potential and Gecor 6 Readings
			Years Service	Average	Std.Dev.	Minimum	Maximum	%>350 mV	Testing Firm
		Control	1985	256 mV	93	120 mV	620 mV	13	MN - DOT
		MCI		292mV	115	150 mV	650 mV	24	MN - DOT
		Control	1986	100 mV	53	000 mV	270 mV	0	MN - DOT
		MCI		141 mV	48	030 mV	300 mV	0	MN - DOT
		Control	1987	292 mV	59	157 mV	444 mV	15	MN - DOT
		MCI		302mV	52	180 mV	479 mV	18	MN - DOT
		Control	1988	299 mV	49	204 mV	466 mV	12	MN - DOT
		MCI		270 mV	50	161 mV	485 mV	5	MN - DOT
		Control	1989	177 mV	64	037 mV	527 mV	2	MN - DOT
		MCI		139 mV	56	009 mV	344 mV	0	MN - DOT
		Control	1990	276 mV	73	162 mV	574 mV	15	MN - DOT
		MCI		238 mV	59	133 mV	512 mV	4	MN - DOT
		Control	1991	149 mV	64	195 mV	237 mV	6	VA TECH
		MCI		179 mV	55	170 mV	235 mV	2,3	VA TECH
		Control	1992	231 mV	64	217 mV	236 mV	NA	VA TECH
		MCI		184 mV	50	173 mV	236 mV	NA	VA TECH
		Control	2000	248 mV	54	134 mV	407 mV	4	AET
		MCI		222 mV	55	145 mV	360 mV	1,4	AET
		Gecor 6 readings
		Control	2000	269 mV	67	175 mV	414 mV	12	AET
		MCI		209 mV	51	144 mV	333 mV	0	AET

Table 2. Criteria for corrosion potential measurement

Potential	Probability of Corrosion
>-200 mV	Less than 10%
-200 mV to -350 mV	50%
>-350 mV	Greater than 90%

Fig.1. Corrosion potential measurements in 2000 year

Corrosion Current Measurements

Corrosion current density (i_corr) estimates were taken with the Gecor 6 device in November 2000. These same estimates were taken in June 1991 and August 1992 using a 3LP device. The i_corr measurement using the 3LP device is proportional to the corrosion rate through Faraday’s Law. The Gecor 6 measures the corrosion rate of steel in concrete by "polarization resistance" or "linear polarization" techniques. This is a non-destructive technique that works by applying a small current to the rebar and measuring the change in the half-cell potential. Then the polarization resistance, R_p, (the change in potential measured by Gecor 6), is divided by the applied current. The Gecor 6 obtains the corrosion rate, i_corr, from the polarization resistance, R_p, by means of the "Stearn and Geary" relationship:

i_corr = B/R_p, where B = 26 mV (1)

The criteria for estimating the reinforcement’s condition in relation to the measured value of the rate of corrosion have been defined as follows in Table 3.

The results of the measurements obtained from 1991 to 2000 are shown in Table 4 and only for 2000 but for different location are shown in Fig.2.

Table 3. Criteria for corrosion rate

Icorr	Intensity of Corrosion
<0.1 to 0.2 µA/cm2	Passive condition
0.2 to 0.5 µA/cm2	Low to moderate corrosion
0.5 to 1.0 µA/cm2	Moderate to high corrosion
>1.0 µA/cm2	High corrosion rate

Table 4. Corrosion rates measurements from 1991 to 2000.

Statistical Parameter				Icorr (mA/ft2)
	Eastbound, LSDC Control				Westbound,LSDC/Cortec MCI 2000
	1991*	1992*	2000**		1991*	1992*	2000**
Mean	1,53	0,74	0,02		1,83	0,51	0,012
Standard Deviation	0,45	0,2	0,01		1,44	0,18	0,008
Number Observati.	11	12	32		11	6	41
*Taken with 3LP meter		**Taken with Gecor 6
1m A/cm2 = 0.929 mA/ft2

Concrete Resistivity Measurements

Concrete resistivity measurements were also taken using Gecor 6 device and concrete resistivity was calculated by means of the formula:

Resistivity = 2 * R * D, where (2)

R = resistance by the "iR drop" from a pulse between the sensor counter-electrode and the rebar network

D = counter-electrode diameter of the sensor

The value of the concrete’s resistance is used as an additional parameter for the interpretation of the rate of corrosion. The interpretation of the results is possible according the criteria by Andrade et all (5,6) given in Table 5. The results of the measurements obtained only in 2000 year are shown in Fig.3.

Fig.2. Corrosion rate measurements in 2000 year

Table 5. Criteria for Concrete Resistivity

Resistivity	Corrosion Rate
>100 to 200 kW · cm	Very low, even with high chloride and carbonation
50 to 100 kW · cm	Low
10 to 50 kW · cm	Moderate to high where steel is active
<10 kW · cm	Resistivity is not the parameter controlling corrosion rate

A graph of the corrosion current and resistance (Icorr-R) results should be used so as to avoid wasting results. Andrade et al (6) recommend that criteria for the estimation of the intensity of corrosion should be used as shown in Fig. 4 in which the real results from the Randolph Avenue Bridge investigation are also given.

Fig.3. Concrete resistivity measurements in 2000 year

Fig.4. Corrosion rate and resistivity relationship

When the research results of a structure are described in a graph such as in Fig. 4., it should be expected that, if corrosion is active, the points of the graph follow the main line and should be in harmony with the degrees of saturation of the concrete structure.

Chloride Contamination Levels

Powdered concrete samples for chloride analysis were taken at mean depths of 1.0, 2.0, and 3.0 in (25, 51, and 76 mm) from 3 different locations on each side of the bridge in 1991, 1992 and 2000. Samples were taken using a rotary impact type drill with a _" sized bit. Three-gram samples that passed through a #20 sieve were obtained from each depth. The powder was then mixed with 20 ml of digestion solution for a total of 3 minutes and then 80 ml of stabilizing solution was added. A calibrated electrode coupled to an Orion Model 720-pH/ISE meter was then immersed in the solution, and the chloride-ion concentration was recorded. This method was consistent with the AASHTO: T260 procedure. The standard deviation for this chloride test was determined by testing the six pulverized concrete Quality Assurance (QA) samples of known chloride content. Each QA sample was tested five times. The results are given in Tables 6 and 7.

Table 6. Chloride — Ion Content (lbs/yd³) for Eastbound lanes, LSDC control

(1991&1992 data: Virginia Technology; 2000 data: American Engineering Testing)

Depth	1991		1992		2000
(in)*	mean	s x	mean	s x	mean	s x
0.5	10.7	2.4	13.7	3.4	-	-
1.0	4.7	2.3	5.7	2.9	17.2	1.8
1.5	2.2	1.8	3.0	1.8	-	-
2.0	2.7	0.9	4.0	2.0	6.2	0.6
2.5	2.3	1.4	3.0	2.0	-	-
3.0	1.4	1.6	1.9	1.8	2.4	0.4
3.5	0.3	0.4	-	-	-	-

(1991&1992 data: Virginia Technology; 2000 data: American Engineering Testing)

*1 in = 2,54 cm

 

In 1991 chloride content was 40% less than the control. The chloride levels increased at all levels in 1992 and 2000.

According to the LIFE-365 Service Life Prediction model (7), and on the basis of the data from chloride content in concrete overlay, Fig. 5. and the LSDC mix design for Randolph Avenue Bridge Overlay, Table 8. the calculation for time until chloride threshold is reached was done and given in Table 9.

 

Table 7. Chloride — Ion Content (lbs/yd³) for Westbound lanes, LSDC/ MCI 2000

(1991&1992 data: Virginia Technology; 2000 data: American Engineering Testing)

Depth	1991		1992		2000
(in)*	mean	s x	mean	s x	mean	s x
0.5	6.0	2.2	9.6	1.8	-	-
1.0	1.1	1.3	3.3	1.5	11.7	0.9
1.5	0	0.1	1.2	1.3	-	-
2.0	0	0	1.0	1.1	1.6	0.5
2.5	0.4	0.6	1.3	0.9	-	-
3.0	1.0	0.9	2.5	1.0	1.3	0.2
3.5	0.9	0.8	-	-	-	-

(1991&1992 data: Virginia Technology; 2000 data: American Engineering Testing)

*1 in = 2,54 cm

a) Eastbound lanes, LSDC Control b) Westbound lanes, LSDC/ MCI 2000

Fig. 5. Chloride contamination levels

 

Table 8. LSDC Mix Design for Randolph Avenue Bridge Overlay
Component					SSD lbs/yd3
Type I Cement					836
Water					270
Design w/c					0,32
Coarse Aggregate					1385
Fine Aggregate					1374
Water Reducing Admixture, PDA25YL					32.0 oz
AEA, Protex					9.3 oz
Note: 1 lb/yd3 = 0.59 kg/m3, 1 oz = 28.4 g

Table 9. Calculation for time until chloride threshold is reached

Control
2.4 lbs/yd3 chlorides at 3" 14 years after overlay was placed. 836 lbs/yd3 of cement in mix design.
2.4 / 836 * 100 = 0.29% Cl- by weight of cement in 14 years.
0.29% / 14 = 0.02% Cl- per year * 19.5 years = 0.40% Chlorides = chloride threshold
1986 + 19.5 = 2005.5 (year we expect chloride threshold to be reached in control concrete)
MCI
1.3 lbs/yd3 chlorides at 3" 14 years after overlay was placed. 836 lbs/yd3 of cement in mix design.
1.3 / 836 * 100 = 0.16% Cl- by weight of cement in 14 years.
0.16% / 14 = 0.011% Cl- per year * 36 years = 0.40% Chlorides = chloride threshold
1986 + 36 = 2022 (year we expect chloride threshold to be reached in MCI treated concrete)
(*uses 0.40% of chlorides by weight of cementious materials.)

Conclusion

The LSDC (Low Slump Dense Concrete) has done an excellent job at reducing the corrosion rate of the steel on both the control and MCI treated sides by reducing the diffusion of chlorides into the concrete. The conditions of the concrete on the bridge were wet and cool, taken after a period of cold weather and rain for several days. This was because the readings with the Gecor 6 are more accurate regarding the corrosion rate in wet conditions. While the LSDC has done an excellent job at protecting the reinforcing steel from corrosion on both sides of the bridge, a difference between readings taken on the control and MCI treated sides can still be seen:

• The average copper/copper sulfate potential value for rebar embedded in the MCI treated overlay was 208.9 mV, 22% less than the average for the control overlay at 269.6 mV. Also, 12% of the readings taken on the control overlay indicated corrosion, while none did on the MCI treated side.
• Rebar on the MCI 2000 treated side had very low corrosion currents, an average of 0.013 m A/cm², approximately 40% below readings taken on control side (average of 0.022 m A/cm²).
• The reduced chloride diffusion can be seen when looking at the data from American Engineering Testing. After fourteen years, the chloride concentration at three inches (76 mm) of depth (near the rebar) is 1.42 kg/m³ on the control side and 0.77 kg/m³ on the MCI treated side.
• According to the LIFE-365 Service Life Prediction model, the chloride threshold (C_t) is 0.05%, this value is commonly used for service-life prediction purposes and is close to a value of 0.40% chloride based on the mass of cementitious materials for a typical structural concrete mix. This is the level of chlorides at which corrosion of the reinforcing steel is initiated. It is estimated that the control will reach 0.05% of chlorides at the rebar in 19.5 years from the application of the overlay, or in the year 2005, if the chloride level continues to increase at its current rate. The MCI 2000 treated side is estimated to reach 0.05% of chlorides in 36 years from the application of the overlay, or in the year 2022.

REFERENCES

1. Miksic, B.A., (1983), Use of Vapor Phase Inhibitors for Corrosion Protection of Metal Products, NACE Corrosion 83, Paper No. 308, Anaheim, California.
2. Bjegović, D., Sipos, L., et al., (1994), Diffusion of the MCI 2020 and 2000 Corrosion Inhibitors into Concrete, International Conference on Corrosion and Corrosion Protection of Steel in Concrete, Sheffield, 865-877.
3. Bjegović, D., Mik_ić, B., Stehly, R., (2000), Test Protocols for Migrating Corrosion Inhibitors (MCI^TM) in Reinforced Concrete, in Emerging Trends in Corrosion Control — Evaluation, Monitoring, Solutions, published by: Akademia Books International, New Delhi, 3-18.
4. COST 521: Corrosion of Steel in Reinforced Concrete Structures, Prevention — Monitoring — Maintenance, Final Report 2002, edited by Romain Weydert, published at IST — Luxembourg University of Applied Sciences, pp 254.
5. Andrade, C., Alonso, M.C., Gonzalez, J.A., "An Initial Effort to Use the Corrosion Rate Measurements for Estimating Rebar Durability", Corrosion Rates of Steel in Concrete, ASTM STP 1065.
6. Andrade, C., Fullea, J., Alonso, C., "The Use of the Graph Corrosion Rate-Resistivity in the Measurement of Corrosion Current", International Workshop MESINA, Instituto Eduardo Torroja, Madrid, Spain, 1999.
7. Thomas, M.D.A. and Bentz, E.C., Life-365 Computer Program for Predicting the Service Life and Life-Cycle Costs of Reinforced Concrete Exposed to Chlorides. University of Toronto, Toronto, Canada, April 21, 2000, pp. 9.