concrete surface or by using migrating corrosion inhibitors. This paper ... Two types of coatings and two applications of migrating corrosion inhibitors were tested.
CORROSION PROTECTION OF EXISTING CONCRETE STRUCTURES
D. Bjegovic and V. Ukrainczyk Faculty of Civil Engineering, University of Zagreb Kaciceva 26, Zagreb, Croatia B. Miksic Cortec Corporation 4119 White Bear Parkway, St. Paul, M N, USA ABS TRACT Corrosion protection of existing concrete structures could be done in two practical ways: by coating the concrete surface or by using migrating corrosion inhibitors. This paper presents experimental results on compatibility of a migrating corrosion inhibitor added to two repair mortars. The two repair mortars are based on an inorganic binder modified with polymers. The influence of a migrating inhibitor on the properties of fresh and hardened mortars was tested. The aim of this study has been also to determine the effect of repair mortars used as coatings and migrating corrosion inhibitors on corrosion of reinforcement in concrete. Two types of coatings and two applications of migrating corrosion inhibitors were tested. Migrating corrosion inhibitors were introduced into the coatings. Migrating corrosion inhibitors topically applied to the concrete surface by brush was done also as one of the applications for corrosion protection of reinforcement embedded in concrete. The effectiveness on reinforcement corrosion protection has been tested according to ASTM G 109. According to the test results, the effectiveness was evaluated. Key words: migrating inhibitor, repair mortar, polymer as admixture INTRODUCTION M uch damage on reinforced concrete structures is caused by reinforcement corrosion. If the bearing capacity of the structure is not affected, the structure is best repaired by ready-made repair material. The building materials industry has offered a range of new materials for the preservation and repairs of already damaged reinforced concrete structures. These materials allow for vapour diffusion while preventing Cl- ions and CO2 diffusion. These materials also allow for better adhesion and polymer-modified repair structures, etc. A special group of corrosion protection measures for concrete reinforcement are migrating corrosion inhibitors added to repair mortars during mixing. According to their action, the inhibitors can be classified into three groups: • Anodic inhibitors, prevent the reinforcement corrosion by affecting the anodic process. M ost widely used compounds that belong to this group are nitrites and chromates. If the anodic inhibitor is not present in sufficient concentration on the complete reinforcement surface, pitting corrosion can appear, creating local damage, which can be more dangerous than no inhibitor at all; this can happen with improper blending of concrete mix. ctp47
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• Cathodic inhibitors, adsorbed on the reinforcement surface create a barrier of molecular thickness, thus enhancing the kinetics of the electrode reactions. Examples of compounds that are cathodic inhibitors include various amines. The initial adsorption of a cathodic inhibitor on the reinforcement surface is influenced by the electrochemical attraction forces between the steel surface and the inhibitor. Cathodic inhibitors do not enhance localized attack, even when concentration drops below the critical level, which is their basic advantage. • M ixed inhibitor influences both the cathodic and anodic processes. Because of the microcell corrosion processes occurring in reinforced concrete, a mixed inhibitor is ideally suited for application. The inhibitor should have good solubility characteristics and rapidly saturate the corroding surface and not leach out. Also the physical and durability of concrete should not be adversely affected. Extensive research is being done to find the optimum corrosion inhibitor for concrete [1-3]. So far, most research has been done on calcium nitrite-based inhibitors [4-8] that have been used for over fifteen years. Their applications are now restricted or even ruled out for ecological reasons. This paper studies the influence of amine-based migrating corrosion inhibitors added into two types of repair mortars used as coatings [9-11], and also applied on the concrete surface for corrosion protection of pre-rusted reinforcement embedded in concrete. RES EARCH PROGRAMME The research program was divided in two phases: I phase - incorporation of migrating corrosion inhibitors into the two types of coatings II phase - migrating corrosion inhibitors topically applied to the concrete surface for corrosion protection of pre-rusted reinforcement embedded in concrete. I PHAS E In order to study the migrating corrosion inhibitor added to coatings, two types of coatings have been selected: - Cement-based thin coating, modified with polymer admixture, maximum aggregate size 1 mm (PCSC Polymer Cement Surface Coating); - Cement-based repair mortar, modified with polymer admixture, maximum aggregate size 4 mm (PCM Polymer Cement M ortar). a) Since repair mortars contain polymer admixtures, compatibility of amine-based migrating inhibitor and repair mortars was tested: • •
On fresh concrete consistency vs. time and influence on porosity On hardened concrete compressive and tensile strength.
Table 1 presents the mortar mix design for a 3-litre test batch. The dosage of migrating inhibitor as an admixture in mortars was 1l/m3 (in accordance with manufacturer's specification) and 3l/m3 for overdose testing. ctp47
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Mix Design* 1 2 3
Powder Component (g)
5076.3 5076.3 5076.3
1 5000 2 5000 3 5000 *1 - mix design without inhibitor 2 - mix design with inhibitor, 1l/m3 3 - mix design with inhibitor, 3l/m3
Liquid component (ml) PCS 200 200 200 PCM 200 200 200
Water (ml)
Migrating Inhibitor (g)
970 1005 1100
1.8 5.3
785 785 785
1.8 5.3
Table 1. Mortar mix design for compatibility testing The compatibility was considered acceptable in cases when migrating inhibitor effect on chemical and physical properties of repair mortars remained within the +10% range. This condition was taken from the specification HRN U.M 1.035, Concrete Admixtures, Quality and Quality Assurance. b) After compatibility testing with two types of repair mortars used as coatings, the systems were tested for corrosion protection properties. Testing was done to evaluate the inhibitor efficiency on inhibiting corrosion of the rebar. Figure 1 represents the scheme for preparation of the specimens for testing. Six specimens were made of concrete base, and three specimens were made for each coating system. The concrete base specimens were made in accordance with ASTM G 109 standards, with a deviation in the concrete composition, because the objective was to make a very permeable concrete. The concrete base was therefore made with 300 kg/m3 cement and the w/c ratio was 0.6. After 28 days concrete specimens were coated with different coating as can be seen in Figure 1. The migrating corrosion inhibitor was added to the coatings in the dosage 1 l/m3.
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concrete base(C)three coats of cement-basedcoating,modified with polymer admixture(PCSC)three c
Figure 1. Scheme for preparation of specimens for testing II PHAS E On existing concrete structures the main problem is to find a way of stopping corrosion after it has started. Cathodic protection has proved to be an effective method for stopping corrosion in reinforced concrete structure but for that all reinforcement must be electrically continuous. This is not so easy to achieve on existing reinforced concrete structure. Therefore one idea was to prove the topical application of migrating corrosion inhibitor on the pre-rusted reinforced concrete specimens. Six specimens were made: three for concrete only base, and three for the migrating corrosion inhibitor topical application. The concrete specimens were made in accordance with ASTM G 109 standard with pre-rusted reinforcement embedded in concrete specimens. The rust layer on the reinforcement was 95 g/m2 formed by exposure of the clean reinforcement to the foggy climate. M igrating corrosion inhibitor was topically applied by brush after 28 days with the concentration suggested by the producer. RES ULTS AND DIS CUS S ION Porosity of fresh mortars for coatings Immediately after mixing, porosity of fresh repair mortars was tested according to HRN U.M 1.031. The results are presented in Figure 2.
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123012345678910Freshconcreteporosity(%)123Coating
Mix DesignPorosityPCSCPCM
Figure 2. Influence of migrating corrosion inhibitor on fresh mortar porosity Consistency vs. time
The consistency was tested by the flow method according to HRN B.C8.042 on the mixes shown in Table 1. The results are presented in Figure 3. 0153045607590105120123155160165170175180185190195200flow (cm)time (min)Consistency vs timeCoati
Figure 3. Influence of migrating corrosion inhibitor on consistency of repair mortar Compressive and tensile strength The influence of corrosion inhibitors on compressive and tensile strength of repair mortars PCSC and PCM was tested on specimens prepared from the mixes in Table 1 which were cured and tested according to HRN B.C8.022. The results are presented in Figure 4.
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123051015202530354045Compressivestrength (N/mm2)123coatings mixdesigna)
Compressive strength (N/mm2)PCSCPCM
Figure 4a. Influence of migrating corrosion inhibitor on strengths of repair mortar 123024681012tensile strength N/mm2)123coatings mixdesignb)
Tensile strength (N/mm2)PCSCPCM
Figure 4b. Influence of migrating corrosion inhibitor on strengths of repair mortar The influence of migrating corrosion inhibitor on the properties of the repair mortars PCSC and PCM is in the range of 10% of less. Overdosage of three times as much as the one recommended by the manufacturer does not have adverse influence on the properties tested. Therefore, the admixture of migrating corrosion inhibitor is compatible with the repair materials PCSC and PCM. Effects of migrating inhibitor on corrosion of reinforcement in concrete Effects of migrating inhibitor on corrosion of the reinforcement in concrete were tested according to standards ASTM C-876 and ASTM G-109. In order to test the effects of the migrating inhibitor as admixture to concrete (ASTM G-109), three reinforcement rods, diameter 14 mm, were embedded in the concrete base on two levels, one rod being placed 3.0 cm below the surface serving as an anode, and the other two connected in series at 2.5 cm and 11.5 cm below the surface serving as cathodes. A tank with 3% NaCl solution was placed on top of the specimen for 14 days. The liquid was removed and the surface allowed to dry for 14 days. The wet/dry procedure was repeated for over a year. A 100-ohm resistor was placed between the anode and the cathode. The change in the resistance between the anode and the cathode was registered, and the current flow was calculated. Measurements were done at least once a month, until the current reached the value of 10 µA. The difference in the potential between the anode and the reference electrode (Cu/CuSO4) was measured during the same time intervals (ASTM C 876). ctp47
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The results of testing the effect of migrating inhibitor on the reinforcement corrosion in the concrete for the specimens from I phase (coatings without and with inhibitors on concrete surface) over 50 weeks are given in Figures 5 and 6. The results of testing the effect of migrating inhibitor topically applied on concrete surface, on the reinforcement corrosion in the concrete for the specimens from II phase over 20 weeks are given in Figures 7 and 8. The results show that the migrating inhibitor added to fresh mix of repair mortars PCSC and PCM was very effective. The value of electrical potential and current are negligible small in the systems containing an admixture of migrating corrosion inhibitor, as compared to the control specimens containing no inhibitor. From the results of testing the effect of migrating inhibitor topically applied on concrete surface according to the measurements of the half-cell potentials it could be said that no influence happened because the potential values were more or less the same during the measurements. But, from the results of corrosion current it should be noticed that the corrosion current slow down with time. From the previous works on the calculation the diffusion rate of migrating corrosion inhibitor [12] is obvious that the testing time of 20 weeks is to short for the true answer. Namely, according to the diffusion calculation certain time is needed for migrating corrosion inhibitor to achieve the protective concentration on the reinforcement level. The concentration of migrating corrosion inhibitor on the reinforcement level is very dependent upon the concrete quality and the moisture level in the concrete. Therefore for the future investigation, before the application of the migrating corrosion inhibitor on the concrete surface the specimens should be cleaned by steel brush to avoid the cement base and to open the surface for better capillary suction. Also on the bases of this testing and the knowing that these migrating corrosion inhibitors are anodecathode type of inhibitors the suggestion is the testing should be performed with the linear polarisation resistance method in the future investigation. CONCLUS ION The influence of migrating amine-based corrosion inhibiting admixture, added to two types of repair mortars during blending was studied. Since repair mortars contain polymer admixtures, compatibility of amine-based migrating inhibitors and repair mortars were tested on fresh and hardened repair mortars. The properties tested have shown that migrating inhibitors and repair mortars PCSC and PCM were compatible. The effect of migrating inhibitor on reinforcement corrosion protection was also tested. The results have shown that migrating inhibitor added to the fresh mix of repair mortars PCSC and PCM is very effective because it delays reinforcement corrosion in concrete. Migrating corrosion inhibitor topically applied on the specimen surface with pre-rusted reinforcement embedded in concrete has shown the promising results but more investigation is needed to ensure that the application could be effective. Because the migrating corrosion inhibitor topically applied is anodecathode type of inhibitors the suggestion is the testing with the linear polarisation resistance method for the future investigation. According to these investigation the more effective results could be achieved when the migrating corrosion inhibitor was added in the fresh repair mortars during the mixing. ctp47
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