INFLUENCE OF WATER REPELLENT TREATMENT ON CORROSION OF STEEL IN CONCRETE

Transcription

1 INFLUENCE OF WATER REPELLENT TREATMENT ON CORROSION OF STEEL IN CONCRETE Tie-jun Zhao (1), Zhao-jun Ren (1) and Zhi-wei Sun (1) (1) Qingdao Technological University, Qingdao P.R.China Abstract Corrosion of steel due to chloride ingress is a worldwide problem in reinforced concrete. In order to control such chloride-induced corrosion and evaluate the effectiveness of silane liquid treatment on reinforced concrete, four groups of reinforced concrete specimens were used to investigate experimentally. In the first group, concrete specimens were made with initial chloride in the mix. The second group is as same as the first one but with water repellent treatment. The third group of concrete specimens was applied migrating chloride from a reservoir of 3% NaCl solution which was placed on the top surface of each specimen. The last group is same to the third one but with silane liquid impregnation in advance. Two water cement ratios of 0.4 and 0.6 in each group of concrete were adopted. And tests were taken with half-cell potential and polarization resistance methods. Results show that the steels in concrete with W/C of 0.6 have more negative half cell potential, higher mass loss ratio and bigger corrosion current density than those with W/C of 0.4, and the effect of improvement through silane liquid surface treatment is evident. Silane liquid surface treatment can provide effective corrosion protection to reinforcing steel in the concrete with migrating chloride, but can not adequately control the corrosion in cases significant internal chloride contamination was present in concrete or corrosion has initiated. 1. INTRODUCTION Steel reinforced concrete structures, such as buildings, bridges, parking decks, etc, represent major private and public investments. The service life and maintenance expenses for such concrete structures are often decisively influenced by corrosion of steel reinforcement. In unfavorable cases severe damage appears only a few years after construction, especially if chloride load is high, as is the case near the sea or where de-icing salts are used [1]. Corrosion of steel due to chloride ingress is a worldwide problem in reinforced concrete. Correspondingly, several methods have been adopted to protect reinforced concrete [2], and one of them is to provide added protection to concrete surface in the form of surface treatment with silane water repellent agent. Steel in concrete is protected naturally by high alkalinity of matrix and this protective quality improves with time due to continued hydration of cement. However, due to the 525

2 interaction with environment, the protection capability of concrete decreases with time. Therefore, extra measures during or after manufacture of concrete may be needed to prevent or slow down the rate of steel corrosion. Surface treatment of concrete with silane is one such measure [3]. The silane reacts with cement matrix and forms a hydrophobic layer on the walls of pores within the concrete, and then concrete will be protected from the ingress of water and water-born salts. It has been reported that surface treatment acts as a barrier between environment and concrete and prevent the entry of harmful substances such as water, chloride, etc. But studies found in technical literatures regarding the effect of water repellent surface treatment on corrosion of steel in concrete have been non-conclusive [4,5]. In addition, it has not been clear how the measured values on steel corrosion, such as half-cell potential and corrosion rate of steel, are affected by presence of water repellent layer on concrete surface at the time when steel is being corroded or uncorroded. This paper will concentrate on the influence of silane water repellent surface treatment on steel corrosion of two water cement ratio concretes with different chloride ingress modes. 2. EXPERIMENTAL PROGRAM 2.1 Test samples Concrete samples are mm 3 with a reservoir of NaCl solution or water on the test surface. The reservoir with size of mm 3 is located in the center of top surface. Upper reinforcing steel is positioned 20 mm from ponded surface and bottom steels are 25 mm from bottom surface. The ends of steel were protected with electroplater s tape and a 200 mm portion in the middle is bare [6]. The concrete specimen is shown as in Figure 1. 3% NaCl or water 75mm 115mm 75mm 150mm 150mm 280mm 115mm Figure 1: Sketches of concrete specimen 2.2 Test methods Half-cell potential measurement Half cell device includes one piece of metal in its own solution, such as copper in CuSO 4 solution. When the half cell is connected to iron in ferrous hydroxide,there will be a potential difference between two half cells, as is illustrated in Figure 2. According to this theory, a copper-copper sulphate half-cell potential device was used to monitor the change in potentials in this paper. The criterion to evaluate corrosion probability by half-cell potential is shown in Table

3 Digital Volmeter Half cell potential mensurement Half cell M M n+ +ne - Fe Fe 2+ +2e - steel 2e - +H 2 O+1/2O 2 2OH - concrete Figure 2: Half cell potential measurement Table 1 ASTM criteria for corrosion of steel in concrete [7] Copper/copper Sulphate Standard hydrogen electrode Corrosion condition >-200 mv >+116 mv Low (10% risk of corrosion) -200 to -350 mv +116 to -34 mv Intermediate corrosion risk <-350 mv <-34 mv High (<90% risk of corrosion) <-500 mv <-184 mv Severe corrosion Linear polarization technology Linear polarization device includes a half cell that is needed to connect to an auxiliary electrode and a high impedance voltmeter as shown in Figure 3. A small potential scan, Δ E(t), defined with respect to corrosion potential ( Δ E = E Ecorr ), is applied to a metal sample in this method. And polarization resistance, Rp, of a corroding electrode is defined from Equation (1) as the slope of a potential versus current density plot at i=0[8]: R p ΔE = i i=0, de / dt 0 Current density is given by i, and corrosion current density, I corr, is related to polarization resistance by Stern-Geary coefficient, B. B (2) I corr = R p The dimension of R p is ohm-cm 2, I corr is ua/cm 2 and B is in mv. Stern-Geary coefficient is related to anodic b a and cathodic b c. Tafel slope is as in Equation (3). (1) 527

4 babc B = 2.303( ba + bc ) The criterion for evaluation of corrosion probability by corrosion current density is shown in Table 2. (3) Digital Control system Guard ring Half cell Auxiliary electrode Fe Fe 2+ +2e - 2e - +H 2 O+1/2O 2 2OH - steel concrete Figure 3: Linear polarization measurement Table 2 Relationship between degree of corrosion and measurable corrosion rate [9] Degree of corrosion Corrosion current density I corr (μa/cm 2 ) Negligible <0.1 Low 0.1 to 0.5 Moderate 0.5 to 1.0 High > Mass loss measurements At the end of exposure period, concrete specimens were carefully destructed and embedded steel bars were recovered. Bars were visually examined to assess their corrosion state qualitatively and then cleaned in an aqueous solution of hydrochloric acid containing a proprietary pickling restrainer [10],which is served to dissolve corrosion products and cementitious debris without causing significant attack on the underlying steel. As a result, mass loss ratio of corrosion steel in concrete can be calculated by Equation (4). W0 W (4) 1 L w = 100% W 0 Where, L w is mass loss ratio of corrosion steel in concrete with dimension of %, W 0 is steel weight before corrosion, and W 1 is steel weight after corrosion, while W 0 and W 1 are in gram. 528

5 2.3 Materials and preparation of specimens Materials and concrete composition Portland cement type P.O.32.5 which has the compressive strength of 15.9MPa and 35.7MPa, bending strength of 3.5MPa and 7.8MPa in 3 days and 28 days correspondingly, coarse aggregates with maximum diameter of 25 mm, sand with maximum diameter of 5 mm are used in two water cement ratio concretes. The concrete compositions are shown in Table 3. Table 3 Concrete composition [kg/m 3 ] W/C Cement Sand Coarse aggregate Water Preparation of specimens For charting laconically, specimen coding is used and two series of tests were carried out, in which I stands for initial chlorides in the mix, M for migrating chloride, W for water repellent material applied, and N stands for no water repellent material applied. Series one: Chloride migrated into specimens with and without water repellent treatment. The process of MW and MN is illustrated in Figure 4. Series two: Initial chloride was in specimens with and without water repellent treatment. The process of IW and IN is illustrated in Figure 5. Casted and demoulded after 24 h 15 kg/m 3 of NaCl has been added to the upper half of fresh concrete Cured for 7d (T=20±3 and R.H 90%) Casted and demoulded after 24h Cured for 7d (T=20±3 and R.H 90%) Stored in the laboratory for 3 weeks (T=20±3 and R.H=50%-70%) Stored in the laboratory for 3 weeks (T=20±3 and R.H=50%-70%) The surface of specimen MW has been treated with water repellent agent (one hour contact with liquid silane) The specimen MN was stored in the laboratory for 1 week (T=20 ± 3 and R.H=50%-70%) The surface of specimen IW has been treated with water repellent agent (one hour contact with liquid silane) The specimen IN was stored in the laboratory for 1 week (T=20 ±3 and R.H=50%-70%) One week was allowed for polymerization of silane One week was allowed for polymerization of the silane Surfaces of specimens MN and MW have been exposed to salt water (3 % NaCl) till the end of whole test Figure 4: Flow chart of MN and MW Then drying-wetting cycles have been applied: surface was exposed to water for 8 hours followed by one week drying in laboratory atmosphere. Figure 5: Flow chart of IN and IW 529

6 2.4 Water repellent treatment on concrete surface Water repellent agent Application of water repellent treatment to concrete surface aims at reducing capillary absorption of water and dissolved aggressive substances. Concrete after hydrophobisation still leaves pores open, so the treatment does not affect the ingress of gaseous species. In this experiment, a type of silane liquid water repellent agent,whose main properties were shown in Table 4,was applied. Its molecule contains alkoxy groups linked to the silicon atoms, which can react with silicates in concrete to form a stable bond as illustrated in Figure 6. Table 4 Main properties of the type of water repellent agent Type Designation Density g/cm 3 Viscosity mpa s Content of silane % Liquid Wacker BS About 99 Hydrolyze reaction a) (OR)S i -R * (OH)S i R * b) R * R * R * R * R * R * OH-S i -OHHO- S i -OHOH-S i -OH OH -Si-O-Si-O-Si- OH OH OH OH O O O OH OH OH Condensation Polymerization Concrete Figure 6: Reaction of silane with concrete substrate [11] Silane liquid impregnation on concrete surface Hydrophobized concrete Sealed surface Liquid silane 5mm Treated surface Figure 7: Water repellent treated and sealed surfaces of concrete specimen The ponded surface of specimen is for silane impregnation, and other surfaces are sealed with paraffin wax in order to impose unidirectional drying and absorption, as as shown in Figure

7 3. EXPERIMENTAL RESULTS AND DISSCUSSION 3.1 Effect of water repellent treatment on steel corrosion with migrating chloride Cu/CuSO 4 half cell potential (mv) MN-0.4 MN-0.6 MW-0.4 MW-0.6 I corr (ua/cm 2 ) MN-0.4 MN-0.6 MW-0.4 MW Time (weeks) Time (weeks) Figure 8: Cu-CuSO4 half cell potential Figure 9: Corrosion current density Mass loss ratio of steel (%) MN-0.4 MN-0.6 MW-0.4 MW-0.6 MN-0.4 (no rust) MN-0.6 (more red rust) MW-0.4 (no rust) MW-0.6 (no rust) Figure 10: Mass loss ratio of corrosion steel Figure 11: Visual condition of steel Results of half-cell potential and corrosion current density measurements for reinforced concrete specimens are given in Figure 8 and Figure 9. They show clearly that for water repellent surface treated concrete, two parameters keep lower (negative) values and have similar trend of development of two water cement ratio concretes throughout the period measured. The concrete specimen of MN-0.6 exhibits highest level of negative corrosion potentials and corrosion current densities. These values obtained mean that corrosion of steel has occurred at 35 weeks of exposure period with corrosion potential of approximately -450 mv and corrosion current density of about 0.35 μa/cm 2, when the corrosion current density of this concrete specimen is approximately 10 times of MW-0.6 specimen. However, specimens of MW-0.4 has relatively higher negative corrosion potential (<-200 mv) and higher corrosion current density (>0.1 μa/cm 2 ), which means some risk of corrosion. 531

8 For the specimens of MW-0.4 and MW-0.6, all corrosion potentials are less negative than mv and corrosion current densities are lower than 0.1 μa/cm 2, which tells that corrosion did not happen in these specimens. The mass loss ratios of corrosion steel in these concrete are shown in Figure 10. The steel in concrete specimen of MN-0.6 has a mass loss ratio of 0.19%, while others in fact have none mass loss ratio. A comparison of visual condition of steel after removal from concrete at the end of exposure period is given in Figure 11. As expected, the appearance of steels in terms of products on the surface, coincides with the results of non-destructive corrosion tests elucidated above. While the steel in specimen of MN-0.6 exhibits much corrosion over the test area. The steels in other specimens do not show any sign of corrosion. From corrosion measurements, a considerable reduction in corrosion activity was noted in the case of surface treated concretes. Especially, the specimens with water cement ratio of 0.6, surface treated specimens appear to have reduced overall corrosion rate significantly and allowed potential of steel reinforcement to reach more positive values. And also corrosion current density and mass loss ratio of corrosion steel are much lower than untreated specimens. 3.2 Effect of water repellent treatment on steel corrosion with initial chloride Results of corrosion potentials measured in the concrete specimens with initial chloride are shown in Figure 12. In the beginning period of experiment (within 8 weeks), the corrosion potentials increased (more negative) rapidly with time and later keep relatively stable and at high values. Corrosion potentials on steel of untreated concrete specimens were more negative than treated ones with silane liquid. The corrosion potential-time curves in Figure 12 were utilized to evaluate the time of corrosion initiation based on the ASTM C 876 criterion. As showed, after 28 days of curing, corrosion potentials of all specimens with initial chloride content are more negative than -400 mv, which indicts that steels in these concretes initiate corrosion. However, for the concrete with same water cement ratio, the treated specimens have less negative corrosion potentials than treated ones, which results silane water repellent treatment reduce the probability of corrosion risk. The corrosion current densities (I corr ) measured on steel in the concrete specimens with initial chloride are shown in Figure 13. The corrosion current densities on steels of series two are initially bigger than 0.1 μa/cm 2, which increase further with time. For the same water cement ratio, the treated concrete specimens have lower corrosion current densities. Figure 14 shows the mass loss ratio of corrosion steel. The specimen of IN-0.4 has approximately 4 times higher values as that of IW-0.4. And for specimen IN-0.6, the mass loss ratio is about 2 times as that of IW-0.6. A visual comparison of corrosion condition of steels is given in Figure 15. All steels exhibit extensive corrosion products over test area. The corrosion degree of all the steels in concrete specimens are in such order: IN-0.6>IN-0.4>IW-0.6>IW

9 Cu/CuSO 4 half cell potential (mv) IN-0.4 IN-0.6 IW-0.4 IW Time (weeks) Figure 12: Cu-CuSO4 half cell potential I corr (ua/cm 2 ) IN-0.4 IN-0.6 IW-0.4 IW Time (weeks) Figure 13: Corrosion current density Mass loss ratio of steel (%) IN-0.4 IN-0.6 IW-0.4 IW-0.6 IN-0.4(a little black rust) IN-0.6(lots of black rust) IW-0.4 (a little red rust) IW-0.6 (lots of red rust) Figure 14: Mass loss ratio of corrosion steel Figure 15: Visual condition of steel 4. CONCLUSIONS Water repellent surface treatment can provide effective corrosion protection to reinforcing steel in concrete contacted with chloride solution, but when significant internal chloride contamination was present in concrete or corrosion already initiated such effect is not conspicuous. In order to prolong the service life of reinforced concrete structures, water repellent treatment can be taken into consideration to reduce the risk of steel corrosion,provided surface treatment is adequately maintained. ACKNOWLEDGEMENTS The authors of this contribution gratefully acknowledge the support of ongoing projects by National Natural Science Foundation of China (Contract No ), Natural Science Foundation of Shandong Province (Contract No. Z2006F02) and Science and Technology Bureau of Qingdao (Contract No jch). 533

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