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Corrosion Mechanism in Concrete

Question:

Discuss about the Corrosion Effects on Civil Engg ?

Corrosion effects on reinforced concrete structure reduce its durability. Corrosion of reinforces steel in concrete is not as complex as it seems rather is similar to electrochemical reaction like that of a battery. Concrete usually provides adequate protection to reinforced steel, which is due to high pH value of 12.5 of the alkaline environment that the concrete provides for protecting the steel from further corrosion (Abbas 2016). The current report evaluates the corrosion mechanism, repair measures and prevention of RCC structures discussing in details the corrosion-testing rate and any specific method with adequate details of experiment. A brief material and methodology has been included followed by application of materials and execution in this report. Finally, the report includes results that have been discussed concluding the report.

Corrosion refers to the chemical process that destructs the materials due to reaction with conditions of environment. According to Aperador (2015), atmospheric corrosion causes rusting of steel and it have been observed that appreciable corrosion starts at a relative humidity exceeding 65%. Below freezing point of water, pure and dry air, corrosion does not occur. Corrosion of steel leads to serious consequences of cross-section area reduction as well as splitting and cracking over concrete. The reduction in cross-section causes reduction of load carrying capacity of concrete structures. It also causes reduction of fatigue strength and reduction of elongation properties.

Corrosion in concrete occurs due to chlorides and sulphates present in reinforcement more than critical limit without obtaining sufficient alkalis within concrete for maintaining steel in a positive condition. Factors that are responsible for corroding steel reinforcement in structures of concrete are concrete quality. The fine aggregate, coarse aggregates water and cement determine concrete quality (Chen 2015). Proper water cement ratio, adequate compaction through vibration, tampering and curing along with proper mixing leads to improved quality of concrete. However if any of the processes is not followed properly, corrosion of reinforcement occurs, hence for preventing steel to corrode, dense concrete also known as high strength concrete is used.

Cover thickness of concrete reinforcement also plays an important role in prevention of corrosion of reinforcement of concrete. Greater thickness of concrete provides higher degree of protection to steel from environmental conditions. Evenness of concrete also plays an important role in protection of steel from corrosion. As per Cui (2016), contamination of reinforcement with salt during placing of concrete on reinforcement also accelerates corrosion rate. Environmental chemicals or concrete secretion causes deterioration process of reinforcement and the chemical effects due to presence of chloride, carbonation, salt and sulphate reacting with tricalcium aluminate (C3A) in concrete causes the corrosion rate to increase. Hydration of cement also develops calcium hydroxide that increases pH value of to approximately 12.5 and hence a thin film of oxide is coated to the reinforcement to protect it from corrosion (Deo 2014). Porosity of concrete also plays a major role depending on degree of compaction, type of cement, concrete age and aggregate grading and size that causes chemicals aggressively penetrating the concrete to reach steels causing its corrosion.

Factors Responsible for Corrosion of Reinforcement


Concrete generally withstands high temperature of 1000 C beyond which concrete starts to deteriorate leading to thermal cracks. These cracks cause the steel to get exposed to environmental harshness that leads to corrosion of concrete. Colder regions causes’ concrete pores to freeze and the formation of ice leads to expansion of volume that consequences bursting pressure with excess pressure on the mass of concrete surrounding due to thawing and freezing condition. Corrosion rate in steel reinforcement of concrete is determined by polarization method of resistance for on-site condition. Gao (2015) commented that the test was applied for bars that are electrically connected with closer layer of the reinforcement facing the counter electrode. During electrical isolation of electrodes the rebars, irrespective of the lying reinforcements there is a measured in depths higher than 1m with testing. The test methods are used at any moment with life service of the structure as well as the kind of climate in provision of the temperature higher than 00C. Highly drying surface of concrete with a resistivity value of greater than 1000 ?m (Haamidh et al. 2016). The pre wetting of surface for concrete is necessary for maintaining the Icorr value of the obtained results. Aim of the experiment is to assess corrosion condition presently for discriminating non-corroding and corroding zones as well as evaluation of repair work affectivity. Calculation of the loss of cross section in rebars through period of propagation with provision of initiation time. Current values are also interpreted through expert personnel in evaluation and testing of the structures. Two concrete slabs are fabricated without use of admixtures adding approximately 3% of CacCl2 of the weight of cement mixing with water (Hsieh 2014). The minimum slab dimension needs to be 1.20 x 1.20 m2 by 10 cm of thickness (Huang 2017). Hence 150 x 150 x 15 cm3 is recommended for the laboratory tests with around 5 to 10 rebars isolated for embedding having a mean for making external electrical between rebars that needs to be provided. As per Islam (2015), the measurements are taken repeatedly through placing the reference electrode at different points along the rebar and calculating the average of the results.

Galvanostatic pulse technique is a polarization technique, which is independent of resistance of concrete. The corrosion rate is determined through 5 – 10 seconds determination of half-cell potential. Galvanised pulse method (GPM) is developed based on equipment software that is developed as user interface between transient analyser response and generator of galvanostatic pulse. Transient analyser response with PSION WorkAbout handheld system provides analysis and results of the GPM performed measurements on highway-bridges that is exposed for de-icing the salts as well as other results of laboratory tests (Jayasree 2016).

Galvanometric pulse method refers to a rapid non-destructive technique of polarization that has been utilized for reinforcement corrosion evaluation on site and in laboratory. Anodic pulse current within a short period has been utilized for galvanostatic reinforcement through a counter electrode which is positioned adjacent to the rebars.  

Prevention and Repair Measures of Corrosion in Reinforced Concrete

Equipments used for the methods contain potentiostat and galvanostat for measuring and controlling the potential and current. Potential measuring circuit are maintained through an electrode potential within a voltage limit of 1mV having a wide range of currents that has a high impedance over 10 M ohm. Such impedance ensures that corroding systems used for measurements helps in minimizing drawn current and the sensitivity of the current circuit is in the order of 0.05µA/cm2 having a sensitivity detection variations of around 0.5mV through a potential range between -1.0 V to +1.0 V (Karar 2015). A sensitivity of around 0.05 µA ranging between 0.05 to 104 µA is used. Auxiliary sensor having counter and references electrodes through metallic materials for producing current in most cost-effective materials for easy maintainability is used.

Reference electrodes are required for obtaining RP, ref. making connections through wires externally among all rebars for reproducing reality (Kim 2017). Taking the calibrated instruments and placing Auxiliary sensors on surface of concrete provides adequate interfacial contact making connections to rebars in providing connection to whole mat. The Rp measurement is again carried out giving a proper value of ohm.cm3 through instructions of equipments in considering polarized current. The experiment is arranged as a preparatory that is necessary in measurement of corrosion rate due to weathering effects. Location of measurement is identified and selected for proper identification. Rebar is then located through proper identification and system coordinate in measurements. According to Kumar (2013), prewetting of surface of concrete preparation through auxiliary sensor fixation and placement connecting reinforcement and equipments executing measurement is also carried out.

Extreme temperature forbids electronic equipments to work with extreme humidity as well as temperature. Temperature below 20C should not be allowed exceeding limits of environment hence below freezing sponge might provide unstable and misleading readings. Below 50C of temperature rate of corrosion reduces to lower values that misinterprets the data (Mao 2016).  Each corrosion rate measurement reading is taken less between 1 to 5 minutes that retards actual condition of corrosion with appropriate measurement methods. Before survey starts, it is necessary for selecting location points of rates of corrosion that requires to be measured. The corrosion points depend on the available time, structural size and access. Measurements are taken at strategic locations of high and low readings from half-cell potentials of concrete resistivity.

Structural importance location regarding corrosion rate measurement duration takes less than approximately 5 minutes. Steel detectors are used for actual geometry of rebar arrangements and the bar pattern is marked on concrete surface as well as cover depth that has been registered and the distance is required for calculation purpose of steel area in total that needs to be polarized. Measurements are then taken in grid with a recommended spacing of around 0.25 m having an exception on small structures or elements having severe exposure condition changes (Mascagni 2014). Prewetting of concrete surface is required prior to application of Auxiliary sensors that avoids contamination of reference electrodes with concrete’s alkaline substances. Clean wetted sponge between concrete surface and sensor is used for making trials on hydrophobically or coated surface. Auxiliary sensor needs to be located directly over known diameter of the rebar, which is either a crossover, or a single bar of specific dimension and length. The surface being horizontal or vertical the steel area is polarized during measurement and calculations taken into account the intersection of two or more bars.

Testing Methods for Corrosion

Electrical connections are made between reinforcements and equipments through opening by excavation, connection or coring through steels exposed and through an attached cable to the rebar. Cleaning of rebar is necessary through using metallic brush and electrical contact avoiding several holes for having electrical networks of reinforcements. Electrical continuity needs to be maintained through steel and connection of rebars being measured. Finally, the connection needs to be verified through checking corrosion potential stability. The stability is very much essential for assuring measurement correctly of the voltage shift after application more than fluctuation between 0.5 mV for every 5 seconds interval (Reddy 2014). After measurement has been carried out until testing time completion, repetition of measurement is necessary taking into account the allowance of potential corrosion recovering the stability.    

Result Discussion/Conclusion

A total number of 7 concrete test blocks were created using to reinforcement bars. The blocks were exposed to 40 days of chlorides and the concentrate was calculated on a regular basis by using a GPM for determining the variation of the quotation rate over time. During the end of the exposure time the blocks destroyed and the reinforcement bars work cleaned. For any kind of corrosion products, loss of weight for the reinforcement bar was determined by using Faraday's law. The weight loss correspondence to the corrosion rate on an average basis that is necessary for integrating the corrosion rate determination by GPM overtime (Singh 2016). For comparing the results it has been seen that there is a good correlation among the quotation which has been determined by the GPM and the rates being determined by the loss of weight.

Weight loss

GPM

Description

Avg. Corrosion rate µA/cm2

mV vs Ag/AgCl`

Avg. Corrosion rate µA/cm2

Bar A

4.6

-345

3.11

Bar B

4.8

-334

2.47

Connected bar A and B

4.7

-345

5.24


Underestimation on the rate of corrosion of the bars not connected is usually due to the balance. As the base is not connected to the spirit of the guard ring, current remains limited and hence influences the confined area. Wang (2016) stated that emphasis for obtaining corrosion rate is an average rate instantaneous method which is important can find area strictly applying to the measuring conditions. Overcoming the problems becomes necessary to integration of frequent production rate measurement over time in comparison the laboratory data of the production rate determined through measurement of weight loss.

Potential Range (mV Vs. Ag/AgCl)

Corrosion rate

   

Laboratory results

Field measurements

-325

-350

150-500

 

-300

-325

50-150

 

-275

-300

150-500

 

-250

-275

50-150

50-150

-225

-250

150-500

 

-200

-225

50-150

 

-175

-200

150-500

 

-150

-175

50-150

 

-125

-150

150-500

 

-100

-125

50-150

 

-75

-100

150-500

150-500

-50

-75

50-150

 

-25

-50

150-500

 

0

 

50-150

 

 Results from the lottery shows highly desirable correlation among quotation rates determined my weight loss and the GPM method. Onsite quotation rates obtained by GPM are compatible to the quotation (Yaldagard 2014). Its average is calculated from the cross-sectional area loss as well as places were the coding area similar as a confined area. For predicting for a lifetime period more detail knowledge of seasonal and daily changes required for obtaining rate of corrosion is essential for a meaningful value. It is also essential for combining the rate of corrosion measurement on site having fixed mounted post personal wireless chloride sensors other non destructive methods for determining the integrity of concrete as well as rates of penetration.

References

Abbas, Y., Nutma, J.S., Olthuis, W. and van den Berg, A., 2016. Corrosion monitoring of reinforcement steel using galvanostatically induced potential transients. IEEE sensors journal, 16(3), pp.693-698.

Galvanostatic Pulse Technique

Aperador, W., Delgado, A. and Bautista-Ruiz, J., 2015. Estimation of the Passivation of Steel Embedded in Alternative Concrete using a Galvanostatic Pulse Technique. INTERNATIONAL JOURNAL OF ELECTROCHEMICAL SCIENCE, 10(7), pp.5238-5248.

Chen, Y., Sun, C., Huang, C., Xu, H. and Wu, J., 2015. A new multi-component composite with gamma polyglutamic acid as corrosion and scale inhibitor. Materials Research Innovations, 19(sup6), pp.S6-13.

Cui, Y., Lan, H.Q., He, R.Y. and Zhang, C.H., 2016. Prediction of the internal corrosion remaining life of a gas pipeline made of high-strength steel. Journal of Pipeline Engineering, 15(3), pp.1-8.

Deo, R.N., Birbilis, N. and Cull, J.P., 2014. Measurement of corrosion in soil using the galvanostatic pulse technique. Corrosion Science, 80, pp.339-349.

Gao, Y., Fan, L., Ward, L. and Liu, Z., 2015. Synthesis of polyaspartic acid derivative and evaluation of its corrosion and scale inhibition performance in seawater utilization. Desalination, 365, pp.220-226.

Haamidh, A., Prabavathy, S., Denny, J., Elyas, M., D’costa, S.A., D’Souza, R.D., Sahu, K., Pradhan, M.K., Aqil, L.A.K., Junaidi, M.A.R. and Kar, R., 2016. A Study on CO 2 Adsorption of Zeolite Plasters and its Effects on Durability Properties of RCC Members, Concrete Tiles and Floorings. European Journal of Advances in Engineering and Technology, 3(5), pp.1-7.

Hsieh, Y.C., Chang, L.C., Tseng, Y.C., Wu, P.W. and Lee, J.F., 2014. Structural characterizations of PtRu nanoparticles by galvanostatic pulse electrodeposition. Journal of Alloys and Compounds, 583, pp.170-175.

Huang, C., Xu, H., Zhang, M., Wu, J. and Huang, C., 2017. Study on Relation Between Industrial Circulating Water Conductivity And Iron Corrosion Velocity. Journal of Residuals Science & Technology, 14(3). pp. 2-6.

Islam, M.A., 2015. Corrosion behaviours of high strength TMT steel bars for reinforcing cement concrete structures. Procedia Engineering, 125, pp.623-630.

Jayasree, S., Ganesan, N. and Abraham, R., 2016. Effect of ferrocement jacketing on the flexural behaviour of beams with corroded reinforcements. Construction and Building Materials, 121, pp.92-99.

Karar, N. and Singh, S.K., 2015. Understanding corrosion in steel reinforced concrete structures: A limited study using TOF-SIMS. Vacuum, 121, pp.5-8.

Kim, S., Kim, J.W. and Kim, J.H., 2017. Enhancement of corrosion resistance in carbon steels using nickel-phosphorous/titanium dioxide nanocomposite coatings under high-temperature flowing water. Journal of Alloys and Compounds, 698, pp.267-275.

Kumar, V., Singh, R. and Quraishi, M.A., 2013. A study on corrosion of reinforcement in concrete and effect of inhibitor on service life of RCC. Journal of Materials and Environmental Sciences, 5, pp.726-731.

Mao, Z., Farkhondeh, M., Pritzker, M., Fowler, M. and Chen, Z., 2016. Dynamics of a Blended Lithium-Ion Battery Electrode During Galvanostatic Intermittent Titration Technique. Electrochimica Acta, 222, pp.1741-1750.

Mascagni, D.B.T., Souza, M.E.P.D., Freire, C.M.D.A., Silva, S.L., Rangel, R.D.C.C., Cruz, N.C.D. and Rangel, E.C., 2014. Corrosion resistance of 2024 aluminum alloy coated with plasma deposited aC: H: Si: O films. Materials Research, 17(6), pp.1449-1465.

Reddy, M.V., Jose, R., Le Viet, A., Ozoemena, K.I., Chowdari, B.V.R. and Ramakrishna, S., 2014. Studies on the lithium ion diffusion coefficients of electrospun Nb 2 O 5 nanostructures using galvanostatic intermittent titration and electrochemical impedance spectroscopy. Electrochimica Acta, 128, pp.198-202.

Singh, D.V., Sachan, A.K. and Rawat, A., 2016. Developments in Corrosion Detection Techniques for Reinforced Concrete Structures. Indian Journal of Science and Technology, 9(30), pp.23-44.

Wang, J., Wang, D. and Hou, D., 2016. Hydroxyl carboxylate based non-phosphorus corrosion inhibition process for reclaimed water pipeline and downstream recirculating cooling water system. Journal of Environmental Sciences, 39, pp.13-21.

Yaldagard, M., Seghatoleslami, N. and Jahanshahi, M., 2014. Preparation of Pt-Co nanoparticles by galvanostatic pulse electrochemical codeposition on in situ electrochemical reduced graphene nanoplates based carbon paper electrode for oxygen reduction reaction in proton exchange membrane fuel cell. Applied Surface Science, 315, pp.222-234.

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