Corrosion Resistance of Steel Reinforcement in Fly Ash Concrete under Chloride Penetration

Idrees ZAFAR Candidate for the Degree of Philosophy in Engineering Supervisor: Prof. Takafumi Sugiyama Division: Field Engineering for the Environment

Introduction requires the durability aspect of the concrete to be verified. In this regard the current research was done to investigate Japan is generating more than 10 million tons of coal ash the performance of fly ash concrete against the corrosion as a by-product from coal fired thermal power plants due to chloride attack during the initiation as well as every year [1]. The quantity of coal ash produced is propagation period. estimated to increase due to the construction of new coal fired power plants to meet the power generation demands. Experimental Methodology Although, fly ash is already being used in many sectors like cement production, road construction, foundation Materials and Specimen Preparation improvements, back-filling and as a fertilizer for agriculture, forestry and fishery sector [2]. In addition to Two sets of fly ash concrete were prepared each for the that fly ash is also used in the construction sector as an initiation and propagation period. The water to cement artificial light-weight aggregate. However, to cope with ratio was kept constant as 0.5 for all the specimens. The the anticipated increase in the generation of coal ash in the detailed mix proportions for all the specimens are shown future, the effective utilization of fly ash needs to be in Table 2. The coarse aggregate used was crushed stone increased for each for the above mentioned sectors. with a maximum size of 13 mm. The absorption and Among several methodologies that are being developed specific gravity of coarse aggregate were 1.62% and 2.64 for effective use of fly ash, the use as an admixture for g/cm³ respectively. The target slump and target air content concrete is considered the most effective one from the was 12 ±1.5 cm and 5 ± 0.5 % respectively. The detailed view point of consuming a large amount of fly ash and of methodology of the specimen preparation and specimen reducing the environmental impact by lowering the configuration are already discussed elsewhere [4]. Three amount of CO2 emission. different series i.e. F30P, F30PL and F30ZL were prepared JIS A 6201 is the standard which details the use of fly ash for both the initiation period and propagation period. F30P for concrete in Japan. According to the latest modification and F30PL for initiation period while F30ZL for in JIS A 6201 (1999), fly ash is categorized into four types propagation period. For the comparison purpose, N2P, named as Type I, Type II, Type III and Type IV [3]. Among F30D and F30DL for initiation while N2Z and F30Z for these Type II is recommended for the use in concrete as an propagation will be used from the already published admixture. researches by the author [4 & 5]. The increase in the use of fly ash in concrete is important Each fly ash concrete series consisted of five specimens. from the view point of the reduction in environmental The embedded lead-lead oxide electrode (Pb/PbO₂) was impact but for the practical implementation in the field it

Table 1. Mix Proportions of Concrete

Unit weight (kg/m3)

Period Type FA/(C+FA)* s/a** Compressive Strength W C FA S G 28 days 91 days (%) (%) Initiation F30P 30 45 145 203 87 852 1037 29.5 39.7 Propagation F30Z 30 43 170 238 102 759 995 27.3 35.6

*: Fly ash replacement ratio, ** :Sand to aggregate ratio placed in the F30P and F30PL specimens for the detection chloride ion concentration in each sliced layer of the of corrosion initiation of rebars in the concrete specimens. concrete [7]. A nonlinear regression analysis was done to Plains rebars were used for specimens of initiation period fit the experimental chloride profile with the solution of while deformed rebars were used for propagation period. the Fick’s 2nd law. The surface chloride ion concentration The increased water content and deformed rebars were (Co) and diffusion coefficient (Do) were obtained from used for the specimens of propagation period to achieve the fitting curve. The obtained values of Co and Do were the specified degree of corrosion within a certain time then used to evaluate the chloride values at the estimated period. The curing period for F30P was 28 days while corrosion level. F30PL and F30ZL were cured for 91 days. The cover depth for all the series was kept same as 20mm and 25 Surface Corroded Area mm for UP and DO rebar except F30P series, which had an increased cover depth by 10mm. After the completion After certain period of chloride application, when the of specified curing period, a transparent canister, specimens of propagation period had reached the specified dimensioning 50 × 100 mm in cross section, was set on degree of corrosion, the specimen were cut to visually the middle of specimen’s top surface and 10% NaCl observe the nature of corrosion products and quantify the solution was poured in that canister to start the chloride corroded surface area of rebars. Corroded surface area exposure to specimens. was measured followed by the cutting and splitting of ASTM C 876 was used for as a criteria for the detection of corrosion test specimens. An image was produced by corrosion initiation while for propagation period the sketching the outline of the corroded area and later this specified values of polarization resistance corresponding image was scanned. Computer image analysis software to certain levels of corrosion current density (RILEM TC (Image J) was used to quantify the corrosion area. 154 2004) were used to indicate a specific corrosion degree [6]. The application of the salt solution was stopped when the specimens had reached the polarization TEST RESULTS AND DISCUSSION resistance within 52 kohm-cm to 26 kohm-cm. 52 kohm-cm (correspond to 0.5 µA/cm²) and 26 kohm-cm (correspond to 1.0 µA/cm²) represent the moderate and Electrochemical Data high degree of corrosion respectively. The half-cell potential was used to detect the onset of corrosion of rebars in fly ash concrete specimens. The Electrochemical Measurements measured values of half-cell potential with reference to

embedded electrode (mV vs Pb/PbO ) and corrosion For the specimens of initiation period the half-cell 2 meter (Ag/AgCl) was standardized to values with potential was continuously monitored by using automatic reference to Copper-Copper Sulphate (mV vs CSE) data logger system [4]. The data logger measured half-cell electrode. The figure 1 and figure 2 show the trend of potential at regular interval of 8 hours with reference to half-cell potential (mV vs CSE) values measured for Pb/PbO₂ electrode embedded in concrete specimens. An F30PL and F30P series, with reference to the embedded AC impedance spectroscope (hereafter called Corrosion electrode and corrosion meter respectively. It was meter) was also used to measure the half-cell potential and observed that after a certain time period of salt impedance data at an interval of about one month. While application, the values of half-cell potential measured by for the specimens of propagation period, the corrosion the embedded electrode drop gradually towards more monitoring was done by using corrosion meter. Corrosion negative value and remain constant within a range of -815 meter was used to measure polarization resistance and ± 15 (mV vs CSE). On the contrary the values of half-cell concrete resistance along with half-cell potential values potential measured by corrosion meter, as shown in figure after every two- week interval. Measurements were made 2, lie well within in the passive zone, indicating no at a location where the saline solution was applied on the corrosion activity at that particular time period of drop top surface of the specimen. shown by embedded electrode. These gradual drops

shown by the embedded electrodes are not the actual Chloride Analysis indication of the corrosion initiation. So, for the current research, the specimens which showed this gradual drop Chloride analysis was conducted after the detection of trend, the initiation of corrosion was estimated by the half- corrosion initiation for specimens of initiation period cell potential values measured by corrosion meter. The while for the specimens of propagation period it was temporal gradual increase of half-cell potential drops carried out after a specified criteria for corrosion level was towards more negativity is mainly because of reduced achieved. The entire chloride application zone (50 mm × oxygen supply due to high degree of saturation around the 100 mm) of concrete specimens was used for chloride embedded electrode region. This behavior of exhibiting analysis. The application zone was sliced in more negative values of half-cell potential is common in approximately 7 mm thick layers. Japanese Industrial the submerged concrete structures because of the limited Standard (JIS A1154) was used to determine the total oxygen supply under the water [4].

(a) (a)

(b) (b)

Figure 1 Trend of half-cell potential for (a) F30PL and (b) Figure 2 Trend of half-cell potential for (a) F30PL and (b) F30P series measured by the embedded electrode F30P series measured by corrosion meter

Figure 3 Trend of polarization resistance for F30PL series Figure 4 Trend of specific concrete resistance for F30PL series

It can be seen from figure 1 and 2 that only 1 specimen in the polarization resistance. Although there is a slight from F30PL series had shown corrosion initiation while decrease in the polarization resistance values over the time rest 4 specimens have not shown corrosion initiation even but the values even after 5-6 years of chloride exposure after the chloride exposure for 6.5 years. On the other are around 1000 kohm-cm. This value of polarization hand 3 specimens for F30P have shown corrosion resistance correspond to corrosion current of 0.02 µA/cm², initiation and rest 2 are still under monitoring. As clearly indicating that specimens are still in the passive mentioned before, N50P and F30DL will be used as a state. reference from already published research by the author The results of specific resistance of concrete can also for the comparison purpose. It was observed that the fly explain this significant increase in the corrosion resistance ash concrete with long curing and plain rebars had shown of fly ash concrete. Figure 4 clearly shows the increase in an increased corrosion resistance as compared to the the specific concrete resistance with time for the fly ash normal concrete specimens with plain rebars. The main concrete specimens. This increase is because of the cause of the enhanced corrosion resistance is the dense pozzolanic reaction which continues even after one year micro structure for fly ash concrete because of the and the resulting products from pozzolanic reaction pozzolanic reactions especially because of the long curing. occupy the pore spaces [8]. This results in more dense The dense micro structure restricts the penetration of micro structure of concrete by reducing the pore size and chloride ions into the concrete and to reach the level of total porosity which can cause a decrease in the oxygen rebar to initiate corrosion. In addition to that F30PL has supply and chloride ion penetration. also showed a longer corrosion initiation time as compared to F30DL series. Although F30DL and F30PL Chloride Threshold Level (CTL) have the same fly ash replacement ratio (30% replacement with cement) and curing period, but the types of rebars In the present study, chloride threshold level is defined as being used are different which resulted in this significant the minimum amount of total chloride ions at the depth of increase. The main cause of increased durability of F30PL reinforcement required to initiate corrosion. The average series compared to F30DL series is the denser steel- values of CTL for N50P, N50D, F30D and F30DL are concrete interface. The use of plain bars has resulted in found to be 5.0, 2.3, 1.2 and 1.9 kg/m³ respectively. These denser and defect free interfacial zone as compared to the average values for normal concrete with deformed bars deformed rebars. are well in line with the previously reported values while It was also noted that, F30P series, fly ash concrete with the mean values for fly ash concrete vary from the increased cover depth by 10 mm also delayed the previous research [9]. This is partially due to the different initiation period as compared to the N50P and F30DL. compositions of fly ash and exposure conditions. However However when F30P is equated to F30PL, there appears the trend of decrease in CTL values against the increase in to be more chances for corrosion to initiate for a higher the replacement ratio of fly ash remained the same. The cover depth with small curing period. This means that for effect of the curing period on CTL values for fly ash fly ash concrete the long curing provides better protection concrete is quite clear from above mentioned values. It against corrosion as compared to the increase in the cover was observed that the average CTL values for fly ash depth by 10 mm. However high cover depths can also concrete increased with an increase in curing period. This provide sufficient internal curing for better resistance increase in CTL values for prolonged curing may be due against chloride penetration. to the dense microstructure, increased binding effects of chlorides and new constituents acting as inhibitors Polarization Resistance and Specific Concrete generated by the reaction. Resistance Propagation Period Polarization resistance is the interface resistance between electrode (rebars) and electrolyte (concrete) while specific As described earlier in the methodology, certain resistance of the concrete in the resistance offered by the polarization resistance limits were specified to simulate a concrete to the flow of current from anode to cathode. specific degree of corrosion. The figure 5 shows the trend Both of these parameters can be used to monitor the state of polarization resistance with time for all the specimens of concrete with regard to corrosion initiation and for F30ZL series. It was observed that the mean time propagation. As the corrosion initiates, there is a sharp duration to reach the specified criteria for F30ZL series drop down in the polarization resistance indicating the was 298 days. The fly ash concrete withthe same fly ash initiation of current flow from anode to cathode. The replacement ratio but a shorter curing period (F30Z, from polarization resistance and specific concrete resistance for reference), has resulted in the reduction of time required F30P and F30PL are shown in figure 3 and figure 4 to reach the specified corrosion criteria. The main reason respectively. It can be seen from figure 3 that polarization for the F30ZL series to show the longest duration of time resistance of the specimens for F30P and F30PL which to initiate as well as reach the criteria of polarization have not corroded yet are on the higher side while the resistance was mainly because in fly ash concrete with ones which have shown corrosion indicate the sharp drop longer curing period the mobility of chloride ions is

Figure 5 Trend of polarization resistance for F30ZL series Figure 6 Relation between surface corroded area and during propagation period polarization resistance

UP

DO

Figure 7 Corrosion state of rebars after reaching the specified degree of corrosion for F30ZL (2) specimen hindered as hydration of the cement matrix along with research should be done to suggest different B values for other pozzolanic reactions proceed, which slow down the different materials like fly ash. anodic process [10]. In addition to that the rate of corrosion depends on the concrete resistance during the propagation Surface Corroded Area period. If the concrete resistance is high it will be difficult for the electrons to pass through the concrete to reach the Figure 6 shows the measured surface corroded area values cathode. The long curing for fly ash concrete with against the polarization resistance values for each deformed rebars, F30DL, had shown an increased specimen at the time of cutting. It was observed that concrete resistance [4]. Although the water content for although the F30ZL had shown the longest time duration F30DL and F30ZL is not the same, but for the comparison to reach the specified degree of corrosion but the surface purposes among F30Z and F30ZL, it can be referred. In corroded area of the corroded specimens were also highest this regard the increased concrete resistance helps to for F30ZL series. In addition to that it was also observed explain the long duration taken by the fly ash concrete that the surface corroded area remained almost constant even with deformed rebars during the propagation period. for all the specimens of F30ZL series. Furthermore it was In addition to that it should also be noted that, according also noted that few specimens of F30ZL series had shown to the present recommendations, the constant value for the corrosion cracks, unlike the other series, when they Stern and Geary ‘B=26mV’ is used in this research. The reached the specified corrosion degree. This behavior is author is of the opinion that the use the same B value for attributed to the low porosity and good interfacial bond normal and fly ash concrete is not justified. Further between concrete and reinforcement of fly ash concrete. Because of the high quality concrete it takes more time for period to reach the specific degree of corrosion. In the chloride ions to reach the reinforcement but it takes addition to that the fly ash concrete with long curing less corrosion volume to produce the corrosion cracks. In had also shown the highest surface corroded area. case of F30ZL series, the void spacing is quite less and there is less space for the corrosion products to accommodate in the pores, so the expanding products REFERENCES exerts the pressure and generate the cracks. While in case of other specimen, the diffusivity of concrete is high 1. Sugiyama, T, 2013. Durability of fly ash concrete in which allows the corrosion products to adjust in the pores. salt laden environment. In Professor Kenji Kawai So a high corrosion volume will be required to generate symposium., Properties of concrete, Proceedings of the corrosion cracks in normal concrete specimens. It should be kept in mind that although the long curing for 3rd International Conference SCMT3, Kyoto, Japan fly ash concrete has increased the time to reach the 18-21 August 2013. Tokyo: JCI. specified degree of corrosion but have also increased the 2. Japan Coal Energy Center (JCOAL) website. surface corroded area. http://www.jcoal.or.jp/eng/ Figure 7 shows the rebars for F30ZL (2) specimen, which 3. JIS A 6201, 1999. Japanese Industrial Standard. Use were taken out after cutting and splitting of concrete of fly ash in concrete. (In Japanese). specimen when it reached the specified degree of 4. Zafar I and Sugiyama T, 2014. Laboratory corrosion. It was noted that in almost all the specimens investigation to study the corrosion initiation of both rebars (UP and DO) were corroded but as shown in rebars in fly ash concrete. Magazine of Concrete figure 7 also, the degree of corrosion for UP rebars was Research, 66(20): 1051-1064. more than DO rebars. This difference in the degree of corrosion is because of the different cover depths. The 5. Zafar I and Sugiyama T, Study on the corrosion of chloride ions will reach the UP rebar first which is having rebars in fly ash concrete during moderate to high the small cover depth as compared to DO rebars. The high corrosion rate. Proceeding of 4th international concentration of all the ingredients required for corrosion symposium on life-cycle civil engineering, IALCCE at the level of UP rebar will enhance the corrosion 2014, 16-19 Nov, Waseda University, Tokyo Japan, propagation as compared to DO rebar. But it should also pp: 1538-1543. be noted that the difference between the cover depths is 6. RILEM TC 154-EMC. Andrade C and Alonso C, only 5 mm, so the DO rebar cannot stay passive for very 2004. Electrochemical Technique for Measuring long period. After certain period of chloride exposure it Metallic Corrosion.Test method for on-site corrosion will also corrode but at a lesser rate as compared to UP rate measurement of steel reinforcement in concrete rebars. It is also an indicator of the fact that experimental setup is working well for the desired goal i.e. to achieve by means of the polarization resistance method. the corrosion first in the UP rebars. Materials and Structures 37: 623-643. 7. JIS A 1154, 2003. Japanese Industrial Standards. Methods of test for chloride ion content in hardened CONCLUSIONS concrete, Japan: 472-482. (In Japanese). 8. Poulsen SL, 2009. Methodologies for measuring the Based on the extensive experimental analysis of this degree of reaction in Portland cement blends with research, the following conclusions were drawn: supplementary cementitious materials by 29 Si and 27 al MAS NMR spectroscopy. PhD Thesis, Aarhus 1. The corrosion resistance was exceptionally improved University, pp. 1-147. for fly ash concrete with 30% cement replacement 9. Thomas M, 1996. Chloride threshold in marine and combination of long curing and plain rebars. The concrete. Cement and Concrete Research 26(4): 513- increased corrosion resistance for fly ash concrete is 519. mainly attributed to the pozzolanic reaction and high 10. Page CL and Havdahl J, 1985. Electrochemical concrete resistance as compared to normal concrete. monitoring of corrosion of steel in micro silica

cement pastes. Materials and Structures 18(1): 41-47. 2. The shorter curing period for fly ash concrete led to

the reduction in the corrosion initiation time along with the chloride threshold values while the longer curing for fly ash concrete has resulted in better performance against corrosion than just increasing cover depth.

3. During the propagation period, the fly ash concrete with a long curing had depicted the extended time