Half Cell Potential Studies for Durability Studies of Concrete Structures in Coastal Environment V L Satish 1 , Engineer SF- ISRO, SDSC SHAR, Sriharikota & Research Scholar JNTU College of Engg Kakinada, [email protected] Abstract: The aspects of concrete durability and performance have become a major subject of discussion especially when the concrete is subjected to severe environment. Due to the saline and aggressive atmospheric conditions at SHAR (Sriharikota, AP,India, the only Space port of India), the durability of structures has become a matter of concern. Reinforced concrete is one of the most common materials used in most of the structures here. The Non Destructive Test (NDT) studies viz, Rebound hammer test, UPV test, Core test, Half cell survey, Chlorides, sulpahtes, Carbonation test etc were conducted on some old buildings at Sriharikota island. Half – cell potential (HCP) survey using Copper-Copper sulphate half-cell was conducted on RCC members, where corrosion symptoms were observed. The HCP values observed indicated Probability of different corrosion stages in reinforcement present in the structure. Studies revealed that Chloride induced reinforcement corrosion is the predominant reason for the early deterioration of RC structures here. This paper attempts to present the results of NDT studies conducted on old buildings at SHAR island with emphasis on the Half-cell Potential Test studies. Keywords: NDT, Coastal environment, Half-cell potential Test, Chloride content, Corrosion. 1. Introduction Durability of cement concrete is defined as its ability to resist weathering action, chemical attack, abrasion or any other process of deterioration. Durable concrete will retain its original form, quality and serviceability when exposed to environment.” [1]. Deterioration of reinforced concrete structures results from external factors or causes within the concrete itself. Deterioration rate of structures depends on the exposure conditions and extent of maintenance. External chemical attack occurs mainly through the action of aggressive ions, such as chlorides, sulphates or carbon dioxide, as well as many natural or industrial liquids and gases. Corrosion of steel bars is a main factor affecting both the concrete durability and strength. Usually, there are two major factors which cause corrosion of rebars in concrete structures, carbonation and ingress of chloride ions. When chloride ions penetrate in concrete more than the threshold value or when carbonation depth exceeds concrete cover, then it initiates the corrosion of RC structures. Concrete normally provides a high degree of protection to the reinforcing steel against corrosion, by virtue of the high alkalinity (pH 13.5) of the pore solution. Under high alkalinity steel remains passivized. In addition, well consolidated and purely cured concrete with low water cement ratio has a low permeability, which minimizes the penetration of corrosion inducing agents such as chlorides, CO2, moisture etc. to steel surfaces. After the initiation of corrosion in steel rebars, products of corrosion tend to expand and occupy a volume of about 6– 10 times greater than that of steel resulting in the formation of cracks and finally spalling of concrete surfaces occurs as shown in Figures 1 and 2. NDE2020, 013, v1: ’Half Cell Potential Studies for Durability Studies of Concrete Structu . . . 1 More info about this article: http://www.ndt.net/?id=26122
Half Cell Potential Studies for Durability Studies of Concrete Structures in Coastal Environment
Apr 07, 2023
Welcome message from author
This document is posted to help you gain knowledge. Please leave a comment to let me know what you think about it! Share it to your friends and learn new things together.
Transcript
UntitledHalf Cell Potential Studies for Durability Studies of Concrete Structures in Coastal Environment
V L Satish1,
Engineer SF- ISRO, SDSC SHAR, Sriharikota & Research Scholar JNTU College of Engg Kakinada, [email protected]
Abstract: The aspects of concrete durability and performance have become a major subject of discussion especially when the concrete is subjected to severe environment. Due to the saline and aggressive atmospheric conditions at SHAR (Sriharikota, AP,India, the only Space port of India), the durability of structures has become a matter of concern. Reinforced concrete is one of the most common materials used in most of the structures here. The Non Destructive Test (NDT) studies viz, Rebound hammer test, UPV test, Core test, Half cell survey, Chlorides, sulpahtes, Carbonation test etc were conducted on some old buildings at Sriharikota island. Half – cell potential (HCP) survey using Copper-Copper sulphate half-cell was conducted on RCC members, where corrosion symptoms were observed. The HCP values observed indicated Probability of different corrosion stages in reinforcement present in the structure. Studies revealed that Chloride induced reinforcement corrosion is the predominant reason for the early deterioration of RC structures here.
This paper attempts to present the results of NDT studies conducted on old buildings at SHAR island with emphasis on the Half-cell Potential Test studies.
Keywords: NDT, Coastal environment, Half-cell potential Test, Chloride content, Corrosion.
1. Introduction Durability of cement concrete is defined as its ability to resist weathering action, chemical attack, abrasion or any other process of deterioration. Durable concrete will retain its original form, quality and serviceability when exposed to environment.” [1]. Deterioration of reinforced concrete structures results from external factors or causes within the concrete itself. Deterioration rate of structures depends on the exposure conditions and extent of maintenance. External chemical attack occurs mainly through the action of aggressive ions, such as chlorides, sulphates or carbon dioxide, as well as many natural or industrial liquids and gases. Corrosion of steel bars is a main factor affecting both the concrete durability and strength. Usually, there are two major factors which cause corrosion of rebars in concrete structures, carbonation and ingress of chloride ions. When chloride ions penetrate in concrete more than the threshold value or when carbonation depth exceeds concrete cover, then it initiates the corrosion of RC structures. Concrete normally provides a high degree of protection to the reinforcing steel against corrosion, by virtue of the high alkalinity (pH 13.5) of the pore solution. Under high alkalinity steel remains passivized. In addition, well consolidated and purely cured concrete with low water cement ratio has a low permeability, which minimizes the penetration of corrosion inducing agents such as chlorides, CO2, moisture etc. to steel surfaces. After the initiation of corrosion in steel rebars, products of corrosion tend to expand and occupy a volume of about 6– 10 times greater than that of steel resulting in the formation of cracks and finally spalling of concrete surfaces occurs as shown in Figures 1 and 2.
NDE2020, 013, v1: ’Half Cell Potential Studies for Durability Studies of Concrete Structu . . . 1
M or
e in
fo a
bo ut
th is
a rt
ic le
2
Fig 1: Cracks in beams indicate corrosion of reinforcement Fig 2 : Spalling of cover concrete & severe corrosion in
beams In a study by Elsener et al., 2003 [2], the evaluation parameters that are required to ensure a sound decision on repair scheme/ developing a maintenance strategy are as follows:
Reasons for damage or loss of protection – degree and amount of damage – progress of damage with time – effect of damage on structural behaviour and serviceability.
Obtaining reliable information on stage and rate of deterioration is done during regular inspections relying on visual examination of the whole structure. If no reliable conclusion on the state of the structure can be obtained, an additional inspection (verification) including non-destructive testing is warranted. Due to the saline and aggressive atmospheric conditions at SHAR (Sriharikota), the durability of structures has become a major concern. As per the report of CECRI Karaikudi, the region of Sriharikota is one of the extremely corrosive places in India. Sriharikota is the most corrosive area with corrosion rate of mild steel of 1.6 mm per year [3]. In the present study, to assess the condition of buildings using the Visual observations and the Non-Destructive (ND) tests, such as Profometer survey, Rebound Hammer test and Ultrasonic pulse velocity test were carried out. Also, the other Partially Destructive tests like Core sampling, Evaluation of Electrochemical parameters of concrete (pH, S, Cl-), Half-cell potential survey etc. were carried out. The details of the Half-cell potential studies, analysis and conclusions are presented in the subsequent sections. 2 Coastal Concrete Structures and Chloride effect Structures in coastal areas are prone to corrosion related problems. Corrosion destructs the parent material by reaction with its environment and it constitutes one of the basic factors of degradation of reinforced concrete structures. The presence of coastal environment, which is rich in chloride ions, is considered as one of the major external factors inducing damage on reinforced concrete structures. The fundamental problem corresponding to the chloride movement is the initial concentration of chlorides when spread in the concrete.
2 NDE2020, 013, v1: ’Half Cell Potential Studies for Durability Studies of Concrete Structu . . .
Fig. 3 Chloride concentration at different depths for bridges exposed to sea (Source: Mohammed, 2018) The general plot of the curve of the changing chloride concentration with depth in the concrete is explained in Figure 3. From this figure, we note that the concentration of chlorides is reduced rapidly whenever propagation occurs inside the concrete. It can also be seen that the chloride concentration with depth varies as a simple parabolic curve.
Fig. 4 Structure Corrosion Risk based on the distance from coast (Source: Mohammed, 2018)
Chloride can be in the form of a spray in the air when the coastal environment surrounds the structure. For example we can consider a structure or a building under construction near the shore in a coastal area wherein the chloride effect can be seen clearly. Figure 4 shows the intensity of risk posed to a structure according to its location in terms of its distance from the coast and also in terms of height. However, these data shall not be taken as it is, but only as a guide to determine the degree of risk, depending on the climatic conditions of the region [4].
3 Half – Cell Electrical Potential Method to measure Corrosion of Reinforcement in Concrete (ASTM C 876- 91)
Since 1978 Half Cell Potential mapping [5] is in vogue for detecting corroding areas on concrete structures in Denmark. The Half Cell Potential Testing method is suggested for diagnosing the probability of reinforcement corrosion in turn which is used for assessment of the durability of reinforced concrete [5,6]. Corrosion inspection of steel can be conducted by many different techniques. Non-destructive technique such as half-cell potential measurement (HCP) is a well known technique for investigation of corrosion in steel [7]. Even though there are many methods for evaluating
NDE2020, 013, v1: ’Half Cell Potential Studies for Durability Studies of Concrete Structu . . . 3
the corrosion, half-cell potential has been considered useful by many researchers as the main method to detect the corrosion activity in RC structures [8]. 3.1 Description of method: Corrosion of steel can be assessed by Half-Cell Potential (HCP) measurement by an electrochemical process as per ASTM C87. In this method, the electrical potential difference between the upper steel rebars and a standard portable reference electrode in contact with the concrete surface is measured. The half-cell is made by a Copper/ Copper Sulphate or Silver/ Silver Chloride cell but other combinations are also used. The concrete acts as an electrolyte and the risk of corrosion of steel reinforcement in immediate region of the test location may be related empirically to the measured potential difference. The schematics of the test and equipment is shown Fig. 5. It consists of a rigid tube composed of dielectric material that is non-reactive with copper or copper sulphate, a porous wooden or plastic plug that remains wet by capillary action, and a copper rod that is immersed within the tube in a saturated solution of copper sulphate. The schematics of the electrode is shown in figure 6. The reference electrode is connected to the positive end of the high impedance voltmeter and the steel reinforcement to the negative. At the anode, the ferrous ions (Fe++) are dissolved and electrons are set free. These electrons drift the steel to the cathode, where they form hydroxide (OH-). This principle creates a potential difference that can be measured by the half-cell method.
Fig 5 Schematics showing the Half Cell Potential Measurement Technique (www.proceq.be)
4 NDE2020, 013, v1: ’Half Cell Potential Studies for Durability Studies of Concrete Structu . . .
Fig.6. Schematic of Copper Sulphate electrode
The reliability of half-cell potential measurement as an indication of corrosion potential has evolved by the good results during the bridge deck corrosion surveys. An indication of the relative probability of corrosion activity was empirically obtained through measurements during the 1970s [9]. This work formed the basis of the ASTM standard C876, which provides general guidelines for evaluating corrosion in concrete structures as outlined in Table 1:
Table 1. Probability of Corrosion according to half-cell potential values
Half Cell Potential Value ( mV) Corrosion activity
More negative than -350 mV >95%
-200 mV to -350 mV 50%
More Positive than -200 mV <5%
Measured half-cell potential values can be plotted to obtain the potential contour for the surface of reinforced concrete structure and this potential contour map as shown in Figure 7 can be used to evaluate the probability of corrosion at different areas on the surface of the concrete structures. Portions of the structures where high corrosion activity is prevalent can be obtained and identified by their high negative potentials.
NDE2020, 013, v1: ’Half Cell Potential Studies for Durability Studies of Concrete Structu . . . 5
Fig. 7. Half Cell Potential Contour
3.2 Equipment Used
Following are the equipment used for corrosion monitoring of reinforcement in concrete (Fig. 8& 9). The measurement of HCP values carried out at site is shown in figure 10.
Fig. 8. Basic Half cell survey equipment Fig. 9. Reference electrode(CuSo4)
Fig. 10. HCP measurement at site
6 NDE2020, 013, v1: ’Half Cell Potential Studies for Durability Studies of Concrete Structu . . .
3.3 Precautions for Taking Half-Cell Potential Measurements A reliable half-cell potential survey for assessing the corrosion damage of a reinforced concrete structure requires good planning and preparation, careful measurement and data correlation. The method specified by ASTM C876 is a satisfactory measurement procedure to follow. The following data should be taken before starting the HCP measurement: • Basic information about the structure such as structural drawings, mix design details, concrete admixture details,
cover meter survey details etc., • The history of previous repair and rehabilitation including patch repair • Earlier conducted condition survey details, visual examination data and delamination survey results, and chloride
content analysis. The half-cell potential data should always be validated by other testing methods before an interpretation of corrosion probability is made. Many methods are commonly used along with the half-cell potential technique in field assessment of corrosion like visual inspection, delamination survey, chloride content measurement, concrete resistance measurement, concrete cover-depth survey, carbonation profile determination etc., 4 Present Study Under this study some buildings which are having an age of approximately 40 years were selected to assess the structural condition of the buildings. The buildings are located at Sriharikota Island, India. It is prone to severe weather phenomena like tropical cyclones, thunderstorms, and squally weather. SHAR being a coastal area, it experiences sea breeze and land breeze also. The half cell potential studies conducted on these buildings are presented and analysed in this paper. 4.1 Case study 1 (Colony PHC). Under this study some residential buildings which were built approximately 40 years back were selected to check the corrosion potential. The buildings are located at Sriharikota Island. These buildings are built during the year approximately during 1975-76. The building units are Ground + 2 units with Load bearing structure with RCC roof slab and other RCC elements like staircase, chajjas etc. The Grade of concrete adopted during construction is M15. Rebound hammer Test and UPV Values. Rebound hammer test values (RHV) on the surface at grid points and on the surface (at Random locations) in the slab portions, wherever taken are uniform. However higher compressive strength in some RCC members indicated carbonated cover concrete. The average compressive strength is about 25 N/Sqmm. It is observed from the Ultrasonic pulse velocity test results that the Slabs, columns & beams tested, have concrete integrity range from doubtful to good. Some of the UPV values indicate that the concrete possess voids (honey combed in the interior portions). Core Test. The mean compressive strength of cubes obtained is about 16 N/ Sqmm which is satisfactory with respect to the design strength of 15 N/ sqmm. Carbonation test & Chloride Tests. The in-situ carbonation tests on Slabs, Beam, and column indicate that reinforcements are corroded due to carbonated layer reaching up to concrete cover thickness after the carbonation test. In the quarters, out of 14 tests, about 9 samples have shown signs of carbonation. The Chloride content as Cl- in the reference quarter is about 0.8 to 1%. The Chloride content as Cl- in the WTP is about 0.6 to 5%. Results of Half –Cell Potential (HCP) Survey (Colony PHC). Half – cell potential survey using Copper-Copper sulphate half-cell was conducted on RCC members, where corrosion symptoms were observed. The HCP values observed show 50% probability of corrosion in some of the quarters tested.
NDE2020, 013, v1: ’Half Cell Potential Studies for Durability Studies of Concrete Structu . . . 7
A total of 18 data sets were collected for reference quarter building. The measurements were taken at random spots over the different structural elements. Refer to Table 2 data showing the minimum and maximum values obtained at the various locations.
Table 2: Maximum and minimum values of the HCP readings (Qrtr Building).
Ref Quarter Bldg
Max Min
Room 1 -177 -290 6
Room 2 -126 -169 6
Room 3 -7 -74 6
Table 3: Maximum and minimum values of the HCP readings (Treatment Plant Building).
WTP Bldg HCP Value ( mV) Data sets
collecetd Max Min
Clarifier -24 -45 6
Flocculator -282 -313 6
Entry area -282 -313 6
It can be seen that the readings ranged from -7 mV to -290 mV. All the data obtained has also been plotted in a histogram shown in Fig. 11. The mean half-cell potential reading was -135 mV for quarter building and -195 for the WTP building. The HCP values observed in SHAR CAMPUS the readings showed more positive. The HCP values of the WTP building indicates Probability of 50% corrosion activity present in the reinforcement.
Fig 11: Histogram of the half-cell potential measurements data (PHC)
4.2 CASE STUDY 2 (Colony DOS- A type qrtr buildings): These are residential buildings which were built approximately 40 years back were selected to check the corrosion potential. The buildings are located at Sullurupeta town which is near by Sriharikota Island. The building units are Ground + 2 units with Load bearing structure with RCC roof slab and other RCC elements like staircase, chajjas etc. The Grade of concrete adopted during construction is M15 to M20.
0
2
4
6
8
10
12
14
Co un
HCP Values (mV)
8 NDE2020, 013, v1: ’Half Cell Potential Studies for Durability Studies of Concrete Structu . . .
Rebound hammer Test and UPV Values. Rebound hammer test values indicated higher compressive strength in some RCC members. The average compressive strength is about 27 N/Sqmm. It is observed from the Ultrasonic pulse velocity test results ranged about 2 km/s indicating doubtful integrity of concrete. Results of Chloride content Tests DOS).The Chloride content as Cl- are very high in some quarters 0.4 to 1% even after hydration. The chloride are normally contributed by added water in the concrete mix. Results of Half –Cell Potential (HCP) Survey (Colony DOS). A total of 90 data sets were collected for reference quarter building (DOS- A type qrtr buildings). The measurements were taken at random spots over the different structural elements. Refer to Table 4 data showing the minimum and maximum values obtained at the various locations. It can be seen that the readings ranged from -130 mV to -408 mV. All the data obtained has also been plotted in a histogram shown in Fig. 12. The mean half-cell potential reading was -294 mV for these quarter buildings. The HCP values indicates Probability of 50% to 95% corrosion activity present in the reinforcement.
Table 4: Maximum and minimum values of the HCP readings (A type Qrtrs- DOS).
Ref Quarter Bldg
Max Min
A26 to A30 -264 -385 18
A4 to A26 -190 -408 42 A26 to A30 staircases -130 -357 12
A2 to A16 -237 -349 18
Fig. 12. Histogram of the half-cell potential measurements data (DOS)
4.3 CASE STUDY 3 (Storage Bldgs I & II) Under this study two industrial buildings which are used for Storage & integration activities in the initial days (year 1980). The building consists of two independent shed structures namely -I & II. The shed structures consist of R.C.C. column – beam framework and are covered with roofing sheets supported on structural steel trusses and purlins. The space available in between two sheds is covered with R.C.C. slab and merged with two sheds. The Grade of concrete adopted during construction is M 20 to M 25.
0
10
20
30
40
50
Co un
Frequency
NDE2020, 013, v1: ’Half Cell Potential Studies for Durability Studies of Concrete Structu . . . 9
Results of Half –Cell Potential (HCP) Survey (Storage Bldgs I & II). Half-cell Potentiometer test is performed at 103 numbers of test locations on columns and east side RCC wall cladding of buildings-I & II. Refer to Table 5 data showing the minimum and maximum values obtained at the various locations. It can be seen that the readings ranged from -11 mV to -198 mV. All the data obtained has also been plotted in a histogram shown in Fig. 13. The mean half- cell potential reading was -90 mV for these quarter buildings. The HCP values indicates 10 % probability (mild risk) of active corrosion.
Fig 13: Histogram of the half-cell potential measurements data (Storage Bldgs I & II)
Results of Chloride content Tests (Storage Bldgs I & II). Chlorides: As per IS: 456:2000, Clause 8.2.5.2, Table 7, limit of chloride content of concrete (maximum total acid soluble chloride content) is 0.025%. The chemical analysis of 13 nos. of concrete powder samples out of 20 samples of VIB-I & II indicates chloride content of 0.1%. 5 Discussions & Conclusions:
The corrosion rate is the main factor to be taken into consideration in framing up the maintenance strategy. The Half Cell Potential Testing method is a technique, used for assessment of the durability of reinforced
concrete and helps in diagnosing reinforcement corrosion. A more negative reading of potential is generally considered to indicate a higher probability of corrosion.
In both the housing units (case study 1 & case study 2) the half-cell potential values indicate 50% probability for corrosion. This can be attributed to the inadequate cover to reinforcement, or lack of compaction of cover concrete resulting in porous micro structure which is visible in the site. Also, there are many leakage areas which might have contributed to the local corrosion spots. There is…
V L Satish1,
Engineer SF- ISRO, SDSC SHAR, Sriharikota & Research Scholar JNTU College of Engg Kakinada, [email protected]
Abstract: The aspects of concrete durability and performance have become a major subject of discussion especially when the concrete is subjected to severe environment. Due to the saline and aggressive atmospheric conditions at SHAR (Sriharikota, AP,India, the only Space port of India), the durability of structures has become a matter of concern. Reinforced concrete is one of the most common materials used in most of the structures here. The Non Destructive Test (NDT) studies viz, Rebound hammer test, UPV test, Core test, Half cell survey, Chlorides, sulpahtes, Carbonation test etc were conducted on some old buildings at Sriharikota island. Half – cell potential (HCP) survey using Copper-Copper sulphate half-cell was conducted on RCC members, where corrosion symptoms were observed. The HCP values observed indicated Probability of different corrosion stages in reinforcement present in the structure. Studies revealed that Chloride induced reinforcement corrosion is the predominant reason for the early deterioration of RC structures here.
This paper attempts to present the results of NDT studies conducted on old buildings at SHAR island with emphasis on the Half-cell Potential Test studies.
Keywords: NDT, Coastal environment, Half-cell potential Test, Chloride content, Corrosion.
1. Introduction Durability of cement concrete is defined as its ability to resist weathering action, chemical attack, abrasion or any other process of deterioration. Durable concrete will retain its original form, quality and serviceability when exposed to environment.” [1]. Deterioration of reinforced concrete structures results from external factors or causes within the concrete itself. Deterioration rate of structures depends on the exposure conditions and extent of maintenance. External chemical attack occurs mainly through the action of aggressive ions, such as chlorides, sulphates or carbon dioxide, as well as many natural or industrial liquids and gases. Corrosion of steel bars is a main factor affecting both the concrete durability and strength. Usually, there are two major factors which cause corrosion of rebars in concrete structures, carbonation and ingress of chloride ions. When chloride ions penetrate in concrete more than the threshold value or when carbonation depth exceeds concrete cover, then it initiates the corrosion of RC structures. Concrete normally provides a high degree of protection to the reinforcing steel against corrosion, by virtue of the high alkalinity (pH 13.5) of the pore solution. Under high alkalinity steel remains passivized. In addition, well consolidated and purely cured concrete with low water cement ratio has a low permeability, which minimizes the penetration of corrosion inducing agents such as chlorides, CO2, moisture etc. to steel surfaces. After the initiation of corrosion in steel rebars, products of corrosion tend to expand and occupy a volume of about 6– 10 times greater than that of steel resulting in the formation of cracks and finally spalling of concrete surfaces occurs as shown in Figures 1 and 2.
NDE2020, 013, v1: ’Half Cell Potential Studies for Durability Studies of Concrete Structu . . . 1
M or
e in
fo a
bo ut
th is
a rt
ic le
2
Fig 1: Cracks in beams indicate corrosion of reinforcement Fig 2 : Spalling of cover concrete & severe corrosion in
beams In a study by Elsener et al., 2003 [2], the evaluation parameters that are required to ensure a sound decision on repair scheme/ developing a maintenance strategy are as follows:
Reasons for damage or loss of protection – degree and amount of damage – progress of damage with time – effect of damage on structural behaviour and serviceability.
Obtaining reliable information on stage and rate of deterioration is done during regular inspections relying on visual examination of the whole structure. If no reliable conclusion on the state of the structure can be obtained, an additional inspection (verification) including non-destructive testing is warranted. Due to the saline and aggressive atmospheric conditions at SHAR (Sriharikota), the durability of structures has become a major concern. As per the report of CECRI Karaikudi, the region of Sriharikota is one of the extremely corrosive places in India. Sriharikota is the most corrosive area with corrosion rate of mild steel of 1.6 mm per year [3]. In the present study, to assess the condition of buildings using the Visual observations and the Non-Destructive (ND) tests, such as Profometer survey, Rebound Hammer test and Ultrasonic pulse velocity test were carried out. Also, the other Partially Destructive tests like Core sampling, Evaluation of Electrochemical parameters of concrete (pH, S, Cl-), Half-cell potential survey etc. were carried out. The details of the Half-cell potential studies, analysis and conclusions are presented in the subsequent sections. 2 Coastal Concrete Structures and Chloride effect Structures in coastal areas are prone to corrosion related problems. Corrosion destructs the parent material by reaction with its environment and it constitutes one of the basic factors of degradation of reinforced concrete structures. The presence of coastal environment, which is rich in chloride ions, is considered as one of the major external factors inducing damage on reinforced concrete structures. The fundamental problem corresponding to the chloride movement is the initial concentration of chlorides when spread in the concrete.
2 NDE2020, 013, v1: ’Half Cell Potential Studies for Durability Studies of Concrete Structu . . .
Fig. 3 Chloride concentration at different depths for bridges exposed to sea (Source: Mohammed, 2018) The general plot of the curve of the changing chloride concentration with depth in the concrete is explained in Figure 3. From this figure, we note that the concentration of chlorides is reduced rapidly whenever propagation occurs inside the concrete. It can also be seen that the chloride concentration with depth varies as a simple parabolic curve.
Fig. 4 Structure Corrosion Risk based on the distance from coast (Source: Mohammed, 2018)
Chloride can be in the form of a spray in the air when the coastal environment surrounds the structure. For example we can consider a structure or a building under construction near the shore in a coastal area wherein the chloride effect can be seen clearly. Figure 4 shows the intensity of risk posed to a structure according to its location in terms of its distance from the coast and also in terms of height. However, these data shall not be taken as it is, but only as a guide to determine the degree of risk, depending on the climatic conditions of the region [4].
3 Half – Cell Electrical Potential Method to measure Corrosion of Reinforcement in Concrete (ASTM C 876- 91)
Since 1978 Half Cell Potential mapping [5] is in vogue for detecting corroding areas on concrete structures in Denmark. The Half Cell Potential Testing method is suggested for diagnosing the probability of reinforcement corrosion in turn which is used for assessment of the durability of reinforced concrete [5,6]. Corrosion inspection of steel can be conducted by many different techniques. Non-destructive technique such as half-cell potential measurement (HCP) is a well known technique for investigation of corrosion in steel [7]. Even though there are many methods for evaluating
NDE2020, 013, v1: ’Half Cell Potential Studies for Durability Studies of Concrete Structu . . . 3
the corrosion, half-cell potential has been considered useful by many researchers as the main method to detect the corrosion activity in RC structures [8]. 3.1 Description of method: Corrosion of steel can be assessed by Half-Cell Potential (HCP) measurement by an electrochemical process as per ASTM C87. In this method, the electrical potential difference between the upper steel rebars and a standard portable reference electrode in contact with the concrete surface is measured. The half-cell is made by a Copper/ Copper Sulphate or Silver/ Silver Chloride cell but other combinations are also used. The concrete acts as an electrolyte and the risk of corrosion of steel reinforcement in immediate region of the test location may be related empirically to the measured potential difference. The schematics of the test and equipment is shown Fig. 5. It consists of a rigid tube composed of dielectric material that is non-reactive with copper or copper sulphate, a porous wooden or plastic plug that remains wet by capillary action, and a copper rod that is immersed within the tube in a saturated solution of copper sulphate. The schematics of the electrode is shown in figure 6. The reference electrode is connected to the positive end of the high impedance voltmeter and the steel reinforcement to the negative. At the anode, the ferrous ions (Fe++) are dissolved and electrons are set free. These electrons drift the steel to the cathode, where they form hydroxide (OH-). This principle creates a potential difference that can be measured by the half-cell method.
Fig 5 Schematics showing the Half Cell Potential Measurement Technique (www.proceq.be)
4 NDE2020, 013, v1: ’Half Cell Potential Studies for Durability Studies of Concrete Structu . . .
Fig.6. Schematic of Copper Sulphate electrode
The reliability of half-cell potential measurement as an indication of corrosion potential has evolved by the good results during the bridge deck corrosion surveys. An indication of the relative probability of corrosion activity was empirically obtained through measurements during the 1970s [9]. This work formed the basis of the ASTM standard C876, which provides general guidelines for evaluating corrosion in concrete structures as outlined in Table 1:
Table 1. Probability of Corrosion according to half-cell potential values
Half Cell Potential Value ( mV) Corrosion activity
More negative than -350 mV >95%
-200 mV to -350 mV 50%
More Positive than -200 mV <5%
Measured half-cell potential values can be plotted to obtain the potential contour for the surface of reinforced concrete structure and this potential contour map as shown in Figure 7 can be used to evaluate the probability of corrosion at different areas on the surface of the concrete structures. Portions of the structures where high corrosion activity is prevalent can be obtained and identified by their high negative potentials.
NDE2020, 013, v1: ’Half Cell Potential Studies for Durability Studies of Concrete Structu . . . 5
Fig. 7. Half Cell Potential Contour
3.2 Equipment Used
Following are the equipment used for corrosion monitoring of reinforcement in concrete (Fig. 8& 9). The measurement of HCP values carried out at site is shown in figure 10.
Fig. 8. Basic Half cell survey equipment Fig. 9. Reference electrode(CuSo4)
Fig. 10. HCP measurement at site
6 NDE2020, 013, v1: ’Half Cell Potential Studies for Durability Studies of Concrete Structu . . .
3.3 Precautions for Taking Half-Cell Potential Measurements A reliable half-cell potential survey for assessing the corrosion damage of a reinforced concrete structure requires good planning and preparation, careful measurement and data correlation. The method specified by ASTM C876 is a satisfactory measurement procedure to follow. The following data should be taken before starting the HCP measurement: • Basic information about the structure such as structural drawings, mix design details, concrete admixture details,
cover meter survey details etc., • The history of previous repair and rehabilitation including patch repair • Earlier conducted condition survey details, visual examination data and delamination survey results, and chloride
content analysis. The half-cell potential data should always be validated by other testing methods before an interpretation of corrosion probability is made. Many methods are commonly used along with the half-cell potential technique in field assessment of corrosion like visual inspection, delamination survey, chloride content measurement, concrete resistance measurement, concrete cover-depth survey, carbonation profile determination etc., 4 Present Study Under this study some buildings which are having an age of approximately 40 years were selected to assess the structural condition of the buildings. The buildings are located at Sriharikota Island, India. It is prone to severe weather phenomena like tropical cyclones, thunderstorms, and squally weather. SHAR being a coastal area, it experiences sea breeze and land breeze also. The half cell potential studies conducted on these buildings are presented and analysed in this paper. 4.1 Case study 1 (Colony PHC). Under this study some residential buildings which were built approximately 40 years back were selected to check the corrosion potential. The buildings are located at Sriharikota Island. These buildings are built during the year approximately during 1975-76. The building units are Ground + 2 units with Load bearing structure with RCC roof slab and other RCC elements like staircase, chajjas etc. The Grade of concrete adopted during construction is M15. Rebound hammer Test and UPV Values. Rebound hammer test values (RHV) on the surface at grid points and on the surface (at Random locations) in the slab portions, wherever taken are uniform. However higher compressive strength in some RCC members indicated carbonated cover concrete. The average compressive strength is about 25 N/Sqmm. It is observed from the Ultrasonic pulse velocity test results that the Slabs, columns & beams tested, have concrete integrity range from doubtful to good. Some of the UPV values indicate that the concrete possess voids (honey combed in the interior portions). Core Test. The mean compressive strength of cubes obtained is about 16 N/ Sqmm which is satisfactory with respect to the design strength of 15 N/ sqmm. Carbonation test & Chloride Tests. The in-situ carbonation tests on Slabs, Beam, and column indicate that reinforcements are corroded due to carbonated layer reaching up to concrete cover thickness after the carbonation test. In the quarters, out of 14 tests, about 9 samples have shown signs of carbonation. The Chloride content as Cl- in the reference quarter is about 0.8 to 1%. The Chloride content as Cl- in the WTP is about 0.6 to 5%. Results of Half –Cell Potential (HCP) Survey (Colony PHC). Half – cell potential survey using Copper-Copper sulphate half-cell was conducted on RCC members, where corrosion symptoms were observed. The HCP values observed show 50% probability of corrosion in some of the quarters tested.
NDE2020, 013, v1: ’Half Cell Potential Studies for Durability Studies of Concrete Structu . . . 7
A total of 18 data sets were collected for reference quarter building. The measurements were taken at random spots over the different structural elements. Refer to Table 2 data showing the minimum and maximum values obtained at the various locations.
Table 2: Maximum and minimum values of the HCP readings (Qrtr Building).
Ref Quarter Bldg
Max Min
Room 1 -177 -290 6
Room 2 -126 -169 6
Room 3 -7 -74 6
Table 3: Maximum and minimum values of the HCP readings (Treatment Plant Building).
WTP Bldg HCP Value ( mV) Data sets
collecetd Max Min
Clarifier -24 -45 6
Flocculator -282 -313 6
Entry area -282 -313 6
It can be seen that the readings ranged from -7 mV to -290 mV. All the data obtained has also been plotted in a histogram shown in Fig. 11. The mean half-cell potential reading was -135 mV for quarter building and -195 for the WTP building. The HCP values observed in SHAR CAMPUS the readings showed more positive. The HCP values of the WTP building indicates Probability of 50% corrosion activity present in the reinforcement.
Fig 11: Histogram of the half-cell potential measurements data (PHC)
4.2 CASE STUDY 2 (Colony DOS- A type qrtr buildings): These are residential buildings which were built approximately 40 years back were selected to check the corrosion potential. The buildings are located at Sullurupeta town which is near by Sriharikota Island. The building units are Ground + 2 units with Load bearing structure with RCC roof slab and other RCC elements like staircase, chajjas etc. The Grade of concrete adopted during construction is M15 to M20.
0
2
4
6
8
10
12
14
Co un
HCP Values (mV)
8 NDE2020, 013, v1: ’Half Cell Potential Studies for Durability Studies of Concrete Structu . . .
Rebound hammer Test and UPV Values. Rebound hammer test values indicated higher compressive strength in some RCC members. The average compressive strength is about 27 N/Sqmm. It is observed from the Ultrasonic pulse velocity test results ranged about 2 km/s indicating doubtful integrity of concrete. Results of Chloride content Tests DOS).The Chloride content as Cl- are very high in some quarters 0.4 to 1% even after hydration. The chloride are normally contributed by added water in the concrete mix. Results of Half –Cell Potential (HCP) Survey (Colony DOS). A total of 90 data sets were collected for reference quarter building (DOS- A type qrtr buildings). The measurements were taken at random spots over the different structural elements. Refer to Table 4 data showing the minimum and maximum values obtained at the various locations. It can be seen that the readings ranged from -130 mV to -408 mV. All the data obtained has also been plotted in a histogram shown in Fig. 12. The mean half-cell potential reading was -294 mV for these quarter buildings. The HCP values indicates Probability of 50% to 95% corrosion activity present in the reinforcement.
Table 4: Maximum and minimum values of the HCP readings (A type Qrtrs- DOS).
Ref Quarter Bldg
Max Min
A26 to A30 -264 -385 18
A4 to A26 -190 -408 42 A26 to A30 staircases -130 -357 12
A2 to A16 -237 -349 18
Fig. 12. Histogram of the half-cell potential measurements data (DOS)
4.3 CASE STUDY 3 (Storage Bldgs I & II) Under this study two industrial buildings which are used for Storage & integration activities in the initial days (year 1980). The building consists of two independent shed structures namely -I & II. The shed structures consist of R.C.C. column – beam framework and are covered with roofing sheets supported on structural steel trusses and purlins. The space available in between two sheds is covered with R.C.C. slab and merged with two sheds. The Grade of concrete adopted during construction is M 20 to M 25.
0
10
20
30
40
50
Co un
Frequency
NDE2020, 013, v1: ’Half Cell Potential Studies for Durability Studies of Concrete Structu . . . 9
Results of Half –Cell Potential (HCP) Survey (Storage Bldgs I & II). Half-cell Potentiometer test is performed at 103 numbers of test locations on columns and east side RCC wall cladding of buildings-I & II. Refer to Table 5 data showing the minimum and maximum values obtained at the various locations. It can be seen that the readings ranged from -11 mV to -198 mV. All the data obtained has also been plotted in a histogram shown in Fig. 13. The mean half- cell potential reading was -90 mV for these quarter buildings. The HCP values indicates 10 % probability (mild risk) of active corrosion.
Fig 13: Histogram of the half-cell potential measurements data (Storage Bldgs I & II)
Results of Chloride content Tests (Storage Bldgs I & II). Chlorides: As per IS: 456:2000, Clause 8.2.5.2, Table 7, limit of chloride content of concrete (maximum total acid soluble chloride content) is 0.025%. The chemical analysis of 13 nos. of concrete powder samples out of 20 samples of VIB-I & II indicates chloride content of 0.1%. 5 Discussions & Conclusions:
The corrosion rate is the main factor to be taken into consideration in framing up the maintenance strategy. The Half Cell Potential Testing method is a technique, used for assessment of the durability of reinforced
concrete and helps in diagnosing reinforcement corrosion. A more negative reading of potential is generally considered to indicate a higher probability of corrosion.
In both the housing units (case study 1 & case study 2) the half-cell potential values indicate 50% probability for corrosion. This can be attributed to the inadequate cover to reinforcement, or lack of compaction of cover concrete resulting in porous micro structure which is visible in the site. Also, there are many leakage areas which might have contributed to the local corrosion spots. There is…
Related Documents
