The chapter delves into the crucial role of electrical resistivity in cement mortar used for repairing reinforced concrete structures. It investigates how different polymers, such as ethylene vinyl acetate (EVA), styrene butadiene rubber (SBR), and polyacrylic ester (PAE), influence resistivity over time. The study also examines the effects of various types of cement, including ordinary Portland cement and ultra-rapid hardening cements, as well as admixtures like ground granulated blast furnace slag (GGBS) and fly ash (FA). The addition of lithium nitrite is explored as a means to enhance ionic conductivity and reduce resistivity. The findings highlight the significant impact of these components on the electrical properties of cement mortar, offering valuable insights for professionals in the field.
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Abstract
In the repair of the reinforced concrete structures deteriorated due to the chloride corrosion of rebars, it is desirable that the electrical resistivity of the patching repair mortar is equal to or lower than that of the existing concrete, considering the case of adopting the cathodic protection in the future. In general, cementitious patching repair mortars contain polymer components, which are thought to increase the resistivity, but there are also components other than polymer components that affect the resistivity. Such components without polymer include admixture components such as ground granulated blast furnace slag and fly ash, cement components such as ultra rapid hardening cement, and corrosion inhibitor components such as lithium nitrite. In this study, the effects of those components on the resistivity of cementitious patching repair mortars up to the age of 26 weeks are clarified. As a result, the electrical resistivity of cementitious patching repair mortar depends greatly on the types and amounts of their components.
1 Introduction
In the repair of the reinforced concrete structures deteriorated due to the chloride corrosion of rebar, patching repair mortars are selected in consideration of the future adoption of the cathodic protection. It is desirable that the electrical resistivity is less than the same level as the existing concrete [1]. Therefore, the resistivity of the cementitious patching repair mortar is desirable to be 50kΩ･cm or less [2]. On the other hand, there are various types of commercially available cementitious patching repair mortar depending on the purpose of application. In past studies, since the resistivity of each commercial product differs, it is necessary to measure the resistivity when applying it to the cathodic protection [3]. In particular, the influence of the polymer component on the resistivity is a concern. In addition, although it has been reported that the admixture of fly ash increases the resistivity over a long period of time [4], there are few reports that have examined the effects of the type and amount of each component constituting the patching repair mortar in detail. In this study, three types of polymer dispersion, an ordinary Portland and three types of ultra-rapid hardening cement as cement, ground granulated blast furnace slag (GGBS) and fly ash (FA) as an admixture, and lithium nitrite aqueous solution (LN) as a corrosion inhibitor were used. Mortar specimens were prepared and the effects of these components on the resistivity of cementitious patching repair mortars were investigated.
2 Experimental Method
2.1 Materials Used
Tap water was used as mixing water, and standard sand specified by JIS R 5201 “Physical testing methods for cement” was used as fine aggregate. Three types of polymer dispersion, which are typical in Japan [5], were used, ethylene vinyl acetate (EVA), styrene butadiene rubber (SBR), and polyacrylic ester (PAE). Polymer cement mortar (PCM) was prepared with three levels of polymer cement ratios of 5, 10, and 20%. Table 1 shows the properties of the polymer dispersions used for this study. Table 2 shows the property of ordinary Portland and 3 types of ultra-rapid hardening cements (UHC) used in the study. UHC-I is a calcium sulfoaluminate cement, UHC-II is an amorphous calcium aluminate cement, and UHC-III is a calcium aluminate cement. The main components that impart ultra rapid hardening properties are different [6].
Table 1.
Properties of Polymer Dispersions
Type
Appearance
Solid content
(%)
Density
(g/m3)
pH
Viscosity
(mPa･s)
EVA
milky liquid
45.3
1.04
6.2
960
SBR
milky liquid
45.1
1.03
9.5
300
PAE
milky liquid
45.0
1.02
8.0
1010
Table 2.
Properties of ordinary Portland and ultra-rapid hardening cements
Type
Density
(g/cm3)
Blaine*
(cm2/g)
Chemical composition (%)
SiO2
Al2O3
Fe2O3
CaO
MgO
SO3
OPC
3.14
3070
20.3
5.2
2.9
64.3
0.9
2.1
UHC-I
3.01
4690
14.4
13.7
2.0
54.5
1.4
11.7
UHC-II
3.00
5400
15.1
9.7
2.1
57.5
0.7
10.1
UHC-III
2.98
6050
11.2
17.2
1.9
52.4
0.8
10.3
*Blaine: Blaine specific surface area
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Table 3 shows the properties of GGBS and FA used in the investigation of the admixture composition. The properties of GGBS were measured according to JIS A 6206 “Ground granulated blast-furnace slag for concrete”, and the properties of FA were measured according to JIS A 6201 “Fly ash for use in concrete”. The amounts of GGBS added were 16%, 46% and 66%, and the amounts of FA added were 8%, 16% and 26%. These mixing ratios are values corresponding to Class A, Class B and Class C in JIS A 5211 “Portland blast-furnace slag cement” and JIS R 5213 “Portland fly-ash cement”, respectively. In order to study the components of the corrosion inhibitor, an aqueous solution of lithium nitrate (LN) with a solid content of 40 wt% (blue transparent liquid, pH 9.2, density 1.24) was used, and the unit amount was adjusted to 55 kg/m3, and OPC, GGBS-46 and FA-16 were used as cements.
Table 3.
Properties of admixture material
Type
Density
(g/cm3)
Blaine*
(cm2/g)
Chemical composition (%)
MgO
SiO2
SO3
Moisture
Ig. Loss
Cl
GGBS
2.91
4160
5.81
―
0.01
―
0.03
0.004
FA
2.29
4070
―
63.6
―
0.2
2.5
―
*Blaine: Blaine specific surface area
2.2 Preparation and Curing of Specimen
Weighing, mixing and molding conformed to JIS R 5201. Sand cement ratio of 3 and a water cement ratio of 0.5 (both mass ratios) were used as the basis for the mix proportions. Mixing amount of each specimen shows Table 4, 5, 6, and 7. The specimen size was 4 × 4 × 16 cm, and the curing was performed in accordance with JIS A 1171 “Test methods for polymer-modified mortar” for 2 days after molding. After demolding, it was cured in water at a temperature of 20 ℃ for 5 days and then in air at a temperature of 20 ℃ for 21 days. Tables 4, 5, 6, and 7 show the mixing amount of the mortar specimens.
Table 4.
Mixing amount of OPC and PCM specimen (Unit g)
Material
OPC
EVA
SBR
PAE
0%
5%
10%
20%
5%
10%
20%
5%
10%
20%
Polymer
0
50
100
200
50
100
200
50
100
200
Water
225
198
170
115
198
170
115
198
170
115
Ordinary Portland Cement 450g and Standard Sand 1350g were common and constant
Table 5.
Mixing amount of OPC and UHC specimen (Unit g)
Material
OPC
UHC-I
UHC-II
UHC-III
Cement
450
450
450
450
Sand
1350
1350
1350
1350
Water
225
225
225
225
Table 6.
Mixing amount of mixed cement specimen (Unit g)
Material
OPC
GGBS
FA
0%
16%
46%
66%
8%
16%
26%
Cement
450
378
243
153
414
378
333
Admixture
GGBS
0
72
207
297
–
–
–
FA
0
–
–
–
36
72
117
Standard Sand 1350g, Water 225g were common and constant
Table 7.
Mixing amount of LN-containing mixed cement specimen (Unit g)
Material
OPC
OPC-LN
GGBS46
GGBS46-LN
FA16
FA16-LN
Cement
450
450
243
243
378
378
Admixture
GGBS
0
0
207
207
–
–
FA
0
–
–
–
36
72
Lithium Nitrite
–
124
–
124
–
124
Water
225
151
225
151
225
151
Standard Sand 1350g was common and constant
2.3 Measurement Method of Resistivity
After 28-day curing, the specimens were subjected to outdoor exposer at Kobe in Japan. For conditioning, the specimens were stored at 20 ℃-room for 3 days before the measurement of the resistivity. Resistivity were measured at 4,8, 13 and 26 weeks after curing, using an AC earth resistance tester (MCMILLER 400A, manufactured by MCMILLER, USA) as shown in Fig. 1. Conductive gel was applied to both ends of the 4 × 4 × 16cm specimen, and the specimen was kept for 10 min [7]. In addition, the data consistency was confirmed by checking the values measured by the JSCE-G 581 “Test method for electrical resistivity of concrete by four electrode method”.
Fig.1.
Outline of Measurement Method of Electrical Resistivity
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3 Result and Discussion
3.1 Effect of Polymer Components on Resistivity
Figures 2, 3, 4 show the relationship between the resistivity and type of polymer and P/C of PCM at 4, 8, and 13 weeks. The resistivity of PCM tended to increase with an increase in the aging and P/C. However, at 4weeks, which is the condition of the standard test, all PCMs except PAE P/C = 20% were 50 kΩ･cm or less. Therefore, it was confirmed that the resistivity of PCM is not as high as generally assumed. However, the resistivity of PCM continued to increase after 4 weeks, and at 13 weeks all PCM of P/C 10% and P/C 20% became greater than 50 kΩ･cm. It seems that the measuring age of the standard test method should be a little longer than 4 weeks.
Fig. 2.
Relationship between resistivity and type of polymer and P/C of PCM at 4 weeks
Fig. 3.
Relationship between resistivity and type of polymer and P/C of PCM at 8weeks
Fig. 4.
Relationship between resistivity and type of polymer and P/C of PCM at 13weeks
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×
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Resistivity of PCM is also affected by the hydrophilic and hydrophobic functional groups contained in the molecular structure shown in Fig. 5 and by the hydrophilic-lipophilic balance (HLB) value of the surfactant contained as a subcomponent [8]. It is presumed that the resistivity of the EVA was lowered due to the water retention of the surfactant contained [8]. However, the EVA also showed high resistivity at P/C = 20% at 13 weeks, which is presumed to be due to progress in drying of the polymer phase.
On the other hand, in SBR and PAE, effect of the hydrophobic functional group is considered to have increased the resistivity. However, SBR uses a nonionic surfactant with a high HLB value, and it is thought that this action improves the water retention, resulting in a lower resistivity than the PAE [8]. It is presumed that PAE showed the highest resistivity because it does not contain hydrophilic functional groups or strongly hydrophilic surfactants.
Fig. 5.
Molecular structure of polymer
×
3.2 Effect of Cement Components on Resistivity
Figure 6 shows the temporal change in resistivity of mortars using OPC, UHC-I, UHC-II and UHC-III from 4 weeks to 26 weeks. OPC is almost constant at 30–70 kΩ･cm, and UHC-I is almost the same as OPC up to 13 weeks, and then increased to 220 kΩ･cm. UHC-II is similarly same as OPC until 13 weeks, and then increased to 420 kΩ･cm. UHC-III shows the resistivity of 150 kΩ･cm at 4 weeks, about three times that of OPC, and then it increased with age to a very large resistivity of about 1000 kΩ･cm. It has been reported that the three types of ultra-rapid hardening cement differ in the type and amount of calcium aluminate that imparts ultra-rapid hardening properties, resulting in different types and amounts of calcium aluminate hydrates that are produced, as well as different microstructural structures [9], and these factors are presumed to have influenced the increase in resistivity with age.
Fig. 6.
Temporal change in resistivity of ordinary and ultra fast hardening cement mortars
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3.3 Effect of Admixture Components on Resistivity
Figure 7 shows the change in the resistivity with GGBS and FA replacement at each age. The resistivity of mortar mixed with any admixture increased with the age. GGBS66 showed the most largest resistivity exceeding 200 kΩ･cm at 26 weeks. In any cases, the increase in resistivity is large from 13 to 26 weeks. Comparing the effects of GGBS and FA on resistivity, the effect of GGBS was slightly greater than FA. Since GGBS has latent hydraulic property and FA consumes calcium hydroxide in the hardening material by pozzolanic reaction, it is considered that the ion concentration in the pore solution decreased and the resistivity of each mortar increased [4].
Fig. 7.
Change in resistivity with GGBS and FA replacement
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3.4 Effect of Lithium Nitrite on Resistivity
Figure 8 shows the change in resistivity when 55 kg/m3 of lithium nitrite is added to OPC, GGBS46 and FA16. Lithium nitrite makes the mortar hygroscopic and increases the moisture content. It is presumed that the mortar containing nitrite ions and lithium ions improves the ionic conductivity and reduces the resistivity. The effect of lithium nitrite addition on resistivity was a 20–30% decrease in OPC, a 20–30% decrease in GGBS46 and FA16 up to 13 weeks, but 40–50% decrease at 26 weeks.
Fig. 8.
Change in resistivity with LN addition
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4 Conclusion
The following findings were obtained as a result of this study.
The resistivity of PCM is increased with an increase in the age and P/C. The degree of increase with increasing P/C differs depending on the polymer component, being the smallest in the SBR and the largest in the PAE.
The resistivity of CSA based and am-CA based ultra rapid hardening cement mortars is similar to that of OPC mortars up to 13 weeks, and then increases. The CA based ultra rapid hardening cement mortar shows a marked increase in resistivity from 4 weeks of age, showing the highest resistivity.
The resistivity GGBS and FA mortar is similar to that of OPC mortar at the early age, but it increases as the age progresses and the mixing ratio increases. It increases greater in mortar mixed with GGBS than FA.
The addition of LN to OPC and mixed with GGBS and FA have the effect of reducing the resistivity.
From the above, it is clear that the electrical resistivity of cementitious patching repair mortar depends greatly on the types and amounts of their components. In addition, we plan to continue this study in the future because the effect changes with the age.
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