Comparison between Destructive and Nondestructive Dynamic Tests in the Presence of Different Ions as the Curing Solution

Corrosion is one of the main causes of the deterioration of reinforced concrete structures. This causes costly repairs and sometimes leads to reconstruction and rehabilitation of the damaged structures. Existence of chloride in pore solution and near the bar accelerates the dioxide reactions of bars [1]. The Electrical Resistivity (ER) is a non-destructive testing method. The corrosion rate of reinforcement and the resistivity of concrete are related. One of the rate-controlling factors is ion transport between anodes and cathodes on the steel surface [2]. Simplifying Bazant’s model [3], the resistivity and corrosion rates are inversed [4,5], although this relationship may be different with concrete composition [6-8]. Within a given structure, areas with low resistivity will be attacked with a relatively high corrosion rate.


Introduction
Corrosion is one of the main causes of the deterioration of reinforced concrete structures. This causes costly repairs and sometimes leads to reconstruction and rehabilitation of the damaged structures. Existence of chloride in pore solution and near the bar accelerates the dioxide reactions of bars [1]. The Electrical Resistivity (ER) is a non-destructive testing method. The corrosion rate of reinforcement and the resistivity of concrete are related. One of the rate-controlling factors is ion transport between anodes and cathodes on the steel surface [2]. Simplifying Bazant's model [3], the resistivity and corrosion rates are inversed [4,5], although this relationship may be different with concrete composition [6][7][8]. Within a given structure, areas with low resistivity will be attacked with a relatively high corrosion rate.
This test enables the evaluation (without estimation) of the strength and the durability of the concrete in aggressive environments [9], especially in marine regions like the Persian Gulf. The ER is one of the durability indexes before the beginning of the corrosion and also one of the factors that determines the rate of the corrosion caused by carbonation or chloride ions in the concrete. Recently, many studies are working on effect of different additives and environmental effects on the destructive and nondestructive dynamic tests of concrete and asphalt pavement [9][10][11]. In fact, the ER is an intrinsic property that quantifies how strongly a given material opposes the flow of electric current in which this property is affected by an external electric field. In some of the references [12], some amounts are listed as the specific ER of the concrete and the probability of the reinforcement corrosion.
The ER of the concrete is one of the inherent and independent characteristics that is affected by some different factors such as the Water to Cement ratio (W/C), paste structure, amount of moisture, temperature, chemical composition of the cement, cement content and the mineral and chemical admixtures [13][14][15][16][17]. Furthermore, the ER may be used as a parameter for controlling the quality of concrete instead of compressive strength tests [18]. Since this test is nondestructive, it can be performed for in-situ controls [19]. By using this test for Quality Control of concrete, some errors that might show during the determining compressive strength can be removed from results. ER method is a fast, easy, and inexpensive, nondestructive measurement technique, which can be used for repetitive quality control of concrete and for the monitoring of structures.

Civil Engineering Research Journal
The results for the chemical analysis of Persian Gulf water are presented in Table 1 [20]. Because of the existence of destructive ions, such as sulfate and chloride, the Persian Gulf is an aggressive environment for the reinforced concrete structures. When the structure has been evaluated for estimating probable damage it should be noted that every ion may cause different damage on the concrete reinforcement structure. This study shows' that NaCl, Na 2 SO 4 , NaOH, and Na 2 CO 3 as electrolytes, have different effects on concrete ER and compressive strength, which are two common tests in assessment.

Mix design
3In this study, four series of mixtures with W/C, 0.4, 0.45, 0.5, and 0.6, were made and tested. The summary of mixtures are given in Table 2. In this study, Type II Portland cement and coarse crushed aggregate with a maximum size of 19mm and natural fine aggregate were used. To achieve optimum performance, the naphthalene-based superplasticizer was used. The chemical composition of cement is presented in Table 3. The gradation of mid-and coarse aggregate is given in Table 4. The tests were done at the Construction Materials Institute at the University of Tehran.

Procedure of tests
The test samples were cylinders with a height and diameter of 70mm. Each test sample was kept for 28 days in the intended solutions with a 24 °C temperature; their amounts of ER were measured at the ages of 14 and 28 days.
Samples were made according to ASTM C192, and were cured in the standard condition according to ASTM C31. Samples were cured in a saturated calcium hydroxide water bath for 7 days. For filling the water pore solution with desired ions, the samples were dried in an oven at a temperature of 110 °C during 3 days, then stored in a desiccator with a vacuum pump during 7 days, at a temperature of 25 °C. After this stage, 48 samples were placed in the NaCl, Na 2 SO 4 , NaOH, and Na 2 CO 3 solutions. Also, 12 samples were kept in a saturated calcium hydroxide water bath as control samples, at a temperature of 24 °C for all environments.

Properties of solutions
Durability of concrete structures in the marine space and sewage environments is very important. Most of the harmful ions of these environments for concrete are Chloride (Cl -), Carbonate (CO 3 2-) and Sulfate (SO 4 2-). Also, the alkaline environment is very effective in sustainability of structures. For these reasons, in this research four solutions were used as hydro-environments for concrete storage. Table 5 shows the concentration of dominant ions in each solution. The two point method has been used for measuring the amount of ER. Figure 1 shows the two point method. For each specimen, two samples were used for measuring the amount of ER. After determining the ER, it is obtained by Equation 1 [20].

The Effect of Ions on the ER
The results are shown in Figure 2-11. According to the results, it can be concluded that the age of samples, W/C, and different electrolytes have an obvious effect on the ER. The W/C is one of the most important parameters on the concrete strength and durability, because this ratio has a very important role on the porous volume of the cement paste. For this reason, the ER of the concrete is affected by this ratio. According to research [21,22], it can be found that, by decreasing the W/C, the concrete strength is increased. According to Figure 2, this result is observed for the concrete with different W/C, and in the presence of different electrolytes. On the other hand, with increasing the age and the W/C, ER increases are visible in Figure 3        According to Figure 3-7, it can be seen that the W/C has an effect on the rate of ER increasing for each code. For reference samples with W/C equal to 0.4, ER is 3.5 times greater than that of samples with W/C equal 0.6. This subject can be seen in other environments. We should notice that the development of ER value within age is different in various environments and W/C. In some codes, the rate of difference is 4 times more. Concrete with lower W/C has less volume capillary pores, especially in early ages. Also, due to the presence of different ions in a different environment, rate of hydration is changed.

Effect of ions in the ER after 14 and 28 days
In Figure 8-11, changes of ER in different environments were shown (when w/c is the same).
The results of Figure 8-11 show that the reference samples have the highest ER when compared with other samples, which means the ER decreases in the presence of different ions. For instance, code B (Alkaline environment) has the minimum ER between other codes where W/C is 0.4; the ER decreases about 70% when compared with reference samples and this reaches 50% for W/C equal to 0.6. The chloride ion has the minimum effect on the decrease in the ER when compared with other samples. For W/C equal to 0.6, the ER decreases 12% when compared with the reference sample and it goes to 5% for W/C equal to 0.4. All the diagrams show that the ER of the D and E samples (Sulfated and Carbonated environments) are almost the same and the results are close to each other.

The Effect of Ions on the compressive strength of 28 day Samples
The compressive strength is affected by different factors such as W/C, curing methods, and many other characteristics [23]. Based on previous research, there are some papers that showed the effect of ions on the compressive strength, but none of them compared the effect of different ions [24]. According to Figure 12, it can be found that the code B samples (Alkaline environment) have more compressive strength, but according to part 3.4, this code has the least ER. This contrast shows that the [OH]-ion has the most electric ion transport in the concrete. In other samples by decreasing the ER, compressive strength also decreases, but with a different rate such as the compressive strength of the E samples (Carbonated environments) is about 10% to 37% lower when compared with the C samples (Chloride environments

Conclusion
The simplest method for measuring the ER of the concrete structures is performing the site experiments; taking attention to this point is very important, since the structures show different ER in different marine, wastewater, alkaline, or experimental environments. This differences in the characteristics of the ions do not show the change between the concrete samples because of the amount of the dominant ion in the pores of the concrete.
a) The samples that are saturated in the NaOH solution have less ER when compared to other solutions. b) When decreasing the water to cement ratio, the difference between ER of other samples decreases.
c) The samples that are placed in the sulfated and carbonated environments have the same ER.