Effect of mixing proportions of concrete on its electrical conductivity and the rapid chloride permeability test (ASTM C1202 or ASSHTO T277) results
Introduction
The rapid chloride permeability test (RCPT)-ASTM C1202 [1] or ASSHTO T277 [2] has been criticized by many scientists and researchers all over the world during the past decade because of its lack of scientific bases and harsh testing conditions [3], [4], [5], [6], [7], [8], [9], [10]. These standards specify the rating of chloride permeability of concrete based on the charge passed through the specimen during 6 h of testing period. ASTM C1202 recognizes that a correlation between the rapid chloride permeability test and the 90-day ponding test results is necessary, while AASHTO T277 does not require this correlation. A comparison has indicated that conventional concretes made with only Portland cement may exhibit coulomb values 6 to 15 times higher, but that the corresponding actual chloride ingress in the 90-day ponding test is only 1 to 2 times higher than the same concrete mixtures but with silica fume [6]. Another study has also found that the general correlation between chloride ingress and coulomb values given in the AASHTO T277 or ASTM C1202 test procedures appears invalid for use with concretes containing silica fume, fly ash and high-range water-reducing admixtures [11].
Recently, several discussions have been published regarding the validity of the test method [12], [13]. The effects on the concentration of conductive ions and the RCPT results from the use of Ca(NO3)2 in concrete are acknowledged. However, the effect of other materials on pore solution chemistry and how the pore solution chemistry affects the RCPT results are not mentioned at all.
Dry concrete is a semiconductor or insulator. Electrical conductivity of water saturated concrete depends on not only the pore structure and but also the chemistry of pore solution. The transport of chloride ions has little to do with the chemistry of pore solutions, but many factors such as cement composition, aggregate, concrete mixing proportions, use of supplementary cementing materials, chemical additives, etc. can have very significant effects on the concentration of conductive ions in the pore solution. RCPT has been used to evaluate the chloride permeability of hardened cement mortars and concretes made with special cements or supplementary cementing materials [14], [15], [16], [17], [18], [19], [20], [21], [22], [23], [24]. It is obvious that the use of RCPT in some of those studies has resulted in some invalid or misleading conclusions. In one study [16], it was found that the inclusion of sands or Class F fly ash decreases chloride permeability significantly, while their effects on water permeability are much smaller. In another study [21], a replacement of 10% cement with class F fly ash can even significantly reduce the chloride permeability of cement mortars at 7 days. Shi [22] used RCPT to test Portland cement and three alkali-activated slag mortars and found that although sodium silicate activated slag cement mortar showed the lowest pore porosity and water permeability among the four cement mortars, it gave much higher passed coulombs than the other three mortars due to the high concentration of conductive ion concentration in the pore solution. In the most recent publication [24], it is found that the RCPT results from the same concrete specimens do not relate directly to the three water-to-cementing material ratios (0.38, 0.45 and 0.52) used, and some concrete specimens with lower water-to-cementing material ratio showed high RCPT values. On other hand, it can be expected that the concrete with a water-to-cement ratio of 0.38 should have a much lower permeability than the concrete with a water-to-cement ratio of 0.52.
This paper has used published results to show how several factors such as cement composition, inclusion of aggregate and use of supplementary cementing materials affect the pore solution chemistry, the electrical conductivity of the pore solution and the RCPT results. The objective of this paper was to further analyze the validity of the RCPT method for evaluation of concrete made with different materials or different proportions.
Section snippets
Specific electrical conductivity of concrete pore solution
For a strong electrolyte, its equivalent conductivity λi decreases with the square root of equivalent concentration Ci[25]:where λi,0 is the equivalent conductivity at infinite concentration. Table 1 lists the equivalent conductivity of aqueous ions commonly identified in concrete pore solution.
For a given temperature, the specific conductivity of a solution can be expressed as follows [26]:where ρ=specific electrical conductivity of aqueous
Rating criteria
ASTM C1202 [1] and ASHATTO T277 [2] specify the rating of chloride permeability of concrete based on the charge passed through the specimen during 6 h of testing period, as listed in Table 3. A charge value of less than 700 to 1000 C is typically specified, which is characterized as very low chloride permeability based on the rating in Table 3. However, some low water-to-cement ratio (0.3 to 0.4) conventional concretes cannot achieve the 700 to 1000 C. Yet, these same concretes exhibit
Electrical conductivity and ion transport
The diffusivity of ions through a water saturated porous medium can be correlated with electrical conductivity as follows [56]:where D=effective diffusivity of an ion in a porous medium, D0=diffusivity of the ion in a solution, σ=electrical conductivity of the solution saturated medium, and σ0=conductivity of the solution.
D0 can be regarded as a constant for a given ion. The electrical conductivity of saturated concrete, σ, relies on the pore structure characteristics and the electrical
Alternative test method
Many test methods have been proposed to assess the chloride permeability of hardened cement and concrete. They can be classified into four catalogues as summarized in Table 4. Streicher and Alexander [7] have reviewed most of those test methods and felt that the rapid migration test (RMT) method developed by Tang and Neilson [57] is a reliable and fast test method. The RMT has been standardized by Nordtest as NT Build 492. The test can be done with a similar apparatus as the RCPT. A
Conclusions
The permeability of concrete depends on the pore structure of concrete, while electrical conductivity or resistivity of concrete is determined by both pore structure and the chemistry of pore solution. Factors that have little to do with the transport of chloride can have great effects on electrical conductivity of concrete. Thus, the electrical conductivity or resistivity of concretes cannot be used as an indication of their permeability. However, it can be used as a quality control indicator
Acknowledgements
Comments on the manuscript from Bryant Mather of US Army Corps of Engineers, Bill Hime of WJE and Kyle Stanish of the University of Toronto are greatly appreciated.
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