EVALUATION OF CONCRETE RESISTANCE TO CHLORIDE PENETRATION BY MEANS OF ELECTRICAL RESISTIVITY MONITORING Marta Kosior -Kazberuk Bialystok Technical University, Wiejska 45E, 15-351 Bialystok, Poland, E-mail:
[email protected] Received
; accepted
Abstract. The changes in electrical resistivity reflect the physical and chemical changes taking place in the concrete. The primary focus of this study was to examine the effect of fly ash addition on Portland cement as well as blast furnace slag cement concrete resistance to chloride ion penetration. Chloride penetration was simulated by subjecting samples to cyclic loading with salt solution and drying or to freeze-thaw cycles in presence of chloride salt. Concrete resistivity development, in chloride exposure regimes, was monitored during 12 months. The resistivity of concrete was found to change with time and this also depended on the type of cement used. Keywords: concrete, fly ash, electrical resistivity, chloride penetrability, charge passed
1. Introduction The ability of concrete to resist the penetration of chloride ions is a critical parameter in determining the service life of steel-reinforced concrete structures exposed to de-icing salts or marine environments. It is apparent the progressive ingress of salts towards the reinforcing steel takes place under alternating wetting and drying or freezing and thawing. The cyclic action accelerates durability problems, for it subjects the concrete to the movement and accumulation of harmful materials. The transport of chlorides in concrete is influenced strongly by the exact sequence of wetting and drying. This sequence varies from location to location, depending on the frost, the wind, the exposure to the sun and the shape and the usage of the construction. Prolonged or repeated ingress can, with time, result in a high concentration of chloride ions near the surface of reinforcing steel [1 - 3]. The electrical resistivity of concrete can be related to two processes involved in corrosion of reinforcement: initiation (chloride penetration) and propagation (corrosion rate). The resisistivity of concrete structure exposed to chloride indicates the risk of early corrosion damage, because a low resistivity is related to rapid chloride penetration and to high corrosion rate [4, 5]. Concrete resistivity is a geometry-independent material property that describes the electrical resistance, that is the ratio between applied voltage and resulting current in a unit cell. The resistivity of concrete may vary from 101 to
105 Ωm, depending on the moisture content in the concrete and the material composition. The current is carried by ions dissolved in the pore liquid. Increased pore saturation (wet concrete) as well as increased number of larger diameter pores (higher water-to-cement ratio) decrease resistivity [6]. For constant moisture content, the resistivity is increased by longer curing (hydration), lower w/c ratio and by addition of reactive materials such as alternative cementing materials: fly ash, blast furnace slag, metakaoline and silica fume [4]. Resistivity also increases when the concrete dries out and when it carbonates, particularly in Portland cement. The resistivity of concrete is related to the susceptibility for chloride penetration. Within a particular structure, more permeable zones will have a comparatively lower resistivity and higher chloride penetration. In order to cover a wide range of conditions in this study, different kinds of cement as well as alternative cementing material fly ash have been used. The results of investigation aiming to monitor the changes in electrical resistivity of concrete subjected to chloride penetration in various moisture and temperature conditions are presented in the paper. The results of long-term monitoring of resistivity changes were confronted with the chloride penetration resistance of concrete determined on the basis of rapid test (RCPT). Based on the test results, the effects of cement type, fly ash content related to cement mass as well as the time of exposition have been discussed.
2. Experimental methods
2. 2. Experimental procedures
2.1. Materials and specimens preparation It is well known that the use of the alternative cementitious materials (ACMs), such as fly ash, ground granulated blast furnace slag or silica fume improve pore structure and reduce permeability of hardened concrete in salt-laden environment [7, 8]. The resistance offered by these concretes has been related to the low mobility of chloride ions due to either the reduction in the number of interconnected pores as a result of the pozzolanic reaction of the ACMs or the chemical binding with the cement hydrates. However, the secondary reaction products are formed slowly in Portland cement concrete containing ACMs, thus the resistance to chloride ions penetration also increases slowly with time [9, 10]. There is not enough available data concerning long-term development of resistance to chlorides penetration of concrete mixtures containing mineral additions. The tests were carried out for concrete with fly ash addition. Two types of cement were used: Portland cement CEM I 32.5 R (C1) and blast furnace slag cement CEM III/A 32.5 NA (C2). The mineralogical composition of clinkers in cements used is presented in Table 1. The blast furnace slag used in C2 cement contained 44.8% CaO, 39.1% SiO2, 7.1% Al2O3, 1.7% Fe2O3, 0.23% SO3, 6.2% MgO. The content of slag in C2 cement was 62%. Table 1. Mineralogical composition of clinkers in cements used
Clinker C1 cement C2 cement
C3S [%] 62.1 60.4
C2S [%] 14.5 15.5
C3A [%] C4AF [%] 8.6 9.3 9.1 8.8
The fly ash from local power plant was used. The phase composition of the fly ash was determined using diffractional X-ray analysis. Two main crystalline phases: β-quartz and mullite were determined. The fly ash also contained anhydrite, calcium oxide and very small amount of calcite and gypsum. The content of CaO (free), obtained from analytical test, was 0.25%. The loss of ignition, monitored during 25 days of coal burning in power plant, did not exceed 4.8%. The specific gravity of mineral addition was 2.23 kg/dm3. The fine aggregate used was river sand with a maximum diameter of 2 mm, the coarse aggregate – basalt grit with a maximum diameter of 8 mm. The tests were carried out on specimens prepared of mixtures with different values of fly ash content: 10, 20 and 30% related to cement mass (FA/C), as well as on unmodified control specimens. The part of fly ash (20%) was taken into account as a binder and the remaining part – as filler. The cement content in control concrete was 350 kg/m3. Water to binder ratio in the all concrete mixtures was constant (w/b = 0.50). The concrete was cast in the moulds and compacted by wibration. The specimens were stored 24 hrs in the moulds and, after demoulding, cured under water at 18 ± 2oC until the test age.
Electrical resistivity measurements are an effective non-destructive method for monitoring the changes in concrete structure. The specimens of 100×100×50 mm with the pairs of stainless steel electrodes, with active surface of 10 cm2, embedded in hardened concrete, were used for evaluating the electrical resistivity [4]. The thickness of concrete cover around electrodes was ∼2 cm. The electrical resistivity development was monitored for concrete specimens exposed in various conditions: • cyclic wetting and drying conditions, permitting moisture movement through concrete pores. The wetting period in 3% NaCl solution was kept for 7 days and the time of drying, at 40% RH and 18± 2oC, was 7 days. During the wetting period, the samples were partially (to 4/5 of their depth) submerged in solution, which should intensify the chlorides migration caused by absorption and capillary suction. • cyclic freezing and thawing in the presence of 3% NaCl solution. The duration of single freeze/thaw cycle was 24 hrs. Before exposition, the samples were stored 90 days in water and then they were saturated with NaCl solution by capillary suction. Every 28 days the resistivity was determined. Since electrical resistivity is a function of the moisture content, just before the measurement all specimens were stored during 48 hrs in climatic chamber at 95% RH and 18oC. The test was conducted during 12 months. The changes in resistance between the pairs of electrodes were measured using 1÷2 kHz AC, electrode polarization effects were reduced to negligible proportions for all mixes [3]. The resistivity values were obtained by multiplying the measured resistance by a conversion factor (the cell constant). The cell constant was calculated considering the effective area of the steel electrodes and their distance of separation. Every series of specimens consisted of four replicates. The resistance to chloride penetration was evaluated qualitatively using a rapid and effectice method suitable for concretes tested characterization. Rapid chloride ion penetrability test (RCPT) was conducted in accordance with ASTM C 1202 [11]. The test method covers the determination of the electrical conductance of concrete to provide a rapid indication of its resistance to the penetration of chloride ions [12, 13] . The RCPT is a resistivity test indeed. The cylindrical sample was positioned in a measuring cell between two fluid reservoirs on each side of the specimen. One reservoir is filled with the solution of 3% NaCl and the other with 0.3 mol/dm3 solution of NaOH. The reservoir containing the sodium chloride is connected to the negative terminal, the sodium hydroxide to the positive terminal of power supply. The electrical currant passing over a period of 6 hours for a voltage 60 VDC is measured. The charge passing (in coulombs) through the sample during 6 hours’ test period is obtained
by integration of the area underneath the curve of currant (in amperes) versus time (in seconds) [11]. The total charge passed has been found to be related to the specimen resistance to chloride ion penetration. The chloride ion penetrability is evaluated qualitatively according to Table 1. Table 2. Chloride ion penetrability based on charge passed [11]
Charge passed [C] > 4000 2000 ÷ 4000 1000 ÷ 2000 100 ÷ 1000 < 100
Chloride ion penetrability high moderate low very low negligible
development at all ages. It is found, in general, that the rate of strength development is slower for mixtures containing fly ash, at early ages. The rate of strength increase in concretes with fly ash is significant between 28- and 180-days. The increase in strength between 28 and 90 days of curing is slower for C2 cement (blast furnace slag cement) concretes but after 180 days of storage the compressive strength of C2 cement concrete with fly ash is comparable with strength of C1 cement (Portland cement) concretes. Finally, the concretes containing the addition reached greater values of compressive strength than control concretes. 3.2. Concrete ability to chloride penetration
The temperature of solutions was monitored during testing. The temperature of the specimens and solutions should be maintained in the range of 20–25 oC. In high temperature the chloride ions transport is significantly accelerated and therefore leads to erroneous conclusions. The concrete disks 50 mm thick with a diameter of 105 mm were used in the test. The specimens were stored under water at temperature 18 ± 2 oC. In order to avoid heterogeneity, the specimens were vacuum-saturated prior to test. Every series of specimens consisted of three replicates. The properties of hardened concrete such as compressive strength, water absorbability, sorptivity and capillary suction of water were also analyzed. 3. Test results and discussion 3.1. Basic properties of hardened concretes The test results of selected physical and mechanical properties of hardened concretes are presented in Table 3. As it is shown from Table 3, the fly ash addition, in considered range of FA/C values, has very little influence on water absorbability of concretes tested but the addition reduces the capillary suction of water and sorptivity of C2 cement concretes. The observed results of concrete strength test prove fly ash influence on the strength
The results of RCP test determined after 28, 90 and 180 days storage are presented in Figs 1 and 2 and the qualitative evaluation of chloride ions penetrability according to [11] is shown in Table 4. The temperature of solutions used for testing exceeded 25 oC in the case of control concrete after 28 days of curing only. The test results showed that the introduction of fly ash into concrete with Portland cement (C1) allows achieving moderate, low and very low chloride permeability (Fig 1), but the cement C1 concrete without fly ash gained low permeability only (Fig 2). All concretes with blastfurnace cement (C2) revealed very low permeability, independently of addition content related to cement mass. However, the influence of FA/C ratio and extended time of curing on the decrease in charge passed is not as univocal as in the case of Portland cement concretes, mentioned above. Probably, the pozzolanic reaction of fly ash [14] was disturbed, at early age, by presence of significant amount of slag in binder. In general, the value of total charge passed through the samples decreased with an increase in curing time. The degree of charge passed reduction was higher for mixtures containing fly ash than for the control mixtures. Independently of curing time, the charge passed through samples with blastfurnace cement was a few times smaller than through samples with Portland cement. Significant differences in total charge passed proved cement type influence on concrete resistance to chloride penetration.
Table 3. Selected properties of concretes with fly ash addition
Cement type
C1
C2
FA/C 0.0 0.1 0.2 0.3 0.0 0.1 0.2 0.3
Capillary suction [% wt.] 1.25 1.33 1.34 1.29 1.59 1.42 1.36 1.25
Water absorb. [% wt.]
Sorptivity [% wt.]
5.57 5.01 5.11 5.41 5.52 5.34 5.57 5.58
2.60 2.90 2.82 2.71 2.75 2.04 1.88 1.57
Compressive strength [MPa] 28 days
90 days
180 days
53.3 52.3 52.1 50.6 46.3 46.7 41.2 43.5
54.0 55.5 59.3 57.5 51.3 53.3 59.1 58.2
55.4 63.5 68.4 62.5 51.9 54.7 65.8 63.8
Table 4. Qualitative evaluation of chlorides penetrability according to [11]
After 28 days of curing Cement type
C1
C2
FA/C
0.0 0.1 0.2 0.3 0.0 0.1 0.2 0.3
After 90 days of curing
After 180 days of curing
Charge Charge Charge Chlorides Chlorides CC/FA Chlorides passed passed CC/FA passed penetrability penetrability penetrability [C] [C] [C] 3289 moderate – 1917 moderate – 1452 low 2445 moderate 1.34 971 low 1.97 601 very low 2274 moderate 1.45 720 very low 2.66 341 very low 1984 moderate 1.66 481 very low 3.98 273 very low 790 very low – 445 very low – 340 very low 845 very low 0.93 313 very low 1.42 245 very low 700 very low 1.13 278 very low 1.60 203 very low 624 very low 1.27 201 very low 2.21 218 very low
CC/FA – 2.41 4.26 5.32 – 1.39 1.67 1.56
were observed, for C2 cement concretes the effect of fly ash on chloride permeability is difficult to be evaluated. Very low penetrability (charge passed < 1000 C) of C2 cement concretes was achieved thanks to blastfurnace cement using. Beneficial effect of fly ash is evident not before 90 days of curing and the decrease in charge passed through samples with fly ash, in comparison with charge passed through unmodified samples, was still observed after 180 days of storage. The amount of increase in CC/FA, with the curing period, was significantly greater for C1 cement concretes.
Fig 1. RCPT total charge passed for C1 cement concretes (vertical bars represent the range of accuracy)
Fig 2. RCPT total charge passed for C2 cement concretes
To analyse the trend followed by the charge passed the factor CC/FA was used [15]. The factor CC/FA is defined as the ratio of the total charge passed through the control mixture to the total charge passed through the concrete mixture with fly ash addition, separately for both cements used. The CC/FA value (Table 4) increase means lower charge passed than the control. After 28 days of curing, for C1 cement concretes imperceptible changes in CC/FA
3.3. Resistivity of concrete subjected to cyclic wetting and drying The changes of electric resistivity of concretes exposed to cyclic wetting in 3% NaCl solution and drying, accelerating moisture movement through concrete pores, are shown in Figs 3 and 4. All the specimens show the increase of concrete resistivity as a function of time, due to the microstructural changes in the hydrated cement paste and the consequent reduction in porosity of the cement paste. Since the rate of change of electric resistivity of the control specimens of both cements is rather low in comparison to that of the fly ash-blended specimens, it could be inferred that it is the pozzolanic reaction of the fly ash that results in a high rate of change of resistivity. The fly ash addition significantly influences the rate of concrete resistivity changes. During wetting/drying cycles, electrical response resulting for concrete specimens containing the addition increased gaining the values even several times greater than the resistivity of control concrete without fly ash [16]. Moreover, the values of resistivity obtained were directly proportional to the addition content in concrete mix (FA/C). Generally, C2 cement (blast furnace slag cement) concretes were characterized by higher value of resistivity than C1 cement (Portland cement) concretes.
Fig 3. Development of electric resistivity of C1 cement concretes exposed to cyclic wetting and drying
Fig 5. Electric resistivity of C1 cement concretes exposed to cyclic freezing and thawing
Fig 4. Development of electric resistivity of C2 cement concretes exposed to cyclic wetting and drying
Fig 6. Electric resistivity of C2 cement concretes exposed to cyclic freezing and thawing
However, comparing the electric resistivity of concretes with fly ash and without addition, the significantly stronger influence of fly ash on resistivity was observed in the case of C1 cement concretes. The resistivity of C1 concrete of FA/C=0.30 gained the value three times greater than the resistivity of control concrete. Such distinct differences were not found in the case of C2 cement concretes. After 9 months of test, the gradual decrease in resistivity of C1 cement concrete without addition was observed. The inconvenient influence of NaCl solution on resistivity of concrete with fly ash was not pointed out. 3.4. Resistivity of concrete exposed to freezing and thawing The data concerning the variation of electric resistivity with FA/C ratio and time of exposition for both cement types, after cyclic freeze/thaw, were plotted in Figs 5 and 6. The synergistic action of freeze/thaw cycles and chloride ions caused significant decrease in electric resistivity, although the curing time of concrete before test was extended. The exposure regime was the main factor affecting concrete resistivity. After initial distinct drop, observed for all concrete specimens, the resistivity
value became stable in aggressive environment. Only in the case of C1 cement concrete without fly ash (FA/C = 0.0) intensive destruction due to freezing and thawing was pointed out. The C2 cement (blast furnace slag cement) concretes exhibited substantially higher electric resistivity than C1 cement (Portland cement) concretes. 4. Conclusions The primary objective of this study was to assess the possibility of evaluating the concrete resistance to chloride penetration based on electric resistivity monitoring. The durability of concrete with fly ash was tested under influence of cyclic wetting and drying as well as freezing and thawing with chlorides. On the basis of investigation the following conclusions can be drawn: 1. The electric resistivity of concrete was found to be closely connected with cement type and mineral addition content related to cement mass as well as the time of storage. During long-term monitoring, it was observed that blending cement with fly ash caused significant changes in electric resistivity of concrete exposed to various conditions simulating service life conditions. The blast furnace slag cement concretes showed substantially higher electric resistivity than
2.
3.
4.
Portland cement concretes and the resistivity value was directly proportional to FA/C ratio. In the content of this study it was shown that fly ash, in considered range of FA/C ratio, resulted in a significant reduction in the amount of chloride ingress into concretes incorporating it as mineral addition. The resistance to chloride penetration, determined in RCPT, was found to increase with time but the chloride penetrability depended on cement type. The long-term beneficial effect on concrete resistance to chloride penetration was found to be better in concretes with the higher percentages of fly ash. The same dependency was observed during monitoring of concrete resistivity changes in different conditions. However, the results of RCPT is not comparable in directly way with the results of resistivity monitoring. The substantial effect of slag and fly ash is reflected in permeability and diffusivity of ions. The introduction of fly ash addition to blast furnace slag cement concrete makes it possible to achieve the required compressive strength and the excellent resistance to chloride penetration. The beneficial effect of fly ash and slag cement combination is evident as early as after 28 days of curing, although the hydration process, pozzolanic reaction and changes in microstructure continue to occur. The use of fly ash or slag will considerably increase the service life of structures exposed to chloride environments. The concrete resistance to chloride penetration can be evaluated using a rapid but destructive method such as Rapid Chloride Ion Penetrability Test but the monitoring of electric resistivity changes gives much more data about concrete properties in different conditions (quality control indicator) in simple way. In order to estimate the durability of structures it is highly desirable to quantify and simulate the chloride diffusion process in concrete. The long-term monitoring can be carried out in different regimes simulating service life conditions.
References 1. Neville, A. Chloride attack of reinforced concrete: an overview. Materials and Structures, 1995, Vol. 28, p. 63-70. 2. Hong, K.; Hooton, R. D. Effects of cyclic chloride exposure on penetration of concrete cover. Cement and Concrete Research, 1999, Vol. 29, p. 1379-1386. 3. Chrisp, T. M.; McCarter, W. J.; Starrs, G.; Basheer, P.A.M.; Blewett, J. Depth-related variation in conductivity to study cover-zone concrete during wetting and drying. Cement and Concrete Composites, 2002, Vol. 24, p. 415-426. 4. Polder, R. B. Test methods for on site measurement of resistivity of concrete – a RILEM TC-154 technical recommendation. Construction and Building Materials, 2001, Vol. 15, p. 125-131. 5. Ampadu, K.O.; Torii, K. Chloride ingress and steel corrosion in cement mortars incorporating low-quality fly ash. Cement and Concrete Research, 2002, Vol. 32, p. 893-901. 6. Basheer, P.A.M.; Gilleece, P.R.V.; Long, A.E.; McCarter, W.J. Monitoring electrical resistance of concrete containing
alternative cementitious materials to assess their resistance to chloride penetration. Cement and Concrete Composites, 2002, Vol. 24, p. 437-449. 7. Bijen, J. Benefits of slag and fly ash. Construction and Building Materials, 1996, Vol. 10, p. 309-314. 8. Antiohos, S.; Maganari, K.; Tsimas, S. Evaluation of blends of high and low calcium fly ashes for use as supplementary cementing materials. Cement and Concrete Composites, 2005, Vol. 27, p. 349-356. 9. Shafiq, N. Effects of fly ash on chloride migration in concrete and calculation of cover depth required against the corrosion of embedded steel reinforcement. Structural Concrete, 2004, No 5, p. 5-10. 10. Roy, D.M.; Jiang, W.; Silsbee, M.R. Chloride diffusion in ordinary, blended and alkali-activated cement pastes and its relation to other properties. Cement and Concrete Research, 2000, Vol. 30, p. 1879-1884. 11. ASTM C 1202-97 Standard Test Method for Electrical Indication of Concrete’s Ability to Resist Chloride Ion Penetration, March 1997. 12. Shi, C. Effect of mixing proportions of concrete on its electrical conductivity and rapid chloride permeability test (ASTM C1202 or ASSHTO T277) results. Cement and Concrete Research, 2004, Vol. 34, p. 537-545. 13. Wee, T.H.; Suryavanshi, A.K.; Tin, S.S. Evaluation of rapid chloride permeability test (RCPT) results for concrete containing mineral admixtures. ACI Materials Journal, 2000, No 2, p. 221-232. 14. Papadakis, V.G.; Tsimas, S. Supplementary cementing materials in concrete. Part I: efficiency and design. Cement and Concrete Research, 2002, Vol. 32, p. 1525-1532. 15. Kosior-Kazberuk, M.; Lelusz, M. Determination of the chloride diffusion coefficient in concrete with fly-ash addition in a migration test. Proc. of the 4th International Conference “Concrete and Concrete Structures”, October 1213, 2005, Zilina, Slovakia, p. 58-63. 16. Kosior – Kazberuk, M.; Lelusz, M. Effect of cyclic chloride exposure on the electrical resistivity development in concrete with fly ash. Proc. of the 4th International Scientific Conference “Quality and Reliability in Building Industry”, October 17-19, 2006, Levoca, Slovakia, p. 203-208.
