EVALUATION OF HIGH PERFORMANCE CONCRETE USING ELECTRICAL RESISTIVITY TECHNIQUE

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1 EVALUATION OF HIGH PERFORMANCE CONCRETE USING ELECTRICAL RESISTIVITY TECHNIQUE Darren T.Y. Lim*, Nanyang Technological University, Singapore Da Xu, Nanyang Technological University, Singapore B. Sabet Divsholi, Nanyang Technological University, Singapore Susanto Teng, Nanyang Technological University, Singapore 36th Conference on OUR WORLD IN CONCRETE & STRUCTURES: August 2011, Singapore Article Online Id: The online version of this article can be found at: This article is brought to you with the support of Singapore Concrete Institute All Rights reserved for CI Premier PTE LTD You are not Allowed to re distribute or re sale the article in any format without written approval of CI Premier PTE LTD Visit Our Website for more information

2 36 th Conference on Our World in Concrete & Structures Singapore, August 14-16, 2011 EVALUATION OF HIGH PERFORMANCE CONCRETE USING ELECTRICAL RESISTIVITY TECHNIQUE Darren T.Y. Lim *, B. Sabet Divsholi #, Da Xu # and Susanto Teng # *School of Civil and Environmental Engineering Division of Structures and Mechanics Nanyang Technological University, Singapore Blk N1, Level 1, 50 Nanyang Avenue, Singapore <csteng@ntu.edu.sg> webpage: Keywords: rapid chloride migration test, high performance, concrete, electrical resistivity Abstract. Using the coefficient of chloride diffusivity to evaluate the quality of the concrete in terms of the durability aspects is a commonly used method. With the inclusion of supplementary cementitious materials (SCM) and a lower water/cementitious ratio, the permeability of high performance concrete (HPC) is reduced significantly. Therefore the testing for chloride diffusivity using the rapid chloride migration test (RCMT) becomes an elaborate affair requiring at least 4 to 5 days of testing instead of the usual 24 hours duration or less. At the early stage of construction, a fast and convenient technique to provide quality checks is required. Electrical resistivity technique is fast and convenient, and is proven to be able to establish a relationship with the chloride diffusivity results for lower grades concrete. In this work, the effectiveness of electrical resistivity technique for HPC to establish a relationship with chloride diffusivity in order to evaluate the quality of the concrete is studied. Six mixes of three different grades of concrete with different SCM such as 30% ultra-fine granulated ground blast-furnace slag replacement and 10% undensified silica fume were cast. Compressive strength, RCMT, and electrical resistivity test results are presented. The correlation of the measurements from electrical resistivity and the chloride diffusivity tests were conducted to study the possibility of establishing a relationship between the results. correlation coefficients (R>0.94) for all the mixes were achieved, representing the feasibility of using electrical resistivity test on HPC. The measurements will be used to determine the quality of the concrete and the corrosion rate of the concrete. # Nanyang Technological University, Singapore

3 1 INTRODUCTION Reinforced concrete structures exposed to sea environments primarily suffers from corrosion of steel bars due to chloride ingress. The penetration of chloride in concrete is controlled by parameters such as the type of cement, quality of the concrete and mix proportions. Conducting chloride diffusivity test for concrete quality control during construction is a time consuming and elaborate affair. Carrying out the Rapid Chloride Migration Test (RCMT) according to Nordtest NT BUILD for Performance Concrete (HPC) with the inclusion of Supplementary Cementitious Materials (SCM) requires at least four to five days of testing. It has been reported that concrete resistivity measurements is a positive method to determine the ability of the concrete to resist chloride penetration 2,3. Measurement of concrete resistivity is a non-destructive test which enables fast and convenient in-place evaluation of concrete quality in the field. It is possible to express a relationship between chloride diffusivity and electrical resistivity of a given concrete by establishing the Nernst- Einstein equation suitable for all porous materials expressed as the following equation 2 :..... where D i is diffusivity for ion i; R is gas constant; T is absolute temperature; Z is ionic valence; F is Faraday constant; t i is transfer number of ion i; γ i is the activity coefficient for ion i; c i is concentration of ion i in the pore water; and ρ is electrical resistivity. For a given type of concrete with given moisture and temperature conditions, equation (1) can be rewritten as 2 : (1). (2) where k represents a constant equivalent to the slope of the linear correlation curve between the diffusivity and the electrical conductivity, which is the inverse of the electrical resistivity. Using equation (2), the routine concrete quality control of the specified chloride diffusivity can be conducted indirectly by carrying out regular measurements of the corresponding electrical resistivity of the concrete. The resistivity of concrete varies depending on the moisture content and the concrete composition. The current is carried by ions in the concrete pore solution. Therefore, increased pore saturation decreases the resistivity of the concrete 4. With the inclusion of SCM such as Ground Granulated Blast-furnace Slag (GGBS) and silica fume, the concrete permeability decreases and a denser paste is achieved. Therefore, the resistance of concrete against chloride penetration is increased 5. It was reported that electrical resistivity technique is effective for quality control of concrete grades below 70MPa 2,3. Thus, the effectiveness of using electrical resistivity technique to evaluate concrete quality in the HPC range was studied in this work. With the inclusion of SCM such as Ultra-Fine GGBS (UFGGBS) and Undensified Silica Fume (USF) coupled with a low water/cementitious (w/c) ratio, HPC with improved mechanical and durability properties was produced. Six mixes with three different w/c ratios and different amounts of cementitious materials were studied. Measurements of electrical resistivity of the concrete were conducted and compared with chloride diffusivity results from the corresponding RCMT. Using a direct two-electrode method to measure the electrical resistivity and compared it with the chloride diffusivities tested according to NT BUILD method for a given concrete, a good correlation (R= ) between the chloride diffusivity and electrical resistivity was obtained. 2 EXPERIMENTAL INVESTIGATION The physical properties and oxide compositions of type 1 Portland cement, UFGGBS and USF used in this study are listed in Table 1. To obtain a minimum slump of 150 mm, a polycarboxylatebased high range water reducing agent (HRWRA) was used.

4 Blaine Surface Area (m 2 /kg) BET Surface Area (m2/kg) Particle Mean Diameter (µm) Cement (I) UFGGBS USF Physical properties < 0.1 Density(kg/m 3 ) Oxide compositions (%) SiO Al 2 O Fe 2 O CaO SO MgO Table 1: Physical and chemical characteristic of cement (I), UFGGBS and USF In this work, a total of six mixes were studied. Three different grades of concrete were designed, first with w/c ratio of 0.35 and 450 kg of cementitious materials, the second with w/c = 0.28 and 520 kg of cementitious materials, and the third with a w/c = 0.25 and 580 kg of cementitious materials. Mix design A was set as a standard concrete mix design; Mix design B maintains similar proportions of constituent materials to Mix design A with the 30% UFGGBS replacement. Mix design C represents a higher concrete grade with lower w/c ratio and higher content of cementitious materials. Mix design D maintains similar proportions of constituent materials to Mix design C with the 30% UFGGBS replacement. Mix design E is the highest grade of concrete among the three grades with the lowest w/c ratio and the highest amount of cementitious materials. There is also a 10% replacement with USF in Mix design E. Mix design F is similar to mix design E except for the 30% UFGGBS replacement. The details of the mix proportions are presented in Table 2. Mix A B C D E F Water / cementitious Aggregate / cementitious UFGGBS replacement (%) USF replacement (%) Total cementitious Coarse / fine aggregate HRWRA / cementitious (%) Aggregate by weight ratio Table 2: Mix proportion details

5 Six sets of specimens were cast in six batches according to each mix design to maintain the consistency of the concrete produced. The samples were water cured in lab temperature close to 25 C for 3, 7, 28, 56 and 90 days and removed from the water prior to testing according to BS EN All specimens were tested between 3 and 90 days after casting. The compressive strength tests were conducted on 100mm cubes using a 2000 kn compression machine according to BS EN RCMT and electrical resistivity tests were carried out on 50 mm thick specimens cut from 100 mm diameter cylinder specimens according to Nordtest method NT BUILD The measurements for electrical resistivity were carried out on the same samples before the samples were used for RCMT. In principle, the migration of chloride ions into the concrete specimen is carried out by applying an external electrical potential across the specimen to force the chloride ions outside the specimen to migrate into the specimen. After certain test duration depending on the quality of the concrete, the specimen is axially split. The chloride penetration depth can be measured via a visible silver chloride precipitation formed when silver nitrate is sprayed onto one section of the split specimen. The measurements of electrical resistivity in this study were conducted using the direct twoelectrode method shown in Figure 1. Given a constant voltage, the current passing through the concrete was recorded. The resistance (R) of the concrete was derived using the following equation: (3) where V is voltage, and I is the current passing through the concrete. The resistivity of the concrete was calculated as (4) where is electrical resistivity, R is electrical resistance, A is surface area, and t is the height of the concrete specimen 8. The chloride migration coefficient is plotted against the electrical conductivity to obtain a correlation between the two sets of results to show the effectiveness of using electrical resistivity techniques to provide a basis for performance-based quality control. Steel plate V 50mm thick concrete specimen R Steel plate Figure 1: Two- electrode method Using the direct two-electrode method for measuring electrical resistivity requires access to two sides of the concrete as shown in Figure 1. However the procedure is not always practical for the evaluation of real size structures with limited access. Therefore a more commonly used method is the four-electrode (Wenner) test method shown in Figure 2. The Wenner test involves a low-frequency alternating electrical current passing between the two outer electrodes while the voltage drop between the two inner electrodes is measured. The apparent electrical resistivity observed on the display is based on the following equation 8 : 2 / (5) where a is electrode spacing, V is voltage drop, and I is the current. This equation is valid for a homogenous semi finite volume of the material.

6 3 TEST RESULTS AND ANALYSIS 3.1 Compressive strength Figure 2: Wenner test equipment Compression test results of cube specimens tested on 3, 7, 28, 56 and 90 days were compiled and plotted in Figure 3. Generally, the specimens containing 30% UFGGBS from Mixes B, D and F achieved higher compressive strength from 3 days onwards compared to their companion mixes without UFGGBS. The inclusion of UFGGBS and USF leads to a higher rate of hydration and pozzolanic reaction, and it also leads to better filling of the interfacial transition zone pores with subsequent improvement in the strength of the concrete. Compressive Strength (MPa) Mix C Mix E Mix A Mix F Mix D Mix B Days Figure 3: Concrete strength (MPa) 3.2 Chloride migration coefficient The chloride migration coefficient can be calculated via the following equation as discussed in NT BUILD D nssm = ) (6) where: D nssm : non-steady-state migration coefficient, x m 2 /s; U : absolute value of the applied voltage, V; T : average value of the initial and final temperatures in the anolyte solution, o C; L : thickness of the specimen, mm; x d : average value of the penetration depths, mm; t : test duration, hour.

7 The computed coefficients are presented in Table 3. It is clear that concrete samples from Mix B, Mix D and Mix F have lower permeability, leading to smaller chloride migration coefficient. This lower concrete permeability is a result of the UFGGBS and USF filling up the pores, restricting the mechanism of chloride transport. As the concrete curing age increases, due to more complete hydration, the permeability of the concrete decreases and the chloride migration coefficient becomes smaller. The duration of the actual migration test is presented in Table 3. Generally, it can be seen that as the quality of the concrete improves, the duration of the test increases. As shown in Table 3, for Mix F at 90 days curing, even after applying a voltage of 60 V for duration of 7 days, the penetration depth was only around 0.15mm. For 28 and 56 days curing, the penetration depth was only around 2-3mm. The measurement error for these small values can be significant. Thus the current arrangement of RCMT setup is reaching its limit for determining the concrete quality for high quality HPC. Figure 4 shows the test setup of RCMT used in this study. Mix A B C D E F Day of testing Initial Current (ma) Voltage (V) Duration of test (hrs) Penetration depth (mm) Chloride diffusivity coefficient < 0.01 Table 3: Details of RCMT and chloride diffusivity coefficient

8 T.Y..D. Lim, B. Sabet Divsholi, D. Xu and S. Teng Figure 4: Test setup of rapid chloride migration test From the RCMT results, it is possible to classify the resistance of the concrete against chloride penetration according to Table 4 9. With reference to Table 4 9, the classification of the concrete samples at different concrete age is listed in Table 5. The resistance of the concrete against chloride penetration increases with the curing age. From the results, it can be shown that for a higher grade concrete, the inclusion of the UFGGBS makes a significant impact on the resistance of the concrete even at an early stage of the hardened concrete. The classification in Table 5 does not reflect significant improvement of the concrete quality with age for Mix E and F as all the values falls in the extremely high category. Chloride diffusivity, D 28 x > < m 2 /s Resistance to chloride penetration Low Moderatee Very highh high Table 4: Resistance to chloride penetration of various types of concrete based on the 28-day chloride diffusivity 9 Day Mix A Mix B 3 Low 28 Moderate Very Mix C Mix D Mix E Moderate Very Very Mix F Table 5: Classification of the concrete for resistance to chloride penetration 3.3 Concrete resistivity Concrete resistivity describess the ability of concrete to oppose the movement of currents through its medium. The higher the electrical resistivity, the more difficult it is for ions to travel through the concrete, thus inhibiting corrosion 2. The pores in the mixes with SCM are generally discontinuous due to the denser microstructure produced. The physical densification of the microstructure and the pozzolanic reaction leading to the formation of secondary CSH could lead to an increase in resistivity 10. Figure 5 presents the measurements of the electrical resistivity of the concrete from each mix. The correlation coefficient between the chloride diffusivity coefficient and the electrical resistivity were obtained and listed in Table 6. The correlation coefficients range generally from , representing the ability to establish a meaningful relationship between the two sets of measurements.

9 1000 Electrical resistivity (kω cm) Mix F Mix E Mix D Mix B Mix A Mix C Day Figure 5: Electrical resistivity of concrete (KΩ cm) Mix Day of testing Electrical Resistivity (KΩ cm) Coefficient of Diffusivity R-squared A B C D E F Table 6: Electrical resistivity of concrete

10 Table 7 11 shows the relationship between concrete resistivity and the corrosion rate of the concrete according to Wenner probe method. Therefore, it is possible to understand the quality of the concrete in terms of the corrosion rate based on the results in Table 6. However Table 7 11, which is based on Wenner probe method, has only four different categories of classifications. As shown in Table 6, the measurement of resistivity using the direct two-electrode method can vary from 5 KΩ cm to 450 KΩcm. Therefore a new set of ranges is proposed in Table 8 for direct measurements of electrical resistivity based on the results presented in Table 6 and the recommendations in Table 4 and 7. Table 9 shows the classification of the mixes based on the new classification presented in Table 8. Resistivity (KΩ cm) Corrosion rate > 20 Low Low to Moderate 5-10 < 5 Very high Table 7: Relationship between concrete resistivity and corrosion rate 11 Direct electrical resistivity measurement RCMT Resistance to chloride penetration (KΩ cm) (From Table 6) < 5 > 15 Low Moderate Very > 40 < 1 Table 8: Recommended classification of concrete based on resistivity Day Mix A Mix B Mix C Mix D Mix E Mix F 3 Moderate Moderate Moderate Very Very 28 Moderate Moderate 56 Moderate 90 Moderate Table 9: Classification of the concrete in terms of corrosion rate The measurements of electrical resistivity using the Wenner method can be correlated against the direct two-electrode method. This will help to improve the classification of Wenner probe measurements for high quality HPC. The current classification in Table 7 simply classifies any concrete with the resistance above 20 KΩ cm as having low corrosion rate. 4 CONCLUSION 1. It is possible to use the measurements from the electrical resistivity technique to establish a relationship with the coefficient of diffusivity to conduct performance-based quality control of the HPC. 2. The correlation coefficient between chloride diffusivity coefficient and concrete resistivity for different grades of concrete is consistently in the range between , representing the suitability of using electrical resistivity technique to evaluate the quality of high performance concrete. ACKNOWLEDGEMENT The authors would like to acknowledge the research funding provided by National Research Foundation of Singapore (NRF) under CRP funded Project Underwater Infrastructure and

11 Underwater City of the Future. Ultra-fine GGBS was provided by EnGro Corporation Limited and the superplasticizer was provided by BASF South East Asia Pte Ltd. REFERENCES [1] NT BUILD 492, Concrete, Mortar and Cement-Based Repair Materials: Chloride Migration Coefficient From Non-Steady-State Migration Experiments. Nordtest Method, [2] Sengul, O. and Gjørv, O.E., Electrical Resistivity Measurements for Quality Control During Concrete Construction, ACI Materials Journal, Vol. 105, No. 6, 2008, pp [3] Smith, K,M., Schokker, A.J. and Tikalsky, P.J., Performance of Supplementary Cementitious Materials in Concrete Resistivity and Corrosion Monitoring Evaluations, ACI Materials Journal, Vol. 101, No. 5, 2004, pp [4] Polder, R.B. and Peelen, W.H.A, Characterisation of Chloride Transport and Reinforcement Corrosion in Concrete Under Cyclic Wetting and Drying by Electrical Resistivity, Cement and Concrete Compositions, Vol. 24, No. 5, 2002, pp [5] Geiseler, J., Kollo, H. and Lang, E., Influence of Blast Furnace Cement on Durability of Concrete Structure, ACI Materials Journal, Vol. 92, No. 3, 1995, pp [6] BS EN : Making and curing specimens for strength tests. [7] BS EN : Testing hardened concrete. Compressive strength of test specimens. [8] Gjørv, O.E., Durability Design of Concrete Structures in Severe Environments, Taylor & Francis, London and New York, 2009, pp [9] Nilsson, L., Ngo, M.H. and Gjørv, O.E., -performance Repair Materials for Concrete Structures in the Port of Gothenburg, Second International Conference on Concrete Under Severe Conditions: Environment and Loading, Vol. 2, 1998 pp [10] Elahi, A., Basheer, P.A.M., Nanukuttan, S.V. and Khan, Q.U.Z., Mechanical and Durability Properties of Performance Concretes Containing Supplementary Cementitious Materials, Construction and Building Materials, 2010, pp [11] ACI Committee 222, Protection of Metals in Concrete Against Corrosion, ACI 222R-01, 2001, pp. 41.