Experimental investigation of concrete curing and strength estimation using piezoelectric admittance measurements

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1 9 th European Workshop on Structural Health Monitoring July 10-13, 2018, Manchester, United Kingdom Experimental investigation of concrete curing and strength estimation using piezoelectric admittance measurements Zhou Sujie 1, Jun Young Jeon 1, Hwee-Kwon Jung 1, Gyuhae Park 1, Cheol Won 2 More info about this article: Abstract 1 Active Structures and Dynamics Laboratory, School of Mechanical Engineering, Chonnam National University, Gwangju, South Korea, cyrano9671@naver.com 2 Platform Technology Research Team, GS E&C Research Institute This paper presents a new technique for monitoring the concrete curing process using embedded piezo electric transducers via admittance measurements. This technique is based on an admittance-based sensor diagnostic process, in which the capacitive values of piezoelectric transducers are dependent on the property of a host structure. To demonstrate the performance of the proposed technique, a series of experimental investigation was carried out with concrete specimens of different compressive strengths (18, 30, 50Mpa). Embedded piezoelectric transducers were used to continuously measure the electrical admittance throughout the curing process. The results demonstrate that there is a clear relationship between the concrete curing status and the capacitive values of piezoelectric transducers, which indicates that the proposed method could be efficiently used for monitoring the curing status of a concrete structure. (gpark@chonnam.ac.kr). 1. Introduction Until today, concrete is one of the most widely used construction materials in the civil engineering industry. Concrete is an excellent material that has been effectively used for construction, especially for large infrastructures (nuclear power plant, dam, highway). At the early-ages of a concrete structure, real-time monitoring of the curing process is very important since it enables a time/cost-efficient construction planning which includes the determination of structural readiness for service and of the proper time to remove shoring or to apply post-tensioning forces. Furthermore, it ensures the safety of both personnel and a structure during the construction work. Collapse of several structures during construction highlights the importance of a concrete curing monitoring. However, it is somewhat challenging to estimate the strength of a concrete structure in real-time because it depends on various factors, including quality of materials supplied to the site, construction technique, and workmanship. Conventional non-destructive methods based on acoustical, electrical, and mechanical properties of the concrete materials have been used to monitor the concrete strength development. However, these methods have some limitations for real world applications because of low accuracy and high cost. In recent years, piezoelectric materials have been applied to monitor concrete curing process through either electro-mechanical impedance (EMI) based methods or vibration-characteristic based methods. In the previous works, Soh and Balla (2005) Creative Commons CC-BY-NC licence

2 suggested concrete strength prediction using the EMI techniques. Shin et al (2007; 2009) carried out an experimental study on monitoring the strength gain using the EMI technique. Lee et al (2010) investigated the variation of the EMI of the PZT patch embedded in the cement paste during the settling process. Park et. al (2013) performed concrete strength monitoring using EMI technique with the wireless sensor node. These studies usually monitor the variation of resonant peaks of EMI measured from a concrete structure during curing process, which may contain some limitations. There are multiple peaks exist in the EMI measurements and it is not clear which resonant peak would be most efficient for curing monitoring. The effects of existing structural damage must be considered when analysing the results. In addition, it is not straightforward to estimate the final status of concrete curing for service. This paper presents a concrete curing monitoring method based on piezoelectric admittance measurements. The basis of this procedure is to track the variation of capacitive value of piezoelectric materials, which shows in the imaginary part of the measured admittances (Park et al., 2006). A series of experimental investigation is implemented to verify the performance of the proposed technique. For experiment, concrete specimens with different compressive strength (18, 30, 50Mpa) are used. Piezoelectric transducers are embedded into each concrete specimen and are used to continuously measure the admittance throughout the curing process. 2. Principles of impedance based concrete curing monitoring The concrete curing monitoring technique is based on the sensor diagnostic process which utilizes the imaginary part of impedance (admittance) in the piezoelectric transducer developed by Park et al (2006). When a PZT patch is surface-bonded to a host structure, The electrical admittance, Y(ω) of the PZT patch is a combined function of the mechanical impedance of host structure and that of the PZT patch, given by ; (1) where ω is the angular frequency, δ is the dielectric loss tangent of piezoelectric material. is the width, length, and the thickness of a PZT patch, is the complex Young s modulus of the PZT material at zero electric field, respectively. A significant observation that can be made from (1) is that one can identify the effect of sensor condition by the measured electrical admittance. The effect of the adhesive layer is obtained by letting be in Eq.(1). (2) = i = - It is clear from (2) that the electrical admittance of a PZT patch would be different if it is under a free-free condition or surface-bonded condition. In this study, this principle was used for monitoring concrete curing. As shown in Eq.(1), the imaginary electrical 2

3 admittance Y(ω) could be represented as a function of structural impedance,. As the concrete curing progresses, the structural impedance increases and it will be manifested as downward shifts of the slope in the electrical admittance of the PZT patch. In other words, the shifts of the slope are related to the change in concrete strength. As the strength of concrete increases, there is a corresponding decrease in the slope, and this feature is used for monitoring the concrete strength. 3. Experimental Setup An initial experiment is conducted on concrete specimens to demonstrate the applicability of the proposed technique. In this section, specimen preparation, test setup, procedures of experiments, including frequency ranges for admittance measurements, are introduced Specimen preparations As shown in Fig. 1, cylindrical standard concrete specimens are fabricated with different strengths (18Mpa, 30Mpa, 50Mpa). Details of the specimens and curing conditions are shown in table 1. For each strength, 18 cylindrical specimens (54 in total) with a diameter of 100 mm and a height of 200 mm are used for compressive strength test. All specimens are stored in the specific chamber, in which the temperature and moisture are controlled during the test. Table 1. Concrete mix proportions Cement ( ) Water ( ) w/c fa/c ca/c A (18 Mpa) B (30 Mpa) C (50 Mpa) * fa : fine aggregate, ca: coarse aggregate, c: cement, w:water Fig 1. concrete specimen 3

4 3.2. Experiment set up and procedure In order to measure electrical admittance during the curing process, a PZT patch (APC 850-copper plate, disk type) bonded to a thin cooper plate is embedded into center of the specimens. Fig. 2 shows the embedded PZT patch. The PZT patches are covered with epoxy (JB weld) to protect them from water. The experimental set up used for electrical admittance acquisitions consists of an impedance circuit and data acquisition system (USB-6366) as shown in Fig. 3 (Pearis et al. 2004) Concrete specimen Epoxy (JB weld) coating Copper plate 10 0 m m PZT patch Fig.2 PZT patch setup Compressive strength tests are taken at 1, 3, 7, 14 and 28 days. Three specimens for each strength group are tested to evaluate the compressive strength. After the 28 days, the compressive strengths for each specimen group (A, B, C) are identified as MPa, MPa and MPa, respectively, by a conventional universal testing unit. Electrical admittances are measured in the frequency range of khz. Specimen PC DAQ Fig.3 Impedance approximating circuit and Experiment set up In order to quantitatively evaluate the slope variation ratio, the variation ratio is defined as, 4

5 (3) where (ω) is the slope of the electrical admittance under free-free condition, Y(ω) represents the difference in the slope of electrical admittance between the freefree and the newly measured electrical admittance. To validate the proposed technique, the results of the compressive strength tests and the variation ratios are compared. 3. Experimental results and discussions 3.1 Signal trend Fig. 4 shows the comparison of the measured admittances of the PZT patch in two difference conditions, free-free and installed in the concrete specimen. The slope of the admittance is decreased by 6.7 %, when the patch is installed in the specimen, which is caused by the effect described in equation (1). During concrete curing monitoring process in specimen A (18 MPa), the slope of the admittance value continuously decreases as shown in Fig. 5. The reason is that the mechanical impedance of the concrete specimen increases during the curing process, which confirms the applicability of the proposed technique. The decrease in the slope could be used as a key feature to monitor the strength of the concrete. After 24 hours of curing, the slope is decreased by 16.4% which is the largest change, and after 7 days, it was observed 18.3 % decreased in the slope. After 28 days, the final decrease in the slope was identified as 21.8 %. This technique could provide several advantages compared to the existing methods based on EMI techniques. The proposed technique does not require the identification of structural resonances. In addition, the selection of the frequency range is straightforward because this method is efficient in the low-frequency range (< 50 khz) Another great advantage of the proposed technique is that this method could provide a quantitative information on the concrete strength, as described in the following section. Fig.4 Imaginary part of admittance under free-free condition and measured data at 0 hours Fig.5 Difference of admittance through each time progresses 3.2 Compressive strength and monitoring result 5

6 Fig. 6 shows the variations of the admittance slope during the entire curing status of 28 days of three different concrete specimens of 15, 30, 50 MPa. Fig. 7 shows the average compressive strength virus curing days measured by a conventional universal testing unit(utm). In figure 6, it is observed that, as the curing progresses, the compressive strength becomes higher and there is a corresponding downshift change in the measured slope. The specimen with 50 MPa produce over 31% change in the variation ratio, while there is a change of 28%, 22% in 30 and 18 MPa compressive strength after 28 days. This method clearly identifies the strength of the concrete specimens. Another comparison of Fig. 6 and 7 reveals that, in the early days (< 7 day), the admittance measurement show much higher changes compared to the compressive strength changes. However, after 15 days, the change becomes smaller that the changes in compressive strength measurement. This could be caused by the ratio between and in equation (1). The optimal size of PZT could improve the performance of monitoring the curing process in this regard. Anyhow, the proposed technique could be efficiently used for monitoring the curing process and could quantitatively monitor the strength of a concrete structures. The selection of the optimal size of piezo-patches and the calibration process for better estimation of the concrete strength is needed, and they remained as our future effort. Fig.6 The variation ratio each time progresses Fig.7 Average Compressive Strength versus curing age 4. Conclusions This paper presents the technique for concrete curing monitoring and assessment of the strength. This technique is based on the admittance-based sensor diagnostic process, in which the capacitive values of piezoelectric transducers are dependent on the strength of a host structure. To demonstrate the performance of the technique, several experiments are performed. The electrical admittance in difference strength concrete specimen is measured during 28 days of the curing process. Based on the experimental results, the compressive strength and the variation of admittance slope show the similar trend during the curing process. This indicates that the proposed technique could be efficiently used for monitoring the curing status of a concrete structure without the need of identifying structural resonances or optimal frequency ranges. 6

7 Acknowledgements This research is supported by the Basic Science Research Program (NRF- 2015R1D1A1A ) through the National Research Foundation of Korea (NRF). References and footnotes 1. J. F. Lamond and J. H. Pielert, Eds.,"Significance of Tests and Properties of Concrete and Concrete-Making Materials," ASTM International, West Conshohocken, PA, (2006): ASTM Test Method for Time of Setting of Hydraulic Cements by Vicat Needle (C 191),Annual Book of ASTM Standards, Vol ,ASTM International, West Conshohocken, PA, (2002): ASTM Test Method for Time of Setting of Concrete Mixtures by Penetration Resistance(C 403/C 403M), Annual Book of ASTM Standards, Vol , ASTM International, West Conshohocken, PA, (2003): C K Soh, S Bhalla Calibration of piezo-impedance transducers for strength prediction and damage assessment of concrete Smart Materials and Structures, Vol. 14, No. 5, (2005): S. W. Shin, A. R. Qureshi, J. Y. Lee and C. B. Yun, "Piezoelectric sensor based nondestructive active monitoring of strength gain in concrete," Smart Materials and Structures, Vol. 17, No. 5, (2008) 6. H. K. Lee, K. M. Lee, Y. H. Kim, H. Yimand D. B. Bae, "Ultrasonic in-situ monitoring of setting process of high-performance concrete," Cement and Concrete Research,, (2004) 34(4): S. D. Bahador and Y. Yaowen, "Monitoring hydration of concrete with piezoelectric transducers," 35th Conference on Our Worldin Concrete & Structures, Singapore (2010) 8. S. Park, S. Ahmad, C. B. Yun and Y. Roh, "Multiple crack detection of concrete structures using impedance-based structural health monitoring techniques," Experimental Mechanics,Vol. 46, Issue 5, (2006): G. Park, H. Sohn, C. R. Farrar and D. J.Inman, "Overview of piezoelectric impedance based health monitoring and path forward,"the Shock and Vibration Digest, Vol. 35, No. 6, (2003): G. Park, C. R. Farrar, A. C. Rutherford and A. N. Robertson, "Piezoelectric active sensor self-diagnostics using electrical admittance measurements," Journal of Vibration and Acoustics, Vol. 128, Issue. 4, (2006): N. Zagrai and V. Giurgiutiu, "Electromechanical impedance method for crack detection in thin plates," Journal of Intelligent Material Systems and Structures, Vol. 12, No. 10, (2001): R. Dugnani, "Dynamic behavior of structure mounted disk-shape piezoelectric sensors including the adhesive layer," Journal of Intelligent Material Systems and Structures,Vol. 20, No. 13, (2009):

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