Crack width monitoring system for reinforced concrete ...

Crack width monitoring system for reinforced concrete beams using piezoceramic sensors D. Hughi & H. Marzouk

Journal of Civil Structural Health Monitoring ISSN 2190-5452 J Civil Struct Health Monit DOI 10.1007/s13349-014-0099-y

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Author's personal copy J Civil Struct Health Monit DOI 10.1007/s13349-014-0099-y

ORIGINAL PAPER

Crack width monitoring system for reinforced concrete beams using piezo-ceramic sensors D. Hughi • H. Marzouk

Received: 9 April 2014 / Accepted: 2 December 2014 Ó Springer-Verlag Berlin Heidelberg 2014

Abstract The following study investigates the development and implementation of a crack width measurement system through the use of piezo-ceramic sensors as part of an active non-destructive health monitoring system for reinforced concrete important structures. As the world demand for economic and sustainable energy rises, the demand for important structures such as long-span bridges, nuclear power containment and offshore platform development has also dramatically increased. These multibillion dollar state-of-the-art structures require the most reliable and durable monitoring system to ensure their most economic yet prudent operation. The proposed system uses permanently embedded low-cost sensors to actively assess the health conditions of concrete beams throughout their lifetime span. In the current investigation, a series of concrete beam specimens have been tested in the lab to examine the adequacy and accuracy of low-cost piezoceramic sensors under a bulk wave system. The estimated crack widths from the results of the proposed system instrument measurements agree very well with the crack gauge records from the theoretical estimates and tested beams. The test results confirm the calibration and validity of the proposed system. The proposed system can provide the means of establishing an active structural health monitoring system that not only provides details on the extent of damage, but also its location within a reinforced concrete member.

D. Hughi H. Marzouk (&) Department of Civil Engineering, Ryerson University, Toronto, ON, Canada e-mail: [email protected]

Keywords Concrete structures Crack detection Structural health monitoring Crack with piezo-ceramic sensors Wave propagation method

1 Introduction Over the last few years, a large number of multibillion dollar long-span bridges, offshore platforms and nuclear power containment structures have been developed in response to the increasing worldwide demand for economic and sustainable energy. The development of such important megastructures sets the standards for safety and extended lifespans (between 50 and 100 years) and requires the highest level of structural health monitoring (SHM) based on reliable and durable sensors. The current study focuses on the development of an effective, yet inexpensive SHM system that measures active crack widths for concrete beams that assist in the integrity of the structural element using a technique that is different than traditional non-destructive techniques. The proposed system uses low-cost embedded piezo-ceramic sensors, to provide real-time assessments of concrete members, and establish the means for preventive health monitoring. This investigation also studies the implementation of these piezo-sensors as permanent, continuous recording instruments that are useful for SHM. Applications of embedded piezo-ceramic sensors in the field of non-destructive testing of concrete members have shown great potential in recent years. Although concrete is a difficult material to work with when subjected to ultrasonic wave signals, successful applications of embedded piezoceramic sensors have emerged in the areas of early-age strength monitoring including: de-bond detection [1, 2], concrete property diagnostics [3], vibration characteristicbased health monitoring [4], and impedance-based health

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Author's personal copy J Civil Struct Health Monit

monitoring [5]. The ease of implementation and quick response characteristic of piezo-ceramics make them ideal for use as part of an active health monitoring system for concrete structures. The present study presents the development and implementation of such a system, along with the results obtained from both of small-scale testing reported in the research work. The interaction between the piezo-ceramic sensor patches and the host structure is recently published for sensor design and data explanation [6].

The amplitudes of the reflected and refracted waves are given by Eqs. (5) and (6), respectively, where Z2 is the acoustic impedance of the material transmitted by the stress wave, and Z1 is the acoustic impedance of the material in which the stress wave is generated. At the concrete/air interface, Z1 represents the concrete impedance and Z2 is the acoustic impedance of air. As the acoustic impedance of air is much smaller than that of concrete, the majority of the induced wave is reflected as a crack which initiates through the signal’s path.

2 Wave propagation

AReflected ¼ Ai

Z2 Z1 Z2 þ Z1

ð5Þ

The principal of waves is most commonly used in the nondestructive testing of steel members and automotive components. Waves are generated using a high-frequency transducer that emits frequency swept harmonic excitations into the test medium. The utilizing of sweep frequency helps one to examine the true vibration characteristic of a medium or ultrasonic signature. The principals of wave propagation are very useful for the analysis of ultrasonic signals. Piezo-ceramic patches can be used to induce stress waves into a test specimen. The stress waves are then analyzed based on frequency, amplitude, and velocity so as to understand the internal conditions of a material. The speed of any wave is given as a function of the wave’s frequency and wavelength. Equation (1) shows that the speed of the wave (C) is equal to the wavelength (k), multiplied by the frequency of the wave (f).

ARefracted ¼ Ai

2Z2 : Z2 þ Z1

ð6Þ

C ¼ k f:

ð1Þ

The material that the wave travels through governs the velocity of a stress wave. The velocity of a shear wave can be determined using Eq. (2). The velocity is proportional to the modulus of elasticity, Poisson’s ratio, and density of the material. This means that the shear wave velocity between two materials is always going to differ. E is the modulus of elasticity, q is the density, and m is the Poisson’s ratio of the material. sffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi E Cs ¼ : ð2Þ 2ð1 þ mÞq Similarly, the pressure wave velocity is calculated based on Eq. (3) sffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi Eð1 mÞ Cp ¼ : ð3Þ ð1 þ mÞð1 2mÞq The velocity of the Rayleigh wave is a function of the shear wave velocity and computed by using Eq. (4). Cr ¼

0:87 þ 1:12m Cs : 1þm

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ð4Þ

3 Embedded piezo-ceramic sensor methodology The proposed piezo-ceramic sensor system utilizes harmonic waves as the basis of its operating principal. The harmonic stress waves are generated at an arbitrary position within the concrete member, and transmitted through the concrete matrix before a secondary piezo-ceramic patch receives them. Since the stress waves are induced by one piezo-ceramic patch (actuator), and received by another piezo-ceramic patch (receiver), the information gathered by each pair would be representative of a straight-line position between the two. The initiation of a crack through a signal path would cause a break in the transmissibility and reduce the signal strength. This reduction in signal strength is directly proportional to the amount of damage detected and forms the basis of the ultrasonic sensing system. Normal strength concrete is found to have an attenuation constant at a rate of 2.80 dB/cm [7]. This means that if the signal is not strong enough, the energy of any stress wave can be quickly dissipated within the concrete matrix, and becomes useless for analysis. Piezo-ceramic patches are very useful, in this regard, as they have the ability to induce stress waves at very high amplitudes and help to eliminate the problems associated with this high attenuation constant. However, the same property that impedes the energy transmission of stress waves in concrete also helps to eliminate any noise that may be experienced in the transmission of waves. The introduction of a wave in a medium does not necessarily follow a linear path, but rather travels spherically. When a wave comes into contact with a concrete/air interface, or the surface of a concrete member, multiple reflections are created due to the large difference in impedance, which is called noise. At attenuation equal to 2.80 dB/cm, the signal’s power is halved for every


centimeter that it travels through the medium. Therefore, the signal that is being reflected off concrete–air interfaces is shadowed by the direct path signal between two piezotransducers, and becomes negligible. sffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi 1 X ðAfn;0 Afn;t Þ2 CWI ¼ : ð7Þ ðAfn;0 Þ2 n¼0 As concrete is an elastic–plastic material, and comprises a complex matrix of granular materials, that means that there is not one specific ultrasonic frequency that would dominate the excitation of this material. To effectively determine the material transmissibility, and be able to evaluate the extent of damage on a signal path between two sensors, a wide band of frequencies needs to be implemented, and thus a frequency sweep was employed. The amplitudes of the received waves were recorded and analyzed using Eq. (7). The equation is a normalization of the commonly accepted root-mean-square deviation equation used for spectral analysis, and has been previously adopted for concrete [8]. A crack width index (CWI) is a function of the difference between the amplitude of frequency ‘n’ prior to the load being applied (Afn,0), and the amplitude of frequency ‘n’ at a given time during the lifetime of a member (Afn,t). The results obtained by CWI are indicative of the vibration characteristic of a section, or its transmissibility, and thus representative of the conditions of a section. The proposed system uses three distinct steps to complete a reading. The desired frequency sweep is generated using an auxiliary function generator, and passed through a custom-made high-output power amplifier before transmitted to a piezo-sensor and translated into a mechanical wave. The stress wave passes through the concrete matrix by generating a frequency spectrum representative of the vibration characteristic of the section. This spectrum is received by a secondary piezo-sensor and translated back into an electrical signal that is sent to a digital oscilloscope where it can be processed to determine the desired CWI values for that specific section. The signal flow chart is illustrated in Fig. 1. The system comprised specially designed high durability piezo-sensors, strategically placed within the concrete matrix of a specific member. The waveform is continuous sweep between 20 and 120 kHz and the peak-to-peak voltage is at 50 V. A wave signal interrogating is provided in Fig. 2. All the sensors were manufactured by the first author. The 27-mm-diameter piezoelectric component was bought from an electronic store, was treated with doublelayer epoxy, and wiring and encasement were attached in our lab. The sensors are attached to the concrete reinforcement by directly using zip ties on the main reinforcement bars placed onto the reinforced concrete member. The piezo-sensor array within the member is wire

connected to an external data acquisition and processing system.

4 Crack width model In previous research work by the second author, a numerical algorithm was developed using a theoretical concrete crack width approach [9] to calculate the crack width for reinforced concrete beams and plates subjected to in-plane bending and tensile forces. This theoretical model is capable for calculating the crack width of reinforced concrete members. This model enables the estimation of the variation in crack width at any section along the crack length. As well, a crack width model is developed that considers the significant effect of the reinforcement distribution, concrete cover, and the level of the strain of the reinforcement. The model is based on equilibrium equations and strain compatibility. The proposed model takes into consideration the effect of bar reinforcement distribution, the concrete cover, and the level of the reinforcement strain. Based on the analytical investigation presented in [9], the following simple design equation is used to evaluate the crack width for concrete beams: wx ¼ 0:17nx a

Cc Sb esm ; db

ð8Þ

where Cc is the concrete cover measured from the concrete surface to the closest bar surface, Sb is the longitudinal bar spacing, db is the bar diameter, a is the ratio of Cc over thickness heff, esm is the mean steel strain, nx is the average increase in strain of reinforcement relative to the adjacent concrete, and wx is the mean or average crack width. The verification of this simple expression is based on comparative studies between the experimental and the theoretical results [9]. The suggested numerical algorithm presents a valuable analytical tool for research and design applications related to the cracking response of thick reinforced concrete panels. The mathematical expression was used to estimate the crack width for all of the tested beams at different loadings.

5 Experimental setup The following experimental program was carried out to evaluate the effectiveness of embedded piezo-sensors in the SHM of reinforced concrete beams during their lifetime. The work consisted of reinforced concrete beams designed in accordance with the Canadian building code CSA-A.23 [10]. Two sets of reinforced concrete beams were tested for the application of piezo-sensors. Four reinforced concrete beams were designed and casted in

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Author's personal copy J Civil Struct Health Monit Fig. 1 Signal flowchart

Fig. 2 A bulk wave signal interrogating graph

Ryerson structural lab for the first set. The four beams were equipped with (50 mm) displacement crack gauges and piezo-sensors to establish the effectiveness of the proposed piezo-sensors toward early crack detection and crack width estimation. The four beams carry the designation ‘‘HS-SM’’, and used for the calibration crack width measurements. However, the second set consisted of two full-scale beams 4 m span and 0.7 m depth as reported by the first author in his thesis [11] and the test results’ details will be published later. The current paper is focused only on the results of the four beams that were used to calibrate the concrete strains and to develop the crack width expression and the crack width index (CWI). 5.1 Concrete beams (HS-S-M) The effectiveness of the proposed SHM system was established using the mentioned four beams. These beams were designed to force the initiation of cracks directly

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through the signal path generated by two embedded piezosensors. The combination of a three-point load setup, and a notch cut into the tension face of the beam across its centerline would facilitate the initiation of cracking. The reinforced concrete design was carried out in accordance with the provisions of the Canadian code [10] to resist a bending moment of 56 kN m and a cracking moment of 16 kN m at a maximum shear capacity that exceeds 150 kN. The schematic for the reinforcement details is presented in Fig. 3. The other three specimens were identical in reinforcement and all the specimens were loaded in increments of 10 kN up to the failure load. Each concrete beam was equipped with a single pair of piezo-sensors and a pair of crack gauges. The crack gauges were attached over the expected cracking region to monitor the initiation of cracks in the member and measure their width. The piezo-sensors were placed at a distance of 300 mm from each support, thus creating a distance of 200 mm in separation between the pair. For consistency,

Author's personal copy J Civil Struct Health Monit Fig. 3 a Piezo-ceramic sensor. b Sensors attached to steel reinforcement

Fig. 4 Dimension and reinforcement detail for a typical tested beam

all the specimens were arranged and equipped using the same sensor spatial position and crack gauge location. The spatial position of the piezo-sensors and casting of the concrete beams are presented in Figs. 4, 5, 6 and 7. The location of the crack gauge is glued to mid span at crack location as shown in Fig. 8b.

6 Experimental results The test results obtained from the experimental investigation are presented herein. The load deflection curves of the four tested beams are shown in Fig. 9. The concrete beams were used to establish a benchmark for the CWI values so as to determine if a section is cracked, and develop a relationship between the experimentally determined CWI and crack width measured values. The developed equation for the calibration of CWI analyzed crack width was then applied to the analysis of all the members, and compared to the results obtained from the numerical algorithm (8) and measured crack widths provided.

Fig. 5 Spatial position of piezo-sensor in a typical tested beam

6.1 Concrete beam test results The results of the concrete beam testing comprised three sets of data. The data obtained from the crack gauges are represented by a load–strain graph (Fig. 10a); by the piezosensors, a load–CWI graph (Fig. 10b); and from visual observation of the cracking behavior of the beams (Table 1). The information gathered from the mechanical crack gauges and the piezo-ceramic sensors allows for a relationship to be drawn between the strain and CWI

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Author's personal copy J Civil Struct Health Monit Fig. 6 Placement of steel reinforcement before casting

Fig. 7 Placement of piezo-sensors in concrete form and notch location

Fig. 8 a Test setup; b three-point load test setup and crack gauge location

values. Table 1 shows the maximum load, failure deflection, visually observed cracking loads, and the load that corresponds to the initiation of cracking for each beam. The four concrete beams exhibited very similar behavior, and had consistent results for the failure loads and deflection. Based on the results obtained from the four tested concrete beams, two limits were set using CWI. The first limit represents the initiation of cracking within a

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member, and the second limit, the visual observation of cracking on the exterior of the member. The initiation of cracking was established once the concrete had exceeded a maximum tensile value of 110 l-strain. The recorded cracking load for each beam is presented in Table 1 as ‘‘Actual Cracking Load’’. The respective CWI values are presented for each case as determined from the load–CWI graph in (Fig. 10b). Based on the data presented in Table 1,

Author's personal copy J Civil Struct Health Monit Fig. 9 Load deflection curves for the four tested beam

it can be concluded that a CWI value which exceeds 0.26 (experimental observation) is strongly indicative of a section that initiated cracking, whereas a CWI value which exceeds 0.48 (experimental observation) is a strong indication that the crack will be visible on the exterior face of the reinforced concrete member. The relationships obtained from each of the four beams are plotted in Fig. 10c, so as to illustrate the trend between the CWI values and concrete strain. The four specimens provide similar results, and show a clear logarithmic trend for the CWI–concrete strain relationship. Among the four tested beams, HS-S-M-2, HS-S-M-3, and HS-S-M-4 demonstrate very consistent results up to a concrete strain of 2,500 l-strain. The HS-S-M-1 beam is found to follow the same trend as the other three beams up to 2,000 l-strain. The average relationship between the CWI values and concrete strain is also shown in Fig. 10c. The results of the four specimens were averaged so as to give one relationship for the CWI values and concrete strain which would summarize the results of the monotonically loaded members. The average CWI–strain relationship could then be analyzed, and used for the development of a model for CWI analyzed crack widths. Due to a slight variation noticed between the cracked and un-cracked specimen behavior, the model is separated into two distinct sections. This is achieved by limiting the values of strain to a maximum reading of 110 l-strain. Equation (9)

approximates the average concrete strain between the two piezo-sensors for both the pre- and post-cracking stages of the concrete section. 0:1217 lnðeÞ 0:2664; 0 e 110 CWI ¼ ð9Þ 0:1335 lnðeÞ 0:287; 110 e 3;500 Equation (9) can be converted into crack width measurements determined through the use of CWI using the effective distance between the two piezo-ceramic sensors to obtain Eq. (10). CWIþ0:2875 e 0:1335 110 c:w ¼ : ð10Þ 2;000;000 The relationship between the CWI and crack width measured values was then verified with the results obtained from the crack gauges. The CWI values obtained from the piezo-ceramic sensors were converted using Eq. (10) with the respective crack widths and plotted, see Fig. 10. The results obtained from the left and right crack gauges were also plotted so as to determine the effectiveness of the piezo-sensors for crack width estimation. The results obtained from HS-S-M-1 are presented in Fig. 11a. The crack gauges of this beam provide accurate results of the effective crack width up to a load of 140 kN, after which the mechanical crack gauges fail, and provide no further information. The

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Fig. 10 a Load versus concrete strains, b load versus CWI values, c CWI versus concrete strains

results provided by the piezo-sensors demonstrate consistent results with the crack gauges up to a load of 140 kN, and continue to provide results up to the failure load of the beam. The results obtained from HS-S-M-2 are presented in Fig. 11b. The installed crack gauges provide results for crack width measurements up to a load of 200 kN, whereas the piezo-sensors provide readings up to the failure load of 260 kN. From the graph on the projected load–crack width, it is noted that the results of the two crack gauges are very similar to one another, and consistent with the results

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obtained from the piezo-sensors. This trend follows up to a load of 200 kN where the crack gauges are once again ineffective. As in the results obtained from HS-S-M-1, the results of the piezo-sensors effectively represent the crack width measurements alongside those provided by the crack gauges, and continue to provide results up to the failure of the member. The results obtained from HS-S-M-3 and HS-S-M-4 are shown in Fig. 11c, d, respectively, and show an excellent example of the effectiveness of the piezo-sensors. HS-S-M3 demonstrates consistent results for the crack width

Author's personal copy J Civil Struct Health Monit Table 1 Summary of results for monotonic small-scale members Member notation

Failure load (kN)

Failure deflection (mm)

Observed cracking load (kN)

CWI at observed crack load

Actual cracking load (kN)

CWI at actual cracking load

HS-S-M-1

235

4.97

75

0.61

45

0.40

HS-S-M-2

250

4.70

65

0.37

50

0.26

HS-S-M-3

250

5.25

70

0.48

50

0.28

HS-S-M-4

260

4.43

60

0.56

55

0.29

Fig. 11 Comparison between crack gauges, numerical algorithm (8) and piezo-ceramic calibration equation

measurements between the two systems. The beam demonstrates that in some cases, the piezo-system will slightly overestimate the crack width, although as the crack width increases, so does the effectiveness of the system. Figure 11d, which represents HS-S-M-4, demonstrates that the

installed crack gauges are effective up to a load of 200 and 160 kN, respectively, for the left and right crack gauges. The data obtained from the piezo-sensors are seen to fall in close proximity with the direct crack measurements up to a load of 150 kN.

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6.2 Crack width test results The piezo-ceramic system is found to provide results that are in good agreement with those obtained from the theoretical mathematical expression, mechanical measured values from the displacement crack gauges, and the instrumentation modeling of the deduced CWI and crack width values. Therefore, CWI values can be effectively used through calibration charts to determine the average concrete crack width in millimeter. The proposed system proves to be a very effective tool for the assessment of the active conditions of reinforced concrete structures. The quick responsiveness and high signal output of the embedded piezo-sensors, along with their durability and ease of integration within the concrete matrix, make the proposed system an ideal component of a health monitoring system. Throughout the preparation of the test specimens, the sensors proved to be very effective and durable. The high heat produced during the curing process of the specimen had no effect on the working conditions of these piezosensors. The stress and strains produced during the casting process were minimal and the sensors did not show any signs of damage to the sensors during and after casting. The piezo-ceramic patches used in the fabrication of the piezosensors have a very high durability to high temperature exposure, and the rugged epoxy encasing makes it ideal for structures where mass concrete is used. The sensors are very cost effective and could be easily implemented into any reinforced concrete beam. The proposed system also has the possibility of embedment into existing structures, although this is not verified in the current experimental work.

7 Conclusion In the proposed development of a crack width measurement system, piezo-sensors are found to be very effective and economical in determining the initiation of cracking within a concrete member. The price of each sensor is estimated to be less than $10 each. The exceptional sensitivity of this system not only detects the initiation of cracking within a concrete beam, but also proves to be a useful tool for predictive health monitoring. By adopting the proposed CWI values, the system provides useful information as to when a member is expected to crack, and automatically provides early warning of distress with signals. The implementation of this active monitoring health

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system for real-time continuous recording with sensors has the potential to facilitate the maintenance of highly important, high-risk facilities by providing engineers with accurate, up-to-date information about the conditions of buildings. Using the information provided by such costeffective sensors, maintenance personnel could prioritize, and effectively assess, these highly important structures. Acknowledgments The authors would like to extend their gratitude to the Natural Sciences and Engineering Research Council for financial support, and the technical staff from the structures lab at Ryerson University, especially Nidal Jaalouk, for their continuous support throughout the completion of this research. Special thanks are extended to Mr. K. Wahba, Mr. A. Sucic and Dr. N. Dawood for their aid, and support throughout the course of this project. The authors would like to thank the Office of Vice President, Research and Innovation, Ryerson University for awarding the 2013 invention award of 25K to patent the SHM instrument for crack width monitoring.

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