CROSS-REFERENCE TO RELATED APPLICATION

[0001] This application claims the benefit of priority to the Singapore application no. 10202251418X filed October 18, 2022, the contents of which are hereby incorporated by reference in their entirety for all purposes.

TECHNICAL FIELD

[0002] This application relates to a concrete monitoring system and concrete monitoring method.

BACKGROUND

[0003] Concrete is a mixture of cement (cement paste), water, and fine/coarse aggregates. In fresh concrete forming or making, the cement reacts with the water through a process called hydration. During the hydration process, the concrete gradually hardens over time. Concrete finishing is a post-forming process for creating a smooth and durable surface on the formed concrete structure. As concrete finishing has to begin prior to the concrete structure being fully hardened, determining the time to begin concrete finishing plays an important role in the final surface finish. Improper finishing may lead to weak, flawed, and unattractive slabs. Therefore, construction operators pay close attention to the concrete in determining the time suitable for concrete finishing. However, current methods, such as Bar Dropping tests, typically lack precision and often adversely affect the concrete structure integrity. SUMMARY

[0004] According to an aspect, disclosed herein a concrete monitoring system. The concrete monitoring system comprises at least one piezoelectric aggregate embeddable in a concrete structure; and a controller. The controller is configured to: perform a plurality of frequency sweeps of the at least one piezoelectric aggregate to obtain a plurality of phase angles; and determine a state of the concrete structure based on a change in phase angle between ones of the plurality of phase angles.

[0005] According to another aspect, disclosed herein a concrete monitoring method. The concrete monitoring method comprises performing a plurality of frequency sweeps of at least one piezoelectric-based aggregate embedded in a concrete structure to obtain a plurality of phase angles; and determining a state of the concrete structure based on a change in phase angle between ones of the plurality of phase angles.

BRIEF DESCRIPTION OF THE DRAWINGS

[0006] Various embodiments of the present disclosure are described below with reference to the following drawings:

[0007] FIG. 1 is a schematic diagram of a concrete monitoring system according to embodiments of the present disclosure;

[0008] FIG. 2 is a flowchart of a concrete monitoring method according to an embodiment; [0009] FIG. 3 is an example output from multiple impedance measurements of a single piezoelectric aggregate;

[0010] FIG. 4 is a detailed view of FIG. 3;

[0011] FIG. 5 is a plot showing a change in peak phase frequency in time;

[0012] FIG. 6 is a plot showing a change in peak phase amplitude in time; [0013] FIG. 7 is a perspective view of a piezoelectric aggregate according to embodiments of the present disclosure;

[0014] FIG. 8 is an exploded view of FIG. 7;

[0015] FIG. 9 is a sectional view of FIG. 8;

[0016] FIG. 10 is a schematic diagram of a piezoelectric structure comprising a circularshaped piezoelectric cylinder and a concrete cylinder according to embodiments of the present disclosure;

[0017] FIG. 11 is a lumped parameter representation of the piezoelectric structure of FIG. 10;

[0018] FIGs. 12Ato 12D are perspective views of a mold 400 for producing the piezoelectric aggregate of FIG. 7.

[0019] FIGs. 13A to 13C are images illustrating a method of making the smart aggregate using the mold of FIG. 12D.

[0020] FIG. 14 is a schematic diagram of a concrete measuring system according to embodiments of the present disclosure;

[0021] FIG. 15 is a schematic diagram of a first wireless scheme of a concrete measuring system according to embodiments of the present disclosure;

[0022] FIG. 16 is a schematic diagram of a second wireless scheme of a concrete measuring system according to embodiments of the present disclosure;

[0023] FIG. 17 illustrates a schematic work flow of the concrete monitoring system according to various embodiments of the disclosure

[0024] FIG. 18 is a flow chart illustrating the algorithm for automatically processing the data;

[0025] FIG. 19 illustrates correlations of the peak phase frequencies to penetration depths; and [0026] FIG. 20 illustrates correlations of the peak phase amplitude to penetration depths.

DETAILED DESCRIPTION

[0027] The following detailed description is made with reference to the accompanying drawings, showing details and embodiments of the present disclosure for the purposes of illustration. Features that are described in the context of an embodiment may correspondingly be applicable to the same or similar features in the other embodiments, even if not explicitly described in these other embodiments. Additions and/or combinations and/or alternatives as described for a feature in the context of an embodiment may correspondingly be applicable to the same or similar feature in the other embodiments.

[0028] In the context of various embodiments, the articles “a”, “an” and “the” as used with regard to a feature or element include a reference to one or more of the features or elements.

[0029] In the context of various embodiments, the term “about” or “approximately” as applied to a numeric value encompasses the exact value and a reasonable variance as generally understood in the relevant technical field, e.g., within 10% of the specified value.

[0030] As used herein, the term “and/or” includes any and all combinations of one or more of the associated listed items.

[0031] For the sake of brevity, the term "phase angle" may be used to refer to any one or more of the terms "peak phase angle", "phase angle curve", “phase angle plot”, "phase angle data points", "maximum phase angle", "peak in the phase angle plot", "peak in the phase angle curve", “measured phase angle”, “database of phase angle”, etc., as will be understood from the context.

[0032] As used herein, the term "aggregates" refers to gravel, sand, crushed rock, and/or various inert materials that are added to concrete, in addition to cement and water. Conventional aggregates may come in fine particular sizes or in coarser granular forms. Aggregates are added to a mixture of cement (e.g., ordinary Portland cement) and water to eventually form part of the hardened concrete structure, and they are known to influence the properties of the fresh (yet to fully harden) concrete as well as the properties of the fully hardened concrete.

[0033] For the sake of brevity, the term "fresh concrete" will refer to a mixture including cement and water before the mixture hardens fully. The term "concrete" will refer more generally to a mixture or a structure including cement and water, including fully-hardened concrete but not limited thereto.

[0034] The term “state" used with respect to a concrete structurewill generally refer to one or more properties of the concrete structure that can be assessed, measured, calculated, estimated, or otherwise determined, and may be described in terms of one or a combination of more than one qualitatively and/or quantitatively determined properties of the concrete structure. The quantitatively determined properties include but are not limited to: water content; porosity; brittleness; mechanical properties such as elasticity, strength, fracture toughness, etc.; electrical properties such as resistance, conductivity, impedance, admittance, dielectric properties, etc.; thermal properties such as coefficient of expansion, heat transfer coefficient, etc.; acoustic properties such as acoustic impedance, stiffness coefficient, etc.

[0035] To aid understanding and not to be limiting, various embodiments of a concrete monitoring system 100 and a concrete monitoring method 200 will be described below with reference to the appended figures. The concrete monitoring system 100 and concrete monitoring method 200 may also be described as a system and a method that can monitor and measure a state of a recently formed concrete structure or fresh concrete structure. For the sake of brevity, the following will describe various examples with respect to the concrete monitoring system and method, but it will be understood that the system 100 and the method 200 may be used in multiple application scenarios pertaining to concrete structure monitoring and is not limited to the specific examples disclosed herein. [0036] FIG. 1 is a schematic diagram of a concrete monitoring system 100 for monitoring a concrete structure 80, according to various embodiments of the disclosure. As illustrated in FIG. 1, the concrete monitoring system 100 may include one or more sensor device (each hereinafter referred to as a "smart aggregate") 110. The piezoelectric aggregate 110 may also be referred to as an electromechanical impedance measurement-based smart aggregate (EIM- SMA). Various numbers of the smart aggregate 110 may be distributed and embeddable at different parts of a concrete structure 90. In some embodiments where the concrete structure 80 is relatively small, one smart aggregate 110 may suffice for monitoring the entire concrete structure 80. In the example illustrated, three smart aggregates 110 are embedded spaced apart from one another in the concrete structure 80 at approximately the same depth from the surface of the concrete structure 80. In other examples, a plurality of the smart aggregate 100 may be embedded at different depths from the same surface of the concrete structure 80. In some examples, each of a plurality of the smart aggregate 110 may be used to determine a state of a localized volume or a localized zone of the concrete structure 80.

[0037] Collectively, a plurality of the smart aggregate 110 may be used to determine a state of a relatively large volume of concrete. In some examples, data collected by the controller 120 from a distribution of the smart aggregate 110 may be used to show different parts of the concrete structure in each of their respective states. Use of the smart aggregate 110 allows for a choice of one or more targeted zones / volumes for monitoring. For example, to monitor the state of the concrete structure 80 closer to the surface, the smart aggregates may be embedded near to the surface of the concrete structure 80. Alternatively, to monitor the state of the concrete structure 80 deep under the surface, the smart aggregates 110 may be embedded deeper in the concrete structure 80.

[0038] In some embodiments, the concrete monitoring system 100 may further include a controller 120. In operation, the controller 120 is in signal communication with one or more of the smart aggregates 110. In some examples, as illustrated in FIG. 1, the controller 120 may be in signal communication with the one or more smart aggregates 110 via one or more wired connections. In other embodiments, the controller may be in signal communication with the smart aggregates 110 via a wireless connection.

[0039] The controller 120 may be configured to perform a method of concrete monitoring. The controller 120 is configured to execute instructions stored in a computer-readable memory and perform a method of determining a state of the concrete structure 80. In some embodiments, the controller 120 may send signals to or receive signals from the respective smart aggregates 110, and process and/or compute an output corresponding to a state of the concrete structure 80 based on the signals. In some embodiments, the concrete monitoring system 100 includes one or more transmitter 130 configured to enable signal communication between the one or more smart aggregate 110 and the controller 120. In some embodiments, the controller 120 may be coupled to a display 121. The controller 120 may be configured to communicate the state of the concrete structure 80 to a user, e.g., via the display 121, to display the output corresponding to the state of the concrete structure 80 to the users or workers.

[0040] In some embodiments, the controller 120 may include an impedance analyzer for processing the signals from the respective smart aggregates 110. The impedance analyzer measures the impedance of the smart aggregates over a range of frequencies. In some embodiments, the controller 120 may include a driver for driving the respective smart aggregates 110. For example, the driver may be a piezoelectric driver or piezoelectric driving circuit, to drive the piezoelectric aggregates.

[0041] In use, the smart aggregate 110 may be disposed or embedded in fresh concrete, e.g., at the time when the cement mixture is newly poured. Each of the smart aggregates 110 may be embedded in the concrete structure 80 during the forming/production process of the concrete structure. In other words, the smart aggregates 110 are placed or disposed interior of the concrete structure 80 when the concrete structure 80 is still relatively fluid/wet and has yet to harden. After embedding the smart aggregates 110 in the fresh concrete structure 80, monitoring of the state of the concrete structure 80 may begin.

[0042] In one aspect, the method of concrete monitoring includes acquiring signals from the smart aggregate 110 continuously or intermittently over a period of time as the fresh concrete structure (with the smart aggregate 110 disposed therein) matures or changes from an initial state towards being a fully-hardened state (mature). In some applications, the controller 120 is configured to determine a recommended time or time period for the user to carry out finishing work on the concrete structure 80. In some embodiments, the state of the concrete structure 80 may be related to a hydration state for the purpose of concrete finishing. As concrete finishing has to begin prior to the concrete structure being fully hardened, determining the hydration state of the concrete structure may have an impact on the final surface finish. Therefore, the method may be performed iteratively over the concrete hardening process, or a maturing process of the concrete structure 80 to determine the state of the concrete structure.

[0043] FIG. 2 illustrates a process flow chart of a method of concrete monitoring 200, according to various embodiments of the disclosure. The method 200 may include, in stage 210, performing a plurality of frequency sweeps of one or more smart aggregates 110, such as piezoelectric aggregates. In some embodiments, the smart aggregates 110 may first be calibrated prior to performing the frequency sweeps. In some embodiments, performing each of the frequency sweeps may include driving each of the piezoelectric aggregates 110 over a range of frequencies, and obtaining a respective electrical signal from each of the piezoelectric aggregates 110 over the same range of frequencies. The electrical signal from each of the piezoelectric aggregates 110 may correspond to an impedance measurement of the respective piezoelectric aggregates 110 over the range of frequencies. In some embodiments, performing each of the frequency sweeps may include measuring an impedance of the respective piezoelectric aggregates 110 over a range of frequencies. In some embodiments, each of the measured impedance may include a respective magnitude of the impedance and a respective phase angle of the impedance. For some embodiments, a plurality of phase angles are obtained upon performing a plurality of frequency sweeps of the piezoelectric aggregate(s) 110. For avoidance of doubt and as examples, the phase angle of each impedance may be a continuous measurement, a discrete measurement, a discrete measurement value, multiple segments of a continuous measurement, multiple segments of a discrete measurement, multiple discrete measurement values, etc.

[0044] Therefore, each frequency sweep may result in a measurement or a signal obtained from the respective piezoelectric aggregates. In some embodiments, multiple frequency sweeps may be performed over a time duration. One frequency sweep of a single aggregate performed at a time instant may produce a unique signal. Therefore, multiple frequency sweeps of the single aggregate performed over a time duration may produce multiple unique signals. Further, a single frequency sweep of multiple aggregates performed over a time instant may also produces multiple unique signals.

[0045] In some embodiments, the plurality of frequency sweeps may be iteratively performed based on a measurement cycle rate. For example, the measurement cycle rate may be described as performing one frequency sweep for each piezoelectric aggregate per 30 minutes or per 10 minutes. In some embodiments, there may be multiple embedded piezoelectric aggregates. The frequency sweeps may be performed staggered, such that at each time instance, only a single frequency sweep is performed. This ensures interferences between the piezoelectric aggregates during the respective frequency sweeps.

[0046] Alternatively, the plurality of frequency sweeps may be sequentially performed based on a measurement schedule. For example, a pre-determined schedule may be first provided to perform frequency sweeps according to a schedule such as 10 minutes, 30 minutes, 80 minutes, 150 minutes, etc after embedding the piezoelectric aggregates.

[0047] In some examples, referring to FIG. 3, each impedance measurement may include a plot of the phase angle across a range of frequencies, for example between 30 kHz to 100kHz. In other examples, the range of frequencies may be between 50 kHz to 80 kHz. The range of frequencies may be determined based on the system configuration, composition of aggregate or grade mix of concrete. In FIG. 3, multiple impedance measurements from respective multiple frequency sweeps are shown. It may be seen that the plot of phase angles may show a peak phase angle or peak phase amplitude at each impedance measurement. The peak phase angle or peak phase amplitude may correspond to a peak phase frequency. As an example, referring to FIG. 4, peak phase amplitude (p _n ) corresponds to peak phase frequency (f _n ). As the phase angle of a typical impedance measurement is bounded by a range of -90 degrees to +90 degrees, it is more convenient to locate the peak phase angle in a frequency sweep, as there may only exist a single peak. Therefore, a shift or change in the peak phase angle may be easily observed. This advantageously enables locating of the peak phase angle for determining the state of the concrete structure in comparison to observing a peak impedance magnitude (magnitude of the impedance) which may exhibit several local peaks.

[0048] In some embodiments, the impedance measurement may be received and output as one or more discrete values, such as a peak phase amplitude, peak phase frequency, a combination of both, etc. In other embodiments, the impedance measurement may be received and output as a table corresponding to one or both of a peak phase amplitude and a peak phase frequency.

[0049] In some embodiments, the method 200 may further include in stage 220, determining a state of the concrete structure based on a change in phase angle between selected ones of the phase angles. The phase angles are obtained from previously performed frequency sweeps. In other words, the state of the concrete structure may be determined based on a change in phase angles of the piezoelectric aggregates upon performing multiple frequency sweeps. In some embodiments, determining the state of the concrete structure is based on a change in phase angle between the selected phase angles obtained from the same piezoelectric aggregate. In some embodiments, the method 200 includes determining the state of the concrete structure during a maturing process of the concrete structure. In some embodiments, each piezoelectric aggregate is individually monitored and observed for a change in the respective phase angle. In other words, signals from each piezoelectric aggregate are collected and analyzed independently. Therefore, for such scenarios, the state of the concrete structure determined from one piezoelectric aggregate may correspond to a “localized” state of the concrete structure or a correspond to a localized zone of the concrete structure. In embodiments where there are multiple piezoelectric aggregates, each “localized” state of the concrete structure may be used to collectively determine a “collective” state of the concrete structure.

[0050] In some embodiments, the method 200 may further include in stage 230, determining a targeted state of the concrete structure based on the change in phase angle between selected ones of the phase angles. In some embodiments, a predetermined threshold or condition of the change in phase angle may be set to determine the targeted state of the concrete structure. In some embodiments, the targeted state of the concrete structure may be a state suitable for postforming processes, such as concrete finishing. Therefore, the targeted state may include a finishing ready state of the concrete structure, wherein in the finishing ready state, the concrete structure is suitable for concrete surface finishing.

[0051] Referring to FIGs. 4 and 5, in some embodiments, the change in phase angle corresponds to a shift in a peak phase frequency between each of the phase angles obtained from the frequency sweeps. The peak phase frequency corresponds to the frequency in which the phase angle plot/curve peaks, for example at fi or f _n . For example, a change in phase angle (Af) corresponds to a shift in peak phase frequency from fi to f _n , or in other cases from f _n to fn+i. It may be appreciated that the shift in peak phase frequency need not be between two consecutive phase angle peaks. In some embodiments, the shift in peak phase frequency is determined between a peak phase frequency of an initial frequency sweep (fi) and a peak phase frequency of a subsequent frequency sweep (such as f _n or f _n +i). The peak phase angle of the initial frequency may be determined during calibration of the piezoelectric aggregates, prior to embedding in the concrete structure. Alternatively, peak phase angle of the initial frequency may be determined based on the first measurement made after embedding the piezoelectric aggregates. In some embodiments, to determine the targeted state of the concrete structure, the shift in the peak phase frequency between the peak phase angle of the initial frequency sweep (fi) and the subsequent frequency sweep (fn+i) is to satisfy a predetermined frequency range, such as between 2kHz to 10 kHz, for example, 6kHz between fi and f _n .

[0052] Still referring to FIGs. 4 and 6, in some embodiments, the change in phase angle corresponds to a change in a peak phase amplitude (Ap) between each of the phase angles obtained from the frequency sweeps. The peak phase amplitude corresponds to the phase amplitude in which the phase angle plot/curve peaks, for example at pi or p _n . For example, the change in peak phase amplitude corresponds to a change or decrease in peak phase amplitude from p _n to p _n +i, or in other cases from pi to p _n . In some embodiments, the change in the peak phase amplitude is determined between respective peak phase amplitudes obtained from two consecutive frequency sweeps, i.e. between p _n and p _n +i. In some embodiments, to determine the targeted state of the concrete structure, the change in the peak phase amplitude between two consecutive frequency sweeps is to satisfy a decrease in the peak phase amplitude within a predetermined amplitude range, such as in a range of 3 to 8 degrees, for example, 6 degrees from p _n to p _n +i. [0053] In some embodiments, in order to determine the targeted state of the concrete structure, both the predetermined conditions of predetermined frequency range and the predetermined amplitude range are to be satisfied. These predetermined conditions may correspond to a concrete bar dropping test of 2 to 5 centimetres in depth of penetration.

[0054] FIGs. 7 to 9 illustrate an example of a piezoelectric aggregate 110 (also referred to as the EIM-SMA or smart aggregate), according to various embodiments of the disclosure. The piezoelectric aggregate may include a piezoelectric member 112 sandwiched between two concrete members 114/116. The two concrete members 114/116 collectively forms a pellet of concrete mass. In some embodiments, the pellet may be a cylindrical or plate-shaped member which has a thickness (Tp) along a thickness direction 90 substantially smaller than a radius along a radial dimension 92. The piezoelectric member 112 may also have a lower thickness in relative to the concrete members 114/116. In some examples, the piezoelectric member 112 may have a thickness (Tp) of 1 to 2 millimetres, and each of the concrete members 114/116 may have a thickness (Tc) of around 5 millimetres. In some examples, a thickness ratio between the piezoelectric member 112 and each of the concrete members 114/116 is in a range of 1 :5 to 2:5.

[0055] In some embodiments, the piezoelectric member 112 may be a piezoelectric thin plate with respective surfaces coated with a metal paint, such as Hammerite - Direct to Rust. In some embodiments, the piezoelectric member 112 may be a piezoelectric thin plate with respective surfaces coated with a thin waterproof layer. The piezoelectric member 112 may have a diameter (Dp) smaller than a diameter (De) of each of the concrete members 114/116. Therefore, allowing the piezoelectric member 112 to be centrally disposed between the two concrete members 114/116 along the thickness direction 90 as well as centrally disposed between the two concrete members 114/116 along the radial direction 92. For example, the two concrete members 114/116 may be formed of the same concrete composition to define a center of mass (M) on the plane of symmetry (P), in which the plane of symmetry is coincidental with the interface between the two concrete members 114/116. For example, the two concrete members 114/116 may be substantially identical such that the pellet defines a plane of mirror symmetry between the two concrete members 114/116. The piezoelectric member 112 may be described as being disposed in the plane of symmetry of the pellet and substantially encased by the pellet 114/116. The piezoelectric member 112 may be substantially surrounded on all sides by the pellet 114/116. It will be understood that this does not preclude the provision of one or more wires extending from the piezoelectric member (disposed inside the pellet 114/116) to the outside of the pellet 114/116. The pellet 114/116 is a fully-hardened concrete mass before the piezoelectric aggregate 110 is put in use, e.g., before the piezoelectric aggregate 110 is disposed in fresh concrete to monitor the curing or hardening of the fresh concrete.

[0056] In some embodiments, each of the concrete members 114/116 may be formed from a mixture of cement; sand; and water. While without any intentions of being limiting, an example volumetric ratio of the mixture of cement; sand; and water may be 1 :0.5:0.4 respectively. In other examples, other combinations of concrete/mortar may also be possible candidates for the concrete members 114/116.

[0057] FIGs. 10 and 11 illustrate a working principle of the piezoelectric aggregate 110. The working principle of the piezoelectric aggregate 110 may be based on the impedance method and the electromechanical coupling theory. According to the electromechanical coupling theory, the dynamic characteristics of a piezoelectric structure 310 which includes a piezoelectric element 312 and a coupling structure 314 are governed by the interactions between the piezoelectric element 312 and the structure 314. The impedance of the piezoelectric structure 310 is determined by the properties of the piezoelectric element 312, the relative location of the piezoelectric element 312 in relation to the structure 314, the boundary conditions of the structure 314, the rigidity of the structure 314, etc. [0058] FIG. 10 illustrates a schematic diagram of a piezoelectric structure 310 including a circular-shaped piezoelectric cylinder 312 and a concrete cylinder 314. To better explain the dynamic interaction of the piezoelectric structure, FIG. 11 illustrates a lumped parameter representation of the piezoelectric structure 310. The piezoelectric element 312 has a certain piezoelectric coupling effect and elastic rigidity. The piezoelectric element 312 of the same model fabricated by a reliable manufacturer, may have their properties (i.e., piezoelectric coupling effect and elastic rigidity) assumed to be substantially similar or even identical when not considering minor manufacturing variances.

[0059] The concrete cylinder 314 is simplified and represented by a single degree of freedom (SDOF) mechanical oscillator with the impedance of: where a> _n = ^K _s /m. m, K _s ,and c are the mass, spring constant, and the damping coefficient of the SDOF mechanical oscillator.

[0060] The admittance (the reciprocal of impedance) of the lumped model as shown in FIG.

11 may be derived as: and thickness of the piezoelectric cylinder, respectively. d.32 is the piezoelectric constant. For the piezoelectric material, p, Y ₂₂ , P and E ₃₃ are the mass density, modulus at zero electric field, mechanical loss factor, and dielectric constant at zero stress of the piezoelectric material respectively.

[0061] Upon mixing cement and water, the hydration reaction takes place, resulting in shrinkage and hardening. In other words, the ‘stiffness’ of the concrete (ks) increases. Referring to Equation (1), the change in concrete stiffness (fa) will alter the dynamic characteristics of the coupled system, which is constituted by the piezoelectric element 312 and the concrete material 314. Based on the above principle, the impedance of the fresh concrete may be employed for concrete monitoring.

[0062] In some embodiments, the piezoelectric aggregate 110 may be an electromechanical impedance measurement (EIM) based smart aggregate (SMA) for monitoring fresh or recently formed concrete. The piezoelectric aggregates may be suitable for mass production. In various embodiments, the smart aggregates are piezoelectric aggregates or piezoelectric-based aggregates, which include and operate using a piezoelectric member made from piezoelectric materials.

[0063] In some embodiments, the concrete monitoring method 200 may include obtaining measured impedance values or results from the piezoelectric aggregates, which are used directly or indirectly via post-processing, to indicate a state of the concrete structure, such as a hardening status of fresh concrete. In addition, the monitoring method 200 is simple and may be automated to alleviate any requirements for human intervention. Therefore, the concrete monitoring system 100 and concrete monitoring method 200 may be widely adopted at building sites or construction sites. Further, due to the simple and automated nature of the system 100 and method 200, workers at the building sites may not be required to have any prior knowledge or understanding of the working principles of the system 100 and method 200.

[0064] FIGs. 12 A to 12D illustrate a mold 400 for producing some embodiments of the piezoelectric aggregate 310 . The mold 400 may be used for rapid production of the proposed EIM-SMA or piezoelectric aggregate. The mold may be presented in various configurations, with the present disclosure being one example. In this embodiment, the mold 400 includes two layers 410/420, a bottom layer 410 as shown in FIG. 12A and a top layer 420 as shown in FIG. 12B. Each layer 410/420 of the mold may be further divided into two components, i.e., the bottom layer 410 may be formed by two components 412/414, and the top layer 420 may be formed by two components 422/424. All of the components 412/414/422/424 may be provided with handles to ease demolding of the piezoelectric aggregate.

[0065] A pair of screw holes may be provided at the edges of the components 412/414/422/424 for assembly and tightening to form a whole mold during the casting process of the piezoelectric aggregate (EIM-SMA) 110. After each of the layers 410/420 is assembled (independently of any other of the layers), the bottom layer may form the base that has a protruded ring with a diameter at the center (FIG. 12 A; while the top layer has a circular hole with an inner diameter of (FIG. 12B). The bottom and top layers 410/420 may be further assembled through a sleeve connection (FIG. 12C). The mold may be fabricated using 3D printers to significantly reduce the bare cost.

[0066] FIG. 13 A to FIG. 13C are images illustrating the steps of using the mold 400 to make piezoelectric aggregates. The EIM-SMA casting procedure may be performed injust a few steps. As shown in FIGs. 13A to 13C, a method of making a piezoelectric aggregate may include:

Step 1 : Pouring concrete into the bottom layer of the mold, and filling the bottom layer;

Step 2: Placing the PZT patch at the center of the bottom layer, and with ends of the wires extending out of the mold; and

Step 3: Fitting the top layer on the bottom layer via a sleeve connection, and filling the top layer with concrete.

[0067] FIG. 14 illustrates an application of the concrete monitoring system 100 in which three piezoelectric aggregates 110 are disposed spaced apart by a minimum spacing (S). The three piezoelectric aggregates 110 may be disposed in a staggered configuration. The minimum spacing (S) minimizes interference or results in minimal interference between the piezoelectric aggregates 110. In some examples, the minimum spacing (S) is around 1 metre. This embodiment may be an alternative solution to staggering the frequency sweeps for each of the piezoelectric aggregates 110, such that a collective state of the concrete structure may be determined collectively at a time instant.

[0068] With reference to FIGs. 15 and 16, according to various embodiments of the concrete monitoring system, in addition to the in-situ or wired mode, two wireless schemes are proposed to make the measurement process more efficient with greater flexibility. For some embodiments, the smart aggregates 110 are embedded in slabs 80a and columns 80b of the concrete structure 80. In some embodiments, the smart aggregates 110 may be embedded away from formwork 82 and rebars 84 of the concrete structure 80.

[0069] FIG. 15 presents a first scheme (Scheme I) in which each smart aggregate 110 may be connected to a transmitter 130 that is placed at the construction site. Alternatively, a multichannel transmitter may be provided, thus allowing a single transmitter to be in communication with multiple smart aggregates. The transmitter 130 may be equipped with the functions of an impedance analyzer. In other words, the transmitter 130 may conduct impedance measurements locally. This may be followed by the impedance measurements or impedance spectrum results being sent to a receiver 122. The receiver 122 may be physically disposed away from the construction site via wireless communication. Upon receiving the data or results, a computer 124 which includes a customized software program at the receiver end, may be configured to interpret and present detailed information relating to the measured results, such as but not limited to a visualization of the impedance spectrum. In Scheme I, the transmitter 130 includes impedance analyzer functionality which is configured to perform local analysis of the impedance measurements.

[0070] FIG. 16 presents a second scheme (Scheme II) in which the impedance analyzer functionality is integrated at the receiver end. This scheme advantageously can reduce the cost and reduce the risk of the analyzer function being damaged (under the relatively harsh conditions on-site). In this embodiment, the transmitter 130 is responsible for collecting and transmitting the raw data, i.e., voltage and/or current signals, and may be configured without the analyzer functions of Scheme 1. The raw data may be sent to the receiver via wireless communication to be received at the receiver 122 for post-processing. Post-processing may include converting the raw data (of voltage and/or current signals) into impedance measurements or impedance spectra. Thereafter, the computer 124 in communication with the analyzer 126 may be used to interpret and present the detailed information of the measured results, such as but not limited to visualization of the impedance spectrum.

[0071] FIG. 17 illustrates a schematic work flow of the concrete monitoring system according to various embodiments of the present disclosure. The workflow may include embedding piezoelectric aggregates or EIM-SMA units in the fresh concrete at a construction site. The wires of the respective piezoelectric aggregates may be connected to the impedance analyzer. As an example, a precision impedance analyzer 6500B from Wayne Kerr Electronics Limited may be used to measure the impedance of the piezoelectric aggregates. As another example, a Sciospec ISX3 may be used as impedance analyzer to measure the impedance of the piezoelectric aggregates. The frequency sweep range of the impedance analyzer can be up to 120 MHZ, with an impedance measurement accuracy is ±0.05%. Measurement data may be saved as .CSV files. In addition to the above settings, the 6500B series impedance analyzer may provide a GPIB interface, i.e., a parallel port designed to enable communication between the instrument and other PC-like control terminals. Moreover, this impedance analyzer also provides a standard RJ45 LAN connector that enables connection to a Fast Ethernet network. In other words, it has an extended function that allows remote control.

[0072] Subsequently, the impedance analyzer may be controlled or configured to take a measurement and record the results at regular time intervals, e.g., every 10 to 30 minutes. The measured results are collected in the raw data format and stored in the same folder. For a given piezoelectric aggregate under free conditions or prior to being embedded in the concrete structure, a frequency swept may be performed for a frequency range from 20 kHz to 200 kHz. Any distinct frequency peak is noted, for example around 60 kHz. After embedding the piezoelectric aggregates into the concrete, it is noted that a resonant frequency of the piezoelectric aggregates under the constrained condition will increase. Therefore, a measurement frequency range is set to be in a range of 40 kHz to 100 kHz to account for such a frequency increase.

[0073] An impedance is a complex number that can be expressed in terms of its real and imaginary parts. Alternatively, an impedance can be expressed in terms of its magnitude and angle. The magnitude of an impedance depends on many factors and can vary in a vast range, as well as the decomposed real and imaginary parts. During the concrete hardening process, the magnitude of the piezoelectric aggregate’s impedance may monotonically increase.

[0074] The peak of an impedance (magnitude of impedance) plot corresponding to the electromechanically coupled resonance may exhibit a local maximum. The peak of the magnitude of the impedance is generally difficult to detect or to distinguish with consistent repeatability.

[0075] In terms of the instrument settings, the present method involves acquiring the magnitude and angle of the impedance as the parameters to be measured. Under the present conditions, the natural limits of the phase angle is between -90 degrees to 90 degrees, and the peak in the phase angle plot is more likely to correspond to a global maximum. Once the instrument settings are determined, they can be stored (during the set-up) in the memory of the controller and/or the computing device. This allows other operators without prior knowledge of the system configuration parameters to load the set-up from the memory for a quick measurement.

[0076] In the experiments conducted, a coded Matlab program was provided to automatically identify the raw data, analyze the impedance evolution, and/or interpret the state of the concrete, e.g., a hydration state of the concrete or a concrete maturity. In other examples, a Python script with Sciospec ISX3 impedance analyzer providing the impedance data, may also be employed for the experiment. The experiments are described in the following as an example solely to aid understanding and not to be limiting. To monitor the state of the concrete, repetitive measurements of the impedance were performed every 10 to 30 minutes. With implementation of an advanced impedance analyzer based on the method proposed herein, the task of determining the strength of a concrete structure can be completed by the instrument in a consistent manner at different time instances over a period of time, automatically and even remotely. This advantageously enables a relatively large collection of data to be acquired and processed without the need for workers to be physically on-site at all times throughout the acquisition of measurement data. .

[0077] In the Matlab program, a file naming scheme is proposed so as to enable automatic identification of the raw data files to be post-processed. The raw data files may be named with the same prefix, e.g., agg4. A separator, such as a hyphen ( - ) or an underline ( > ), may follow after the prefix. The time information is preferably indicated after a separator. Below are some examples of acceptable filenames: “agg4-30mins.csv”, “agg5_210mins.csv”, etc.

[0078] Prior to executing the Matlab algorithm, the program files were placed in the same folder with the test data files. FIG. 18 is a flow chart illustrating an example of the algorithm for automatically processing the data. The algorithm first searches all the .CSV files in the current directory. If there is no .CSV files in the current directory, the algorithm will display an error message to the user. According to the prefixes and the separators that appeared in the file names, the algorithm will screen all the files first and identify the test data files. Subsequently, the data files will be sorted according to the test time information contained in the file names. The algorithm will read the data in those files by sequence. If the data files are incorrectly formatted, the algorithm will display an error message to remind the users. Assuming that the data in the files are successfully read by the algorithm, they will be stored in Matlab working space. A peak search algorithm will then be used to identify the peaks and extract the corresponding frequency & magnitude information. Thereafter the peak evolution trend will be plotted out with respect to time to reflect the hydration state or maturity development of the concrete. The plots will be saved as .jpeg format figures in the current directory.

[0079] Two separate Matlab programs were developed to implement the above algorithm. Matlab Program I is responsible for identifying and sorting the .CSV files in the current directory. Matlab Program II is responsible for post-processing the data in the identified .CSV files. Detailed descriptions of the functionalities, inputs, and outputs of the Matlab programs are provided in Tables 1 and 2 below.

Table 1. Matlab Program I Table 2. Matlab Program II

Calibration and correlation analysis

[0080] According to various embodiments of the present disclosure, the method proposed herein includes predicting the maturity or the hardening status, based on at least the phase angle of the measured impedances. The prediction may be based on one or more reference data developed to facilitate interpretation of the impedance measurement results. For example, in the experiments conducted, a set of refernece data was developed based on conventional bardropping tests. Alternatively or additionally, reference data related to concrete maturity and based on other tests may be used. As an illustrative example, the bar-dropping test was selected for the experiment as the users were used to describing concrete maturity in terms of bar- dropping test results. The present method does not preclude the use of reference data acquired via other ways. The present method does not preclude the possibility that correlation back reference becomes unnecessary for the interpretation of the impedenace measurement results, e.g., the present method becomes widely accepted in the industry and/or becomes codified in standards. In the experiments conducted, the impedances measured were correlated to the bardropping depths to indicate concrete maturity. Table 3 lists the mixture proportions of the constituent materials for the concrete tested and monitored in the experiment.

Table 3. Summary of mixture proportions

[0081] In order to ensure the established correlation mapping validates for a plurality of the piezoelectric aggregate, the piezoelectric aggregates were calibrated to exhibit the same initial impedance characteristics under the free condition, i.e., before embedding into the fresh concrete. A plurality of piezoelectric aggregates that have been calibrated against a common reference may be used in different scenarios without re-calibration. This means that changes in concrete maturity can be monitored over a period of time with more consistent and quantifiable results, e.g., in contrast to the bar-dropping test which may be influenced to some extent by the inherently random way in which an article is released by human hands and the inherently random way in which the article lands. Bar-dropping test results can include non-trival or large fluctuations, e.g., although the penetration depth overall decreases with time, actual measurement results show that the change in penetration depth does not necessarily change monotonically. In a possible application scenario, the piezoelectric aggregate may be mass producted and calibrated to a standard reference such that the piezoelectric aggregates can be immediately be put to use by construction workers on-sitewithout the need for the construction workers to carrry out a separate calibration for each concrete structure to be monitored. This will help to address the fluctuations in bar-dropping tests that are carried out on-site.

[0082] As an example, FIGs. 19 and 20 illustrate the correlation mapping between a series of impedance results (acquired from the present method and piezoelectric aggregate) and results from conventional bar-dropping tests. In the example, the tests were performed on G40 concrete. The impedance phase peak includes both aspects of the peak phase frequency shift and the peak phase amplitude change. Two graphs, FIGs. 19 and 20, were plotted to illustrate the correlations of the peak phase frequencies (acquired using the proposed piezoelectric aggregates) and the peak phase amplitudes (aquired using the proposed piezoelectric aggregates) to penetration depths (acquired using the conventional bar-dropping tests), respectively. The penetration depth can be observed to decrease with time, as the concrete gets hardened.

[0083] It may be observed that the peak phase frequency or impedance peak frequency monotonically increases in this process, indicating the increase of concrete ‘stiffness’, which is in line with the hardening of the concrete. The peak phase amplitude monotonically decreases in the course of this concrete hardening process, and provides another indicator to reflect the degree of concrete maturity or concrete hardening status. The correlation or calibration includes establishing a look-up table or a fitting function suitable for correlating the impedance results (acquired from the piezoelectric aggregate) with the bar-dropping test or other reference results. [0084] Once calibrated, given an impedance measurement result (peak phase frequency and peak phase amplitude), the state of the concrete or concrete hardening status may be interpreted by referring to the look-up table or by using the fitting function. Once the correlation mapping is established, references in the form of look-up tables, fitting functions, etc., may be employed to interpret the impedance measurement results for any standardized piezoelectric aggregate. As used herein, "standardized piezoelectric aggregate" refers to piezoelectric aggregates or EIM-SMA units that have been calibrated (pre-calibrated) against the same or similar reference. The peak phase frequency shifts and peak phase amplitude may be extracted. Subsequently, the state of the concrete or maturity development status of the concrete may be obtained by referring to the correlation mapping. The larger the peak frequency shift is, the higher the concrete solidification is; and the larger the magnitude change is, the higher the concrete solidification is.

[0085] Several beneficial applications of the proposed system and method can be appreciated from the description provided above. For example, for the purpose of monitoring concrete structures, the conventional bar-dropping test is known to provide relatively inconsistent measurement values. Further, bar-dropping tests may cause irreversible damages to the concrete structures, such as formation of internal cracks or damages to the external surfaces. Another shortcoming of the bar-dropping test is the need to be conducted repetitively by workers at the site. These and other issues can be addressed in practical terms by using the system and method proposed herein instead.

[0086] The bar-dropping test may be affected by many external factors. Therefore, several in-parallel bar-dropping tests often yield results of non-negligible differences. As compared to bar-dropping test results, the impedance curve is relatively smoother and has better monotonicity. In addition, the impedance measurement may be performed by the instrument automatically without human intervention. This indicates that it is possible to avoid or sigificantly reduce operator-related external factors, and that the proposed system and method is a more reliable solution for monitoring the state of a concrete structure.

[0087] In one aspect, various embodiments of the present disclosure includes a concrete monitoring system. The concrete monitoring system comprises at least one piezoelectric aggregate embeddable in a concrete structure; and a controller. The controller is configured to: perform a plurality of frequency sweeps of the at least one piezoelectric aggregate to obtain a plurality of phase angles; and determine a state of the concrete structure based on a change in phase angle between ones of the plurality of phase angles. In various embodiments, the state of the concrete structure corresponds to a hydration state of the concrete structure. In various embodiments, the state of the concrete structure includes a finishing ready state of the concrete structure, wherein in the finishing ready state, the concrete structure is suitable for concrete surface finishing. In various embodiments, the change in phase angle between ones of the plurality of phase angles includes the change in phase angle between the ones of the plurality of phase angles of the same piezoelectric aggregate. In various embodiments, the change in phase angle includes a shift in a peak phase frequency between the ones of the plurality of phase angles. In various embodiments, the shift in the peak phase frequency is determined between an initial frequency sweep and a subsequent frequency sweep. As examples, the shift in the peak phase frequency is in a range of 2kHz to 10 kHz.

[0088] In various embodiments the change in phase angle includes a change in a peak phase amplitude between the ones of the plurality of phase angles. In various embodiments, the change in the peak phase amplitude is determined between respective ones of the plurality of phase angles of two consecutive frequency sweeps. In various embodiments the change in the peak phase amplitude corresponds to a decrease in the peak phase amplitude in a range of 3 degrees to 8 degrees. In various embodiments, the plurality of frequency sweeps are sequentially performed based on a measurement schedule.

[0089] In various embodiments, the plurality of frequency sweeps are iteratively performed based on a measurement cycle rate. For example, the measurement cycle rate is one frequency sweep per 30 minutes or shorter. In some examples, each of the plurality of frequency sweep is performed over a sweeping frequency range, the sweeping frequency range is between 30kHz to 100kHz. [0090] In various embodiments, each of the at least one piezoelectric aggregate comprises a piezoelectric member sandwiched between two concrete members. In some examples, a thickness ratio between the piezoelectric member and each of the concrete members is 2:5 or lower. In various embodiments, the piezoelectric member is a piezoelectric thin plate with respective surfaces coated with a metal paint. In various embodiments, the respective surfaces of the piezoelectric member are coated with a thin waterproof lay er.In various embodiments of the concrete monitoring system, the at least one piezoelectric aggregate includes two piezoelectric aggregates spaced apart by a minimum spacing, wherein the minimum spacing corresponds to minimal interference between the two piezoelectric aggregates. In various embodiments, the controller is further configured to determine the state of the concrete structure during a maturing process of the concrete structure.

[0091] According to another aspect of the present disclosure, a concrete monitoring method is disclosed in various embodiments. The method comprises performing a plurality of frequency sweeps of at least one piezoelectric-based aggregate embedded in a concrete structure to obtain a plurality of phase angles; and determining a state of the concrete structure based on a change in phase angle between ones of the plurality of phase angles. In various embodiments, the method further comprises determining a targeted state of the concrete structure based on the change in phase angle, wherein in the targeted state, the concrete structure is suitable for concrete surface finishing. In various embodiments, the method further includes determining the state of the concrete structure during a maturing process of the concrete structure.

[0092] All examples described herein, whether of apparatus, methods, materials, or products, are presented for the purpose of illustration and to aid understanding, and are not intended to be limiting or exhaustive. Modifications may be made by one of ordinary skill in the art without departing from the scope of the invention as claimed.