JP2001289829A - Non-destructively measuring technique of concrete deterioration by impulse elastic wave - Google Patents

Abstract

PROBLEM TO BE SOLVED: To accurately and easily non-destructively measure dynamical characteristics and a deterioration degree of a concrete of a structure. SOLUTION: A surface of the structure is oscillated by using a striking unit such as a hammer or the like. Its wave is received by a plurality of pickups (sensors) similarly on the surfaces. Signals collected by the respective sensors are analyze and compared based on propagation characteristics of a surface wave. Thus, the dynamical characteristics or the deterioration degree of the concrete are measured.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a method for diagnosing a concrete structure. More specifically, the present invention relates to a method for measuring or estimating the degree of deterioration of concrete and mechanical characteristics (such as strength).

[0002]

2. Description of the Related Art Concrete that has passed time undergoes material deterioration due to chemical action and physical action. In terms of mechanical properties, the deformation coefficient and strength of concrete decrease. In concrete strength tests using coring, there are many cases where strength is reduced by a factor of several due to deterioration. In addition, the permeability of concrete may increase due to deterioration. Therefore, early diagnosis of soundness by nondestructive testing is desirable.

However, it cannot be said that an effective non-destructive exploration method has been established for the degree of deterioration of concrete. As a method for non-destructively estimating the compressive strength of concrete, conventionally, a resilience method (Schmidt hammer method), an ultrasonic wave (sonic speed) method, a composite method, and the like have been implemented.
However, at present, there is a large variation between the estimated value and the measured value in any of the methods, which is insufficient as an evaluation method, and the establishment of a more reliable estimation method is required.

The method of measuring the compressive strength by the Schmidt hammer method is most frequently used at present because the measurement is simple and can be applied irrespective of the shape and dimensions of the object to be measured. However, in the case of non-homogeneous materials such as concrete, the degree of resilience can be determined by the impact of local impacts on the surface of the material, such as a Schmidt hammer. , Surface weathering), and many problems remain.

[0005] Techniques for estimating the internal defect of concrete, measuring the dynamic elastic modulus of the material, and estimating the compressive strength based on the propagation speed and propagation time of the ultrasonic pulse (P wave) are often used. However, it is pointed out that the strength estimation accuracy is not very good for various reasons. (B) Although the directivity improves as the frequency used increases,
The attenuation of the sound wave also increases. (B) Unlike metallic materials, sound waves traveling through concrete are greatly attenuated. In addition, since the transmitted energy of ultrasonic waves is small, it is difficult to measure civil engineering structures having large dimensions such as dams and tunnels. (C) Although the sound velocity measurement at the place where oscillation and reception are performed with a concrete member in between is high in accuracy, if only the same surface such as a dam or tunnel surface can be measured, the measurement accuracy of the sound velocity is considerably reduced. (D) It may be affected by internal rebar. (E) It is also affected by the composition of the concrete and the water content. (F) Theoretically, the P-wave velocity is related to the dynamic elastic modulus and the density of the material, but the strain is small, so that the P-wave velocity has little direct relation to the strength of the material.

In order to improve the estimation accuracy, there is a so-called composite non-destructive test method in which two or more types of non-destructive test methods are used in combination. For example, there is a method of exploration in combination with the Schmidt hammer method and the ultrasonic sound velocity method. However,
The strength determination formula has not yet been established by such a composite method, and each method itself still has its own shortcomings.

[0007] In addition to the conventional sound velocity, the correlation between the amplitude or frequency of a sampled signal and the concrete strength has been studied using an ultrasonic measuring device. It is clear that the progress of concrete degradation reduces the amplitude of the sampled signal and lowers the predominant frequency. However, it is pointed out that the measurement varies depending on the method of contact between the probe and the concrete surface. Although there is a method using shock elastic waves, the frequency of the vibration wave is low and the rise is very slow, so that the time setting of the wave start becomes very unstable. In addition, it is almost impossible to estimate the strength of concrete by the electromagnetic method.

[Problems to be solved by the invention]

A concrete structure or concrete has the following characteristics. (B) The dimensions of structures such as tunnels and dams are often large. (B) Work can be performed only on a surface with a structure. For example, there is a downstream surface of a dam and an inner surface of a tunnel. (C) Concrete is not a uniform material such as metal but a composite material composed of aggregates, binders, and voids. (D) In some cases, concrete contains rebar and moisture. (E) Degradation of concrete occurs when it comes from the surface (for example, neutralization) and when it comes from the inside (for example, alkali-aggregate reaction and expansion due to rust of reinforcement). Occurs). Therefore, the properties of the concrete surface and the interior do not always match.

Therefore, as described above, existing non-destructive measurement methods for concrete (such as a resilience method and an ultrasonic method) have a serious problem. An object of the present invention is to overcome the above-mentioned drawbacks of the prior art, apply to structures such as dams and tunnels, and provide a new diagnosis method for concrete deterioration.

[0010]

SUMMARY OF THE INVENTION The present invention has been developed with a focus on voids and cracks existing in concrete, which have the greatest effect on the mechanical properties (such as strength and elastic modulus) and permeation properties of concrete. Things.

There are several types of voids (eg, capillary voids, air bubbles, etc.) in concrete due to various causes. As a result of the load, drying shrinkage, and creep, microcracks occur in the concrete (particularly, the boundary layer between the aggregate and the cement paste, that is, the transition zone). Increasing the load increases the size and number of microcracks.

It is considered that such voids and microcracks govern the strength of concrete. This is because stress concentration and the resulting collapse when a load is applied start from large voids and microcracks that exist without exception. Needless to say, the more voids and microcracks, the lower the strength of concrete. Since voids and microcracks do not easily transmit stress (particularly shear stress), they have a large effect on the elastic modulus of concrete. or,
The voids and microcracks are interconnected and increase the permeability of the concrete, thus affecting the durability of the concrete. Therefore, the degree (largeness or size) of the voids or microcracks is considered to be the most important influencing factor of concrete deterioration. However, how to grasp this nondestructively is a major issue.

The present invention solves the above-mentioned problem by using a surface wave (Rayleigh wave) that oscillates on a surface of a structure using a hitting device and is transmitted along the surface of the structure. This is a method for measuring the deterioration of the metal (FIG. 3).
Unlike the conventional method, the present method has the following features. Since a shock wave is generated by hitting with a hammer or the like instead of ultrasonic oscillation, energy is significantly increased. The wave used is not a conventional body wave but a surface wave. Further, the physical quantity used is the attenuation (loss coefficient) of the surface wave energy in addition to the conventional running speed. That is, the present invention is, by finding in the elastic wave traveling through the concrete, the propagation velocity of the surface wave to reflect to the voids or microcracks and (phase velocity) V _r loss factor H _r, mechanical properties of concrete ( This is a method for measuring the degree of deterioration.

In order to measure _Vr and _Hr , a measurement system as shown in FIG. 4 has been developed. The measurement system includes a plurality of sensors (acceleration pickups), a charge amplifier, a terminal panel, an A / D board-mounted personal computer, and the like. In order to respond to the site, a system that supplies power using a battery was developed.

The impact elastic wave is generated by hitting the surface of the structure with an oscillation mechanism such as a hammer. A sensor attached to the location closest to the impact location is set as a trigger. The trigger sensor picks up the wave signal and activates the entire measurement system. Therefore. Each sensor acquires a wave signal in a predetermined order. The wave signal collected by the sensor is converted into an electric signal, amplified by the charge amplifier, and sent to the personal computer via the terminal panel. The digital signal is converted into a digital signal by the A / D board in the personal computer and stored in the hard disk, thereby completing the signal sampling process. The mechanical properties (for example, strength) of the concrete are estimated by calculating the collected wave signal and obtaining _Vr and _Hr . The principle of this calculation processing is shown as follows.

On the surface of the concrete structure, the waves generated by the impact include various components (longitudinal waves, transverse waves, surface waves, plate waves depending on places). The energy transmitted along the surface of the structure in it is mainly surface waves (love waves and Rayleigh waves are among the surface waves,
In the case of concrete structures, Rayleigh waves are dominant). Surface waves have the following properties. (B) Surface waves are the most important waves on the surface of a structure. A considerable part (theoretically, about 67%) of energy is transmitted in the form of a surface wave in waves generated by hitting the surface of a structure. Note that, depending on the type of transmission (the surface wave is cylindrical and the body wave is spherical), the geometric attenuation of the surface wave is much smaller than that of the body wave. Moreover, when actually exploring, the sensor can only be attached to the surface of the structure.
Therefore, most of the energy of the waves picked up by the sensors is occupied by surface waves, except for the sensors close to the source. (B) The effect of surface waves decreases rapidly with depth. In concrete, the amplitude of the surface wave falls to about 0.2 at a wavelength depth of 1 assuming that the surface of the elastic body is 1. Therefore, it can be seen that the surface wave mainly exists only within the range of the wavelength of one time the depth. (C) Surface waves in an elastic body greatly depend on the shear characteristics (shear constant) of the material. Shear properties are greatly affected by voids and microcracks in concrete. (D) When concrete is used as an elastic body, the propagation velocity (phase velocity) of the surface wave traveling in it is almost the same as the propagation velocity of the shear wave.

[0017] While it is complicated to solve the theoretical solution of the phase velocity V _r of the surface wave, V _r in a semi-infinite elastic body is substantially the same as the traveling speed of the shear wave (a few percent lag) can be seen. Therefore, the phase velocity of the surface wave is affected by the shear characteristics (dynamic shear elastic modulus, that is, shear stiffness, etc.) of the material in the same manner as the shear wave velocity. In concrete, the shear stiffness is determined by the aggregate particles (generally having high rigidity), the mortar matrix (generally having low rigidity), and a transition zone connecting them. Therefore, the phase velocity of the surface wave is affected by the concrete mix, the type of aggregate, and the condition of the microcracks in the transition zone. Softening of the mortar matrix or aggregate, the progress of microcracks is believed that V _r due the presence of the gap becomes slow. Therefore, it can be seen V _r is one parameter to reflect the mechanical properties of the concrete.

As described above, since it is very difficult to determine the start time of a shock elastic wave, it is expected that an error will be large when simply measuring the traveling speed of the wave using the start time. Therefore, when measuring the _{V r,} the sensor of the plurality (> 2) are used. Taking into account the similarity of the phase, using the method of least squares method, the measurement error of V _r is considered to be contained to a minimum.

[0019] However, there is simply concrete deterioration about at V _r, even where it is insufficient to evaluate, particularly for strength. Speed measurement uses only signals that arrive early. Therefore, when a healthy part and a damaged part (deteriorated part) coexist in concrete, the damaged part may be missed. _Vr is greatly affected by the content of the aggregate and its hardness. Although the strength of cement mortar containing no aggregate is often high, _Vr is slow because of its low rigidity.

In general, aggregates are stronger than transition zones and cement matrices, so excluding very hollow, low-strength aggregates, such as pumice aggregate, have a direct effect on concrete strength. Since there is no effect, the strength of concrete is determined by the strength of the transition zone and the strength of the cement matrix. From the viewpoint of strength, the strength of the cement matrix depends on the porosity therein, and the strength of the transition zone depends on the microcracks therein. Therefore, understanding the degree of such porosity and microcracks is a major point in nondestructive measurement of concrete strength. For this purpose, a method for diagnosing concrete strength deterioration using the loss coefficient of surface waves was developed.

During propagation of an elastic wave, the kinetic energy of the wave is gradually converted to heat due to viscosity, friction, or a foreign substance (for example, a crack) of the material, and the elastic wave is weakened. This is called material damping. Generally, the material attenuation of a hard and homogeneous material (such as steel) is small, while the material attenuation of a soft and discrete material such as soil is considerably large. The damping of a semi-infinite elastic body by a homogeneous material is expressed as follows.

(Equation 1) Here, x _r: vibration amplitude x of a point away r from the oscillation source _0: vibration amplitude at the point of reference closer to the oscillation source (distance r ₀₎ lambda: wavelength of the wave h: represents the attenuation properties of the material, the loss Called coefficient. The order of metal materials → rock / concrete → soil is approximately 0.001 →
The value is 0.01 → 0.1 → 0.5.

The main factors affecting the loss coefficient are mainly the viscosity of the material, friction between material particles, reflection of waves between foreign materials (medium), and the like. In concrete, wave reflection and radiation, especially between solid parts and voids or microcracks, have a large effect on the loss factor of materials. Of course, the greater the number of voids and microcracks, the greater the loss factor of concrete. Note that the loss factor is calculated by decreasing the amplitude, and correlates with the change in wave energy.
As a result, even if a healthy part and a damaged part coexist in concrete, energy is attenuated at the damaged part.
It appears in the decrease in the measured amplitude. That is, the larger the loss coefficient, the greater the degree of material degradation. Even if there is a reinforcing bar in the concrete, its area ratio (rebar ratio) is small (generally only a few percent), so the effect on the overall energy is small. Since the shear rigidity of water is very small, the effect on surface waves is almost the same in the air gap. Therefore, a technique by loss factor H _r of the surface wave to evaluate the degradation of the concrete, as compared to the method of evaluating at existing speeds, with significant benefits.

However, regarding the determination of the loss coefficient, the most difficult point is considered to be the variation in signal sampling due to the mounting of the sensor. To determine the speed, it is simply determined at the time of arrival, whereas to determine the loss factor, the amplitude of the signal must be known. However, the signal amplitude is greatly affected by the mounting method of the receiving sensor, the position and the inherent error of the channel (sensor, cord, charge amplifier, cable, A / D conversion, etc.). It is extremely difficult to accurately determine the loss coefficient.

The above errors are basically divided into two types. (B) Error due to random noise: An error mainly caused by electrical noise and environmental noise. In order to remove such errors, a band-pass filter (BP
F) etc. are used. In addition, exploration is performed using multiple oscillations at the same location and multiple survey lines. Theoretically, if the results of the Nth order measurement are averaged, the signal-to-noise ratio (S / N) (B) Fixed error: This is due to the sensor mounting method, location, and characteristics of each channel. Various ideas were devised to solve this problem. First, a plurality of sensors are arranged and, for example, in the case of eight sensors, analysis is performed using an average value of four sensors before and after (FIG. 4). Next, the sensor is attached to the concrete surface by polishing the mounting surface of the sensor. Note that calibration is performed for each channel before the test. In the present invention, a bidirectional oscillation technology was developed to drastically solve this problem (FIG. 5). The principle of error elimination by this method is shown as follows using an example of two sensors (FIG. 6).

The magnitude of the oscillated signal is S ₀ , the signals collected in channels 1 and 2 are S ₁ and S ₂ , respectively, and the attenuation factor of concrete is f ₀ , and the attenuation factors of channel 1 and channel 2 are: The rates are f ₁ f
If _2, in the case of the oscillation from the channel -1, the signal ratio eta ₁ is

(Equation 2) It is. Similarly, when oscillating from channel-2, the signal ratio η ₂ is

(Equation 3) It is. If averaging as Equation 4 the two signal ratio, the true value f ₀ is determined.

(Equation 4)

According to these methods, it is possible to eliminate the measurement error to a considerable extent and measure the surface wave loss coefficient _Hr with high accuracy. Phase velocity V _r and mating surface waves, damage microcracks in the concrete, or material (binder, aggregate) is believed to reflect the changes in the.

BEST MODE FOR CARRYING OUT THE INVENTION The measurement of concrete deterioration by the method of the present invention is performed in the following order.

Oscillation occurs by hitting using a hammer or the like.
It is possible to change the wavelength of the oscillating surface wave (particularly the part that is forcibly vibrated by impact) depending on the weight, material and tip shape of the hammer. In other words, the heavier and softer the hammer, the longer the wavelength of the generated surface wave. Shorter wavelengths are desirable when measuring shallow surface degradation of concrete.
On the other hand, a long wavelength is desirable for measuring deep.

An oscillation signal is collected. The configuration of the signal sampling system designed and produced here is shown in FIG. The sensor used is an acceleration pickup whose resonance frequency is 30 kHz or higher, and the frequency range is fc to 5 kHz ±
0.5 dB, fc to 12 KHz ± 3 dB. A / D
The conversion speed of the board can be input at 1 μs / ch and up to 16 channels. At present, 8 channels are used. The trigger is set by software, and the signal sampling is performed correspondingly (FIG. 7). The collected acceleration signals are converted to digital signals and stored on the hard disk of the personal computer. If necessary, remove environmental noise using filters (electrical filter and digital filter).

The concrete deterioration is estimated by the analysis.
The degree of concrete deterioration is estimated by analyzing the collected data in the following order. (B) Correct the measured travel time of the wave. Since sampling is performed for each channel by the A / D board in order, a slight shift between the channels (this time shift corresponds to the sampling cycle of the A / D board) occurs. Correction for this deviation is performed. (B) Calculate the surface wave phase velocity _Vr

(Equation 6) Here, L and t are the traveling distance and traveling time of the surface wave, respectively. In the case of a plurality of sensors, calculation is performed by the least square method.

(Equation 7) Here, the superscript "-" means an average value. (C) The acceleration signal is converted into a speed signal by integration. Since the speed signal directly correlates with the energy of the surface wave, the measured acceleration signal is integrated as follows and converted into a speed signal.

(Equation 8) However, the integrated speed also performs baseline correction. (D) Compensate for geometric attenuation. The geometric attenuation of the surface wave velocity amplitude is corrected by the following equation.

(Equation 9) Here, A is the amplitude, and r is the distance from the vibration source (hitting point). Footnote ₀ represents a reference point (trigger channel). (E) Correct the effects of shear wave components. Theoretically, in the wave oscillating on the elastic body surface, about 26% of the energy is occupied by the shear wave. Since the shear wave is attenuated at r- ² along the surface of the elastic body, it can be seen that the component of the shear wave almost disappears when it is separated from the oscillation source to some extent. However, a signal taken at a location (sensor) close to the source contains a certain shear wave. This component will also be removed. Calculating the (f) material damping _{H r.}

(Equation 10) It is represented by λ is the wavelength of the surface wave.

[0030]

DESCRIPTION OF THE PREFERRED EMBODIMENTS At present, examples of measurement on concrete blocks or structures are shown in FIGS. As a whole, the apparent state and the measurement result agree well. It should be _noted that Hr tends to be lower as _Vr is faster.

It is known that the unconfined compressive strength σ _{c of the} concrete is partially contained therein. σ _c ＶV _r and σ _c ＨH
The relationship with _r is shown in FIGS. 8 and 9, respectively. Basically, both have a certain correlation.

[0032] shown in this, survey results for M dam, despite the very slow V _r compared to other data, H _r is not so high, the data that has the particularity to deviate from other data You can see that there is. Therefore, concrete density and strength tests were conducted by core sampling. The test results showed that the density of concrete was 1.9 to 2.1 g / c.
m ³ only without, usually concrete (density is generally 2.
^{3 to} 2.4 g / cm ³ ). However, its strength is 200 kg / cm ²
Having the above, it was about the same as the average concrete strength. From this result, it became clear that the concrete of M dam is unique compared to other concrete. The present measurement results reflect that, confirming the effectiveness of the present invention.

In the measurement for the N dam, a test was performed in which the wavelength of the oscillating surface wave was changed by changing the oscillation mechanism. FIG. 2 shows the average value of the wavelength, phase velocity, and loss coefficient of the oscillated surface wave. Due to the propagation characteristics of surface waves in the depth direction, it is considered that the concrete at the measured location has a higher strength or elastic modulus on the inside than on the surface.

At present, active research is being conducted on the quantitative correlation between V _r and H _r and the mechanical properties of concrete, and in particular, on the relationship with the cause of deterioration. Note that the measurement results accuracy, who V _r is higher than H _r, sometimes the variation is small. Therefore, we are aiming for a more reliable non-destructive measurement technique for concrete material degradation in consideration of the dispersion of measurement results.

[0035]

As described in detail above, the present invention makes it possible to accurately and nondestructively measure the mechanical properties of concrete of a structure.

[Brief description of the drawings]

FIG. 1 is a list of concrete deterioration investigation examples according to the present invention. The italic characters in the table are due to unidirectional oscillation.

FIG. 2 is a list of wavelength changes of surface waves oscillated by an oscillation mechanism with respect to an N dam, and their phase velocities and loss coefficients.

FIG. 3 is a diagram showing a measurement image of a structural concrete mechanical property in the method of the present invention.

FIG. 4 is a diagram showing a configuration of a signal sampling system according to the method of the present invention.

FIG. 5 is a diagram illustrating a method of setting a trigger and a sampling order in order to collect a vibration signal according to the method of the present invention.

FIG. 6 shows the principle of a technique for eliminating errors by bidirectional oscillation.

FIG. 7: V _{r コ ン ク リ}ー ト for concrete material according to the invention
It is a survey of H _r.

FIG. 8 is a measured σ _c ＶV _r relationship for a concrete material according to the present invention.

FIG. 9 is a measured σ _c ＨH _r relationship for a concrete material according to the present invention.

 ──────────────────────────────────────────────────続 き Continuing from the front page (72) Inventor Kenichiro Egawa 1-2-13 Motoyokoyamacho, Hachioji-shi, Tokyo F-term in Central Giken Co., Ltd. (Reference) 2G047 AA10 BA04 BC02 BC03 BC09 CA03 CB03 2G061 AA13 BA15 CA08 EA06 EA07 EA09 EB02 EB08 EC02

Claims (3)

[Claims] 1. The oscillated elastic wave is collected by a sensor on the concrete surface (FIG. 3), and the mechanical properties and the degree of deterioration of the concrete are non-destructively determined by the traveling speed (phase speed) and the loss factor of the surface wave therein. Measure with 2. The two-way oscillation technology reduces the effect of mounting a sensor and the measurement error of a measuring machine. 3. Deterioration in the concrete depth direction is measured by changing the oscillation mechanism or changing the wavelength of the elastic wave using a filter (electric or digital).