Method for estimating strength of lining concrete before removal from form, and method for determining time to remove lining concrete from form

Abstract

To more easily estimate the strength of lining concrete before removal from a form.SOLUTION: The above problem is solved by a method for estimating the strength of lining concrete 3 before removal from a form, characterized in that an impact position 2H is set on an inner surface of an arch 2 of a tunnel in a strength estimation area, and a vibration measuring device 10 is installed at a position away from the impact position 2H, and after the lining concrete 3 is poured and before removal from the form, the impact position 2H on the inner surface of the arch 2 of the tunnel is struck with a hammer 5 or the like to vibrate the arch 2 of the tunnel and the lining concrete 3, and the post-pouring vibration is measured with the vibration measuring device 10, a vibration characteristic value correlated with the compressive strength of the lining concrete 3 is obtained from the post-pouring vibration, and the compressive strength of the lining concrete 3 in the strength estimation area is estimated from this vibration characteristic value.SELECTED DRAWING: Figure 2

Description

[Technical Field]

[0001] The present invention relates to a method for estimating the strength of lining concrete before demolding, and a method for determining the timing of demolding lining concrete. [Background technology]

[0002] In the construction of lining concrete for mountain tunnels, ordinary concrete is poured sequentially from the side walls towards the top, so the top, which is the final pouring point, is the youngest concrete. Consequently, the concrete at the top, being the youngest concrete, tends to develop less strength upon demolding, raising concerns about spalling and delamination. Furthermore, depending on the pouring conditions and environmental conditions, the strength development of concrete other than the top may also be delayed. Moreover, when high-flow (self-compacting) concrete is injected from a low position, the top is not necessarily the youngest concrete.

[0003] For this reason, methods have been proposed, such as measuring the cumulative temperature of the lining concrete before demolding and measuring the compressive strength of the lining concrete based on the correlation between this temperature and the compressive strength (see Patent Document 1).

[0004] However, this method directly measures the temperature of the concrete inside the formwork (steel mold), which requires either modifying part of the formwork or leaving sensors or other equipment inside the hardened concrete. [Prior art documents] [Patent Documents]

[0005] [Patent Document 1] Japanese Patent Publication No. 2012-26734 [Overview of the Initiative] [Problems that the invention aims to solve]

[0006] Therefore, the main objective of the present invention is to provide a method for more easily estimating the strength of the lining concrete before demolding, and so on. [Means for solving the problem]

[0007] The method for estimating the strength of the lining concrete before demolding, which solves the above problems, is as follows. <First aspect> A striking position is set on the inner surface of the centering in the intensity estimation region, and a vibration measuring instrument is installed at a position away from the striking position. After pouring the lining concrete and before demolding, the striking position on the inner surface of the centering is struck. Te Se The pouring platform and the lining concrete are vibrated, and the post-placement vibration is measured with the vibration measuring instrument. From the post-casting vibrations, a vibration characteristic value that correlates with the compressive strength of the lining concrete is determined. From these vibration characteristic values, the compressive strength of the lining concrete in the strength estimation region is estimated. A method for estimating the strength of lining concrete before demolding, characterized by the features described above.

[0008] (Effects and Benefits) According to this method for estimating the strength of the lining concrete before demolding, the strength of the lining concrete can be estimated at any location on the inner surface of the centering mold where the above-mentioned impact and vibration measurements are possible, and there is no need to directly access the concrete. Furthermore, by impacting the inner surface of the centering mold as in this method, even unhardened lining concrete can be reliably impacted and vibration measured without damage. Therefore, the strength of the lining concrete before demolding can be estimated more easily than with conventional methods.

[0009] <Second aspect> Before placing the lining concrete, the striking position on the inner surface of the centering is struck to vibrate the centering and the lining concrete, and the pre-placement vibration is measured with the vibration measuring instrument. From the vibration before placing and the vibration after placing, the frequency spectrum before placing and the frequency spectrum after placing are obtained respectively. Let the recorded waveform at the age t of the material be Y t (f), let the recorded waveform before placing be Y0(f), let the impact be X(f), and let the vibration medium at the age t of the material be D t (f), let the vibration medium before placing be D0(f), let the vibration measuring instrument be U(f), let the centor be S(f), and let the concrete at the age t of the material be C t (f). When "·" is regarded as the product in the frequency domain, the frequency response function H t (f), which is a characteristic quantity and correlates with the compressive strength of the concrete, is obtained from the frequency spectrum before placing and the frequency spectrum after placing. H t (f)=Y t (f) / Y0(f) ={X(f)·D t (f)·U(f)} / {X(f)·D0(f)·U(f)} =D t (f) / D0(f) ={S(f)·C t (f)} / S(f) =C t (f) …(1) Estimate the compressive strength of the covering concrete in the strength estimation region from the above characteristic quantity. A method for estimating the strength of covering concrete before demolding in the first aspect.

[0010] (Function and effect) The index for estimating the compressive strength can be obtained from the vibration measurement by centor impact, and is not particularly limited as long as it is a vibration characteristic value that correlates with the compressive strength of the covering concrete. However, if it is the frequency distribution of the gain of the frequency response function of this aspect, it is preferable because the compressive strength of the covering concrete can be estimated without being affected by the thickness of the steel plate at the impact position of the centor and the impact direction.

[0011] <The third aspect> Before placing the lining concrete, the striking position on the inner surface of the centering is struck to vibrate the centering and the lining concrete, and the pre-placement vibration is measured with the vibration measuring instrument. From the pre-casting vibration and the post-casting vibration, the pre-casting frequency spectrum and the post-casting frequency spectrum are obtained, respectively. The recorded waveform at age t is Y t Let (f) be the recorded waveform before concrete placement, let X be the impact, and let D be the vibrating medium at age t. t Let (f) be the vibration medium before placement, let U(f) be the vibration measuring instrument, let S(f) be the centering, and let C be the concrete at age t. t (f) When "·" is the product in the frequency domain, the gain increase rate determined as the coefficient of the logarithmic approximation curve in the frequency response function shown in equation (1) below is obtained from the frequency spectrum before and after concrete placement. H t (f) = Y t (f) / Y0(f) ={X(f)·D t (f)·U(f)} / {X(f)·D0(f)·U(f)} =D t (f) / D0(f) ={S(f)·C t (f) / S(f) =C t (f) …(1) The compressive strength of the lining concrete in the strength estimation region is estimated from the gain increase rate. A method for estimating the strength of lining concrete before demolding according to a first embodiment.

[0012] (Effects and Benefits) The index for estimating compressive strength can be obtained from vibration measurements caused by centering, and is not particularly limited as long as it is a vibration characteristic value that correlates with the compressive strength of the lining concrete. However, the gain increase rate in this embodiment is preferable because it allows for the estimation of the compressive strength of the lining concrete without being affected by the thickness of the steel plate at the centering impact position or the direction of impact.

[0013] <Fourth aspect> The thickness of the aforementioned lining concrete is 200 to 800 mm. A method for estimating the strength of lining concrete before demolding, according to any one of the three embodiments described above.

[0014] (Effects and Benefits) The above-described method for estimating the strength of the lining concrete is particularly effective when the thickness of the lining concrete is within the range of this embodiment.

[0015] <Fifth aspect> The aforementioned striking device is a hammer equipped with a steel head whose striking surface is a spherical shape with a diameter of 30 to 70 mm. A method for estimating the strength of lining concrete before demolding, according to one of the four embodiments described above.

[0016] (Effects and Benefits) In order to vibrate the steel plate and the lining concrete located on the back of it as a single unit, and to generate a clean vibration centered on the striking point, the hammer of this embodiment is suitable for the above-described method of estimating the strength of the lining concrete.

[0017] <Sixth aspect> A striking position is set on the inner surface of the centering in the intensity estimation region, and a vibration measuring instrument is installed at a position away from the striking position. After pouring the lining concrete and before demolding, the striking position on the inner surface of the centering is struck. Te Se The pouring platform and the lining concrete are vibrated, and the post-placement vibration is measured with the vibration measuring instrument. From the post-casting vibrations, a vibration characteristic value that correlates with the compressive strength of the lining concrete is determined. The demolding time is determined when this vibration characteristic value reaches the vibration characteristic value corresponding to the predetermined target compressive strength. A method for determining the timing of demolding of lining concrete before demolding, characterized by the above.

[0018] (Effects and Benefits) According to this method for determining the timing of demolding the lining concrete before demolding, the timing of demolding the lining concrete can be determined at any location on the inner surface of the centering mold where the above-mentioned impact and vibration measurements can be performed, and there is no need to directly access the concrete. Furthermore, by impacting the inner surface of the centering mold as in this method, even unhardened lining concrete can be reliably impacted and vibration measured without damage. Therefore, the timing of demolding the lining concrete before demolding can be determined more easily than with conventional methods. [Effects of the Invention]

[0019] The present invention offers advantages such as being able to more easily estimate the strength of the lining concrete before demolding. [Brief explanation of the drawing]

[0020] [Figure 1] This is a schematic diagram showing the tunnel cross-section during the placement of the lining concrete. [Figure 2] This is a schematic cross-sectional view showing the measurement status at the top of the surface. [Figure 3] (a) A plan view and (b) a cross-sectional view of the test specimen. [Figure 4] This is an explanatory diagram showing how to calculate the gain increase rate. [Figure 5] This graph shows the relationship between age, gain increase rate, and uniaxial compressive strength for (a) s6c100, (b) s6c300, (c) s9c100, and (d) s9c300. [Figure 6] (a) This graph shows the relationship between age, gain increase rate, and uniaxial compressive strength for c100 and c300. [Figure 7] (a) This graph shows the relationship between age, gain increase rate, and uniaxial compressive strength for the cases of s6 and s9. [Figure 8] (a) This graph shows the relationship between uniaxial compressive strength and gain increase rate for c100 and (b) c300. [Figure 9] This is the frequency spectrum for each Δt interval. [Figure 10] This is a graph of the frequency response function for each Δt. [Figure 11] This is a graph showing the gain increase rate for each Δt in the case of c100. [Figure 12] This is a graph showing the gain increase rate for each Δt in the case of c300. [Modes for carrying out the invention]

[0021] Figure 1 shows a cross-section of tunnel 1 with centering 2 installed, assuming the state after the pouring of lining concrete 3 is complete and before demolding. In the figure, reference numeral 1A indicates the top of the tunnel, 1B the left shoulder, 1C the right shoulder, 1D the left spring line, and 1E the right spring line, while reference numeral 4 indicates the natural ground.

[0022] To estimate the compressive strength (uniaxial compressive strength) of the lining concrete 3 before demolding, a striking position 2H is set on the inner surface of the centering 2 in the strength estimation area, as shown in Figure 2, and a vibration measuring instrument 10 is installed at a distance from the striking position 2H. The setting of the striking position 2H and the installation of the vibration measuring instrument 10 can be done before the placement of the lining concrete 3, or after the placement of the lining concrete 3, as long as vibration measurement is performed. The strength estimation area is the area to be measured for strength estimation, and can be selected from, for example, the top surface 1A, left shoulder 1B, right shoulder 1C, left spring line section 1D, and right spring line section 1E, one or more locations depending on the placement conditions and environmental conditions, or all locations. One or more striking positions 2H can be set in one strength estimation area, and one or more vibration measuring instruments can be installed. If there are multiple strength estimation areas, a vibration measuring instrument 10 can be installed at each location. As is well known, ordinary concrete is usually filled from the lap side to the gable end and from the spring line to the top 1A, and the top 1A (especially the gable end), which is the final pouring location, will be the youngest at the time of demolding. Therefore, the demolding time can be determined by estimating the compressive strength of the lining concrete 3 at least at the top 1A. On the other hand, when high-flow (self-filling) concrete is injected from a low position, the top 1A may not be the youngest. Therefore, the demolding time can be determined by setting the strength estimation area to multiple locations (for example, all locations at the top 1A, left shoulder 1B, right shoulder 1C, left spring line 1D, and right spring line 1E) and estimating the compressive strength of the lining concrete 3 at each location. The placement of the striking position 2H and the vibration measuring instrument 10 can be determined as appropriate as long as they are within the strength estimation area, but it is desirable to place them in the center of the strength estimation area as much as possible.

[0023] The vibration measuring instrument 10 can use an accelerometer, which is widely used in measuring impact elastic waves, as well as a speedometer, displacement meter, microphone, etc. The performance of the vibration measuring instrument 10 is not particularly limited, but when using an accelerometer, its sensitivity should be 5 to 20 mV / (m / s²). 2A vibration measuring instrument with a frequency range of approximately ±3dB: 3Hz to 10kHz, or a wider range, can be suitably used. Depending on the vibration characteristic values ​​used to estimate the compressive strength, multiple vibration measuring instruments 10 can be installed at intervals.

[0024] In estimating the compressive strength, after placing the lining concrete 3 and before demolding, the striking position 2H on the inner surface of the centering 2 in the strength estimation region is struck with a hammer 5 or the like to vibrate the centering 2 and the lining concrete 3, and the post-placement vibration is measured with a vibration measuring instrument 10. From the measured post-placement vibration, a vibration characteristic value that correlates with the compressive strength of the lining concrete 3 is obtained, and then the compressive strength of the lining concrete 3 in the strength estimation region is estimated from this vibration characteristic value. When recording the change in compressive strength at each age, or when the compressive strength has not reached the strength required for demolding, the striking and vibration measurement at the same position can be repeated at predetermined time intervals as needed, or the striking and vibration measurement can be performed again at the same position after a desired time has elapsed. In addition, to avoid the effects of measurement variability, multiple striking and vibration measurements can be performed at the same age and position, and the compressive strength can be obtained as the average value (this includes not only estimating the compressive strength based on the average value of feature quantities such as the gain increase rate described later, but also using the average value of the compressive strength as the final estimated value). In this case, it is natural that the distance between the impact position 2H and the position of the vibration measuring instrument 10 is kept constant. According to this estimation method, the strength of the lining concrete 3 can be estimated non-destructively at any location on the inner surface of the centering 2 where the above-mentioned impact and vibration measurements can be taken, and there is no need to directly access the lining concrete 3. Therefore, the strength of the lining concrete 3 before demolding can be estimated more easily than in the conventional method.

[0025] The correlation between the compressive strength of the lining concrete 3 and the vibration characteristic values ​​measured by impact (such as the gain increase rate described later) varies depending on the site conditions. Therefore, it is desirable to determine the correlation through preliminary experiments at the site or under the same conditions (e.g., temperature). Specifically, the necessary number of test specimens are prepared in advance at the site or under the same conditions (e.g., temperature) using concrete with the same mix as that used for the lining. The vibration characteristic values ​​and compressive strength of these specimens are measured sequentially at predetermined time intervals (e.g., 1 hour). Based on this measurement data, a relationship equation (e.g., an approximate straight line) between the vibration characteristic values ​​and compressive strength is determined. By substituting the vibration characteristic values ​​obtained from impact and vibration measurements of the centering during actual construction into this relationship equation, the compressive strength of the lining concrete during construction can be estimated. Furthermore, the demolding target strength (e.g., 2 N / mm²) can be obtained from this relationship equation. 2 By determining the target vibration characteristic value at the time of construction, the demolding time can be determined when the vibration characteristic value obtained based on impact and vibration measurements of the centering during actual construction reaches the target vibration characteristic value.

[0026] The thickness T of the lining concrete 3 is not particularly limited, but it is especially effective when it is between 200 and 800 mm. Also, the thickness S of the steel plate at the striking position 2H of the centering 2 is not particularly limited, but it is preferable when it is between 6 and 9 mm.

[0027] As long as the centering 2 and lining concrete 3 can be vibrated by impact, the impact method is not particularly limited, and a manual hammer 5 in which a worker holds the handle and strikes with brute force, or a hammer device that performs impact using a drive source such as a solenoid can be used. The hammer 5 can be any hammer that is widely used in the measurement of impact elastic waves, but in order to reduce the influence of the material and weight of the hammer 5, it is desirable to select a hammer 5 that is suitable for the vibration characteristic value used to estimate the compressive strength through prior experiments. From the experimental results described later, a hammer 5 with a steel head having a spherical impact surface with a diameter of 30 to 70 mm can be suitably used.

[0028] The vibration characteristic values ​​correlated with the compressive strength of the lining concrete 3 are not particularly limited as long as they can be determined from the vibrations measured in the measurement process, but the gain increase rate described below is preferable.

[0029] (Estimation of compressive strength based on gain increase rate) To determine the gain increase rate, before placing the lining concrete 3 and after placing it but before demolding, the striking position 2H on the inner surface of the centering 2 is struck with a hammer 5 or the like, as shown in Figure 2, to vibrate the centering 2 and the lining concrete 3, and the vibration before and after placing is measured with a vibration measuring instrument 10. Figure 4(a) shows an example, where 0h is the waveform of the vibration before placing, 1h is the vibration after 1 hour, and 24h is the vibration after 24 hours. Then, from the vibration before and after placing, the pre-placing frequency spectrum and post-placing frequency spectrum are obtained by FFT processing, etc., as shown in Figure 4(b), and based on these pre-placing frequency spectrum and post-placing frequency spectrum, the recorded waveform at age t is Y t Let (f) be the recorded waveform before concrete placement, let X be the impact, and let D be the vibrating medium at age t. t (f) Let's put it before pouring oh Let D0(f) be the vibration medium, U(f) be the vibration measuring instrument, S(f) be the centering, and C be the concrete at age t. t Let (f) be the product in the frequency domain, then the gain increase rate determined as the coefficient of the logarithmic approximation curve in the frequency response function shown in equation (1) below can be found. H t (f) = Y t (f) / Y0(f) ={X(f)·D t (f)·U(f)} / {X(f)·D0(f)·U(f)} =D t (f) / D0(f) ={S(f)·C t (f) / S(f) =C t (f) …(1) Figure 4(c) is a graph of the frequency response function showing the frequency spectrum after concrete placement with reference to the pre-placement state. The higher the frequency at which the amplitude change (gain) is large compared to the pre-placement state, the larger the gain increase rate determined as the coefficient a in the equation of the logarithmic approximation curve of the frequency response function. Figure 4(d) is a graph showing the change in the gain increase rate with age. The frequency range used to determine the logarithmic approximation curve of the frequency response function can be appropriately determined as long as the coefficient a and the compressive strength are correlated. For example, it can be the frequency range in which vibration can be detected as defined in the specifications of the vibration measuring instrument 10 (3Hz to 10kHz in the case of the accelerometer described later) or a narrower range (for example, 1 to 10kHz as described later).

[0030] As can be seen from the experimental examples described later, the rate of gain increase increases with the progression of age, resulting in a compressive strength of 2N / mm². 2 The compressive strength tends to increase up to (the compressive strength that serves as a guideline for demolding in actual construction). This is thought to be because, in response to the change in the elastic modulus accompanying the hardening of the lining concrete 3, the frequency response (gain) decreases around 1 kHz and increases around 10 kHz. Therefore, the compressive strength can be estimated by using the above-mentioned gain increase rate as an indicator. Furthermore, the demolding timing can also be determined. In addition, as can be seen from equation (1), the term related to the vibration of the steel plate disappears when calculating the frequency response function, so when estimating the compressive strength of the lining concrete 3 using this gain increase rate as an indicator, there is an advantage in that it does not have to be affected by the thickness of the steel plate at the impact position 2H of the centering 2.

[0031] The gain increase rate can be considered one of the features of the frequency response function that correlates with the compressive strength of concrete. Therefore, the gain increase rate is an effective indicator, but other features of the frequency response function that correlate with compressive strength, such as the centroid frequency or the maximum amplitude frequency, can also be used.

[0032] When using the gain increase rate as an indicator, the distance X between the striking position 2H and the vibration measuring instrument 10 is not particularly limited, but it is preferable to have it be 100 mm or less, for example.

[0033] <Experiment to estimate compressive strength using gain increase rate as an indicator> (Specimen) As shown in Figure 3, a top surface test specimen 20, which simulates the top surface of the lining concrete 3, was fabricated by erecting a formwork 22 around a flat steel plate 21 measuring 650 x 650 mm, which mimicked Center 2, and pouring concrete into the formwork 22. The thickness of the steel plate 21 was set to two levels, 6 mm and 9 mm (referred to as s6 and s9), and the thickness of the concrete T was set to two levels, 100 mm and 300 mm, which is the standard design thickness for lining (referred to as c100 and c300). The top surface test specimen 20 was lifted and supported using a stand 23 so that impact and vibration measurements could be taken on the underside of the steel plate 21.

[0034] The concrete mix design is shown in Table 1. Each top surface specimen 20 was left to stand in a constant temperature and humidity environment of 20°C and 60%RH after concrete placement. In addition, cylindrical specimens with a diameter of 100 mm and a height of 200 mm were prepared using the same concrete as that used to prepare the top surface specimens 20, and were used for compressive strength testing.

[0035] [Table 1]

[0036] (measurement) For the measurement, the center of the underside of the steel plate 21 of the top surface specimen 20 shown in Figure 3 was struck with a hammer 5, which had a handle attached to a head made of a steel ball. A vibration measuring instrument 10 (Fuji Ceramics Co., Ltd. accelerometer "SAF51A", sensitivity: 5mV / (m / s)) was placed 100mm away from the striking position 2H. 2 The signal was recorded within a frequency range of ±3dB (3~10,000Hz).

[0037] Table 2 shows the experimental parameters, test age, and level (abbreviations), and Table 3 shows the test items for each level. For s6c100 (steel plate thickness 6 mm, roll thickness 100 mm), steel balls of φ19, 30, and 40 mm were used to strike the steel plates. For s6c300 (steel plate thickness 6 mm, roll thickness 300 mm), steel balls of φ40 and 63 mm were used to aim for a higher energy input. For s9c100 (steel plate thickness 9 mm, roll thickness 100 mm) and s9c300 (steel plate thickness 9 mm, roll thickness 300 mm), steel balls of φ30, 40, and 63 mm were used to strike the steel plates. [Table 2] [Table 3]

[0038] Here, the resolution Δf of the frequency spectrum calculated by FFT (Fast Fourier Transform) is given by equation (2) below, where T is the sampling time, S is the number of samples, and Δt is the sampling time interval. Δf = 1 / T = 1 / (S·Δt) …(2)

[0039] From equation (2), the frequency resolution depends on the number of samples and the time interval. In the s6 series, signals were recorded with S:20,000 and Δt:0.1μs, and FFT was performed with 16 and 384 samples, resulting in a frequency resolution of Δf ≈ 0.61kHz. In the s9 series, waveforms were recorded with S:20,000 and Δt:0.1, 0.5, and 2.5μs, resulting in frequency resolutions Δf of 0.61, 0.12, and 0.024kHz.

[0040] In parallel with the impact elastic wave measurement, compressive strength tests were conducted at the ages of s6c100 and s9c100 shown in the underlined sections of Table 2. For the compressive strength test, the embedded surface of the cylindrical specimen was capped with gypsum, and the loading rate was set to 0.01 N / mm². 2 It was executed as / s.

[0041] (Calculation result of gain increase rate) Figures 5(a) to (d) show the changes in gain increase rate with age and the changes in compressive strength (measured by compressive strength tests) for s6c100 (steel ball diameters 19, 30, 40 mm), s6c300 (steel ball diameters 40, 63 mm), s9c100 (steel ball diameters 30, 40, 63 mm), and s9c300 (steel ball diameters 30, 40, 63 mm), respectively, calculated based on vibration measurement results (sampling time interval 0.1 μs, frequency resolution 0.61 kHz).

[0042] The overall trend in these results is that the gain increase rate increases with age, and it was found that the gain increase rate correlates with the compressive strength of the concrete. Furthermore, as shown in Figure 8, the gain increase rate was 2 N / mm² in both the c100 and c300 cases. 2 The trend is increasing up to 2N / mm 2 From this point onward, the gain increase rate was used to determine the demolding target strength of 2 N / mm² for the actual construction. 2 Strength estimation and determination of demolding time (for example, when the gain increase rate reaches about 40 dB / dec for a 300 mm thick lining, the compressive strength of the lining concrete should be set to 2 N / mm²) 2 It was found that it is possible to estimate the degree of damage, or to determine the timing of demolding the lining concrete.

[0043] However, as shown in Figure 6, the gain increase rate increased gradually in the range of -10 to 30 dB / dec for a concrete thickness of 100 mm, while it showed a somewhat sharper increasing trend in the range of -10 to 50 dB / dec for a concrete thickness of 300 mm. Here, assuming that the main component of the vibration obtained by the accelerometer is the deflection vibration of the whole (steel plate and concrete), the vibration of a rectangular thin plate fixed on all four sides is proportional to the square root of the bending stiffness, and the bending stiffness is proportional to the cube of the plate thickness. That is, the bending stiffness of a concrete thickness of 300 mm (c300) is 27 times higher than that of a concrete thickness of 100 mm (c100) at the same age, and it is thought that c300 exhibits a higher frequency than c100. For this reason, it is thought that the frequency spectrum shifted to the high-frequency side for c300 compared to c100, and the gain increase rate was evaluated as larger. Furthermore, assuming that the adhesion characteristics (integrity) between the concrete and the steel plate improve as the concrete develops strength over time, it is thought that at young ages, the adhesion characteristics between the concrete and the steel plate are poor, and the effect of steel plate vibration is dominant, resulting in similar gain increase rates under all conditions. As the concrete develops strength over time, the steel plate and concrete vibrate together, leading to a tendency for the gain increase rate to differ depending on the concrete thickness. Therefore, when using the gain increase rate as an indicator for determining compressive strength or demolding timing, it is preferable to determine the correlation between the gain increase rate and compressive strength through prior experiments, as this indicator may change depending on the concrete thickness.

[0044] On the other hand, compared to the effect of concrete thickness, the influence of steel plate thickness and the diameter of the steel balls used on the frequency response function was not clearly observed, as shown in Figure 7. This is thought to be because, as shown in equation (1), when calculating the frequency response function, the influence of terms common to both before and after concrete placement was canceled out.

[0045] On the other hand, Figures 9 and 10 show the frequency spectra and frequency response functions obtained for each age of material at different sampling time intervals Δt (for S9C300, steel ball diameter 40 mm). From Figure 9, it can be seen that this measurement contains almost no components below 0.1 kHz. Considering that the frequency range in which the accelerometer has a constant sensitivity is 0.003 kHz or higher, it can be concluded that vibrations below 0.1 kHz did not occur.

[0046] Next, as shown in Figure 10, the frequency response function was minimum around 1 kHz, and as the concrete's strength developed, the frequency response decreased in the frequency range below approximately 2 kHz, and in the range above that, the frequency response tended to increase up to around 10 kHz. This trend was similar regardless of the concrete thickness and steel ball diameter. Therefore, a coefficient (gain increase rate) was calculated from the logarithmic approximation curve of the gain from 1 to 10 kHz in the frequency response function, and the following investigation was conducted using this gain increase rate as a strength estimation index. Specifically, Figures 11 and 12 show the relationship between the gain increase rate obtained at each level with varying Δt and the estimated strength of the top specimen 20 for each concrete thickness. Here, the legend is φ (steel ball diameter) Δt (sampling time interval). Figures 11 and 12 show an increasing trend in the gain increase rate similar to Figure 8, and no clear change in the gain increase rate due to the difference in frequency resolution Δf was observed. Therefore, considering the frequency acquisition region shown in Figures 9 and 10, it is considered desirable to measure with a sampling time interval of 0.5 to 2.5 μs, i.e., a sampling time length of 10 to 50 ms, when the number of samples is 20,000.

[0047] Furthermore, a mock-up was prepared, and a mock-up formwork mimicking natural ground was installed at regular intervals (400 mm thickness) around the outer surface of the mock-up to create a simulated environment. Except for pouring lining concrete in the same manner as in actual construction, the same experiment was conducted under the same conditions as the compressive strength estimation experiment using the gain increase rate as an indicator, as described above. The result showed that the gain increase rate showed a similar increasing trend to that of c300. [Industrial applicability]

[0048] This invention can be used to estimate the strength of the lining concrete before demolding, that is, to determine the timing of demolding. Even when estimating compressive strength by the centering impact method described above, it is also possible to use conventional methods for estimating compressive strength based on the cumulative temperature of the lining concrete in conjunction with this method, if necessary. [Explanation of Symbols]

[0049] 1...Tunnel, 1A...Top of the tunnel, 1B...Left shoulder, 1C...Right shoulder, 1D...Left spring line, 1E...Right spring line, 2...Centering, 3...Lining concrete, 4...Natural ground, 2H...Impact position, 10...Vibration measuring instrument, T...Wall thickness, 5...Hammer, X...Separation distance, 11...First vibration measuring instrument, L...Second distance, 12...Second vibration measuring instrument, 21...Steel plate, 22...Formwork, 23...Support structure, 20...Top of the tunnel test specimen.

Claims

1. A striking position is set on the inner surface of the centering in the intensity estimation region, and a vibration measuring instrument is installed at a position away from the striking position. After pouring the lining concrete, and before demolding, the striking position on the inner surface of the centering is struck to vibrate the centering and the lining concrete, and the post-pouring vibration is measured with the vibration measuring instrument. From the post-casting vibrations, a vibration characteristic value that correlates with the compressive strength of the lining concrete is determined. From these vibration characteristic values, the compressive strength of the lining concrete in the strength estimation region is estimated. A method for estimating the strength of lining concrete before demolding, characterized by the features described above.

2. Before placing the lining concrete, the striking position on the inner surface of the centering is struck to vibrate the centering and the lining concrete, and the pre-placement vibration is measured with the vibration measuring instrument. From the pre-casting vibration and the post-casting vibration, the pre-casting frequency spectrum and the post-casting frequency spectrum are obtained, respectively. The recorded waveform at age t is Y ｔ (f) Let the recorded waveform before concrete pouring be Y ０ Let (f) be the force of the impact, and let D be the vibrating medium at age t. ｔ (f) Let D be the vibration medium before concrete placement. ０ Let (f) be the vibration measuring instrument, let S(f) be the centering, and let C be the concrete at age t. ｔ Let (f) be the frequency response function H given by equation (1) below, where "•" represents the product in the frequency domain. ｔ (f) is a feature quantity that correlates with the compressive strength of the concrete, and is determined from the pre-casting frequency spectrum and the post-casting frequency spectrum. H ｔ (f)=Y ｔ (f) / Y ０ (f) ={X(f)・D ｔ (f)・U(f)} / {X(f)・D ０ (f)・U(f)} =D ｔ (f) / D ０ (f) ={S(f)・C ｔ (f)} / S(f) =C ｔ (f) …(1) The compressive strength of the lining concrete in the strength estimation region is estimated from the aforementioned feature quantities. The method for estimating the strength of lining concrete before demolding, as described in claim 1.

3. Before placing the lining concrete, the striking position on the inner surface of the centering is struck to vibrate the centering and the lining concrete, and the pre-placement vibration is measured with the vibration measuring instrument. From the pre-casting vibration and the post-casting vibration, the pre-casting frequency spectrum and the post-casting frequency spectrum are obtained, respectively. The recorded waveform at age t is Y ｔ (f) Let the recorded waveform before concrete pouring be Y ０ Let (f) be the force of the impact, and let D be the vibrating medium at age t. ｔ (f) Let D be the vibrating medium that can be placed before concrete pouring. ０ Let (f) be the vibration measuring instrument, let S(f) be the centering, and let C be the concrete at age t. ｔ (f) and when "•" is the product in the frequency domain, the gain increase rate determined as the coefficient of the logarithmic approximation curve in the frequency response function shown in equation (1) below is obtained from the frequency spectrum before and after concrete placement. H ｔ (f)=Y ｔ (f) / Y ０ (f) ={X(f)・D ｔ (f)・U(f)} / {X(f)・D ０ (f)・U(f)} =D ｔ (f) / D ０ (f) ={S(f)・C ｔ (f)} / S(f) =C ｔ (f) …(1) The compressive strength of the lining concrete in the strength estimation region is estimated from the gain increase rate. The method for estimating the strength of lining concrete before demolding, as described in claim 1.

4. The thickness of the aforementioned lining concrete is 200 to 800 mm. A method for estimating the strength of lining concrete before demolding, according to claim 1 or 2.

5. The aforementioned striking device is a hammer equipped with a steel head whose striking surface is a spherical shape with a diameter of 30 to 70 mm. A method for estimating the strength of lining concrete before demolding, according to claim 1 or 2.

6. A striking position is set on the inner surface of the centering in the intensity estimation region, and a vibration measuring instrument is installed at a position away from the striking position. After pouring the lining concrete, and before demolding, the striking position on the inner surface of the centering is struck to vibrate the centering and the lining concrete, and the post-pouring vibration is measured with the vibration measuring instrument. From the post-casting vibrations, a vibration characteristic value that correlates with the compressive strength of the lining concrete is determined. The demolding time is determined when this vibration characteristic value reaches the vibration characteristic value corresponding to the predetermined target compressive strength. A method for determining the timing of demolding of lining concrete before demolding, characterized by the above.

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