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 form stripping, and a method for determining the time to strip the lining concrete. [Background technology]

[0002] In the construction of lining concrete in mountain tunnel construction, ordinary concrete is poured in order from the side walls to the top, so the top, which is the last place to be poured, is the youngest. Therefore, at the top, where the concrete is at a young age, it is easy for the concrete to not develop its strength when it is removed from the form, and there is a concern that the concrete may peel off or fall off. Also, depending on the pouring conditions and environmental conditions, the concrete other than the top may not develop its strength easily. Furthermore, when high-flow (self-compacting) concrete is pumped in from a low position, the top is not necessarily the youngest.

[0003] For this reason, for example, a method has been proposed in which the accumulated temperature of the lining concrete before removal from the form is measured, and the compressive strength of the lining concrete is measured based on the correlation between this and the compressive strength (see Patent Document 1).

[0004] However, because this method directly measures the temperature of the concrete inside the center (steel formwork), it is necessary to either partially modify the center or to leave sensors or other equipment inside the hardened concrete. [Prior art documents] [Patent documents]

[0005] [Patent Document 1] JP 2012-26734 A Summary of the Invention [Problem to be solved by the invention]

[0006] Therefore, the main object of the present invention is to provide a method for more easily estimating the strength of lining concrete before it is removed from the form. [Means for solving the problem]

[0007] The method for estimating the strength of lining concrete before removal from the form, which solves the above problems, is as follows. <First aspect> A striking position is set on the inner surface of the center in the strength estimation area, and a vibration measuring device is installed at a position away from the striking position. After pouring the lining concrete, before demolding, the striking position on the inner surface of the center is struck to vibrate the center and the lining concrete, and the vibration after pouring is measured with the vibration measuring device. A vibration characteristic value having a correlation with the compressive strength of the lining concrete is obtained from the vibration after pouring, The compressive strength of the lining concrete in the strength estimation region is estimated from the vibration characteristic value. A method for estimating the strength of lining concrete before removal from the form.

[0008] (Action and effect) According to this method for estimating the strength of lining concrete before form stripping, the strength of the lining concrete can be estimated at any location on the inner surface of the center as long as the impact and vibration measurement are possible, and there is no need to directly access the concrete. Furthermore, by striking the inner surface of the center as in this method, it is possible to reliably strike and measure vibration even on unhardened lining concrete without damaging it. Therefore, it is possible to estimate the strength of the lining concrete before form stripping more easily than in the past.

[0009] <Second aspect> Before pouring the lining concrete, the striking position on the inner surface of the center is struck to vibrate the center and the lining concrete, and the vibration before pouring is measured with the vibration measuring device. A pre-casting frequency spectrum and a post-casting frequency spectrum are obtained from the pre-casting vibration and the post-casting vibration, respectively; The recorded waveform at material age t is Y t (f), the recorded waveform before pouring is Y0(f), the impact is X(f), and the vibration medium at material age t is D t (f), the vibration medium before pouring is D0(f), the vibration measuring instrument is U(f), the center is S(f), and the concrete at age t is C t (f), and "·" is the product in the frequency domain. The frequency response function H t (f) A feature quantity correlated with the compressive strength of concrete is obtained from the frequency spectrum before pouring and the frequency spectrum after pouring; 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) Estimating a compressive strength of the lining concrete in the strength estimation region from the feature amount. A method for estimating the strength of lining concrete before removal from the form according to the first embodiment.

[0010] (Action and effect) The index for estimating the compressive strength can be obtained from vibration measurements by striking the center, and is not particularly limited as long as it is a vibration characteristic value that has a correlation with the compressive strength of the lining concrete. However, it is preferable to use the frequency distribution of the gain of the frequency response function of this embodiment, because it allows the compressive strength of the lining concrete to be estimated without being influenced by the thickness of the steel plate at the striking position of the center or the striking direction.

[0011] <Third aspect> Before pouring the lining concrete, the striking position on the inner surface of the center is struck to vibrate the center and the lining concrete, and the vibration before pouring is measured with the vibration measuring device. A pre-casting frequency spectrum and a post-casting frequency spectrum are obtained from the pre-casting vibration and the post-casting vibration, respectively; The recorded waveform at material age t is Y t (f), the recorded waveform before pouring is Y0(f), the impact is X(f), and the vibration medium at material age t is D t (f), the vibration medium before pouring is D0(f), the vibration measuring instrument is U(f), the center is S(f), and the concrete at age t is C t (f) where "·" 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 the following equation (1) is obtained from the frequency spectrum before pouring and the frequency spectrum after pouring, 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) Estimating the compressive strength of the lining concrete in the strength estimation region from the gain increase rate. A method for estimating the strength of lining concrete before removal from the form according to the first embodiment.

[0012] (Action and effect) The index for estimating the compressive strength can be obtained from vibration measurements by striking the center, and is not particularly limited as long as it is a vibration characteristic value that has a correlation with the compressive strength of the lining concrete. However, the gain increase rate in this embodiment is preferable because it allows the compressive strength of the lining concrete to be estimated without being influenced by the thickness of the steel plate at the striking position of the center or the striking direction.

[0013] <Fourth aspect> The thickness of the lining concrete is 200 to 800 mm. A method for estimating strength of lining concrete before removal from the form according to any one of first to third aspects.

[0014] (Action and effect) The above-mentioned method for estimating the strength of the lining concrete is particularly effective when the thickness of the lining concrete roll is within the range of this embodiment.

[0015] <Fifth aspect> The impact is performed by a hammer having a steel head with a spherical impact surface having a diameter of 30 to 70 mm. A method for estimating strength of lining concrete before removal from the form according to any one of the first to fourth aspects.

[0016] (Action and effect) This type of hammer is suitable for the above-mentioned method of estimating the strength of lining concrete, as it vibrates the steel plate and the lining concrete located on the back side of it as a whole, and generates clean vibrations centered on the impact position.

[0017] <Sixth aspect> A striking position is set on the inner surface of the center in the strength estimation area, and a vibration measuring device is installed at a position away from the striking position. After pouring the lining concrete, before demolding, the striking position on the inner surface of the center is struck to vibrate the center and the lining concrete, and the vibration after pouring is measured with the vibration measuring device. A vibration characteristic value having a correlation with the compressive strength of the lining concrete is obtained from the vibration after pouring, When the vibration characteristic value reaches a vibration characteristic value corresponding to a preset target compressive strength, it is determined that it is time to remove the mold. A method for determining when to remove lining concrete before removal of the form, comprising:

[0018] (Action and effect) According to this method for determining when to remove the lining concrete before it is removed from the form, it is possible to determine when to remove the lining concrete from any location on the inner surface of the center as long as the impact and vibration measurement are possible, and there is no need to directly access the concrete. Furthermore, by striking the inner surface of the center as in this method, it is possible to reliably strike and measure vibration even in unhardened lining concrete without damaging it. Therefore, it is possible to determine when to remove the lining concrete before it is removed from the form more easily than in the past. Effect of the Invention

[0019] According to the present invention, advantages are provided such as making it easier to estimate the strength of the lining concrete before it is stripped of its form. [Brief description of the drawings]

[0020] [Figure 1] FIG. 2 is a schematic diagram showing the cross section of a tunnel when lining concrete is poured. [Diagram 2] FIG. 13 is a schematic cross-sectional view showing the measurement status of the top end portion. [Diagram 3] (a) Plan view and (b) cross-sectional view of the specimen. [Figure 4] FIG. 11 is an explanatory diagram showing a method for calculating a gain increase rate. [Diagram 5] 13 is a graph showing the relationship between material age, gain increase rate, and unconfined compressive strength for (a) S6C100, (b) S6C300, (c) S9C100, and (d) S9C300. [Figure 6] 1 is a graph showing the relationship between material age, gain increase rate, and unconfined compressive strength for (a) c100 and (b) c300. [Figure 7] 13 is a graph showing the relationship between the material age and the gain increase rate and the unconfined compressive strength in the cases of (a) s6 and (b) s9. [Figure 8] 1 is a graph showing the relationship between unconfined compressive strength and gain increase rate for (a) c100 and (b) c300. [Figure 9] This is the frequency spectrum for each Δt. [Figure 10] 1 is a graph of a frequency response function for each Δt. [Figure 11] 13 is a graph showing the gain increase rate for each Δt in the case of c100. [Figure 12] 13 is a graph showing the gain increase rate per Δt for c300. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0021] Figure 1 shows a cross section of a tunnel 1 with a center tunnel 2 installed, assuming a state after the pouring of the lining concrete 3 has been completed and before removal of the form. In the figure, reference numeral 1A denotes the top end, 1B the left shoulder, 1C the right shoulder, 1D the left spring line, and 1E the right spring line, respectively, and reference numeral 4 denotes the natural ground.

[0022] In order to estimate the compressive strength (uniaxial compressive strength) of the lining concrete 3 before demolding, an impact position 2H is set on the inner surface of the center 2 in the strength estimation area as shown in FIG. 2, and a vibration measuring device 10 is installed at a position away from the impact position 2H. The setting of the impact position 2H and the installation of the vibration measuring device 10 can be performed not only before pouring the lining concrete 3, but also after pouring the lining concrete 3, as long as it is before vibration measurement. The strength estimation area is an area where strength estimation is performed, and one or more locations can be selected from the top end 1A, left shoulder 1B, right shoulder 1C, left spring line 1D, and right spring line 1E, depending on the pouring conditions and environmental conditions, or all locations can be selected. One or more impact positions 2H can be set in one strength estimation area, and one or more vibration measuring devices can be installed. If there are multiple strength estimation areas, a vibration measuring device 10 can be installed in each location. As is well known, normal concrete is usually filled from the lap side to the gable side and from the spring line to the top end 1A, and the top end 1A (especially the gable), which is the final pouring location, is the youngest when the form is removed, so the time to remove the form can be determined by estimating the compressive strength of the lining concrete 3 at least at the top end 1A. On the other hand, when high-flow (self-compacting) concrete is pressed in from a low position, the top end 1A is not necessarily the youngest, so the strength estimation area is set to multiple locations (for example, all locations of the top end 1A, left shoulder 1B, right shoulder 1C, left spring line 1D, and right spring line 1E), and the compressive strength of the lining concrete 3 at each location can be estimated to determine the time to remove the form. The impact position 2H and the installation position of the vibration measuring device 10 may be appropriately determined as long as they are within the strength estimation area, but it is preferable to set them in the center of the strength estimation area as much as possible.

[0023] The vibration measuring instrument 10 may be an accelerometer, which is widely used in measuring impact elastic waves, as well as a speedometer, a displacement meter, a microphone, etc. The performance of the vibration measuring instrument 10 is not particularly limited, but when an accelerometer is used, its sensitivity is preferably 5 to 20 mV / (m / s 2) and a frequency range of about ±3 dB: 3 Hz to 10 kHz, or a wider range than these can be suitably used. A plurality of vibration measuring instruments 10 can be installed at intervals depending on the vibration characteristic value used to estimate the compressive strength.

[0024] In estimating the compressive strength, after pouring the lining concrete 3, the striking position 2H on the inner surface of the center 2 in the strength estimation area is struck with a hammer 5 or the like before demolding to vibrate the center 2 and the lining concrete 3, and the post-casting vibration is measured with a vibration measuring device 10. From the measured post-casting vibration, a vibration characteristic value correlating with the compressive strength of the lining concrete 3 is obtained, and the compressive strength of the lining concrete 3 in the strength estimation area is estimated from this vibration characteristic value. When recording the change in compressive strength at each age, or when the compressive strength does not reach the strength required for demolding, it is possible to repeat the striking and vibration measurement at the same position at a predetermined time interval as necessary, or to perform the striking and vibration measurement at the same position again after a desired time has elapsed. In addition, in order to avoid the influence of measurement variations, it is also possible to perform the striking and vibration measurement multiple times at the same age and at the same position and obtain the compressive strength as the average value (including not only estimating the compressive strength based on the average value of the characteristic amount 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 goes without saying that the distance between the impact position 2H and the position of the vibration measuring device 10 is 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 center 2 where the above-mentioned impact and vibration measurement are possible, and there is no need to directly access the lining concrete 3. Therefore, the strength of the lining concrete 3 before stripping can be estimated more easily than in the past.

[0025] Since the correlation between the compressive strength of the lining concrete 3 and the vibration characteristic value (such as the gain increase rate described later) measured by impact varies depending on the conditions at the site, it is desirable to obtain the correlation by a preliminary experiment at the site or under the same conditions as at the site (for example, temperature). That is, a required number of test specimens are prepared in advance at the site or under the same conditions as at the site (for example, temperature) using the same mix of concrete as that used for the lining, and the vibration characteristic values ​​and compressive strength of these test specimens are measured in sequence at a predetermined time interval (for example, one hour), and a relational expression (for example, an approximate straight line) between the vibration characteristic value and the compressive strength is obtained based on the measurement data, and the compressive strength of the lining concrete during construction can be estimated by substituting the vibration characteristic value obtained based on the impact of the center and vibration measurement into this relational expression during the actual construction. Also, the target strength for demolding (for example, 2 N / mm 2 ) the target vibration characteristic value is calculated in advance, and when the vibration characteristic value calculated based on the impact of the center and vibration measurement during actual construction reaches the target vibration characteristic value, it can be determined that it is time to remove the formwork.

[0026] The winding thickness T of the lining concrete 3 is not particularly limited, but is particularly effective when it is 200 to 800 mm. In addition, the thickness S of the steel plate at the impact position 2H of the center 2 is also not particularly limited, but is preferably 6 to 9 mm.

[0027] As long as the center 2 and the lining concrete 3 can be vibrated by striking, the striking means is not particularly limited, and may be a manual hammer 5 that a worker holds with a handle and strikes with arm strength, or a hammer device that strikes with a drive source such as a solenoid. As the hammer 5, any hammer that is widely used in measuring impact elastic waves may be used without particular limitation, but in order to reduce the influence of the material and weight of the hammer 5, it is desirable to select a hammer 5 suitable for the vibration characteristic value used to estimate the compressive strength through a prior experiment or the like. From the experimental results described below, a hammer 5 with a steel head whose striking surface is a sphere having a diameter of 30 to 70 mm can be suitably used.

[0028] The vibration characteristic value having a correlation with the compressive strength of the lining concrete 3 is not particularly limited as long as it can be obtained from the vibration measured in the measurement step, but the gain increase rate described below is preferable.

[0029] (Estimation of compressive strength based on gain increase rate) In finding the gain increase rate, before pouring the lining concrete 3 and after pouring and before demolding, striking position 2H on the inner surface of the center 2 as shown in Figure 2 is done with a hammer 5 or the like to vibrate the center 2 and the lining concrete 3, while measuring the pre- and post-pouring vibrations with a vibration measuring device 10. Figure 4(a) shows an example, where 0h is the waveform of the pre-pouring vibration, 1h is the waveform after 1 hour, and 24h is the waveform after 24 hours. Then, from the pre- and post-pouring vibrations, a pre- and post-pouring frequency spectrum is found by FFT processing or the like as shown in Figure 4(b), and the recorded waveform at material age t is converted into Y based on these pre- and post-pouring frequency spectra. t (f), the recorded waveform before pouring is Y0(f), the impact is X(f), and the vibration medium at material age t is D t (f), the vibration medium placed before pouring is D0(f), the vibration measuring instrument is U(f), the center is S(f), and the concrete at age t is C t (f) and "·" is the product in the frequency domain, the gain increase rate can be determined as the coefficient of the logarithmic approximation curve in the frequency response function shown in the following equation (1). 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) FIG. 4(c) is a graph of a frequency response function showing a frequency spectrum after pouring based on that before pouring, and the higher the frequency at which the amplitude change (gain) is greater compared to before pouring, the greater the gain increase rate determined as the coefficient a of the logarithmic approximation curve of the frequency response function. FIG. 4(d) is a graph showing the change in gain increase rate due to material age. The frequency range in which the logarithmic approximation curve of the frequency response function is calculated can be determined as appropriate as long as it is within a range in which the coefficient a and the compressive strength are correlated. For example, it can be the vibration detectable frequency range determined in the specifications of the vibration measuring instrument 10 (3 Hz to 10 kHz for the accelerometer described later) or a narrower range (for example, 1 to 10 kHz as described later).

[0030] As will be seen from the experimental example described later, the gain increase rate increases with increasing material age, with compressive strength increasing by 2N / mm 2 The gain increase rate shows an increasing tendency up to or above 1kHz (the compressive strength that is the benchmark for form stripping in actual construction). This is thought to be due to the frequency response (gain) decreasing around 1kHz and increasing around 10kHz in response to the change in the elastic modulus accompanying the hardening of the lining concrete 3. Therefore, by using the above-mentioned gain increase rate as an index, the compressive strength can be estimated. In addition, the time for form stripping 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 index, there is an advantage in that it is not affected by the thickness of the steel plate at the impact position 2H of the center 2.

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

[0032] When the gain increase rate is used as the index, the distance X between the impact position 2H and the vibration measuring device 10 is not particularly limited, but is preferably, for example, 100 mm or less.

[0033] <Experiment to estimate compressive strength using gain increase rate as an index> (Specimen) As shown in Fig. 3, a formwork 22 was erected on all four sides of a flat steel plate 21 measuring 650 mm in length and 650 mm in width that imitates the center 2, and concrete was poured into the formwork 22 to prepare a top specimen 20 that imitates the top of the lining concrete 3. 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 T of the concrete was set to two levels, 100 mm and 300 mm (referred to as c100 and c300), which is the standard design thickness of the lining. The top specimen 20 was lifted and supported by a stand 23 so that impact and vibration measurements could be taken on the underside of the steel plate 21.

[0034] The concrete mix is ​​shown in Table 1. After pouring the concrete, each top specimen 20 was left in a constant temperature and humidity environment of 20°C and 60% RH. In addition, a cylindrical specimen with a diameter of 100 mm and a height of 200 mm was prepared using the same concrete as that used to prepare the top specimen 20, and used for the compressive strength test.

[0035] [Table 1]

[0036] (measurement) For the measurement, the center of the underside of the steel plate 21 of the top specimen 20 shown in FIG. 3 was struck with a hammer 5 having a handle attached to a head made of a steel ball, and the vibration measuring device 10 (Fuji Ceramics Co., Ltd. accelerometer "SAF51A", sensitivity: 5 mV / (m / s 2 ), and signals were recorded in the frequency range ±3 dB: 3–10,000 Hz.

[0037] The experimental parameters, test age and level (abbreviation) are shown in Table 2, and the test items for each level are shown in Table 3. For S6C100 (steel plate thickness 6 mm, winding 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, winding thickness 300 mm), steel balls of φ40 and 63 mm were used to strike the steel plates in order to input a larger amount of energy, and for S9C100 (steel plate thickness 9 mm, winding thickness 100 mm) and S9C300 (steel plate thickness 9 mm, winding 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 expressed by the following formula (2) using the sampling time length: T, the number of samples: S, and the sampling time interval: Δt. Δf = 1 / T = 1 / (S Δt) …(2)

[0039] According to formula (2), the frequency resolution depends on the number of samples and the time interval. In the s6 series, the signal was recorded with S: 20,000 samples and Δt: 0.1 μs, and FFT was performed with 16,384 samples, so Δf ≒ 0.61 kHz. In the s9 series, the waveform was recorded with S: 20,000 samples and Δt: 0.1, 0.5, and 2.5 μs, so the frequency resolution Δf was 0.61, 0.12, and 0.024 kHz.

[0040] In parallel with the impact elastic wave measurement, compressive strength tests were also conducted at the ages of S6C100 and S9C100, as shown in the underlined part of Table 2. The compressive strength tests were performed with the cylindrical specimens with the driving surface capped with gypsum and the loading rate set to 0.01 N / mm 2 The experiment was carried out as / s.

[0041] (Gain increase rate calculation result) Figures 5(a) to (d) show the change in gain increase rate with material age and the change in compressive strength (measured by compressive strength test) 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), calculated based on the vibration measurement results (sampling time interval 0.1 μs, frequency resolution 0.61 kHz).

[0042] The overall trend of these results is that the gain increase rate increases with age, and it is found to be correlated with the compressive strength of concrete. Also, as shown in Figure 8, in both cases of c100 and c300, the gain increase rate was 2N / mm 2 The trend is to increase to 2N / mm 2 Since it became almost constant after that, the gain increase rate was calculated based on the target strength for removal of the actual work, which is 2N / mm 2 (For example, when the gain increase rate reaches about 40 dB / dec for a roll thickness of 300 mm, the compressive strength of the lining concrete is reduced to 2 N / mm 2 It was found that it is possible to estimate the extent of the damage, or determine the time when the lining concrete should be removed from its form.

[0043] However, as shown in Fig. 6, the gain increase rate increases gradually in the range of -10 to 30 dB / dec for a concrete thickness of 100 mm, whereas it shows a somewhat rapid increase in the range of -10 to 50 dB / dec for a concrete thickness of 300 mm. If we assume that the main component of the vibration obtained by the accelerometer is the bending vibration of the whole (steel plate and concrete), the vibration of a rectangular thin plate with four edges fixed is proportional to the square root of the bending rigidity, and the bending rigidity is proportional to the cube of the plate thickness. In other words, the bending rigidity 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 material age, and c300 is considered to show a higher frequency than c100. For this reason, it is considered that the frequency spectrum of c300 shifts to the higher frequency side compared to c100, and the gain increase rate was evaluated to be larger. Also, assuming that the bond properties (integrity) between the concrete and the steel plate increase due to the development of concrete strength as the material ages, the bond properties between the concrete and the steel plate are poor at an early age, and the effect of the vibration of the steel plate is prominent, so the gain increase rate under all conditions shows similar values, and it is considered that the gain increase rate tends to differ depending on the concrete thickness because the steel plate and concrete vibrate together as the material ages and the concrete develops its strength. Therefore, when the gain increase rate is used as an index for determining the compressive strength or the time to remove the formwork, the index may change depending on the concrete thickness, so it is preferable to find the correlation between the gain increase rate and the compressive strength by a preliminary experiment.

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

[0045] On the other hand, the frequency spectrum and frequency response function (for S9C300, steel ball diameter 40mm) obtained for each material age by sampling time interval Δt are shown in Figures 9 and 10. It can be seen from Figure 9 that this measurement contains almost no components below 0.1kHz. Considering that the frequency range with a certain sensitivity of the accelerometer is 0.003kHz or more, it can be concluded that no vibration below 0.1kHz is occurring.

[0046] Next, from Fig. 10, it was confirmed that the frequency response function was at a minimum near 1 kHz, and that the frequency response decreased in the frequency range below about 2 kHz as the concrete strength developed, and in the range above that, the frequency response tended to increase up to around 10 kHz. This tendency was the same regardless of the concrete thickness and the steel ball diameter. For this reason, a coefficient was calculated from the logarithmic approximation curve of the gain from 1 to 10 kHz in the frequency response function (gain increase rate), and the following study was performed using this gain increase rate as a strength estimation index. That is, Figs. 11 and 12 show the relationship between the gain increase rate obtained at each level of changing 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). Figs. 11 and 12 show the same increasing tendency of the gain increase rate as Fig. 8, and no clear change in the gain increase rate due to differences in the frequency resolution Δf was observed. For this reason, taking into consideration the frequency acquisition range shown in FIG. 9 and FIG. 10, it is considered desirable to perform measurements with a sampling time interval of 0.5 to 2.5 μs for a sampling number of 20,000, that is, a sampling time length of 10 to 50 ms.

[0047] In addition, a center tube to be used in the actual construction was prepared, and a mock ground formwork simulating the ground was installed at a fixed interval (400 mm roll thickness) from the outer periphery of the center tube to create a simulated environment. Covering concrete was poured in the same way as in the actual construction. Except for this, a similar experiment was conducted under the same conditions as the compressive strength estimation experiment using the gain increase rate as an index, and the gain increase rate showed a similar increasing trend to that of c300. [Industrial Applicability]

[0048] The present invention can be used to estimate the strength of the lining concrete before it is stripped, i.e., to determine when it is time to strip it. Even when estimating the compressive strength using the center strike method described above, it is also possible to use the conventional estimation of compressive strength based on the accumulated temperature of the lining concrete in combination, if necessary. [Explanation of symbols]

[0049] 1...tunnel, 1A...top end, 1B...left shoulder, 1C...right shoulder, 1D...left spring line section, 1E...right spring line section, 2...center, 3...lining concrete, 4...ground, 2H...impact position, 10...vibration measuring instrument, T...winding 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...frame, 20...top end specimen.

Claims

1. A striking position is set on the inner surface of the center in the strength estimation area, and a vibration measuring device is installed at a position away from the striking position. After pouring the lining concrete, before demolding, the striking position on the inner surface of the center is struck to vibrate the center and the lining concrete, and the vibration after pouring is measured with the vibration measuring device. A vibration characteristic value having a correlation with the compressive strength of the lining concrete is obtained from the vibration after pouring, The compressive strength of the lining concrete in the strength estimation region is estimated from the vibration characteristic value. A method for estimating the strength of lining concrete before removal from the form.

2. Before pouring the lining concrete, the striking position on the inner surface of the center is struck to vibrate the center and the lining concrete, and the vibration before pouring is measured with the vibration measuring device. A pre-casting frequency spectrum and a post-casting frequency spectrum are obtained from the pre-casting vibration and the post-casting vibration, respectively; The recorded waveform at material age t is Y ｔ (f), and the recorded waveform before pouring is Y ０ (f), the impact is X(f), and the vibration medium at material age t is D ｔ (f), and the vibration medium before pouring is D ０ (f), the vibration measuring instrument is U(f), the center is S(f), and the concrete at age t is C(f). ｔ (f) and "·" is a product in the frequency domain, the frequency response function H ｔ (f) A feature quantity correlated with the compressive strength of concrete is obtained from the pre-pouring frequency spectrum and the post-pouring 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) Estimating a compressive strength of the lining concrete in the strength estimation region from the feature amount. A method for estimating strength of lining concrete before removal from the form as claimed in claim 1.

3. Before pouring the lining concrete, the striking position on the inner surface of the center is struck to vibrate the center and the lining concrete, and the vibration before pouring is measured with the vibration measuring device. A pre-casting frequency spectrum and a post-casting frequency spectrum are obtained from the pre-casting vibration and the post-casting vibration, respectively; The recorded waveform at material age t is Y ｔ (f), and the recorded waveform before pouring is Y ０ (f), the impact is X(f), and the vibration medium at material age t is D ｔ (f), and the vibration medium that can be placed before pouring is D ０ (f), the vibration measuring instrument is U(f), the center is S(f), and the concrete at age t is C(f). ｔ (f) and "·" 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 the following formula (1) is obtained from the frequency spectrum before pouring and the frequency spectrum after pouring. 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) Estimating the compressive strength of the lining concrete in the strength estimation region from the gain increase rate. A method for estimating strength of lining concrete before removal from the form as claimed in claim 1.

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

5. The impact is performed by a hammer having a steel head with a spherical impact surface having a diameter of 30 to 70 mm. A method for estimating strength of lining concrete before removal from the form according to claim 1 or 2.

6. A striking position is set on the inner surface of the center in the strength estimation area, and a vibration measuring device is installed at a position away from the striking position. After pouring the lining concrete, before demolding, the striking position on the inner surface of the center is struck to vibrate the center and the lining concrete, and the vibration after pouring is measured with the vibration measuring device. A vibration characteristic value having a correlation with the compressive strength of the lining concrete is obtained from the vibration after pouring, When the vibration characteristic value reaches a vibration characteristic value corresponding to a preset target compressive strength, it is determined that it is time to remove the mold. A method for determining when to remove lining concrete before removal of the form, comprising:

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