Methods for estimating prestress

Abstract

This invention provides a method for accurately estimating the prestress of a PC structure without damaging the concrete and using a simple method. [Solution] The method is characterized by the following steps: selecting two locations (upper and lower parts of the transverse girders 4a to 4e) as preliminary measurement locations, where the design stress is known, and which are formed of concrete of the same material as the concrete of the location to be estimated (infill slab 3), and where the magnitude of the design stress is different; measuring the propagation time of ultrasonic pulses at these preliminary measurement locations and calculating the propagation speed; obtaining an equation representing the relationship between design stress and propagation speed from the design stress value and the propagation speed value at the preliminary measurement locations; and measuring the propagation time at the location to be estimated, and estimating the stress at the location to be estimated using the calculated propagation speed value and the equation representing the relationship between design stress and propagation speed.

Description

Technical Field

[0001] The present invention relates to a method for estimating the stress at a site where prestress acts in a prestressed concrete (PC) structure, and more particularly, to a method for estimating prestress (stress in the prestress direction) by measuring the propagation speed of ultrasonic pulses.

Background Art

[0002] PC generally involves tensioning PC steel materials arranged in a concrete member and applying the reaction force as prestress to the concrete. By utilizing this principle, it is possible to increase the span and reduce the weight of the member. Therefore, PC is utilized in various structures such as bridges.

[0003] By the way, in the maintenance and management of PC structures, it is necessary to grasp and evaluate the prestress situation of PC steel materials. However, since PC steel materials are arranged inside the concrete, it is difficult to directly grasp the prestress. Therefore, a method (stress release method) for grasping prestress by releasing the stress of concrete and steel bars has been put into practical use (Patent Document 1, etc.).

Prior Art Documents

Patent Documents

[0004]

Patent Document 1

Patent Document 2

Summary of the Invention

Problems to be Solved by the Invention

[0005] Figure 1 is a partial cross-sectional perspective view showing the structure of a typical PCT girder bridge 1. This PCT girder bridge 1 is composed of a main girder 2 (T-girder), a slab 3, and a transverse girder 4. In this PCT girder bridge 1, as shown in the figure, numerous main-direction PC steel members 21 are densely and eccentrically arranged inside the lower edge side of the main girder 2, and transverse PC steel members 31 (post-tensioned type) are arranged one by one at equal intervals near the centroid inside the slab formed by the upper flange of the main girder 2 and the slab 3. In addition, one (or more) transverse PC steel members 41 are also arranged under the transverse girder 4.

[0006] The stress relief method disclosed in Patent Document 1 can be suitably applied to the lower edge of the main girder 2 in the case of a PCT girder bridge 1 as shown in Figure 1, but it is difficult to apply to parts where the prestress is small and the stress state is complex, such as the infill deck slab 3. Furthermore, the stress relief method has problems such as causing localized failure of the concrete and requiring a high level of skill and investigation time.

[0007] Furthermore, while it may be possible to properly evaluate prestress in any part of an element such as the infill slab 3 by applying an ultrasonic method such as the one disclosed in Patent Document 2 (a method for evaluating prestress by measuring the propagation speed of ultrasonic pulses transmitted through the inside of concrete), it is necessary to note that when using the ultrasonic method, the obtained measurement values ​​will fluctuate due to various factors.

[0008] For example, factors that can cause measurement errors include (1) the accuracy of the measuring device, (2) the smoothness of the measuring surface, (3) the contact condition of the probe, and (4) the skill of the person performing the measurement. In addition, factors that can affect the propagation speed due to the prestress condition in the propagation path include (5) the distance between transmission and reception, (6) the measurement frequency, (7) the moisture content of the measuring surface, (8) the concrete material, and (9) the progression of shrinkage and creep.

[0009] Of these, factors (1) to (4) can be addressed to some extent by using measuring devices with small measurement errors, and factors (5) to (7) can be addressed to some extent by matching the measurement conditions. However, sufficient countermeasures have not yet been established for factors (8) and (9).

[0010] More specifically, as countermeasures for factors (8) and (9), methods have been considered to cancel out influencing factors by measuring propagation velocities in two directions (the prestress direction and a different direction) and to use measurement values ​​other than propagation velocity. However, the method of canceling out influencing factors by measuring propagation velocities in two directions has the problem that although prestress can be considered as the difference between the propagation velocities in two directions, the absolute value becomes small, making it susceptible to variability (uncertainty), and a considerable number of measurement values ​​must be obtained to obtain measurement accuracy. Furthermore, in the case of the infill slab 3 shown in Figure 1, if there is a crack 32 extending in the bridge axis direction (a direction perpendicular to the prestress direction), the method of measuring propagation velocities in two directions cannot be applied. Moreover, the physical relationship with the amount of prestress is unclear for methods that use measurement values ​​other than propagation velocity.

[0011] The present invention aims to solve the problems of the prior art and provides a method for estimating prestress in a PC structure that can estimate the prestress of the PC structure with high accuracy without damaging the concrete and using a simple method. [Means for solving the problem]

[0012] The present invention is a method for estimating the prestress of a PC structure by measuring the propagation time of ultrasonic pulses in the prestress direction using an ultrasonic measuring instrument at the target site, and estimating the prestress of the target site based on the measurement results, comprising the steps of: selecting two or more sites with different magnitudes of design stress as preliminary measurement sites in an element that constitutes the PC structure, has a known design stress, is made of concrete of the same material as the concrete of the target site, and has PC steel arranged thereon; measuring the propagation time in the prestress direction at these preliminary measurement sites and calculating the propagation velocity at the preliminary measurement sites from the measured values ​​of the propagation time obtained; obtaining an equation representing the relationship between design stress and propagation velocity from the value of the design stress at the preliminary measurement sites and the value of the propagation velocity at the preliminary measurement sites; and measuring the propagation time in the prestress direction at the target site, calculating the propagation velocity from the measured values ​​of the propagation time obtained, and estimating the stress at the target site using this propagation velocity value and the equation representing the relationship between design stress and propagation velocity.

[0013] Furthermore, it is also possible to obtain an equation that represents the relationship between the design stress and the propagation time at the preliminary measurement site, and then configure the system to estimate the stress at the target site using the propagation time value of the target site and the aforementioned equation.

[0014] Furthermore, it is preferable to measure the propagation time of the ultrasonic pulse using an ultrasonic measuring instrument with an ultrasonic pulse transmission / reception distance of 100 to 1000 mm and an ultrasonic pulse propagation time measurement accuracy of less than ±1%. Moreover, it is preferable to select a smooth surface as the preliminary measurement site and the estimation target site where the propagation speed in a direction different from the prestress direction (a direction forming an angle of 70 to 90°) is 90% or more of the propagation speed in the prestress direction (i.e., no damage such as cracks). It is also preferable to select a site with a moisture content of 1 to 5% as the preliminary measurement site and the estimation target site.

[0015] Furthermore, if there is a temperature difference of 6°C or more between the preliminary measurement site and the estimated target site, it is preferable to apply a temperature correction to the propagation velocity value of the preliminary measurement site and obtain an equation representing the relationship between the design stress and propagation velocity from the design stress value of the preliminary measurement site and the propagation velocity value of the preliminary measurement site after temperature correction. [Effects of the Invention]

[0016] The prestress estimation method according to the present invention can estimate the prestress of a PC structure without destroying the concrete and in a simple manner. Furthermore, it can be applied without problems even to areas where the prestress is small, the stress state is complex, and it is difficult to apply the stress relief method.

[0017] Furthermore, the prestress estimation method according to the present invention can be measured more simply than the conventional ultrasonic method (only the propagation time in the prestress direction needs to be measured, and the number of measurement lines and measurements required is small), and even if there are cracks extending in a direction perpendicular to the prestress direction in the concrete being measured, the stress can be estimated with sufficient accuracy, and the possibility of fracture of the PC steel can be determined.

[0018] Furthermore, according to the prestress estimation method of the present invention, when a PC steel material has fractured, the impact of the fracture can be diagnosed within the range of insufficient grout filling from the location of the fracture. While conventional methods required measurements to be taken at the location immediately adjacent to the fracture of the PC steel material, this method allows for indirect detection of fracture even when measurements are taken at a different location where insufficient grout filling occurs. [Brief explanation of the drawing]

[0019] [Figure 1] Figure 1 is a partial cross-sectional perspective view showing the structure of a typical PCT girder bridge 1. [Figure 2] Figure 2 is a graph plotting the measurement results of the propagation velocity of the crossbeam 4 in an experiment (example) conducted using the prestress estimation method according to the present invention.

Best Mode for Carrying Out the Invention

[0020] The prestress estimation method according to the present invention measures the propagation time of ultrasonic pulses in the prestress direction at a specific part (estimation target part) of a PC structure, and based on the measurement result, estimates the prestress (stress in the prestress direction) of the part. It is carried out by the following steps: 1. Preparation and adjustment of an ultrasonic measuring instrument; 2. Preliminary measurement; 3. Obtaining an expression representing the relationship between the design stress and the propagation speed; 4. This measurement and prestress estimation. This prestress estimation method can be applied even to parts where the prestress is small, the stress state is complex, and it is difficult to apply the stress relief method, such as the slab 3 between the spans of the PCT girder bridge 1 shown in FIG. 1. Hereinafter, an example of applying the present invention to the slab 3 between the spans of the PCT girder bridge 1 (a method for estimating the prestress of the slab 3) will be described as an embodiment of the present invention.

[0021] Step 1 (Preparation and adjustment of an ultrasonic measuring instrument) As an ultrasonic measuring instrument, prepare a measuring instrument having two probes (transmission-reception distance: 100 to 1000 mm) fixed to the main body at intervals, and in a state where these probes are in contact with the measurement object, measure the time (propagation time) until an ultrasonic pulse (for example, frequency 40 to 200 kHz) output from one probe reaches the other probe.

[0022] Next, prepare a reference test piece, conduct measurement of the propagation time, and confirm that the accuracy of the ultrasonic measuring instrument is less than ±1%. For example, if the measurement result for a reference test piece with a propagation time of 55×10 -6 seconds for a transmission-reception distance of 150 mm is between 54.5×10 -6 seconds and 55.5×10 -6 seconds, it can be confirmed that the accuracy of the ultrasonic measuring instrument is less than ±1%.

[0023] Step 2 (Preliminary measurement) Before performing the measurement of the ultrasonic pulse propagation time (main measurement) for the estimated target area (the infill deck slab 3 of PCT girder bridge 1 shown in Figure 1), a preliminary measurement (measurement of propagation time for areas other than the estimated target area) is performed to obtain an equation that represents the relationship between the design stress and propagation velocity of the concrete constituting this PCT girder bridge 1.

[0024] First, an element (measurement target) is selected from any of the elements constituting the PCT girder bridge 1, where the design stress is known, PC steel members are installed, and the element includes parts with different magnitudes of design stress. Two parts of this element with different magnitudes of design stress are then selected as preliminary measurement sites (first preliminary measurement site, second preliminary measurement site). For example, the transverse girder 4 (4a-4e) shown in Figure 1 is selected as the measurement target, the upper part of transverse girder 4 (where the design stress is small) is selected as the first preliminary measurement site, and the lower part of transverse girder 4 (where the design stress is large) is selected as the second preliminary measurement site. More than two preliminary measurement sites can be selected.

[0025] Furthermore, the measurement targets (and preliminary measurement areas) selected here must be made of concrete of the same material as the concrete of the estimated target area (infill slab 3) (concrete with the same raw materials and mix, and manufactured at the same time). This is because the propagation speed is affected by the concrete material, shrinkage, creep progression, etc. Whether the concrete is of the same material can be confirmed from the construction records (design calculations) of PCT girder bridge 1.

[0026] The measurement surface of the preliminary measurement area is preferably a smooth surface where the propagation velocity in a direction different from the prestress direction (a direction forming an angle of 70 to 90°) is 90% or more of the propagation velocity in the prestress direction (i.e., there is no damage such as cracks 32). Furthermore, the moisture content of the preliminary measurement area is preferably 1 to 5%.

[0027] In the first and second preliminary measurement areas, it is preferable to set multiple measurement locations (measurement lines) for each measurement target (crossbeam 4a to 4e) (for example, 3 locations), and it is preferable to perform measurements multiple times (for example, 3 times) for each measurement line. Specifically, the probe of the ultrasonic measuring instrument is pressed against the measurement surface (concrete surface), and trial measurements are repeated several times. After the measurement value stabilizes, multiple measurements are performed, and the propagation time is read each time. The measurement line (an imaginary line in which the two probes of the ultrasonic measuring instrument are brought into contact with each end) is set to be parallel to the direction of the stress to be determined.

[0028] After completing the measurement of propagation time at the preliminary measurement site, the propagation velocity is calculated from the measured values. In this case, if there is a variation (variation) of 10% or more among multiple measurement locations or multiple measurements, the data for that measurement location or measurement line is discarded.

[0029] Step 3 (Obtaining an equation that represents the relationship between design stress and propagation velocity) From the propagation velocity values ​​obtained in Step 2 (preliminary measurement) at the first and second preliminary measurement sites, the relationship between the design stress and propagation velocity of the concrete constituting PCT girder bridge 1 is determined.

[0030] The design stress of the concrete constituting PCT girder bridge 1 will be determined from the design calculations at the time of construction. If the design calculations are unavailable, a reconstruction design will be performed. (Even if the design stress of the concrete can be determined by performing a reconstruction design, it will be considered that "the design stress is known," and the selection requirements for the measurement target will be met in Step 2 (preliminary measurement).)

[0031] Furthermore, if there is a temperature difference in the measurement target (especially if the temperature difference is 6°C or more), temperature correction will be applied to the propagation velocity values ​​at the first and second preliminary measurement sites obtained in Step 2 (preliminary measurement).

[0032] Concrete expands as the temperature rises (coefficient of linear expansion: 10 × 10 -6As concrete expands (by 10°C), the propagation speed decreases. More specifically, if the temperature of concrete rises by 10°C, 1 meter of concrete will expand by 100 × 10 -6 When the material expands, the propagation speed decreases by approximately 2% (rate of change in speed per 10°C temperature difference: approximately 2%). Therefore, if the timing of Step 2 (preliminary measurement) and Step 4 (main measurement) differs, or if a temperature difference occurs in the object being measured due to other factors, the estimation accuracy can be improved by applying an appropriate temperature correction to the measured propagation speed obtained in Step 2 (preliminary measurement).

[0033] For example, if the concrete temperature during Step 4 (main measurement) is 10°C higher than during Step 2 (preliminary measurement), the propagation velocity measured during Step 4 (main measurement) will decrease by approximately 2%. Therefore, a temperature correction is applied to reduce the measured propagation velocity obtained in Step 2 (preliminary measurement) by 2%. Conversely, if the concrete temperature during Step 4 (main measurement) is 10°C lower, a temperature correction is applied to increase the measured propagation velocity obtained in Step 2 (preliminary measurement) by 2%. If the temperature difference in the measurement target is 6°C or less, it is considered to have little effect on the propagation velocity, and the application of temperature correction is unnecessary.

[0034] Let "x" be the design stress values ​​for the upper and lower parts of crossbeam 4 obtained from the design calculations, and "y" be the propagation velocity value (measured value or temperature corrected value) obtained in step 2. If we perform linear regression on these values ​​using the least squares method, we obtain a linear function equation of the form "y = ax + b". This is transformed into the equation "x = (yb) / a" which calculates the estimated stress (x) from the propagation velocity (y).

[0035] Step 4 (Main measurement and prestress estimation) Measurements are taken by pressing the probe of an ultrasonic measuring instrument against the concrete surface (on a measuring line parallel to the prestress direction) of the estimated target area (the infill slab 3 of PCT girder bridge 1 shown in Figure 1) (main measurement). The propagation velocity (y) is calculated from the measured propagation time and substituted into the equation (x=(yb) / a) obtained in step 3 to calculate the estimated stress (x) in the prestress direction.

[0036] By comparing the obtained estimated stress (x) value with the design stress value of the infill slab 3, it is possible to determine whether the lateral PC steel members 31 are in a sound state or fractured at the location where this measurement was performed. Furthermore, if the lateral PC steel members 31 are fractured, the measurement results of this measurement on the surrounding area allow for the detection and evaluation of the extent of the fracture's impact (including insufficient grout filling, etc.).

[0037] In this explanation, we have described an example of applying the present invention to the infill deck slab 3 of the PCT girder bridge 1 shown in Figure 1. However, the application of the present invention is not limited to the infill deck slab 3 of the PCT girder bridge 1. For example, it can also be applied to the lower part, upper part (deck slab), and web of the main girder 2 of the PCT girder bridge 1, as well as the transverse girder 4, etc. It can also be suitably applied to various elements of bridges such as box girder bridges and composite girder bridges.

[0038] Furthermore, in the above embodiment, in step 2, the "propagation speed" is calculated from the measured value of the "propagation time" at the preliminary measurement site, in step 3, an equation representing the relationship between the design stress and the "propagation speed" is obtained, and in step 4, the estimated stress in the prestress direction of the target site is calculated using the "propagation speed" of the target site and the equation obtained in step 3. However, it is also possible to calculate the estimated stress based on "propagation time" instead of "propagation speed," that is, in step 3, an equation representing the relationship between the design stress and the "propagation time" at the preliminary measurement site is obtained, and in step 4, the estimated stress in the prestress direction of the target site is calculated using the measured value of the "propagation time" of the target site and the equation obtained in step 3. [Examples]

[0039] Here, the results of a test conducted by the inventors to confirm the effectiveness of the prestress estimation method according to the present invention (method for estimating the prestress of the infill slab 3) will be described as an embodiment of the present invention.

[0040] Step 1 (Preparation and adjustment of the ultrasonic measuring instrument) An ultrasonic measuring instrument with a distance of 150 mm between the two probes (transmitting / receiving distance) was prepared, and it was confirmed that the accuracy was less than ±1%.

[0041] Step 2 (Preliminary Measurement) The transverse beams 4 (4a-4e), which are made of the same concrete as the infill slab 3 (see Figure 1), which is the estimated target area, were selected as the measurement target. The upper part of transverse beam 4 (design stress 1.1) was selected as the first preliminary measurement area, and the lower part of transverse beam 4 (design stress 3.1) was selected as the second preliminary measurement area. After confirming that these preliminary measurement areas were smooth surfaces and had a moisture content of 1-5%, propagation time was measured using an ultrasonic measuring instrument, and the propagation velocity at the preliminary measurement areas was calculated from the obtained measured values.

[0042] Table 1 shows the propagation time measurement results at the first preliminary measurement site (upper part of the crossbeams 4a to 4e) and the propagation speed calculated based on these results, and Table 2 shows the propagation time measurement results at the second preliminary measurement site (lower part of the crossbeams 4a to 4e) and the propagation speed calculated based on these results. The propagation speed (unit: m / sec) was calculated by dividing the transmission / reception distance (0.15m) by the propagation time (average value of measured values, unit: seconds). [Table 1] [Table 2]

[0043] Step 3 (Obtaining an equation that represents the relationship between design stress and propagation velocity) Since it was confirmed that the concrete temperature during Step 4 (main measurement) was 20°C higher than during Step 2 (preliminary measurement), a temperature correction was applied to reduce the propagation velocity values ​​obtained in Step 2 (preliminary measurement) by 4% each (see Tables 1 and 2).

[0044] Figure 2 shows a graph with design stress on the x-axis and propagation velocity on the y-axis, plotting the design stress values ​​obtained from the design calculations (upper part of crossbeam 4: 1.1, lower part of crossbeam 4: 3.1) and the propagation velocity values ​​of crossbeam 4 (after temperature correction) obtained in step 2 (preliminary measurement). By performing linear regression of these values ​​using the least squares method, the following linear function equation was obtained. Equation 1: y = 137.6x + 3912

[0045] Transforming the above equation into an equation for calculating the estimated stress (x) from the propagation velocity (y), we obtain the following: Formula 2: x = (y - 39¹²) / 137.6

[0046] Step 4 (Main measurement and prestress estimation) When the probe of an ultrasonic measuring instrument was pressed against the concrete surface of the estimated target area (the infill deck slab 3 of the PCT girder bridge 1 shown in Figure 1) (on a measurement line parallel to the direction in which the transverse PC steel members 31 extend (prestress direction) shown in Figure 1) and measured, the propagation time was 33.6 × 10⁻¹⁴. -6 At 2 seconds, the propagation speed (y) was 4464 m / s. By substituting this value of propagation speed (y) into equation 2 above, the estimated stress (x) is 4.0 N / mm². 2 The following value was obtained.

[0047] When the estimated stress (x) value obtained for the infill slab 3 was compared with the design stress value for the infill slab 3 obtained from the design calculations of the PCT girder bridge 1 at the time of construction, it was confirmed that they were in close agreement. Therefore, it was possible to evaluate that the lateral PC steel members 31 were in a sound condition in the area where this measurement was performed. 2If the stress level is below (for example, 50% or less of the design stress), it can be concluded that there is a high probability that the lateral PC steel member 31 has fractured. [Explanation of Symbols]

[0048] 1:PCT girder bridge, 2: Main girder, 3: Infill slabs, 4,4a~4e: crossbeam, 21: Main direction PC steel material, 31: Lateral PC steel material, 32: Cracks, 41: Lateral PC steel material

Claims

1. A method for estimating the prestress of a PC structure by measuring the propagation time of ultrasonic pulses in the prestress direction using an ultrasonic measuring instrument at the target site of the PC structure, and estimating the prestress of the target site based on the measurement results, The steps include selecting two or more locations with different magnitudes of design stress as preliminary measurement locations within an element that constitutes the aforementioned PC structure, has a known design stress, is made of concrete of the same material as the concrete in the estimated target area, and has PC steel members arranged therein, and measuring the propagation time in the prestress direction for these preliminary measurement locations, The steps include obtaining an equation that represents the relationship between design stress and propagation time from the design stress value at the preliminary measurement site and the propagation time value in the prestress direction at the preliminary measurement site, The process involves measuring the propagation time in the prestress direction for the target area, and estimating the stress in the prestress direction of the target area using this propagation time value and an equation that expresses the relationship between the design stress and the propagation time. A method for estimating prestress, characterized by including [a specific element].

2. A method for estimating the prestress of a PC structure by measuring the propagation time of ultrasonic pulses in the prestress direction using an ultrasonic measuring instrument at the target site of the PC structure, and estimating the prestress of the target site based on the measurement results, The steps include: selecting two or more locations with different magnitudes of design stress as preliminary measurement locations within an element that constitutes the aforementioned PC structure, has a known design stress, is made of concrete of the same material as the concrete in the area to be estimated, and has PC steel members arranged therein; measuring the propagation time in the prestress direction at these preliminary measurement locations; and calculating the propagation velocity in the prestress direction at the preliminary measurement locations from the measured values ​​of the obtained propagation time; The steps include obtaining an equation representing the relationship between design stress and propagation velocity from the design stress value of the preliminary measurement site and the propagation velocity value in the prestress direction at the preliminary measurement site, The process involves measuring the propagation time in the prestress direction for the area to be estimated, calculating the propagation velocity from the measured propagation time, and estimating the stress in the prestress direction of the area to be estimated using this propagation velocity value and the equation representing the relationship between the design stress and the propagation velocity. A method for estimating prestress, characterized by including [a specific element].

3. A method for estimating prestress according to claim 1 or claim 2, characterized in that the propagation time of an ultrasonic pulse is measured using an ultrasonic measuring instrument having an ultrasonic pulse transmission / reception distance of 100 to 1000 mm and an ultrasonic pulse propagation time measurement accuracy of less than ±1%.

4. A method for estimating prestress according to claim 1 or 2, characterized in that a smooth surface is selected as a preliminary measurement site and an estimation target site, wherein the propagation velocity in a direction 70 to 90° different from the prestress direction is 90% or more of the propagation velocity in the prestress direction.

5. A method for estimating prestress according to claim 1 or claim 2, characterized in that a part with a water content of 1 to 5% is selected as a preliminary measurement site and a site to be estimated.

6. The method for estimating prestress according to claim 1, characterized in that, when there is a temperature difference of 6°C or more between the preliminary measurement site and the estimation target site, a temperature correction is applied to the propagation time value of the preliminary measurement site, and an equation representing the relationship between design stress and propagation time is obtained from the design stress value of the preliminary measurement site and the propagation time value of the preliminary measurement site after temperature correction.

7. The method for estimating prestress according to claim 2, characterized in that, when there is a temperature difference of 6°C or more between the preliminary measurement site and the estimation target site, a temperature correction is applied to the propagation velocity value of the preliminary measurement site, and an equation representing the relationship between design stress and propagation velocity is obtained from the design stress value of the preliminary measurement site and the propagation velocity value of the preliminary measurement site after temperature correction.

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