JP7481300B2 - Method for estimating stress in concrete members - Google Patents

Description

The present invention relates to a method for estimating stress in a concrete member, and in particular to a method for estimating prestress in a concrete member.

Conventionally, two strain gauges, one in the X direction and one in the Y direction, are attached to the measurement surface of the concrete part of a concrete member, and a cylindrical core is cut vertically from the measurement surface using a core drilling machine to extract the core. The released strain, which is the difference between the strain before and after the cut obtained through the strain gauge, is determined, and the prestress is estimated from the released stress based on this released strain (see, for example, Patent Document 1).

Japanese Patent No. 5095258

However, unless a special core drilling machine is used to make the cut, it is not possible to obtain measurement data from the strain gauges during cutting. This is because the wiring extending from the strain gauges to the measuring instruments gets in the way of the cutting by the core drill of the core drilling machine. Therefore, strain data is measured before cutting, the wiring is temporarily removed before cutting, and the wiring is reconnected after cutting is completed to obtain strain data.

Disconnecting and reconnecting the wiring in this way takes time. In addition, because strain data cannot be obtained during cutting, defects such as cracks in the concrete that occur during cutting cannot be detected, and there is a risk that the accuracy of prestress detection will be reduced.

In addition, when measuring using conventional methods with two or more sets of strain gauges on the same measurement surface, each set consisting of two strain gauges in the X and Y directions, it is necessary to space them at least 500 mm apart and take one piece of data for each measurement position to avoid the effects of released strain at each measurement position or the effects of minute cracks caused by core drilling. However, there are few cases where it is possible to ensure such a distance.

In view of the above, the present invention aims to provide a method for estimating stress in concrete members that can simplify the work and improve the accuracy of strain data.

The method for estimating stress in a concrete member of the present invention comprises the steps of attaching at least one strain gauge to the measurement surface of the concrete member, making a cut in the concrete portion of the concrete member so as to surround the locations where all of the strain gauges are attached and extend in the direction of the extension of the measurement surface of the concrete member, detecting the strain of the concrete member in the attachment direction of the strain gauge via the strain gauge, and estimating the stress of the concrete portion in the attachment direction of the strain gauge based on the detected strain.

According to the method of estimating stress in a concrete member of the present invention, cuts are made in the concrete portion so as to extend in the direction of the measurement surface of the concrete member, surrounding the locations where all of the strain gauges are attached. As a result, unlike the conventional case described above, the wiring of the strain gauges does not get in the way when making the cuts with a core drilling machine or the like, so strain data can be continuously acquired from the strain gauges from the start of the cut to its completion. Therefore, by continuously grasping the behavior of the strain, defects such as cracks in the concrete that occur during the cut can be detected, making it possible to improve the accuracy of stress estimation.

In the method of estimating stress in a concrete member of the present invention, it is preferable that the attachment of the at least one strain gauge includes attaching a first strain gauge in an arbitrary first direction and attaching a second strain gauge in a second direction intersecting the first direction.

In this case, it becomes possible to measure strain in two different directions, and estimate stress in two different directions.

In addition, in the method for estimating stress in a concrete member of the present invention, it is preferable to attach multiple pairs of the first strain gauge and the second strain gauge.

In this case, strain in two different directions can be measured with even greater precision, making it possible to improve the precision of estimating stress in two different directions.

In addition, in the method for estimating stress in a concrete member of the present invention, it is preferable that one of the at least one or more strain gauges is attached in the direction of the prestress acting on the concrete member.

In this case, it is possible to measure the strain in the direction of the prestress, and thus to detect the stress in the direction of the prestress.

In addition, in the method of estimating stress in a concrete member of the present invention, when multiple strain gauges are attached, it is preferable that each strain gauge is spaced apart in the direction in which the cuts are made.

In this case, the strain gauges are spaced apart, making it possible to measure strain with high accuracy through the strain gauges. In addition, because cuts are made in the concrete portion so that they extend in the direction of the measurement surface, the measurement surface onto which the strain gauges can be attached can be ensured to extend in the direction of the cut, making it possible to attach multiple strain gauges at a distance. It is also possible to attach two or more sets of strain gauges.

In addition, in the method for estimating stress in a concrete member of the present invention, it is preferable that the direction in which the cuts are made in the concrete member is perpendicular to the direction of prestress.

In this case, when comparing the strains detected through multiple strain gauges, the prestress can be compared at the same position. Also, it is structurally difficult to make cuts in the concrete in a direction parallel to the prestress direction, so it is preferable to make cuts in a perpendicular direction.

In addition, in the method of estimating stress in a concrete member of the present invention, it is preferable that the wiring connected to the strain gauge extends on the side opposite to the side where the incision begins.

In this case, it is possible to prevent interference between the strain gauge wiring and the core drill of the core drilling machine when making the cut.

In addition, in the method of estimating stress in a concrete member of the present invention, a cut is made in the concrete portion so that it extends in the direction of the measurement surface. This means that the temperature rise in the concrete when making the cut is smaller and the effect of temperature distortion is also smaller, compared to the conventional method in which the cut is made so that it extends perpendicular to the measurement surface.

Furthermore, since the temperature rise of the concrete is small, the temperature difference occurring between a pair of strain gauges is small, and it can be assumed that the temperature strain is the same. As will be described later, in the method for estimating stress in a concrete member of the present invention, the attachment of the at least one or more strain gauges includes attaching an X-direction strain gauge in the X direction and attaching a Y-direction strain gauge in the Y direction perpendicular to the X direction. When the strain detected via the X-direction strain gauge is εX', the strain detected via the Y-direction strain gauge is εY', and the Poisson's ratio of the concrete portion is ν, it is also possible to calculate the temperature strain εT, the X-direction strain εX with the temperature strain εT removed, and the Y-direction strain εY with the temperature strain εT removed based on the following equations (1) to (3).
εT = (εY' + vεX')/(1 + v) ... (1)
εX = (εX' - εY') / (1 + v) ... (2)
εY = ν (εX' - εY') / (1 + ν) ... (3)

FIG. 4 is a schematic diagram showing a state in which a notch is made in concrete in the method for estimating stress in a concrete member according to the embodiment of the present invention. 11 is a graph showing an example of strain data before and after cutting.

The method for estimating stress in a concrete member according to an embodiment of the present invention will be described with reference to Figure 1. Note that Figure 1 is a diagram for explaining this embodiment in a schematic manner, and the dimensions have been exaggerated.

The concrete member 10 that is the subject of this method is, for example, a member made of concrete 12 prestressed with PC steel members 11, as shown in Figure 1. The concrete member 10 is particularly a PC (prestressed) concrete member in which the PC steel members 11 are tensioned to apply tension and then fixed to the concrete 12. PC concrete members are used in PC girders of PC bridges, etc.

In such PC concrete members, it may be necessary to estimate the actual prestress in order to confirm whether the desired prestress is acting, or because design documents do not exist and the prestress is unknown. This method is suitable for estimating such prestress.

In this method, first, a process is performed in which strain gauges 14, 15 are attached to the measurement surface 13 of the concrete portion 12 of the concrete member 10. However, it is preferable to attach the strain gauges 14, 15 in a position that is not close to the PC steel member 11.

The strain gauges 14 and 15 are attached in close contact with the measurement surface 13, and strain data for detecting strain in the attachment direction occurring on the measurement surface 13 can be obtained from a measuring instrument (not shown).

Here, a first strain gauge 14 is attached to the measurement surface 13 in the X direction to detect strain in the X direction (left-right direction in FIG. 1), and a second strain gauge 15 is attached to the measurement surface 13 in the Y direction to detect strain in the Y direction (front-back direction in FIG. 1), which is perpendicular to the X direction.

Furthermore, here, multiple sets of the first and second strain gauges 14, 15, specifically two sets, are attached. By installing multiple sets of strain gauges 14, 15 in this manner, the behavior of multiple strain data can be compared to rationally determine the suitability of the strain measurement values, thereby improving measurement accuracy.

Here, the strain gauges 14, 15 are attached with a gap between them so that they are aligned in the direction in which the measurement surface 13 extends, specifically in the X direction. The direction in which the measurement surface 13 extends is any direction, preferably perpendicular to the PC steel material 11 on which the prestress acts, in which the prestress is applied at the same position. When the measurement surface 13 is the bottom surface of the concrete member 10 as in this embodiment, the direction in which the measurement surface 13 extends is preferably the X direction. When the measurement surface is a curved surface, the direction in which the measurement surface extends is the direction in which the measurement surface extends as a whole.

Next, with the strain gauges 14, 15 attached, a process is performed in which incisions 16 are made in the concrete 12 so that they extend in the direction of the measurement surface 13. In addition, a process is performed in which the strain is measured via each strain gauge 14, 15. These processes are performed simultaneously as described above, but it is also possible to simply measure the strain before and after each incision.

The incision 16 is made using a commercially available core drill (not shown). The core drill is equipped with a cylindrical core drill (core bit) and is typically used to cut out a cylindrical core from concrete in the conventional manner described above.

In this embodiment, this core drilling machine is used to make a semicircular incision 16 in the concrete portion 12 parallel to the measurement surface 13 from left to right in FIG. 1, to cut out a semicylindrical core 17. However, the incision 16 may also be made from right to left in FIG. 1. In the cut semicylindrical core 17, all the strain gauges 14, 15 are positioned in a straight line on the surface of a rectangular plane extending in the central axis direction (left-right direction) so as to extend in that direction.

It is also possible to attach strain gauges to the underside of the concrete member 10 in the front-to-back direction (Y direction) and make cuts in the front-to-back direction, but this is not preferable because it results in a cross section with a different stress state. It also makes it difficult to perform drilling work.

The incisions 16 are then made so that the areas where all the strain gauges 14, 15 are attached are on the inside, and the periphery is cut off so that the core 17 is released from prestress. When the prestress is released, the stress acting on the measurement surface 13 changes, and the strain on the measurement surface 13 changes, so the strain data acquired by the strain gauges 14, 15 also changes. As an example is shown in Figure 2, the difference in strain before and after the incisions is the released strain.

The semicircular cuts 16 are made so that cutting is performed with half of the core drill exposed to the outside of the concrete 12. This allows the chips to be discharged more smoothly than in the conventional circular cut method, making it less likely that the core 17 will experience a temperature rise due to friction between the chips. Furthermore, in the exposed area, the core drill comes into contact with the outside air and dissipates heat, suppressing a temperature rise in the core 17.

Concrete expands and undergoes thermal distortion when its temperature rises, so it is preferable that the temperature difference in the core 17 before and after the incision is small. In addition, excessive temperature rise is undesirable, as there is a risk that the moisture contained in the concrete will evaporate and cause shrinkage. As described above, this method can suppress the temperature rise in the core 17 caused by the incision 16, so it is possible to suppress the temperature distortion of the core 17 before and after the incision, and also to shorten the waiting time until the temperature of the core 17 after the incision approaches the temperature before the incision.

The semi-cylindrical core 17 can be easily pulled out, so that relevant information that affects the measurement accuracy, such as the internal aggregate of the concrete part 12, the carbonation depth, and the state of cracks, can be confirmed from the pulled out core 17. Similar measurements can also be made on the cut side.

The core 17 does not necessarily have to be cut out. It is sufficient to simply make a cut 16 that extends beyond the area where the strain gauges 14, 15 are attached, so that no prestress is applied to any of the areas where the strain gauges 14, 15 are attached.

In addition, the core 17 is not limited to a semi-cylindrical shape, and may have a shape in which a part of the cylinder has been removed so that the cross-sectional shape is larger or smaller than a semicircle. However, if the cross-sectional shape is larger than a semicircle, it will be difficult to remove the core 17 unless a cut is made along the entire length of the measurement surface.

The wiring 18 connected to each strain gauge 14, 15 extends to the side opposite the side where the core drilling machine starts making the incision 16, and is connected to a measuring instrument (not shown). As a result, the wiring 18 of the strain gauges 14, 15 do not interfere with the cutting by the core drilling machine, so that strain data can be continuously acquired from the start of the incision to its completion, as shown in FIG. 2, and the behavior of strain can be continuously understood. This makes it possible to detect defects such as cracks in the concrete 12 that occur during the incision, and improves the accuracy of strain measurement and therefore the accuracy of stress detection.

However, the strain may be detected only before the cutting starts and after the cutting is completed. Even in this case, it is possible to determine the released strain from the difference between these strains.

It is also preferable to attach the strain gauges 14, 15 at an appropriate interval so that they do not interfere with or overlap each other. Instead of making a cut perpendicular to the measurement surface as in the conventional case described above, the cut 16 is made to extend in the direction in which the measurement surface 13 extends, so that the measurement surface 13 can be secured over a long range, and multiple strain gauges 14, 15 can be attached to the measurement surface 13 at appropriate intervals.

Concrete is a non-homogeneous material made from a mixture of gravel, sand, cement, and water, and its properties are particularly affected by aggregates such as gravel. The aggregate of concrete used in civil engineering structures is generally 20mm to 40mm, and it is considered preferable that the dimensions of a strain gauge that can measure while minimizing the effects of this aggregate be about three times the maximum dimension of the aggregate.

Therefore, for example, it is preferable that the dimensions of the strain gauges 14, 15 used in this embodiment are 60 mm to 75 mm, and the diameter of the core 17 is about 100 mm. The distance between the strain gauges 14, 15 is about 75 mm or more, so that multiple strain gauges 14, 15 can be installed close to each other.

Then, a process is carried out to estimate the stress in the concrete based on the measured strain.

Below, an example of stress estimation that takes temperature strain into account is described. However, the method of stress estimation is not limited to this, and conventional methods may be used.

It is considered that the X-direction strain εX' detected through the X-direction strain gauge 14 is composed of the X-direction stress εX and the X-direction temperature strain εXT, and the Y-direction strain εY' detected through the Y-direction strain gauge 15 is composed of the Y-direction stress εY and the X-direction temperature strain εYT. As a result, the following equations (1) and (2) are established.
εX′=εX+εXT … (1)
εY′=εY+εYT … (2)

The relationship between the stress εX in the X direction and the stress εY in the Y direction is expressed by the following equation (3) using the Poisson's ratio ν of the concrete 12.
εY = -ν εX … (3)

In the method for estimating stress in a concrete member according to this embodiment, the temperature rise in the concrete 12 is small as described above when the core drilling machine makes the incision 16. Therefore, the temperature difference between the strain gauges 14, 15 is small, and it can be assumed that the temperature strains are the same. Therefore, it is presumed that the temperature strain εXT in the X direction and the temperature strain εYT in the X direction are equal, and the following formula (4) is established.
εXT = εYT ... (4)

From the above formulas (1) to (4), the temperature strain εT, the x-direction strain εX, and the y-direction strain εY can be calculated based on the following formulas (5) to (7).
εT = εXT = εYT = (εY' + vεX')/(1 + v) ... (5)
εX = (εX' - εY') / (1 + v) ... (6)
εY = ν (εX' - εY') / (1 + ν) ... (7)

The x-direction strain εX is the strain in the X-direction obtained by subtracting the temperature strain εT from the measured strain εX' in the X-direction obtained through the X-direction strain gauge. When the measured strain εX' is the released strain, which is the difference in strain before and after the cut, it represents the released strain in the X-direction that was present in the concrete.

In the above-described strain calculation method, the temperature strains εXT and εYT can be removed from the detected strains εX' and εY', so the strains εX' and εY can be estimated regardless of the temperature rise of the core 17. Therefore, it is not necessary to wait until the core 17 cools before measuring the strain, and it is possible to shorten the measurement time.

Note that the present invention is not limited to the above-mentioned method for estimating stress in concrete members, and can be modified as appropriate.

For example, a case has been described in which a strain gauge 14 is attached in the X direction and a strain gauge 15 is attached in the Y direction, and strain is determined via these strain gauges 14, 15. However, the strain gauge may be, for example, a single sheet. In this case, for example, by attaching a strain gauge in the direction of prestress, it is possible to determine the prestress.

In addition, when attaching multiple strain gauges, the strain gauges do not necessarily need to be attached so as to detect strain in mutually orthogonal directions. This is because, if the direction of strain detected by the strain gauges is known, it is possible to determine the strain in the direction in which prestress acts, for example, even if it is not orthogonal. In addition, although the concrete member 10 that is the subject of this method has been described as a prestressed member, it may also be a non-prestressed concrete member.

10...Concrete member, 11...PC steel, 12...Concrete, concrete part, 13...Measurement surface, 14...Strain gauge, first strain gauge, X-direction strain gauge, 15...Strain gauge, second strain gauge, Y-direction strain gauge, 16...Notch, 17...Core, 18...Wiring.

Claims (8)

A step of attaching at least one strain gauge to a measurement surface of a concrete member;
making a cut in the concrete portion of the concrete member so as to surround all of the locations where the strain gauges are attached and extend in the direction in which the measurement surface of the concrete member extends;
detecting a strain of the concrete member in a direction in which the strain gauge is attached via the strain gauge;
and estimating the stress of the concrete portion in the direction in which the strain gauge is attached based on the detected strain. The method for estimating stress in a concrete member according to claim 1, characterized in that the attachment of the at least one strain gauge includes attaching a first strain gauge in an arbitrary first direction and attaching a second strain gauge in a second direction intersecting the first direction. The method for estimating stress in a concrete member according to claim 2, characterized in that multiple sets of the first strain gauge and the second strain gauge are attached. The method for estimating stress in a concrete member according to any one of claims 1 to 3, characterized in that one of the at least one or more strain gauges is attached in the direction of prestress acting on the concrete member. 5. The method for estimating stress in a concrete member according to claim 1, wherein a direction in which the concrete member is cut is perpendicular to a direction of prestress. The method for estimating stress in a concrete member according to any one of claims 1 to 5, characterized in that when multiple strain gauges are attached, the strain gauges are spaced apart in the direction in which the cuts are made. The method for estimating stress in a concrete member according to any one of claims 1 to 6, characterized in that the wiring connected to the strain gauge extends on the side opposite to the side where the incision is started. The attachment of the at least one strain gauge includes attaching an X-direction strain gauge in an X-direction and attaching a Y-direction strain gauge in a Y-direction perpendicular to the X-direction,
The method for estimating stress in a concrete member according to any one of claims 1 to 7, characterized in that, when the strain detected via the X-direction strain gauge is εX', the strain detected via the Y-direction strain gauge is εY', and the Poisson's ratio of the concrete portion is ν, temperature strain εT, strain in the X-direction εX with temperature strain εT removed, and strain in the Y-direction εY with temperature strain εT removed are calculated based on the following equations (1) to (3).
εT = (εY' + vεX')/(1 + v) ... (1)
εX = (εX' - εY') / (1 + v) ... (2)
εY = ν (εX' - εY') / (1 + ν) ... (3)

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