JP3975255B2 - Concrete embedded strain gauge and concrete strain measurement method - Google Patents

Description

[0001]
BACKGROUND OF THE INVENTION
The present invention relates to an embedded concrete strain gauge and a concrete strain measurement method using the strain gauge, and more specifically, cement kneading useful for elucidating the mechanism of cracking phenomenon due to self-shrinkage of concrete. The present invention relates to a concrete-embedded strain gauge capable of continuously measuring strain in the process from the initial stage to hardening, and a concrete strain measurement method using the strain gauge.
[0002]
[Prior art]
In general, concrete used in structures gradually solidifies with the passage of time from the fluidized state (cement paste state) at the time of placing, and after several days to several tens of days, depending on the type of concrete, It cures and is completely cured after several years. The cementitious material undergoes self-shrinkage immediately after placement, and in order to measure this self-shrinkage, a method of measuring the strain by directly contacting a strain meter with concrete is most desirable.
However, when a conventional embedded strain gauge is used, the Young's modulus of the strain gauge itself is large, so that the shrinkage behavior immediately after placing concrete with low strength and Young's modulus cannot be measured. there were.
[0003]
That is, the Young's modulus of the conventional buried strain gauge there 39N / mm ^2, also In the Young's modulus of the mold of the strain gauge, there is also about 2750N / mm ^2, the initial behavior of the self-contraction after hitting設直However, it was difficult to measure.
For this reason, a non-contact measurement method using a laser or eddy current has been proposed ("Proceedings of the Japan Society of Civil Engineers"). 571 / V-36, pp. 211-219, 19977.8 “Study on Prediction Method of Self-Shrinkage Strain of Concrete” and “Concrete Engineering Annual Papers, Vol.20, No.2, 1998, (“ Shrinkage and Restraint Stress of High-Strength Concrete ” the study").
[0004]
[Problems to be solved by the invention]
However, these non-contact measurement methods require a reference point for measurement, and it is impossible to observe the shrinkage behavior of an actual structure.
In addition, when measuring the self-stress proposed by the Japan Concrete Institute, an embedded strain gauge is used because the gauge plug cannot be embedded in the concrete. (Research Committee Report, pp. 22-45, 1996) However, since the conventional embedded strain gauge has a large Young's modulus as described above, it cannot measure the shrinkage behavior immediately after placing concrete. was there.
On the other hand, the processing performance of high-performance concrete such as high strength and high fluidity concrete has been increasing recently.
[0005]
However, even in such high-performance concrete, in the case of concrete having a small water bonding ratio and a large number of unit bonding materials, self-shrinkage becomes prominent, and it is necessary to consider self-shrinkage for crack control and durability design. Is pointed out. Against this background, various factors such as materials used for self-shrinkage and curing conditions have been studied, but research reports on the shrinkage properties immediately after kneading, which are important for elucidating the mechanism of self-shrinkage, are extremely Few.
This is thought to be due to the fact that there is no or a lack of the ability to accurately and stably measure self-shrinkage immediately after kneading and placing concrete.
The present invention has been made in view of the above-mentioned circumstances, and the object of the present invention is that it is possible to measure the behavior of concrete self-shrinkage immediately after placing the cementitious material, and the cracking phenomenon due to the self-shrinkage of concrete. It is to provide a concrete embedded strain gauge and a concrete strain measurement method that are extremely useful for elucidating the mechanism of cracks and controlling and designing cracks.
[0006]
[Means for Solving the Problems]
Concrete implantable strain gage according to claim 1 of the present invention, in order to achieve the above object, the strain gauge detects the strain occurring in the concrete and embedded in the subject the concrete to be measured, shed Zumi gauge elements Cement is cured immediately after kneading by embedding it in low-elasticity silicone resin so that the Young's modulus is such that it displaces following the behavior of the concrete immediately after kneading. It is configured to be able to measure the behavior of body expansion or contraction.
The strain gauge element in the concrete embedded strain gauge according to claim 2 is made of a metal foil or a metal wire, and a lead wire covered with vinyl chloride or the like is connected to the end of the strain gauge element. The entire element and the vicinity of the connecting portion of the lead wire are embedded in a silicone resin.
[0007]
The strain gauge element in the concrete embedded strain gauge according to claim 3 is embedded in the middle of the thickness direction of the silicone resin.
Further, the silicone resin in the concrete embedded strain gauge according to claim 4 is one in which iron powder is mixed into the silicone resin and the specific gravity is adjusted to 1.8 to 2.2 close to the specific gravity of concrete. It is a feature.
The concrete embedded strain gauge according to claim 5 is characterized in that a strain gauge element and a thermocouple disposed in the vicinity thereof are molded with a low-elasticity silicone resin.
[0008]
In addition, in the concrete strain measuring method according to claim 6, in order to achieve the above-described object, a step of laying a low-friction coefficient sheet made of fluororesin on the inside of the mold having an open upper end;
Placing a cement paste formed by kneading cement having a known water-cement ratio and other conditions into the mold,
The droplet設直or after hitting under construction Nihi Zumi gauge element and a thermocouple placed in the vicinity of the strain gauge elements by molding coated with a low elasticity of silicone over down resin, following the behavior of the concrete immediately after kneading Embedding a thermocouple-attached strain gauge that has a Young's modulus to the extent of displacement in the mold center,
Sealing the upper end surface of the mold with the strain gauge embedded therein;
Connecting the strain gauge element and the thermocouple to a measuring instrument, and continuously measuring strain from immediately after placing the concrete to hardening, together with the temperature in the vicinity thereof,
It is characterized by comprising.
[0009]
[Action]
By adopting the configuration as described above, the strain gauge coated with the silicone resin has low elasticity, so that it follows (responds) to the behavior of the cement immediately after kneading in the embedded concrete, and is displaced. Since the resistance value changes, by measuring the resistance change using a strain measuring instrument, not only the hardening state of the concrete but also the behavior, that is, the strain, even in the flow state immediately after setting and before hardening. Can be measured.
In particular, in the case of a strain gauge with a thermocouple in which a strain gauge element and a thermocouple disposed in the vicinity thereof are coated with a low-elasticity silicone resin, the heat of hydration of cement during the measurement of self-shrinkage However, since the strain information is obtained together with the temperature information of the same location, the influence on the strain due to the temperature change can be appropriately corrected.
[0010]
DETAILED DESCRIPTION OF THE INVENTION
A concrete embedded type strain gauge according to an embodiment of the present invention will be described with reference to the accompanying drawings.
1A and 1B show an external configuration of a concrete-embedded type according to a first embodiment of the present invention. FIG. 1A is a plan view and FIG. 1B is a front view thereof.
In FIG. 1, 1 is a strain gauge element, such as a metal foil or a metal wire made of Cu-Ni alloy, Ni-Cr alloy, etc. on a thin sheet-like paper, a gauge hold sheet such as polyester or phenol. For example, although not shown in the figure, the resistance material is processed by a method such as photoetching of a serpentine grit and a wide gauge tab formed at the end thereof.
[0011]
Reference numeral 2 denotes a lead wire covered with vinyl chloride or the like connected to a gauge tab formed at the end of the strain gauge element 1 by means of soldering or the like.
3 is a low elasticity (low elastic modulus) silicone resin that covers (molds) the entire strain gauge element 1 and the vicinity of the connecting portion of the lead wire 2 to follow the behavior of the concrete immediately after kneading. It is a silicone mold body having a Young's modulus to such an extent that it can be displaced . This silicone mold body 3 is thin and has a relatively narrow width but a long rectangular shape in the direction of sensitivity (in the case shown in FIG. 1, the thickness is 2 mm, the width is 10 mm, and the length is 80 mm. .)
The strain gauge element 1 and the silicone mold body 3 are collectively referred to as a strain gauge 4.
[0012]
Next, a manufacturing method of the strain gauge 4 will be described with reference to FIG.
In FIG. 2, reference numeral 5 denotes a unit mold for molding one silicone mold body 3, and a plurality of unit molds 5 are connected to 5 ′, 5 ″. The opened large formwork 6 is partitioned by the partition plate 7. Of the surfaces constituting the formwork 6, the surface from which the lead wire 2 is drawn out (the front surface in FIG. 2) is evenly spaced upward and downward. The detaching panels 6a and 6b are divided into two, and the detaching panels 6a and 6b are provided with semicircular notch openings so that openings 6c for drawing out the lead wires 2 are formed in a circular shape at the top and bottom. It is provided.
Each unit mold 5, 5 ′, 5 ″... Is treated with a fluororesin (so-called Teflon (trademark)) so that the filling silicone does not adhere.
[0013]
The unit frames 5, 5 ′, 5 ″... Constituting the mold 6 having such a structure are filled with low-elasticity silicone resin, but initially the lower half with the removal panel 6 a in the upper half removed. In this state, a silicone mold body having a thickness of 1/2 is formed, so that the strain gauge element 1 attached on the gauge hold sheet and The lead wire 2 connected to the gauge tab is placed so as to be flat, and the lead wire 2 is first led out through a semicircular opening formed in the removal panel 6b.
Next, after the upper removal panel 6a is mounted on the upper front surface of the mold 6, the low elasticity silicone resin is again applied to the unit molds 5, 5 ', 5 ". Fill and continue until the silicone resin reaches the upper surface of the mold 6. If necessary, remove the silicone resin raised from the upper surface of the mold 6 with a top plate or scraping it off with a flat plate. Processing is performed so that the upper surface is a flat surface.
[0014]
After the silicone resin is solidified, the removal panels 6a and 6b are removed, and the molded silicone mold body 3 is taken out from the respective unit molds 5, 5 ', 5 ". It will be completed.
In addition, without providing the partition plate 7 in the mold 6, after molding the silicone resin in the above-described manner on the entire mold 6, it may be cut into individual mold bodies.
The top plate of the mold 6 can be opened and closed. When the lower half of the mold body is molded, the top is opened, and the lower half is molded, and the strain gauge element 1 and the like are placed and then closed. It may be.
[0015]
The molding method of the silicone mold body 3 is not limited to the above-described method. For example, the strain gauge 1 with a lead wire is placed in advance in a mold so as to hold a predetermined position, and a silicone resin is used. Injection or press-fitting may be performed, and in this case, mass production becomes easy.
The strain gauge 4 manufactured in this manner has low elasticity (the Young's modulus is 1.2 N / mm ^{2 in} the example shown in the figure), and a conventional embedded strain gauge (Young's modulus 39 N / mm). Compared with ² ), an extremely small Young's modulus can be obtained, so that the self-shrinkage change can be measured immediately after cement mixing.
[0016]
Next, a second embodiment of the present invention will be described with reference to FIG.
In FIG. 3, the strain gauge element 1, the lead wire 2, and the silicone mold body 3 are common in the basic configuration to the first embodiment shown in FIG. 1.
The difference is that a thermocouple 9 is disposed in the vicinity of the strain gauge element 1 and the thermocouple lead wire 10 is connected to the thermocouple 9 by means of soldering, brazing or the like at a place where there is no temperature change. When molding as the silicone mold body 3, the entire strain gauge element 1 and the thermocouple 9 having such a configuration, and the vicinity of the connection portion between the lead wire 2 and the thermocouple lead wire 10 connected thereto, The molding method is the same as described above, except that molding is performed so as to enclose.
The strain gauge 8 with thermocouple formed as a mold body in this way is provided with the strain gauge element 1 and the thermocouple 9 at the center in the thickness direction (vertical direction in FIG. 3B). It will be.
[0017]
In addition, since the vicinity of the connection portion between the strain gauge element 1 and the lead wire 2 and the thermocouple lead wire 10 is integrally sealed by the silicone mold body 3, the outside air and liquid are passed through the lead wires 2 and 10. Therefore, the strain gauge element 1 does not have a risk of causing a decrease in insulation or deterioration due to oxidation due to moisture absorption or contact with outside air.
Although not shown in the figure, as a third embodiment, iron powder is mixed in a silicone resin as a material constituting the silicone mold body 3, and the specific gravity of the silicone mold body 3 is made to be the specific gravity of concrete. It is desirable to use what was adjusted to 1.8-2.2 near. If iron powder is mixed in the silicone resin, the specific gravity of the mold body 3 will approach the specific gravity of the concrete, and it will become familiar, and the silicone mold body 3 can be obtained only by the silicone resin due to the reaction between iron and concrete. This is because the strain transmission efficiency is improved as compared with the case of the structure, and as a result, the initial behavior of the self-shrinkage immediately after placing the concrete can be measured more accurately.
[0018]
Next, a method for measuring the strain of concrete using the thermocouple-equipped strain gauge 8 shown in FIG. 3, for example, will be described with reference to FIG. 4 and FIG.
FIG. 4 is a longitudinal sectional view schematically showing an apparatus for measuring the strain of concrete using the concrete embedded strain gauge according to the present invention.
In FIG. 4, a mold 11 filled with a cement specimen (same for a concrete specimen) 13 has a cylindrical shape with an open upper end, and a low elastic sheet made of a fluororesin, for example, A Teflon sheet 12 (“Teflon” is a trademark) is laid to reduce the friction generated between the mold 11 and the cement specimen 13 to be measured.
[0019]
At the time of measurement, the strain gauge with thermocouple 8 is positioned at a substantially central portion of the mold 11. However, the cement specimen 13 may be embedded during or after placement.
The cement specimen 13 has a water-cement ratio (for example, 0.3), cement material (for example, ordinary Portland cement), kneaded water (for example, ion-exchanged water), and in the case of concrete, the type of aggregate, etc. The cement is kneaded together with water and others, and the mold 11 is filled (placed).
The timing of embedding the strain gauge with thermocouple 8 depends on the conditions of the cement specimen, but may be after placement if the cement paste is soft. It is desirable to arrange with.
When a predetermined amount of cement paste is filled in the mold 11, the upper end face is sealed to prevent moisture from escaping. In the present embodiment, liquid paraffin is dropped on the upper end surface of the cement specimen 13 to prevent moisture from escaping.
[0020]
Immediately after placing the cement, the lead wire 2 and the thermocouple lead wire 10 are respectively connected to a strain measuring device and a temperature measuring device (both not shown), and the strain immediately after placing the cement paste and its strain. Measure the temperature near the measurement site continuously.
Since the embedded strain gauges according to the first and second embodiments of the present invention have low elasticity (low Young's modulus), self-shrinkage change immediately after kneading and placing cement (or concrete) Can be measured accurately.
FIG. 5 shows an apparatus for measuring the strain of concrete. However, the difference from the case of FIG. 4 shows a case where an embedded strain gauge is placed horizontally.
In placing the cement specimen 13, when the half of the mold 11 was placed, the strain gauge 8 with thermocouple was placed sideways (horizontal), and then cement paste was placed in the upper half near the upper end of the mold 11. Then, the upper end surface of the mold 11 is sealed, and the subsequent steps are the same as described with reference to FIG.
[0021]
As described above, the following has been found depending on whether the strain gauges 4 or 8 are arranged in the vertical direction or in the horizontal direction.
That is, there was a difference between the horizontal cement specimen and the vertical cement specimen at any water cement ratio, but the water cement ratio (W / C) was 0.3, 0.35, Of the three types of 0.40, the most significant difference was observed at W / C = 0.40 with a large amount of bleeding. However, bleeding and subsidence are length changes that occur before the crosslinked structure of the hydrated product is formed in the mortar due to hydration of the cement. it is conceivable that. It is estimated that the above difference depending on the embedding direction is due to a change in the distribution of the water cement ratio in the paste. Therefore, in the case of horizontal embedment, the water cement ratio varies depending on the measurement position, and it is expected that measurement errors will occur. It is thought to reflect the overall self-contraction.
[0022]
By the way, FIG. 6 shows a comparison between the strain measurement results obtained by the strain gauge element 4 and the strain gauge 8 with thermocouple according to the present invention and the measurement results of the conventional laser displacement meter and spring type displacement meter. From FIG. 6, the initial strain is very large when using a laser displacement meter and a spring displacement meter, but when using a laser displacement meter, the cement mark is hardened by the laser mark. It seems that the initial strain was overestimated because the steel plate for receiving the reaction force of the spring settled in the case of using a spring type displacement meter due to the settling before. However, when evaluated as values after 12 hours of age when it is considered that sedimentation has stopped, these values are almost the same, with no significant difference.
[0023]
FIG. 7 is a graph showing the effect when the iron powder is mixed into the silicone resin constituting the strain gauge according to the present invention and when the iron powder is not mixed.
As can be seen from FIG. 7, it is shown that a larger strain output (strain output change) is obtained when iron powder is put in a silicone resin, and more accurate strain measurement is realized.
This is because the iron powder mixed in the silicone resin increases the bonding force with the concrete (or cement), and the adhesion between the silicone mold body 3 and the concrete increases. As a result, the strain transmission rate as a strain gauge is improved. It is connected.
The present invention is not limited to the one described above and shown in the drawings, and modifications can be made without departing from the scope of the invention.
[0024]
【The invention's effect】
As is apparent from the above description, according to the present invention, even if the object to be measured is not only in a hardened state of concrete, but also in a state of having fluidity immediately after placing the concrete, the strain of the concrete at that time It is possible to provide a concrete-embedded strain cage capable of continuously measuring accurately.
Therefore, cracks with the use of concrete embedded strain gauge according to the present invention, it is possible to measure the behavior of the self-contraction of cementitious material after punching設直, by extension, perform the elucidation of the self-contraction mechanism Control and durability design.
[0025]
Further, according to the concrete embedded strain gauge according to the invention of claim 4, by mixing iron powder into the silicone resin, the strain gauge output characteristics are greatly improved without increasing the cost so much. This enables more accurate strain measurement of concrete.
In addition, according to the concrete embedded strain gauge according to the invention of claim 5, since the temperature at the strain measurement site in the concrete can be measured simultaneously with the strain measurement, an accurate strain measurement result with temperature compensation is obtained. It is done.
Further, according to the invention of claim 6, the self-shrinkage behavior of the concrete can be accurately measured immediately after placing the cementitious material, the mechanism of the cracking phenomenon due to the self-shrinkage of the concrete is clarified and the crack control It is possible to provide a strain measurement method of concrete that is extremely useful for designing.
[Brief description of the drawings]
BRIEF DESCRIPTION OF DRAWINGS FIG. 1 schematically shows an external configuration of a concrete embedded strain gauge according to a first embodiment of the present invention, in which (a) is a plan view and (b) is a front view.
FIG. 2 is a perspective view for explaining a method for manufacturing a concrete embedded strain gauge according to FIG. 1;
FIGS. 3A and 3B schematically show an external configuration of a concrete embedded strain gauge according to a second embodiment of the present invention, where FIG. 3A is a plan view and FIG. 3B is a front view.
FIG. 4 is a longitudinal sectional view schematically showing an apparatus for measuring the strain of concrete when the embedded concrete strain gauge according to the present invention is arranged in the longitudinal direction.
FIG. 5 is a longitudinal sectional view schematically showing a device for measuring the strain of concrete when the embedded concrete strain gauge according to the present invention is disposed in the horizontal (horizontal) direction.
FIG. 6 is a graph showing a comparison of strain output characteristics of a concrete embedded strain gauge according to the present invention and a conventional strain gauge.
FIG. 7 is a graph showing the effects when iron powder is mixed into the silicone resin constituting the strain gauge according to the present invention and when it is not mixed.
[Explanation of symbols]
DESCRIPTION OF SYMBOLS 1 Strain gauge element 2 Lead wire 3 Silicone mold body 4 Strain gauge 5,5 ', 5 "Unit mold 6,11,14 Mold 6a, 6b Removal panel 6c Opening 7 Partition plate 8 Strain gauge 9 with thermocouple 9 Thermocouple 10 Lead wire for thermocouple 12 Teflon sheet 13 Cement specimen

Claims (6)

The strain gauge to detect the buried in the object to be measured in concrete distortion occurs in the concrete, the ratio Zumi gauge elements of the low-elasticity silicone resin is molded, the degree of displacement to follow the behavior of the concrete immediately after kneading Concrete embedding type strain characterized by being configured to measure the expansion or contraction behavior of hardened concrete immediately after kneading by embedding it in the concrete to be measured immediately after kneading , with Young's modulus gauge.   The strain gauge element is made of a metal foil or a metal wire, and a lead wire coated with vinyl chloride or the like is connected to the end of the strain gauge element. The entire strain gauge element and the vicinity of the connection portion of the lead wire are connected with a silicone resin. The embedded concrete strain gauge according to claim 1, which is embedded in the concrete.   The strain-embedded strain gauge according to claim 1 or 2, wherein the strain gauge element is embedded substantially in the middle of the thickness direction of the silicone mold body.   The silicone resin used is one in which iron powder is mixed in the silicone resin and the specific gravity is adjusted to 1.8 to 2.2 close to the specific gravity of concrete. The concrete embedded strain gauge described in 1. 5. The embedded concrete strain gauge according to claim 1, wherein a strain gauge element and a thermocouple disposed in the vicinity thereof are molded with the low-elasticity silicone resin. Laying a sheet of low coefficient of friction made of fluororesin on the inside of the mold with an open top;
Placing a cement paste formed by kneading cement having a known water-cement ratio and other conditions into the mold,
The droplet設直or after hitting under construction Nihi Zumi gauge element and a thermocouple placed in the vicinity of the strain gauge elements by molding coated with a low elasticity of silicone over down resin, following the behavior of the concrete immediately after kneading Embedding a strain gauge with a thermocouple that has a Young's modulus to the extent of displacement in the center of the mold,
Sealing the upper end surface of the mold with the strain gauge embedded therein;
Connecting the strain gauge element and the thermocouple to a measuring instrument, and continuously measuring the strain from immediately after placing the concrete to hardening, together with the temperature in the vicinity thereof;
A method for measuring strain of concrete, comprising:

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