JP7729534B2 - Pressure jig for concrete specimen and freeze-thaw test method using the same - Google Patents

Description

The present invention relates to a pressure jig for concrete specimens used to measure deterioration caused by repeated freezing and thawing of concrete, and in particular to a freeze-thaw test method using the same, for conducting freeze-thaw tests on concrete specimens to which external compressive stress is applied.

Concrete is an artificial stone made by mixing cement with aggregates such as sand and gravel and water, and is used in a variety of structures such as building foundations and agricultural irrigation channels.

However, concrete can deteriorate rapidly depending on usage and environmental conditions. Frost damage in cold regions is one such deterioration phenomenon, where the repeated freezing and thawing of the water contained in the concrete causes cracks and peeling.

Various countermeasures have been taken to prevent this type of frost damage. For example, Japanese Patent Application Laid-Open No. 2015-83767 proposes a method for preventing freeze and salt scaling, which involves spraying onto the surface of the hardened cement body an antifreeze agent that contains at least one salt selected from the group consisting of sodium salts, potassium salts, and calcium salts but does not contain magnesium, and a water-soluble metal salt that contains at least one metal selected from the group consisting of magnesium, iron, and aluminum and reacts with an alkaline aqueous solution to form the hydroxide of the metal, thereby forming a coating made of the hydroxide of the metal between the surface of the hardened cement body and the ice adhering to the surface of the hardened cement body (Patent Document 1).

Furthermore, Japanese Patent Application Laid-Open No. 2019-156700 proposes an invention relating to a frost damage inhibitor made from a hardened cement product containing antifreeze polysaccharides, in order to reduce the possibility of corrosion caused by the use of salts, as in the invention described in Patent Document 1 (Patent Document 2).

Furthermore, the Japanese Industrial Standards (JIS) has established the Underwater Freezing and Thawing Test Method (JIS A1148A Method) as a standard for evaluating the performance of concrete that has been treated to prevent frost damage.

JP 2015-83767 A JP 2019-156700 A

The inventors have previously conducted actual research into frost damage deterioration of concrete structures and have found that applying compressive stress to concrete from the outside makes it less likely to crack due to frost damage.

However, the current underwater freeze-thaw test method (JIS A1148A) does not specify a test method for concrete subjected to external compressive stress. As a result, it is not possible to quantitatively evaluate the level of externally applied pressure or the relationship between that pressure and the composition of the concrete, and there is a growing need for a quantitative evaluation method.

The present invention was made to solve the above-mentioned problems, and aims to provide a pressure jig for concrete specimens, which can apply compressive stress to concrete specimens from the outside and quantitatively evaluate the effects of repeated freezing and thawing on the concrete specimens, and a freeze-thaw testing method using the same.

The concrete specimen pressure jig of the present invention applies compressive stress from the outside to a concrete specimen used in a concrete freeze-thaw test, solving the problem of being able to apply compressive pressure to a concrete specimen from the outside and then remove that pressure. It comprises a first constraining plate on which the concrete specimen is placed, multiple guide rails erected on the outer edge of the first constraining plate, a second constraining plate that slides along the guide rails and restrains the concrete specimen together with the first constraining plate, and pressure application/relief means that applies pressure to the concrete specimen by pressing against the second constraining plate, or removes pressure from the concrete specimen by releasing that pressure.

In one aspect of the present invention, to solve the problem of easily applying and releasing pressure to a concrete specimen, the first and second constraining plates may be formed in a substantially square shape in a plan view, the guide rails may be erected at the four corners of the first constraining plate, the second constraining plate may have insertion holes formed therein that allow it to slide along the guide rails, and the pressure application/removal means may include a pressure plate that is slidable along the guide rails above the second constraining plate and can be fixed at any height, and that has a female threaded hole that penetrates vertically at its center, and a pressure bolt that screws into the female threaded hole of the pressure plate and uses its tip to press downward against the center of the upper surface of the second constraining plate, thereby applying pressure to the concrete specimen.

Furthermore, as one aspect of the present invention, in order to solve the problem of accurately measuring the pressure applied to a concrete specimen, a pair of strain gauges may be provided at axisymmetric positions on the axial side of the pressure bolt to measure the amount of axial contraction of the pressure bolt in response to the applied pressure.

The freeze-thaw test method of the present invention uses a concrete specimen pressure jig to quantitatively evaluate the effects of repeated freezing and thawing of a concrete specimen to which external compressive stress has been applied using the concrete specimen pressure jig. The freeze-thaw test method uses the concrete specimen pressure jig, and includes the following steps: a specimen initial value measurement step, which measures a numerical value that serves as an indicator of the strength of the concrete specimen before freezing and thawing; a specimen pressurization step, which restrains the concrete specimen by sandwiching it between the first and second restraint plates and applies a predetermined pressure; a test liquid injection step, which places the concrete specimen in a restrained and pressurized state using the concrete specimen pressure jig in a test tank and injects test liquid into it until the concrete specimen is completely submerged; a freeze-thaw circulation step, which controls the temperature of the test liquid to perform temperature management consisting of a freezing process from a first temperature above zero to a second temperature below zero and a thawing process from the second temperature to the first temperature, and repeats freezing and thawing the concrete specimen a predetermined number of times; and The method includes a post-freeze-thaw measurement step in which, after a predetermined number of freeze-thaw cycles, the concrete specimen is removed from the concrete specimen pressure jig and the numerical value of the index is measured; a step number determination step in which it is determined whether the number of steps in the freeze-thaw cycling step has exceeded a predetermined number; a relative dynamic modulus of elasticity calculation step in which, if the predetermined number of steps has not been exceeded in the step number determination step, the relative dynamic modulus of elasticity is calculated from the numerical value of the index before the freeze-thaw cycle and the numerical value of the index after the freeze-thaw cycle; and a relative dynamic modulus of elasticity determination step in which it is determined whether the calculated relative dynamic modulus of elasticity is equal to or less than a predetermined numerical value. If the relative dynamic modulus of elasticity is equal to or less than a predetermined numerical value, the concrete specimen is restrained by the concrete specimen pressure jig and a predetermined pressure is applied to it, and the freeze-thaw cycling step is terminated if the predetermined number of steps has been exceeded in the step number determination step, or if the relative dynamic modulus of elasticity is equal to or less than a predetermined numerical value in the relative dynamic modulus of elasticity determination step.

The freeze-thaw test method of the present invention uses a concrete specimen pressure jig to quantitatively evaluate the effects of repeated freezing and thawing of a concrete specimen to which external compressive stress has been applied using the concrete specimen pressure jig. The freeze-thaw test method uses the concrete specimen pressure jig, and includes a specimen pressurization step in which a concrete specimen before freezing and thawing is sandwiched between the first and second confinement plates and restrained, and a predetermined pressure is applied; a specimen initial value measurement step in which a numerical value that serves as an index of the strength of the concrete specimen in a state pressurized by the concrete specimen pressure jig; a test liquid injection step in which the concrete specimen in a state restrained and pressurized by the concrete specimen pressure jig is placed in a test tank and test liquid is injected until the concrete specimen is completely submerged; and a temperature management step in which the temperature of the test liquid is controlled to perform a freezing process from a first temperature above zero to a second temperature below zero, and a thawing process from the second temperature to the first temperature, as one cycle, and the concrete specimen is subjected to a predetermined number of freezing and thawing cycles. the concrete specimen is subjected to a predetermined number of freeze-thaw cycles, and then a post-freeze-thaw measurement step is performed in which the index value is measured while the specimen is pressurized by the concrete specimen pressurizing jig; a step count determination step is performed in which the number of steps in the freeze-thaw cycling step exceeds a predetermined number; a relative dynamic modulus of elasticity calculation step is performed in which, if the predetermined number of steps is not exceeded in the step count determination step, the relative dynamic modulus of elasticity is calculated from the index value before the freeze-thaw cycle and the index value after the freeze-thaw cycle; and a relative dynamic modulus of elasticity determination step is performed in which the calculated relative dynamic modulus of elasticity is equal to or less than a predetermined value. If the relative dynamic modulus of elasticity is not equal to or less than a predetermined value, the freeze-thaw cycling step is performed in which the concrete specimen is restrained by the concrete specimen pressurizing jig and a predetermined pressure is applied; and the freeze-thaw test is terminated if the predetermined number of steps is exceeded in the step count determination step, or if the relative dynamic modulus of elasticity is equal to or less than a predetermined value in the relative dynamic modulus of elasticity determination step.

The present invention makes it possible to apply compressive stress to a concrete specimen from the outside and quantitatively evaluate the effects of repeated freezing and thawing on the concrete specimen.

1 is a perspective view showing a first embodiment of a pressurizing jig for a concrete specimen according to the present invention, in which a concrete specimen is restrained. 1 is a front view showing a pressure jig for a concrete specimen according to the first embodiment. FIG. 1 is a plan view showing a pressure jig for a concrete specimen according to the first embodiment of the present invention. FIG. FIG. 2 is a bottom view showing the pressure jig for a concrete specimen of the first embodiment. FIG. 1 is a flow chart showing the procedure of a first embodiment of a freeze-thaw test method according to the present invention. FIG. 10 is a flowchart showing the procedure of a second embodiment of the freeze-thaw test method according to the present invention.

The following describes a first embodiment of a pressure jig for a concrete specimen and a freeze-thaw test method using the same according to the present invention, using the drawings.

The concrete specimen pressure jig 1 is a jig used in freeze-thaw tests of concrete, and is capable of applying compressive stress to a concrete specimen 10 from the outside. As shown in FIG. 1, the concrete specimen pressure jig 1 in this first embodiment comprises a first constraining plate 2 on which the concrete specimen 10 is placed, multiple guide rails 3 erected on the outer periphery of the first constraining plate 2, a second constraining plate 4 that slides along the guide rails 3 and constrains the concrete specimen 10 together with the first constraining plate 2, a pressure applying/removing means 5 that applies pressure to or removes pressure from the concrete specimen 10, and a strain gauge 6 that measures the pressure applied by the pressure applying/removing means 5. Each component is described in detail below.

The first confinement plate 2 is a plate on which the concrete specimen 10 is placed, and which, together with the second confinement plate 4, clamps and constrains the concrete specimen 10. In this first embodiment, the first confinement plate 2 is formed in a roughly square shape in plan view so that it can be installed in a test tank used in conventional freeze-thaw testing methods, and is made from a square steel plate with sides of approximately 100 mm and a thickness of approximately 5 mm.

In addition, insertion holes are opened at the four corners of the first restraint plate 2 for inserting the guide rails 3, and the underside of the inserted guide rails 3 is configured as legs 21 whose height can be adjusted by screwing in long nuts.

The shape and dimensions of the first restraint plate 2 are not particularly limited and may be selected appropriately depending on factors such as the size of the test tank used in the freeze-thaw test method.

The guide rail 3 is a rail that guides the second restraint plate 4 so that it can slide up and down, and is erected on the outer edge of the first restraint plate 2. In this first embodiment, the guide rail is formed from a long bolt approximately 530 mm long so that it can be installed in a test tank used in conventional freeze-thaw testing methods.

In addition, in this first embodiment, as shown in FIG. 1, a total of four guide rails 3 are erected, one at each of the four corners of the first restraint plate 2. Specifically, each guide rail 3 is inserted into an insertion hole opened at the four corners of the first restraint plate 2 and is erected by being screwed onto a long nut that forms the leg 21.

The guide rail 3 is not limited to being made of long bolts, but may be selected appropriately from round or square bar material without male threads.

The second constraining plate 4 slides along the guide rails 3 to restrain the concrete specimen 10 together with the first constraining plate 2. In this first embodiment, like the first constraining plate 2, it is formed from a square steel plate with sides of approximately 100 mm and a thickness of approximately 5 mm. At the four corners of this second constraining plate 4, insertion holes 41 with an inner diameter large enough to insert the guide rails 3 are opened at positions corresponding to those of the guide rails 3. By inserting the guide rails 3 into each insertion hole 41, the second constraining plate 4 can move up and down while remaining parallel to the first constraining plate 2.

The shape and dimensions of the second constraining plate 4 are not particularly limited, and, like the first constraining plate 2, may be selected appropriately depending on the size of the test tank used in the freeze-thaw test method.

The pressure application/removal means 5 is a means for applying pressure to push down the second restraint plate 4 and releasing that pressure. In this first embodiment, as shown in Figure 1, it includes a pressure plate 51 installed above the second restraint plate 4 and a pressure bolt 52 screwed into this pressure plate 51.

The pressure plate 51 serves as a base for applying pressure to the second restraint plate 4, and in this first embodiment is made of a roughly square steel plate with sides of approximately 100 mm and a thickness of approximately 10 mm so that it can be installed in a test tank used in conventional freeze-thaw testing methods. To enable this pressure plate 51 to slide along the guide rail 3 above the second restraint plate 4 and be fixed at any height, in this first embodiment, insertion holes are formed in the four corners through which the guide rail 3 can be inserted, and the pressure plate is fastened in place by being clamped from above and below using nuts threaded onto the guide rail 3.

Furthermore, a female threaded hole 511 that penetrates vertically is formed in the center of the pressure plate 51 to allow the pressure bolt 52 to be screwed in. In this first embodiment, the female threaded hole 511 is formed to allow an M12 bolt to be screwed in.

The shape and dimensions of the pressure plate 51 are not particularly limited, and may be selected appropriately from shapes that allow it to slide along the guide rail 3 and apply pressure to press down on the first restraint plate 2.

The pressure bolt 52 is a bolt that is threaded into the female threaded hole 511 of the pressure plate 51 to apply pressure to the second restraint plate 4. In this first embodiment, as shown in Figures 1 and 3, it is an M12 hexagonal bolt with a hexagonal head in plan view and a shaft length of approximately 90 mm. The side of this pressure bolt 52 has a threaded portion 521 with a male thread formed approximately 30 mm from the head, and the portion distal to the threaded portion 521 is an unthreaded portion 522 without a thread to accommodate the strain gauge 6.

The shape of the pressure bolt 52 is not limited to a hexagonal bolt, and may be selected appropriately from a square bolt that is square when viewed from above, a bolt with a hexagonal recess in the center of the top surface into which a hexagonal wrench can be inserted, or a bolt with a cross-shaped recess into which a screwdriver can be inserted.

Furthermore, the pressure application/removal means 5 is not limited to a configuration consisting of a pressure plate 51 and pressure bolts 52, but may be selected appropriately from means that can apply pressure to press down on the second restraint plate 4 and release that pressure, such as a hydraulic jack.

The strain gauge 6 measures the amount of axial contraction of the pressure bolt 52 in response to the applied pressure (applied force), and is capable of outputting the amount of contraction as an electrical signal. Although not shown, it is connected to an analog/digital converter or computer, which receives the output electrical signal and inputs it into a mathematical formula corresponding to the strain gauge 6, thereby allowing the applied force to be calculated.

Furthermore, in this first embodiment, as shown in Figures 1 and 2, a pair of strain gauges 6 are provided at axisymmetric positions on the axial side of the unthreaded portion 522 of the pressure bolt 52. In this way, in this first embodiment, by providing a pair of strain gauges 6 at axisymmetric positions and using the average value of the values measured by each strain gauge 6 when calculating pressure, it is possible to suppress the influence of measurement errors in the strain gauges 6 caused by bending deformation of the pressure bolt 52 when pressure is applied.

The method for measuring the applied pressure is not limited to using a strain gauge 6, but may be selected as appropriate from methods based on the tightening torque value of the pressure bolt 52, angle methods based on the tightening angle, and ultrasonic bolt axial force methods that use ultrasonic waves to measure the expansion and contraction of the pressure bolt 52.

Next, the function of each component of the pressure jig 1 for concrete specimens of this first embodiment will be explained along with the freeze-thaw test method of this first embodiment.

The freeze-thaw test method of this first embodiment is a freeze-thaw test method using a concrete specimen pressure jig 1. As shown in FIG. 5, it includes a specimen initial value measurement step S1, in which a numerical value serving as an index of the strength of the concrete specimen 10 is measured; a specimen pressurization step S2, in which a predetermined pressure is applied to the concrete specimen 10; a test liquid injection step S3, in which the concrete specimen 10 is placed in a test tank and test liquid is injected until the concrete specimen 10 is entirely submerged; a freeze-thaw cycling step S4, in which the concrete specimen 10 is repeatedly frozen and thawed a predetermined number of times; a post-freeze-thaw measurement step S5, in which the numerical value of the index of the concrete specimen 10 after the predetermined number of freeze-thaw cycles is measured; a step number determination step S6, in which the number of steps in the freeze-thaw cycling step S4 is determined; a relative dynamic modulus of elasticity calculation step S7, in which the relative dynamic modulus of elasticity is calculated; and a relative dynamic modulus determination step S8, in which it is determined whether the calculated relative dynamic modulus of elasticity is equal to or less than a predetermined value. Each step is described below.

The specimen initial value measurement step S1 is a step for measuring a numerical value that serves as an indicator of the strength of the concrete specimen 10. In this first embodiment, the primary resonance frequency of the concrete specimen 10, which is a vertically elongated rectangular parallelepiped, is measured before freezing and thawing and before a predetermined pressure is applied.

The primary resonance frequency is a value measured during non-destructive testing to determine a value that serves as an indicator of concrete strength, and is a value used to calculate the relative dynamic modulus of elasticity. In this first embodiment, the resonance frequency of the flexural vibration is measured as the primary resonance frequency. In addition to the resonance frequency of the flexural vibration, the mass and ultrasonic propagation time of the concrete specimen 10 are also measured.

In addition, the numerical value that indicates the strength of concrete is not limited to the primary resonance frequency; the ultrasonic propagation time is also a numerical value that indicates strength and can be used to calculate the relative dynamic modulus of elasticity.

The specimen pressurization step S2 is a step in which the concrete specimen 10 is restrained by the concrete specimen pressurization jig 1 and a predetermined pressure is applied. Specifically, as shown in Figure 1, the concrete specimen 10 is sandwiched between the first restraint plate 2 and the second restraint plate 4 of the concrete specimen pressurization jig 1. At this time, the height at which the pressure plate 51 is fixed is adjusted as necessary.

Then, the pressure bolt 52 is rotated in the tightening direction, and its tip presses downward against the center of the top surface of the second constraining plate 4. The second constraining plate 4 slides downward along the guide rail 3 while remaining parallel to the first constraining plate 2. As a result, the concrete specimen 10 is constrained by the first constraining plate 2 and the second constraining plate 4, and pressure is applied.

The strain gauge 6 measures the amount of shrinkage of the pressure bolt 52, which shrinks in response to the pressure (applied pressure) applied to the concrete specimen. Specifically, the strain gauge 6 outputs an electrical signal corresponding to the amount of shrinkage of the pressure bolt 52 and sends it to a computer or the like. The computer or the like calculates the applied pressure from the received electrical signal and displays it on a monitor or the like. In this first embodiment, the pressure applied to the concrete specimen 10 is managed based on the applied pressure displayed on a monitor or the like.

In addition, in this first embodiment, a pair of strain gauges 6 are provided at axially symmetric positions on the unthreaded portion 522 of the pressure bolt 52, and even if the pressure bolt 52 is bent and deformed by the pressure, the effect of the bending deformation can be canceled out by using the average value based on the electrical signals of each strain gauge 6, making it possible to accurately calculate the pressure.

Test liquid injection step S3 is a step in which the concrete specimen 10, restrained and pressurized by the concrete specimen pressure jig 1, is placed in the test tank and test liquid is injected until the entire specimen is submerged.

The test tank in this first embodiment is large enough to accommodate multiple concrete specimen pressurization jigs 1, allowing freeze-thaw tests to be performed on multiple pressurized concrete specimens 10 at once. The test liquid used in this first embodiment is Nybrine liquid, which contains ethylene glycol and propylene glycol as its main components.

In addition, in this first embodiment, although not shown, the concrete specimen 10, restrained and pressurized by the concrete specimen pressure jig 1, is placed in a test tank, the concrete specimen 10 is entirely submerged in test liquid, and the output value of the strain gauge 6 is confirmed with the temperature of the test liquid set to a first temperature, which will be described below.

The freeze-thaw cycle step S4 is a step in which the concrete specimen 10 is repeatedly frozen and thawed a predetermined number of times. Specifically, the temperature of the test liquid is controlled to perform temperature management in which one cycle consists of a freezing process from a first temperature above zero to a second temperature below zero, and a thawing process from the second temperature to the first temperature, and freezing and thawing are repeated a predetermined number of times.

In this first embodiment, the first temperature is 5°C, the second temperature is -18°C, and the temperature of the test liquid is controlled so that the time required per cycle is 3 hours or more and 4 hours or less, in accordance with the JIS A1148A method. At this time, the temperature of the center of the concrete specimen 10 at the first temperature is within the range of 5±2°C, and the temperature of the center of the concrete specimen 10 at the second temperature is within the range of -18±2°C.

The number of cycles must not exceed 36 cycles in accordance with the JIS A1148A method, and in this first embodiment, the number of cycles is appropriately selected from 30, 32, 34, or 36 cycles.

The post-freeze-thaw measurement step S5 is a step in which the primary resonance frequency is measured as an index value after a predetermined number of freeze-thaw cycles. In this first embodiment, the concrete specimen 10 is removed from the concrete specimen pressurizing jig 1 and measurement is performed to match the measurement conditions of the specimen initial value measurement step S1. Because the concrete specimen 10 is restrained by the concrete specimen pressurizing jig 10 of this first embodiment, it can be easily removed from the concrete specimen pressurizing jig 10 by loosening the pressure bolts 52.

Then, the primary resonance frequency (resonance frequency of flexural vibration) of the removed concrete specimen 10 is measured. In this first embodiment, the mass and ultrasonic propagation time are also measured. At this time, it is preferable to measure the primary resonance frequency and the like in an indoor environment where the temperature is controlled to 5°C, the same as the first temperature, so that the temperature of the concrete specimen 10 does not rise above the first temperature before the next freeze-thaw cycle step S6.

Step number determination step S6 is a step for determining the first condition for terminating the freeze-thaw test, and determines whether the number of steps in freeze-thaw cycling step S4 has exceeded a predetermined number. In this first embodiment, step number determination step S6 determines whether the number of cycles is 300 cycles or the smallest number of steps exceeding 300 cycles.

For example, if the number of cycles per step is 30, the predetermined number of steps is 10 (30 cycles x 10 steps = 300 cycles), and if the number of cycles per step is 34, the predetermined number of steps is 9 (34 cycles x 9 steps) = 306 cycles.

In other words, in this first embodiment, if the number of cycles per step is 30 to 33, the predetermined number of steps is 10, and if the number of cycles per step is 34 to 36, the predetermined number of steps is 9.

Then, in step S6, if it is determined that the number of steps exceeds the predetermined number (S6: YES), the freeze-thaw test is terminated. On the other hand, if it is determined that the number of steps does not exceed the predetermined number (S6: NO), the process proceeds to step S7, which calculates the relative dynamic modulus of elasticity.

The relative dynamic modulus of elasticity calculation step S7 is a step for calculating the relative dynamic modulus of elasticity, which is one of the indicators of concrete deterioration. In this first embodiment, the relative dynamic modulus of elasticity is calculated by comparing the primary resonance frequency before freeze-thawing measured in the specimen initial value measurement step S1 with the primary resonance frequency after freeze-thawing measured in the most recent post-freeze-thaw measurement step S5. The calculated relative dynamic modulus of elasticity is expressed as a percentage.

The relative dynamic modulus of elasticity can also be calculated from the ratio of the ultrasonic propagation time before freezing and thawing to the ultrasonic propagation time after freezing and thawing.

The relative dynamic modulus of elasticity determination step S8 is a step for determining the second condition for terminating the freeze-thaw test when the number of steps does not exceed a predetermined number, and determines whether the calculated relative dynamic modulus of elasticity is equal to or less than a predetermined value. In this first embodiment, the relative dynamic modulus of elasticity determination step S8 determines whether the relative dynamic modulus of elasticity is equal to or less than 60%.

If it is determined in the relative dynamic modulus of elasticity determination step S8 that the relative dynamic modulus of elasticity is not equal to or less than the predetermined value (60%) (S8: NO), the concrete specimen 10 is again constrained using the concrete specimen pressure jig 1 and a predetermined pressure is applied (S9), and the process returns to the freeze-thaw cycling step S4, where steps S5 to S8 are repeated. In this first embodiment, the pressure constraining the concrete specimen 10 is measured and managed using a pair of strain gauges 6 that can accurately measure the applied pressure, so the freeze-thaw test can be performed again under the same pressurization conditions.

On the other hand, if the relative dynamic modulus of elasticity determination step S8 determines that the relative dynamic modulus of elasticity is below the predetermined value (60%) (S8: YES), even if the number of steps in the freeze-thaw cycling step S4 has not exceeded the predetermined number, the concrete specimen 10 is deemed to have been sufficiently deteriorated by freezing and thawing, and the freeze-thaw test is terminated.

According to the first embodiment described above, the following effects can be achieved.
1. By using the pressure jig 1 for concrete specimens, a freeze-thaw test can be performed while applying compressive stress from the outside to the concrete specimen 10, and the effects of repeated freeze-thaw cycles can be quantitatively evaluated.
2. The concrete specimen pressure jig 1 can restrain the concrete specimen 10 and apply pressure to it, and can also be easily removed.
3. Since the concrete specimen 10 can be easily restrained and removed using the pressure jig 1 for concrete specimens, measurements of numerical values that serve as indicators of strength after freezing and thawing in freeze-thaw tests can be carried out smoothly.
4. The pair of strain gauges 6 can accurately measure the applied pressure, and the concrete specimen 10 can be restrained with an equal pressure even when it is repeatedly attached and detached.
5. Since the numerical values of the above indexes can be measured before and after freeze-thawing without being restrained by the concrete specimen pressurizing jig 1, an accurate relative dynamic modulus of elasticity can be calculated.

Next, a second embodiment of the freeze-thaw test method according to the present invention will be described. Note that among the components and steps of this second embodiment, those that are the same as or correspond to those of the first embodiment described above will be assigned the same reference numerals and will not be described again.

The freeze-thaw test method of this second embodiment is a method in which a numerical value serving as an index of strength for calculating the relative dynamic modulus of elasticity is measured while the concrete specimen 10 is pressurized by the concrete specimen pressurizing jig 1, and the freeze-thaw test is performed without removing the specimen when measuring the numerical value serving as an index of post-freeze-thaw strength. That is, as shown in FIG. 6 , the specimen pressurizing step S2' is performed before the specimen initial value measuring step S1', and in the post-freeze-thaw measuring step S5', the index value is measured while the concrete specimen 10 is pressurized by the concrete specimen pressurizing jig 1.

Specifically, in the specimen pressurization step S2' in this second embodiment, the concrete specimen 10 before freezing and thawing is sandwiched between the first constraining plate 2 and the second constraining plate 4 to restrain it, and a predetermined pressure is applied. A strain gauge 6 is used to manage the applied pressure.

Furthermore, in the specimen initial value measurement step S1' in this second embodiment, a numerical value that serves as an index of strength is measured while the concrete specimen 10 remains attached to the concrete specimen pressurizing jig 1. In this second embodiment, the ultrasonic propagation time is measured as a numerical value that serves as an index of strength. This is preferably performed in an indoor environment that is temperature-controlled at 5°C, the same as the first temperature.

The numerical value that serves as an indicator of strength is not limited to the ultrasonic propagation time, but may also be the primary resonance frequency, as in the first embodiment.

Next, proceed to test liquid injection step S3 and freeze-thaw circulation step S4.

In the post-freeze-thaw measurement step S5', after a predetermined number of freeze-thaw cycles, the concrete specimen 10 is pressurized using the concrete specimen pressurizing jig 1, and the ultrasonic propagation time is measured as a numerical value that serves as an indicator of strength.

The number of steps in the freeze-thaw cycling step S4 is determined in the step count determination step S6. If the step count determination step S6 determines that the number of steps exceeds the predetermined number (S6: YES), the freeze-thaw test is terminated.

On the other hand, if it is determined that the number of steps has not exceeded the predetermined number (S6: NO), proceed to step S7 for calculating the relative dynamic modulus of elasticity.

The relative dynamic modulus of elasticity calculation step S7 is a step for calculating the relative dynamic modulus of elasticity. In this second embodiment, the ultrasonic propagation time before freezing and thawing measured in the specimen initial value measurement step S1' and the ultrasonic propagation time after freezing and thawing measured in the post-freezing and thaw measurement step S5' are both measured for the concrete specimen 10 in a pressurized state, so the relative dynamic modulus of elasticity can be calculated by comparing them.

Then, in the relative dynamic modulus determination step S8, it is determined whether the calculated relative dynamic modulus of elasticity is equal to or less than a predetermined value. If it is determined in the relative dynamic modulus determination step S8 that the relative dynamic modulus of elasticity is not equal to or less than the predetermined value (60%) (S8: NO), the process returns to the freeze-thaw cycle step S4 and continues steps S5 to S8.

In this second embodiment, the concrete specimen 10 remains restrained by the concrete specimen pressure jig 1, so it can return to the freeze-thaw circulation step S4 as is, allowing the work to be carried out smoothly.

On the other hand, if it is determined in the relative dynamic modulus of elasticity determination step S8 that the relative dynamic modulus of elasticity is equal to or less than the predetermined value (S8: YES), the freeze-thaw test is terminated.

As described above, the freeze-thaw test method of the second embodiment, like the first embodiment, allows for quantitative evaluation of the effects of repeated freezing and thawing of a concrete specimen 10 to which external compressive stress has been applied. In particular, because the test is conducted while the concrete specimen 10 is restrained in the concrete specimen pressure jig 1, which continues to apply a constant pressure, the effect on strength caused by the release of pressure during the test is eliminated, and the need for installation and removal work is eliminated, resulting in smoother testing.

The pressure jig for concrete specimens and the freeze-thaw test method using the same according to the present invention are not limited to the above-described embodiments and can be modified as appropriate.

For example, the shape of the first constraining plate 2 and second constraining plate 4 that constrain and apply pressure to the concrete specimen 10 is not limited to a square shape, and may be selected appropriately from rectangular, circular, or other shapes to match the shape of the concrete specimen 10. Furthermore, the direction in which the concrete specimen 10 is constrained and pressure is applied is not limited to the longitudinal direction of the concrete specimen 10, and the concrete specimen 10 may be constrained and pressure applied in the lateral direction, allowing for a quantitative evaluation of the effects of the resulting repeated freezing and thawing action.

REFERENCE SIGNS LIST 1 Pressure jig for concrete specimen 2 First restraint plate 3 Guide rail 4 Second restraint plate 5 Pressure application/removal means 6 Strain gauge 10 Concrete specimen 21 Leg 41 Insertion hole 51 Pressure plate 52 Pressure bolt 511 Female thread hole 521 Threaded portion 522 Non-threaded portion

Claims (4)

A concrete specimen pressure jig that applies compressive stress from the outside to a concrete specimen used in a concrete freeze-thaw test,
a first restraint plate on which the concrete specimen is placed;
a plurality of guide rails provided upright on an outer peripheral edge portion of the first restraint plate;
a second constraining plate that slides along the guide rail and constrains the concrete specimen together with the first constraining plate;
and a pressure applying/relieving means for applying pressure to the concrete specimen by pressing the second restraint plate, or for removing pressure from the concrete specimen by releasing the pressing force,
the first constraining plate and the second constraining plate are formed in a substantially square shape in a plan view, the guide rails are provided upright at four corners of the first constraining plate, and an insertion hole is formed in the second constraining plate such that the second constraining plate can slide along the guide rails,
The pressure application/relief means
a pressure plate that is slidable along the guide rail above the second restraint plate and can be fixed at any height, and that has a female screw hole that penetrates in the vertical direction at a central position;
a pressure bolt that is screwed into the female screw hole of the pressure plate and that uses its tip to press downward the center position of the upper surface of the second restraint plate, thereby applying pressure to the concrete specimen. The pressure jig for concrete specimens described in claim 1, wherein a pair of strain gauges are provided at axisymmetric positions on the axial side of the pressure bolt to measure the amount of axial contraction of the pressure bolt in response to the pressure applied. A freeze-thaw test method using a pressurizing jig for a concrete specimen that applies compressive stress from the outside to a concrete specimen used in a freeze-thaw test of concrete, the pressurizing jig for a concrete specimen having a first constraining plate on which the concrete specimen is placed, a plurality of guide rails erected on the outer periphery of the first constraining plate, a second constraining plate that slides along the guide rails and constrains the concrete specimen together with the first constraining plate, and a pressure applying/removing means that applies pressure to the concrete specimen by pressing the second constraining plate, or removes pressure from the concrete specimen by releasing the pressing force,
a test piece initial value measurement step for measuring a numerical value that is an indicator of the strength of the concrete test piece before freezing and thawing;
a specimen pressurizing step of sandwiching and restraining the concrete specimen between the first constraining plate and the second constraining plate and applying a predetermined pressure;
a test liquid injection step of placing the concrete specimen restrained and pressurized by the concrete specimen pressurizing jig in a test tank and injecting test liquid into the test tank until the entire concrete specimen is submerged;
a freeze-thaw cycle step in which the temperature of the test liquid is controlled to perform a temperature management cycle consisting of a freezing step from a first temperature higher than zero to a second temperature lower than zero and a thawing step from the second temperature to the first temperature, and the concrete specimen is repeatedly frozen and thawed a predetermined number of times;
a post-freeze-thaw measurement step of removing the concrete specimen from the concrete specimen pressurizing jig after a predetermined number of freeze-thaw cycles and measuring the numerical value of the index;
a step number determination step of determining whether the number of steps of the freeze-thaw cycle step has exceeded a predetermined number;
If the number of times does not exceed a predetermined number in the step number determination step, a relative dynamic modulus of elasticity calculation step of calculating a relative dynamic modulus of elasticity from the numerical value of the index before the freeze-thaw and the numerical value of the index after the freeze-thaw;
a relative dynamic modulus of elasticity determination step of determining whether the calculated relative dynamic modulus of elasticity is equal to or less than a predetermined value,
If the relative dynamic modulus of elasticity is not equal to or less than the predetermined value, the freeze-thaw circulation step is continued while the concrete specimen is restrained by the pressure jig for the concrete specimen and a predetermined pressure is applied, the concrete specimen is removed from the pressure jig for the concrete specimen and the post-freeze-thaw measurement step is continued, the number of steps determination step, the relative dynamic modulus of elasticity calculation step, and the relative dynamic modulus of elasticity determination step are continued,
The freeze-thaw test method, wherein while the freeze-thaw circulation step, the post-freeze-thaw measurement step, the step number determination step, the relative dynamic elastic modulus calculation step, and the relative dynamic elastic modulus determination step are being performed, the freeze-thaw test is terminated if the step number determination step exceeds a predetermined number, or if the relative dynamic elastic modulus becomes equal to or less than a predetermined value in the relative dynamic elastic modulus determination step. A freeze-thaw test method using a pressurizing jig for a concrete specimen that applies compressive stress from the outside to a concrete specimen used in a freeze-thaw test of concrete, the pressurizing jig for a concrete specimen having a first constraining plate on which the concrete specimen is placed, a plurality of guide rails erected on the outer periphery of the first constraining plate, a second constraining plate that slides along the guide rails and constrains the concrete specimen together with the first constraining plate, and a pressure applying/removing means that applies pressure to the concrete specimen by pressing the second constraining plate, or removes pressure from the concrete specimen by releasing the pressing force,
a specimen pressurizing step of sandwiching and restraining a concrete specimen before freezing and thawing between the first restraint plate and the second restraint plate and applying a predetermined pressure;
a test piece initial value measurement step of measuring a numerical value that is an index of the strength of the concrete test piece in a state pressurized by the concrete test piece pressurizing jig;
a test liquid injection step of placing the concrete specimen restrained and pressurized by the concrete specimen pressurizing jig in a test tank and injecting test liquid into the test tank until the entire concrete specimen is submerged;
a freeze-thaw cycle step in which the temperature of the test liquid is controlled to perform a temperature management cycle consisting of a freezing step from a first temperature higher than zero to a second temperature lower than zero and a thawing step from the second temperature to the first temperature, and the concrete specimen is repeatedly frozen and thawed a predetermined number of times;
a post-freeze-thaw measurement step of measuring the numerical value of the index while applying pressure with the concrete specimen pressure jig after a predetermined number of freeze-thaw cycles;
a step number determination step of determining whether the number of steps of the freeze-thaw cycle step has exceeded a predetermined number;
If the number of times does not exceed a predetermined number in the step number determination step, a relative dynamic modulus of elasticity calculation step of calculating a relative dynamic modulus of elasticity from the numerical value of the index before the freeze-thaw and the numerical value of the index after the freeze-thaw;
a relative dynamic modulus of elasticity determination step of determining whether the calculated relative dynamic modulus of elasticity is equal to or less than a predetermined value,
If the relative dynamic modulus of elasticity is not equal to or less than the predetermined value, the concrete specimen is constrained by the pressure jig for the concrete specimen and a predetermined pressure is applied, and the freeze-thaw cycling step, the post-freeze-thaw measurement step, the step number determination step, the relative dynamic modulus of elasticity calculation step, and the relative dynamic modulus of elasticity determination step are continued;
The freeze-thaw test method, wherein while the freeze-thaw circulation step, the post-freeze-thaw measurement step, the step number determination step, the relative dynamic elastic modulus calculation step, and the relative dynamic elastic modulus determination step are being performed, the freeze-thaw test is terminated if the step number determination step exceeds a predetermined number, or if the relative dynamic elastic modulus becomes equal to or less than a predetermined value in the relative dynamic elastic modulus determination step.