JPH08338838A - Method and apparatus for measuring unit quantity of water in concrete - Google Patents

Abstract

PURPOSE: To measure an estimated unit quantity of water in a short time. CONSTITUTION: An enclosable pressure reducing chamber 2 is formed in which heater 6 for heating a mortar 13 sampled from fresh concrete is disposed. The pressure reducing chamber 2 is coupled with a vacuum pump 8 and an electronic balance for measuring the weight of sample 13 is provided. The sample 13 is enclosed in the pressure reducing chamber 2 and the vacuum pump 8 is operated to reduce the pressure in the pressure reducing chamber 2. The sample 13 is heated under pressure reduced state thus evaporating water at low boiling point in a short time.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a method and apparatus for measuring the unit water content of concrete for measuring the unit water content of fresh concrete.

[0002]

2. Description of the Related Art Conventionally, since the strength of concrete is judged by a value obtained by a compressive strength test four weeks after it is placed, a concrete structure has already been completed at the time of judgment, and should the strength be insufficient. When it occurs, destroy the structure,
There is a problem that it will have to be reconstructed.

The strength of the concrete after solidification is
It is known that it can be determined by the water-cement ratio of concrete before it has set, that is, fresh concrete. For this purpose, various methods of measuring the water-cement ratio of concrete and judging the strength of concrete have been proposed. However, all of them had poor practicality. For example, as a method for measuring the unit water amount of fresh concrete, Japanese Patent Laid-Open No. 20448/1988 discloses
In concrete that does not solidify yet, quantitatively add an easily liquid substance to water, after the substance diffuses in the concrete, extract the liquid from the concrete and measure the concentration of the substance in the liquid, A method for measuring the water content of concrete before solidification for measuring the water content of concrete has been proposed. Further, in Japanese Patent Laid-Open No. 62-106368, a part of concrete or mortar that has not yet solidified is taken out as a sample, and Of the specified ion, the concentration of the specified ion is measured, the ion of the same kind in the sample of the specified ion is added to the sample in a fixed amount, and the concentration of the added ion is measured for the sample after the ion addition, Measure the ion concentration before and after adding ions, and determine the unit water amount contained in concrete or mortar that has not solidified yet by measuring the change in ion concentration before and after adding ions. Although methods for measuring the amount of water in concrete such as concrete have been proposed, these methods involve adding the above substances or adding ions, which complicates the measurement work and results in a water mixture of cement and water. It is expected that the sum action will make accurate measurement difficult.

Therefore, as described in JP-A 64-20448, page 1, left column, lines 16 to 19, when moisture content in concrete is measured, the moisture content in concrete is heated by heating. It has been attempted to elevate the measurement accuracy of this heating and drying method and measure an accurate unit water amount by using a method of evaporating and comparing the weight difference of concrete before and after evaporation of the same moisture, that is, a so-called heating and drying method.

[0005]

The above-mentioned conventional heating and drying method has problems that a large-scale heating device is required, a relatively long drying time is required, and the heating temperature becomes high. It was Especially when the heating time becomes long and the heating temperature becomes high, the hydration of the cement is promoted and water is taken into the cement as binding water, so even if the weight difference of concrete before and after evaporation is measured, the correct amount of unit water There was a problem that it became difficult to measure.

[0006] Therefore, an object of the present invention is to provide a concrete unit water amount measuring method and an apparatus therefor capable of performing measurement in a short time and accurately measuring an estimated unit water amount.

[0007]

According to a first aspect of the present invention, a mortar sample is collected from fresh concrete, the weight of the sample is measured, and the sample is heated in a decompressed space to evaporate water, and then the sample is sampled. Is a unit water amount measuring method of concrete, in which the estimated unit water amount is obtained by measuring the weight of the sample and calculating the estimated unit water amount by the weight difference of the sample after water evaporation.

Further, the invention of claim 2 is a method for measuring a unit water amount of concrete, wherein the absolute pressure of the depressurized space is 160 mmHg or less.

Furthermore, the invention of claim 3 is a method for measuring the unit water content of concrete, wherein the temperature of the sample is maintained at 50 ° C. (50 ° C.) or less and heated.

The invention according to claim 4 is a decompression chamber which can be sealed,
Unit water content measurement of concrete provided with heating means for heating a mortar sample taken from fresh concrete placed in this decompression chamber, a suction device for decompression connected to the decompression chamber, and weighing means for measuring the weight of the sample It is a device.

[0011]

In the structure of the above-mentioned claim 1, since the sample is heated in the decompression space, the boiling point of water is lowered, and water is evaporated at a relatively low temperature in a short time. In addition, since water evaporates at low temperature in a short time, the generation of bound water due to hydration of cement is suppressed, and by measuring the weight of the sample before and after evaporation, an accurate estimated unit water amount with less influence of bound water Can be obtained.

In the structure of the above-mentioned claim 2, since the absolute pressure of the decompression space is 160 mmHg or less, the boiling point of water decreases,
Water evaporates at low temperatures.

In the structure of claim 3, the sample is heated under reduced pressure and the heat corresponding to the heat of vaporization of water is supplied to heat the sample at 50 ° C. or less, so that the hydration action of the cement is suppressed. can do.

In the structure of the above-mentioned claim 4, the sample is housed in the decompression chamber in a sealed state, the decompression suction device is operated to decompress the decompression chamber, the sample is heated in this decompression state, and water is reduced at a low boiling point. Can be evaporated in time.

[0015]

Embodiments of the present invention will be described below with reference to the accompanying drawings. 1 to 6 show an embodiment of the present invention,
The measuring apparatus of the present invention will be described. The drying apparatus 1 shown in FIGS. 1 and 2 has a decompression chamber 2 in the upper part, and the decompression chamber 2 has a bottomed box shape with an open upper part. An opening / closing lid 3 capable of sealing the decompression chamber 2 is provided in the upper part of the, and an observation window 4 for visually recognizing the inside of the decompression chamber 2 is provided at the center of the opening / closing lid 3. A rear portion of the opening / closing lid 3 is pivotally attached to the rear portion of the decompression chamber 2, and a handle 5 is provided on the front upper surface of the opening / closing lid 3. Further, a heater 6 as a heating means is provided in the decompression chamber 2, and the heater 6 is an electrothermal heater having a heating wire (not shown) therein.
The circular upper surface 7 is formed flat. The heater 6 is equipped with a thermocouple (not shown) and can be heated at a constant temperature by a temperature control device (not shown). A vacuum pump 8 as a vacuum suction device is built in the lower part of the drying device 1, and a pipe 9 connected to a suction port of the vacuum pump 8 is connected to the vacuum chamber 2. A steam condenser 10 is provided in the middle of the pipe 9, and a cold trap for steam is used in the steam condenser 10. For example, a condensation chamber larger than the pipe 9 is provided in the middle of the pipe 9. The condensation chamber is held in a cooled state, and the water vapor sent from the decompression chamber 2 through the pipe 9 is condensed in the condensation chamber and the water obtained is removed to the vacuum pump 8. Prevents the transmission of water vapor. Further, the heater 6 and the electric vacuum pump 8 are electrically connected to a household power source or the like via a power cord 11. Further, on the front surface of the drying device 1, an operation panel 12 for adjusting the on / off operation and operation time of the heater 6 and the vacuum pump 8 and the temperature of the heater 6 and the suction force of the vacuum pump 8 is provided. In addition, 3A and 2A in the figure
Is a heat insulating material for the opening / closing lid 3 and the decompression chamber 2.

A tray 14 for holding the sample 13 of mortar T is made of stainless steel, and the tray 14 is the upper surface 7 of the heater 6.
It has a circular bottom surface 14A and a side surface 14B which are substantially the same shape as the above, and is provided with a handle 14C on the side surface 14B, and the bottom surface 14A is formed flat.

3 and 4 is provided with a weighing chamber 22 at an upper portion thereof, and a weighing table 24 of an electronic balance 23 as a weighing means is provided in the weighing chamber 22 so as to face the weighing chamber 22. Room
An opening / closing cover 25 that covers 22 is provided, a rear portion of the opening / closing cover 25 is pivotally attached to the weighing / calculating device 21, and a handle 26 is provided on the front surface of the opening / closing cover 25.
An observation window 27 that allows the inside of the weighing chamber 22 to be visually recognized is provided on the upper surface of 25. The weighing / calculating device 21 incorporates a calculating means 28 including a CPU and the like, and on the front surface thereof, an operation panel 29 and a paper discharge port 31 of a printer 30 for printing the calculation result are provided. Further, the electronic balance 23, the computing means 28, and the printer 30 are electrically connected to a household power source or the like via a power cord 32.

Next, the measuring method using the devices 1 and 21 and the constitution of the calculating means 28 will be explained. First, fresh concrete is prepared by mixing water, cement, fine aggregate, coarse aggregate and admixture. Mix and use those with a known water-cement ratio, and, for example, fresh concrete that is fresh concrete to be shipped on the same day, or part of the fresh concrete that was brought into the site by a concrete mixer truck. A sample 13 of mortar T is collected by wet screening from this fresh concrete. In this wet screening, a mortar sample 13 is obtained by separating the mortar T and the coarse aggregate using a 5 mm sieve (not shown), that is, separating the coarse aggregate from the green concrete. Further, in the actual work, fresh concrete is sieved and put into a receiving container (not shown) of the mortar T, and the mortar T in the receiving container is put into the receiving tray 14 with a spoon (not shown) or the like. Eg 4
About 100 grams is put in, but the equipment used for this work, such as the sieve, receiving container, and spoon, is used in a wet state wiped with a damp towel to prevent the occurrence of a water error associated with the sampling operation. .

The mortar sample 13 thus collected from the fresh concrete is put together with the pan 14 into the electronic balance.
The sample 13 is placed on a weighing table 24, the open / close cover 25 is closed, and the weight M ₁ of the sample 13 before drying is measured. In this case, the weight of the saucer 14 is measured in advance, and the weight before drying M ₁ of the sample 13 is obtained by subtracting the weight of the saucer 14 from the measured weight, and the calculating means 28 performs the calculation and The weight M ₁ before drying is stored.

Next, by using the drying device 1, the sample 13 was depressurized.
To dry. First, set the pan 14 containing the sample 13 to the heater 6
The bottom surface 14A is fixed to the top surface 7 in a close contact state by a clamp mechanism (not shown). Due to such close contact, the heat of the heater 6 is uniformly transferred to the sample 13, and the heat of the heater 6 is converted into the heat of vaporization of water without waste. After closing the opening / closing lid 3 and sealing the inside of the decompression chamber 2, the operation panel 12 is operated to operate the heater 6 and the vacuum pump 8. Then, the heating temperature of the heater 6 is 250 ° C., and the vacuum chamber 8 is continuously sucked to heat and dry the sample 13 in the decompression chamber 2 at an absolute pressure of 160 mmHg or less. In the graph of FIG. 6 showing this state, in the experiment under the same conditions, the horizontal axis shows time,
The temperature is plotted on the vertical axis, and changes in the temperature of the heater 6, the decompression chamber 2, the sample 13 and the room temperature are shown. When heating by the heater 6 and the operation of the vacuum pump 8 are started at the same time, the decompression chamber takes about 1 minute. The absolute pressure of 2 becomes 160 mmHg or less, the boiling point of water falls under the reduced pressure close to this vacuum, and the water of sample 13 evaporates at a temperature of about 20 to 30 ° C below 50 ° C. The reason why the temperature of the heater 6 gradually rises is that the heat of the heater 6 is removed as vaporization heat of water, and the temperature of the sample 13 is kept at about 20 to 30 ° C. for about 12 minutes. The constant indicates that the water is vaporizing, and sample 13
The temperature of the heater 6 reached 250 ° C. after 12 minutes when the vaporized water in the sample disappeared, and at 250 ° C., the temperature became almost constant by the temperature control device.
In No. 13, the temperature of the remaining cement, fine aggregate, etc. itself rises, and the slope gradually rises as shown in the graph. And almost 400
This method using gram of sample 13
After 2 minutes, the water excluding the bound water of the sample 13 was completely evaporated, and 18 minutes, which was longer than 12 minutes, was set as the reduced pressure heating time. In this way, the water in the sample 13 is evaporated at a low temperature of about 20 to 30 ° C. in a relatively short time, so that the hydration action of the cement during heating can be suppressed. In addition, 1 atm is an absolute pressure of 760 mmHg.

Subsequently, the sample 13 after the reduced pressure heat treatment was subjected to an electronic balance.
The sample 13 is placed on the table 23 and the weight M ₂ of the sample 13 after drying is measured. The calculating means 28 calculates the weight of the sample 13 and stores the obtained weight M ₂ after drying. In this way before drying,
Calculating the weight M _1, M ₂ after drying, known cement, prior to drying of the water and fine aggregate, after drying weight M _1, M ₂ from the calculating means 28 calculates the like estimated unit water. In the present invention, terms such as unit water amount, unit cement amount, unit aggregate amount and the like are known by concrete mixing, and terms such as estimated unit water amount and estimated water cement ratio are calculated by this measurement method. Show things.

Further, in order to improve the accuracy of the estimated unit water amount, the inventor further conducts the following preliminary experiments, and
The estimated unit water amount obtained from the above measurement is corrected based on the data, and this is incorporated into the computing means 28 in advance as data or is input from the operation panel 29 to obtain a more accurate estimated unit water amount and estimation based on this. It was possible to calculate the water cement ratio. The experiment and data are described below.

First, in the present method, it was possible to suppress the hydration of cement during heating by using the reduced pressure heat treatment. For example, when carrying fresh concrete to the site by a concrete mixer truck, etc. Then, after the concrete time is loaded on the mixer truck, the measurement is carried out after a predetermined time has passed, so that water is taken into the cement as bound water during that time. Therefore, in order to correct the amount of water taken in as bound water in the sample 13, the ignition loss test of the sample 13 after the reduced pressure heating treatment for 18 minutes was performed, and from this ignition loss test, the reduced pressure heating for 18 minutes was performed. The ratio of the bound water of the treated sample 13 is calculated for each water cement ratio.

[0024]

[Table 1]

Table 1 above shows, as an example, the ignition loss value X of Sample 13 having a water-cement ratio of 50% due to the mixing of concrete by the ignition loss test after the reduced pressure heating treatment for 18 minutes. Moreover, the value X of the loss on ignition with respect to the cement mass is shown as a percentage, assuming that the kneading time before the reduced pressure heat treatment is 0 minutes, 60 minutes, and 90 minutes later. That is, the value X indicates the weight percentage of water contained as the bound water in the cement in the sample 13 which was subjected to the reduced pressure heat treatment after each kneading time. And besides the above Table 1, different water cement ratios (40, 45, 50, 55, 6
0%, etc.) of the sample 13 is, for example, after each kneading time every 30 minutes, by the same ignition loss test,
The weight percentage of water remaining as bound water in the cement after the reduced pressure heat treatment is calculated, and these values X are stored in the arithmetic processing means 28. Alternatively, when the elapsed time and the water-cement ratio are input, the arithmetic processing means 28 performs arithmetic using the value X corresponding to those conditions. The ignition loss test is carried out based on the soil ignition loss test method of the Japan Society of Soil Engineering (JSFT221-1990), and a part of the sample 13 after the reduced pressure heat treatment is put into a crucible to obtain one of the samples 13. The crucible is weighed together with the parts, and the crucible is set to 700 to 800 °.
Heat at C for 2-4 hours, heat until a portion of the sample 13 reaches a constant weight, and determine the reduced weight as a percentage of the weight of the portion of the sample.

Secondly, correction is made based on the water absorption rate of the fine aggregate used as the sample. The fine aggregate of the mix design is based on the fine aggregate in the surface dry state, and actually, the fine aggregate that contains water is mixed to produce concrete. Therefore, the reduced pressure heat treatment also evaporates the water content corresponding to the water absorption rate of the fine aggregate. Therefore, the water absorption rate P of the fine aggregate mixed in the concrete
Based on JIS, specific gravity and water absorption test method of fine aggregate (JIS A
1109-1993) is applied. Then, in the actual work, the water absorption rate P of the concrete fine aggregate to be used is input to the calculating means 28 by the operation panel 29. By this input, the calculating means 28 calculates the estimated unit quantity Wa, the estimated water cement ratio Ha, the concrete strength σ, etc. based on the following respective equations, and prints them on the printer 30.

The symbols in the following formulas 1, 2 and 3 indicate respective values when concrete is mixed, C is a unit amount of sentinum (Kg / m ³ ), W is a unit amount of water (Kg / m ³ ), and S is Unit fine aggregate amount (Kg / m ³ ), P indicates the water absorption rate of the fine aggregate.

[0028]

[Equation 1]

[0029]

[Equation 2]

[0030]

(Equation 3)

Explaining Equation 1 above, based on the pre-drying weight M ₁ and post-drying weight M ₂ obtained before and after the reduced pressure heat treatment, the unit cement amount C, the unit water amount W and the unit fine aggregate amount known by the concrete mixing are known. The estimated unit water amount contained in the unit mortar amount obtained by adding S is calculated, and the value obtained by multiplying the ignition loss value X obtained in the above-mentioned experiment by the unit cement amount C is added to the cement after the reduced pressure heat treatment. The amount of water contained as bound water is corrected, and further, the water contained in the fine aggregate at the time of blending is reduced based on the known water absorption rate P for each fine aggregate used. In addition, as described above, the correction by the water absorption rate P is as follows.
Since the fine aggregate in the surface dry state is used, the water contained in the fine aggregate is evaporated by the reduced pressure heat treatment, and the estimated unit water amount increases by this amount. Make a correction to reduce the included unit water volume.

Equation 2 is an equation for calculating the estimated water cement ratio Ha from the unit water amount Wa obtained from Table 1 above.

The above mathematical formula 3 is an example of a regression equation for calculating the concrete strength σ, which is used based on the results of the ready-mixed concrete plant, and is different for each ready-mixed plant, but it is based on the unit cement amount C and the unit water amount W. In addition, if the concrete strength can be obtained and the unit water volume W is replaced with the estimated unit water volume,
It can be seen that the estimated concrete strength can be calculated.

The following Table 2 shows the estimated unit water amount Wa and the estimated water cement of 15 kinds of samples with each water cement ratio C, each unit water amount W, and different composition of fine aggregate, coarse aggregate and admixture. The ratio Ha was measured and calculated by this method, and it was revealed that the ratio Ha can be measured with high accuracy.

[0035]

[Table 2]

As described above, in this embodiment, according to claim 1, a mortar sample 13 is taken from fresh concrete, the weight M _{1 of the} sample 13 is measured, and the sample 13 is heated in a decompressed space, After evaporation of water, weight of sample 13 M ₂
Is measured and the estimated unit water amount is obtained by the weight difference of the sample 13 after evaporation of water, so that the sample 13
Since the boiling point of water is lowered, the water evaporates at a relatively low temperature in a short time, and the water evaporates at a low temperature in a short time, so that the generation of bound water due to hydration of cement is suppressed. By measuring the weight of the sample 13 before and after evaporation, it is possible to obtain an accurate estimated unit water amount with little influence of bound water.

Further, according to the second aspect, since the absolute pressure of the decompression space is 160 mmHg or less, the boiling point of water is lowered and the water evaporates at a low temperature.

Further, according to claim 3, since the temperature of the sample 13 is maintained at 50 ° C. or less and heated, the sample 13 is heated under reduced pressure.
By heating the sample 13 and supplying heat corresponding to the heat of vaporization of water, the sample 13 is heated at 50 ° C. or less, so that the hydration action of the cement can be suppressed.

As described above, in this embodiment, according to the fourth aspect, the sealable decompression chamber 2 and the heater 6 as a heating means for heating the mortar sample 13 taken from the fresh concrete contained in the decompression chamber 2 are used. And a vacuum pump 8 which is a vacuum suction device connected to the decompression chamber 2 and an electronic balance 23 which is a weighing means for measuring the weight of the sample 13. Therefore, the sample 13 is sealed in the decompression chamber 2. The vacuum pump 8 is continuously operated during heating to reduce the pressure in the decompression chamber 2, and the sample 13 is heated in this depressurized state to evaporate water at a low boiling point in a short time.

Further, as an effect of the embodiment, by reducing the ignition loss of the sample 13 subjected to the reduced pressure heat treatment, the ignition loss values X of various samples 13 are calculated in advance, and the ignition loss values X are corrected by the ignition loss values X. It is possible to obtain an accurate estimated unit water amount Wa and the like, and further, by using the water absorption rate P of the fine aggregate to be used to correct the amount of water in the fine aggregate evaporated in the reduced pressure heat treatment, it becomes more accurate. The estimated unit water amount Wa can be calculated, and the ignition loss value X is calculated in advance for each elapsed time.
Since it is calculated and used, the estimated unit water amount Wa of the concrete brought into the site by a mixer truck or the like can be accurately measured. Further, since the vacuum pump 8 is continuously operated during the reduced pressure heat treatment, evaporation of water can be promoted.
Further, since the bottom surface 14A of the tray 14 and the top surface 7 of the heater 6 are formed flat and the two are made in close contact with each other by the clamping means, and the bottom surface 14A and the top surface 7 are formed in substantially the same shape, the heater 6 The heat is efficiently transferred to the sample 13, that is, as shown in FIG. 6, even if the heater 6 is heated, the temperature of the decompression chamber 2 does not rise so much, and the heat of the heater 6 is efficiently converted to the heat of vaporization of water. can do.

The present invention is not limited to the above embodiment, and various modifications can be made within the scope of the gist of the present invention. For example, the drying device and the weighing calculation device may be integrated, and an electronic balance may be incorporated in the heater to measure the weight of the pan placed on the upper surface of the heater. Further, it is also possible to use, for each unit amount of concrete, a printed record submitted by a ready-mixed concrete supplier. Further, the heating temperature can be selected as appropriate. Further, a temperature sensor for measuring the temperature of the sample is provided in the decompression chamber, and when the temperature of the sample rises by this temperature sensor, it can be seen that the evaporation of water is completed. May be.

[0042]

According to the first aspect of the present invention, a mortar sample is taken from fresh concrete, the weight of the sample is measured, and the sample is heated in a decompressed space to evaporate water, and then the weight of the sample is measured. However, it is a unit water amount measuring method of concrete that obtains the estimated unit water amount by the weight difference of the sample after water evaporation, and the unit water amount of concrete that can perform the measurement in a short time and can accurately measure the estimated unit water amount. A measurement method can be provided.

The invention according to claim 2 is a method for measuring a unit water amount of concrete in which the absolute pressure of the depressurized space is 160 mmHg or less, the measurement can be performed in a short time, and the estimated unit water amount is accurately measured. It is possible to provide a method for measuring the unit water amount of concrete that can be performed.

Furthermore, the invention of claim 3 is a method of measuring the unit water content of concrete, in which the temperature of the sample is maintained at 50 ° C. or lower, and the unit water content can be measured in a short time.
It is possible to provide a method for measuring the unit water amount of concrete, which can accurately measure the estimated unit water amount.

According to a fourth aspect of the invention, a decompression chamber capable of being closed is provided,
Unit water content measurement of concrete provided with heating means for heating a mortar sample taken from fresh concrete placed in this decompression chamber, a suction device for decompression connected to the decompression chamber, and weighing means for measuring the weight of the sample It is an apparatus, can perform measurement in a short time, and can provide a unit water amount measuring device for concrete that can accurately measure an estimated unit water amount.

[Brief description of drawings]

FIG. 1 is a sectional view of a drying device showing an embodiment of the present invention.

FIG. 2 is a perspective view of a drying device showing an embodiment of the present invention.

FIG. 3 is a cross-sectional view of a weighing / calculating device showing an embodiment of the present invention.

FIG. 4 is a perspective view of a weighing / calculating device showing an embodiment of the present invention.

FIG. 5 is a flow chart illustrating a measurement procedure according to an embodiment of the present invention.

FIG. 6 is a graph showing the temperature of a sample and a heater in a reduced pressure heating state showing an example of the present invention.

[Explanation of symbols]

1 Drying device 2 Decompression chamber 6 Heater (heating means) 8 Vacuum pump (decompression suction device) 13 Sample 21 Measuring / calculating device 23 Electronic balance (measuring means) M ₁ Weight before drying M ₂ Weight after drying Wa Estimated unit water amount

 ─────────────────────────────────────────────────── ─── Continuation of the front page (72) Inventor Akifumi Yamada 1603-1 Kamitomioka-cho, Nagaoka-shi, Niigata Nagaoka University of Technology

Claims (4)

[Claims] 1. A mortar sample is taken from fresh concrete, the weight of the sample is measured, and the sample is heated in a decompressed space to evaporate water, and then the weight of the sample is measured to obtain a sample after evaporation of water. A method for measuring the unit water volume of concrete, characterized in that the estimated unit water volume is obtained from the difference in weight. 2. The concrete water content measuring method according to claim 1, wherein the absolute pressure of the decompression space is 160 mmHg or less. 3. The method for measuring the unit water content of concrete according to claim 1, wherein the sample is heated while being kept at a temperature of 50 ° C. or lower. 4. A hermetic decompression chamber, a heating means for heating a mortar sample collected from fresh concrete placed in the decompression chamber, a decompression suction device connected to the decompression chamber, and a weight of the sample. A unit water content measuring device for concrete, comprising a measuring means for measuring.