Method for testing water-cement ratio of concrete by utilizing near-field microwaves

Abstract

The invention discloses a method for testing a water-cement ratio of concrete by utilizing near-field microwaves, and relates to testing of the water-cement ratio of the concrete. Firstly, a concrete sample with an unknown water-cement ratio is obtained; then, scanning a probe of a near-field microwave detection device from a set height of the surface of the sample to a position 10 microns below the surface by a point scanning method, detecting to obtain a quality factor Q, and taking a reciprocal 1 / Q of the quality factor Q (Q is a ratio of stored peak energy to dissipated energy in each period during system resonance); and finally, the water cement ratio is calculated according to the formula 1 / Q = 0.296 * W / C-0.05. According to the method, testing needs to be carried out when water and cement are mixed for 30 min, and a probe needs to be coated with a medium or provided with a protective sleeve to prevent conduction failure. An equation is obtained by fitting the 1 / Q values of multiple groups of standard samples and the water cement ratio, so that the water cement ratio of the concrete is conveniently tested.

Description

Technical Field

[0001] The invention relates to a method for testing the water-cement ratio of concrete, and is specifically a method for testing the water-cement ratio of concrete by utilizing near-field microwaves. Background Art

[0002] The water-cement ratio (W / C) refers to the mass ratio of water to cement in concrete, typically expressed as W / C, where W represents the mass of water and C represents the mass of cement. This ratio directly impacts concrete strength. Generally, a lower W / C ratio results in higher concrete strength. However, a too low W / C ratio can lead to cracking due to autogenous shrinkage or drying shrinkage. A too high W / C ratio can lead to insufficient concrete strength, bleeding, and segregation, resulting in a laitance layer on the surface and compromising quality. Therefore, properly controlling the W / C ratio is crucial, and determining the W / C ratio of existing slurries is a critical scientific challenge that needs to be addressed.

[0003] Currently, the commonly used methods for testing water-cement ratio include: gravimetric method, slump method, pressure method, and chemical analysis method. The gravimetric method directly weighs water and cement and is used before mixing. It is not suitable for concrete in a slurry state. The slump method measures the water-cement ratio as reflected by the slump value of concrete in a slurry state, but the test range is very limited (not suitable for highly fluid or self-compacting concrete) and is affected by the tester's experience, resulting in large errors. The chemical analysis method determines the water-cement ratio using chemical instruments, but the operation is relatively complex, expensive, and requires sample destruction, making it impossible to achieve non-destructive testing. Summary of the Invention

[0004] The purpose of the present invention is to provide a method for in-situ measurement and non-destructive detection of concrete water-cement ratio using near-field microwaves with fast response and simple operation.

[0005] To achieve the above object, the present invention relates to a method for testing the water-cement ratio of concrete using near-field microwaves, the method comprising:

[0006] Step 1: Obtain a concrete sample with unknown water-cement ratio;

[0007] Step 2: Scan the probe of the near-field microwave detection device from the sample surface at a set height using a point scanning method until it reaches a position 10 μm below the sample surface. Detect the sample and obtain the quality factor Q0, and take its reciprocal.

[0008] Among them, the quality factor Q0 represents the ratio of the peak energy stored in the system at resonance to the energy dissipated in one cycle, that is,

[0009] Step 3: Use the following formula to correct the quality factor;

[0010]

[0011] Among them, Q is the corrected quality factor, k T Indicates the temperature correction coefficient, k RH Indicates the humidity correction coefficient, T indicates the current ambient temperature, and RH indicates the current ambient humidity;

[0012] Step 4: Calculate the water-cement ratio of the concrete sample using the following formula:

[0013]

[0014] Furthermore, the sample is tested in step 2 at 30 minutes after the water and cement are mixed.

[0015] Furthermore, in step 2, the probe of the near-field microwave detection device is first coated with a dielectric, or a protective cover is installed to prevent the probe from conducting electricity when in contact with the sample, thereby preventing measurement failure.

[0016] Furthermore, in step 2, multiple groups of sample probes are taken and the Value, then five groups of samples The values ​​are averaged.

[0017] Furthermore, in step 3:

[0018] when When k T =0.015, k RH =0.008;

[0019] when When k T =0.008, k RH =0.004;

[0020] In other cases, k T =0.012, k RH =0.006.

[0021] Compared with the existing technology, the present invention has the advantages of low cost, operation suggestions, in-situ testing, strong universality, quick response and non-destructive testing. BRIEF DESCRIPTION OF THE DRAWINGS

[0022] Figure 1 Flow chart of the method of the present invention.

[0023] Figure 2 Schematic diagram of the structure of the measuring device involved in step 2.

[0024] Figure 3 This is the standard curve of concrete water-cement ratio obtained by fitting in step 3. DETAILED DESCRIPTION

[0025] See also Figure 1 The specific engineering process of a method for testing water-cement ratio using near-field microwaves in one of the numerous embodiments of the present invention includes four steps: preparing five sets of water-cement ratio samples, near-field microwave testing, data image processing, and data image analysis. The specific operation steps are as follows:

[0026] Step 1 (Preparing five sets of standard concrete samples): Weigh 20g of P·O42.5R grade ordinary Portland cement into a small beaker and 6g of deionized water into another beaker. Use a syringe to draw the deionized water into the cement beaker, record the time, and stir thoroughly with a stirring rod. Pour 20g of the slurry into a Petri dish to obtain a sample with a water-cement ratio of 0.30. Wait 30 minutes before measuring. Using the same method, keep the amount of cement constant and weigh 8g, 9g, 10g, and 11g of deionized water to prepare samples with water-cement ratios of 0.40, 0.45, 0.50, and 0.55, respectively. Record the time when the water is added, then mix and measure after 30 minutes.

[0027] Step 2 (near-field microwave test): Instruments and equipment see Figure 2 In this embodiment, the resonant cavity is a 1 / 4 wavelength resonant cavity. The instrument is preheated and the frequency is adjusted to the optimal test state. The sample is placed on the instrument stage. The needle tip is adjusted to the sample position. A downward point scan is performed from a position at a certain height from the sample. The point scan is completed when the needle tip is inserted into the position 10 μm below the sample. The quality factor Q measured by the vector network analyzer is read 6 times at intervals of 10 μm using a computer, and the reciprocal is taken to obtain value;

[0028] Step 3 (obtaining a standard curve from near-field microwave testing): Take the inverse of the quality factor in step 2 Take five groups of samples where the probe just touches the sample surface. Value, that is, z=0 in step 2 test data The z coordinate is the vertical movement distance of the probe, and Origin software is used to fit the five groups of Regarding the value of water-cement ratio, a linear standard curve of concrete water-cement ratio is obtained. This standard curve is - water-cement ratio (W / C), the equation of this standard curve is: R of the fitted curve 2 It is 0.998, which is close to 1, indicating a good fitting effect.

[0029] Step 4 (near-field microwave test to determine water-cement ratio): According to the method in step 1, weigh 20g of P·O42.5R grade ordinary Portland cement, add appropriate amount of water (unknown water-cement ratio), mix and let it stand for 30 minutes, then test the quality factor Q value of the sample according to the method in step 2 and take the reciprocal According to step 3, take the test data with z=0 The value was calculated by using Origin software on the known concrete water-cement ratio standard curve. value, determine the water-cement ratio of the sample and mark it on the standard curve (such as Figure 3 shown).

[0030] Step 5 (correct the standard curve formula for temperature and humidity): The standard curve relationship between the quality factor Q value and the water-cement ratio W / C under fixed temperature and humidity conditions is as follows:

[0031]

[0032] and The relationship should be:

[0033]

[0034] Regarding the correction of humidity, that is, the effective water content θ eff Correction:

[0035] θ eff =φ(W / C)·S(RH)·γ(T)

[0036] γ(T)=1-α(T-298)(α=0.005K -1 )

[0037] Where φ(W / C) is the expression of porosity with respect to water-cement ratio, S(RH) is the expression of saturation with respect to humidity, and γ(T) is the correction factor of temperature on adsorption;

[0038] Regarding temperature correction:

[0039] The increase in water temperature causes the energy storage factor ε′ to decrease:

[0040] ε′(T)=ε′ dry +θ eff [80-β(T-298)](β=0.2K -1 )

[0041] Influence on conductivity σ:

[0042]

[0043] The corrected loss factor ε″ is:

[0044]

[0045] Finally, the simplified value is obtained after temperature and humidity correction. The relationship with the water-cement ratio W / C should be:

[0046]

[0047] RH is humidity, which should be in the range of 40-80%, and temperature T is in the range of 20-40℃. T and k RH The reference values ​​are given in Table 1 below.

[0048] Table 1k T and k RH Reference value of

[0049] frequency <![CDATA[k T (K -1 )]]> <![CDATA[k RH (%RH -1 )]]> Applicable conditions 500MHz 0.015 0.008 W / C>0.5 1GHz 0.012 0.006 General conditions apply 2GHz 0.008 0.004 W / C<0.4

[0050] The standard curve of the results in this article is now corrected.

[0051]

[0052] The original data was obtained at 25°C and 50% RH. The actual measurement environment was 35°C and 70% RH. The results are:

[0053]

Claims

1. A method for testing the water-cement ratio of concrete using near-field microwaves, the method comprising: Step 1: Obtain a concrete sample with unknown water-cement ratio; Step 2: Scan the probe of the near-field microwave detection device from the sample surface at a set height using a point scanning method until it reaches a position 10 μm below the sample surface. Detect the sample and obtain the quality factor Q0, and take its reciprocal. Among them, the quality factor Q0 represents the ratio of the peak energy stored in the system at resonance to the energy dissipated in one cycle, that is, Step 3: Use the following formula to correct the quality factor; Among them, Q is the corrected quality factor, k T Indicates the temperature correction coefficient, k RH Indicates the humidity correction coefficient, T indicates the current ambient temperature, and RH indicates the current ambient humidity; Step 4: Calculate the water-cement ratio of the concrete sample using the following formula:

2. The method for testing the water-cement ratio of concrete using near-field microwaves according to claim 1, wherein: The time for testing the sample in step 2 is 30 minutes after the water and cement are mixed.

3. The method for testing the water-cement ratio of concrete using near-field microwaves according to claim 1, wherein: In step 2, the probe of the near-field microwave detection device is first coated with a dielectric or provided with a protective cover to prevent electrical conduction when the probe contacts the sample, thereby preventing measurement failure.

4. The method for testing the water-cement ratio of concrete using near-field microwaves according to claim 1, wherein: In step 2, multiple groups of sample probes are taken and just contacted with the sample. Value, then five groups of samples The values ​​are averaged.

5. The method for testing the water-cement ratio of concrete using near-field microwaves according to claim 1, wherein: In the step 3: when When k T =0.015, k RH =0.008; when When k T =0.008, k RH =0.004; In other cases, k T =0.012, k RH =0.006.

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