JP6990955B2 - Early detection method for concrete deterioration - Google Patents

Description

The present invention relates to a method of analyzing a digital image of concrete to detect deterioration of concrete at an early stage.

Factors that cause concrete deterioration include alkali-silica reaction, delayed formation of ettringite (DEF: Delayed Ettringite Formation), freeze-thaw, drying shrinkage, and corrosion of reinforcing bars.
Of these, in the alkali silica reaction, silica in the reactive aggregate and alkali metal ions in the concrete react under high pH conditions to form alkali silica gel, and this gel absorbs water and expands to form concrete. It is a phenomenon that cracks occur in the concrete. Since the cracks caused by this alkali-silica reaction have a hexagonal shape, they can be distinguished from other cracks caused by deterioration by the shape of the cracks.
In addition, delayed formation of ettringite is a phenomenon in which ettringite may be concentrated and generated several years after steam curing of concrete, and this ettringite expands the concrete and the concrete collapses.
Freezing and thawing is a phenomenon in which water in concrete repeats freezing and thawing for many years, and the volume expansion of water (ice) causes cracks in concrete. Further, the drying shrinkage is a phenomenon in which the moisture in the concrete evaporates due to the drying of the concrete and the concrete shrinks to cause cracks. Further, corrosion of reinforcing bars is a phenomenon in which the passivation film of reinforcing bars is damaged by neutralization or salt damage, the reinforcing bars rust and expand, and the concrete is cracked.
In these concrete deterioration phenomena, the deterioration is often too advanced when the cracks become apparent and the cracks are found. Therefore, in order to effectively prevent the deterioration of concrete, it is necessary to detect the factors of deterioration at an early stage before the cracks become apparent and take measures according to each factor.

By the way, conventionally, several methods for detecting the deterioration of the concrete have been proposed.
In the crack inspection method for a concrete structure described in Patent Document 1, an undercoat layer, a peeling prevention sheet layer, and a topcoat layer are sequentially laminated on a substrate constituting the concrete structure, and then a topcoat is further applied. A high-elasticity coating layer mixed with a fluorescent dye that emits light by excitation light and a low-elasticity coating layer mixed with a shielding agent that blocks the transmission of excitation light are sequentially laminated on the layer to form a concrete structure. This is a method of irradiating the structure with excitation light after the start of service to detect cracks generated in the substrate due to deterioration over time.
Further, in the method for detecting a concrete deterioration factor described in Patent Document 2, a visible light image is acquired by imaging a concrete surface, and on the other hand, the concrete surface is irradiated with infrared light and a scanning device is used to scan the reflected light from the concrete surface. It is input to the spectroscope through the spectroscope, and the absorbance based on the light intensity of a specific wavelength for detecting a specific deterioration factor is detected by the spectroscope, and the absorbance is converted into the concentration of the deterioration factor and the concentration is quantum. It is a method of acquiring a deterioration factor image by expressing it in shades or colors according to the concrete surface to be measured based on the quantified value, and synthesizing the deterioration factor image and the visible light image.

However, the method described in Patent Document 1 is a method of detecting a crack after a crack is generated, and further high elasticity after sequentially laminating an undercoat layer, a peeling prevention sheet layer, and a topcoat layer. The coating film layer and the low elasticity coating film layer must be sequentially laminated, which is complicated. Further, the method described in Patent Document 2 is specified for detecting a deterioration factor by irradiating infrared rays and inputting reflected light from the concrete surface to a spectroscope in addition to acquiring a visible light image of the concrete surface. It is necessary to detect the absorbance based on the light intensity of the wavelength of the above, and the work is also complicated.

Japanese Unexamined Patent Publication No. 2014-85200 Japanese Unexamined Patent Publication No. 2007-85850

Therefore, an object of the present invention is to provide a method for detecting deterioration of concrete at an early stage and easily.

When the present inventor examined a detection method that meets the above object, he found that the cause of deterioration can be detected quickly and easily from the time course and distribution of the strain on the concrete surface grasped by using the digital image correlation method. Completed the invention. That is, the present invention is an early detection method for deterioration of concrete having the following configuration.

[1] An early detection method for concrete deterioration, which detects the deterioration of concrete by using the pattern appearing in the image of the distribution of the maximum principal strain obtained through the following steps (A) and (B).
An early detection method for concrete deterioration, which detects deterioration of concrete based on the following criteria (a) to (e) using the pattern.
(A) Image acquisition step of acquiring a digital image of the surface to be acquired of concrete over time (B) Strain is calculated using the digital image correlation method based on the digital image, and the distribution of the maximum principal strain is distributed based on the strain. Maximum principal strain distribution acquisition process
(A) The portion where the hexagonal pattern appears is detected as an alkali-silica reaction.
(B) When a uniform pattern appears as a whole except for the aggregate portion, it is detected as delayed generation of ettringite.
(C) When a speckled pattern appears, it is detected as freeze-thaw.
(D) When a linear pattern appears in a place where a reinforcing bar does not exist, or a linear pattern appears in an oblique direction with respect to a member such as a column or a beam of a structure, it is detected as a drying shrinkage.
(E) When a linear pattern appears directly above the reinforcing bar, it is detected as corrosion of the reinforcing bar.
[2] The method for early detection of concrete deterioration according to the above [1], which detects deterioration of concrete based on the positive and negative distribution of strain in addition to the pattern.
[3] The method for early detection of concrete deterioration according to the above [2], wherein the deterioration of concrete is detected based on the following criteria (f) to (j) using the positive and negative distributions of the pattern and strain.
(F) In the case where a hexagonal pattern appears, the line portion of the hexagonal pattern shows a larger positive (expansion) strain, and the substrate portion (part other than the pattern) is also positive (expansion) as a whole. ) Is detected as an alkali-silica reaction.
(G) When a uniform pattern appears as a whole except for the aggregate part, the aggregate part does not show any strain, but the part made of other paste or mortar is totally positive (expansion). ) Is detected as delayed generation of ettringite.
(H) When a spot-like pattern appears and the spot portion shows a large positive (expansion) strain, it is detected as freeze-thaw.
(I) When a linear pattern appears in a place where the reinforcing bar does not exist, or a linear pattern appears in an oblique direction with respect to a member such as a column or a beam of a structure, and the linear portion of the pattern is a plus. When the strain of (expansion) is shown and the substrate portion shows negative (shrinkage) strain as a whole, it is detected as dry shrinkage.
(J) When a linear pattern appears directly above the reinforcing bar and the line portion of the pattern shows positive (expansion) strain, it is detected as corrosion of the reinforcing bar.

The method for early detection of concrete deterioration of the present invention can detect the cause of concrete deterioration early and easily.

It is a figure which shows the test body (gray part) of the block-shaped test body, and the test body (gray part) of the plate cut out from there to the thickness 50 mm. However, the unit of the numerical value in FIG. 1 is mm. It is a figure which shows the pattern of steam curing. It is a figure which shows the measurement range of the test piece by the digital image correlation method, and the sticking position of the contact gauge chip. However, the unit of the numerical value in FIG. 3 is mm. It is a figure which shows the test piece used for the freeze-thaw test. It is a figure which shows the restraint form used for the drying shrinkage test. It is a photograph which shows the scanner for digital image acquisition used in an Example. It is a figure which shows the distribution of the maximum principal strain generated by the alkali-silica reaction in the promotion (curing) period of 13, 28 days, and 56 days. However, the numerical value on the left side of FIG. 7 is abbreviated as "× ^10-6 ". It is a figure which shows the distribution of the maximum principal strain caused by the delayed formation of ettringite in the underwater curing period of 7 days and 28 days. However, the numerical value on the left side of FIG. 8 is abbreviated as "× ^10-6 ". It is a figure which shows the distribution of the maximum principal strain caused by freezing and thawing in 30 cycles, 300 cycles, and 480 cycles. However, the numerical value on the left side of FIG. 9 is abbreviated as "× ^10-6 ". It is a figure which shows the distribution of the maximum principal strain caused by the drying shrinkage in the drying period of 1, 2, and 7 days. However, the numerical value on the left side of FIG. 10 is abbreviated as "× ^10-6 ". The white arrow indicates the cracked part.

As described above, the method for early detection of concrete deterioration of the present invention uses the pattern appearing in the image of the distribution of the maximum principal strain obtained through the steps (A) and (B) to accelerate the deterioration of concrete. It is a method of detection. Hereinafter, the method for early detection of deterioration of concrete of the present invention will be described in detail.

(A) Image acquisition step The step is a step of acquiring a digital image of the surface to be acquired of concrete (test piece) over time.
Here, the concrete is not particularly limited, and is not particularly limited, and is ordinary concrete, watertight concrete, hot concrete, cold concrete, mass concrete, fluidized concrete, high-fluidity concrete, high-strength concrete, low heat-generating concrete, expanded concrete, prestressed concrete, and low. Included are shrink concrete, fibrous reinforced concrete, lightweight concrete, and polymer concrete.
In order to obtain a good digital image, it is preferable to polish the surface to be acquired of concrete. In addition, the digital image of the surface to be acquired is acquired before and after the concrete is deformed. In addition, since the accumulated strain is released when cracks occur, the image is preferably acquired after the concrete has hardened and at least before the cracks occur. Here, if moisture adheres to the acquisition surface of the concrete before deformation, the color contrast becomes small and color unevenness occurs, and the correlation with the image acquired after drying is lowered. May be done. In order to avoid this deterioration of the correlation, before acquiring the image, compressed air or the like is sprayed onto the imaged surface of the concrete before drying to remove the moisture on the imaged surface, or the image is allowed to stand until the imaged surface is free of moisture. Perform pretreatment such as air drying. The pretreatment is an arbitrary treatment performed when water is attached to the acquisition surface of the image.

ただし、実際は、矩形に設定した変形前のサブセットに対し、変形後のデジタル画像そのものが変形しているため、サブセットが矩形にならない場合がある。この場合、これを補正するため、サブセット内部において変位勾配が一定と仮定して、変形前後の座標(x,y)および(x^*,y^*)には下記（２）式を用いる。

以上の計算は、市販の画像解析用ソフトウエア（例えば、digital：Correlated solutions社製）を用いて行なうことができる。 (B) Maximum Principal Strain Distribution Acquisition Step This step is a step of calculating a strain using a digital image correlation method based on the digital image and obtaining a maximum principal strain distribution based on the strain. The digital image is a digital image before and after the deformation of concrete, and the maximum principal strain after the deformation is calculated by using the digital image correlation method.
The digital image correlation method is a method of calculating the amount of movement on concrete and converting it into the maximum principal strain based on the distribution of the brightness values of the digital images acquired before and after the deformation due to the strain.
Specifically, the strain is calculated through the following calculation process.
(I) Find the luminance value distribution within a subset centered on an arbitrary position in the digital image before deformation.
(Ii) Search for a subset of the digital image before deformation, which has the brightness value distribution with the highest correlation with the brightness value distribution of the digital image after deformation, and regard the center point as the position after the point of interest is displaced. , The amount displaced from the point of interest to the center point is calculated, and the displaced amount is further converted into the maximum principal strain. The correlation of the subset before and after the deformation is expressed by using the correlation coefficient R of the following equation (1).

However, in reality, the subset may not be a rectangle because the digital image itself after the transformation is deformed with respect to the subset before the transformation set in the rectangle. In this case, in order to correct this, the following equation (2) is used for the coordinates (x, y) and (x ^* , y ^* ) before and after the deformation, assuming that the displacement gradient is constant inside the subset.

The above calculation can be performed using commercially available image analysis software (for example, digital: manufactured by Correlated solutions).

Next, the criteria for detecting the cause of deterioration of concrete used in the present invention are described below.
(A) As shown in FIG. 7 described later, the portion where the hexagonal pattern appears is detected as an alkali silica reaction (ASR). In addition, the line portion of the pattern shows a larger positive (expansion) strain, and the substrate portion (part other than the pattern) also shows a positive (expansion) strain as a whole.
(B) As shown in FIG. 8 described later, when a uniform pattern appears as a whole except for the aggregate portion, it is detected as delayed generation of ettringite. In addition, the aggregate part does not show strain, but the other paste part or mortar part shows positive (expansion) strain as a whole. In FIG. 8, the above judgment is made because the aggregate is exposed, but in the actual structure where the aggregate is not exposed, the paste portion or the mortar portion on the concrete surface is an overall plus (in the entire acquired image). (Expansion) strain is shown.
(C) As shown in FIG. 9 described later, when a speckled pattern appears, it is detected as freeze-thaw. In addition, the spotted part shows a large positive (expansion) strain. The parts other than the spots cannot be used to detect deterioration because the strain becomes positive (expansion) or negative (contraction) depending on the concrete compounding conditions and installation conditions.
(D) As shown in FIG. 10 below, when a linear pattern appears in a place where a reinforcing bar does not exist, or a linear pattern in an oblique direction appears with respect to a member such as a column or a beam of a structure. , Detected as dry shrinkage. In addition, the part where the reinforcing bar does not exist in the structure can be confirmed in advance by using an existing method such as a drawing or an electromagnetic wave radar. Further, the diagonal pattern with respect to a member such as a column or a beam does not limit the angle with respect to each member. In addition, the line portion of the pattern shows a positive (expansion) strain, and the substrate portion shows a negative (contraction) strain as a whole.
(E) When a linear pattern appears directly above the reinforcing bar (the covered portion of the reinforcing bar), it is detected as corrosion of the reinforcing bar. The position of the reinforcing bar can be confirmed in advance by using an existing method such as a drawing of a structure or an electromagnetic wave radar. In addition, the line part of the pattern shows positive (expansion) strain, and the other parts have positive (expansion) or negative (contraction) strain depending on the concrete compounding conditions and installation conditions, so deterioration can be detected. Not available.
In the black-and-white figure (photograph) shown in the [Drawing] column, the plus (expansion) and minus (contraction) of the strain cannot be distinguished. Therefore, a color photograph in which the plus and minus of the strain can be distinguished is separately submitted.
Further, in detecting various causes of deterioration, if the image acquisition location and specific examples shown in Table 1 are selected as the target for acquiring the digital image of concrete, the deterioration phenomenon can be detected more efficiently.

As described above, since the method for early detection of deterioration of concrete of the present invention can detect various factors of deterioration of concrete at an early stage, it is possible to take measures against deterioration at an early stage, which contributes to the maintenance and life extension of concrete. can do.

Hereinafter, the present invention will be described with reference to Examples, but the present invention is not limited to these Examples.
1. 1. Composition of materials used and concrete (test piece) Table 2 shows the materials used, and Table 3 shows the composition of concrete.
Formulation No. in Table 3 1 to 4 have the following purposes (intentions).
(1) No. 1 is a formulation of an alkali silica reaction test piece. In order to promote the generation of the alkali-silica reaction, NaOH was used by dissolving it in kneaded water so that the amount of R ₂ O including the amount of alkali derived from cement was 8.6 kg / m ³ . The cement used in No. 1 is ordinary Portland cement, and the target air volume is shown in Table 3.
(2) No. 2 is a formulation of a test piece for delayed generation of ettringite. To promote the occurrence of delayed production of ettringite, K ₂ SO ₄ was added so that the amount of SO ₃ was 2% by mass. The cement used in No. 2 is early-strength Portland cement, and the target air volume is shown in Table 3.
(3) No. 3 is a formulation of a test piece for freezing and thawing. The cement used in No. 3 is ordinary Portland cement, and the target air volume is shown in Table 3.
(4) No. 4 is a formulation of a test piece for drying shrinkage. In order to promote the progress of drying shrinkage, coarse aggregate was removed from the poorly mixed concrete with a unit water volume of 300 kg / m ³ and W / C of 60%. In addition, in order to suppress bleeding, limestone fine powder was used instead of the fine aggregate. The cement used in No. 4 is usually Portland cement. The limestone fine powder is also indicated as S in Table 3.

2. 2. Preparation of test piece
Using the formulations of No. 1 to No. 3, block-shaped concrete having a length of 400 mm, a width of 400 mm, and a thickness of 300 mm shown in FIG. 1 was prepared, and then the concrete was cured under the conditions shown in Table 4. After this curing, a plate having a thickness of 50 mm was cut out from the concrete, and the measurement surface was polished to prepare an alkali silica reaction test piece, an ettringite delayed generation test piece, and a freeze-thaw test piece, respectively. The test piece for freezing and thawing using the formulation of No. 3 was prepared by coating the five surfaces excluding the measurement surface with aluminum tape as shown in FIG. 4 after the polishing.
In addition, in order to simulate the state in which concrete is restrained in the actual structure, the test piece for drying shrinkage using the formulation of No. 4 is placed after placing mortar in the restraint form shown in FIG. The test piece was prepared by curing at 20 ° C. and 80% relative humidity for 3 days, removing only the bottom surface, and then curing at 20 ° C. and 80% relative humidity for 2 days.

3. 3. Acquisition of digital image of the test piece and distribution of maximum principal strain The digital image of the test piece is taken at the base length immediately after the end of the pre-curing (second time), at the base length, and in each period shown in FIGS. 7 to 10. , The range shown in FIGS. 3 and 4 was scanned and acquired by using the line sensor type digital image acquisition scanner (total field strain measuring device) shown in FIG. In order to avoid changes in strain due to temperature and changes in luminance value due to the influence of moisture, the test piece was stored for 20 hours or more in an environment of 20 ° C. and 60% relative humidity before acquiring an image.
Next, the calculation of the distribution of the maximum principal strain is analyzed by the digital image correlation method using the acquired digital image, and the maximum principal strain on the surface of the test piece is subjected to the following calculation processes (i) and (ii). Distribution was calculated. The distribution of the maximum principal strain calculated here was obtained (i) on the digital image of the test piece at the time of basal length and after accelerated curing, and the luminance value distribution within the subset centered on an arbitrary position was obtained.
(Ii) Search for a subset of the digital image of the test piece before drying, which has the highest correlation with the brightness value distribution of the digital image of the test piece after accelerated curing, and focus on the center point. The distance (displacement amount) moved from the point of interest to the center point was calculated by regarding the position after the movement (displacement), and the moved distance was further converted into the maximum principal strain. Figures 7 to 10 show the distribution of the maximum principal strain.
Further, as a reference example, in order to grasp the change in the overall length of the test piece, as shown in FIG. 3, contact gauge tips are attached to the remaining three sides of the test piece excluding the driving surface, and JIS A 1129 -2 In accordance with "Mortar and concrete length change measurement method-Part 2: Contact gauge method", the overall length change of the test piece was measured and averaged in each period shown in Table 5. The value was calculated. The results are shown in Table 5.

As shown in the rightmost figure of FIG. 7, in the alkali-silica reaction, a hexagonal pattern appeared in the image of the distribution of the maximum principal strain even before cracking occurred. Moreover, the positive (expansion) strain was shown as a whole, and the line part of the pattern showed a larger positive strain.
Further, as shown in FIG. 8, in the delayed generation of ettringite, a uniform pattern appeared as a whole except for the aggregate portion. In addition, the aggregate part showed no strain, but the other paste or mortar part showed positive (expansion) strain as a whole. It should be noted that the large positive strain with large local dots is noise.
Further, as shown in FIG. 9, in freeze-thaw, high spot-like strain appeared before deterioration phenomena such as scaling and pop-out occurred. The large positive strain that enters the horizontal is noise.
Further, as shown in FIG. 10, some diagonal linear patterns appeared on the concrete-constraining members such as columns or beams of the structure before cracks occurred in the drying shrinkage. In addition, the line part of the pattern showed a positive (expansion) strain, and the other substrate parts showed a negative (contraction) strain as a whole.

Claims (3)

It is an early detection method of concrete deterioration, which detects the deterioration of concrete by using the pattern appearing in the image of the distribution of the maximum principal strain obtained through the following steps (A) and (B).
An early detection method for concrete deterioration, which detects deterioration of concrete based on the following criteria (a) to (e) using the pattern.
(A) Image acquisition step of acquiring a digital image of the surface to be acquired of concrete over time (B) Strain is calculated using the digital image correlation method based on the digital image, and the distribution of the maximum principal strain is distributed based on the strain. Maximum principal strain distribution acquisition process
(A) The portion where the hexagonal pattern appears is detected as an alkali-silica reaction.
(B) When a uniform pattern appears as a whole except for the aggregate portion, it is detected as delayed generation of ettringite.
(C) When a speckled pattern appears, it is detected as freeze-thaw.
(D) When a linear pattern appears in a place where a reinforcing bar does not exist, or a linear pattern appears in an oblique direction with respect to a member such as a column or a beam of a structure, it is detected as a drying shrinkage.
(E) When a linear pattern appears directly above the reinforcing bar, it is detected as corrosion of the reinforcing bar. The method for early detection of concrete deterioration according to claim 1, wherein in addition to the above pattern, deterioration of concrete is detected based on positive and negative distribution of strain. The early detection method for concrete deterioration according to claim 2, wherein the deterioration of concrete is detected based on the following criteria (f) to (j) using the positive and negative distributions of the pattern and strain.
(F) In the case where a hexagonal pattern appears, the line portion of the hexagonal pattern shows a larger positive (expansion) strain, and the substrate portion (part other than the pattern) is also positive (expansion) as a whole. ) Is detected as an alkali-silica reaction.
(G) When a uniform pattern appears as a whole except for the aggregate part, the aggregate part does not show any strain, but the part made of other paste or mortar is totally positive (expansion). ) Is detected as delayed generation of ettringite.
(H) When a spot-like pattern appears and the spot portion shows a large positive (expansion) strain, it is detected as freeze-thaw.
(I) When a linear pattern appears in a place where the reinforcing bar does not exist, or a linear pattern appears in an oblique direction with respect to a member such as a column or a beam of a structure, and the linear portion of the pattern is a plus. When the strain of (expansion) is shown and the substrate portion shows negative (shrinkage) strain as a whole, it is detected as dry shrinkage.
(J) When a linear pattern appears directly above the reinforcing bar and the line portion of the pattern shows positive (expansion) strain, it is detected as corrosion of the reinforcing bar.

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