JP5631788B2 - Method for predicting diffusion state of chemical species in concrete, and method for predicting corrosion occurrence time of steel in concrete using the same - Google Patents

Description

  The present invention relates to a method for predicting the diffusion state of chemical species such as chloride ions penetrating into concrete from the outside, and a method for predicting the corrosion occurrence time of steel in concrete using the same.

Steel materials such as reinforcing steel in concrete are not easily corroded because they are covered with a passive film while in an alkaline atmosphere. However, when the concrete is neutralized by the penetration of carbon dioxide, or chloride ions or sulfate ions penetrate into the concrete and come into contact with the steel material, the passive film on the surface of the steel material is destroyed and the steel material is easily corroded. Among these chemical species, chloride ions are known to have the strongest corrosive action of reinforcing bars.
Therefore, when an attempt is made to design and construct a reinforced concrete structure that requires durability for a certain period of time in an environment where corrosion (salt damage) of the steel due to chloride ions is a concern, an appropriate cover depth of the steel is required. In order to determine the thickness, it is necessary to predict the corrosion occurrence time of the steel material in the structure.

As a method for predicting the corrosion occurrence time of the steel material, a method is known that uses a solution of a diffusion equation based on Fick's second law (a function including a period and a distance as variables and an apparent diffusion coefficient). That is, this method measures the distribution of chloride ion concentration in concrete immersed in an aqueous solution of sodium chloride for a short period of time, obtains an apparent diffusion coefficient from the measured value and the function, and the diffusion coefficient and the function. Is used to calculate the chloride ion concentration at the position where the steel material is buried, and the time when the calculated value reaches the lower limit concentration of chloride ion that corrodes the steel material (hereinafter referred to as the “corrosion lower limit concentration”). The process of predicting as follows.
As a method for obtaining the apparent diffusion coefficient, for example, Non-Patent Document 1 discloses that a cement hardened body is immersed in an aqueous sodium chloride solution for a predetermined period, and then the chloride ion concentration C at a distance x in the hardened body is set. After the measurement, C / (2C ₀ ) and x are plotted on the normal probability paper, the slope of the straight line of the following equation (1) is obtained, and the method of calculating the apparent diffusion coefficient D from the slope is described. Yes.

In Non-Patent Document 1, as a method for measuring the distribution of chloride ions, a method for measuring the concentration of chloride ions and a method for measuring the penetration distance of chloride ions are described. Among these methods, the method for measuring the concentration of chloride ions is to scrape a hardened cement body soaked in a sodium chloride aqueous solution for a predetermined period of time with a metal saw at intervals of about 0.05 cm from the surface of the hardened body. This is a method of pressing into a tablet and quantifying by fluorescent X-ray analysis (page 128).
Further, the method for measuring the penetration distance of chloride ions is to cut the side surface of the hardened cement body, and then apply a fluorescein sodium aqueous solution and a silver nitrate aqueous solution to the cut surface to develop chloride ions, This is a method of measuring the distance from the surface to the colored boundary (pages 128 and 129).

  In Non-Patent Document 2, the total chloride ion concentration per unit mass of concrete is measured at each depth position in a concrete specimen immersed in an aqueous sodium chloride solution for a predetermined period, and the following equation (2) is obtained. A method of calculating an apparent diffusion coefficient by performing a regression analysis based on this is described.

In Non-Patent Document 2, the method for measuring the distribution of chloride ion concentration is based on JIS A 1154 or JSCE G 574 (page 321).
Specifically, in the method according to JIS A 1154, test pieces cut out from five or more (principle) specimens are pulverized to 0.15 mm or less, and then chloride ions are extracted using nitric acid. It measures the amount of ions. Further, the method according to JSCE G 574 is to flatten the analysis surface of the analysis sample cut out from the specimen, and then polish the surface sequentially from coarse to fine, changing the abrasive, and then the surface. A conductive material is deposited on the surface, and the amount of chloride ions is measured by surface analysis using various apparatuses such as EPMA. Both methods require complicated and time-consuming work.

By the way, as shown in Table 1 (Table 3 published on page 131 of Non-Patent Document 1), the apparent diffusion coefficient is a variable that decreases with time (the age of the cured body), and is not a constant. Therefore, in order to accurately predict the corrosion occurrence time of the steel material by the method described in Non-Patent Document 1 or Non-Patent Document 2 (hereinafter referred to as “conventional method”), the apparent age of each hardened body is apparent. The diffusion coefficient must be determined. In particular, when it is intended to predict the corrosion occurrence time of a steel material over a long period of time, in order to obtain the distribution of chemical species, it is necessary to perform the above-described time-consuming measurement work over a long period of time.
Therefore, it has been extremely difficult to predict the distribution of chemical species over a long period of time with high accuracy using conventional methods.

Goto Seishi et al. “Diffusion of chlorine ions in hardened cement paste”, Journal of Ceramic Industry Association, Vol.87, No.3, pp.126-133, 1979 "Test method for apparent diffusion coefficient of chloride ions in concrete by immersion (draft) (JSCE-G 572-2010)", concrete standard specification [standards], Japan Society of Civil Engineers, 2010

  Therefore, the present invention has been made in view of these problems, and provides a method and the like that can predict the long-term distribution of chemical species in concrete with high accuracy without measuring the concentration of the chemical species. The purpose is to do.

  As a result of earnest research to solve the above problems, the present inventor has (1) the distance from the color boundary where the chemical species and the colorant have reacted to the surface of the concrete (hereinafter referred to as “penetration distance”). ), And then (2) find the coefficient of the regression equation using the distance, etc., and (3) express the diffusion coefficient related expression that includes the coefficient and uses time as a variable. By using the prediction formula, the long-term distribution of the chemical species in the concrete can be predicted with high accuracy without measuring the concentration of the chemical species in the concrete. Discovered and completed the present invention.

That is, the present invention provides the following [1] to [4].
[1] A method for predicting a long-term diffusion state of a chemical species in concrete based on a short-term diffusion state, including at least the following steps (A) and (B): Of predicting diffusion state
(A) After exposing concrete to chemical species for a short period of time and allowing chemical species to penetrate into concrete, the chemical species in the concrete are colored by a color reaction, and the penetration distance of the chemical species in the concrete is measured. Chemical species penetration distance measurement step (B) Chemical species concentration calculation step including at least the following steps ( B1), (B2) and (B3)
(B1) The infiltration period T of the chemical species is used as an explanatory variable, the infiltration distance X of the chemical species is used as an objective variable, and regression analysis is performed using the following equation (1) as a regression equation, and A, b, c, and n Coefficient A, b, c, n determining process for determining values
(B2) Coefficient E calculation step of calculating E value from the following equation (2) and A value
(B3) Substituting the values of E, b, c, and n into the following formula (3) to obtain a prediction formula for the concentration of chemical species, and based on the prediction formula, chemistry at an arbitrary distance x and time t A concentration C calculating step for calculating the concentration C (x, t) of the seed;

[2] A method for predicting the diffusion state of chemical species in concrete, wherein the chemical species according to [1] is chloride ions.
[3] the species in the concrete, on the basis of the diffusion condition in a short period of time, a method of predicting the corrosion occurrence time of the steel in the concrete, was predicted using a prediction method according to [1], steel The time at which the concentration of the chemical species at the burial site is the same as the lower limit concentration of the chemical species that corrodes the steel (hereinafter referred to as the “corrosion lower limit concentration”) is predicted as the corrosion occurrence time of the steel. How to predict when corrosion will occur.
[4] A method for predicting the corrosion occurrence time of a steel material in concrete, wherein the chemical species according to [ 3 ] is chloride ion.

  According to the present invention, without measuring the concentration of the chemical species in the concrete, by using a simpler method than the conventional method and a prediction formula that reflects the change over time of the diffusion coefficient, It is possible to predict the diffusion state over the time and the corrosion occurrence time of the steel material in the concrete with high accuracy.

It is a figure which shows the state of fitting of the plot of the measured value of the osmosis | permeation distance of a chloride ion, and an osmosis | permeation period, and the curve represented by a prediction formula ((3) Formula). It is the figure which predicted the density | concentration of the chloride ion in the predetermined position of a reinforcing bar for 100 years after chloride ion began to infiltrate into concrete, (a) is a prediction formula (Formula (5)) concerning the present invention. (B) is a diagram in the case of prediction using a prediction formula (Equation (15)) in which the diffusion coefficient is a constant.

As described above, the present invention includes at least (A) a chemical species penetration distance measuring step and (B) a chemical species concentration calculation step, and a method for predicting the diffusion state of chemical species in concrete, and the method This is a method for predicting the corrosion occurrence time of steel materials in concrete.
The present invention will be described below.

(A) Chemical species permeation distance measuring step As described above, this step involves exposing the concrete specimen to the chemical species for a short period of time, allowing the chemical species to permeate the concrete, and then coloring the chemical species in the concrete. This is a step of measuring the permeation distance of chemical species in the concrete.
(1) Concrete specimen The cement used for the specimen is not particularly limited, and examples thereof include Portland cement such as ordinary Portland cement, mixed cement such as blast furnace cement and fly ash cement, and ordinary ecocement.
Moreover, the aggregate used for the specimen is not particularly limited. For example, coarse aggregate such as gravel, crushed stone, slag coarse aggregate, lightweight coarse aggregate, river sand, mountain sand, land sand, sea sand, crushed sand, etc. , Fine aggregates such as cinnabar, slag fine aggregate, lightweight fine aggregate. Such coarse aggregates and fine aggregates can use natural aggregates as well as recycled aggregates.
Examples of the curing method for the specimen include standard curing, wet air curing, and sealed curing.
In addition, since a diffusion coefficient changes also with conditions, such as a kind of concrete material, a mixing | blending, and a curing method, it is preferable to produce a test piece on the same conditions as the concrete actually used.

(2) Penetration of chemical species Examples of the chemical species include chloride ions, sulfate ions, carbon dioxide, and the like.
Moreover, the method of making a chemical species penetrate | invade a specimen, for example, when chemical species is (i) chloride ion, the method of JSCE-G 572-2010 described in the nonpatent literature 2 is mentioned, ( ii) In the case of sulfate ions, the JIS draft “Testing method for chemical resistance by solution immersion of concrete (draft)” can be cited. (iii) In the case of carbon dioxide, JIS A 1152 “Concrete neutralization depth Measurement method ".

(3) Coloring of chemical species and measurement of penetration distance The chemical coloration method is, for example, by splitting the specimen and, on the splitting surface, when the chemical species is (i) chloride ion, an aqueous solution of sodium fluorescein. (Ii) In the case of sulfate ions, spray or apply a chlorophosphonazo III aqueous solution and then spray or apply an aqueous barium chloride solution. (Iii) In the case of carbon dioxide, there is a method in which a phenolphthalein solution is sprayed or applied to cause coloration.
Then, the length from the boundary of the colored portion to the surface of the specimen is measured to determine the penetration distance of the chemical species. The measurement of the penetration distance needs to be performed in a plurality of penetration periods in order to accurately determine the coefficient of the regression equation including the penetration distance and the penetration period as variables in the step (B1) described later. It is preferable to carry out in a period.
The density at which a chemical species is colored generally has a lower limit value specific to the chemical species (hereinafter referred to as “coloring lower limit density”), and the concentration of the chemical species at the boundary of the colored portion is considered to be the lower limit value. . Then, measuring the penetration distance also means measuring the concentration of the chemical species at that point. Therefore, there is an advantage that the conventional laborious measurement of the concentration of chemical species can be omitted by measuring the penetration distance. Incidentally, the minimum coloration concentration of chloride ion by silver nitrate is 0.15% by mass per unit cement mass.

Considering the fact that the apparent diffusion coefficient in the above equations (3) to (5) changes with time, the diffusion coefficient is expressed by a function D (t) whose variable is only the infiltration period t of chemical species. The diffusion equation is expressed by the following equation (6).

次に、変数ｔを下記（７）式により変数変換する。
dT = D(t)dt ……（７）
該変数変換により、上記（６）式は下記（８）式に変形される。

Next, the variable t is converted into a variable by the following equation (7).
dT = D (t) dt (7)
By the variable conversion, the above equation (6) is transformed into the following equation (8).

ここで、境界条件として、コンクリート表面における化学種の濃度をC_s（一定）とし、初期条件として、コンクリートに初めから含まれている化学種の濃度をC_i（一定）とすると、上記（８）式の解析解は下記（９）式になる。

Here, if the concentration of chemical species on the concrete surface is C _s (constant) as the boundary condition, and the concentration of chemical species contained in the concrete from the beginning is C _i (constant), the above (8 The analytical solution of equation (9) is the following equation (9).

上記（９）式において、ｘ＝X（化学種の呈色境界とコンクリート表面との距離）、Ｃ＝C_NS（呈色部分の境界における化学種の濃度）を代入した後、Xを表す式に変形すると、下記（１０）式になる。

In the above equation (9), after substituting x = X (distance between the color boundary of the chemical species and the concrete surface) and C = C _NS (the concentration of the chemical species at the boundary of the colored portion), an equation representing X Is transformed into the following equation (10).

ここで、浸透期間ｔを変数とする前記の関数Ｄ（ｔ）を、見掛けの拡散係数と浸透期間との関係から経験的に新規に得られた下記（１１）式を用いて表す。

Here, the function D (t) with the permeation period t as a variable is expressed using the following equation (11) newly obtained empirically from the relationship between the apparent diffusion coefficient and the permeation period.

次に、上記（１１）式を前記（７）式に代入して積分すると、下記（１２）式が得られる。

Next, when the above equation (11) is substituted into the equation (7) and integrated, the following equation (12) is obtained.

さらに、上記（１２）式を上記（１０）式に代入すると、下記（３）式が得られる。

Further, when the formula (12) is substituted into the formula (10), the following formula (3) is obtained.

ここで、上記（３）式中のＡは下記（４）式で表される定数である。

Here, A in the above formula (3) is a constant represented by the following formula (4).

また、上記（１２）式を上記（９）式に代入すると、下記（５）式が得られる。

Further, when the formula (12) is substituted into the formula (9), the following formula (5) is obtained.

(B) Chemical Species Concentration Calculation Step As described above, after calculating the regression equation coefficient using the infiltration distance and infiltration period, the step uses the solution of the diffusion equation based on Fick's second law. A formula for predicting the concentration of a species, which includes a coefficient related to a diffusion coefficient including the coefficient and having time as a variable, is obtained as one of the terms, and further using the prediction formula, an arbitrary distance and This is a step of calculating the concentration of chemical species over time.
The step is preferably
(B1) Regression analysis (fitting) is performed using the chemical species permeation period T as an explanatory variable, the chemical species permeation distance X as a target variable, and the above equation (3) as a regression equation, and A, b, c And a coefficient A, b, c, n determining step for determining the value of n.
(B2) a coefficient E calculation step of calculating an E value from the above equation (4) and the A value;
(B3) Substituting the values of E, b, c and n into the above equation (5) to obtain a prediction formula for the concentration of chemical species, and based on the prediction formula, the chemistry at an arbitrary distance x and time t And a concentration C calculating step for calculating the concentration C (x, t) of the seed.
Below, each process is demonstrated.

(B1) Coefficient A, b, c, n determining step This step takes the penetration period T (explanatory variable) on the horizontal axis (or vertical axis) and the penetration distance X (target variable) on the vertical axis (or horizontal axis). After plotting (T, X) obtained by actual measurement on the coordinates, fitting the curve represented by the above equation (3) to this, the coefficients A, b, c of the equation (3) And determining the value of n. Here, the fitting is an operation for determining the values of A, b, c, and n so that the objective function F (A, b, c, n) represented by the following equation (13) is minimized. .

(B2) Coefficient E calculation step In this step, the chemical species concentration C _{s on the} surface of the specimen, the coloration lower limit concentration C _{NS of} the chemical species, and the chemical contained in the specimen from the beginning are expressed in the above equation (4). This is a step of calculating the coefficient E by substituting the seed concentration C _i and the coefficient A obtained in the step (B1).

(B3) Concentration C Calculation Step In this step, the concentration C _s of the chemical species on the surface of the specimen, the concentration C _{i of the} chemical species contained in the specimen from the beginning, (B2) Substituting the coefficient E calculated in the process and the coefficients b, c, and n, a prediction formula for the concentration C (x, t) of the chemical species with the distance x and the time t as variables is obtained, and an arbitrary penetration distance x And calculating the concentration C (x, t) of the chemical species in the infiltration period t.
Through the above steps, it is possible to easily and accurately predict the diffusion state of chemical species in the concrete at any penetration distance and penetration period.

Method for predicting corrosion occurrence time of steel in concrete The method is based on the prediction method described in the above [1] or [2], and the concentration of chemical species at the place where steel is buried is the lower limit corrosion concentration of steel. Is a method for predicting the time when the corrosion of the steel material occurs. Incidentally, when the chemical species is chloride ion and the cement is ordinary Portland cement, the lower limit corrosion concentration is 0.4% by mass per unit cement mass.
Further, the corrosion occurrence time of the steel material may be obtained as an approximate solution by using a numerical calculation method such as Newton-Raphson method for the following equation (14) in which the above equation (5) is rewritten into an equation form.

EXAMPLES The present invention will be specifically described below with reference to examples, but the present invention is not limited to these examples.
1. Preparation of specimen for immersion test Using blast furnace cement type B, concrete with water-cement ratio (W / C) of 74%, 63% and 53%, respectively, is mixed and poured into a mold, and the material age is 1 day. After demolding, a prismatic specimen having a length of 10 cm, a width of 10 cm, and a length of 40 cm was obtained. Subsequently, the specimens were subjected to standard curing until the material age of 7 days, and then subjected to air curing at 20 ° C. and 60% RH until the material age of 28 days.
Next, the specimen after curing was coated with an epoxy resin except for one side that was in contact with the side of the mold, to prepare a specimen for immersion test.

2. Immersion test The specimen for immersion test was immersed in an aqueous sodium chloride solution having a liquid temperature of 20 ° C. and a concentration of 3% by mass for 3 days, and then placed in a constant temperature and humidity chamber at 20 ° C. and 60% RH for 4 days. Dried.
This immersion and drying operation for a total of 7 days (1 week) is taken as 1 cycle, and immersion, drying operations are carried out for 1, 4, 8, 13 and 26 cycles (weeks) to obtain a specimen infiltrated with chloride ions. It was.
Next, a 0.1 mass% sodium fluorescein aqueous solution is sprayed on the splitting surface obtained by splitting the specimen, and then a 0.01 M silver nitrate aqueous solution is sprayed to present a region in which chloride ions have permeated. Colored (light emission).
The distance from the surface of the specimen to the boundary through which chloride ions permeated was measured with five calipers, and the measured values were averaged to obtain the chloride ion penetration distance. The results are shown in Table 2.

3. Regression analysis (fitting)
The penetration period and penetration distance in Table 2 were plotted on the coordinates, with the penetration period of chloride ions on the horizontal axis and the penetration distance of chloride ions on the vertical axis. A curve represented by the equation (3) is fitted to the plot so that the objective function F (A, b, c, n) of the equation (13) is minimized, and A, b, c and n The value of was determined. The results are shown in Table 3.
Moreover, the plot of the penetration distance and penetration period in the specimens of all water cement ratios, and the curve represented by the equation (3) substituted with the values of A, b, c and n shown in Table 3 The state is shown in FIG. As shown in FIG. 1, the fitting state was very good in the specimens of all water cement ratios.

4). A method for predicting the corrosion occurrence time of reinforcing bars Substituting the A value into the equation (4) to obtain the E value, and further substituting the E value and the values of b, c and n into the equation (5), The prediction formula C (x, t) of the diffusion state of the chemical species in the inside was obtained. By using this prediction formula C (x, t), the distribution of chloride ions at an arbitrary distance x and time t can be predicted.
Accordingly, as an example, substituting depth of the reinforcing bar x = 4 cm, C _s = 3 mass%, and C _i = 0 mass% is substituted into the prediction formula C (x, t), and chloride ions are added to the concrete. The concentration of chloride ions was calculated for 100 years from the beginning of the penetration of steel, and the corrosion occurrence time of the reinforcing bars was predicted. The result is shown in FIG.

Here, assuming that the lower corrosion limit concentration of reinforcing steel bars is 0.4% by mass per unit cement mass, from the curve of FIG. 2 (a), the corrosion occurrence time of reinforcing steel bars is in the case of a specimen with a water-cement ratio of 74%. In 7.5 years, if the ratio is 63%, it will be 54.6 years, if the ratio is 53%, it will be 88.9 years. The result was.
In general, it is known that concrete made of the same concrete material has a higher resistance to chloride ion penetration and higher durability as the water-cement ratio is lower. Is consistent with

Here, as a comparative example, an example in which a change with time of the diffusion coefficient is not considered is shown. That is, the apparent diffusion coefficient D _a obtained by fitting a curve represented by the following formula ( 15 ) where the diffusion coefficient is a constant against the plot of the chloride ion penetration distance and the penetration period in Table 2 Using the values of C _s and C _i and the following equation ( 16 ), chloride ions at a position where the depth of the reinforcing steel cover is d = 4 cm for 100 years from the start of penetration of chloride ions into the concrete The concentration of steel was calculated to predict the corrosion occurrence time of the reinforcing bars. The result is shown in FIG.
The following equation (15) corresponds to the inverse function of the above equation (1) and the above equation (2), and the following equation (16) is substantially the same as the above equation (1) and the above equation (2). It is.

When the corrosion lower limit concentration of the reinforcing bar is the same as that of the example, from the curve of FIG. 2 (b), the corrosion occurrence time of the reinforcing bar is 7.1 years in the case of a specimen having a water-cement ratio of 74%. When the ratio was 63%, it was 20.7 years, and when the ratio was 53%, it was 14.2 years.
Originally, the apparent diffusion coefficient, which decreases with the passage of time, was set to a constant value in the comparative example, so the following contradiction occurred in the comparative example. That is,
(1) Despite the fact that the water-cement ratio of the specimen decreased from 63% to 53%, the result that the corrosion occurs earlier from 20.7 to 14.2 Clearly contradicts.
(2) As a result of the chloride ion permeation rate in the comparative example becoming faster than that in the example, the corrosion occurrence time was earlier than in the example.
As described above, the method of the comparative example does not take into account the temporal change of the apparent diffusion coefficient, and thus the result is different from the actual result as described above. Therefore, the reliability of the prediction is significantly lower than the method of the present invention. I can say that.

  The method of the present invention can be applied to the prediction of the long-term diffusion state of chemical species in concrete and the prediction of the corrosion occurrence time of steel in concrete.

Claims (4)

A method for predicting a long-term diffusion state of a chemical species in concrete based on a short-term diffusion state, comprising at least the following steps (A) and (B): Prediction method.
(A) After exposing concrete to chemical species for a short period of time and allowing chemical species to penetrate into concrete, the chemical species in the concrete are colored by a color reaction, and the penetration distance of the chemical species in the concrete is measured. Chemical species penetration distance measurement step (B) Chemical species concentration calculation step including at least the following steps ( B1), (B2) and (B3)
(B1) The infiltration period T of the chemical species is used as an explanatory variable, the infiltration distance X of the chemical species is used as an objective variable, and regression analysis is performed using the following equation (1) as a regression equation, and A, b, c, and n Coefficient A, b, c, n determining process for determining values
(B2) Coefficient E calculation step of calculating E value from the following equation (2) and A value
(B3) Substituting the values of E, b, c, and n into the following formula (3) to obtain a prediction formula for the concentration of chemical species, and based on the prediction formula, chemistry at an arbitrary distance x and time t A concentration C calculating step for calculating the concentration C (x, t) of the seed;

A method for predicting a diffusion state of chemical species in concrete, wherein the chemical species according to claim 1 is chloride ion. A method for predicting the corrosion occurrence time of steel in concrete based on the diffusion state of chemical species in concrete in a short period, which is predicted using the prediction method according to claim 1 , A method for predicting the corrosion occurrence time of steel in concrete, in which the time when the concentration of the chemical species is the same as the lower limit concentration of the chemical species that corrodes the steel is predicted as the corrosion occurrence time of the steel. A method for predicting the occurrence of corrosion of steel in concrete, wherein the chemical species according to claim 3 is chloride ion.

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