KR100884578B1 - Rebar Corrosion Critical Chloride Measurement Method - Google Patents

Abstract

The present invention is an acid solution input step of adding an acid solution to the precipitate consisting of concrete specimens and distilled water step by step; Converting the concentration of the administered acid when the pH of the precipitate decreases to 10.0 by the concentration of hydrogen ions; Calculating a ratio of [Cl ⁻ ] of the precipitate to [H ⁺ ] of the administered acid; [Cl ^-]: by a ratio of [H ^+] and determining the corrosion critical chloride content; and by presenting the corrosion critical chloride content measurement method that includes, considering each chemical properties of different concrete corrosion threshold chloride The quantity can be determined so that the value can be quantified according to the characteristics of the structure and the environment in which it is located.

Description

Rebar Corrosion Critical Chloride Measurement Method {CHLORIDE THRESHOLD VALUE METHOD FOR STEEL CORROSION IN CONCRETE}

TECHNICAL FIELD The present invention relates to the field of construction, and in particular, to a method for measuring the amount of chloride that causes corrosion of concrete internal reinforcing bars.

Concrete is used as a material in civil engineering and structures, so it is widely used. For this reason, research and development for securing the performance and stability is steadily in progress. In particular, with the development of high-strength concrete in recent years, a lot of research has been conducted to extend the life and predict the life.

In particular, when concrete structures are exposed to marine environments such as bridges, harbors, and coastal roads, the problem of reinforcing corrosion caused by the penetration of chloride concrete has been raised. Reinforcing corrosion not only causes stress in the concrete, causing cracking and dropping of concrete, but also causes structural defects due to the reduction of the cross section of the steel, which is directly related to safety problems.

On the contrary, studies on the critical chloride content of reinforcing bars, which are the main factors of reinforcing bars in concrete, are being actively conducted. In our country, regardless of the formulation or the performance of concrete, but always limited and concrete per batch 1.2kg / m ^3, it is true does not make sense.

In the United States and Europe, the amount of corrosion critical chloride is limited to 0.4% compared to the binder in consideration of the anti-rust performance of the binder. However, it is different depending on the site conditions. It is true.

In other words, it is difficult to express the corrosion critical chloride amount in one unified value because it is difficult to evaluate the anti-rust performance of cement paste and hydrate in concrete, and the pH of pore water generated at the surface of reinforcing bar is usually 10.0. This is because it does not take into account the falling chemical properties.

The present invention has been made to solve the above problems, by expressing the amount of critical corrosion of the reinforcing corrosion corrosion beforehand by the chemical method, according to the mixing and performance of the concrete, by expressing the amount of critical corrosion of the reinforcing steel corrosion through a new method, Its purpose is to provide key factors for predicting remaining and critical lifetimes that can be free from corrosion.

In order to achieve the object as described above, the present invention, an acid solution input step of adding an acid solution to the precipitate consisting of concrete specimens and distilled water step by step; Converting the concentration of the administered acid when the pH of the precipitate decreases to 10.0 by the concentration of hydrogen ions; Calculating a ratio of [Cl ⁻ ] of the precipitate to [H ⁺ ] of the administered acid; The [Cl ^-]: determining a corrosion critical chloride content by the ratio of [H ^+]; proposes a corrosion critical chloride content measuring method comprising a.

The [Cl ^-]: when the ratio of [H ^+] in the range of 0.008 ~ 0.010, the [Cl ^-]: to determine the corrosion critical chloride content by the ratio of [H ^+] is preferred.

The [Cl ^-]: when the ratio of [H ^+] in the range of 0.0087 ~ 0.0092, the [Cl ^-]: to determine the corrosion critical chloride content by the ratio of [H ^+] is more preferred.

It is preferable to further include the step of converting the critical corrosion chloride amount calculated by the molarity as a percentage of the binder.

In order to increase the accuracy of the measurement, the concrete specimen is preferably in the form of a powder.

In addition, it is more preferable that the step of eluting the compound in the cement hydrate by precipitating the concrete powder in distilled water before the acidic solution injection step.

After the acidic solution input step, it is more preferable to further include the step of stabilizing the precipitate for a certain time to ensure a stable pH measurement.

In the present invention, since the aggregate is not assumed to be chemically reactive, the concrete specimen is more preferably used mortar or cement paste.

Since concrete exposed to the air usually has a change in chemical properties, it is preferable to collect and use an inner one at least 10 mm away from the structure surface when collecting concrete specimens.

According to the present invention, as shown in FIG. 10, since the critical corrosion amount of reinforcing corrosion can be determined in consideration of the chemical properties of concrete, the value can be quantified according to the characteristics of the structure and the environment in which it is disposed.

Therefore, it is possible to predict more accurate corrosion start time and repair time.

Hereinafter, the principles and specific examples of the present invention will be described in detail.

In the present invention, the effects of the chloride immobilization properties and corrosion inhibiting ability of cement on the critical chloride content of the corrosion of reinforcing steel in mixed cement were evaluated.

General Portland cement, 30% fly ash, 60% blast furnace slag powder, and 10% silica fume were used as a binder of the mixed cement.

Critical chloride amounts were evaluated by corrosion rate measurements using mortar specimens containing chloride in the range of 0.0 to 3.0%.

Chloride immobilization characteristics were measured by water extraction method after 200 days of curing, and the corrosion inhibition ability of cement against pH decrease was measured by acid neutralization test.

As a result, the corrosion rate increased with the increase of chlorine concentration of mortar, and the critical chloride content was 1.03 for portland cement, 0.65 for 30% fly ash, 0.45 for 60% blast furnace slag, and 0.98% for 10% silica fume. .

In addition, while the chlorine ion immobilization ability with respect to the critical chloride amount did not show a clear relationship, it was found that the value of the critical chloride amount increased as the corrosion inhibiting ability increased.

In the present invention, as a new method for indicating the critical chloride amount, a molar ratio of hydrogen ions to the molar concentration of the total chloride amount at a pH value of 10, [Cl ⁻ ]: [H ⁺ ], is proposed. The ratio calculated a fixed value of 0.009 regardless of the type of binder through the experiment.

Chloride, which causes corrosion of reinforcing bars, greatly affects the durability life of coastal concrete structures because corrosion products such as rust cause the reinforcement to greatly expand the reinforcement volume, causing stress in the concrete and causing structural failure.

Thus, studies on the corrosion of reinforcing steel in concrete have been conducted in various fields.

However, the corrosion mechanism of concrete structures is not clearly explained due to the complex chemical action and corrosion resistance behavior around rebar.

It is only hypothesized that the alkaline environment of pore water in concrete exists in the form of a thin protective film on the surface of the rebar and will be destroyed by chlorine ions.

Thus, the ambiguity of the reinforcing steel corrosion mechanism in concrete, Along with the rate of chlorine ion migration, this contributes to a wide range of critical chloride amounts for rebar corrosion in concrete, which is an important factor in assessing when to start corrosion.

The possibility of local pH drop around rebars has been neglected, so the role of chlorine ions as a catalyst in the early stages of corrosion has received great attention.

Chlorine ions causing pitting corrosion of the reinforcing bar surface include two phases: pit formation and growth.

The cause of the pit generation is still in dispute, but when the pit starts to be formed, the passivation film damaged by the cement component is reformed.

Pits are caused by a drop in pH, and the constant supply of chlorine ions prevents the regeneration of the passivation film.

At the onset of corrosion, the hydrogen ions generated by the electrochemical reaction around the reinforcing bar lowers the pH of the pore water as shown in Formula 1, causing acidification.

_{FeCl 2 + 2H 2 O → Fe} (OH) 2 + 2H + + 2Cl -

The decrease in pH makes the passivation film unstable and chlorine ions promote iron dissolution. As the pH of the pore water decreases, most of the fixed chlorine ions are liberated with free chlorine ions to consume more hydroxide ions.

In the present invention, considering the chemical action of chlorine ions and hydrates, the rebar corrosion critical chloride amount for the mixed cement was derived.

Four kinds of binders, general Portland cement, fly ash, blast furnace slag powder and silica fume were selected to evaluate the effect of mixed concrete binders on corrosion resistance.

The chlorine ion immobilization characteristics and the corrosion inhibition ability of cement for the liberalization of fixed chlorine ions due to pH drop were compared by measuring acid neutralization capacity against acidification.

The critical chloride content was determined by measuring the corrosion rate of the rebar in mortar specimens containing chloride in the cement weight ratio of 0.0 to 3.0%.

As a binder, mixed cement was substituted with ordinary Portland cement (OPC), 30% fly ash (PFA), 60% blast furnace slag fine powder (GGBS) and 10% silica fume (SF).

The composition of the mixed cement is shown in Table 1.

CaO SiO ₂ Al ₂ O ₃ Fe ₂ O ₃ MgO Na ₂ O K ₂ O Mn ₂ O ₃ TiO ₃ SO ₃ OPC 64.7 20.7 4.6 3.0 1.0 0.13 0.65 - - 3.0 PFA 1.7 48.7 18.8 7.7 1.0 0.4 1.9 - 0.9 0.64 GGBS 41.2 34.2 11.7 1.43 8.81 0.29 0.31 0.3 0.58 - SF 0.31 94.9 0.23 0.07 0.04 0.15 0.56 - - 0.17

Cement paste was used on the assumption that the aggregate did not affect the chemical reaction of pore water, and mortar specimens were prepared for the corrosion test. The water-binder ratio was 0.4.

Using a NaCl standard reagent, a square cement paste (100 × 100 × 200 mm) was poured by mixing 10 steps of chlorine ions in a binder water ratio of 0.0 to 3.0% in the blended water.

Paste specimens were cured by spinning at 6 rpm for 24 hours after placing to block minimal separation of binder and chlorine ions.

The specimens were then demolded and blocked with evaporation of water using polyethylene films to prevent leaching of chlorine and hydroxide ions, followed by curing at 20 ± 1 ° C. for 6 days in a curing room. Curing period was 7, 28, 60, 200 days.

The specimens were dried in a 104 ° C. drying furnace for 24 hours prior to the experiment to prevent drying of residual amounts of alkali and chlorine ions during powdering. Thereafter, the specimen was made into a powder using a grinder and a 300 μm sieve, and 1.0 g to 3.0 g of the powder specimen were mixed with 50 ml of distilled water at 50 ° C.

The mixed solution was stirred for 4 minutes and then stabilized for 30 minutes to extract dissolved chlorine ions equivalent to free chlorine ions. The solution passed through the filter paper was measured for the concentration of chlorine ions using a potentiometric titrator.

In order to measure the inhibition ability of the cement against pH drop, a cement paste containing 1.5% chlorine ions was poured as in the chlorine ion immobilization experiment.

After 200 days of curing, the paste was powdered and 3 g of powder specimens were mixed with a solution consisting of 28 moles of 2 mole nitric acid and distilled water (40 ml) to produce a solution of different pH levels.

The mixed solution is to be measured for 10 minutes after stirring until there is no change in pH. In the present invention, the chlorine ion concentration of the mixed solution according to the concentration of nitric acid was also measured at the same time. The method for determining the chloride ion concentration has already been described above.

Mortar specimens were placed using a rectangular mold (50.0 × 50.0 × 90.0 mm) with a steel rod of 10.0 mm in diameter in the center. Mortar was poured with weight ratio cement: water: fine aggregate = 1.00: 0.40: 2.45, and 10-step chloride was added at a concentration of 0.0 to 3.0%.

Both ends of the rebar were covered with cement paste and then covered with rubber once more. One end of the rebar was laid out and poured to connect the wires.

Specimens were cured for 28 days at 20 ± 1 ℃ using a polyethylene film.

In the present invention, the corrosion rate of the rebar was measured by the anodic polarization technique.

Mortar specimens were immersed in 0.5M NaCl solution for 24h prior to measurement to stabilize the mortar's electrical resistance, and the upper part of the specimen was immersed in the air for oxygen supply.

Experimental apparatus for measuring the corrosion rate is shown in FIG.

The corrosion potential was fixed between +25 mV and -25 mV and the scanning rate was 0.1 mV / sec. The mortar's IR drop was lost due to the use of a resistor. First, the polarization resistance was measured, and the corrosion rate was calculated by the following equation.

here,

I: Corrosion Rate (mA / m ² )

R _{P :} polarization resistance (Ωm ² )

B: Corrosion potential (mV) 26mV when corrosion

Fixed chloride amount is defined as the total chloride amount minus the dissolved chloride amount.

Figure 2 shows the chlorine ion immobilization capacity of 7,28,60,200 days of curing of OPC, 30% PFA, 60% GGBS, 10% SF.

The chlorine ion immobilization characteristics of all pastes increased with a long curing period from 7 to 200 days, and the immobilization capacity of 7 days was the value regardless of the binder when comparing the immobilization ability of total chloride 0.5-0.6% and 3.0%. There was little difference.

The immobilization properties at 28 days were also unaffected by the binder. The immobilization characteristics of 60 days of age were greater in 30% PFA and 60% GGBS than in OPC and 10% SF. On the last measurement date, 200 days of age, the immobilization properties were influenced by the binder type.

The order of chlorine ion immobilization properties is 60% GGBS> 30% PFA> OPC> 10% SF. Chlorine ion immobilization characteristics can be expressed by Langmuir or Freundlich isotherms which express the relationship between free and fixed chlorine ions using equations (3) and (4).

Langmuir isotherm:

Langmuir isotherm:

Freundlich isotherms:

Freundlich isotherms:

Where C _b is the amount of fixed chlorine ions and C _f is the amount of free chlorine ions. The constants k, n, α and β are shown in Table 2.

Binder Curing (days) Freundlich isotherm Langmuir isotherm k n R ² α β R ² OPC 7 0.4131 3.3557 0.97 3.7728 7.3416 0.98 28 0.4759 2.5383 0.97 1.8639 2.6011 0.93 60 0.5718 2.5310 0.98 2.8643 3.5751 0.98 200 1.7176 2.1219 0.90 8.5575 4.0878 0.94 30% PFA 7 0.4038 2.9355 0.97 2.8804 5.5241 0.97 28 0.5802 2.7711 0.97 3.6441 4.8501 0.96 60 0.7860 1.9281 0.97 2.0970 1.5078 0.99 200 1.8677 1.8728 0.96 6.8786 2.8681 0.99 60% GGBS 7 0.4392 2.0936 0.97 1.0578 1.2922 0.98 28 0.5257 1.8599 0.95 0.9042 0.6755 0.93 60 0.8336 2.5578 0.96 3.1151 2.5366 0.96 200 2.3043 1.5782 0.94 5.6029 1.5633 0.96 10% SF 7 0.3735 2.5733 0.96 1.6639 2.9990 0.96 28 0.4703 3.1853 0.94 3.0892 5.0384 0.87 60 0.5159 1.9835 0.98 1.4732 1.5909 0.99 200 0.9710 1.8360 0.96 2.3010 1.2566 0.96

These equations were used to explain the chlorine ion immobilization characteristics in advance. The experimental results of the present invention were consistent with Langmuir and Freundlich isotherms and the accuracy of the results was confirmed.

Figure 3 representatively shows the pH change measured at 60 days GGBS suspension of the acid concentration 0, 10, 20 mol / kg at the age of 1, 6, 12, 16, 21 to confirm the stability of the pH measurement.

The pH gradually increased over time, and little changed after 12 days.

The initial increase in pH can be explained by the leaching of alkali metals such as K ⁺ , Na ⁺ and other alkali components.

In the present invention, a pH value stable for 21 days was used to evaluate the corrosion inhibition ability of the cement and the chemical action associated with cement paste suspension.

4 shows the relationship between the concentration of nitric acid and the pH of the suspension for each binder.

Increasing the nitric acid concentration of the suspension reduced the pH regardless of the binder type.

For the suspension of OPC, 10% SF, the initial pH value without addition of acid was about 12.65, slightly higher than the 12.50 pH value of 30% PFA and 60% GGBS.

This is presumably because the chemical action of cement, which is a pozzolanic material, is related to lowering the generation of alkali metal or calcium hydroxide.

At the same nitric acid concentrations, such as 12.50 and 11.50 of OPC, the pH drop was very small.

This means that cement or pore water has high resistance to pH reduction.

The final pH of the suspension with 20 mol / kg nitric acid was varied depending on the type of binder.

OPC, 10% SF, decreased to approximately 10.00 and 30% PFA, 60% GGBS decreased below pH 4.00.

The resistance of cement to pH reduction is expressed as the ratio of the concentration change of nitric acid added to the suspension to the change in pH, defined as acid neutralization capacity (ANC).

The acid neutralization capacity of OPC, 30% PFA, 60% GGBS, and 10% SF is shown in FIG. 5.

At certain pH values, peaks indicative of the resistance of cement hydrate to pH drop were strongly generated.

The location and size of the peaks depend on the binder, but at pH 12.5, peaks generally occur regardless of the binder.

It is caused by calcium hydroxide, which is one of the hydrates that dissolves in the pore water and exhibits alkalinity of about pH 12.5-12.6, thereby suppressing the pH drop at about pH 12.5, and occurs around the reinforcing bar at the onset of corrosion.

These results indicate that the stronger the resistance of the cement to pH reduction, the lower the risk of corrosion of the steel in concrete.

However, cement hydrates corresponding to other peaks have not yet been clearly identified.

It can be seen that 30% PFA, 60% GGBS showed a prominent peak at pH 11.50 or lower, while OPC had a peak (pH drop resistance) at pH 11.00.

This clearly demonstrates that the hydrates that inhibit pH drop depend on the binder.

6 shows the liberalization of fixed chlorine ions with increasing acid through the relationship between the ratio of free chlorine ions to total chlorine ions and nitric acid concentration.

Increasing the concentration of nitric acid liberates fixed chlorine ions, it can be seen that the amount of free chlorine gradually increases.

However, when the acid concentration was increased to more than 15 mol / kg, 85-95% of free chlorine ions were generated, after which the increase of free chloride was no longer observed.

Until now, the amount of chloride measured by titration has been underestimated, which is important for the evaluation of the total amount of chloride in concrete.

The chloride dissolved in the acid determined by the titration method will be equal to 85-95% of the total chloride.

Corrosion rate of the reinforcing steel in the mortar measured by the anode polarization technique is shown as a value of the amount of chlorine ions as shown in FIG.

The corrosion rate before the onset of corrosion increased with increasing chloride, and the increase in corrosion rate was also affected by the binder.

The corrosion rate of OPC and 10% SF was lower than that of 30% PFA and 60% GGBS at the passivation layer around the rebar, but the corrosion rate of OPC and 10% SF was greater after the onset of corrosion.

This result suggests that pozzolanic materials do not affect corrosion with increasing critical chloride content but slow down the corrosion propagation rate.

It is well known that the corrosion rate exceeds 1-2 mA / m ² at the onset of steel corrosion in concrete. This hypothesis provides guidelines for calculating the critical chloride content for corrosion. Fixed linear equations were used to determine the range of chlorine ions before and after the onset of corrosion.

Critical chlorine ion values, chlorine ion ranges, and fixed equations are shown in Table 3. The order of the critical chloride amounts is equal to OPC> 10% SF> 30% PFA> 60% GGBS.

Binder Threshold value (%, binder) Range of chlorides (%, binder) Fit equation R ² OPC 0.81-1.24 0.8-1.5 Y = 2.46 x -1.06 0.98 30% PFA 0.48-0.81 0.4-0.8 Y = 3.32 x -0.66 0.93 60% GGBS 0.35-0.55 0.2-0.6 Y = 4.84 x -0.69 0.94 10% SF 0.77-1.18 0.6-1.5 Y = 2.41 x -0.85 0.99

The chlorine ion immobilization properties have been considered important in assessing the risk of corrosion of concrete structures due to the assumption that fixed chlorine ions are removed from the pore water and do not contribute to corrosion progress.

As a result, it is thought that the higher the chlorine ion immobilization characteristic, the lower the risk of corrosion. However, studies of corrosion risks reported in the past show no correlation between critical chloride values and chlorine ion immobilization properties.

For example, Hansson and Sorenson announced that concrete specimens made of low C ₃ A-based cements, such as sulfate-resistant cement (SRPC), did not always have low critical chloride values measured by macrocells.

Breit and Schiessl have reported similar critical chloride amounts in the range of 0.2 to 0.4% cement weight ratio for OPC and SRPC.

These results suggest that the chlorine ion immobilization properties have a small effect on the amount of critical chlorides in the continuous corrosion process, which is due to the involvement of fixed chlorine ions in corrosion with the decrease of the pH around the rebar surface due to the cement properties. Can be.

In the present invention, the corrosion inhibition ability of the cement against the pH reduction by nitric acid was measured.

FIG. 8 shows the effect of corrosion inhibition capacity on the critical chloride amount compared to the effect of chlorine ion immobilization.

This result does not imply that chlorine ion immobilization implies a low risk of corrosion; in fact, the pH-lowering resistance of the binder (the ability of the cement hydrate to inhibit corrosion) increases the critical chloride content, while there is no specific effect on corrosion. do.

In conclusion, while the hydrate of cement restrains the pH of the pore water from dropping to pH 10 due to the liberalization of fixed chlorine ions, it can be said that chlorine ion immobilization has no direct effect on the inhibition or mitigation of the reinforcing steel in concrete. .

The results of this experiment are important findings that if the resistance to pH drop, which is related to the critical chloride value, is large, the corrosion resistance of the reinforcing steel in the concrete structure due to the action of chlorine ions is high.

The critical chloride content for corrosion derived from research over the last 20-30 years is of great significance for assessing the risk of chlorine ions causing corrosion of concrete structures.

Gouda, through experiments made by the concrete pore water contained in the first chlorine ions artificially [Cl ^{-]: -} proposed a method of presenting a threshold concentration of chloride in a molar concentration ratio [OH].

The relationship between chlorine and hydroxide ions in the corrosion process can be explained by the following equation.

Where n and K are constants. This indicates that the [Cl ⁻ ] ⁿ : [OH ⁻ ] concentration ratio is expressed as a constant for corrosion protection.

^{[Cl -]: [OH -} ] concentration ratio has been considered a risk of corrosion, as well as chlorine ions generated by the chlorine ions generated as carbonation. However, because the chlorine ions present in the concrete are fixed or adsorbed in some cement and removed from the pore water, this representation is not appropriate for the concrete environment.

When applied to concrete, this ratio may appear smaller at the same risk of corrosion due to fixed chlorine ions and may be evaluated as not at risk of corrosion progression. However, this expression has the drawback that it ignores the dangers from fixed chlorine ions.

As a result, the expression of critical chloride content method ^{[Cl -]: [OH -} ] as an alternative of the molar concentration ratio of the free chlorine ions only and corrode the concrete in reinforced because it destroys the passive film, the concentration of free chloride ion Has been used.

While the pit is increased, fixed chlorine ions are liberated around the rebar, reducing the pH.

In addition, unlike an environment of pore water, cement paste provides a corrosion inhibiting effect. Therefore, this presentation method is not suitable for expressing corrosion critical chloride concentrations and corrosion risks.

The method of expressing the amount of critical chloride by total chloride ion concentration is the most widely used and adopted standard.

The critical chloride content, expressed as binder weight ratio of total chloride content, has been used because it is relatively easy to determine and includes the corrosion risk of fixed chlorine ions and the corrosion inhibitory effect of cement hydrates.

This method describes the total amount of chlorine ions more accurately in terms of corrosion risk and the corrosion inhibition properties of cement, and is more reflected by the total amount of cement than the pH of pore water.

However, this method of expression also has limitations because only the amount of binder does not reflect all of the corrosion inhibiting properties of cement.

Hydration's resistance to pH reduction is greatly affected by the amount of binder as well as the type of binder.

It is well known that cement hydrates have corrosion inhibitory properties for chlorine ions that alleviate the drop in pH around the rebar in concrete, causing corrosion.

This was confirmed through the results that the corrosion inhibiting ability in the present invention is different depending on the type of the binder and the degree of corrosion risk also varies depending on the type of the binder.

In a recent study, a more suitable representation of the corrosion-inhibiting properties and deterioration factors of concrete was proposed by acid neutralization capacity (ANC) and the amount of chlorine ions in acid solutions, but it is not adequate to reflect both the critical chloride content and the ANC representing it. not.

Table 4 shows the procedure for calculating the critical chloride amount by the ratio of [Cl ⁻ ]: [H ⁺ ] molar concentrations.

Binder Threshold value (%, binder) Buffering capacity (mol / kg) ^{Threshold ratio of [Cl -]:} [H +] Max Min Mean Max Min Mean OPC 1.24 0.81 1.03 23.30 0.010708 0.006995 0.008895 30% PFA 0.81 0.48 0.65 14.00 0.011641 0.006898 0.009341 60% GGBS 0.55 0.35 0.45 10.50 0.010539 0.006707 0.008623 10% SF 1.18 0.77 0.98 21.50 0.011043 0.007206 0.009171

The amount of acid required to reduce the pH of the concrete to a certain pH and the amount of cement face in the solution were calculated in the present invention.

Acids required to decrease to pH 10 (moles H ⁺ / kg binder) were calculated to be 23.3 for OPC, 14.0 for 30% PFA, 10.5 for 60% GGBS, and 21.5 mol / kg for 10% SF, respectively (see Figure 5). ).

The various corrosion critical chlorides, which represent the total weight of chloride in the binder weight ratio, were determined to be 1.03 for OPC, 0.65 for 30% PFA, 0.45 for 60% GGBS, and 0.98% for 10% SF, respectively (see Table 3).

Critical chloride content of the measured data in the OPC, 30% PFA, 60% GGBS, 10% SF on the basis of the [Cl ^-]: the value of the range, such as 0.0090 were calculated when nd represented by [H ^+] molar concentration ratio.

This can be said to be the best representation of the corrosion critical chloride content because the total chloride ratio for ANC takes into account both important corrosion inhibition properties (cement cargo) and deterioration factors (total chloride content).

This representation of the critical chloride content has two advantages.

(1) the prediction of critical chloride content and the resulting corrosion onset time, and (2) the reflection of corrosion inhibition properties of cement hydrates.

The critical chloride amount will have a wide range, even if the exposed external environment and the concrete mix ratio are the same, because the degree of resistance to corrosion with time (i.e. the ability to inhibit corrosion against pH drop) varies with the variation in cement hydration.

Once the ANC of the cement is measured, the critical chloride amount can be easily calculated and the corrosion start time can also be predicted by measuring the chlorine ion rate.

Furthermore, by expressing in this way, it is possible to reflect all corrosion inhibitory properties and deterioration factors for corrosion and thus to reflect the influence factors on the corrosion risk.

Many papers have reported that a wide range of critical chloride amounts is caused by variables affecting the threshold.

External factors such as concrete humidity and temperature also affect the critical chloride content, but in practice it is almost impossible to control such an environment for the rise of critical chloride.

Past research has indicated that exposure temperatures have a significant effect on the critical chloride content.

Increasing the temperature from 20 ° C. to 70 ° C. reduces the critical chloride amount by five times.

In addition, an increase in temperature reduces the amount of fixed chloride amount and lowers the pH value of the pore water.

In addition, since the amount of wetting affects the mobility of chlorine ions and the potential of the environment, the concentration of chlorine ions in the pore water, the amount of critical chloride is affected by the amount of wetting of the concrete.

It has been reported that a high amount of wet in an environment with sufficient oxygen reduces the amount of critical chloride. This may be due to a decrease in the resistance of the concrete.

However, in spite of its significant impact on the critical chloride content, only a few studies have been considered.

The amount of alkali in the cement appears to affect the critical chloride amount, but is hardly affected.

In the study of Hussain et al in an increased critical chloride content during periods of high amount of alkali [Cl ^{-]: -} expresses the ratio [OH].

This result may be due to the pH of the pore water increased by the alkali of the cement.

However, the critical chloride amount expressed as cement weight ratio total chloride amount was found to be unaffected by the alkali amount.

It should also be remembered that a large amount of alkali in the cement can increase the alkali-silica reaction (ASR) dl.

To avoid the potential risk of ASR, most specifications and standards limit the maximum alkali content to 0.6% by weight cement when using potentially reactive aggregates.

Chlorine ions (concrete internal chlorine ions and infiltrated external chlorine ions) have also been reported to affect corrosion.

It should be considered that both internal and external ions act in the same form for corrosion.

However, the excessive presence of internal chlorine ions has been questioned by the results of studies showing that the risk of corrosion is greater when compared with the same amount of chlorine ion penetration from the outside.

If the internal chlorine ion starts to corrode before the protective film is formed around the reinforcing bar, the corrosion increase can occur very rapidly in an environment where oxygen and water are sufficiently provided.

Fig. 10 is a schematic representation of the method for calculating the rebar corrosion critical chloride amount used in the present invention. The ratio of the acid concentration until the pH of each binder falls to 10.0 and the amount of corrosion critical chloride expressed as a ratio of the amount of cement or the amount of binder is determined. The ratio always shows a constant value regardless of the type of binder. Calculate the amount of critical corrosion chloride of the corrosion of the concrete structure or specimen using this.

In the present invention, the resistance of rebar in mortar to chlorine ions causing corrosion was evaluated by the chlorine ion immobilization ability, the corrosion inhibiting ability of the hydrate, and the corrosion rate of the specimen containing various concentrations of chlorine ions.

In addition, a new method of expressing the critical chloride content for rebar corrosion in concrete considering the inhibitive properties of cement hydrate is proposed. The conclusions from the experimental study are as follows.

(1) The chlorine ion immobilization capacity greatly depends on the type of binder, curing period, and concentration of chlorine ions.

GGBS pastes showed high chlorine ion immobilization ability due to the high content of aluminum oxide (Al ₂ O ₃ ), which is an important oxide for the formation of C ₃ A in hydration.

The chlorine ion immobilization capacity of the binder is in the order 60% GGBS> 30% PFA> OPC> 10% SF.

In addition, an increase in curing period or a decrease in the amount of chlorine ions contained increased the immobilization capacity. Immobilization ability could be explained using Langmuir or Freundlich isotherms.

(2) Corrosion inhibiting ability of cement hydrate was used by suspension of powder paste and resistance of pH was decreased at specific pH despite the increase of acid in suspension, which was expressed as peak in acid neutralization capacity (ANC) curve.

The location and size of the peaks in the acid neutralization curves varied with the binder except for the peaks generated at pH 12.5.

(3) Corrosion rate of rebar in mortar increased regardless of the binder depending on the amount of chlorine ions mixed.

Assuming that the corrosion rate exceeds 1-2 mA / m ² , the average value of the critical chloride content for corrosion is calculated as 1.03 for OPC, 0.65 for 30% PFA, 0.45 for 60% GGBS, and 0.98% for 10% SF. It became.

(4) Regarding the relationship between the chlorine ion immobilization / inhibition ability and the corrosion risk, no specific relationship between the chlorine ion immobilization property and the critical chloride amount was observed, while the high corrosion inhibiting ability of the cement hydrate to pH decrease was high. It was found to be related to the amount of chloride.

(5) The critical chloride amount was expressed as the molar ratio of [Cl ⁻ ]: [H ⁺ ].

The concentration of chlorine ions was derived from the threshold expressed in total chloride content and the concentration of hydrogen ions calculated from the acid required when the suspension was reduced to pH 10.

As a result, the molar ratio of [Cl ⁻ ]: [H ⁺ ] molar ratios of OPC, 30% PFA, 60% GGBS, and 10% SF was constant at 0.0090.

Since the above has been described only with respect to some of the preferred embodiments that can be implemented by the present invention, the scope of the present invention, as is well known, should not be construed as limited to the above embodiments, the present invention described above It will be said that both the technical idea and the technical idea which together with the base are included in the scope of the present invention.

1 is a block diagram of a test apparatus for measuring the corrosion rate.

Figure 2 is a graph showing the chlorine ion immobilization capacity of 7,28,60,200 days of curing of OPC, 30% PFA, 60% GGBS, 10% SF.

Figure 3 is a graph showing the pH change measured at 60 days GGBS suspension of the acid concentration 0, 10, 20 mol / kg at the age of 1, 6, 12, 16, 21 to confirm the stability of the pH measurement.

Figure 4 is a graph showing the relationship between the concentration of nitric acid and the pH of the suspension for each binder.

5 is a graph showing the acid neutralization capacity of OPC, 30% PFA, 60% GGBS, 10% SF.

6 is a graph showing the degree of liberalization of fixed chlorine ions with increasing acid through the relationship between the ratio of free chlorine ions to total chlorine ions and the concentration of nitric acid.

7 is a graph showing the corrosion rate of the rebar in the mortar measured by the anode polarization method as a value of the amount of chlorine ions.

FIG. 8 is a graph showing the effect of corrosion inhibition capacity on the critical chloride amount compared to the effect of chlorine ion immobilization.

9 is a drop in the pH of the ordinary portland cement and 30% fly ash, 60% blast furnace slag fine powder, 10% silica fume added cement pool precipitated in distilled water, 0.2M nitric acid solution step by step as an acid solution Graph showing.

10 is a flow chart showing a method for calculating the corrosion amount of the critical corrosion chloride used in the present invention.

Claims (9)

An acidic solution input step of adding an acidic solution to the precipitate consisting of concrete specimens and distilled water step by step; Converting the concentration of the administered acid when the pH of the precipitate decreases to 10.0 by the concentration of hydrogen ions; Calculating a ratio of [Cl ⁻ ] of the precipitate to [H ⁺ ] of the administered acid; Determining a critical corrosion amount of reinforcing bar corrosion based on the ratio of [Cl ⁻ ]: [H ⁺ ]; Rebar corrosion critical chloride measurement method comprising. The method of claim 1, The [Cl ^-]: when the ratio of [H ^+] in the range of 0.008 ~ 0.010, the [Cl ^-]: corrosion threshold, characterized in that for determining the corrosion critical chloride content by the ratio of [H ^+] Chloride content measurement method. The method of claim 2, The [Cl ^-]: when the ratio of [H ^+] in the range of 0.0087 ~ 0.0092, the [Cl ^-]: corrosion threshold, characterized in that for determining the corrosion critical chloride content by the ratio of [H ^+] Chloride content measurement method. The method of claim 1, The method of claim 1, further comprising the step of converting the critical corrosion chloride amount calculated by the molarity as a percentage of the binder. The method of claim 1, The concrete specimen is a method of measuring the critical corrosion amount of reinforcing corrosion corrosion, characterized in that the powder form. The method of claim 5, Before the acid solution injection step, And eluting the compound in cement hydrate by precipitating the concrete powder in distilled water. The method of claim 5, After the acid solution injection step, Reinforcing corrosion of the critical corrosion chloride method characterized in that it further comprises the step of stabilizing the precipitate for a certain time to ensure the measurement of a stable pH. The method of claim 1, The concrete specimen is a method of measuring the amount of critical corrosion chloride reinforcing corrosion characterized in that the mortar or cement paste. The method of claim 1, The concrete specimen is a method of measuring the critical corrosion amount of the reinforcing corrosion of the reinforcing bar, characterized in that to take the inside of the structure more than 10 mm.

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