A method for passivation and curing of reinforced concrete test pieces for indoor corrosion tests

Abstract

The application discloses a reinforced concrete test piece passivation maintenance method for indoor corrosion test, relates to the concrete maintenance field, and aims at the problems of slow steel bar passivation, weakened passivation performance and distorted corrosion test results caused by insufficient oxygen supply of traditional full immersion water maintenance method.The three-layer collaborative maintenance system of bottom shallow water support and surface sponge covering is adopted: the test piece after form removal is erected on the support in the water tank, the water surface height is controlled to be 1 / 3 of the test piece height, the test piece surface is tightly covered with saturated water absorption sponge, and maintenance is carried out for 28 days under the environment of 20+ / -2 DEG C and relative humidity of 50%-70%.The application meets the collaborative development demand of concrete hydration and steel bar passivation, makes the steel bar reach the expected passivation performance (corrosion current density is stably kept at 0.2 muA / cm 2 The following passivation interval), reduces the real initial state, and is simple in operation, low in cost and applicable to reinforced concrete durability test in batches.

Description

Technical Field

[0001] This invention relates to the field of concrete curing technology, and in particular to a passivation curing method for reinforced concrete specimens used in indoor corrosion tests. Background Technology

[0002] In reinforced concrete durability studies, accelerated corrosion tests in the laboratory must accurately simulate the actual initial corrosion state of the reinforcing steel in engineering structures. Engineering practice shows that when corrosive media such as chlorides penetrate to the surface of the reinforcing steel, the steel has already undergone a natural passivation process over many years in the alkaline environment inside the concrete. The dense oxide film (γ-Fe2O3 / Fe3O4) formed on its surface can significantly delay corrosion. Therefore, concrete specimens must undergo sufficient passivation pretreatment before conducting indoor corrosion tests to ensure that the initial state matches the actual engineering conditions. Currently, the industry commonly uses water curing to maintain reinforced concrete specimens. However, substantial experimental evidence shows that traditional water curing methods have fundamental flaws: the dissolved oxygen concentration in the aqueous environment is severely insufficient, leading to hindered passivation and even passivation degradation.

[0003] The oxygen deficiency problem in water curing stems from the physicochemical mechanism of oxygen dissolving in water. At standard curing temperature (20 ℃), the saturated dissolved oxygen concentration in water is only 8.11 mg / L, while the oxygen concentration in the pore gas phase of concrete is as high as 210 mg / L (approximately 21% of the atmospheric oxygen concentration). When the specimen is completely submerged in water, oxygen transport to the steel / concrete interface faces three obstacles: first, oxygen must cross the water-air interface and dissolve in water (mass transfer coefficient k...). L ≈ 3×10 -5 (m / s), and then diffuses through the concrete protective layer (diffusion coefficient Dc ≈ 1.5 × 10⁻⁶ m / s). -12 m 2 The oxygen concentration at the rebar interface is suppressed to less than 2 mg / L, far below the critical value of 5 mg / L required for stable growth of the passivation film. This oxygen-deficient state will directly disrupt the kinetic equilibrium of the rebar passivation reaction 4Fe + 3O2 + 6H2O → 4Fe(OH)3: on the one hand, the decrease in oxygen partial pressure causes the forward reaction rate constant k1 to decrease by 62% (from 2.5 × 10⁻⁶ ppm). -9 m 3 / mol·s decreased to 0.9×10 -9 m 3 On the other hand, the continuous infiltration of water molecules leads to a 92% increase in the membrane hydrolysis rate constant k2 (from 1.2 × 10⁻⁶ mol·s⁻¹). -7 s -1 Increased to 2.3×10 -7 s -1The final result was that the corrosion current density of the steel bars increased rather than decreased after 28 days of curing, averaging 0.35 ± 0.12 μA / cm². 2 This far exceeds the passivation threshold of 0.2 μA / cm. 2 Further X-ray photoelectron spectroscopy (XPS) analysis confirmed that the protective component Fe in the passivation film of this type of specimen was... 2+ The content is only 8-12 at% (normal passivation film formation can reach 35 at%), and there are a large number of donor defects.

[0004] Therefore, there is an urgent need for a passivation curing method for reinforced concrete specimens used in indoor corrosion tests to improve the above-mentioned problems. Summary of the Invention

[0005] To address the aforementioned issues, this application proposes a method for passivating and curing reinforced concrete specimens for indoor corrosion testing. This method aims to resolve the problem of insufficient passivation and distorted initial state of reinforced concrete specimens in indoor corrosion tests due to the difficulty in coordinating cement hydration and steel passivation in traditional full-immersion water curing methods. Specific details include: The passivation curing method for reinforced concrete specimens uses water storage containers, supports, and porous, breathable, and moisturizing materials as curing tools. By constructing a three-layer synergistic curing system with shallow water moisturization at the bottom, air circulation in the middle, and breathable and moisturizing coverage at the top, it simultaneously achieves continuous hydration of concrete and efficient passivation of steel reinforcement.

[0006] The passivation curing method for reinforced concrete specimens includes the following steps: S1. At least two supporting members are arranged in parallel inside the water storage container, with a gap between the supporting members to support the reinforced concrete specimen; S2. The demolded reinforced concrete specimen is placed on the supporting member so that a continuous air flow gap is formed between the bottom of the specimen and the bottom of the water storage container, and an air flow distance is reserved between adjacent specimens. S3. Fill the water storage container with water until the water level is higher than the top surface of the supporting component and lower than 1 / 2 of the height of the reinforced concrete specimen, forming a shallow water layer that keeps the bottom moist. S4. After soaking the breathable and moisture-retaining material with an open porous structure into water to saturation, apply it tightly to cover all exposed outer surfaces of all reinforced concrete specimens to form a surface curing layer that combines water retention and breathability. S5. Regularly replenish the breathable and moisturizing material with water to maintain its saturated moisture state, and control the maintenance environment temperature at 18°C. 22℃, relative humidity 40% After maintaining 80% of the material until the preset passivation cycle, remove the breathable and moisturizing material and proceed directly with the corrosion test.

[0007] This method can maintain the oxygen concentration at the steel-concrete interface at a stable level above the passivation critical value of 5 mg / L, avoiding problems such as passivation degradation and loss of interfacial alkalinity caused by traditional full immersion water curing.

[0008] Preferably, in step S3, water is poured into the water storage container until the water level is one-third of the height of the reinforced concrete specimen, or 1-3 cm above the bottom of the specimen. This ensures the supply of humidity for the hydration of the bottom concrete while preventing the water level from being too high and blocking the air circulation in the middle.

[0009] Preferably, the thickness of the breathable and moisturizing material in S4 is 20 mm. 30 mm is the optimal thickness range that balances continuous moisture retention and oxygen permeability, ensuring that air can freely penetrate to the surface of the specimen without localized alternation between dry and wet conditions.

[0010] Preferably, the breathable and moisture-retaining material is any one of wood pulp cotton, polyurethane sponge, or non-woven fabric, with a porosity ≥80% and good water absorption and retention properties.

[0011] Preferably, the breathable and moisturizing material is soaked in 75% alcohol for 20 minutes before use. Sterilization is performed for 40 minutes to prevent microbial growth from contaminating the surface of the specimen and clogging the pores of the concrete surface.

[0012] Preferably, the spacing between adjacent reinforced concrete specimens in S2 is 8. The spacing between the support components is 12 cm, which is adapted to the length of the specimen to ensure that the air can circulate freely around the specimen and there are no dead corners due to lack of oxygen.

[0013] Preferably, in step S5, water is evenly injected onto the surface of the breathable and moisturizing material once a day, while the water level in the water storage container is monitored in real time to keep the water level stable within a preset range.

[0014] Preferably, the preset passivation period is 28 days, and after curing, the corrosion current density of the steel reinforcement stabilizes and decreases to 0.2 μA / cm. 2 Below, passivation film Fe 2+ Content is 8 12at%, consistent with the actual natural passivation state of the project.

[0015] Preferably, the water storage container is an open-top polyethylene turnover box with a maximum volume of 30 cubic meters. 50 L; The supporting components are construction timber.

[0016] Preferably, the reinforced concrete specimen is a concrete prism with embedded ordinary carbon steel bars, the diameter of which is 8 mm. 12 mm, the thickness of the concrete cover for the reinforcing steel is 15 mm. 25 mm.

[0017] In summary, the passivation curing method for reinforced concrete specimens used in indoor corrosion tests according to the present invention has the following advantages compared with traditional technologies: 1. By utilizing the open porous structure of breathable and moisture-retaining materials to maintain 100% relative humidity on the surface of the specimen to ensure full hydration of concrete while allowing free air penetration, the oxygen concentration at the steel / concrete interface is stably maintained above 5 mg / L, meeting the critical requirement for stable growth of the passivation film and thus achieving a synergistic balance between breathability and moisture retention. 2. The inefficient oxygen supply mode of traditional water curing, which involves water phase dissolution and liquid phase diffusion, is transformed into a rapid oxygen supply mode dominated by gas phase diffusion. The oxygen transport path is reconstructed, which increases the forward passivation reaction rate by 62% and reduces the membrane hydrolysis rate by 92%. This solves the problem of slow passivation and degradation of reinforced concrete in water curing from the root of the kinetics. 3. After 28 days of curing, the corrosion current density of the steel reinforcement stabilized and decreased to 0.2 μA / cm². 2 Below, the passivation film protects Fe 2+ The content is approximately 35 at%, forming a dense oxide layer with a composition and structure highly consistent with naturally formed passivation films.

[0018] The technical method of the present invention will be further described in detail below with reference to the accompanying drawings and embodiments. Attached Figure Description

[0019] Figure 1 This is a schematic diagram of the curing process for reinforced concrete specimens. Figure 2 This is a schematic diagram of an absorbent sponge. Figure 3 A schematic diagram of a reinforced concrete specimen and its conductor; Figure 4 This is a schematic diagram of the sleepers; Figure 5 Impedance spectra of reinforced concrete specimens after 28 days of passivation under different curing methods. Figure 5 (a) shows the sponge maintenance method, and (b) shows the traditional water maintenance method. Figure 6 Linear polarization diagrams of reinforced concrete specimens after 28 days of passivation under different curing methods. Figure 6 (a) shows the sponge maintenance method, and (b) shows the traditional water maintenance method.

[0020] Figure Labels 1. Open-top polyethylene water tank; 2. Reinforced concrete specimen; 3. Sleepers; 4. Absorbent sponge; 5. Connecting wires for embedded steel bars. Detailed Implementation

[0021] The technical method of the present invention will be further described below with reference to the accompanying drawings and embodiments. It should be noted that, unless otherwise specifically stated, the relative arrangement, numerical expressions, and values ​​of the components and steps described in these embodiments do not limit the scope of this application.

[0022] The following description of at least one exemplary embodiment is merely illustrative and is in no way intended to limit the scope of this application and its application or use.

[0023] Techniques, systems, and equipment known to those skilled in the art may not be discussed in detail, but where appropriate, they should be considered part of the instruction manual.

[0024] In all the examples shown and discussed herein, any specific values ​​should be interpreted as merely exemplary and not as limitations. Therefore, other examples of exemplary embodiments may have different values.

[0025] Unless otherwise defined, the technical or scientific terms used in this invention shall have the ordinary meaning as understood by one of ordinary skill in the art to which this invention pertains.

[0026] Example 1 This embodiment provides a method for curing reinforced concrete specimens used in indoor corrosion tests using a breathable and moisture-retaining sponge. The curing device is as follows: Figures 1-4 As shown, it includes a polyethylene open-top water tank 1, a reinforced concrete specimen 2, sleepers 3, absorbent sponges 4, and wires 5 connecting the embedded reinforcing bars.

[0027] Preparation of maintenance tools: Water storage container: Commercially available polyethylene open-top turnover box, outer diameter 730 mm × 560 mm × 230 mm, maximum volume 40 L, check for leaks before use.

[0028] Support components: 700 mm × 70 mm × 40 mm construction timber (sleepers) are used. The bottom is sanded before use to ensure that the top surface is flat and the structure is stable.

[0029] Breathable and moisture-retaining material: 200 mm × 100 mm × 30 mm wood pulp cotton is used. Before use, it is completely immersed in 75% alcohol for 30 minutes for sterilization. After that, it is taken out and dried for later use.

[0030] Auxiliary tools: a fine-flow watering can, a sprinkling can, and a water level gauge.

[0031] Specimen preparation: A 50 mm × 50 mm × 200 mm reinforced concrete prism specimen was prepared, with 10 mm diameter ordinary carbon steel bars embedded inside, and a steel bar cover thickness of 20 mm. After the specimen was poured, it was cured in a standard curing room for 24 hours before demolding, and the surface laitance and burrs were removed.

[0032] The complete maintenance process is as follows: Step 1: Check the water tank for leaks and place the sleepers parallel to each other in the water tank at fixed intervals (10 cm), ensuring that the 20 cm long concrete test block can be placed on the sleepers.

[0033] Step 2: Place the demolded concrete test blocks at fixed intervals (10 cm) on the parallel sleepers in the water tank, and check whether the concrete test specimens are stable to prevent the test specimens from falling into the water during the later stages of curing, which would lead to insufficient oxygen supply during curing and thus affect the passivation effect of the steel bars.

[0034] Step 3: Use a slow-flowing water jug ​​to fill the water tank with water so that the water level is just above the sleeper and maintained at 2 cm above the bottom of the concrete specimen.

[0035] Step 4, apply a 20mm thick material. After soaking a 30 mm absorbent sponge in water until saturated, it is tightly applied to cover all the surfaces of the specimens.

[0036] Step 5: Spray water onto the sponge surface daily using a watering can to maintain sponge humidity. Keep the ambient temperature at 20±2℃ and the relative humidity at 50%. 70%; after 28 days of continuous maintenance, the sponge was removed and corrosion tests were conducted.

[0037] Note: The absorbent sponge must be sterilized by soaking in 75% alcohol for 30 minutes before use to prevent microbial growth and contamination of the test specimen surface.

[0038] During the curing process, pay attention to monitoring the water level in the water tank to avoid the water level being too low, which would result in insufficient humidity for concrete hydration, or too high, which would result in insufficient oxygen supply for the passivation of the steel reinforcement inside the concrete.

[0039] It is recommended to clean all equipment before and after each experiment.

[0040] Verification of maintenance effect: Electrochemical impedance spectroscopy (EIS) and linear polarization (LPR) tests were performed on the specimens after curing using a KOST CS310H electrochemical workstation. The test results are as follows: Figure 5 and Figure 6 As shown, the specimens were compared with those cured by the traditional full immersion water curing method in the same batch.

[0041] Electrochemical impedance spectroscopy (EIS) analysis: The Nyquist plot of the sponge curing specimen from this invention shows a complete and full large-diameter capacitive arc, with a charge transfer resistance of 125 kΩ·cm. 2 The Nyquist plot of specimens prepared by the traditional water-cultivation method showed a significantly smaller diameter for capacitive arc resistance and compressive deformation, with a charge transfer resistance of only 32 kΩ·cm. 2 .

[0042] Linear polarization test: The polarization resistance of the specimen cured by sponge method is 118 kΩ·cm. 2 The corrosion potential was -210 mV (vs. SCE), and the corrosion current density was 0.15 μA / cm. 2 Stable below 0.2 μA / cm 2 Passivation threshold; the polarization resistance of specimens prepared by traditional water-based methods is 28 kΩ·cm. 2 The corrosion potential was -380 mV (vs. SCE), and the corrosion current density of the specimen obtained by the traditional water curing method was 0.35 μA / cm. 2 Exceeding 0.2 μA / cm 2 The passivation threshold has not been met.

[0043] Test results show that the method of the present invention can form a dense and stable passivation film in the steel bars of concrete specimens, and the passivation effect is significantly better than that of the traditional water curing method.

[0044] Comparative Example 1 Traditional full immersion water culture method.

[0045] Reinforced concrete specimens from the same batch as those in Example 1 were completely immersed in clean water at 20±2℃ for 28 days, with the water changed daily during the curing period. After curing, electrochemical tests were performed using the same method. The test results, as described above, showed that the passivation effect was significantly inferior to that of the method of this invention.

[0046] Example 2 The difference between this embodiment and Embodiment 1 is that: the breathable and moisture-retaining material is a 20 mm thick polyurethane sponge with a porosity of 85%; the curing environment temperature is 18℃ and the relative humidity is 70%; the preset passivation period is 28 days. All other operating steps are exactly the same as in Embodiment 1.

[0047] After curing, the test results showed that the corrosion current density of the specimen was 0.16 μA / cm. 2 (Below 0.2 μA / cm) 2 Passivation threshold), polarization resistance is 109 kΩ·cm 2 It also achieved a good passivation effect.

[0048] Example 3 The difference between this embodiment and Embodiment 1 is that the diameter of the embedded steel bars in the reinforced concrete specimen is 12 mm, and the thickness of the protective layer is 25 mm; the placement interval between adjacent specimens is 12 cm; and the breathable and moisture-retaining material is sterilized by soaking in 75% alcohol for 40 minutes before use. The remaining operating steps are exactly the same as in Embodiment 1.

[0049] After curing, the test results showed that the corrosion current density of the specimen was 0.15 μA / cm. 2 (Below 0.2 μA / cm) 2 The passivation threshold is within a good passivation state, meeting the requirements of indoor corrosion testing.

[0050] Depend on Figure 5 The Nyquist plots of (a) sponge curing and (b) traditional water curing clearly show that the electrochemical impedance characteristics of the reinforced concrete specimens differ significantly between the two curing methods: the Nyquist plot of the sponge curing specimens shows a complete and full capacitive arc morphology, and the diameter of the capacitive arc is several times larger than that of the specimens cured by traditional water curing. This indicates that the charge transfer resistance of the specimens cured by sponge curing is greatly increased, the electrochemical reaction resistance on the surface of the steel reinforcement is significantly increased, the passivation film is complete and dense, and the protective effect on the steel reinforcement is significant. On the other hand, the Nyquist plot of the specimens cured by traditional water curing shows a severely smaller diameter of the capacitive arc, an incomplete arc shape, and obvious compression deformation. This reflects that its charge transfer resistance is smaller, the electrochemical reaction on the surface of the steel reinforcement is more likely to occur, the passivation film structure is loose and has a large number of defects, and an effective passivation protective layer is not formed.

[0051] Figure 6 The linear polarization results of (a) sponge curing and (b) traditional water curing further confirm the differences in the passivation performance of steel bars in concrete under different curing methods. The polarization resistance of the specimens cured by sponge curing is significantly higher than that of the specimens cured by traditional water curing, and the corrosion potential is more positive, indicating that the passivation film on the surface of the steel bars is more stable and has better corrosion resistance. On the other hand, the polarization resistance of the specimens cured by traditional water curing is lower and the corrosion potential is more negative, reflecting that the protective effect of the passivation film on the surface of the steel bars is weak, the steel bars are in a state of easy corrosion, and effective passivation has not been achieved.

[0052] Electrochemical impedance spectroscopy and linear polarization tests show that the sponge curing method enables the steel reinforcement in concrete specimens to quickly form a protective, dense, and stable passivation film, achieving excellent and effective passivation. In contrast, using the traditional water curing method, within the same curing period, it is difficult for the steel reinforcement in concrete specimens to form a complete and dense passivation film, failing to achieve the expected passivation effect. This stark contrast directly demonstrates the significant advantages of the sponge breathable and moisture-retaining curing technology of this invention in improving the passivation performance of steel reinforcement during concrete curing.

[0053] In summary, this method eliminates the need for complex operating procedures and expensive machinery. It ensures high efficiency and low cost while effectively passivating the reinforcing steel bars embedded in concrete, guaranteeing the accuracy and reliability of indoor corrosion tests on concrete specimens. The method utilizes simple, safe, and easy-to-operate tools, enabling the curing of large batches of concrete specimens. During the curing process, the method ensures comprehensive contact between circulating air and the specimens, guaranteeing sufficient oxygen supply for steel bar passivation. This invention provides users with a safe and reliable method for passivating and curing reinforced concrete.

[0054] Finally, it should be noted that the above embodiments are only used to illustrate the technical methods of the present invention, and are not intended to limit them. Although the present invention has been described in detail with reference to preferred embodiments, those skilled in the art should understand that modifications or equivalent substitutions can still be made to the technical methods of the present invention, and these modifications or equivalent substitutions should not cause the modified technical methods to deviate from the spirit and scope of the technical methods of the present invention.

Claims

1. A passivation curing method for reinforced concrete specimens used in indoor corrosion tests, characterized in that, The method, which uses water-retaining containers, support components, and porous, breathable, and moisture-retaining materials as maintenance tools, includes the following steps: S1. At least two supporting members are arranged in parallel inside the water storage container, with a gap between the supporting members to support the reinforced concrete specimen; S2. The demolded reinforced concrete specimen is placed on the supporting member so that a continuous air flow gap is formed between the bottom of the specimen and the bottom of the water storage container, and an air flow distance is reserved between adjacent specimens. S3. Fill the water storage container with water until the water level is higher than the top surface of the supporting component and lower than 1 / 2 of the height of the reinforced concrete specimen, forming a shallow water layer that keeps the bottom moist. S4. After soaking the breathable and moisture-retaining material with an open porous structure into water to saturation, apply it tightly to cover all exposed outer surfaces of all reinforced concrete specimens to form a surface curing layer that combines water retention and breathability. S5. Regularly replenish the breathable and moisturizing material with water to maintain its saturated moisture state, and control the maintenance environment temperature at 18°C. 22℃, relative humidity 40% After maintaining 80% of the material until the preset passivation cycle, remove the breathable and moisturizing material and proceed directly with the corrosion test.

2. The passivation curing method for reinforced concrete specimens used in indoor corrosion tests according to claim 1, characterized in that, In S3, water is poured into the storage container until the water level is one-third of the height of the reinforced concrete specimen, or 1-3 cm above the bottom of the specimen.

3. The passivation curing method for reinforced concrete specimens used in indoor corrosion tests according to claim 2, characterized in that, The thickness of the breathable and moisture-retaining material described in S4 is 20 mm. 30 mm.

4. The passivation curing method for reinforced concrete specimens used in indoor corrosion tests according to claim 3, characterized in that, The breathable and moisture-retaining material is any one of wood pulp cotton, polyurethane sponge, or non-woven fabric, with a porosity ≥80%.

5. The passivation curing method for reinforced concrete specimens used in indoor corrosion tests according to claim 4, characterized in that, Before use, the breathable and moisturizing material is soaked in 75% alcohol for 20 minutes. Sterilize after 40 minutes.

6. The passivation curing method for reinforced concrete specimens used in indoor corrosion tests according to claim 5, characterized in that, The spacing between adjacent reinforced concrete specimens in S2 is 8. 12 cm.

7. The passivation curing method for reinforced concrete specimens used in indoor corrosion tests according to claim 6, characterized in that, In S5, water is evenly injected onto the surface of the breathable and moisturizing material once a day, while the water level in the water storage container is monitored in real time to maintain a stable water level.

8. The passivation curing method for reinforced concrete specimens used in indoor corrosion tests according to claim 7, characterized in that, The preset passivation period is 28 days.

9. A passivation curing method for reinforced concrete specimens used in indoor corrosion testing according to claim 8, characterized in that, The water storage container is an open-top polyethylene turnover box with a volume of 30 cubic meters. 50 L; the supporting component is building timber.

10. A passivation curing method for reinforced concrete specimens used in indoor corrosion testing according to claim 9, characterized in that, The reinforced concrete specimen was a concrete prism with embedded ordinary carbon steel bars, the bars having a diameter of 8 mm. 12mm, the thickness of the concrete cover for the reinforcing steel is 15mm. 25 mm.

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