Research on improving the durability performance of damaged concrete based on ultra high performance concrete repair

This article uses ultra‐high performance concrete (UHPC) to repair damaged concrete and analyzes the durability performance of the repaired concrete. Among them, three erosion solutions with a mass concentration of 2.8% NaCl + 0.29% Na2SO4, 5% NaCl, and 5% NaCl + 10% Na2SO4 were designed to soak the concrete specimens for 1 to 3 months. Taking the SWC test group as an example, when the concrete test block undergoes 30 dry wet cycles and the erosion depth is 20 mm, the difference in chloride ion concentration under different diffusion dimensions is not significant. After 90 dry wet cycles, at a depth of 20 mm, the chloride ion concentration inside the concrete in the three‐dimensional diffusion state reached 3 times that of one‐dimensional diffusion, and the chloride ion concentration in the two‐dimensional diffusion state also reached 2 times that of one‐dimensional diffusion. At a depth of 30 mm, a small amount of chloride ion content was also found in the two‐dimensional diffusion and three‐dimensional diffusion.


INTRODUCTION
2][3][4][5][6][7] For example, in the 1990s, the total cost of cement in the United States was $6 trillion, and the annual maintenance and reconstruction costs were $300 billion; In the United States, out of 253,000 cement bridges in 1987, one panel was damaged within 20 years; According to "The 2002 Corruption Situation of the United States Bureau of Economic Analysis," corrosion of foundations and houses causes an annual loss of $70 billion.The completed Central Expressway in England includes eight elevated roads with a length of 21 km.The cost of these elevated roads is only £28 million, but the maintenance cost is 1.6 times its cost.[10][11][12] Chlorides and sulfates are one of the most common corrosive substances in concrete. 13In practical engineering, the surface of concrete is corroded by chloride and sulfate, which can lead to a decrease in the mechanical properties of concrete, and even cause damage to the concrete.5][16] Firstly, chloride ions will enter the interior of the concrete and combine with calcium ions in the cement stone to form calcium chloride, which will damage the structure of the cement stone in the concrete. 17At the same time, sulfate will react with calcium ions in concrete to form gypsum, thereby damaging the cement stone structure of concrete. 18The combined action of chloride and sulfate can increase the concentration of chloride ions and sulfate inside the concrete, further damaging the cement structure in the concrete. 19Secondly, the combined action of chloride and sulfate can also have an impact on the pore structure of concrete.The pore structure in concrete has a significant impact on its mechanical properties.][22][23] In addition, chloride salts and sulfates can also cause an increase in the size of pores and porosity in concrete, which can also lead to a decrease in the mechanical properties of concrete. 24Sulfate solution can also replace some of the chloride ion products Friedel salts in concrete to form needle shaped ettringite crystals, and ettringite often accumulates together in clusters, radiating from the center to the surrounding areas.This type of crystal often has a larger expansion volume, which is easy to generate internal stress inside the concrete and cause certain damage to the concrete structure. 25he combined corrosion of sulfate and chloride can also have a certain impact on the durability of concrete. 26On the one hand, both chloride and sulfate can lead to a decrease in the alkalinity of concrete, thereby losing its ability to protect steel bars and promoting the occurrence of steel corrosion.8][29] Specifically, the combined action of chloride and sulfate can lead to changes in the pore structure of concrete, leading to an increase in the internal porosity of the concrete.The penetration of chloride ions in cement causes chemical changes in the cement paste, resulting in volume expansion and increased porosity. 30In addition, in cement, sulfate reacts with the cement matrix, causing the cracking of materials such as ettringite in the cement matrix, thereby increasing the porosity of the cement matrix.The existence of voids not only affects the strength and stability of concrete, but also increases its impermeability and corrosion resistance. 31In addition, the synergistic corrosion of sulfates and chlorides is also the main factor causing corrosion of steel bars in concrete. 32,33herefore, exploring the multidimensional transport mechanism of chloride ions under the coupling of multiple factors and mastering the migration law of chloride salts inside concrete is of great significance for improving the durability of concrete.

Mix ratio
The strength design of concrete in this experiment refers to GB/T 50476-2019 "Durability Design Standards for Concrete Structures".The exposure environment of concrete structures is selected as a marine chloride environment where chloride corrosion causes steel corrosion.The chloride corrosion location is in areas such as bridge piers, bearing platforms, and docks affected by tidal and splash zones.The design service life is 100 years, and the minimum concrete strength that meets the durability requirements is C50, The concrete used in this experiment is the damaged concrete, and high-strength concrete is used for repair.The mix ratio is shown in Table 1.

Erosion environment
To ensure that the immersed salt solution closely resembles the real marine environment, three seawater samples were collected from Tong'an District of Xiamen City for this experiment.The concentrations of Cl − and SO4 used for configuration.A dry-wet cycle system with a cycle of 12 h natural drying and 12 h of solution immersion (dry-wet ratio 1:1) was selected.To consider the impact of composite salt solution erosion on concrete durability, four groups with different erosion environments were designed, as shown in Table 2.

Erosion test
To ensure the stability of ion concentration in the solution, record the salinity changes of the solution with a seawater salinity meter every week, and replace the solution once a month.The soaking solution used for the number group SWC and SW is the same.When drying is required, a small water pump is used to extract the salt solution from the SWC soaking box into the SW soaking box.When the solution needs to be soaked, a portion of the salt solution in the SW soaking box is then refluxed along the outlet into the SWC soaking box by gravity.The entire dry wet cycle ensures that the salt solution can pass through the top surface of the test piece.The test blocks of number groups Cl5 and Cl510 are immersed in the corresponding salt solution.The concrete erosion test device and the dry wet cycle method are shown in Figure 1.

Sampling
Refer to Appendix C of JGJ/T 322-2013 "Technical Specification for Detection of Chloride Ion Content in Concrete" to determine the water-soluble chloride ion content in hardened concrete.The detection method is silver nitrate titration and potassium chromate indicator.
For concrete specimens studying the three-dimensional diffusion of chloride ions, a marble cutting machine is used for cutting and grinding powder sampling.First, mark the location on the concrete surface where sampling is required, and then use a marble cutting machine to cut out small cubes as shown in the figure from the concrete test block.Use a grinding machine to grind the inner corners of the cubes.Use a handheld impact drill to drill the powder from the corresponding positions on the concrete test block and the inner corners of the cubes, and collect the powder obtained from both methods together.
To ensure more accurate data on the sampled powder, only 5-8 g of mortar powder was collected at each location.Under the same service conditions, there were two concrete specimens for two-dimensional diffusion of chloride ions and three specimens for three-dimensional diffusion, which can avoid the problem of collecting multiple powders in the same concrete specimen.Figure 2 shows the collected concrete powder.

Testing
To determine the water-soluble chloride ion content in hardened concrete, weigh 5 g of mortar powder and add 50 mL (V1) of distilled water.Shake well and place it on a test electric furnace with a wire gauze.Boil for 5 min, then stop heating and cover the bottle.Let it stand for 24 h, and filter the mixture through filter paper to obtain the filtrate.Transfer two 20 mL (V2) portions of the filtrate into two triangular flasks, add phenolphthalein indicator, and neutralize it with nitric acid solution until it becomes colorless.Before titration, add 10 drops of potassium chromate indicator to the two filtrates.
Then titrate with silver nitrate standard solution until the pink-yellow color disappears.Record the volume of silver nitrate solution consumed for each titration and take the average value V3 as the measurement result.The water-soluble chloride ion content in hardened concrete is calculated using the following formula: where, W W Cl − is the percentage of water-soluble chloride ions in hardened concrete to the mass of mortar (%);C AgNO 3 is concentration of Silver nitrate Standard solution (mol/L); V 3 -standard dosage of Silver nitrate solution during titration (mL); G-Mass of mortar sample (g); V 1 -Amount of distilled water used for soaking the sample (mL); V 2 -The amount of filtrate extracted during each titration (mL).

SWC test group
Figure 3A-C shows the one-dimensional, two-dimensional, and three-dimensional diffusion of chloride ions inside the concrete specimens soaked in the SWC test group for 1, 2, and 3 months, respectively.The corresponding number of dry and wet cycles for 1, 2, and 3 months is 30, 60, and 90, respectively.It can be observed that after undergoing wet and dry cycles, the internal chloride ion content of the concrete specimen significantly increases, especially at the depth of 20-30 mm inside the concrete.With the increase of wet and dry cycles, the change in chloride ion content becomes more pronounced.It can be seen that the increase in the number of dry and wet cycles also has a significant promoting effect on the high-dimensional diffusion of chloride ions.When the concrete specimen undergoes dry and wet cycles, there is no significant difference in chloride ion concentration at a depth of 20 mm in concrete erosion under the influence of one-dimensional, two-dimensional, and three-dimensional diffusion, all of which are less than 0.1%.After 90 dry and wet cycles, at a depth of 20 mm for concrete erosion, the chloride ion concentration inside the concrete in the three-dimensional diffusion state reached three times that of one-dimensional diffusion, and the chloride ion concentration in the two-dimensional diffusion state also reached twice that of one-dimensional diffusion.Moreover, at a depth of 30 mm, a small amount of chloride ion content was also found in the two-dimensional diffusion and three-dimensional diffusion.
The SWC test group simulates the concrete structures in the tidal and splash areas in ocean engineering.The concrete structures in this area are subjected to the influence of alternating drying and wetting of seawater every day.These concrete test blocks are also affected by the deepest erosion, especially in areas with sufficient oxygen content.When enough chloride ions enter into the concrete at a certain depth, redox will occur with the reinforcement in the concrete, causing corrosion and cracking, ultimately affecting the load-bearing capacity of the structure.From this, we can see that the diffusion rate of chloride ions is very significant under the combined action of dry-wet cycles and multi-dimensional diffusion of chloride ions.When analyzing this working condition, we cannot ignore the impact of diffusion dimensions on the chloride ion erosion of concrete.It is essential to consider the effects of multi-dimensional diffusion and dry-wet cycles on the chloride ion concentration inside the concrete structure to accurately evaluate the durability and service life of concrete structures in tidal and splash areas.

SW test group
Figure 4A-C shows the one-dimensional, two-dimensional, and three-dimensional diffusion of chloride ions inside the concrete specimens soaked in the SW test group for 1, 2, and 3 months, respectively.It can be observed that as the soaking time increases, the chloride ion concentration on the concrete surface also increases.Comparing the chloride ion concentration on the concrete surface under different diffusion dimensions, there is no significant difference.The impact of diffusion dimensions on the chloride ion concentration inside the concrete is mainly reflected in the change in the corrosion depth range of 0-20 mm inside the concrete.At an erosion depth of 10 mm and a soaking time of 3 months, the three-dimensional diffusion chloride ion concentration in the SW test group was 2.55 times that of one-dimensional diffusion.However, under the same three-dimensional diffusion conditions, at a concrete erosion depth of 10 mm, the chloride ion concentration in the SW test group soaked for 3 months was 1.76 times that of the chloride ion concentration in the SW test group soaked for 1 month.
The SW experimental group simulated the concrete structure affected by the underwater zone in marine engineering.The transfer of chloride ions to the interior of the concrete in this area is mainly caused by the difference in ion concentration.However, from the experimental results, we can infer that the effect of this concentration difference is limited.The concrete itself has a tight structure, and as the erosion depth increases, the difference in chloride ion concentration in different sections of the concrete gradually decreases.When the concentration of chloride ions decreases to a certain range, it becomes difficult to transport further deeper inside the concrete.Moreover, the underwater area has a relatively low oxygen content, making the structure of this area relatively safer compared to the tidal splash area.Therefore, the risk of corrosion and cracking in underwater concrete structures may be lower than that in tidal splash areas.However, it is still important to consider the effects of chloride ion diffusion and erosion on the durability and service life of concrete structures in underwater zones in marine engineering.

Cl5 test group
Figure 5A-C shows the one-dimensional, two-dimensional, and three-dimensional diffusion of chloride ions inside concrete specimens soaked in the Cl5 test group for 1, 2, and 3 months, respectively.It can be observed that the chloride concentration on the concrete surface increases significantly with the increase of chloride concentration in the erosion environment.In the area of 0-20 mm erosion depth, the chloride content is also higher.Differences in chloride concentration inside the concrete can be observed under different dimensional diffusion.Under the same immersion time, the chloride concentration in the three-dimensional diffusion concrete reaches twice that of the one-dimensional diffusion at a 10 mm erosion depth.The trend of chloride ion concentration variation in concrete is that it first rapidly decreases and then slowly decreases with the increase of erosion depth.
Comparing Figures 4 and 5, it can be observed that the trend of change in both is almost the same.The higher the concentration of chloride ions in the corrosive environment, the more chloride ions will be transported to the interior of the concrete under the effect of concentration difference.On the surface of the concrete, the change in chloride ion concentration is very obvious as the erosion time increases.Moreover, as the erosion depth increases, the variation of chloride ion concentration over time is very small, which also indicates that the larger the concentration difference, the more chloride ion concentration enters the concrete interior.However, this is only the basic condition for chloride ion diffusion into the concrete interior and is not the main cause of steel corrosion inside the concrete.Steel corrosion inside the concrete is caused by the combination of chloride ion diffusion and the presence of oxygen and moisture.Therefore, it is important to consider not only the chloride ion diffusion but also the presence of oxygen and moisture in the concrete when evaluating the durability and service life of concrete structures in corrosive environments.

Cl5S10 test group
Figure 6A-C shows the one-dimensional, two-dimensional, and three-dimensional diffusion of chloride ions inside concrete specimens soaked in the Cl5S10 test group for 1, 2, and 3 months, respectively.According to Figure 6, the chloride ion concentration on the surface of the Cl5S10 immersion group is high, but in the erosion depth of 0-20 mm, the ion concentration decreases rapidly.Moreover, at an erosion time of 1 month, the change in chloride ion concentration inside the concrete is consistent with the slope of three-dimensional diffusion and one-dimensional diffusion.When the immersion time reaches 3 months, there is a slight difference, but it is not significant.Compared with the change of chloride ion concentration in the concrete of the test group after immersion for 1, 2, and 3 months under the three-dimensional diffusion condition, it was found that the test group was less sensitive to time.
During the immersion period, sulfate ions replaced part of the chloride ions to generate Ettringite, forming certain expansion products that filled the pores in the concrete and slowed down the transmission of chloride ions in the concrete.Therefore, the Cl5S10 test group showed a certain degree of resistance to chloride ion diffusion, which can effectively reduce the risk of corrosion and cracking in concrete structures.

CONCLUSION
(1) The main reason for the transfer of chloride ions into the interior of concrete is caused by the difference in ion concentration.The larger the concentration difference, the more chloride ions enter the interior of the concrete.
(2) Compared to external multi field conditions, the effect of this concentration difference is limited, especially in complex environments where chloride ions diffuse multi-dimensionally.Concentration difference is the basic condition for chloride ions to diffuse into the interior of concrete.It is not the main cause of steel corrosion inside concrete.
(3) In the future, we will conduct full-scale experiments on chloride ion diffusion in concrete, which can better analyze the three-dimensional diffusion of chloride ions without considering size effects.

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A) A set of concrete experimental designs (B) Test soaking device (C) Dry wet cycle method (D) Compound salt solution erosion F I G U R E 1 Concrete erosion test equipment (A) a set of concrete experimental designs (B) test soaking device (C) dry wet cycle method (D) compound salt solution erosion.F I G U R E 2 Actual sampling diagram.

F I G U R E 3
Change of chloride ion concentration with erosion depth in SWC test group (A) soak for 1 month (B) soak for 2 month (C) soak for 3 month.

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Change of chloride ion concentration with erosion depth in SW test group (A) soak for 1 month (B) soak for 2 month (C) soak for 3 month.

F I G U R E 5
Change of chloride ion concentration with erosion depth in Cl5 test group (A) soak for 1 month (B) soak for 2 month (C) soak for 3 month.

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Change of chloride ion concentration with erosion depth in Cl5S10 test group (A) soak for 1 month (B) soak for 2 month (C) soak for 3 month.