Prestressed concrete tank with thermal spray coating

Abstract

To provide a tank made of pre-stressed concrete that can lower permeation speed of a storage liquid and secure durability.SOLUTION: A tank made of pre-stressed concrete has crack resistant property by controlling a pre-stress introduction amount to 5-10 kgf / cm2. The tank made of pre-stressed concrete has a spray coating film formed therein by spray-coating a metal or resin covering material at a surface of the tank.SELECTED DRAWING: None

Description

Technical Field

[0001] The present invention relates to a prestressed concrete tank having a corrosion-resistant or penetration-resistant film formed by spraying a metal or resin resistant to corrosive substances or penetrating substances on the prestressed concrete tank.

Background Art

[0002] Oils such as fuel oil and crude oil, which are storage liquids in concrete tanks, penetrate into the concrete and cause the deterioration rate to increase. Therefore, it is common to apply a steel lining material (steel plate) or a coating by painting to the concrete surface to prevent the penetration of oil.

[0003] In order to suppress the water absorption expansion of reactive aggregates incorporated in concrete due to alkali-silica reaction, which is a factor of concrete deterioration, measures to suppress water intrusion by surface coating of concrete are common.

[0004] Also, concrete is strongly alkaline, and this alkali prevents the corrosion of steel materials. On the other hand, calcium hydroxide in the concrete reacts with carbon dioxide in the air to produce calcium carbonate, and the concrete loses its alkaline state and tends to become acidic, which is called neutralization. This corrodes steel materials such as reinforcing bars. On the surface of the reinforcing bars in a highly alkaline environment, oxygen is chemisorbed and a dense oxide layer is formed, thereby forming a passive film, and the reinforcing bars in the concrete are protected from corrosion. However, when chloride ions (Cl - ) above the allowable concentration exist in the concrete, the passive film on the surface of the reinforcing bars is destroyed and the reinforcing bars are corroded. Therefore, as a measure to suppress the intrusion of deterioration factors such as carbon dioxide that causes neutralization and salts that cause salt damage, a protective layer is formed by surface coating or the like.

[0005] Furthermore, while the water-cement ratio of concrete is often increased to improve fluidity and workability, this results in the creation of microscopic voids (porous areas). When such microscopic voids occur, in structures where a storage liquid such as oil is in direct contact with the inner surface of the concrete, the storage liquid penetrates into the concrete, accelerating the rate of concrete deterioration, reducing liquid tightness, and causing the storage liquid to leak.

[0006] In light of the above issues, measures to protect concrete from deterioration factors include coating the concrete surface with paint or covering it with metal materials.

[0007] When paint is used to cover a concrete surface, it can reduce the rate at which stored liquid penetrates into the concrete, thereby slowing down the rate of deterioration. However, the paint can leach out depending on the contents (especially oil). Furthermore, paint that cannot follow the distortion and cracking of the concrete will peel off, shortening the lifespan of the concrete.

[0008] Furthermore, as an example of using metal materials to cover concrete surfaces, there is a PS-type double-walled low-temperature tank in which a seal metal (steel lining material) about 3 mm thick is attached to the inner surface of the prestressed concrete outer tank. However, the installation of the seal metal is complicated, and there is a problem of increased construction costs.

[0009] Furthermore, fuel storage facilities have a structure where the outside of steel tanks is covered with concrete, which provides high liquid-tightness, but it has the problem of increasing construction costs.

[0010] Incidentally, a conventional technique for improving the durability and corrosion resistance of concrete is already known: applying a metal or other material to the concrete surface via thermal spraying. By applying a metal or other material thermal spray coating to the concrete surface, the penetration rate of the storage liquid into the concrete is significantly reduced. Patent documents 1 and 2 are known as conventional techniques for applying metal or resin thermal spraying to the concrete surface.

[0011] Patent Document 1 discloses a cathodic protection method for reinforced concrete structures, which involves a galvanic anode method in which the surface of the reinforced concrete structure is blast-surface-treated, and then thermal spraying is performed to form a thermal-sprayed metal layer consisting of an anode with a thickness ranging from 270 μm to 330 μm.

[0012] Patent Document 2 describes a method in which a glossy glass layer is uniformly fused to the required surface of a long prestressed concrete material reinforced by tensioning metal wires, thereby enabling the concrete material to maintain its original high strength without thermal degradation.

[0013] The prior art described in Patent Documents 1 and 2 does not relate to techniques for preventing damage or peeling of thermal spray coatings due to cracking during concrete service. [Prior art documents] [Patent Documents]

[0014] [Patent Document 1] Japanese Patent Publication No. 2009-263739 [Patent Document 2] Japanese Patent Application Laid-Open No. 63-100085 [Overview of the Initiative] [Problems that the invention aims to solve]

[0015] Reinforced concrete and unreinforced concrete are prone to cracking due to tensile loads, sunlight, ultraviolet rays, etc. Therefore, when thermal spraying is applied to the surface of reinforced concrete, there is a high risk that the thermal spray coating will be damaged and peel off in response to cracks in the surface caused by the aging deterioration of the reinforced concrete.

[0016] Furthermore, if paint is applied to the parts of the concrete surface that come into contact with the liquid, the paint film may dissolve or peel off due to the stored liquid. This allows the stored liquid, which is one of the factors that cause concrete deterioration, to penetrate into the concrete from the exposed surface, reducing the strength of the concrete itself. In addition, if paint is applied to the concrete surface, changes in ambient temperature can accelerate the peeling of the paint film. The exposed concrete areas are directly exposed, accelerating localized deterioration. [Means for solving the problem]

[0017] This invention relates to a prestress introduction rate of 5-10 kgf / cm². 2 The present invention provides a prestressed concrete tank that is controlled to have crack resistance, wherein a metal or resin coating material is thermally sprayed onto the surface of the tank to form a thermal spray coating.

[0018] Furthermore, the present invention provides a prestressed concrete tank for storing oil, wherein a thermal spray coating of relatively low-melting-point aluminum, zinc, or lead is formed on the inner surface of the tank by thermal spraying.

[0019] Furthermore, the present invention provides a prestressed concrete tank for storing liquid ammonia, wherein a thermal spray coating of rigid polyvinyl chloride resin is formed on the inner surface of the tank by thermal spraying.

[0020] The present invention provides a prestressed concrete tank for use in coastal areas, in which a thermal spray coating of seawater-resistant aluminum bronze or zinc-aluminum alloy is formed on the surface of the tank.

[0021] Furthermore, the present invention provides a prestressed concrete tank in which different coating materials are thermally sprayed onto the wetted surface of the inner surface of the tank that is in contact with the stored liquid and the gas phase portion that is exposed to the atmosphere, wherein a coating material having corrosion resistance to the stored liquid is thermally sprayed onto the wetted surface and a coating material having seawater resistance is thermally sprayed onto the gas phase portion. [Effects of the Invention]

[0022] The prestressed concrete tank sprayed with the coating material of the present invention combines the crack resistance characteristics of prestressed concrete with the penetration resistance and durability improvement characteristics of the sprayed film, thereby suppressing the damage and peeling of the sprayed film due to the cracking of the concrete, reducing the penetration rate of the stored liquid into the prestressed concrete tank, and ensuring durability.

Embodiments for Carrying Out the Invention

[0023] In addition to the fact that the tensile strength of concrete is significantly smaller, about one-tenth of the compressive strength, tensile stress is generated not only by loads such as drying shrinkage and changes in outside air temperature, and cracking is likely to occur. Prestressed concrete (hereinafter referred to as PC) tank structures for storing water and sewage or low-temperature liquefied gas, in addition to the tensile stress generated by the action of the load, control the amount of prestress introduced to be 5 to 10 kgf / cm 2 to improve the crack resistance characteristics. Prestress is generally applied in a horizontal direction parallel to the bottom surface (bottom plate) and a vertical direction perpendicular to the bottom surface, but it is not limited thereto. If it is less than 5 kgf / cm 2 , the effect of preventing cracking is small. If it is 10 kgf / cm 2 , for example, it has been found that it is sufficient even if the tank contains stored substances such as water or oil, and it can be increased, but increasing it does not improve the expected effect, while the economy is inferior. Since the PC tank has crack resistance characteristics compared to other reinforced concrete, etc., even if spraying is performed, the sprayed layer is difficult to peel off.

[0024] A coating material is sprayed onto the surface of this PC tank to form a protective film. The coating material sprayed onto the tank surface can be a metal or resin with a relatively low melting point that has excellent corrosion resistance in the alkaline environment of concrete. In this application, a relatively low melting point means a melting point of 1100 degrees Celsius or less. If a metal with a relatively low melting point is used, the thickness of the sprayed film can be increased, and sufficient protection for the stored contents can be obtained because the concrete surface is porous. In addition, the surface temperature will not rise to a level that would cause thermal deterioration of the concrete substrate.

[0025] Furthermore, such metal or resin coatings are preferably made of materials that have corrosion resistance to deterioration factors affecting concrete, such as storage liquids. In addition, it is preferable that the coating material is capable of suppressing the penetration of deterioration factors into the concrete and can accommodate expansion and contraction due to ambient temperature changes.

[0026] Deterioration factors are those that penetrate into the concrete and accelerate its deterioration rate. For example, in the case of a precast concrete (PC) tank that stores oil and where the oil comes into contact with the concrete surface, the oil becomes a deterioration factor. Seawater splashes and rainwater that come into contact with the concrete surface also become deterioration factors.

[0027] Furthermore, a higher water-cement ratio in concrete results in a larger unit water content, higher fluidity, and improved workability, but it also lowers the compressive strength after hardening. Therefore, the upper limit for the water-cement ratio in civil engineering works is set at 55% for reinforced concrete and 60% for unreinforced concrete. In PC tanks, to increase fluidity during construction, admixtures that generate fine air bubbles (such as surfactants) or admixtures with fine particles (fly ash, silica fume, blast furnace slag fine powder) are added. This creates a ball-bearing effect, ensuring fluidity while controlling the water-cement ratio to 45% or less. By keeping it below 45%, the carbonation depth of the PC tank is reduced, extending its service life to an extremely long period (approximately 200 years). After concrete hardening, the voids (porous areas) created by the drying of water in the concrete are reduced, resulting in denser concrete and suppressing carbonation. In this way, the reduction of fine air bubbles and voids on the concrete surface improves the adhesion of the covering material, prevents peeling, and extends the lifespan of the PC tank.

[0028] Admixtures used in small quantities, whose volume is not included in the final volume of the concrete, etc., can be used to improve the fluidity of the concrete and suppress carbonation. These admixtures include air-entraining agents, water-reducing agents, and superplasticizers. Admixtures used in relatively large quantities, whose volume is included in the final volume of the concrete, etc., can include blast furnace slag powder with latent hydraulic properties and expansive agents that compensate for a portion of the drying shrinkage after hardening.

[0029] The following describes coating materials for thermal spraying onto PC tanks used for oil storage. For example, if the liquid stored in a PC tank is oil, the metal sprayed onto the wetted surface of the tank can be low-melting-point aluminum (melting point approximately 660°C), zinc (melting point approximately 420°C), or lead (melting point approximately 328°C), which have excellent corrosion resistance to oil. As for aluminum, in addition to pure aluminum (purity of 99% or more), there is also aluminum-silicon, which is mixed with 12% silicon. If the Si content is 18% or less, the melting point is lower than that of pure aluminum. As for zinc, in addition to pure zinc, zinc-aluminum alloy (melting point 440°C), which is mixed with 15% aluminum, can also be used. Here, "oil" refers to all industrial oils, including crude oil, petroleum, and heavy oil.

[0030] In this way, by covering the wetted surfaces of the PC tank with a metal that has corrosion resistance to the stored liquid, the PC does not come into direct contact with the stored liquid, the thermal spray coating does not dissolve, liquid tightness is improved, the rate at which the stored liquid penetrates into the PC can be slowed, and a decrease in the strength and cracking of the PC can be suppressed.

[0031] In the case of PC tanks located in coastal areas, there are concerns about seawater spray, such as splashes and the introduction of atmospheric seawater spray into the tank through ventilation devices. Therefore, aluminum bronze (melting point approximately 1050°C), an alloy of copper and aluminum with excellent seawater resistance, can be used as the metal sprayed onto the tank surface. As for aluminum bronze, it is preferable to use, for example, special aluminum bronze (ARMS Bronze®), which has significantly improved seawater resistance. Special aluminum bronze is an alloy in which small amounts of iron, nickel, and manganese are added to a copper alloy containing 5-12% aluminum. In addition to aluminum bronze, aluminum-zinc alloy can also be used. The surface of the zinc-aluminum alloy spray coating is harder than the paint film, so it has wear resistance and exhibits excellent corrosion resistance, especially in coastal environments. As for this aluminum-zinc alloy, for example, zinc-aluminum alloy wire for thermal spraying sold as Zn85Al15 can be used. The product consists of 85% by weight zinc and 15% by weight aluminum, and is corrosion-resistant to sodium chloride (salt) and sulfur dioxide in marine environments. It is suitable for arc and flame spraying processes.

[0032] By covering the surface of PC tanks with a thermal spray coating made of seawater-resistant metal, deterioration of the tanks in corrosive environments containing salt can be suppressed, preventing a decrease in PC strength and cracking. This is also effective for PC structures in snowy and cold regions where de-icing and de-icing agents are sprayed.

[0033] Thermal spray materials come in two forms: wire and powder. For thermal spraying, aluminum wire drawn from special grade 2 aluminum ingots (Al: 99.85 mass% or higher) can be used. For thermal spraying, zinc wire can be drawn from the purest zinc ingot (Zn 99.995 mass% or higher, Fe 0.001 mass% or lower for corrosion protection) to become welding wire (Zn 99.9% mass% or higher, Cu 0.05 mass% or lower). Lead powder material with a Pb content of 99.5 mass% or higher can be used for thermal spraying. For thermal spraying, zinc-aluminum alloy wires can be used, which are alloyed using zinc of the highest purity or higher, and aluminum of special grade 2 or higher, with a zinc-to-aluminum mass ratio between 95:5 and 70:30, and then drawn for thermal spraying. In particular, alloy wires with a zinc-to-aluminum ratio of 85:15 exhibit optimal corrosion resistance against salt and marine environments. Furthermore, aluminum alloy wire containing 3% Ti also exhibits excellent corrosion resistance in coastal areas.

[0034] Depending on the corrosive environment of each part of the PC tank, different coating materials can be sprayed. For example, different metals can be used to coat the wetted surfaces of the tank and the gas phase areas exposed to the outside air. For instance, if the stored liquid in a PC tank is oil, an oil-resistant aluminum coating can be used on the wetted surfaces, while seawater-resistant aluminum bronze can be used on the gas phase areas.

[0035] For surface preparation before thermal spraying, a primer containing silica sand as fine powder mixed with a specially modified epoxy resin can be used to prevent the influence of moisture, water, and alkali from the substrate, improve the adhesion of the thermal spray coating to the substrate, and ensure the required film thickness.

[0036] The appropriate thermal spraying method will be selected considering the type of coating material and ease of application. Thermal spraying can be performed after removing the laitance from the surface of the PC tank and applying the primer mentioned above. Flame spraying, arc spraying, and cold spraying can be used as thermal spraying methods.

[0037] Flame spraying using a fused wire is used for spraying metals and alloys such as aluminum and zinc. In flame spraying using a fused wire, the spray material is continuously fed from the central hole of the spray gun and melted by a flame of oxygen-fuel gas mixture. The molten spray material is then atomized into droplets by a jet of compressed air from the surroundings and sprayed onto the surface of a PC tank to form a coating.

[0038] Arc spraying is widely used for metal spraying of aluminum, zinc, and other materials. Because the material is melted by an arc, it allows for high-volume and high-efficiency spraying, and is used to form sprayed coatings on large PC tanks. Arc spraying, also known as wire arc spraying, is a spraying method in which an arc is generated between two metal wires, the heat of the arc melts the wires, and while feeding the wires according to the melting rate, gas injection such as compressed air is used to atomize the molten material, which is then sprayed onto the surface of the PC tank to form a coating.

[0039] Cold spraying is used for thermal spraying of lead. This method involves supersonicing a gas at a temperature lower than the melting point or softening temperature of the spray material, introducing powdered spray material into the flow, accelerating it, and causing it to collide with the surface of a PC tank while still in a solid phase to form a coating.

[0040] The film thickness of the coating material is adjusted and controlled so that there are no voids (porous areas) on the surface of the coating material. The thickness of the metal thermal spray coating is preferably around 100 to 500 μm, and more preferably 120 to 250 μm. By applying a metal thermal spray coating to a PC tank, it is possible to maintain penetration resistance and durability for at least the following number of years. For example, if there is a lot of moisture inside the tank and the thickness of the aluminum or zinc thermal spray coating is 150 μm, the service life of the thermal spray coating (the number of years until the first repair is required) is known to be very long, more than 20 years. Also, if the PC tank is located in an area where seawater is scattered or where salt is frequently present, and the thickness of the zinc thermal spray coating is 250 μm, the service life of the thermal spray coating is known to be 10 to 20 years. Furthermore, because PC tanks possess crack-resistant properties, unlike when thermal spraying is applied to other types of concrete, the lifespan of the metal thermal spray coating itself can be reliably maintained without having to consider damage or peeling of the metal thermal spray coating due to cracking caused by the aging deterioration of the concrete.

[0041] When the stored liquid in a PC tank is liquid ammonia, the coating material sprayed onto the wetted surface of the tank can be rigid polyvinyl chloride resin (melting point approximately 85-210°C), which has a lower melting point than metal and excellent corrosion resistance to liquid ammonia. Rigid polyvinyl chloride can be sprayed using the flame spraying method, where polyvinyl chloride resin powder is heated and melted with a flame and sprayed onto the surface of the PC tank to form a film of 500 μm or more. The thickness of the sprayed film can range from 0.5 mm to 7.0 mm. Other resin materials can also be used. In the case of water supply, ultraviolet light is irradiated as a measure against Cryptosporidium, so for tanks used for tap water, in addition to weather resistance to temperature changes, materials that do not deteriorate due to ultraviolet light are preferable.

[0042] If the stored material is a gas such as propane, ethylene, or methane, metal spraying can be used.

[0043] This invention combines the crack-resistant properties of PC with the penetration-resistant and durability-enhancing properties of thermal spray coatings, thereby suppressing damage and peeling of the thermal spray coating associated with concrete cracking, reducing the penetration rate of stored liquid into the PC tank, and ensuring durability.

[0044] Furthermore, the present invention has the following advantages compared to the prior art: it can suppress deterioration over time due to carbonation, salt damage, and ASR (alkali-aggregate reaction) of concrete structures, which were previously a concern. Compared to conventional PC tanks coated with paint, the risk of peeling and damage to the thermal spray coating is reduced, increasing the service life of PC tanks and enabling long-term use. In addition, compared to thermal spraying on ordinary reinforced concrete, the thickness of the coating material required to resist cracking of the substrate can be reduced, making construction and management easier. Compared to conventional coating with steel lining material, construction costs can be significantly reduced. [Industrial applicability]

[0045] By replacing the seal metal (steel lining material) on the inner surface of the PC outer tank of a conventional PS-type double-walled cryogenic tank with a thermal spray coating made of metal or other coating material, construction time and costs can be reduced. Furthermore, replacing the conventional structure of covering the outside of a steel tank with PC with a structure that covers the inside of the PC tank with a coating of metal or other material can reduce construction time and costs. Furthermore, by applying it to seawater and wastewater treatment facilities, etc., corrosion resistance and permeability can be improved.

Claims

1. A prestressed concrete tank having crack resistance, reduced voids in the concrete after hardening, and reduced carbonation depth, achieved by adding an admixture that generates fine air bubbles or an admixture that consists of fine particles, thereby controlling the water-cement ratio to 45% or less, and thereby reducing the voids in the concrete after hardening. The tank is formed by thermal spraying a metal or resin coating onto the surface of the tank, which is based on a primer made of epoxy resin mixed with silica sand, to create a tightly adhering thermal spray coating.

2. A prestressed concrete tank for storing oil, wherein a thermal spray coating of aluminum, zinc, or lead is formed by thermal spraying aluminum, zinc, or lead to cover the liquid-contacting surface of the tank, wherein the aluminum is pure aluminum with a purity of 99% or more and a melting point of 660°C, or aluminum silicon mixed with silicon to have a lower melting point than pure aluminum; the zinc is pure zinc with a melting point of 420°C, or a zinc-aluminum alloy mixed with aluminum to have a lower melting point than pure aluminum; and the lead is pure lead with a melting point of 328°C, as described in claim 1.

3. A prestressed concrete tank for storing liquid ammonia, wherein a thermal spray coating of rigid polyvinyl chloride resin is formed by thermal spraying the wetted surface of the tank, wherein a flame spraying method is used to heat and melt polyvinyl chloride resin powder with a flame and spray it, forming a coating of 500 μm or more on the wetted surface of the prestressed concrete tank.

4. A tank made of prestressed concrete in which seawater splashes come into contact with the surface of the tank, wherein a thermal spray coating is formed on the surface of the tank by thermal spraying of seawater-resistant aluminum bronze, zinc-aluminum alloy, or Ti-aluminum alloy, wherein the aluminum bronze is an alloy of a copper alloy containing 5 to 12% aluminum with small amounts of iron, nickel, and manganese added, the zinc-aluminum alloy consists of 85% by weight zinc and 15% by weight aluminum, and the Ti-aluminum alloy contains 3% Ti in aluminum, as described in any one of Claims 1 to 3.

5. A tank for storing oil in which seawater splashes come into contact with the inner surface of prestressed concrete, wherein different coating materials are sprayed onto the liquid-contacting surface of the inner surface of the tank that comes into contact with the stored liquid and the gas phase portion that is exposed to the atmosphere, wherein an aluminum coating material having corrosion resistance to the oil is sprayed onto the liquid-contacting surface and an aluminum bronze coating material having seawater resistance is sprayed onto the gas phase portion, as described in Claim 1.