JPS6410457B2 - - Google Patents

Description

[Detailed description of the invention]

The present invention relates to improvements in carbon fiber reinforced concrete. The inherent brittle nature of cementitious matrices can be significantly improved by dispersing them with appropriate amounts of suitable fibrous materials, such as carbon fibers. Thanks to the development of inexpensive pitch-based carbon fibers, carbon fiber-reinforced concrete is being put into practical use, creating new structures with strength, deformation, and elasticity properties that were not possible with conventional cement concrete. There are great expectations for it as a material. The present inventors have been involved in the development of this carbon fiber-reinforced concrete for many years, and have found that there are fundamental problems in construction using this material that are not found in ordinary concrete. That is, when metal is in contact with this carbon fiber reinforced concrete, metal corrosion (metal oxidation)
This is a phenomenon in which the process progresses markedly. For example, in this carbon fiber reinforced concrete construction,
When reinforcing bars, steel frames, steel formwork, tie wires, anchor fasteners, spacers, and other metals are used, the surfaces where these metals come into contact with carbon fiber-reinforced concrete can cause rapid damage that would be unimaginable with ordinary concrete. Corrosion progresses. The present invention aims to solve this problem. For this purpose, the present inventors have conducted intensive research to elucidate the corrosion behavior described above, and have found that various causes interact with each other, but the basic principle is that carbon fiber is extremely conductive. The potential is as noble as that of a noble metal, and when this carbon fiber comes into contact with a less noble metal (usually iron or iron alloy), a local battery is formed here, and this local It can be determined that battery action is the main cause of metal corrosion, and when reinforced concrete with carbon fibers dispersed at 0.2 to 10% by volume in a cementitious matrix is hardened at the contact surface with metal, If a resin layer is interposed on this contact surface and an insulating layer with an electrical resistance of at least 100Ω is formed between the carbon fiber reinforced concrete and the metal and then cured, metal corrosion specific to this carbon fiber reinforced concrete can be prevented. It turns out that the problem can be almost completely solved. The present invention interposes an organic resin layer between the contact surfaces of carbon fiber reinforced concrete and metal, thereby cutting off direct contact between the carbon fibers in the cement matrix and the metal. It is characterized by preventing the local battery effect caused by the carbon fibers formed on the surface. In other words, when a carbon fiber-reinforced concrete mixture that has not hardened yet is cured while in contact with reinforcing bars, steel frames, metal formwork, binding wires, anchor fasteners, spacers, and other metal objects, the surface of these metals baser than carbon An electrically insulating layer is formed so that the carbon fibers in the cement matrix do not come into direct contact with each other, and a resin layer with high insulation resistance is used to form this electrically insulating layer. There are various types of resins with high insulation resistance, and basically any resin that exhibits insulation can be applied to the present invention regardless of its type. However, in actual construction, the use depends on the metal components used, such as reinforcing bars, steel frames, anchors, etc., which require strong adhesion to concrete, and forms, spacers, etc., which do not require as much adhesion strength. It is necessary to consider the type and form of the resin used. In order to form this resin layer, if adhesion strength between the concrete and the metal member is not required, a sheet-like resin film molded product may be wrapped or covered over the surface of the metal member, but in practice In terms of construction, it is convenient to apply resin coating. More specifically, it is most effective to form this insulating layer by powder-coating an epoxy resin on the metal surface, and in this case, sufficient adhesion strength to concrete can be obtained. In this way, the present invention prevents local battery action due to contact with carbon fibers by interposing a highly insulating resin layer on the contact surface with a metal less base than carbon fibers. This insulating layer must be sufficiently insulative to interrupt the flow of current generated by the local cell, ie, sufficiently functional to break contact between the metal and the carbon fibers. The value varies depending on the type of resin, but according to the tests conducted by the present inventors, 0.2% of carbon fiber is added to the cement matrix.
It was found that in the case of reinforced concrete dispersed at ~10% by volume, it is sufficient to form an insulating layer of at least 100Ω. Although it is possible to block current flow to some extent and prevent corrosion even if the resistance is less than 100Ω, it is extremely difficult to uniformly form a very thin resin layer with a resistance of less than 100Ω, and the coating may partially break. A situation arises in which a defective part occurs and the carbon fibers come into direct contact to form a local battery. Normally, when applying epoxy resin coating to iron surfaces, the average
A film thickness of about 100 μm exhibits an electrical resistance of approximately 500Ω or more, and the thicker the film, the higher the insulation resistance. Therefore, for the most commonly used iron-based metal materials, the objective can almost be achieved even by applying a thin epoxy resin coating with an average thickness of about 100 μm. In actual construction, some oxide film (black scale) is formed on the surface of iron-based metal materials, although the amount and thickness are not uniform, and black scale is actively generated. Since it is common to use reinforced steel bars, this oxide film layer may also act as an insulating layer. However, this oxide film may peel off in places during transportation or construction, exposing the metal surface, and its thickness may vary, so this alone cannot be expected to completely prevent local battery formation. Furthermore, if the scale layer is too thick, it may cause a decrease in the adhesion strength between the concrete and the steel surface. Therefore, since the iron oxide layer formed on the iron-based surface cannot function as an insulating layer with a resistance of 100Ω or more, it is important to interpose a resin layer. An insulating layer of 100Ω or more means that the insulation resistance is that of the resin alone, excluding this oxide film. When applying epoxy resin coating, it is practically difficult to maintain the continuity of the coating unless it is coated with a film thickness of at least 100 μm on average;
By forming more than one resin layer, this resin layer alone can produce 100
It is desirable to have an electrical resistance of Ω or more. The magnitude of the current flowing from the carbon fiber to the iron, assuming that one of the carbon fibers uniformly dispersed in the cement matrix is in contact with the exposed surface of the iron at its tip in the presence of water. increases as the surface area of the carbon fibers that comes into contact with water increases. Therefore, the longer the fibers are, the larger the current will flow through them. However, when carbon fibers are dispersed in the same amount, corrosion may actually progress more quickly if short fibers are used. This may be due to the increased number of contact points where the fiber tip contacts the metal surface. The results of experiments conducted by the present inventors show that when carbon fibers are dispersed in a cement matrix in an amount of 0.2 to 10% by volume, which is the usual amount of carbon fibers for carbon fiber reinforced concrete, the length of the carbon fibers is However, no matter what type of material it is, if an insulating layer of at least 100Ω or more is formed between it and the ferrous metal surface, the contact between the metal and the carbon fiber will be cut off, and the corrosion current that forms a galvanic cell will be suppressed. We confirmed that it is possible to completely block out In addition, the carbon fiber reinforced concrete with an insulating layer interposed between it and the metal according to the present invention has carbon fibers dispersed in a cement matrix, and the presence or absence of aggregate such as sand or gravel is not required. size of quantity,
In addition, regardless of the presence or absence of various additives and admixtures, and regardless of their amount, and regardless of the type of cement, it exhibits the effect of preventing metal corrosion due to the formation of local batteries due to contact with carbon fibers. be. The content of the present invention will be explained in more detail below based on the test results. [Corrosion potential and polarization curve] Place a cement mortar in a wooden frame, insert a carbon fiber specimen, a steel specimen, a stainless steel mesh bar, or a couple of these into the mortar, and insert the redox potential measuring device. (using saturated KCl solution as reference electrode, counter electrode: platinum) and potentiostat (using saturated KCl solution, potential sweep speed)
The corrosion potential and polarization curve of each test material in cement were measured using 40 mV/min (counter electrode: platinum). The measurement results of each corrosion potential are shown in Table 1 on the next page.

[PH and redox potential of cement mixture]

PH of cement mixture using the compounding materials shown in Table 2
The redox potential was measured and the results shown in Table 3 were obtained.

【table】

[Table] As shown in Table 3, the PH value of the cement mixture is almost constant regardless of the blend, and is in the range of 13.4 to 13.7. In addition, the oxidation-reduction potential is − immediately after mortar is placed.
It is about 0.15 to -0.22V, but it shifts slightly to the negative side during steam curing. This means that the oxidizing nature of the environment decreases over time. If the redox potential of the environment is determined by the redox reaction of oxygen, the upper limit of that potential is determined by the redox equilibrium potential of oxygen, which is expressed by the following equation. E _p = 1.23-0.06PH + 0.015log Po ₂ (VvsSHE) = 0.99-0.06PH + 0.015log Po ₂ (VvsSCE) Substituting Po ₂ = 0.2atm and PH = 13.5, E = 0.19 (VvsSCE) This value is significantly higher than the redox potential values measured with Pt. Considering the high overpotential of the oxygen reduction reaction, the redox potential of the system is determined by the oxygen reduction reaction unless other effective oxidizing agents (e.g. Fe ³⁺ ) are present in the system. You can think of it as a thing. From the above test results, it was found that the presence of CF (carbon fiber) in the cement matrix has a negative effect on the corrosion of iron (steel) in contact with the cement matrix. This is thought to be due to galvanic corrosion due to contact between steel and CF, since CF has good electrical conductivity and exhibits a potential as noble as that of noble metals such as Pt. In other words, the presence of CF in the cement matrix increases the cathode area of the galvanic corrosion cell, forming what is called a small anode and a large cathode, thereby promoting corrosion. If this is electrochemically diagrammed, it will be as shown in Figure 3. In other words, initially, steel maintains its corrosion resistance at a potential of , but when the oxide film is locally destroyed by the presence of ions such as Cl ^-1 ,
The potential is transferred to and corrodes. On the other hand, the presence of CF increases the cathode reaction, which transfers the potential and accelerates corrosion. On the steel surface that is the anode of this galvanic cell, the pH decreases due to the reaction shown by the following equation, so a stable film cannot be maintained. This causes corrosion to grow. Fe ²⁺ +H ₂ O→Fe(OH) ⁺ +H ⁺ (PH decrease) However, in the case of contact between CF and stainless steel, the potentials of both are close and in the passive region, so there is no galvanic corrosion. Example Carbon fiber reinforced concrete 1 with the composition shown in Table 4
As shown in Fig. 4, various test reinforcing bars 2 were buried in the tank, and after being steam-cured at 40°C for 5 hours, they were heated at 180°C and 10 atm in an autoclave.
A 5-hour accelerated corrosion test was performed 1, 3, and 5 times.
This test reinforcing bar 2 is a normal reinforcing bar without black skin,
We used hot-dip galvanized steel, SUS304 stainless steel, and ordinary reinforcing steel coated with epoxy resin (approximately 200 μm thick).

[Table] After steam curing or after each autoclave treatment, each test reinforcing bar was taken out of the concrete and its corrosion status was examined. The results are shown in Table 5. However, A: No rust, B: Spotted rust, C: Several spots of rust, D: Partial red rust, and E: Red rust on 50% or more of the area.

[Table] A similar corrosion acceleration test was conducted using ordinary concrete with the composition shown in Table 4, except that carbon fiber was not added. It did not occur.

[Brief explanation of drawings]

Figure 1 shows the change in corrosion potential over time with and without the addition of carbon fiber to cement mortar, Figure 2 shows the polarization curve with and without the addition of carbon fiber to cement mortar, and Figure 3 shows the change in corrosion potential with and without the addition of carbon fiber to cement mortar. Fig. 4 is a diagram showing the dimensions and shape of the test specimen subjected to the accelerated corrosion test of reinforcing bars. 1... Carbon fiber reinforced concrete, 2... Test reinforcing bar.

Claims (1)

[Claims] 1. 0.2 carbon fibers in cement matrix
When the reinforcing concrete dispersed at ~10% by volume is cured on the contact surface with the metal, an insulating resin layer is interposed on the contact surface so that the electrical resistance between the reinforced concrete and the metal is at least 100
Carbon fiber reinforced concrete formed with an insulation layer of Ω or more and hardened. 2. The carbon fiber reinforced concrete according to claim 1, wherein the metal is iron or an iron alloy.