JPS6410458B2 - - 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, binding wires, anchor fasteners, spacers, and other hardware are used, the surfaces where these metals come into contact with carbon fiber-reinforced concrete may experience rapid corrosion that would be unthinkable with ordinary concrete. It 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 this carbon fiber reinforced concrete is cured after forming an insulating layer made of an inorganic material with an electrical resistance of at least 100Ω or more on the surface of this metal, the problem of metal corrosion peculiar to this carbon fiber reinforced concrete will be solved. It turns out that the above problem can be almost completely resolved to our advantage. The present invention provides an inorganic insulating layer between the contact surface of carbon fiber reinforced concrete and the metal to cut 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 an inorganic material with high insulation resistance is used to form this electrically insulating layer. There are various types of inorganic materials with high insulation resistance, and basically any inorganic material other than metals or carbon-based materials exhibiting insulation properties can be applied to the present invention. However, in actual construction, it depends on the relationship with the metal components used, such as reinforcement, rib arrangement, anchors, etc., which require strong attachment to concrete, and forms, spacers, etc., which do not require as much attachment strength. , it is necessary to consider the type and form of the inorganic material used. In order to form this inorganic insulating layer, if adhesion strength between the concrete and the metal member is not required, a sheet-like inorganic material layer may be wrapped or covered on the surface of the metal member, but in practice From a construction standpoint, it is also convenient to apply an insulating layer of a predetermined thickness to the metal surface before contacting it with the carbon fiber reinforced concrete. More specifically, it is most convenient to form an electrically insulating layer by applying cement mortar or cement paste to the metal surface and hardening it. You can get something. Furthermore, instead of this type of cement-based material, a glass-based material or a metal oxide-based material may be used. In this way, the present invention prevents local battery action due to contact with carbon fibers by interposing a highly insulating inorganic layer on the contact surface with a metal less base than carbon fibers. This insulating layer must have sufficient insulating properties to interrupt the flow of current generated by the local cell, ie, to break contact between the metal and the carbon fibers. The insulation resistance value varies depending on the type of inorganic material used, but
According to tests conducted by the present inventors, in the case of reinforced concrete in which carbon fibers are dispersed at 0.2 to 10% by volume in a cement matrix, if an insulating layer of at least 100 Ω or more is formed, contact between metal and carbon fibers is prevented. It turns out that it is possible to cut it off. 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 reinforcing bars, this oxide film layer may also function 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. Further, when the oxide film peels off and a portion where the metal surface is exposed is formed, a local battery is formed between the oxide film and the metal surface, which may lead to accelerated corrosion. 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, in carrying out the present invention, it is important to separately provide an inorganic insulating layer, and in the present invention, an insulating layer having an electrical resistance of at least 100Ω refers to an inorganic material alone excluding this oxide film.
This means that it is an insulating layer with an electrical resistance of 100Ω or more. To most conveniently carry out the invention, a cement mortar or cement paste having a cementitious composition similar to that of carbon fiber-reinforced concrete is applied to the metal surface and the applied layer is cured (this is a fully cured layer). (Although it may be in a semi-hardened state, there is virtually no problem with it), so it is preferable to bring it into contact with carbon fiber reinforced concrete that has not hardened yet. In this case, it is possible to enjoy the advantage that there is no problem of decrease in adhesion strength between the carbon fiber reinforced concrete and the metal, and the construction is easy. 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 fiber 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. According to the results of experiments conducted by the present inventors, 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 No matter what the carbon fiber 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 the galvanic cell will be completely eliminated. We confirmed that it can be blocked. 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 is free from the presence or absence of aggregate such as sand or gravel. 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, counter electrode: platinum) and potentiostat (using saturated KCl electrode as reference electrode, 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 (V vsSHE) = 0.99-0.06PH + 0.015log Po ₂ (V vsSCE) Substituting Po ₂ = 0.2atm and PH = 13.5, E
= 0.19 (V vs SCE) This value is considerably higher than the redox potential value 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. The above test results show 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 a noble metal 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,
Hot-dip galvanized steel, SUS304 stainless steel, and ordinary reinforcing bars coated with a cement mortar layer (approximately 2 mm film layer) and hardened were used.

[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 reinforcing concrete dispersed at ~10% by volume is cured on the contact surface with metal, an insulating layer made of an inorganic material with an electrical resistance of at least 100Ω or more is formed on the surface of the metal in advance and then cured. carbon fiber reinforced concrete. 2. The carbon fiber reinforced concrete according to claim 1, wherein the metal is iron or an iron alloy. 3. The carbon fiber reinforced concrete according to claim 1 or 2, wherein the insulating layer made of an inorganic material is a hardened or semi-hardened layer of cement mortar or cement paste.