Concrete evaluation method and concrete specimen used therefor

Abstract

To provide a concrete specimen and an evaluation method for further accurately measuring the properties of concrete containing an expansive material.SOLUTION: The concrete specimen is composed of constrained reinforcing bars and concrete formed of a concrete composition. The constrained reinforcing bars are provided with a tie bar formed in the shape of a coil spring and a plurality of main reinforcing bars having straight portions and hook-shaped folded portions at both ends of the straight portions, the straight portions being arranged in parallel with the central axis of the coil spring and the folded portions being arranged to face the inside of the coil spring, and the straight portions and the folded portions are joined. The constrained reinforcing bars are enclosed by the concrete. A concrete evaluation method uses the concrete specimen. This concrete specimen is suitable for evaluating the water permeability of concrete.SELECTED DRAWING: Figure 1

Description

[Technical Field]

[0001] The present invention relates to a method for evaluating concrete and a concrete specimen used therefor. [Background technology]

[0002] In order to improve the durability of concrete structures, it is effective to prevent cracking in the concrete. In recent years, in the fields of civil engineering and construction, the use of expansive concrete additives in concrete compositions has been increasing in order to prevent drying shrinkage cracking.

[0003] When such expansive concrete is formed using such expansive additives, it is usually internally placed with restraining rebars (e.g., steel bars). Therefore, when evaluating expansive concrete, it is desirable to evaluate it under restrained conditions. A widely known method for evaluating expansive concrete with such restraining rebars is the restrained expansion test method for expansive concrete under uniaxial restraint conditions (Method A), specified in Appendix B (Reference) of JIS A 6202. This test method involves restraining the concrete specimen with steel plates at both ends. However, in practice, expansive concrete is generally internally restrained by internally placed rebars, and it is rare for the concrete to be used under complete restraint conditions, such as when both ends are fixed with steel plates, as specified in the JIS standard. Therefore, such a test method is difficult to call a realistic test method and may not be appropriate for evaluation. For this reason, a method for evaluating expansive concrete using internal restraining rebars, which simulates the actual usage conditions of expansive concrete, has also been proposed (Patent Document 1).

[0004] On the other hand, it has been reported that expansive concrete with internal restraining rebars consolidates the concrete and forms a hardened body with high watertightness. However, methods for evaluating the watertightness of expansive concrete have not yet been standardized, and the evaluation methods used in each report are not completely consistent, making it difficult to fully compare them, and sufficient knowledge is currently lacking. Several methods for evaluating expansive concrete with restraining rebars have been proposed. For example, Non-Patent Document 1 presents a case study examining the water permeability of expansive concrete. This water permeability test involves preparing a cylindrical specimen with a hollow space, passing water through the hollow space, and evaluating the water permeability from the inside to the outside of the concrete specimen. However, because the test is performed on a specimen with a special shape, it is difficult to use the results obtained by this method to evaluate the watertightness of general expansive concrete. [Prior art documents] [Patent documents]

[0005] [Patent Document 1] JP 2015-108513 A [Non-patent literature]

[0006] [Non-Patent Document 1] "Watertightness of Expansive Concrete", Proceedings of the Symposium on Cracks in Concrete Structures, Japan Concrete Institute, 1977, pp. 177-180 Summary of the Invention [Problem to be solved by the invention]

[0007] The present invention provides a method for better evaluation of concrete under restraint. [Means for solving the problem]

[0008] A method for evaluating concrete formed from a concrete composition according to the present invention includes: A hoop formed in the shape of a coil spring, A plurality of main reinforcements each having a straight portion and a folded portion formed into a hook shape at both ends of the straight portion; a restraining rebar having a structure in which the straight portion is arranged parallel to the central axis of the coil spring and the folded portion is arranged toward the inside of the coil spring and joined together; A solid concrete specimen is prepared by incorporating the restraining rebar using the concrete composition; and evaluating the concrete using the concrete specimen.

[0009] Further, a method for evaluating the water permeability of concrete formed from a concrete composition according to the present invention includes the steps of: A hoop formed in the shape of a coil spring, A plurality of main reinforcements each having a straight portion and a folded portion formed into a hook shape at both ends of the straight portion; a restraining rebar having a structure in which the straight portion is arranged parallel to the central axis of the coil spring and the folded portion is arranged toward the inside of the coil spring and joined together; A solid concrete specimen is prepared using the concrete composition, and the confining rebar is embedded therein; After curing and forcibly drying the concrete specimen, The method comprises allowing water to permeate from the bottom surface of the concrete specimen.

[0010] Further, a concrete specimen according to the present invention is used in a method for evaluating concrete, The concrete structure is composed of a confining reinforcing bar and concrete formed from a concrete composition, The restraining steel bar is A hoop formed in the shape of a coil spring, A plurality of main reinforcements each having a straight portion and a folded portion formed into a hook shape at both ends of the straight portion; However, the straight portion is parallel to the central axis of the coil spring, and the folded portion is disposed toward the inside of the coil spring and joined together, The restraining steel bars are solid and enclosed within the concrete. [Effects of the Invention]

[0011] According to the present invention, it is possible to more appropriately evaluate properties of concrete under restraint, such as watertightness, frost resistance, carbonation resistance, performance in protecting steel, chloride penetration resistance, alkali-aggregate reaction resistance, crack resistance, fire resistance, abrasion resistance, strength, durability, weather resistance, expansion resistance, chemical resistance, seawater resistance, and shrinkage. [Brief explanation of the drawings]

[0012] [Figure 1] 1 is a perspective view of a restraining bar according to one aspect of the present invention. FIG. [Figure 2] FIG. 2 is a side view of a restraining bar according to one aspect of the present invention. [Figure 3] 1 is a schematic diagram illustrating a concrete specimen according to one embodiment of the present invention. DETAILED DESCRIPTION OF THE INVENTION

[0013] Hereinafter, embodiments of the present invention will be described in detail.

[0014] <Concrete specimen> The concrete specimen according to the present invention includes a confining rebar having a specific shape. Fig. 1 is a perspective view of one embodiment of the confining rebar that can be used in the present invention, and Fig. 2 is a side view. The confining rebar 10 is A hoop 11 formed in a coil spring shape; A plurality of main reinforcements 12 each having a straight portion and a folded portion formed into a hook shape at both ends of the straight portion; It consists of:

[0015] The hoop has a coil spring shape. A coil spring shape refers to a shape similar to a compression coil spring. Specifically, the reinforcing bar is spirally wound around a central axis, with the ends forming ring-shaped upper and lower surfaces. This configuration is preferable because it can be realized by molding a single reinforcing bar. It may also be formed by connecting two rings made of reinforcing bar with a reinforcing bar shaped like a spiral. This type of configuration is preferable because it provides good followability to deformation when concrete is formed from the concrete composition, and the spring effect more effectively restrains expansion in the vertical direction.

[0016] The main reinforcement has a straight portion and folded portions at both ends. The folded portions are formed into a hook shape, and these folded portions restrain the expansion of the concrete in the vertical direction. The length of the straight portion of the main reinforcement is preferably longer than the length in the central axis direction of the coil spring shape formed by the hoops. The shape of the folded portions is not particularly limited as long as it restrains the expansion of the concrete, but for example, a semicircular or hook-shaped shape can be selected. For example, if the folded portion is semicircular, its radius is preferably 10 to 50 mm. Furthermore, it is preferable that the main reinforcement, including the folded portions, is formed in a plane, i.e., not twisted. This shape is preferable because it improves the ability to follow deformations when the concrete is formed.

[0017] The main reinforcement is joined so that its straight portion is parallel to the central axis of the coil spring shape formed by the hoops, and its folded portion faces the inside of the coil spring. The main reinforcement is preferably placed inside the coil spring shape, and its straight portion is preferably joined to the inside of the hoops. The hoops and main reinforcement can be joined by any method. Specific examples include fixing with fasteners, restraining with iron wire, or welding.

[0018] The restraining rebars have multiple main bars. The number of main bars attached to each restraining rebar is preferably 2 to 8, and more preferably 3 to 6. Furthermore, when a concrete specimen is prepared using these restraining rebars, the reinforcing steel ratio is preferably 0.5 to 1%.

[0019] Examples of the reinforcing bars that make up the restraining reinforcing bars used in the present invention include, but are not limited to, round steel reinforcing bars, deformed steel bars, and stainless steel reinforcing bars. The reinforcing bars can be made of any known material, such as SR235, SD295A, or SD345. The thickness of the steel can be selected as appropriate within a range that does not impede the functionality of the present invention, and examples include round steel bars with a diameter of 6 mm.

[0020] The size of the restraining steel bars can also be determined arbitrarily depending on the evaluation method, etc., but it is preferable that the length of the hoop in the central axis direction is 7 to 11 cm and the diameter of the ring-shaped bottom surface of the hoop is 8 to 12 cm, for example.

[0021] Fig. 3 is a schematic diagram of a concrete specimen 20 according to the present invention. Generally, concrete specimens are formed from a concrete composition, but the concrete specimen 20 according to the present invention is solid, with the above-described confining rebar 10 contained within concrete 21 formed from the concrete composition. The size and shape of the concrete specimen can be freely set according to the test to be evaluated.

[0022] The restraining rebar is preferably placed approximately in the center of the concrete specimen. This is because the restraining force is applied evenly throughout the concrete specimen. To adjust the position of the restraining rebar in the concrete specimen, it is preferable to fix it to the formwork with wire or binding wire. There are no particular restrictions on the wire or binding wire, as long as it can fix the restraining rebar to approximately the center of the concrete specimen.

[0023] A concrete specimen according to the present invention is manufactured, for example, by the following procedure. First, a formwork for a concrete specimen, a confining rebar, and a concrete composition are prepared. The concrete composition is prepared by blending cement, aggregate, water, admixtures, and chemical admixtures, and thoroughly mixing them in a concrete mixer or the like. When evaluating the properties of expansive concrete, an expansive additive for concrete specified in JIS A 6202 is used as the admixture. Next, the confining rebar is placed in the formwork, and the concrete composition is poured into the formwork and hardened while the confining rebar is enclosed by the concrete composition, thereby manufacturing a concrete specimen. The concrete composition is preferably filled into the formwork while applying vibration using a table vibrator or the like. The cover of the rebar is preferably 1 to 3 cm, for example, approximately 2 cm. When filling the concrete composition, it is preferable that there is a gap between the confining rebar and the formwork to allow the aggregate to enter. By setting the cover to such a size, uneven distribution of the aggregate in the concrete specimen can be prevented.

[0024] After the concrete specimen has been sufficiently hardened, it is removed from the formwork and, if necessary, cured for a predetermined period before use in evaluation. In order to prevent moisture from escaping from the concrete specimen after removal from the formwork, a moisture-impermeable sheet such as a polyester sheet may be placed inside the formwork in advance, and the specimen may be covered with the sheet immediately after preparation.

[0025] <Concrete evaluation method> The concrete evaluation method of the present invention comprises preparing a concrete specimen having a specific shape and using it to evaluate concrete. Here, "evaluating concrete" includes evaluating properties of concrete, such as watertightness, frost resistance, carbonation resistance, steel protection performance, chloride intrusion resistance, alkali-aggregate reaction resistance, crack resistance, watertightness, fire resistance, abrasion resistance, strength, durability, weather resistance, expansion resistance, chemical resistance, seawater resistance, and shrinkage. Since the concrete specimen of the present invention contains restraining rebar, the method is particularly suitable for evaluating expansive concrete in a restrained state.

[0026] The method for evaluating the performance and characteristics is not particularly limited except for using the concrete specimens described above. For example, when evaluation is performed for quality assurance of concrete raw materials, quality control can be performed by evaluating the concrete specimens described above using an original standardized method. However, in order to obtain more versatile evaluation results, it is preferable to combine the concrete specimens of the present invention with an evaluation method conforming to JIS standards or the like.

[0027] <Method for evaluating the water permeability of concrete> The concrete specimen of the present invention can be used to evaluate the water permeability of concrete. Specifically, the evaluation is performed in accordance with JIS A 1404 "Testing methods for cement waterproofing agents for construction." This will be explained in detail below. 1) Prepare a concrete specimen (φ15cm x 12cm) containing restraining steel bars. 2) The concrete specimens that have been cured to the specified age are forced to dry (in a dryer at approximately 80°C for 7 days) until they reach a constant mass. The mass (m0) of the specimens after drying is measured. 3) Place the concrete specimen in the permeability test device. Attach rubber gaskets with a thickness of at least 1 cm, each with a 5 cm diameter circular permeable hole in the center, to the top and bottom of the specimen, then place the gaskets in the permeability test device and tighten them evenly. The permeability test device used in JIS A 1404 "Testing methods for cement waterproofing agents for construction" can be used as the test device. 4) Water pressure (3.0 × 10 5 Pa) and maintain for a specified time. 5) After the test is completed, remove the specimen from the test device, quickly wipe off excess water on the surface of the specimen, and immediately measure the mass (m t :t is the time the water pressure is applied). 6) The permeability (p) of the specimen is calculated using the following formula. p=m t -m0 7) The permeability of concrete is evaluated based on the relationship between the time water pressure is applied and the amount of water permeable.

[0028] The present invention will be described below with reference to various examples, but the present invention is not limited to the following examples. [Example]

[0029] 1. Preparation of concrete specimens The concrete composition used to prepare the concrete specimens was prepared using the materials listed in Table 1. The concrete composition (EX-RC) listed in Table 2 was also prepared using these materials. The expansive additive was used as a replacement for fine aggregate, and the standard addition amount was 20 kg / m. 3 For comparison, a plain concrete composition (PL) without any expansive additive was also prepared, as shown in Table 2. The fresh properties of each concrete composition were set to a target slump of 12±2.5 cm and a target air content of 4.5±1.5%.

[0030] [Table 1]

[0031] [Table 2] *s / a is the fine aggregate ratio

[0032] A formwork was prepared with restraining rebar (φ10cm x 9cm; rebar diameter 6mm) installed inside, as shown in Figure 1. A concrete composition (EX-RC) mixed for 2 minutes with a concrete mixer was filled into the formwork while vibrating it with a table vibrator, and then allowed to harden to prepare a test specimen. The test specimen size was φ15cm, height 12cm, and the cover of the restraining rebar was set to 2cm. A comparative concrete composition (PL) was also filled into a formwork without restraining rebar and allowed to harden to prepare a comparative test specimen.

[0033] 2. Test Results 2-1 Permeability test The concrete specimens were removed from the formwork when they were 2 days old, cured underwater until they were 7 days old, and then sealed and cured for 14 days. After curing, the concrete specimens were forced to dry in a dryer at approximately 80°C for 7 days. The mass (m0) of the specimens after forced drying was measured and they were used for a watertightness evaluation test (permeability test). Rubber gaskets with a thickness of at least 1 cm and a circular permeable hole with a diameter of 5 cm in the center were attached to the top and bottom of the concrete specimens, which were then placed in a permeability test device and tightened uniformly. A water pressure (3.0 x 10) was applied to the underside of the specimens. 5 After 1 hour, the specimen was removed from the test device and the mass (m 1h The permeability of the specimen (p:m 1h -m0) was calculated. Furthermore, tests were conducted in which water pressure was applied for a specified period of time (from 3 hours to 7 days). The watertightness of the concrete was evaluated based on the relationship between the time water pressure was applied and the amount of water permeation. The results are shown in Table 3. The amount of water permeation of the expansive concrete (EX-RC) was large up to one day after the start of the water permeation test, but the increase in water permeation thereafter was slight, and the amount of water permeation from three days onwards was smaller than that of plain concrete. The difference in water permeation is thought to be due to the fact that the expansion of the concrete is suppressed by the restraining steel bars, and it is thought that the concrete evaluation method of the present invention will provide more reasonable results. [Table 3]

[0034] 2-2 Observation of permeable areas After conducting a seven-day permeability test, the concrete specimens were split and a developer that reacts with water to turn pink was applied, and the permeable areas were observed. The permeable areas were confirmed by the color that was generated by the developer, and a permeability test was conducted using restraining rebar. It was found that the expansive concrete (EX-RC) had less permeable areas than the plain concrete (PL). [Explanation of symbols]

[0035] 10 Reinforcement bars 11 Stirrup 12 Main reinforcement 20 Concrete specimen 21 Concrete

Claims

1. 1. A method for evaluating concrete formed from a concrete composition, comprising: A hoop formed in the shape of a coil spring, A plurality of main reinforcements each having a straight portion and a folded portion formed into a hook shape at both ends of the straight portion; a restraining rebar having a structure in which the straight portion is arranged parallel to the central axis of the coil spring and the folded portion is arranged toward the inside of the coil spring and joined together; A solid concrete specimen is prepared using the concrete composition to contain the restraining rebar; evaluating the concrete using the concrete specimen.

2. 1. A method for evaluating the permeability of concrete formed from a concrete composition, comprising: A hoop formed in the shape of a coil spring, A plurality of main reinforcements each having a straight portion and a folded portion formed into a hook shape at both ends of the straight portion; a restraining rebar having a structure in which the straight portion is arranged parallel to the central axis of the coil spring and the folded portion is arranged toward the inside of the coil spring and joined together; A solid concrete specimen is prepared using the concrete composition, and the confining rebar is embedded therein; After curing and forcibly drying the concrete specimen, The method comprises permeating water from the bottom surface of the concrete specimen.

3. A concrete specimen used in a method for evaluating concrete, comprising: The concrete structure is composed of a confining reinforcing bar and concrete formed from a concrete composition, The restraining steel bar is A hoop formed in the shape of a coil spring, A plurality of main reinforcements each having a straight portion and a folded portion formed into a hook shape at both ends of the straight portion; However, the straight portion is parallel to the central axis of the coil spring, and the folded portion is disposed toward the inside of the coil spring and joined together, A concrete specimen in which the restraining steel bars are solid and enclosed within the concrete.

4. The concrete specimen according to claim 3, comprising 2 to 8 main reinforcements.

5. 5. The concrete specimen according to claim 3, wherein the length of the tie in the central axis direction is 7 to 11 cm, and the diameter of the circular bottom surface of the tie is 8 to 12 cm.

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