Externally restrained thermal crack prevention method

Abstract

To provide an external restraint type temperature crack prevention method that uses a curing accelerator and a curing retarder in a mass concrete structure, does not extend the process, has a high degree of freedom in process control, and can reduce temperature stress.SOLUTION: In this external restraint type temperature crack prevention method, a mass concrete structure is divided into a lower concrete connected to a restraint and an upper concrete on the lower concrete, and a curing retarder is added to the lower concrete and a curing accelerator is added to the upper concrete before placing concrete. By adjusting the amount of the curing retarder and the amount of the curing accelerator, relative difference is generated between the Young's modulus of the lower concrete and the Young's modulus of the upper concrete after the concrete is placed, to reduce temperature stress due to external restraint during shrinkage in a mass concrete structure.SELECTED DRAWING: Figure 2

Description

[Technical field]

[0001] The present invention relates to a method for preventing externally restrained thermal cracking in concrete structures. [Background technology]

[0002] Slab-like structures with a thickness of 80cm or more and wall-like structures with restrained lower ends are defined as mass concrete, and there is concern about thermal cracks caused by heat generated by hydration of cement. Such thermal cracks are broadly divided into internally restrained thermal cracks caused by temperature differences between the inside and outside of the structure, and externally restrained thermal cracks caused by volumetric changes during shrinkage of the structure being restrained by restraining bodies such as bedrock or existing concrete. In particular, the latter type is more likely to become penetrating cracks in wall-like structures with restrained lower ends, which can reduce the long-term durability of the structure, so it is necessary to pay close attention to its occurrence.

[0003] Countermeasures against thermal cracks include the use of low-heat cement to suppress the heat of concrete hydration and the installation of pipe cooling to lower the temperature of concrete after pouring (Patent Documents 1 and 2), and the use of induction joints and expansive materials is also known. Non-Patent Document 1 discloses a method of significantly reducing temperature stress and preventing thermal cracks by pouring concrete containing a hydration heat suppressing super retarder into a section 400 to 600 mm from the bottom end of the wall. [Prior art documents] [Patent documents]

[0004] [Patent Document 1] JP 2015-155881 A [Patent Document 2] JP 2016-89357 A [Non-patent literature]

[0005] [Non-Patent Document 1] Japan Concrete Technology Co., Ltd., Kazunori Kono, "ND Retarder Method" (6th Concrete Technology Exchange Meeting - Technical Presentation - November 18, 2016) http: / / www.jc-tech.co.jp / html / siryou / paper / contech06_kono.pdf Summary of the Invention [Problem to be solved by the invention]

[0006] Since thermal cracks occur relatively early after concrete is poured, it is necessary to predict their occurrence using temperature stress analysis when planning the concrete pouring process and to prevent them by taking appropriate measures.

[0007] Conventional measures such as the use of low-heat cement and pipe cooling to lower the temperature of concrete after casting have the problem of high costs. In addition to the above methods, there are also methods using induction joints and expansive materials to suppress externally restrained thermal cracking, but the use of these is often subject to restrictions.

[0008] In the method of Non-Patent Document 1, concrete containing a hydration heat suppressing super-retarder is poured into the lower part of the wall, 400 to 600 mm below the bottom end. However, because the hardening of the lower part is delayed, it takes longer for the entire wall to harden, resulting in the problem of extending the construction period. Furthermore, because only a hardening retarder is used, the freedom of process management is reduced.

[0009] In view of the problems of the conventional technology as described above, the present invention aims to provide an externally restrained thermal crack prevention method for mass concrete structures, which uses a hardening accelerator and a hardening retarder according to the target area, does not extend the process, allows a high degree of freedom in process management, and can reduce temperature stress. [Means for solving the problem]

[0010] The method for preventing externally restrained thermal cracks to achieve the above object is a method for preventing externally restrained thermal cracks caused by a wall-like mass concrete structure constructed by pouring concrete against a restraining body being externally restrained by the restraining body at the bottom end of the wall-like mass concrete structure during shrinkage after pouring the concrete, comprising: The mass concrete structure is divided into a lower concrete connected to the restraining body and an upper concrete on the lower concrete, and before the concrete is poured, a hardening retarder is added to the lower concrete and a hardening accelerator is added to the upper concrete; By adjusting the amount of the hardening retarder and the hardening accelerator added, a relative difference is created between the Young's modulus of the lower concrete and the Young's modulus of the upper concrete after the concrete is poured, thereby reducing the temperature stress due to the external restraint during shrinkage in the mass concrete structure and preventing thermal cracking.

[0011] According to the above-mentioned externally restrained thermal crack prevention method, by adjusting the amount of hardening retarder added to the lower concrete and the amount of hardening accelerator added to the upper concrete, a relative difference is generated between the Young's modulus of the lower concrete and the Young's modulus of the upper concrete after concrete pouring, thereby reducing the temperature stress caused by external restraint during shrinkage in the concrete structure and preventing thermal cracks. In addition, since the amount of hardening retarder added to the lower concrete and the amount of hardening accelerator added to the upper concrete can be adjusted, the degree of freedom of adjustment is high and the degree of freedom of process management is high, and the time to complete hardening of the lower concrete can be shortened compared to when hardening retarder is added only to the lower concrete, and the problem of extending the process does not occur.

[0012] In the above-mentioned external restraint type thermal crack prevention method, it is preferable to adjust the amount of the hardening retarder and the hardening accelerator added so that the time difference between the time from pouring the lower concrete until the Young's modulus reaches its upper limit and the time from pouring the upper concrete until the Young's modulus reaches its upper limit is a predetermined time or more.

[0013] In other words, the expansion of the lower concrete before reaching the upper limit is restrained by the upper concrete, so that a compressive stress according to the time difference is introduced into the lower concrete, thereby reducing the temperature stress. Also, the restraint stress during the shrinkage of the upper concrete within the time difference is reduced. Effect of the Invention

[0014] According to the present invention, by using a hardening accelerator and a hardening retarder in a concrete structure, the time until the hardening of the lower concrete is completed can be shortened compared to when a hardening retarder is added only to the lower concrete, and an externally restrained thermal crack prevention method can be provided which does not extend the process, allows a high degree of freedom in process management, and can reduce temperature stress. [Brief description of the drawings]

[0015] [Figure 1] 1 is a perspective view showing a schematic view of a concrete structure to which an external constraint type thermal crack prevention method according to an embodiment of the present invention can be applied. FIG. [Diagram 2] 2 is a graph showing a schematic diagram of a temperature history curve and a change in Young's modulus over time in an upper concrete and a lower concrete after the concrete structure of FIG. 1 is poured; DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0016] Hereinafter, an embodiment of the present invention will be described with reference to the drawings. Fig. 1 is a perspective view showing a concrete structure to which the external constraint type thermal crack prevention method according to the present embodiment can be applied. Fig. 2 is a graph showing a temperature history curve and a change in Young's modulus over time in the upper concrete and the lower concrete after the concrete is poured in the concrete structure of Fig. 1.

[0017] As shown in FIG. 1, the mass concrete structure 10 is integrally formed into a wall shape by a lower concrete 11 and an upper concrete 12, and is constructed on an existing concrete structure S such that the lower concrete 11 is connected to the existing concrete structure S. The height of the lower concrete 11 is shorter than the height of the upper concrete 12, and is within a range of, for example, about 50 cm to 1 m. The thickness w of the mass concrete structure 10 is at least 80 cm, and the existing concrete structure S acts as a restraining body after concrete is poured, externally restraining the lower end of the mass concrete structure 10 during shrinkage of the concrete structure, which may cause externally restrained thermal cracking in the mass concrete structure 10. However, in this embodiment, such thermal cracking is suppressed as follows.

[0018] The lower concrete 11 in Fig. 1 is constructed by pouring concrete with a concrete hardening retarder added, and then the upper concrete 12 is constructed by pouring concrete with a concrete hardening accelerator added. At this time, concrete with hardening retarder added or concrete with hardening accelerator added is also manufactured when concrete is manufactured by mixing cement, aggregate, water, etc. at a ready-mix concrete plant, so no special burden is placed on the construction site and no special process is required.

[0019] The suppression of thermal cracking will be described with reference to Figure 2. When concrete containing a hardening retarder is poured to construct the lower concrete 11, the concrete generates heat due to the hydration reaction of the cement, and as shown by the temperature history curve a shown by the dashed line in Figure 2, the temperature of the lower concrete rises over time, reaches a peak temperature a1, then starts to cool and gradually decreases. The concrete expands and contracts in accordance with this temperature fluctuation, and the Young's modulus of the concrete increases almost linearly over time, as shown by the thick solid line b in Figure 2, and when it reaches the peak temperature a1, it reaches an upper limit near the upper limit point b1, becoming an almost constant upper limit value.

[0020] Next, when concrete containing a hardening accelerator is poured on top of the lower concrete 11 to construct the upper concrete 12, heat is generated by the hydration reaction, and the temperature of the upper concrete rises over time, as shown by the temperature history curve c shown by the dashed line in Figure 2, and after reaching a peak temperature c1, it begins to cool and gradually decreases. The concrete expands and contracts in accordance with this temperature fluctuation, and the Young's modulus of the concrete changes almost linearly over time, as shown by the solid line d in Figure 2, and when it reaches the peak temperature c1, it reaches an upper limit near the upper limit point d1, becoming an almost constant upper limit value.

[0021] The hardening period of the lower concrete 11 and the upper concrete 12 corresponds to the period during which Young's modulus increases in FIG. 2, and the hardening of the lower concrete 11 and the upper concrete 12 is almost completed near the upper limits b1 and d1 of Young's modulus.

[0022] As can be seen from Fig. 2, the Young's modulus of the lower concrete 11 increases relatively slowly due to the addition of a hardening retarder, whereas the Young's modulus of the upper concrete 12 increases relatively rapidly due to the addition of a hardening accelerator. For this reason, the Young's modulus of the upper concrete 12 reaches the upper limit point d1 in a shorter time than that of the lower concrete 11, and the Young's modulus of the lower concrete 11 reaches the upper limit point b1 with a time difference A delay from that of the upper concrete 12. In other words, a relative difference occurs between the Young's modulus of the lower concrete 11 and the Young's modulus of the upper concrete 12 until the Young's modulus of the upper concrete 12 reaches the upper limit point d1. The time difference A corresponds to the difference between the time when the lower concrete 11 reaches the peak temperature a1 and the time when the upper concrete 12 reaches the peak temperature c1.

[0023] When the upper concrete 12 hardens and shrinks early, the lower concrete 11 has not yet hardened within the time difference A in Figure 2, has a small Young's modulus and is relatively soft, so the degree of external restraint from the existing concrete structure S is small and the restraint stress is reduced.In addition, the upper concrete 12 is less restrained by the lower concrete 11 during shrinkage, so the occurrence of thermal cracks in the upper concrete 12 is suppressed.

[0024] In addition, in the lower concrete 11, since the upper concrete 12 hardens and shrinks during the expansion process within the time difference A, the expansion of the lower concrete 11 is restrained by the upper concrete 12, a compressive stress according to the time difference A is introduced, and the temperature stress is reduced, thereby suppressing the occurrence of thermal cracks.

[0025] 2 is related to the decrease in temperature stress in the upper concrete 12 and the introduction of compressive stress in the lower concrete 11, and requires a relatively long time. For this reason, the length of time difference A can be adjusted by adjusting the amount of hardening accelerator added to the upper concrete 12 to adjust the time it takes for the Young's modulus of the upper concrete 12 to reach the upper limit point d1, and by adjusting the amount of hardening retarder added to the lower concrete 11 to adjust the time it takes for the Young's modulus of the upper concrete 11 to reach the upper limit point d1.

[0026] That is, the time it takes for the Young's modulus of the upper concrete 12 to reach the upper limit d1 depends on the amount of hardening accelerator added, with a short time when a large amount is added and a long time when a small amount is added, and conversely, the time it takes for the Young's modulus of the lower concrete 11 to reach the upper limit b1 depends on the amount of hardening retarder added, with a long time when a large amount is added and a short time when a small amount is added, so the time difference A can be adjusted by the amount of hardening accelerator and the amount of hardening retarder added. Such adjustment of the time difference A provides a higher degree of freedom in adjusting the time difference A and increases the degree of freedom in process management than when only a hardening accelerator is added to the upper concrete 12 or when only a hardening retarder is added to the lower concrete 11.

[0027] In actual construction, a process time difference B occurs between the completion of pouring the lower concrete 11 and the completion of pouring the upper concrete 12, and the temperature history curve c and the change line d of the Young's modulus of the upper concrete in Figure 2 are shifted to the right by the process time difference B, but it is desirable to confirm the process time difference A when each concrete is poured simultaneously with the assumed amount of hardening accelerator and hardening retarder added, confirm the process time difference B taking into consideration the time required to complete pouring according to the pouring amount of the upper concrete 12, and determine the amount of hardening accelerator and hardening retarder added so that the process time difference A can be secured while also determining the start time of pouring the upper concrete. Securing the time difference A taking into consideration the process time difference B can be achieved more reliably by adding the hardening accelerator and hardening retarder than by adding only the hardening accelerator or only the hardening retarder.

[0028] Furthermore, if adding a hardening retarder to the lower concrete 11 causes a problem of extending the process, the amount of hardening retarder added can be reduced to an extent that does not cause a problem of extending the process, and the amount of hardening accelerator added to the upper concrete 12 can be increased to offset the corresponding time change, thereby dealing with the problem of extending the process. Thus, according to this embodiment, the hardening completion time of the lower concrete 11 can be shortened compared to the case where a hardening retarder is added only to the lower concrete.

[0029] In this embodiment, various types of hardening retarders and hardening accelerators can be used, but hardening accelerators that do not contain nitric acid compounds are preferred from the viewpoint of long-term durability of the mass concrete structure.

[0030] In addition, since the speed of strength development differs depending on the type of cement, it is preferable to confirm in advance by experiment, etc., the appropriate amounts of hardening retarder and hardening accelerator to be added depending on the type of cement to be used.

[0031] Although the embodiments for carrying out the present invention have been described above, the present invention is not limited to these, and various modifications are possible within the scope of the technical concept of the present invention. For example, by applying the present invention to various mass concrete structures having a thickness of at least 80 cm, it is possible to prevent externally restrained thermal cracking. In addition, in FIG. 1, the restraining body is described as an existing concrete structure S, but the restraining body is not limited to this, and may be, for example, a hard rock. [Industrial Applicability]

[0032] According to the present invention, by using a hardening accelerator and a hardening retarder in a mass concrete structure, the process can be extended without increasing the degree of freedom in process management, and temperature stress can be reduced. This makes it possible to prevent externally restrained thermal cracking efficiently at low cost without relying on the conventional costly use of low-heat cement or pipe cooling, and without using induction joints or expansive materials. [Explanation of symbols]

[0033] 10 Mass Concrete Structures 11 Lower Concrete 12 Upper Concrete S Existing concrete structure, restraint a Temperature history curve after pouring of the lower concrete b Change line of Young's modulus of the lower concrete c Temperature history curve after pouring of upper concrete d Change in Young's modulus of the lower concrete a1,c1 Peak temperature b1,d1 upper limit point

Claims

1. A method for preventing externally restrained thermal cracking caused by external restraint by a restraining body at the bottom end of a wall-shaped mass concrete structure constructed by pouring concrete against a restraining body during shrinkage after pouring of the concrete, comprising: The mass concrete structure is divided into a lower concrete connected to the restraining body and an upper concrete on the lower concrete, and before the concrete is poured, a hardening retarder is added to the lower concrete and a hardening accelerator is added to the upper concrete; By adjusting the amount of the hardening retarder and the amount of the hardening accelerator added, a relative difference is created between the Young's modulus of the lower concrete and the Young's modulus of the upper concrete after the concrete is poured, thereby reducing the temperature stress due to the external restraint during shrinkage in the mass concrete structure and preventing thermal cracking.

2. A method for preventing externally restrained thermal cracks as described in claim 1, in which the amount of hardening retarder and the amount of hardening accelerator added are adjusted so that the time difference between the time from pouring the lower concrete until the Young's modulus reaches its upper limit and the time from pouring the upper concrete until the Young's modulus reaches its upper limit is a predetermined time or more.

3. The external restraint type thermal crack prevention method described in claim 2, wherein the expansion of the lower concrete before it reaches the upper limit is restrained by the upper concrete, thereby introducing compressive stress according to the time difference into the lower concrete and reducing the temperature stress.

4. 4. The method for preventing external restraint type thermal cracking according to claim 2 or 3, wherein the restraint stress during shrinkage in the upper concrete is reduced within the time difference.

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