JP2000248743A - Mass concrete placing method - Google Patents

Abstract

PROBLEM TO BE SOLVED: To restrain the width of crack caused on a structure surface below a specific value by forming a lower layer concrete layer using ordinary Portland cement in a lower layer in the whole layer thickness of a mat, and layering a concrete layer using low heating cement on the lower layer concrete layer. SOLUTION: In a mat-like concrete main body 10, a concrete lower layer 11 using ordinary Portland cement is placed on the ground. A concrete surface layer 12 using low heating Portland cement is placed on the concrete lower layer 11. Then, the ordinary concrete layer 11 is set to a layer thickness H1, and is integrally placed on the ground. This layer 11 is continuously placed by being divided into several layers so that a horizontal placing joint is not formed. Then, the low heating concrete layer 12 is continuously placed. In that case, the concrete layer 12 is set to H2, and a ratio to the whole layer thickness H is set to 15 to 10 % to thereby sufficiently control the occurrence of surface crack and the crack width.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a method for placing mass concrete, and more particularly to a method for placing mass concrete in which the generation of surface cracks and the control of the crack width of mass concrete are realized by an inexpensive method.

[0002]

2. Description of the Related Art In a concrete structure having a large mat shape or the like, a temperature change occurs in the structure due to heat of hydration of cement used for casting concrete. Temperature cracks often occur on the concrete surface due to the temperature stress accompanying this temperature change. If the width of the cracks exceeds a certain value, it may not be possible to maintain the water stopping property, leading to a serious defect of the structure such as a deterioration in structural strength and durability. Conventionally, such concrete in which thermal stress due to heat of hydration of cement is problematic is treated as mass concrete, and design and construction are performed in consideration of examination of crack generation and the like.

[0003] As a countermeasure in construction to prevent the occurrence of temperature cracks in mass concrete, low heat type cements capable of reducing the rise in temperature during concrete hardening are often used. Examples of low heat type cement include medium heat Portland cement, low heat generation cement mixed with blast furnace slag fine powder and fly ash, and belite (C
Low heat generation type Portland cement obtained by mixing ₂ S) at a constant rate has been developed. These low heat-generating cements are used as appropriate, and the construction is performed while controlling the concrete placement height, the placement temperature, and the like.

[0004]

However, these low heat type cements have a higher material unit cost than ordinary Portland cement. For this reason, in a structure using a large amount of cement, such as mass concrete, reducing the amount of low-heating cement used within a range that is not affected by temperature cracks leads to a reduction in construction costs.

[0005] Therefore, an object of the present invention is to solve the above-mentioned problems of the prior art and to reduce the width of temperature cracks generated on the surface of a mass concrete structure to a certain value or less, thereby improving durability. It is an object of the present invention to provide a mass concrete casting method for constructing an economic mass concrete structure which can be obtained sufficiently.

[0006]

Means for Solving the Problems To achieve the above object, the present invention relates to a method for casting a mat-shaped mass concrete structure, wherein ordinary portland cement is used as a lower layer of the total thickness of the mat. The present invention is characterized in that a used lower concrete layer is formed, and concrete is cast so as to laminate a concrete surface layer using low heat generation type cement on the lower concrete layer.

Preferably, the concrete surface layer is set to a thickness of 15 to 10% of the total thickness of the mat.

[0008]

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS An embodiment of a method for placing mass concrete according to the present invention will be described below with reference to the accompanying drawings. FIG. 1 shows a part of a section of a mat-like concrete cast by the method for casting mass concrete of the present invention. For example, the bottom slab concrete of an underground LNG tank may have a bottom slab thickness of 4.0 m or more. In concrete casting, the type of cement, the concrete placement temperature, etc. are managed as mass concrete. In this specification, the following description will be made on the assumption that a similar scale is used as an embodiment. The mat-like concrete body 10 shown in FIG. 1 has a total thickness H = 4.5 m, and a concrete lower layer 11 using ordinary Portland cement as a compounding cement is cast on the ground. The lower layer 11 is usually called a concrete layer 11). Furthermore, the ordinary concrete layer 11
A concrete surface layer 12 made of Portland cement with low heat generation is cast on the surface (hereinafter, this concrete surface layer 12 is referred to as a low heat generation concrete layer 12).

The ordinary concrete layer 11 has a layer thickness H ₁ = 3.
It is set to 75 to 4.0 m, and is integrally cast on the ground. The plain concrete layer 11 so that it can not be horizontal strokes seam are driven divided into several layers in a given section of the layer thickness H ₁ continuously. Further, a low heat-generating concrete layer 12 as a concrete surface layer is cast continuously from the ordinary concrete layer 11. The low heat concrete layer 12 is in this embodiment the layer H ₂ = 0.75~0.
5 m, and the ratio to the total layer thickness H is about 15 to 10
%It has become.

The low heat generation concrete layer 12 has a thickness H _2.
Is set as a ratio to the total layer thickness H. The value is preferably determined based on the result of concrete temperature stress analysis by numerical analysis such as the finite element method under the conditions of the type and amount of cement used, the shape of the concrete structure, the surrounding environmental conditions, and the like. The optimum ratio of the layer thickness can be obtained from the analysis results described later. To minimize the tensile stress on the concrete surface, the ratio of the surface layer thickness to the total layer thickness is 15 to 10%.
It is preferable to set Incidentally, the layer thickness of H ₂ low heat concrete layer 12 from the viewpoint of construction is preferably not less than 0.1 m.

As the low heat type cement to be used, a known medium heat Portland cement or a low heat heat cement obtained by mixing blast furnace slag fine powder and fly ash in two or three components with a base cement is used for the target structure. It can be used appropriately according to the requirements. Further, as for the base cement to be used, it is preferable to change the mixing ratio of the cement component by controlling the amount of belite.

Here, concrete temperature analysis for verifying the effect of the concrete placing method of the present invention will be described. In the present embodiment, a temperature analysis model (FIG. 2)
(A)) and a finite element method analysis using a stress analysis model (FIG. 2 (b)) of the structure based on the temperature distribution obtained by this temperature analysis model. In FIG. 2, mesh division is not shown for simplification of the drawing. As shown in FIG. 2A, the temperature analysis model has a total thickness H according to the full-scale structure.
= 4.50 m, ordinary concrete layer H ₁ = 3.25-
4.20 m, low heat generation concrete layer H ₂ = 1.25-1.25
It was set by changing within the range of 0.30 m. The upper boundary is a heat transfer boundary, and the ground lower boundary is an adiabatic boundary and a fixed temperature boundary.

The thermophysical properties (concrete adiabatic temperature rise characteristics, concrete thermal conductivity, thermal diffusivity, specific heat) and the like used in the analysis are determined by the type of cement used, the amount used, the composition, and the implantation temperature. In this analysis, low heat Portland cement with controlled belite content was used as low heat type cement, and various constants of this cement were used. As described above, in the concrete temperature analysis, a highly accurate analysis result can be obtained by setting an analysis model and an analysis condition adapted to the shape, the constraint condition, the type of cement used, and the like of the target mass concrete structure.

Table 1 shows the conditions of the analysis cases depending on the type of cement and the thickness of the casting layer, and the maximum tensile stress (8 days old) of the concrete surface layer which is considered to have an effect on the occurrence of cracks on the concrete surface as the analysis result. It is a list.

[0015]

[Table 1]

FIGS. 3 to 7 show temporal changes in concrete temperature corresponding to cases 1 to 5 shown in Table 1. FIG. In Case 3, the temperature of the concrete surface and the temperature directly below the concrete surface which cause the occurrence of surface cracks (nodes 45 and 45)
37) can be sufficiently suppressed.

FIGS. 8 to 12 show temporal changes in concrete stress in Cases 1 to 5, and FIGS. 13 to 17 show FIGS.
FIG. 13 is a concrete stress distribution at a target cross section near the concrete surface corresponding to the stress state of FIG. In the analysis model (Case 3 to Case 5) in which the low heat-generating concrete layer is formed on the surface layer by the casting method of the present invention, the maximum tensile stress on the concrete surface (node 125) is 1 in comparison with the case of Case 1. / 2, which is lower than that of Case 2 using low heat generation type cement for the entire thickness. As shown in FIG. 11, FIG. 12, FIG. 16, and FIG. 17, even when the stress on the concrete surface is equal, if the layer thickness of the H ₂ layer is small, the tensile stress on the upper part of the ordinary concrete starts to increase. Therefore, the optimum thickness range of the H ₂ layer is 0.75.
It is preferable to set it to about 0.5 m (15% to 10%).

[0018]

As shown in Table 1, in the method of the present invention (Case 3 to Case 5), the occurrence of surface cracks and the occurrence of surface cracks were greater than in the case where low-heating type cement was used for the entire thickness (Case 2). The width of the crack can be sufficiently controlled. In particular, at the stage where the concrete tensile strength does not sufficiently appear in the short-term age, the concrete surface stress is reduced, so that a sufficient effect can be expected to reduce the crack width.

[Brief description of the drawings]

FIG. 1 is a partial cross-sectional view showing an embodiment of a method for placing mass concrete.

FIG. 2 is a partial cross-sectional perspective view showing an example of an analysis model in an analysis method of a mass concrete placing method.

FIG. 3 is a diagram showing the results of concrete temperature analysis (case 1 temperature change over time).

FIG. 4 is a diagram showing the results of concrete temperature analysis (case 2 temperature change over time).

FIG. 5 is a diagram showing the results of concrete temperature analysis (case 3 temperature change over time).

FIG. 6 is a diagram showing the results of concrete temperature analysis (case 4 temperature change over time).

FIG. 7 is a diagram showing the results of concrete temperature analysis (case 5 temperature change over time).

FIG. 8 is a diagram showing a result of concrete temperature analysis (case 1 stress change over time).

FIG. 9 is a diagram showing the results of concrete temperature analysis (case 2 stress change over time).

FIG. 10 is a diagram showing the results of concrete temperature analysis (case 3 stress change over time).

FIG. 11 is a diagram showing the results of concrete temperature analysis (case 4 stress change over time).

FIG. 12 is a diagram showing the results of concrete temperature analysis (case 5 stress change over time).

FIG. 13 is a diagram showing the results of concrete temperature analysis (case 1 concrete stress cross-sectional distribution).

FIG. 14 is a diagram showing a result of concrete temperature analysis (case 2 concrete stress cross-sectional distribution).

FIG. 15 is a diagram showing a result of concrete temperature analysis (case 3 concrete stress cross-sectional distribution).

FIG. 16 is a diagram showing the results of concrete temperature analysis (case 4 concrete stress cross-sectional distribution).

FIG. 17 is a diagram showing the results of concrete temperature analysis (case 5 concrete stress cross-sectional distribution).

[Explanation of symbols]

 10 Matt-shaped concrete body 11 Normal concrete layer 12 Low heat-generating concrete layer

 ────────────────────────────────────────────────── ─── Continuing from the front page (72) Inventor Hisaichi Maruyama 1603-1 Kami-Tomiokacho, Nagaoka-shi, Niigata F term in Nagaoka University of Technology 2E172 AA01 DB11 EA13

Claims (2)

[Claims] When a mat-shaped mass concrete structure is cast, a concrete lower layer using ordinary Portland cement is cast on a base of the total thickness of the mat, and the concrete lower layer is formed on the concrete lower layer. A method for placing mass concrete, wherein the concrete surface layer using low heat type cement is placed so as to be laminated. 2. The mass concrete casting method according to claim 1, wherein said concrete surface layer has a thickness of 15 to 10% of the total thickness of said mat.