JPH0415346B2 - - Google Patents

Description

[Detailed Description of the Invention] <<Industrial Application Field>> This invention relates to a method for preventing cracks on the surface of mass concrete (concrete with a large thickness of cast), and more specifically, a method for preventing cracks on the surface of mass concrete (concrete with a large thickness of cast), and more specifically, The present invention relates to a method for preventing cracks that occur on the surface of concrete (including the surface where it is placed, that is, the surface that contacts the ground) due to heat of hydration generated by chemical reactions.

<<Prior Art>> FIG. 2 is an explanatory diagram showing the cross-sectional temperature distribution due to the generation of hydration heat when the mass concrete shown in FIG. 1 is cast, and the state of stress generated accordingly.

Concrete appears to be in a hardened state one to two days after pouring, but the hydration reaction (chemical reaction between water and cement) continues after that, and its strength increases as well.

By the way, heat (heat of hydration) is generated during the hydration reaction. When mass concrete is placed, the heat of hydration on the concrete surface is radiated to the outside,
The heat of hydration in the center is not dissipated to the outside because of the thick pouring thickness, so a cross-sectional temperature distribution 2 occurs in which the temperature in the center is high and the temperature in the surface is low.

Due to the nature of thermal expansion, each part stretches in response to changes in temperature, but since the coefficient of linear expansion is proportional to the change in temperature, the center area with a high temperature is prevented from elongating by the surface area with a low temperature, and conversely, the surface area is prevented from elongating by the surface area with a low temperature. As a result, tensile stress is generated in the surface area and compressive stress is generated in the center area, which is proportional to the cross-sectional temperature distribution 2 based on a constant reference line 4.

When the tensile stress f generated on the surface exceeds the tensile strength of the surface at the time of the temperature distribution, cracks occur on the surface.

Therefore, in order to prevent cracks on the surface, the temperature difference between the temperature rise at the center, a ₁ °C, and the temperature rise at the surface, b ₁ °C, must be
The tensile stress f generated on the surface may be reduced by making a ₁ °C−b ₁ °C smaller.

For this purpose, pre-cooling methods,
Pipe cooling method etc. were used.

FIG. 4 is an explanatory diagram of the pre-cooling method. The pre-cooling method is a method of pouring concrete 6 that has been cooled in advance. Immediately after pouring, the cooling temperature is _t1 , but the temperature of each part of the cross section increases due to the generation of hydration heat. Since there is almost no heat dissipation at the center of the cross section, the temperature rise from the cooling temperature at the center a ₄ °C is the temperature rise from room temperature at the center of the cross section when room temperature mass concrete is poured as shown in Figure 1. a Approximately equal to ₁ °C. On the other hand, the temperature of the surface part decreases due to heat dissipation, but it cannot drop below room temperature, so the temperature rises from the cooling temperature of the surface b ₄ ℃ is the room temperature of the surface when room temperature mass concrete is poured Temperature rise from b greater than ₁ °C. Therefore, the temperature difference between the center and the surface is a ₄ °C − b ₄ °C
is smaller than the temperature difference a ₁ °C - b ₁ °C between the center and the surface in the normal case without cooling, and therefore the tensile stress generated on the surface can be reduced.

FIG. 5 is an explanatory diagram of the pipe cooling method. The pipe cooling method is a method in which a pipe 8 is embedded during mass concrete placement, and after the placement, cold water 10 is circulated within the pipe 8 to exchange heat between the cold water and the heat of hydration, thereby suppressing the temperature rise of the concrete. That is. If the temperature that decreases when cold water takes away the heat of hydration is c°C, then the temperature rise at the center of the cross section a ₅ °C is the value obtained by subtracting c°C from the temperature rise a ₁ °C at the center of the cross section during normal pouring. a ₁ ℃−
approximately equal to c°C. On the other hand, the temperature of the surface area is low due to heat dissipation, and because of the heat exchange between that low temperature and the cold water, the heat of hydration taken away by the cold water is not as much as the center of the cross section, and therefore the cold water takes away the heat of hydration. The temperature that decreases due to this is smaller than the temperature decrease c° C. at the center of the cross section. Therefore, the temperature difference between the center and the surface is a ₅ ℃−b ₅
°C is the temperature difference between the center and the surface in the normal case a ₁ °C
−b ₁ °C, and therefore the tensile stress generated on the surface can be reduced.

≪Problems to be solved by the invention≫ However, the conventional pre-cooling method and pipe cooling method can reduce the temperature difference between the center and the surface compared to normal pouring. However, there will still be a significant temperature difference, which will result in significant tensile stress on the surface, and when the tensile stress exceeds the current tensile strength, cracks will occur.

In addition, the conventional pre-cooling method,
In the pipe cooling method, the former requires pre-cooling of the entire poured concrete, and the latter requires the installation of pipes for pipe cooling, resulting in a large cost for cooling.

This invention was made in view of these problems, and its purpose is to further reduce the temperature difference between the center and surface of poured mass concrete compared to the conventional pre-cooling method or pipe cooling method. It is an object of the present invention to provide a pre-cooling method that can reduce the amount of water and thus further prevent surface cracking and reduce the cost for cooling.

<<Means for solving the problem>> In order to achieve the above object, the method for preventing cracks on the surface of mass concrete according to the present invention is as follows:
When placing mass concrete, cooled concrete is placed only in the middle layer.

≪Operation≫ Since cooled concrete is placed only in the middle layer of poured mass concrete, the temperature distribution after the temperature rises is a state in which the temperature peak in the middle layer is depressed compared to the temperature distribution of concrete poured at room temperature. Therefore, the difference between the maximum temperature rise and the surface temperature rise is extremely small.

<<Example>> FIG. 2 is an explanatory diagram showing an example of the method of precooling only the middle layer of mass concrete according to the present invention. As shown in FIG.
6 is poured with ordinary uncooled concrete.

The cooling temperature of the middle layer and the thickness of the middle layer relative to the lower and upper layers are the maximum temperature rise a ₂ ℃ based on room temperature.
It is experimentally and analytically determined to minimize the temperature difference a ₂ ℃−b ₂ ℃ between the temperature rise b ₂ ℃ and the surface temperature rise b 2 ℃.

Immediately after the mass concrete placed by this method is poured, the lower and upper layers are at room temperature and the middle layer is at a cooling temperature, so the temperature rise in the middle layer starts from the cooling temperature, and therefore the cross-sectional temperature after the temperature rise As shown in Figure B, the peak of the temperature distribution in the middle layer of the room-temperature poured concrete shown in Figure 1 is in a depressed state, and therefore the temperature between the maximum temperature rise a ₂ °C and the surface temperature rise b ₂ °C is as shown in Figure B. The difference a ₂ °C - _{b 2} °C is extremely small compared to a ₄ °C - b ₄ °C when the whole is precooled.

In addition, in Figure 3, the mass concrete is divided into multiple layers, more than three layers, and the surface part is set to room temperature p℃, and as it advances to the center layer, p℃＞q ₁ ℃＞r℃, p℃＞q ₂
A pre-cooling method is shown in which the cooling temperature is gradually lowered so that ℃>r℃. By pre-cooling in this manner, a finer cross-sectional temperature distribution can be obtained, and the temperature difference between the maximum temperature and the surface temperature can be further reduced.

≪Effects of the Invention≫ Since this invention places cooled concrete only in the middle layer of mass concrete,
The difference between the maximum cross-sectional temperature and the surface temperature can be made smaller than in the case of conventional pre-cooling or pipe cooling of the entire mass concrete, and therefore cracking of the mass concrete surface can be more completely prevented.

Moreover, since this invention places cooled concrete only in the middle layer of mass concrete, cooling costs can be reduced compared to the case where the entire concrete is pre-cooled or pipe-cooled.

[Brief explanation of drawings]

Figure 1 is an explanatory diagram showing the cross-sectional temperature distribution due to the generation of heat of hydration and the stress generation state associated with it in the case of mass concrete placement using the normal method.
The figure is an explanatory diagram showing an example of the method of placing cooled concrete only in the middle layer of mass concrete according to the present invention, and Figure 3 is an explanatory diagram showing an example of the method of pouring cooled concrete only in the middle layer of mass concrete. FIG. 4 is an explanatory diagram showing a cooling method, FIG. 4 is an explanatory diagram showing a pre-cooling method in a conventional example, and FIG. 5 is an explanatory diagram showing a pipe cooling method in a conventional example. 22...lower layer, 24...middle layer, 26...upper layer.

Claims (1)

[Claims] 1. A method for preventing cracks on the surface of mass concrete, characterized by pouring cooled concrete only in the middle layer when pouring mass concrete.