JP2008044806A - Crack suppressing concrete and method of suppressing crack of concrete - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To suppress the occurrence of residual stress or crack in the concrete which is caused mainly by temperature distortion. <P>SOLUTION: The crack suppressing concrete contains cement, aggregate and an expanding material which are blended to give ≤80 N/mm<SP>2</SP>compressive strength to the concrete and in which 10-100 vol% of aggregate is light weight aggregate. <P>COPYRIGHT: (C)2008,JPO&INPIT

Description

  The present invention relates to a crack suppressing concrete and a crack suppressing method for concrete.

  Cracks in concrete are caused by drying shrinkage caused by water evaporation, self-shrinking caused by hydration reaction, and concrete temperature rise and drop caused by the progress of cement hydration reaction. It is thought that this is caused by the temperature distortion.

  The temperature strain is caused by the temperature inside the concrete rising due to heat generated by the hydration reaction of the cement and expanding, and then shrinking due to the subsequent temperature drop. In particular, in concrete structures with a relatively large cross section (mass concrete structure) and concrete structures with a large amount of unit cement (unit binder amount) even if the cross section is not so large, heat generated by the hydration reaction of cement is generated in the external environment. Therefore, there is a problem that a temperature strain is likely to occur due to difficulty in being released to the surface, and a crack having a large crack width called “temperature crack” is likely to occur.

  On the other hand, the self-shrinkage is considered to be caused by the consumption of water in the pores due to the hydration reaction of cement, especially in high-strength concrete with a small water-cement ratio. Shrinkage is noticeable.

Conventionally, as a method for suppressing self-shrinkage of high-strength concrete, a method has been proposed in which 30% by volume or less of coarse aggregate is replaced with special high-strength artificial lightweight aggregate and blended in concrete (patent document) 1). According to the method described in Patent Document 1, a special high-strength artificial lightweight aggregate is blended in concrete to replenish moisture consumed by the hydration reaction with moisture held by the artificial lightweight aggregate. Thus, it is disclosed that ultra-high-strength concrete having a compressive strength exceeding 100 N / mm ² can be obtained while reducing self-shrinkage due to self-drying.

JP 2005-22931 A

  However, although the method described in Patent Document 1 has a certain degree of effectiveness for reducing self-shrinkage strain caused by self-drying and suppressing cracks caused by the self-drying, the temperature as described above is used. It is not possible to effectively prevent the occurrence of cracks due to strain and the residual stress.

  In view of such problems of the prior art, an object of the present invention is to suppress cracks mainly due to temperature strain and to reduce residual stress in the concrete.

In order to solve the above-mentioned problems, the present invention is composed of a cement, an aggregate, and an expansion material, and is composed of a blend having a compressive strength of 80 N / mm ² or less, and 10 to 100% by volume of the aggregate is lightweight. A crack-suppressing concrete characterized by being an aggregate is provided.
Further, the present invention uses cement, aggregate, and an expansion material, and prepares a blend having a compressive strength of 80 N / mm ² or less, and 10 to 100% by volume of the aggregate is a lightweight aggregate. A concrete crack control method is provided.

According to the crack-suppressing concrete according to the present invention, the expansion reaction due to the hydration of the expansion material proceeds when the temperature rises due to the hydration reaction of the cement, and the lightweight aggregate has a lower compression modulus than the cured cement paste or the cured mortar. Compressive elastic strain is accumulated on the surface. And in the blending of concrete with a compressive strength of 80 N / mm ² or less, such a lightweight aggregate is blended so as to be 10 to 100% by volume of the total aggregate amount. This lightweight aggregate will be in the state which accumulated sufficient compressive strain. Thereafter, when the concrete is about to shrink due to a temperature drop, the compression elastic strain stored in the lightweight aggregate is released, and the amount of return strain of the lightweight aggregate is larger than that of the hardened cement paste or hardened mortar. Therefore, it is possible to reduce the temperature strain caused by the shrinkage, and it is possible to suppress the cracking of the concrete that occurs mainly as a result.

In addition, in the blending of concrete with a compressive strength of 80 N / mm ² or less, the lightweight aggregate is blended so that the amount of the lightweight aggregate is 10 to 100% by volume of the total aggregate amount. The water that exhibits the storage function and is impregnated in the voids of the lightweight aggregate is gradually supplied to the hydration reaction of the cement and the expansion material during the hardening process of the concrete. Thus, in the temperature drop process, the water supplied from the lightweight aggregate can sustain the expansion action of the expansion material and the expansion action due to the hydration reaction of the cement, and the effect of reducing the shrinkage is also exhibited. It is done.

  As described above, according to the crack-suppressing concrete according to the present invention, the residual stress in the concrete mainly due to temperature strain is reduced, and cracks caused by these are effectively suppressed, and The effect is that the width of cracks generated is reduced or the number of cracks generated in concrete is reduced.

  In particular, the crack-suppressing concrete according to the present invention exhibits a remarkable effect in a mass concrete structure having a relatively large cross section and a concrete structure having a large amount of unit cement.

The concrete according to the present invention includes cement, an aggregate, and an expansion material, and is composed of a blend having a compressive strength of 80 N / mm ² or less, and 20 to 100% by volume of the aggregate is a lightweight aggregate. It is characterized by this.

  As the lightweight aggregate used in the present invention, the same as the known lightweight aggregate conventionally used for concrete can be used. Examples of the lightweight aggregate include inorganic foams, foam glass, waste foam glass, and the like, which are prepared by adjusting the particle size of granulated slag, shirasu, obsidian, etc. The glass powder etc. which grind | pulverized and adjusted the particle size can be mentioned. In addition to these artificial lightweight aggregates, natural lightweight aggregates can also be used.

The lightweight aggregate preferably has an absolutely dry density of 1.2 to 2.0 g / cm ³ . Further, the lightweight aggregate preferably has a crushing load of 500 to 1000 N, and more preferably 600 to 900 N.

  By using such a lightweight aggregate with a relatively low crushing load, the expansion of concrete caused by the hydration reaction of the expanded material or cement is likely to accumulate as compressive elastic strain in the lightweight aggregate, and the temperature strain In addition, temperature cracks can be more effectively suppressed.

  Furthermore, the lightweight aggregate used in the present invention can be any lightweight aggregate with any water content regardless of whether it is impregnated with water in advance.

The lightweight aggregate is blended so as to be 10% by volume or more and 100% by volume or less of the aggregate constituting the concrete, preferably the lower limit is 15% by volume, more preferably the lower limit is 18% by volume. It mix | blends so that it may become.
The particle size of the lightweight aggregate is not particularly limited, and may be either coarse aggregate or fine aggregate.

In addition, the expansion material used in the present invention is not particularly limited as long as it can expand in volume by a hydration reaction. Examples of the expansion material include those that expand by producing ettringite (3CaO.Al ₂ O ₃ .3CaSO ₄ .32H ₂ O) from calcium sulfoaluminate, and expansion by hydration expansion due to the formation of calcium hydroxide. And the like.
In addition, as the expansion material, an expansion material conforming to JIS A 6202 can be suitably used.

The amount of the expansion material is preferably 60 kg or less, more preferably 10 to 50 kg per 1 m ^{3 of} concrete.

In addition, the concrete according to the present invention can be obtained by blending with various water binder ratios depending on the use of the concrete as long as the concrete has a compressive strength of 80 N / mm ² or less. Concrete prepared with a water binder ratio of 35 to 80% by weight is preferred. If it is concrete of such a water binder ratio, the temperature distortion reduction effect and temperature crack suppression effect which are show | played by this invention will be exhibited more notably.

  Further, the cement used in the present invention is not particularly limited, and various ordinary portland cements such as normal strength, ultra-high strength, low heat, moderate heat, sulfate resistance, blast furnace slag fine powder, fly ash, silica fume, limestone fine powder. Various mixed cements in which admixtures such as powder are mixed in an arbitrary ratio can be used.

The crack-suppressing concrete according to the present invention is intended for concrete having a compressive strength of 80 N / mm ² or less, and is particularly suitable for concrete having a compressive strength of 20 to 50 N / mm ² .

  Moreover, it is also possible to mix | blend another material with the concrete which concerns on this invention in the range which does not inhibit the objective of this invention. Other materials include chemical admixtures such as water reducing agents, high performance water reducing agents, AE water reducing agents, high performance AE water reducing agents, various admixtures such as blast furnace slag fine powder, fly ash, silica fume, limestone powder, etc. can do.

  EXAMPLES Hereinafter, although an Example is given and this invention is demonstrated further in detail, this invention is not limited to these Examples.

(Examples 1 and 2 and Comparative Examples 1 to 4)
Cement, expanded material, coarse aggregate, lightweight aggregate, water, fine aggregate, and admixture were mixed in the composition shown in Table 1 below to prepare concrete of examples and comparative examples. The materials used are as follows.
Cement: Sumitomo Osaka Cement, ordinary Portland cement (density 3.15 g / cm ² )
Expansion material: manufactured by Denka Co., Ltd., trade name “CSA # 20” (density 2.98 g / cm ² )
Lightweight aggregate: Nippon Mesalite Kogyo Co., Ltd., artificial lightweight aggregate, trade name “Mesalite Coarse Aggregate” (absolute density 1.64 g / cm ² , crushing load 800-900N)
Coarse aggregate: Crushed stone (density 2.6g / cm ² )
Fine aggregate: River sand (density 2.63g / cm ² )
Admixture: NMB, water reducing agent

(Measurement of temperature strain)
About the prepared concrete of the Example and the comparative example, stress was measured using the test apparatus as shown in FIG. That is, a concrete test piece 1 is prepared using the above concrete, and both ends of the concrete test piece 1 are fixed by a crosshead 2, and a stepping motor 3 is used for one of the crossheads 2. A load cell 4 whose position can be adjusted in the longitudinal direction was installed adjacent to it. Then, the relative distance of the crosshead 2 is measured using a displacement meter, and when the amount of displacement of the relative distance becomes 0.5 μm, the stepping motor 3 is operated to force the load cell 4 to the original position. At that time, the load acting on the load cell 4 (the load due to expansion or contraction) was measured. Moreover, the thermometer was installed in the inside of this concrete test piece 1, and the temperature change was measured. In this way, the stress due to expansion and contraction of the concrete test piece 1 and the internal temperature were measured until about 200 hours after the concrete was filled.
FIG. 2 shows a temperature measurement result, and FIG. 3 shows a stress measurement result.

From FIG. 2, it is recognized that the temperature rise of about 30 ° C. occurs after about 15 hours in any of the concrete of the example and the comparative example. In particular, it can be seen that in Comparative Example 4 produced by blending high-strength concrete having a compressive strength exceeding 100 N / mm ² , a temperature increase exceeding 60 ° C. occurs.

  In addition, from FIG. 3, it is recognized that the compressive stress increases in each concrete until about 15 hours have passed, so as to correspond to such a temperature increase. Furthermore, it can be seen that after the compressive stress exceeds the peak value, that is, as the internal temperature of the concrete decreases, the compressive stress changes to tensile stress. In the concrete of Comparative Examples 1 to 3, a tensile stress of 2 to 3 MPa was generated after about 200 hours, and in the high strength concrete of Comparative Example 4, a tensile stress of about 3 MPa was generated after only 50 hours. It has become a state. On the other hand, in the concrete of Examples 1 and 2, the generated stress is about 0 to 1.5, and it is recognized that the stress caused by the temperature change is effectively suppressed.

(Examples 3-9 and Comparative Examples 5-8)
Similarly, concretes of Examples 3 to 9 and Comparative Examples 5 to 8 were prepared according to the formulation shown in Table 2 below.

(Measurement of temperature strain)
The prepared concretes of Examples 3 to 9 and Comparative Examples 5 to 8 were placed in a state of being restrained along the H-shaped steel via a plurality of bolts, and restrained when 200 hours passed after placing. The internal stress generated in each concrete specimen was evaluated by measuring the strain generated in the H-shaped steel as a body. The results are also shown in Table 2.

  As shown in Table 2, in the examples in which the ratio of the lightweight aggregate in the coarse aggregate exceeds 10% by volume, the internal stress generated in the concrete test piece is smaller than in the comparative example in which the ratio is less than 10% by volume. A trend was observed. In particular, in Examples 6 to 9 in which the proportion of the lightweight aggregate exceeded 18% by volume, the internal stress generated in the concrete test piece was very small, and it was confirmed that the effect of the present invention was remarkably exhibited.

Schematic of a temperature strain measuring device. The graph which showed the temperature measurement result of the concrete in Examples 1, 2 and Comparative Examples 1-4. The graph which showed the stress measurement result of Examples 1 and 2 and Comparative Examples 1-4.

Claims (5)

Crack suppression characterized by containing cement, aggregate, and expansive material, and having a compressive strength of 80 N / mm ² or less, and 10 to 100% by volume of the aggregate is a lightweight aggregate. concrete.   The crack-suppressing concrete according to claim 1, wherein the water binder ratio is 35 to 80% by weight.   The crack-suppressing concrete according to claim 1 or 2, wherein a crushing load of the lightweight aggregate is 1000 N or less. The crack-suppressing concrete according to any one of claims 1 to ³ , wherein the expansion material is blended at a rate of 60 kg or less per 1 m ^{3 of} concrete. A concrete crack characterized by using cement, an aggregate, and an expansion material, and preparing a blend having a compressive strength of 80 N / mm ² or less, and 10 to 100% by volume of the aggregate is a lightweight aggregate. Suppression method.

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