KR100697221B1 - Composition for Shotcrete Having Excellent Performance Using Metakaolin - Google Patents

Abstract

The present invention relates to a high-performance shotcrete composition, by using fine powder metakaolin (Metakaolin) as a blending material while using a general portant cement and alkali-free fastener, as well as the overall mechanical properties such as compressive strength and bending strength of the shotcrete as well as dropping of the shotcrete It provides a high performance shotcrete composition that can prevent and enhance long term durability.

Shotcrete, metakaolin, silica fume

Description

Composition for Shotcrete Having Excellent Performance Using Metakaolin}

1 is a photograph of contrasting the morphology of meta kaolin (a) and metakaolin (b) produced domestically produced abroad,

2 is a graph comparing the results of the setting time of the mixing conditions of the shotcrete, each of which is mixed with metakaolin and silicon fume as the admixture of the present invention,

Figure 3 is a graph comparing the compressive strength results of the mixing conditions of the shotcrete, each of which is mixed with metakaolin and silicon fume as a mixed material of the present invention,

4 is a graph comparing the results of bending strength for each blending condition of shotcrete in which metakaolin and silicon fume are respectively blended as the admixture of the present invention;

5 is a graph comparing the chlorine ion passage charge (water permeability) results of the mixing conditions of the shotcrete, each of which is mixed with metakaolin and silicon fume as the admixture of the present invention;

6 is a graph comparing the results of freeze-thawing resistance according to the mixing conditions of the shotcrete, each of which is mixed with metakaolin and silicon fume as the admixture of the present invention.

The present invention relates to a high-performance shotcrete composition using metakaolin, and more particularly, high reactivity metakaolin, which is a fine powder admixture, prevents the shotcrete from falling off and improves long-term durability as well as general mechanical properties such as compressive strength and bending strength of the shotcrete. A high performance shotcrete composition.

Although shotcrete is a temporary support member for stabilizing the excavation section immediately after excavation of the tunnel / underground space and used as a permanent support material after completion of the tunnel / underground space, At 28 days of age, the compressive strength of domestic shotcrete is 180kgf / cm2 or 210kgf / cm2, which is considerably lower than the strength standard of 300kgf / cm2 or more applied in Norway and Switzerland. Due to the low strength level of the domestic shotcrete, the construction speed and construction cost are increased due to the increase in the thickness of the shotcrete required to stabilize the tunnel / underground space compared to the foreign countries, which hinders the efficiency and economic efficiency of the shotcrete construction. Moreover, no technical considerations have been made in the long term durability of shotcrete in Korea.

In addition, high-strength and high durability of shotcrete, which are well-known for the long-term stability of tunnels / underground spaces, are required.In addition to the existing methods of additional secondary concrete lining after the shotcrete is placed, the secondary concrete lining is omitted and high strength and high durability In order to apply the single-shell tunneling method to apply high-performance shotcrete with permanent lining, shotcrete high-performance technology is essential.

In foreign countries, silica fume, which is a fine powder admixture, is essentially used for high performance of shotcrete. However, since the production of silica fume requires a lot of power, production is not performed at all in the country where the power cost is high, and the price itself is up to 10 times more than the unit price of cement. Moreover, when a large amount of silica fume is used, the unit quantity increases in order to obtain the same dough concentration, which may cause problems such as initial cracking.In the case of incorporation of silica fume, since the bleeding is small, the amount of reserves increases. As a result, the use of high-performance water reducing agents, which are large in shrinkage and also expensive, had a problem of increasing proportionally.

The present invention has been made to solve the above problems, an object of the present invention is to provide a high-quality high-strength shotcrete composition that improves the overall mechanical properties such as compressive strength and bending strength of the shotcrete and enhances long-term durability.

In addition, the present invention, by using metakaolin instead of industrial fume silica fume for high-intensity expression to significantly reduce the unit cost of shotcrete and to prevent the dropping of shotcrete and at the same time provide a high-strength shotcrete composition.

In order to achieve the above object, the present invention

Per 1㎥ of shotcrete,

Coarse aggregate of 500 to 700 kgf weight with a maximum dimension of 10 to 15 mm;

Fine aggregate weighing 1,000-1,200 kgf;

300-450 kgf weight cement;

Water having a water-cement ratio of 40 to 60% by weight;

Liquid type fastener of 4 to 10% by weight of the cement weight; And

3-10% by weight of metakaolin admixture of the cement weight;

Provided is a high performance shotcrete composition comprising a.

EMBODIMENT OF THE INVENTION Hereinafter, this invention is demonstrated in detail.

The shotcrete composition of the present invention is composed of coarse aggregate, fine aggregate, cement, water, admixture and a fastener.

The coarse aggregate has a maximum dimension of 10 to 15 mm and uses 500 to 700 kgf weight per 1 m 3 of shotcrete and can be changed according to design. If the maximum size of the coarse aggregate is too large, it is preferable to select coarse aggregate within the above range because there is a large amount of rebound during shotcrete construction and the hose may be closed.

The fine aggregate uses a weight of 1,000 ~ 1,200kgf per 1㎥ of shotcrete, the compounding amount can be changed according to design.

Portland cement is generally used as the cement, and in some cases, if it is necessary to generate strength early after construction or cannot sufficiently take the curing period after construction, crude steel portland cement, cemented carbide etc. may be used. The amount of cement is appropriate to use 300 ~ 450kgf weight per 1㎥ of shotcrete, but can be changed according to the design.

The shotcrete composition of the present invention is suitable to use water having a water-cement ratio of 40 to 60% by weight and can be used by adding or subtracting according to design within the above range.

The fastener is a component used when constructing shotcrete with a thickness of 5 to 15 cm or more upward, such as tunnel construction, in the present invention, it is preferable to use an alkali free (Alkali-Free) liquid quickener that has been generally used in the present invention. . The addition amount of the alkali-free liquid quickener is preferably determined by a test within the range of 4 to 10% by weight of the weight of the cement used normally.

The admixture uses metakaolin which can prevent shotcrete from falling off while enhancing long-term durability with securing mechanical properties such as compressive strength and flexural strength, instead of silica fume admixture, which is an industrial by-product, which has been used for high strength.

The metakaolin is an amorphous material that behaves as an artificial pozzolanic and has the following characteristics in strength and workability when such metakaolin is used for shotcrete.

First of all, silica (SiO ₂ ) and alumina (Al ₂ O ₃ ) are high in content and both of them are amorphous and have a large specific surface area. Therefore, calcium silicate hydrate combined with calcium hydroxide produced as a result of cement hydration (Calcium Silicate Hydrate) ), Calcium aluminate hydrate (Calcium Aluminate Hydrate), and calcium alumino-silicate hydrate (Calcium Alumino-Silicate Hydrate) has a mechanism to form more hydration products and reaction with calcium hydroxide reduces harmful alkali action It has a better potential for weathering as well as a distinct water tightness when incorporated into cement paste formulations, which enhance strength and reduce water permeability.

Such metakaolin can be used both domestically and domestically manufactured products, but because the domestic resources of kaolin, which is a raw material of metakaolin, are abundant in Korea, the unit price is low, so there is no big difference from the unit price of cement. Moreover, since the conventional silica fume is an industrial by-product, metakaolin is produced under good management and control, it is more preferable to use domestic metakaolin because it has the advantage that the change in physical and chemical properties is very small. Do.

On the other hand, meta kaolin produced in Korea has a relatively small specific surface area compared to products manufactured overseas, has a high component of Fe ₂ O ₃ is pale yellow, and the loss of ignition is also somewhat high (see Fig. 1).

For reference, the physical and chemical properties of domestic metakaolin are summarized in Table 1 below.

The addition amount of such metakaolin admixture is preferably determined by a test within the range of 3 to 10% by weight of the weight of the cement, similar to the usual silica fume admixture, and considering the permeability of the prepared shotcrete 4 It is more preferable to use within the range of -8% by weight.

On the other hand, the high-strength shotcrete composition of the present invention can be configured by adding a high performance water reducing agent. The high performance sensitizer is a surface active agent with excellent dispersibility and little foaming property and is used as a high strength or fluidizing agent. When the high performance water reducing agent is added to the shotcrete, it is possible to greatly improve the workability of the shotcrete construction without deteriorating the strength. The content is 4 ~ 10kgf weight per 1 m 3 of shotcrete, the compounding amount can be changed by design.

 Physical properties  Powder  importance  Particle Size [44㎛ Pass (%)]  Moisture content (%)  10000 (㎠ / g)  2.53  90  0  Chemical properties  Chemical component (%) SiO ₂ Al ₂ O ₃ Fe ₂ O ₃ TiO ₂  52  40  3  0.4 K ₂ O Na ₂ O  CaO  MgO  0.6  0.4  1.2  0.5

Since the composition using metakaolin according to the present invention is relatively large particles, there is an advantage of using a small amount of a high performance water reducing agent than conventional portland cement concrete. That is, at the same water-cement ratio (W / C), metakaolin concrete can reduce the content of high performance water reducing agents by at least 20% and up to 50% than silica fume concrete and is less sticky than conventional silica fume compositions. Workability and finish are also shown.

According to the composition according to the present invention, since the high-performance / high durability shotcrete composition is possible using metakaolin, it is also applicable to the single-shell tunnel method using the shotcrete lining as a permanent support material.

Furthermore, the present invention is the biggest disadvantage of silica fume by using metakaolin instead of silica fume, which is generally used as a mixture for high-strength expression as described above in a general shotcrete composition composed of cement, water, aggregate, admixture and fastener. It overcomes the disadvantage that the amount of high-performance water reducing agent that has high drying shrinkage and high price increases proportionally and contributes to the increase of construction ability of shotcrete.

In addition, the shotcrete is required to use a fastener for early strength development, and unlike concrete, it is sprayed, and thus, when placed on a tunnel wall, significant rebound occurs, and metakaolin admixture is applied as in the present invention. This also has the advantage of preventing the shotcrete from falling off.

Hereinafter, exemplary embodiments of the present invention will be described in detail by way of example, and the present invention is not intended to be limited thereto.

<Example>

1) raw material

In this example, H type 1 ordinary portland cement suitable for commercially available KS L 5201 was used as cement, and coarse aggregate was used as crushed stone having a maximum dimension of 13 mm, and fine aggregate had specific gravity of 2.6, absorption rate of 0.67, and assembly rate of 2.86. Instructor was used.

The particle size of the fine aggregate used in the present invention did not deviate from the KS particle size standard range.

In addition, a commercially available alkali-free liquid type fastener was used in Korea, and metakaolin was used as a mineral admixture in Korea. In the silica fume used in the comparative example, the content of SiO ₂ in ASTM C 1240-95 was 85% or more, and the average particle diameter was 0.2 μm and the powder degree was about 240,000 cm 2 / g.

2) Formulation Design

 Table 2 is a formulation table used to test the effect of the high strength shotcrete composition of the present invention. When blending, the slump was fixed at 10 ± 2cm, air volume 4 ± 1%, water cement ratio 45%, and fine aggregate 68%. At this time, silica fume (SF) and metakaolin (MK) were substituted with 4% and 8% of the cement weight, respectively.

3) Experiment

As described above, the eight specimens designed and blended were subjected to the condensation time, compressive strength, flexural strength, chlorine ion permeation test, and freeze-thawing test, respectively, to compare their performances.

  At this time, the setting time was measured by KS F 2436 "Test method for setting the setting time of concrete by penetration resistance needle".

In addition, compressive strength was implemented by KS F 2405 "Concrete compressive strength test method". That is, a specimen of φ100 × 200 mm was prepared and repeated three times for 1 day, 3 days, 7 days and 28 days of age. In the universal testing machine, the specimen was placed in the center of the test plate, and the specimen was pressurized until the specimen was destroyed at a constant speed. The average load of the specimen was divided by the cross-sectional area, and the average value was defined as the compressive strength. The specimens were cured under water at 23 ± 2 ° C and constant temperature condition at 23 ± 2 ° C and 50% relative humidity for 1 day.

 combination  Maximum size of coarse aggregate (mm)  Slump range (cm)  Air volume range (%)  W / C (%)  Residual rate (%)  Unit material amount (㎏ / ㎥)  water  Cement (C)  Silica Hume (SF)  Metakaolin (MK)  Fine ash  Coarse aggregate  Glidant (g / ㎥)  Quick payment (C × 7%)  P      13      10 ± 2      4 ± 1      49      68      205.8  420  -  -      1124      545  3360      21  SF4  403.2 16.8 (SF = 4% × C) -     4200  SF8  386.4 33.6 (SF = 8% × C) -  MK4  403.2 - 16.8 (MK = 4% × C)  MK8  386.4 - 33.6 (MK = 8% × C)

In order to evaluate the bending performance of each admixture, the bending performance test was carried out according to KS F 2408 "Testing method of bending strength of concrete". Two specimens were prepared for 1 day, 3 days, 7 days, and 28 days by casting directly into a square mold of 150 × 150 × 550㎜. After initial curing, they were cured under constant temperature of 23 ± 2 ℃. The experiment was conducted.

In addition, the increase in permeability in the shotcrete not only lowers the strength due to the expansion of the crack, but also worsens the durability of freezing, melting and the like, so that the permeability has a significant meaning not only in strength but also in durability. In order to find out the indirect permeability of shotcrete according to the mixture of admixtures, the experiment was conducted according to the method of ASTM C 1202 (Electrical Indication of Concrete's Ability to Resist Chloride Ion Penetration), which measures the permeation of chlorine ions as shown in Table 3 below. It was.

 Coulomb (C)  Chlorine Ion Permeability  Condition  〉 4000 400 ～ 2000 2000 ～ 1000 1000 ～ 100 〈100  High Moderate Low Very Low  High W / C (Water Cement Ratio) 0.4 ~ 0.5 (Water Cement Ratio) W / C (Water Cement Ratio) <0.4 Latex Modified Concrete Polymer Concrete

Finally, the shotcrete may undergo repeated freezing and thawing with the change of winter temperature, and the freezing and thawing repetitive action freezes the free water in the shotcrete and expands its volume so that the tensile stress due to the large expansion pressure inside the shotcrete is increased. In order to measure the freezing and melting resistance of the shotcrete according to the mixture of admixtures, in this embodiment, 100 × 100 × 400㎜ square specimens were prepared and cured in a curing bath at 23 ° C. for 14 days. The freezing and thawing test was performed according to KS F 2456, "Method B of the Test Method for Resistance to Concrete for Rapid Freezing-Temperature-A Test Procedure for Rapid Freezing in Air and Melting in Water".

The reference specimens were fabricated, and the freezing effect of dropping the temperature at the center of the specimen from 4 ° C. to 18 ° C. and the melting effect of raising the temperature again to 4 ° C. were repeated. Relative dynamic modulus was measured every 30 cycles, and the test results were prepared by using two specimens.

4) Experiment result

The results for the test are shown in FIGS. As can be seen from Figures 2 to 6, the present invention using a metakaolin admixture showed an improvement in performance in terms of overall mechanical properties such as compressive strength, flexural strength and the like compared to the comparative example using an expensive admixture called silica fume.

Specifically, referring to FIG. 2, which shows the test results of the mixing time of the shotcrete according to the admixture mixing conditions, the setting times were similar regardless of the types and mixing of the admixtures. There was no delay.

This is because the fastening characteristics of the fasteners, which form the net structure and promote the hydration reaction of cement, influence the setting time of shotcrete from the initial reaction with portland cement and water mixture. do.

3 shows the compressive strength test results of the shotcrete according to the admixture blending conditions, the compressive strength of the admixtures containing admixtures showed a large difference in the early age compared to the formulation (P) containing only the fastener. However, as age increases, the strength expression is excellent and the compressive strength is higher.

This is believed to be due to the reaction characteristics of silica fume and metakaolin, which react with calcium hydroxide produced during cement hydration to form an additional CSH gel to cause a pozzolanic reaction.

In addition, the 28-day compressive strength when using metakaolin and silica fume showed up to 50% or more increase in strength compared to the formulation (P) without mixing admixtures, especially shotcrete compressive strength when using metakaolin Was similar to the compressive strength of shotcrete with silica fume or about 2 ~ 3 MPa maximum.

This suggests that metakaolin having a larger particle size than silica fume enhances the rigidity of the microstructure, thereby increasing the compressive strength of the shotcrete thereof. As a result, it was found that although the domestic production is possible and the unit price is considerably lower than that of silica fume, metakaolin has a performance equivalent to that of silica fume.

In addition, as a result of testing the bending strength of each shotcrete according to the admixture mixture conditions, as shown in Figure 4, the 28-day bending strength when using the metakaolin and silica fume is up to 40% or more compared to the formulation (P) containing no admixture It showed strength enhancing effect. In particular, it was confirmed that the flexural strength of the blend containing metakaolin (MK4 and MK8) compared to the blend containing silica fume (SF4 and SF8) showed a maximum increase in strength of 1 MPa.

As a result of the chlorine ion permeation test of each shotcrete according to the admixture mixture conditions, Figure 5 shows that the water permeability of the shotcrete of the formulation without admixtures is "high permeability", but the permeability of the admixture containing the admixture is " It can be seen that the impermeability is improved from what appears as "low permeability" or "very low permeability".

  In particular, the average particle diameter of silica fume is 0.1 μm, which is about 100 times smaller than that of cement particles (10 μm). Thus, when the amount of admixtures is the same, due to the micro-filter effect of filling the voids between the cement particles. In the case of mixing and mixing silica fume, it was confirmed that (SF4 and SF8) showed the lowest permeability.

In contrast, the average particle diameter of metakaolin is about 1 to 2 μm smaller than cement particles (10 μm) but larger than silica fume (0.1 μm), so that the metakaolin single incorporation compared to the permeability of the silica fume-incorporated shotcrete using the same amount Shotcrete's permeability was somewhat large. However, the permeability of shotcrete by metakaolin does not show a significant difference compared to the shotcrete incorporating silica fume, and is less than "low permeability", which is sufficient to prevent groundwater leakage in tunnels and underground spaces.

Therefore, the shotcrete mixed with metakaolin exhibits the same permeability-related durability as the shotcrete mixed with the expensive silica fume. As described above, the mechanical properties (compressive strength, flexural strength, etc.) can be obtained more positively. appear.

Finally, as a result of the freeze-thaw resistance test of each shotcrete according to the admixture conditions, as shown in FIG. 6, the freeze-thaw resistance showed a great difference depending on the use of the admixture, as in the previous chlorine ion passing charge. The larger the amount used (8% of the weight of the cement), the higher the freeze-thawing resistance.

In addition, the freeze-thaw resistance according to the use of metakaolin and silica fume showed a very good freeze-thaw resistance, as the relative dynamic modulus of elasticity measured at 300 times was almost 90% or more. Moreover, even in the absence of admixtures, the relative dynamic modulus measured at 300 times showed excellent freeze-thawing resistance of more than 85%.

The above experimental results are summarized as follows.

First, due to the effect of metakaolin improving the stiffness of the microstructure, the mechanical properties (compressive strength and flexural strength) of the shotcrete incorporating metakaolin showed almost equal or better performance improvement compared to the shotcrete incorporating silica fume. It was.

Second, regardless of the type of admixture and presence of admixture, it showed similar setting time and there was no delay effect of setting time according to mixing condition of admixture. Therefore, there was no problem in the construction and early strength of shotcrete mixed with metakaolin.

Third, the durability of shotcrete mixed with metakaolin was not significantly different from that of shotcrete using silica fume. In both blends, the shotcrete's permeability was either "low permeability" or "very low permeability". In addition, all showed excellent freeze-thawing resistance of more than 90%. However, in terms of permeability, the lower the permeability of the shotcrete, the higher the durability of the shotcrete. However, as the permeability of the shotcrete becomes too low, the ground water pressure that the shotcrete must support increases, thereby increasing the strength of the shotcrete. There may be a problem to increase the thickness of the shotcrete. This problem can be solved by adjusting the substitution rate (cement substitution rate) of metakaolin according to the durability criteria of shotcrete desired by the designer / user.

Therefore, in the shotcrete composition using fine powder metakaolin as admixture while using general portland cement and alkali-free fastener, the content of metakaolin is in the range of 3 to 10% by weight of the cement weight as in the conventional silica fume admixture. It is preferable to use, and considering the permeability of the prepared shotcrete, it is more preferable to use within the range of 4 to 8% by weight of the cement weight.

Since the composition using metakaolin according to the present invention is relatively large particles, there is an advantage of using a small amount of a high performance water reducing agent than conventional portland cement concrete. That is, at the same water-cement ratio, metakaolin concrete can reduce the content of high-performance sensitizer by at least 20% and up to 50% than silica fume concrete, and it is less sticky than the composition using conventional silica fume. Together.

According to the composition according to the present invention, since the high-performance / high durability shotcrete composition is possible using metakaolin, it is also applicable to the single-shell tunnel method using the shotcrete lining as a permanent support material.

Furthermore, the present invention is the biggest disadvantage of silica fume by using metakaolin instead of silica fume, which is generally used as a mixture for high-strength expression as described above in a general shotcrete composition composed of cement, water, aggregate, admixture and fastener. It overcomes the disadvantage that the amount of high-performance water reducing agent that has high drying shrinkage and high price increases proportionally and contributes to the increase of construction ability of shotcrete.

In addition, the shotcrete is required to use a fastener for early strength development, and unlike concrete, spraying is required so that significant rebound occurs when placed on the tunnel wall, as in the present invention. The application also has the advantage of preventing the shotcrete from falling off.

Claims (3)

Per 1㎥ of shotcrete, Coarse aggregate of 500 to 700 kgf weight with a maximum dimension of 10 to 15 mm; Fine aggregate weighing 1,000-1,200 kgf; 300-450 kgf weight cement; Water having a water-cement ratio of 40 to 60% by weight; Liquid type fastener of 4 to 10% by weight of the cement weight; And 3-10% by weight of metakaolin admixture of the cement weight; High performance shotcrete composition using metakaolin comprising a. The high performance shotcrete composition of claim 1, wherein the metakaolin is used in an amount of 4 to 8% by weight of the cement weight. The high performance shotcrete composition using metakaolin according to claim 1 or 2, wherein a high performance water reducing agent having a weight of 4 to 10 kgf per 1 m 3 of the shotcrete is further added.

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