• Journal of the Korea institute for structural maintenance and inspection
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• v.15 no.6
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• pp.195-200
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• 2011

KS F 2782 (for shotcrete accelerators) standard cross-section of tunnel construction or repair is a reinforcement used in the field of shotcrete accelerators as a criterion in assessing shotcrete performance. Thus, KS F 2782 by standard accelerator will determine the nature of the product of concrete accelerators that will be used to record variations in the product roles, through determination of the quality of the experimental method to identify only the quality of the many variables that exist. This evaluation standard has so far distinguished accelerator products in indoor experiments that do not meet quality standards but were mostly for an on-site accelerator mixed with the shotcrete after being quite satisfied with the level of quality in a certain number of products. This observation is derived from the results of an indoor experiment considered to verify whether the site is suitable for indoor experiments, and whether its actual location in the city is relevant to the accelerator quality, to find a way to test if it fits. This study centers on the material conditions of the shotcrete accelerator and a variety of experimental results, and used the Gilmore needle to compare the compressive strength and KS F 2782 specification of the accelerator as a means to ensure product quality conformity analysis and for further research experiments. In conclusion, a portion of KS F 2782 standard that fixes the problems that can be resolved from the ground up as a whole is not a review for the domestic reality. As an indoor experiment to ensure uniformity in the field when applied in a sufficient correlation, complement must be in place.

• Journal of the Korea Concrete Institute
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• v.17 no.6 s.90
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• pp.879-884
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• 2005

The setting time is a very important factor affecting the quality of tunnel lining and reinforcement of inclined slope etc. Currently, however, the quality criteria of accelerating admixture to improve it is not established well. In this study, evaluation on setting time measuring methods of cement mixed a accelerating admixture (AA) was performed using Gillmore and Vicat needle test methods. For both test methods, the setting time for addition at a time was better than post addition regardless of initial setting and final setting. For Gillmore needle test method, two types of measuring methods were selected and it is noted that setting time with cement type under the same accelerating admixture can be different. Accordingly, manufacturing company shall develop a less sensitive accelerating admixture to cement type. For Vicat needle test method, six types of measuring methods were used and a proper measuring method of the admixture were proposed as follows: (1) the temperature of materials used shall be controlled exactly and (2) to evaluate its properties, an admixture usage of $5\%$ (ratio of cement weight) is recommended.

• KSCE Journal of Civil and Environmental Engineering Research
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• v.26 no.2C
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• pp.121-131
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• 2006

The strength standard of shotcrete in Korea is relatively lower than that in Europe where high-strength shotcrete has been developed and actively applied to the common practice, so it is hard to test a quality of high-strength shotcrete. In this study, field test was performed to find a solution improving the strength and a long-term durability on domestic shotcrete. In field test, a high-strength shotcrete was produced using high-quality additions and accelerators, and an effect of additions and accelerators was observed. In addition, quality test based on EFNARC was also performed. As a result of field test, a promotion ratio of early strength is 90~97% in case of using alkali-free accelerators, therefore, alkali-free accelerators had an effect on an increase of early strength on shotcrete. A compressive strength of shotcrete using Micro-silica fume was 45.2~55.8MPa and flexible strength was 5.01~6.66MPa, so a promotion ratio of strength was 37~79%, 17~61% respectively. It was showed that increment effect of strength by the silica fume replacement of 7.5~10% for cement mass was remarkable. As a result of test, it was possible to apply high-strength shotcrete to the domestic practice using high-quality additions such as Micro-silica fume and accelerators such as alkali-free.

• Journal of Korean Tunnelling and Underground Space Association
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• v.16 no.2
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• pp.135-148
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• 2014

This study was performed to evaluate the performance about Improved C12A7 based mineral accelerator (ICM) increased in initial and long-term strength. ICM was developed to overcome the long-term strength decrease in existing accelerator. To evaluate the performance of ICM according to addition rate, setting time, compressive strength, and flexural strength tests were conducted in laboratory. In results, initial setting time was slower, final setting time was faster than existing $C_{12}A_7$ based mineral accelerator (CM) when usage of ICM 6%. In compressive and flexural strength, existing CM was higher than ICM at 3hours and 1day. After 7days, strength of shotcrete using ICM was increased. Rebound test, compressive strength and flexural strength test with optimum addition rate through the laboratory test were conducted in field. Field experiment results were the same as laboratory test. Long-term strength performance of ICM was superior to existing accelerator.

• Journal of the Korea Concrete Institute
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• v.17 no.3 s.87
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• pp.427-434
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• 2005

An experimental investigation was carried out in order to verify the compressive strength, flexural strength, equivalent bending strength, rebound rate of shotcrete according to silicate accelerator, aluminate accelerator, cement mineral accelerator respectively and to especially evaluate the performance of shotcrete using cement mineral accelerator for high quality. The test result of compressive strength was showed that all accelerators were satisfied the required test value for each age, for the requirement of having the $75\%$ or higher compressive strength ratio to plain concretes at 28 days, cement mineral accelerator with $87\%$ compressive strength ratio was only satisfied. In flexural strength test, cement mineral accelerator was satisfied the flexural strength requirement in steel fiber reinforced shotcrete for each age. Aluminate type was conformed to the requirement for 28 days, but not at 1 day, silicate type was failed to satisfy standard requirement. Rebound rate was measured between $11{\~}19\%$ and cement mineral accelerator was showed comparatively lower rebound rate. Based on the test results, cement mineral accelerator exhibited excellent strength improvement and lower rebound rate compared to the conventional accelerator, its result is showed the possibility of making high performance shotcrete.

• Journal of the Korea Concrete Institute
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• v.16 no.1 s.79
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• pp.70-78
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• 2004

Although set accelerating agents are used generally in New Austrian Tunneling Method, the standards for test methods and quality of set accelerating agents are not prescribed domestically. In this study, the proprieties of the various standards and the characteristics of set accelerating agents for shotcrete were evaluated. The alkali contents of set accelerating agents based on silicate, aluminate and cement were higher than those of alkali-free ones. From the result, it is thought that the quality control of aggregate should be enhanced and that the number of test cycle of alkali-aggregate reaction should be increased. The setting times of cement paste with set accelerating agents based on silicate and alkali-free ones were different largely with mixing methods. Compressive strength of mortar with set accelerating agents based on silicate, aluminate and cement at one day satisfied the specifications of Korea Concrete Institute. However, the strength ratio compared to control mix at 28 days showed as $50{\~}65\%$ except for the alkali-free set accelerating agents. As a results of setting time and strength test, the establishment of domestic standards that can reflect the characteristics of materials and construction methods of tunnels and that can increase quality of set accelerating agents is required immediately.

• Journal of Korean Tunnelling and Underground Space Association
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• v.12 no.4
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• pp.285-294
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• 2010

As there are increasing number of tunneling works these days, shotcrete is used as primary supports in order to secure the stability of tunnels. The quality of shotcrete has a direct influence on tunnels, because it is a primary support which secures the stability of completed tunnels as well as the stability of tunnels under excavation. Especially in case that ordinary shotcrete is used under weak rock conditions or at water gushing sections, more shotcrete is needed and rebound ratio increases. As a result, it is hard to keep economic feasibility. In addition to it, in subway construction, there are cases of separated excavation and it may have a bad influence on construction period or quality. Therefore, in this study, we are going to evaluate the early strength of powder type accelerator.

• Journal of the Korean Geotechnical Society
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• v.22 no.10
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• pp.77-91
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• 2006

Domestic practices in shotcrete use have developed in many respects even now, but it still has issues about material, construction, quality standard and so on. In overseas, the construction using high strength shotcrete with $39.2{\sim}58.8 MPa$ of compressive strength is becoming common based on the shotcrete technology of high strength and durability. However, domestic shotcrete design strength is low at around 20.6 MPa of compressive strength and a long term durability is also insufficient. In this paper, field tests using high-quality additives and accelerators were performed to obtain the improvement of shotcrete strength and EFNARC standard was used to evaluate the field test results. In addition, deterioration test combined with the freezing-thawing and carbonation was also performed in order to investigate a long-term durability of high-strength shotcrete. As a result of the field test, the promotion ratio of early strength was $90{\sim}97%$ in case of using alkali-free accelerators. And the compressive strength of the shotcrete using Micro-silica fume was $45.2{\sim}55.8MPa$ and flexible strength was $5.01{\sim}6.66MPa$, so the promotion ratio of strength was $37{\sim}79%$ and $17{\sim}61%$ respectively. The promotion effect of strength by silica fine additives ratio of $7.5{\sim}10%$ for cement mass was much superior to the other cases. It was especially examined that using Micro-silica fume reduced deterioration due to mixed steel fiber and improved a long-term durability of shotcrete.

• Journal of the Korea Academia-Industrial cooperation Society
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• v.13 no.10
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• pp.4884-4890
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• 2012

In this dissertation, the shrinkage and durability of shotcrete were examined. Analysis and comparison of shrinkage against the strength of shotcrete was investigated after performing actual shotcrete shooting for each mix of different percentage of expansive agent. The research on restrained shrinkage with different percentages of expansive agent used (3%, 7%, 11% and 15%) shows that the maximum shrinkage in all samples occurred in between 13-15 hrs after shotcrete shooting. And it is noticed that increasing the percentage of expansive agent can lower the shrinkage. And hence, the 15% of expansive agent not only lowered the shrinkage But it also lowered the strength.