practices for crack control of concrete in · pdf fileregarding control of thermal effect at...

CONCRACK 3 – RILEM-JCI International Workshop on Crack Control of Mass Concrete and Related Issues Concerning Early-Age of Concrete Structures, 15-16 March 2012, Paris, France

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PRACTICES FOR CRACK CONTROL OF CONCRETE IN JAPAN

Taira Yamamoto (1) and Takeshi Ohtomo (1)

(1) Civil Engineering Division, TAISEI Corporation, JAPAN

Abstract

Several Japanese practices for crack control are presented at first. Although usage of low heat type cement is one of most common applications, associated construction technologies are also playing important roles to realize good quality concrete. Bridge foundation concrete for the world longest suspension bridge, underground LPG tank and world deepest immersed tube tunnel are presented. Secondly, procedures to predict thermal cracking of concrete based on JCI (Japan Concrete Institute) Guideline are presented. The objective structure is LNG tank with 80 m in diameter and 36 m in height. By using 3-D FEM analysis, countermeasures to mitigate thermal cracking are studied. Since such quantitative study is considered to be a rational approach and can be studied in advance (in design stage), the method presented in JCI Guideline has recently been becoming a prevailing procedure for early age crack control in Japan.

Résumé On présente tout d’abord plusieurs pratiques employées au Japon pour la maîtrise de la

fissuration. Si l’utilisation de ciment à basse chaleur d’hydratation est une des méthodes les plus courantes, les techniques constructives associées jouent aussi un rôle important pour réaliser un béton de bonne qualité. Ceci est illustré avec le béton de fondation du plus long pont suspendu du monde, un réservoir souterrain de gaz pétrolier liquéfié, et le tube du tunnel immergé le plus profond du monde. On présente ensuite les procédures permettant de prédire la fissuration d’origine thermique du béton sur la base des recommandations du Japan Concrete Institute (JCI). L’ouvrage considéré est un réservoir de gaz naturel liquéfié de 80 m de diamètre et 36 m de haut. A partir d’une analyse tridimensionnelle aux éléments finis, on étudie les mesures de prévention et de limitation de la fissuration d’origine thermique. Dans la mesure où une telle étude quantitative apparaît comme rationnelle et permet l’anticipation (au stade de la conception), la méthode des recommandations du JCI est en passe de devenir la procédure la plus répandue au Japon pour assurer la maîtrise de la fissuration.

1. INTRODUCTION

Regarding control of thermal effect at early age, various practices have been carried out in Japan. Low-heat Portland cement (LH) was standardized in Japanese Industrial Standards (JIS) in 1997. Since then, a lot of practices to mitigate thermal effects by using LH have been


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conducted. In chapter 2, several practices with LH/low heat type cement and their associated technologies are presented.

As for prediction method for thermal crack at early age, some methodologies are described in “Guidelines for Control of Cracking of Mass Concrete 2008” (hereinafter, JCI Guideline), Japan Concrete Institute (JCI), and “Standard Specification for Concrete Structures”, Japan Society of Civil Engineers (JSCE). Those methods are rationally utilized to predict thermal crack at early age quantitatively by computational analysis and to mitigate thermal effect by considering countermeasures appropriately. Especially in the method of JCI Guideline, utilizing of 3-dimensional FEM analysis is premised, and various complicated conditions, such as structural dimension, construction procedures and temperature histories, etc., can be taken into account. In recent years, prediction of early age crack by computational analysis in design stage makes it possible to take appropriate actions in advance to mitigate thermal crack of concrete structure. In chapter 3, as an application example of JCI Guideline, design and construction of LNG tank is presented.

2. PRACTICES USING LOW-HEAT TYPE CEMENT AND ASSOCIATED TECHNOLOGY

In the following, low heat type cement application and associated technology are presented.

2.1 Akashi-Kaikyo Bridge Pier Foundation Concrete Akashi-Kaikyo Bridge is a world longest suspension bridge with its centre span of 1991 m

connected between Honsyu (main) island and Awaji island. The foundation of its main tower with 80 m in diameter and 70 m in height is a huge reinforced concrete structure, constructed by steel caisson method.

Underwater concrete was used for the lower part of the foundation, and the rest is normal concrete placement in the air. Underwater concrete was anti-washout concrete whose cement combination was 3-component low heat type using blast-furnace slag and fly ash, and adiabatic temperature rise was restricted to 30 degree. Further, pre-cooling system was introduced to control the concrete temperature after mixing below 20 degree. Cold water is used to cool coarse/fine aggregate together with depressurize cooling by centrifugal dehydration.

1) Construction 2) PIC form

Figure 1: Akashi-Kaikyo Bridge


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When upper part concrete was placed, steel caisson was used as an outer form. Top portion of the foundation is surrounded by PIC (Polymer Impregnated Concrete) board as a permanent concrete form in order to keep long time durability under the marine environment. Low heat type cement was used to mitigate thermal effect, and furthermore, pre-cooling method was also introduced. Outer temperature effect was reduced by water curing method. Especially in winter season, hot water curing was utilized.

2.2 Underground LPG Tank Base Slab with Bifurcation Pipe Pumping Method 60,000 m3 underground LPG tank was constructed in Yokohama, Japan. Its diameter and

depth are 45 m and 38 m, simultaneously. Although the bottom slab is not water resisting type, high sectional forces are expected because temperature of LPG is -45 ℃. Therefore, higher quality concrete was required for the bottom slab. Thickness of bottom slab is 1.5 m, their design strengths of outer and centre portions are 40 N/mm2 and 27 N/mm2, simultaneously.

Three-component low heat type cement including blast-furnace slag and fly ash was used. In order to construct high quality homogeneous concrete, bifurcation piping method was introduced. Since pumping pipe is equally diverged one after another from one mobile concrete pump, concrete is equally distributed to the outlets. Accordingly, concrete can be equally placed for a wide-ranged area in terms of placing height.

Although the objective tank was rather thin bottom slab of 1.5 m, the larger the thickness of bottom slab is, the higher the usefulness of this method becomes.

Figure 2: Bifurcation Pipe Pumping Method

2.3 Soil Retaining Diaphragm Wall using High Strength Concrete For the excavation to construct the underground LPG tank described in subsection 2.2,

cylindrical diaphragm wall was constructed. Since this wall was designed as a permanent water tight structure, requirement of concrete quality was very high.

As concrete design strength of lower part was 60 N/mm2, LH was used. Since it was very difficult to control curing conditions for diaphragm wall in general, analytical study for element division length was carried out in advance to mitigate crack risks. Since cracks tend to occur in secondary element due to constraint from the primary element, analytical study was conducted in order to mitigate crack risks in secondary element.


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Figure 3: Diaphragm Wall using High Strength Concrete

2.4 Immersed Tube Tunnel under Bosphorus Strait using Floating Placement Method

About 1.4 km immersed tube tunnel for railway was constructed 60 m beneath the Bosphorus strait in Istanbul, Turkey, which is world deepest immersed tube tunnel. The 1.4 km tunnel section is a part of the whole tunnel section of 13 km, and other tunnel sections have been constructed by Tunnel Boring Machine or NATM. This tunnel was designed based on Japanese design codes and standards. Box shaped reinforced concrete structure with 15.3 m in width and 8.6 m in height was constructed element by element, whose maximum length was 135 m. Due to restriction of element fabrication yard, upper half of concrete was placed by floating placement method. Since long time durability of 100 years was required, early age crack was not allowed by the employer. Three-component low heat type cement including fly ash and micro silica was used. Not only temperature effects but also effects due to floating placement were considered in the study for the effects and countermeasures during construction. Although 2-D FEM analysis by using CP method was required by the employer, 3-D FEM analysis according to JCI guideline was also conducted in order to predict the early age crack rationally since construction procedure and conditions were not so simple.

1) Tunnel Element 2) Floating Placement

Figure 4: Floating Placement Method for Immersed Tube Tunnel


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Figure 5: 3D FEM Analysis for Immersed Tube Tunnel

3. JCI GUIDELINE APPLICATION FOR LNG TANK In this chapter, design and construction of 180,000 m3 LNG tank is presented in terms of

study of the thermal effect during construction as an example of JCI guideline application.

3.1 Outlines LNG tank is composed of inner and outer steel layer and outer concrete layer (tank wall

and bottom slab). The objective concrete layer is a cylindrical structure with its inner diameter of 80 m and liquid height of 36 m. Thickness of upper/lower tank wall is 0.6 m / 1.2 m, simultaneously (Fig. 6). Since 1st lift (lowest part) of tank, which is 2.8 m in height, tends to be affected most severely in terms of early age crack, the study in design stage shows the result that LH needs to be used together with pipe cooling method. Tank wall is required to prevent LNG leakage from inside to outside in case of occurrence of steel tank leakage. Therefore, early age cracks need to be prevented as much as possible.

Figure 6: Dimension of Tank

3.2 Design standard Tank wall is designed based on “Guidelines for LNG Aboveground Storage Tank”

(hereinafter, LNG Guideline), Japan Gas Association. In this guideline, the method for


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studying the early age crack is described as to follow the method described in “Standard Specification for Concrete Structures”, Japan Society of Civil Engineers (JSCE). This method is CP method, and can be applicable to rather simple structure. However, tank wall needs to be studied in more complicated conditions, such as three dimensional constraint conditions and pipe cooling conditions, etc. Therefore, JCI Guideline is applied in this study.

3.3 Target thermal cracking index In LNG Guideline, thermal cracking probability of 5 % or less is required. This

corresponds to thermal cracking index of 1.85 in JCI Guideline.

3.4 Analysis for verification In order to determine the cement material and associated countermeasures, verification

studies are conducted to satisfy the target thermal cracking index of 1.85. The analysis was conducted by axi-symmetric FEM analysis. The analysis model is shown in Figure 7. The analysis consists of two phases, heat transfer analysis and stress analysis. In heat transfer analysis, non-steady state analysis is conducted, and then temperature history of concrete is obtained. The history data is succeeded to the stress analysis and is used to calculate stress history.

3.4.1 Conditions for heat transfer analysis The following conditions for heat transfer analysis are considered. a) Ambient air temperature: Average daily temperature is used obtained from the

Meteorological Agency. b) Time of concrete placement: Concrete placement is carried out in middle of June.

Ambient air temperature is 24℃. c) Heat transfer coefficient, thermal properties of concrete and thermal properties of

ground: Those are determined using recommended values in JCI Guideline. d) Adiabatic temperature rise of concrete: Adiabatic temperature test is conducted. The

results of the test are revised to be fit to the adiabatic temperature curve described in JCI Guideline. Figure 8 shows the adiabatic temperature rise used in this study.

05

101520253035404550

0 1 2 3 4 5 6 7 8 9 10

Age（day）

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ture

rise

(ºC)

Figure 7: Analysis Model Figure 8: Adiabatic Temperature Rise


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3.4.2 Conditions for stress analysis The following conditions for heat transfer analysis are considered. a) Compressive strength of concrete: Compressive strength tests are conducted. The

results of the tests are revised to be fit to the compressive strength curve described in JCI Guideline.

b) Tensile strength and Young’s modulus of concrete: Both tests are conducted and their results are revised to be fit to the curves as a function of compressive strength described in JCI Guideline.

0

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Ft=0.18f'ck0.785

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E=9420f'ck0.332

Figure 9: Conditions of Stress Analysis

3.5 Results of Analysis Figure 10 shows the maximum temperature distribution and the minimum crack index

distribution. From the analysis, pipe cooling method is indispensable to satisfy the target thermal cracking index of 1.85.

1) Maximum Temperature 2) Minimum Crack Index

Figure 10: Analysis Results

3.6 Comparison between analysis and measurement Concrete placement started 9:00 a.m. and finished 5:00 p.m. Pipe cooling started 4:30 p.m.

and lasted about 2 days. Figure 11 shows the comparison between analysis and measurement


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results in terms of temperature history. From this result, analysis conditions seem to be slightly severe because both temperature rise and drop of analysis are greater than actual measurement data.

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0 24 48 72 96 120 144 168 192 216 240 264 288 312 336Time (Hr)

Tem

p. (º

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Result of Analysis

Start of cooling in Analysis

End of cooling in Analysis

Result of measurement

Outer Temperature

Pipe cooling Pipe cooling

Figure 11: Results of Measurement

4. CONCLUSIONS In this paper, various Japanese practices in terms of thermal crack control are presented. As

for JCI guideline, it has been becoming a prevailing procedure to predict/control thermal cracking at early age in Japan. In order to achieve more accurate prediction, accumulation of application data and results has been carried out.

REFERENCES [1] Kashima, S., Sakamoto, M, Okada, R., Iho, T. and Nakagawa, Y., ‘Automation and robotics of

underwater concreting in huge scale steel caisson, the main tower foundations of the Akashi Kaikyo bridge’, 1991 Proceedings of the 8th ISARC, Stuttgart, 1991, 179-188.

[2] Ohtomo, T., Sakamoto, J. and Shindoh, T, ‘Applications of high quality underground diaphragm wall using self-compacting concrete to various underground constructions’, Proceedings of the 1st fib congress, Osaka, 2002, 107-114.

[3] Aoyagi, Y. and Goto, S., ‘Developments in concrete technology for LNG in-ground storage tanks in Japan’, 21st Conference on our World in Concrete & Structure, 1996.

[4] Gokce, A., Koyama, F., Tsuchiya, M. and Gencoglu, T., ‘The challenges involved in concrete works of Marmaray immersed tunnel with service life beyond 100 years’, Tunnelling and Underground Space Technology, Vol. 24, Issue 5, 2009, 592-601.

[5] Japan Concrete Institute, ‘Guidelines for control of cracking of mass concrete 2008’, 2011.

practices for crack control of concrete in · pdf fileregarding control of thermal effect at...

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