Properties of Expansive-Ultra ete

Transcription

1 SP Properties of Expansive-Ultra High-Str Stren ength Conc oncret ete by M. Suzuki, I. Maruyama, and R. Sato Synopsis: In order to decrease cross sectional area of structural members, ultra high strength concrete with compressive strength over 150 MPa is required for building structural members, which needs no steam curing. In the present study, concrete is made of silica fume cement which is composed of low heat type cement and silica fume and demonstrates high compactability. Compressive strength of the concrete with water to binder ratio of 0.15 and the effect of hydration heat of binder on compressive strength are investigated experimentally. Effectiveness of expansive additive on reduction of autogenous shrinkage is also investigated. According to the experiment, compressive strength over 150 MPa is gained by adopting appropriate aggregates without steam curing at early ages, while the strength of full sized specimens decreased about 10 % at the age of 91 days. Autogenous shrinkage was reduced from more than 700x10-6 to 0 by expansive additive and shrinkage-reducing admixture. However, expansive additive leads to strength reduction of about 10 %. Keywords: autogenous shrinkage; expansive additive; heat of hydration; high-strength concrete; shrinkage-reducing admixture 1159

2 1160 Suzuki et al. Masahiro Suzuki is a section manager of Technical Research Institute, P.S. Mitsubishi Construction Co., Ltd., Odawara-City, Kanagawa, Japan. His research interests include autogenous shrinkage of high-strength concrete and prestressed concrete members. He is a member of JCI, JSCE, and PCEA. Ippei Maruyama is a research associate of Social and Environmental Engineering Department, Graduate School of Engineering, Hiroshima University, Higashi-Hiroshima, Japan. He received his Doctor of Engineering degree from the University of Tokyo in His research interests include modeling of cement hydration, shrinkage, and creep. He is a member of JCI, JSCE, and AIJ. Ryoichi Sato is a professor of Social and Environmental Engineering Department, Graduate School of Engineering, Hiroshima University, Higashi-Hiroshima, Japan. He received his Doctor of Engineering degree from Tokyo Institute of Technology in His field of interest covers long-term behaviors of reinforced and prestressed concrete members, creep and shrinkage of concrete, thermal effects on structures, and analytical modeling of RC members. He is a member of ACI, IABSE, Fib, and RILEM. INTRODUCTION Ultra-high strength concrete, when restrained, may undergo thermal cracking due to excessive hydration heat when the water to binder ratio is as low as 0.15 that corresponds the unit binder content of more than 1000 kg/m 3. Silica fumes introduced for higher strength often result in larger autogenous shrinkage strain compared with the ordinary concrete (1). Thus, the reduction of the autogeneous shrinkage strain is a crucial task to avoid cracking. Application of shrinkage-reducing agents and expansive additives has been proposed to reduce the autogenous shrinkage strain (2, 3). Autogenous shrinkage behavior of silica fume admixed high strength concrete with an equivalent temperature history as that of a real structure has been studied within a limited range (4). However, little research has been conducted on the reduction of the autogenous shrinkage strain under high temperature history. Excessive use of expansive agent may sacrifice the compressive strength. This study deals with experiments on the autogenous shrinkage strain of silica fume-admixed, high strength concrete subjected to the high temperature history. Effects of the differences in temperature increase on the autogenous shrinkage strain, and of expansive agent and shrinkage reducing agent on compressive and tensile strengths were examined.

3 High-Str Stren ength/high-p th/high-per erform ormanc ance Conc oncret ete 1161 EXPERIMENTS Materials used and mix proportions Materials used in this experiment are shown in Table 1, and the chemical composition of the cement is shown in Table 2. Silica fume substituted for 10.5 percent of the cement was premixed with the cement; the density of premix-cement (hereafter referred to as SF-LC) was 3.08 g/cm 3. The expansive agent was a lime type and generally dosed 20 kg per 1 m 3. Fine and coarse aggregates were selected for the use of high strength concrete prior to the experiments. The mix proportions are shown in Table 3. Dosage of the superplasticizer was determined to produce a slump flow of 60±5 cm, an air content of 2±1 percent. Among three mix proportions, SFLC refers to the control the mix, SFLC-E refers to a mix where the expansive additive was added, and SFLC-E-R refers to a mix where expansive additive and a shrinkage reducing admixture were added. Unit water content and coarse aggregate content were common to all the mixes, 155 kg/m 3 and 0.53 m 3 /m 3 taking into account the smooth passage of the aggregate between reinforcing steel bars. Dosage of the shrinkage reducing admixture was normal while that of the expansive additive was 1.5 times larger than the standard. Method of mixing Method of mixing is shown in Fig. 1. Mortar was mixed for 180 seconds prior to the introduction of coarse aggregate and subsequently the concrete was mixed for 60 seconds. A dual-axis mixer with a capacity of 2000 liters was used for the full-scale model column mixing 1100 liters per batch, and a mixer with a capacity of 55 liters was used for specimens mixing 40 liters per batch. The superplasticizer and the anti-foaming agent were introduced with the mixing water. Test methods Two series of tests were executed: different curing condition, and compressive strength and the autogenous shrinkage strain tests for the specimens using different types of the autogenous shrinkage strain reducing materials. Method of each test is as follows. Fresh concrete -- Slump flow test was based on Japanese Industrial Standard (JIS) A 1150 and air content was determined according to JIS A Setting test -- Setting test was performed to identify the time when the autogenous shrinkage strain starts based on JIS A 6204 appendix 1. Specimens sampled from the fresh concrete were screened to pass a 5-mm sieve under vibration of a rod vibrator. The initial and the final setting were

4 1162 Suzuki et al. determined when the penetration resistance reached 3.5 MPa and 28 MPa respectively. Compressive strength, Young's modulus of elasticity and splitting tensile test The dimensions of the specimens cured under the standard conditions were 100 mm in diameter and 200 mm in length. Compressive strength tests were executed according to JIS A Dimensions of the full scale model column are shown in Fig. 2. In a specimen with a dimension of 900 x 900 x 1100 mm, a core was sampled at a position 100 mm from the center. The shape of an insulation box used in the simplified adiabatic curing is shown in Fig. 3. It has been proven that the insulation box can provide an equivalent thermal environment with that of the full scale model specimens (5). Wire strain gauges 60 mm in length were adhered on the specimen for the compressive strength test. The splitting tensile strength was executed on the basis of JIS A 1113 with a specimen that was cured under the standard condition and had a dimension of 150 mm in diameter and 200 mm in length. Autogenous shrinkage strain at 20 C -- Autogenous shrinkage strain test at a temperature of 20 C is illustrated in Fig. 4. Dimensions of the specimen were 100 x 100 x 394 mm. Test method was based on the JCI method as specified in the Test Method of Autogenous Shrinkage and Expansion of Cement Paste, Mortar and Concrete (revised in 2002)(6) by the JCI Technical Committee for Autogenous Shrinkage. Polyester film was used to cover the top of a specimen after placing. The specimen was demolded on the next day after mixing and subjected to the sealed curing with an aluminum tape of 0.05 mm in thickness to avoid drying shrinkage. A mold type strain gauge was used with an apparent elastic modulus as low as 40 MP minimizing the restriction by the mold gauge. Number of specimen was three for each mix. Autogenous shrinkage strain under the simplified adiabatic condition -- Configurations of autogenous shrinkage strain test under the simplified adiabatic condition are illustrated in Fig. 5. Dimensions of the specimen were 200 x 200 x 600 mm. The thickness of the insulation, 300 mm, was determined to maintain the same maximum temperature as in the full scale model column. The specimen was removed from the insulation box when the concrete temperature approached to the room temperature, and then subjected to the sealed curing wrapped with the aluminum tape of 0.05 mm in thickness. Number of specimen was three for each mix. Temperatures at the center of the specimen were recorded with a thermocouple and were used to compensate the overall strain with the linear thermal expansion coefficient of concrete 10 x Taking into account the maturity the age of specimen is expressed in terms of Arrhenius' formula in Eq. (1) t = t exp / 273 T ( t ) / t (1) i where it is the effective age and ti is the number of day when the temperature is T C (To=1 C) i 0

5 High-Str Stren ength/high-p th/high-per erform ormanc ance Conc oncret ete 1163 RESULTS AND DISCUSSION Behavior of fresh concrete and setting Properties and setting test results of fresh concrete are shown in Table 4. Temperature of concretes Temperature histories at the center of the model column and the specimen cured under the simplified adiabatic condition for the compressive strength test are shown in Fig. 6. Nearly the same result can be seen for both concretes in the maximum temperature and in the rate of temperature increase to the maximum temperature. Difference between the maximum temperature and temperature of the concrete as mixed was 40 C. Temperature histories of specimens made with the SFLC mix for the autogenous shrinkage strain measurement and for the compressive strength test subjected to the simplified adiabatic curing are shown in Fig. 7. The temperature of the specimen for autogenous shrinkage strain measurement is an average of three specimens. The effective age calculated with Eq. 1 corresponding to the initial setting is also shown as a dotted line in Fig. 7. Nearly the same temperature history can be seen for both concretes, and this tendency was also seen in SFLC and in SFLC-E. Difference between the maximum temperature and mixed-up temperature of concrete as mixed was 45 C. It can be concluded that the specimen cured under the simplified adiabatic condition shows nearly the same temperature history as the model column whose development of autogenous shrinkage can be represented by the specimen. It has been reported that the region of a rapid increase in concrete temperature can be regarded as the initial setting; this was also in this experiment. Thus the start of autogenous shrinkage strain of the specimen cured under the simplified adiabatic condition can be evaluated with Eq. 1 in terms of the effective age and the initial and the final setting at a constant temperature of 20 C. Mechanical properties Development of compressive strength of the specimen made with the SFLC mix is shown in Fig. 8. Specimens cured under the standard condition, the simplified adiabatic condition and cored from the model column are marked in triangle, diamond and round respectively. The marker in white represents the experiment comparing the specimens cured under the standard condition and those cured under the simplified adiabatic condition. The marker in black represents the other experiment including cores sampled from the model column. The 91-day compressive strengths were 170 MPa, 168 MPa and 155 MPa for specimens cured under the standard condition, cured under the simplified adiabatic condition, and cored from the model column respectively satisfying the targeted compressive strength of 150 MPa. The other experiment is under way (markers in white);

6 1164 Suzuki et al. but it shows similar tendency and the reproducibility of the simplified adiabatic curing method may be confirmed. Comparison of the compressive strength between specimens cured under the standard condition, the simplified adiabatic condition and cored from the model column is shown in Fig. 9 as normalized with the 91-day compressive strength of the standard cured specimen. Compressive strength of the cored specimen became 10 percent lower than that cured under the standard condition and 5 percent lower than that cured under the simplified adiabatic condition. An appropriate compensation may be necessary when estimating the compressive strength of a structure by the specimens cured under the standard condition and those cured under the simplified adiabatic condition. Effects of the expansive additive and the shrinkage reducing admixture on the compressive strength are shown in Table 5 by each mix and by ratios to the compressive strength of SFLC mix. Specimens cured under the standard condition showed approx. 10 percent lower strength in SFLC-E and SFLC-E-R mixes, while almost no difference was observed, regardless of the mix, for specimens cured under the simplified adiabatic condition. Since there was no difference between compressive strengths of SFLC-E and SFLC-E-R mixes at the same age, the decrease in compressive strength of specimen cured under the standard condition may be attributed to the use of expansive additive: a dosage 1.5 times greater than the standard may result in a coarser concrete pore structure. Young's modulus of elasticity and splitting tensile strength of specimens cured under the standard condition are shown in Table 6. The 28-day Young's modulus of elasticity was approx. 50 GPa and the splitting tensile strength was approx. 6 MPa. Autogenous shrinkage strain Development of averaged autogenous shrinkage strains in the specimens cured under the simplified adiabatic condition and under a constant temperature of 20 C are shown in Fig. 10. The effective age was calculated with Eq. 1. The starting point of autogenous shrinkage strain has been defined as the time of initial setting in the JCI method while the final setting was used in this study because the collection of basic data on the autogenous shrinkage stress estimation has been intended (7). The autogenous shrinkage strain of SFLC specimens, without any expansive additive and shrinkage reducing admixture, cured under the simplified adiabatic condition was 700x10-6 at the effective age of 40-day, while that cured under constant temperature of 20 C was 450x10-6 exhibiting a marked difference due to the curing conditions. On the other hand, the autogenous shrinkage strain of specimens with expansive additive and/or shrinkage reducing admixture showed almost no difference, by the curing conditions at the effective age of 40-day. Relationship between effective age and shrinkage reduction is shown in Fig. 11. Specimens were made with SFLC mix and cured under a constant temperature of 20 C, as well as under the simplified adiabatic condition. Both the expansive additive and shrinkage reducing admixture were effective

7 High-Str Stren ength/high-p th/high-per erform ormanc ance Conc oncret ete 1165 in reducing the autogenous shrinkage strain, which was 280x10-6 at the effective age of 40-day with the expansive additive and 400x10-6 with the expansive additive and shrinkage reducing admixture for the specimen cured under a constant temperature of 20 C. This may be attributed to a synergetic effect of the admixtures: a direct constraint of the autogenous shrinkage strain by the expansive additive and reduction of surface tension of free water that may induce the autogenous shrinkage strain by shrinkage reducing admixture. The autogenous shrinkage strain of specimen cured under the simplified adiabatic condition was 430 x 10-6 in SFLC-E (solely by the expansive additive) and 650 x 10-6 in SFLC-E-R (by both) exhibiting a marked difference with those cured under a constant temperature of 20 C. This may imply the effect of temperature history on the reduction of the autogenous shrinkage strain even though the expansive additive and/or the shrinkage reducing admixture is introduced. Effect of the shrinkage reducing admixture by curing conditions is shown in Fig. 12. Effectiveness was expressed by subtracting the shrinkage reduction of SFLC-E form that of SFLC-E-R. Shrinkage reducing effect showed maximum at the effective age approx. 2-day and no subsequent development was observed regardless of the curing conditions. The expansive additive seemed to contribute to the shrinkage reduction in a sustained manner as no increase in the autogenous shrinkage was observed in SFLC-E and SFLC-E-R mix. CONCLUSIONS Major findings of this study can be summarized as follows. (1) A high strength concrete of water-binder ratio of 0.15 was prepared with a low-heat portland cement and a silica fume of 10-percent substitution. The 91-day compressive strength of the concrete was 170 MPa when cured under the standard condition while that subjected to the high temperature history of approx. 70 C became 10 percent lower. (2) Reduction in 28-day compressive strength due to the dosage of the expansive additive, 1.5 times greater than the standard, was 10 percent when cured under the standard condition and 5 percent when cured under the simplified adiabatic condition. (3) Autogenous shrinkage strain at the effective age of 40-day depended on the curing conditions and was 700x10-6 when cured under the simplified adiabatic condition whose temperature history was equivalent to the full-scale model column, and 450x10-6 when cured under the standard condition at a constant temperature of 20 C. (4) Both the expansive additive and the shrinkage reducing admixture were able to reduce the autogenous shrinkage strain to a substantial degree. Depending on the curing condition of specimens, the autogenous shrinkage strain reducing effect was more notable when the specimen was cured in the simplified adiabatic condition rather than cured in the standard condition at a constant temperature of 20 C. (5) The expansive additive was effective in minimizing the development of the autogenous shrinkage strain.

8 1166 Suzuki et al. REFERRENCES 1. E. Tazawa, S. Miyazawa, Influence of Binder and Mix Proportion on Autogenous Shrinkage of Cementitious Materials, Journal of Materials, Concrete Structures and Pavements of JSCE No.502, V-25, 1994, pp M. Tanimura, H. Hyodo, T. Sato, R. Sato, An Investigation of Reducing Shrinkage of High Strength Concrete, Proceedings of The Japan Concrete Institute, Vol.22, No.2, pp , M. Tanimura, H. Hyodo, R. Sato, Autogenous Expansion/Shrinkage Properties of High-Strength Concrete Using Expansive Admixture Proceedings of The Japan Concrete Institute, Vol.24, No.1, pp , H. Hashida, N. Yamazaki, A Study on Composed Deformation of Autogenous Shrinkage and Thermal Expansion due to Heat of Hydration in High-strength Concrete, Concrete Research and Technology of The Japan Concrete Institute, Vol.13, No.1,pp.25-32, S. Odaka, F. Sakuramoto, K. Suzuki, K. Yanagida, Experimental Study on Strength in Structures of High Strength Concrete Using Belite Rich Cement, Summaries of Technical Papers of Annual Meeting Architectural Institute of Japan, pp , Work of JCI Committee on Autogenous Shrinkage of Concrete, pp.51-54, H. Cheong, H. Kawano, H. Watanabe, S. Sato, Coefficient of Thermal Expansion of High Strength Concrete, Proceedings of The Japan Concrete Institute, Vol.22, No.2, pp , 2000

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10 1168 Suzuki et al. Fig. 1 Method of mixing Fig. 2 Dimensions of the full-scale model column and the core sampling position

11 High-Str Stren ength/high-p th/high-per erform ormanc ance Conc oncret ete 1169 Fig. 3 Simplified adiabatic curing box Fig. 4 Autogenous shrinkage strain test at a temperature of 20 C Fig. 5 Autogenous shrinkage strain test under the simplified adiabatic condition

12 1170 Suzuki et al. Fig. 6 Temperature histories of the model column and the specimen cured under the simplified insulated condition Fig. 7 Temperature histories of specimens for the autogenous shrinkage strain measurement and for the compressive strength test subjected to the simplified insulated curing

13 High-Str Stren ength/high-p th/high-per erform ormanc ance Conc oncret ete 1171 Fig. 8 Development of the compressive strength of the specimen made with the SFLC mix Fig. 9 Comparison of the compressive strength between specimens cured under the standard condition, the simplified adiabatic condition and cored from the model column

14 1172 Suzuki et al. Fig. 10 Development of averaged autogenous shrinkage strains

15 High-Str Stren ength/high-p th/high-per erform ormanc ance Conc oncret ete 1173 Fig. 11 Relationship between effective age and shrinkage reduction Fig. 12 Effect of the shrinkage-reducing admixture by curing conditions

16 1174 Suzuki et al.