Dogbone Test 9 Korea Paper).pdf

Key Engineering Materials ISSN: 1662-9795, Vols. 417-418, pp 649-652 doi:10.4028/www.scientific.net/KEM.417-418.649 © 2010 Trans Tech Publications, Switzerland

Online: 2009-10-08

Comparison of Tensile Strengths with Different Test Methods in Ultra High Strength Steel-Fiber Reinforced Concrete (UHS-SFRC) Su-Tae Kang1, a, Jung-Jun Park1,b, Gum-Sung Ryu1,c Gyung-Taek Koh1,d, and Sung Wook Kim1,e 1

Structural Engineering & Bridges Research Division, Korea Institute of Construction Technology, Goyang, 411-712, Korea a

[email protected], [email protected], [email protected], [email protected], [email protected]

Keywords: High strength, Steel fiber, Concrete, Tensile strength, Cracking

Abstract. Ultra High Strength Steel-Fiber Reinforced Concrete (UHS-SFRC) is characterized by very high compressive and tensile strength that is about 8 times of ordinary concrete, and high ductility owing to the addition of steel fibers. This paper investigates the relationship existing among the direct tensile strength, flexural tensile strength and splitting tensile strength of UHS-SFRC. Differently from ordinary concrete, it is found that the first cracking strengths in UHS-SFRC obtained through direct tensile test and splitting tensile test are similar, while the strength obtained from flexural tensile test is significantly larger than those from other tests. Based on the experimental results, relationships between the direct tensile strength and flexural tensile strength, between the first cracking strengths in direct tensile test and in flexural tensile test, and between the first cracking strength in direct tensile test and the flexural tensile strength are proposed. Introduction The improved tensile strength and high toughness of Ultra High Strength Steel-Fiber Reinforced Concrete (UHS-SFRC) can be pointed out as its most advantageous features [1,2]. UHS-SFRC develops direct tensile strength exceeding 10MPa and exhibits high ductility and toughness through its strain hardening behavior as shown in Fig. 1. Fig. 1 compares the typical responses under uniaxial stress state observed in Ordinary Concrete (OC), Fiber Reinforced Concrete (FRC) or High Strength Fiber Reinforced Concrete (HSFRC), Engineered Cementitious Figure 1 Comparison of typical response in uniaxial Composites (ECC) and Ultra High Strength stress state [3] Fiber-Reinforced Concrete (UHS-SFRC) [3]. FRC and HSFRC are experiencing strain softening immediately after the initiation of cracking whereas ECC and UHS-SFRC are showing strain hardening. Moreover, UHS-SFRC distinguishes from ECC by the development of extremely high tensile strength larger than 10MPa. Despite of the numerous direct and indirect tension tests that have been performed on ordinary concrete and FRC to date, the absence of consensus on standardized method for the evaluation of the tensile behavior is noteworthy. Even if direct tensile test of concrete is a direct method, this method presents drawbacks due to its complexity, the fluctuation of the experimental results produced by diverse factors like the occurrence of secondary bending stress after cracking, and problems relevant to the discrepancies inherent to the selected test method. Accordingly, preference is often given to indirect methods such as the splitting tensile test or flexural tensile test for analysis or design by converting the obtained values onto direct tensile strength using proposed relationships. For example, All rights reserved. No part of contents of this paper may be reproduced or transmitted in any form or by any means without the written permission of Trans Tech Publications, www.ttp.net. (#69787861, Pennsylvania State University, University Park, USA-16/09/16,15:13:45)

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CEB-FIP 90 [4] defines the relationship between the direct tensile strength and the splitting tensile or flexural tensile strength as follows. (1)

(2) is the mean direct tensile strength; is the mean splitting tensile strength; is the where mean flexural tensile strength; and takes value of 100mm, and is the depth of beam (mm). Recalling that UHS-SFRC exhibits strain hardening behavior while ordinary concrete shows quasi-brittle behavior, these different behaviors have obviously extreme influence on the structural performance. Therefore, need is to distinguish and define the strength at first cracking and ultimate strength as well as to clarify the relationship between the strengths obtained through different test methods. Mix proportion and test methods The composition of matrix used is given in Table 1 and 2% vol. steel fibers were used. The flow test showed the flowability of about 230mm. Table 1. Mix design of concrete (relative ratios*) Cement Water Silica fume Fine aggregates Filler Superplasticizer Steel fiber 1.00 0.25 0.25 1.10 0.30 0.018 2% vol. * mass ratio to cement, except steel fiber expressed as volumetric ratio to the whole volume. Tensile test was executed by applying 3 methods that are direct tensile test, splitting tensile test, and flexural tensile test. Direct tensile strength test was carried out on fabricated dog-bone shaped specimens, as shown in Fig. 2(a), using the test apparatus illustrated in Fig. 2(b). Splitting tensile strength test was performed on cylindrical specimens as in Fig. 3 and the strength could be obtained using the relationship Flexural tensile strength test was executed through four-point bending test on fabricated specimens with dimensions of 100mm×100mm×400mm, as shown in Fig. 4, and the strength Figure 2. Direct tensile test was computed using the relationship

Figure 3. Splitting tensile test

.

Figure 4. Flexural tensile test

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Experimental results Figure 5 plots the results of the direct tensile test. The first cracking strength obtained experimentally provided a mean value of 11.5MPa with standard deviation of 1.21MPa and the tensile strength (maximum value) presented mean of 13.5MPa and standard deviation of 0.71MPa. Figure 6 describes the results of the flexural tensile test. The first cracking strength exhibits mean value of 16.4MPa with standard deviation of 0.59MPa, and the flexural tensile strength presents mean value of 38.1MPa with standard deviation of 2.77MPa. Figure 7 plots the stress-strain diagram measured in the splitting tensile test which gives a mean of 11.9MPa and standard deviation of 1.30MPa for the splitting tensile strength. In view of Fig. 7, even if the initiation of the first cracks can be estimated, the maximum tensile strength cannot be derived due to the continuous increase of the load induced by the widening of the loaded area provoked by the crushing of the specimen after cracking. 16

16

Direct tensile stress (MPa)

Direct tensile stress (MPa)

Initial area

12

8

4

0

12

8

4

0

0

4 8 Crack width (mm)

12

0

0.2

0.4 0.6 Crack width (mm)

0.8

(a) Direct tensile behavior (b) initial area including first cracking Figure 5. Direct tensile behavior 50

50

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Flexural tensile stress (MPa)

Flexural tensile stress (MPa)

initial area

30

20

10

0

40

30

20

10

0 0

2

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Deflection at center (mm)

6

0

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1

Deflection at center (mm)

(a) Flexural tensile behavior (b) initial area including first cracking Figure 6. Flexural tensile behavior The first cracking strengths and maximum tensile strengths of UHS-SFRC resulting from the 3 test methods were compared. Comparison reveals the similarity of the first cracking strengths obtained from direct tensile test and splitting tensile test, and the large difference exhibited by the first cracking strength obtained from the flexural tensile test, which showed value larger by about 35~40% than the

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Splitting tensile stress (MPa)

two other methods. On the other hand, the maximum 30 tensile strengths derived from direct tensile test and flexural tensile test appear to be respectively 1.17 times and 2.32 times larger than the first cracking strength. The reason for 20 the significantly larger maximum tensile strength provided by the flexural tensile test can be attributed to the tensile strain hardening or strain softening behavior in UHS-SFRC due to the fiber bridging even after cracking, which is clearly 10 different to the quasi-absence of post-cracking resistance in ordinary concrete. A comparison of the maximum tensile strengths obtained from direct tensile test and flexural 0 tensile test shows that the latter produces strength 2.82 times 0 2000 4000 6000 8000 10000 Strain (µm/m) larger than that of the direct tensile test. Considering that tensile test of fiber reinforced concretes is generally Figure 7. Splitting tensile behavior executed by means of flexural tensile test, the relationship between the flexural tensile strength and the first cracking strength and maximum tensile strength under direct tensile behavior can be expressed as follows. and

(3)

is the first cracking strength obtained from direct tensile test; and, where the result obtained for a specimen with dimensions of 100mm×100mm×400mm.

corresponds to

Conclusions This paper evaluated the tensile strength of UHS-SFRC using 3 methods that are direct tensile test, splitting tensile test, and flexural tensile test. Results were analyzed by distinguishing the first cracking strength and the maximum tensile strength. The flexural tensile test produced first cracking strength with values larger by approximately 35~40% than those obtained by the other methods, and maximum tensile strength 2.82 times larger than the first cracking strength obtained by the direct tensile test. Besides, relationships were proposed so as to enable the estimation of the first cracking strength and maximum tensile strength of UHS-SFRC under direct tensile behavior from the results of the flexural tensile test. Acknowledgements The work presented in this paper was funded Korea Institute of Construction Technology(KICT) via the project ‘Hybrid Cable-Stayed Bridge 2020’ and by Korea Institute of Construction and Transportation Technology Evaluation and Planning (KICTEP) under the Ministry of Land, Transport and Maritime Affairs (MLTM) via the project ‘Center for Concrete Corea’(05-CCT-D11). References [1] P. Richard and M. Cheyrezy, in: Composition of Reactive Powder Concrete, Cement and Concrete Research, Vol. 25, No. 7, 1995, pp. 1501-1511. [2] O. Bonneau et al., in: Mechanical Properties and Durability of Two Industrial Reactive Powder Concretes, ACI Materials Journal, Vol. 94, No. 4, 1997, pp. 286-290. [3] V. C. Li and G. Fischer, in: Reinforced ECC - An Evolution from Materials to Structures, Proceedings of the 1st fib Congress - Concrete Structures in the 21st Century, pp. 105-122, Osaka, 2002. [4] CEB-FIP, in: CEB-FIP model code 90, pp. 34-36, 1991.