SP FRC PERFORMANCE COMPARISON: UNIAXIAL DIRECT TENSILE TEST, THIRD-POINT BENDING TEST, AND ROUND PANEL TEST

Transcription

1 SP FRC PERFORMANCE COMPARISON: UNIAXIAL DIRECT TENSILE TEST, THIRD-POINT BENDING TEST, AND ROUND PANEL TEST Shih-Ho Chao, Jae-Sung Cho, Netra B. Karki, Dipti R. Sahoo, and Nur Yazdani Synopsis: The evaluation of the properties of FRC mixtures is of prime importance for these mixtures to be used effectively and economically in practice. Although currently there are various standards or testing methods for evaluation of the properties of FRC, there is no agreement on which standard is the best for a specific structural application. This can be a major reason that has inhibited the introduction of FRC into structural design code. This study investigated three major different material evaluation methods, i.e. uniaxial direct tensile test, third-point bending test, and round panel test, as well as behavior of specimens tested by the three methods. The advantages and limitations of those methods are discussed. Keywords: ASTM C1550, ASTM C1609, fiber reinforced concrete, residual strength, shear, uniaxial direct tensile test 5.1

2 Chao et al. ACI member Shih-Ho Chao is Assistant Professor in the Department of Civil Engineering at the University of Texas at Arlington. He received his PhD in structural engineering from the University of Michigan, Ann Arbor in He is an associate member of ACI Committee 544, Fiber Reinforced Concrete, Joint ACI-ASCE Committee 423, Prestressed Concrete, and ACI Committee 408, Bond and Development of Reinforcement. His research interests include fiber reinforced concrete, prestressed concrete, and seismic behavior of structural members. Jae-Sung Cho is doctoral student in the Department of Civil Engineering at the University of Texas at Arlington. He received his B.S. in Civil engineering from the Kookmin Univeristy, Seoul, Korea in 2002 and his ME degree from the University of Texas at Arlington in His research interests include fiber reinforced concrete and prestressed concrete. Netra B. Karki is doctoral student and graduate teaching assistant in the Department of Civil Engineering at the University of Texas Arlington. He received his M.S. degree in Structural Engineering from Tribhuvan University, Nepal. His research work is in the behavior of fiber reinforced prestressed concrete flexural members. Dipti R. Sahoo is Assistant Professor at the Indian Institute of Technology at Bhubaneswar. He was a former postdoctoral Fellow in the Department of Civil Engineering at the University of Texas at Arlington. He received his PhD in Structural Engineering from Indian Institute of Technology Kanpur, India. His research interests include fiber reinforced concrete, seismic behavior of structural members, and seismic strengthening of structures using supplemental energy dissipation devices. ACI member Nur Yazdani is Professor and Chair of Civil Engineering at the University of Texas at Arlington. He is currently chairing ACI-SEI joint committee on Concrete Bridge Design. He is a member of ACI committees on Fiber Reinforced Concrete and Durability. His research interests include concrete bridge materials, design and rehabilitation, hazard mitigation and engineering education.

3 FRC Performance Comparison: Uniaxial Direct Tensile Test, Third-Point Bending Test, and Round Panel Test INTRODUCTION Due to the presence of fibers and its interaction with concrete matrix, many mechanical properties, such as tensile, compressive, flexural, and shear of fiber reinforced concrete (FRC) are quite different from those of plain concrete. Behavior of structural members with identical FRC mixture can also exhibit significantly different behavior when subjected to different types of loadings. As a consequence, various test methods for evaluating the FRC mechanical properties have been developed. However there is no agreement on which standard is the best for a specific structural application. This is a major reason that has inhibited the introduction of FRC into structural design code. This study investigated three major FRC evaluation methods, i.e. uniaxial direct tensile test, third-point bending test (ASTM C1609, ASTM, 2007), and round panel test (ASTM C1550, ASTM, 2005), as well as the behavior and the response of specimens. The advantages and limitations of those methods are discussed. RESEARCH SIGNIFICANCE Several material testing methods have been used for evaluating the performance of FRC; these methods can show very different results for the same FRC material. A particular method should be carefully selected and the results should be interpreted with caution. This study investigated three commonly used FRC test methods and discusses the potential problems they have. BASIC REQUIREMENTS FOR A MATERIAL TEST METHOD Although material properties measured by a standard test are not necessarily representative of the fiber concrete in the structure, those test results can ensure that the FRC was batched properly and can give indications of performance if used in structures. An ideal material test method for FRC needs to account for many factors. For example, Mindess et al. [2003] have suggested that the toughness or residual strength parameters obtained from FRC material tests should satisfy the following criteria: 1. It should have a physical meaning that is both readily understandable and of fundamental significance if it is to be used for the specification or quality control of FRC. 2. The end-point used in the calculation of the toughness parameters should reflect the most severe serviceability conditions anticipated in the particular application. 3. The variability inherent in any measurement of concrete properties should be low enough to give acceptable levels of both within-batch and between-laboratory precision. 4. It should be able to quantify at least one important aspect of FRC behavior (e.g. strength, toughness, or crack resistance) and should reflect some characteristics of the load vs. deflection curve itself. 5. It should be as independent as possible of the specimen size and geometry. It has been recognized that none of the standard tests used for obtaining FRC toughness or residual strength is able to fulfill all the criteria described above [Bentur and Mindess, 2007]. As a result, it is important to understand the limitation and difficulties encountered when using a particular material test method. In this study, three types of FRC evaluation methods were employed for the same FRC mixture and their features as well as limitations are discussed. Uniaxial Direct Tensile Test This test type can identify the key properties of FRC such as strain-hardening or strain-softening, elastic modulus, and stress versus strain relationships under tension, which are the constitutive properties of FRC that are useful for modeling and design of FRC structural members [Naaman, et al., 2007]. However currently there is no 5.3

4 Chao et al. standard method for this test in the U.S., in part because it is difficult to provide a gripping arrangement which will not lead to specimen cracking at grips. Specimens used in this study were specifically designed so that a pin-pin loading condition is created at the ends (Figure 1). The advantages of this adopted end condition are : 1) a pure axial load is applied since the additional end moment, if any, could be largely minimized; 2) no specific treatment, such as adhesives, would be needed to fix the ends to the setup. Both ends are strengthened by the double dog-bone geometry and steel meshes to ensure that cracking would only occur at the central portion within the gauge length. The double dog-bone shape was used to mitigate the stress concentration resulted from the reduction of cross-section. The central portion has a square cross-section with a dimension of 102x102 mm (4x4 in.). This dimension was selected to reduce the size effect [Naaman and Reinhardt, 2006] while maintaining a suitable weight for laboratory handling. The strains were measure by a pair of linear variable differential transformers (LVDTs) with a gauge length of approximately 178 mm (7 in.). Tests were carried out by a closed-loop, servo-controlled machine with a loading rate of approximately 0.05 mm/min (0.002 in./min). Third-Point Bending Test (ASTM C1609) This test method is used to evaluate the flexural performance of FRC by using parameters derived from the load-deflection curve obtained by testing a simply supported beam under third-point loading. The bending test setup used in this study was based on ASTM C1609 [ASTM, 2007], as shown in Figure 2. Specimens have a prism shape with a dimension of 150x150x500 mm (6x6x20 in.). The width and depth of test specimens are greater than three times the length of the steel fiber used in this study, so the preferential fiber alignment effect is minimized [ASTM, 2007]. A pair of LVDTs was mounted on a jig based on the ASTM C1609 requirement to ensure accurate determination of the net deflection at the mid-span, exclusive of the effects of seating or twisting of the specimen on its supports. Tests were carried out by a closed-loop, servo-controlled machine with a loading rate of approximately 0.05 mm/min (0.002 in./min). Round Panel Test (ASTM C1550) This recently developed bending test method [ASTM, 2005] was based on extensive studies carried out by Bernard [2000, 2001a, 2001b, 2002,]. This test involves the central point loading of a large round panel, 800 mm (31.5 in.) in diameter and 75 mm (2.95 in.) thick, supported on three symmetrically arranged pivots as shown in Figure 3. Although this method was originally developed for fiber reinforced shotcrete, it can also be used for evaluating plate-like members such as concrete slabs-on-grade. The test panel experiences bi-axial bending in response to the central point load. The performance of FRC specimens tested by this method is quantified in terms of the energy absorbed between the onset of loading and selected values of central deflection. Some suggested performance-based specification are [Bernard, 2002] : 1) Energy absorbed up to a deflection of 5 mm (0.2 in.) to indicate performance for applications in which crack control is important; 2) Energy absorbed up to a deflection up to 40 mm (1.6 in.) to evaluate performance for applications where large cracks can be tolerated. The radius of the hemispherical end of the loading piston and the supports were fabricated according to the dimensions and criteria given in ASTM C1550 [ASTM, 2005]. A closed-loop, servo-controlled machine is required by ASTM C1550 for performing this test. However the specimen size, plus the steel support, is too large to fit into the available machines at the structures laboratory at UT Arlington. Instead, the tests were carried out under a stiff reaction frame through a hydraulic cylinder with careful control of the loading rate, which was approximately 4 mm/min (0.16 in./min) as specified by ASTM C1550.

5 Materials and Specimen Preparation FRC Performance Comparison: Uniaxial Direct Tensile Test, Third-Point Bending Test, and Round Panel Test EXPERIMENTAL PROGRAM, TEST RESULTS, AND DISCUSSION The steel fiber used in this study is shown in Figure 4 and Table 2 gives mix proportions for the concrete mixture. As shown in Figure 4, different from many steel fibers with double end bends which provide better mechanical bond and better composite performance, the steel fiber used has only a single bend at the both ends. Two fiber volume fractions, 0.5% and 1.5% were used. Three round panel, six ASTM beams, and six tensile specimens were cast for both fiber contents. Mixing and casting of all specimens (Figure 5) were done on the same day with three batches due to the capacity limitation of the drum mixer used. All specimens were then moved to the curing room with a controlled environment (27 o C (80 o F) and 100% RH). Tests were conducted approximately 30 days after casting. Comparison between Three FRC Evaluation Methods Conducted in Phase 1 a. Uniaxial Direct Tensile Test Figure 6 (a) presents typical response of two replicate (in terms of geometry and mix composition) tensile specimens with 1.5% fiber content and Figure 6 (b) shows locations of cracks in four of the replicate specimens. The following observations can be made: 1. In general the locations of major cracks were confined within the intended gauge length, by using the double dog-bone ends and steel mesh reinforcement. 2. While the first cracking stresses of two replicate specimens were close, the post-cracking response and residual strength showed significant variability. 3. Consistent location of cracks and propagation path cannot be well controlled. This is the major factor that led to the variability in the post-cracking response. b. Third-Point Bending Test (ASTM C1609) Figure 7 (a) shows load versus deflection response for three replicate specimens. As can be seen, while the first cracking loads, approximately 45 kn (10,000 lbf), were the same, the peak flexural strength and the postcracking responses deviated significantly. This high variability issue in third-point bending test has been recognized as an inherent problem due to the lack of control over the position of cracks [Bernard, 2002], which was also observed in this study as shown in Figure 7 (b). Another factor leading to the variability is the non-uniform fiber distribution [Dupont and Vandewalle, 2004]. Experimental evidence shown by Bernard [2002] also indicated that the residual strength of a third-point loaded beam is the least attractive parameter because it displays very poor reliability in that the coefficient of variation is generally greater than 20%. This large scatter in the residual strength can be a major problem if characteristic values have to be determined. c. Round Panel Test (ASTM C1550) Previous studies [Bernard, 2002] indicated that the variation in cracking load, peak load, or energy absorbed up to a specified central deflection from this test is generally low (coefficient of variation between 5% to 13%). This could be due to the following: 5.5

6 Chao et al. 1. Location of cracks as well as crack patterns can be well controlled: panels tested by this method almost always break into three segments upon failure, at angles of about 120 o, as seen in Figure 8 (b). This is due to the fact that this type of failure consumes the least amount of energy [Bernard and Pricher, 2000]. 2. Increased cracked area: the three major cracks give a somewhat average mechanical behavior to minimize the influence of non-uniform fiber distribution. On the contrary, in both the unixial direct tensile test and ASTM third-point bending test, the performance is usually governed by one major crack, which could be largely affected by the extent of fiber distribution. The load versus central deflection curves of two replicate panel specimens are shown in Figure 8(a); scatter in the post-cracking response is noticeable. This was probably the consequence of specimens not being cast from the same batch, eccentric loading, or complicated cracking propagation path, etc. Some issues with this type of test are: 1. As mentioned earlier in this paper, the specimen together with the steel support fixture is too large to fit into many commonly used testing machines [Bentur and Mindess, 2007]. As a result, some previous tests had to be carried out by 80% scaled (both the diameter and thickness) specimens [Xu et al., 2004]. In addition, the specimen itself is too heavy to be handled in the laboratory; one specimen weights 888 N (200 lbf). 2. When central deflection of the tensile surface of the panel is measured directly with a LVDT, an incomplete or erroneous deflection record may occur if the crack opening becomes too large, as shown in Figure 9. ASTM C1550 has suggested using a LVDT with a probe approximately 20 mm (0.8 in.) wide if the opening becomes an issue. Greater probe width is not recommended because off-center cracks may induce exaggerated apparent deflections if they occur adjacent to a wide probe [ASTM, 2005]. However, the opening in the center could be greater than 20 mm (0.8 in.) at large deflections (as shown in Figure 9) and thus lead to incorrect measurement. Performance of Specimens with Different Fiber Volume Fractions Figures 10 through 12 show the performance comparison of specimens with 0.5% and 1.5% fiber content. As can be seem, the different fiber volumes were distinguished well in all three tests. The energy indices suggested by ASTM C1550 for performance evaluation were also calculated up to 25 mm (1in.) central deflection and shown in Figure 12. Figure 13 gives the responses from the three tests for specimens with the same mixture and 0.5% fiber content. It is noted that Mix 1 with 0.5% Type 1 steel fibers exhibited a sudden drop in strength after the first cracking in all three tests. However different testing methods showed distinct post-cracking behavior. For specimens under uniaxial tension, the residual strength rapidly dropped to 15% of the peak strength after the first cracking, yet the peak residual strength ratios increased to 45% and 75% when subjected to one-way bending (third-point bending test) and bi-axial bending (round panel test), respectively. Increasing fiber volume fraction from 0.5% to 1.5% made the descending curve more gradual from all three tests, as revealed by Figure 14. It is interesting to note that, while the specimen under direct tension exhibited a softening behavior after the first cracking, more ductile response was observed in the specimens subjected to one-way and bi-axial bending. Deflection hardening response up to 4 mm (0.16 in.) central deflection occurred in the round panel specimen, which indicates significant stress redistribution after first cracking, before deflection softening response occurred. It is noted that tests conducted by Xu et al [2004] using 80% scaled specimens gave somewhat opposite results; that is, significant deflection-hardening behavior was observed in beam-type tests, but only minor hardening behavior was noticed in their round panel tests. Observation obtained from this study suggests that different material evaluation methods may be required for members under a particular loading situation.

7 FRC Performance Comparison: Uniaxial Direct Tensile Test, Third-Point Bending Test, and Round Panel Test SUMMARY AND CONCLUSIONS This study investigated three major different material evaluation methods for FRC, uniaxial direct tensile test, third-point bending test (ASTM C1609), and round panel test (ASTM C1550), as well as the behavior of specimens tested by the three methods. The advantages and limitations of those methods were reviewed. The following conclusions are drawn from this study: 1. The three major material evaluation methods (uniaxial direct tensile test, third-point loading bending test, and round panel test) can give significant different performance for the same FRC material. A strain-softening FRC under direct tension can exhibit significant deflection-hardening behavior under bi-axial bending. 2. This study may raise an interesting question if load-deflection curves of FRC is a material property or if they are testing-method dependent. Which test data is more representative of material property? Can we compare tension directly with bending since the cracking or failure mechanism is different? Based on the test results from this study, it seems that the behavior of FRC depends on how the FRC members are carrying the loading. The response from a uniaxial direct tensile test can be more representative of the behavior of a FRC member subjected to direct tension or pure shear (diagonal tension); while the responses from third-point loading bending test and round panel test are more representative of the behavior of members subjected to uni- and bi-axial bending, respectively. Direct comparison between tensile and bending test may not be meaningful since the load-carrying mechanisms are different. However, based on the test results, one can ascertain that the uni- and biaxial bending behavior of an FRC member will be better (or more ductile) if the uniaxial direct tensile test gives good performance. 3. Both the uniaxial direct tensile test and the third-point loading bending test show very high coefficient of variations in the post-cracking responses. This can be attributed to the lack of control of crack locations and non-uniform fiber distribution. Further improvement of these two material tests to achieve higher level of reliability in post-cracking performance is needed. Although previous studies suggest that round panel test could considerably reduce the variability in testing data, its use in ordinary laboratories could be limited due to the specimen (plus support) size and weight. 4. The high deviation in the test results calls for a better FRC material test method that required less number of specimens and still gives small deviation. An average of load-deflection curves of many replicates can still be misleading if the coefficient of variation is too high. ACKNOWLEDGEMENTS Materials used in this investigation were provided by Hanson Pipe & Precast at Grand Prairie, Texas. Their help is gratefully appreciated. This research was supported in part by grants from the Texas Department of Transportation (Project ). Any opinions, findings, and conclusions expressed in this study are those of the authors, and do not necessarily reflect the views of the sponsors. REFERENCES 1. ASTM C1609/C 1609M-07, Standard Test method for Flexural Performance of Fiber-Reinforced Concrete (Using Beam with Third-Point Loading), ASTM International, West Conshohocken, PA, 2007, 9 pp. 2. ASTM C , Standard Test method for Flexural Toughness of Fiber Reinforced Concrete (Using Centrally Loaded Round Panel), ASTM International, West Conshohocken, PA, 2005, 13 pp. 5.7

8 Chao et al. 3. Bentur, A. and Mindess, S., Fiber Reinforced Cementitious Composites, 2 nd Edition, Taylor & Francis, 601 pp. 4. Bernard, E. S., Behaviour of Round Steel Fibre Reinforced Concrete Panels under Point Loads, Materials and Structures, 33, 2000, pp Bernard, E. S. and Pircher, M., Influence of Geometry on Performance of Round Determinate Panels Made with Fibre Reinforced Concrete, Engineering Report No. CE10, School of Civil Engineering and Environment, UWS Nepean, Kingswood NSW, Australia, January, Bernard, E. S., The Influence of Strain Rate on Performance of Fiber-Reinforced Concrete Loaded in Flexure, Cement, Concrete, and Aggregates, CCAGDP, American Society for testing and Materials, Vol. 23, No. 1, June 2001, pp Bernard, E. S. and Pircher, M., The Influence of Thickness on Performance of Fiber-Reinforced Concrete in a Round Determinate Panel Test, Cement, Concrete, and Aggregates, CCAGDP, American Society for testing and Materials, Vol. 23, No. 1, June 2001, pp Bernard, E. S., Correlations in the Behaviour of Fibre Reinforced Shotcrete Beam and Panel Specimens, Materials and Structures, 35, 2002, pp Dupont, D. and Vandewalle, L., Comparison between The Round Plate Test and The RILEM 3-Point Bending Test, 6 th RILEM Symposium on Fibre-Reinforced Concrete (FRC) BEFIB 2004, September 2004, Varenna, Italy, pp Mindess, S., Young, J. F., and Darwin, D., 2003, Concrete, 2 nd Edition, Prentice Hall, Upper Saddle River, NJ. 11. Naaman, A. E. and Reinhardt, H. W., Proposed Classification of HPFRC Composites based on their Tensile Response, Materials and Structures, 39, 2006, pp Naaman, A. E., G. Fischer, and Krstulovic-Opara, N., Measurement of Tensile Properties of Fiber Reinforced Concrete: Draft Submitted to ACI Committee 544, High Performance Fiber Reinforced Cement Composites (HPFRCC5), Mainz, Germany, July 10-13, 2007, pp Xu, H., Mindess, S. Banthia, N., Toughness of Polymer Modified, Fiber Reinforced High Strength Concrete: Beam Tests vs. Round Panel Tests, International RILEM Symposium on Concrete Science and Engineering: A Tribute to Arnon Bentur, Editors: Weiss, J., Kovler, K., Marchand, J., and Mindess, S., RILEM Publications SARL, Hooked-end steel fiber (single-bend) Table 1 Properties of steel fiber used in this study Length (L) [1] Diameter (D) [1] Aspect ratio (L/D) [1] Tensile Strength [2] 39 mm (1.55 in.) 0.97 mm (0.038 in.) 40 [1] Measured; [2] Provided by manufacturers 1034 MPa (150 ksi ) Cement (Type-I) Fly Ash (Class C) Table 2 Mix proportion (by weight) and average compressive strength Sand [3] Coarse Aggregate [4] Water Superplasticizer [5] Steel Fibers [6] (or [7] ), Type 1 fiber Average Compressive Strength 65.4 MPa (9.5 ksi ) [3] ASTM Natural River Sand (Fineness Modulus = 2.57); [4] Maximum size = 3/4 in. (19 mm); [5] High Range Water Reducing Admixture; [6] 1.5% by volume; [7] 0.5% by volume [1 MPa = ksi]

9 FRC Performance Comparison: Uniaxial Direct Tensile Test, Third-Point Bending Test, and Round Panel Test 5.9

10 Chao et al. Figure 1 Geometry and dimensions of the direct tensile specimen Figure 2 Setup and specimen for ASTM C1609 third-point bending test (a) (b)

11 FRC Performance Comparison: Uniaxial Direct Tensile Test, Third-Point Bending Test, and Round Panel Test (c) (d) (e) Figure 3 Setup and specimen for ASTM C1550 round panel test Figure 4 Steel fibers (single-bend hook) used in this study 5.11

12 Chao et al. (a) (b) (c) (d) Figure 5 Specimen casting and curing

13 FRC Performance Comparison: Uniaxial Direct Tensile Test, Third-Point Bending Test, and Round Panel Test (a) Replicate specimen results (b) Location of major cracks Figure 6 Direct tensile test results 5.13

14 Chao et al. (a) Replicate specimen results SP#1 SP#2 SP#3 (b) Location of major cracks Figure 7 Third-point bending test results

15 FRC Performance Comparison: Uniaxial Direct Tensile Test, Third-Point Bending Test, and Round Panel Test (a) Replicate specimen results SP#1 SP#2 (b) Location of major cracks Figure 8 Round panel test results 5.15

16 Chao et al. Figure 9 Opening of cracks and location of LVDT Figure 10 Stress versus strain (crack opening) responses of specimens with different fiber fractions under direct tensile test

17 FRC Performance Comparison: Uniaxial Direct Tensile Test, Third-Point Bending Test, and Round Panel Test Figure 11 Load versus deflection responses of specimens with different fiber fractions under third-point bending test Figure 12 Load versus central deflection responses of specimens with different fiber fractions under round panel test as well as absorbed energy up to 25 mm deflection 5.17

18 Chao et al Stress (psi) Uniaxial Direct Tensile Test Mix #1; Type 1 steel fiber V f = 0.5% Stress (MPa) Uniaxial Direct Tensile Test Peak Post-Cracking Strength = 15% Peak Strength Strain (up to peak strength)(%) Third-Point Bending Test Peak Post-Cracking Strength = 45% Peak Strength Round Panel Test Peak Post-Cracking Strength = 75% Peak Strength Figure 13 Performance of specimens with same FRC mixture (0.5% V f ) under different type of material tests

19 FRC Performance Comparison: Uniaxial Direct Tensile Test, Third-Point Bending Test, and Round Panel Test Uniaxial Direct Tensile Test Elastic behavior up to first cracking, then followed by softening response Third-Point Bending Test Minor stress redistribution after first cracking, then followed by deflection softening response Round Panel Test Significant stress redistribution after first cracking (deflection hardening response up to 4 mm central deflection), then followed by deflection softening response Figure 14 Performance of specimens with same FRC mixture (1.5% V f ) under different type of material tests 5.19