INVESTIGATION OF THE SIZE EFFECT IN SHEAR OF STEEL FIBER REINFORCED CONCRETE (SFRC) SLENDER BEAMS

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1 High Performane Fiber Reinfored Cement Composites (HPFRCC7), INVESTIGATION OF THE SIZE EFFECT IN SHEAR OF STEEL FIBER REINFORCED CONCRETE (SFRC) SLENDER BEAMS M. Zarrinpour (1), J.-S. Cho (2) and S.-H. Chao (1) (1) University of Texas at Arlington, USA (2) Engineer at Stueve Constrution Co. Algona, Iowa, USA Abstrat Signifiant shear strength redution with the inrease of the height of slender plain onrete beams with no web reinforement is a well-known phenomenon alled size effet. The Amerian Conrete Institute s ACI 318 Building Code allows steel fiber reinfored onrete (SFRC) to replae onventional minimum shear reinforement in slender beams; however, the beam s height is limited to 610 mm (24 in.) This is due to the fat that, exept for a very few tests, the majority of test results on shear behavior of SFRC beams has been evaluated on speimens with an effetive depth of 178 mm (7 in.) to 572 mm (22.5 in.). The objet of this researh was to investigate the size effet of SFRC beams in shear as well as the long-term shear performane (up to 33 months after asting) of SFRC beams by experimentally testing four pairs of SFRC slender beams with an extended observane time after asting. These beams had total heights of 457 mm (18 in.), 610 mm (24 in.), 915 mm (36 in.), and 1220 mm (48 in.), and one pair of beams was 457 mm (18 in.) high ompanion beams made of plain onrete. The only variable parameter was the overall depth, while the remaining key fators were held onstant inluding shear span to effetive depth ratio of 3.5, longitudinal reinforement ratio (approximately 2.7%), ompressive strength of onrete (targeted at 42 MPa), steel fiber volume fration (0.75%), and types of steel fiber (hooked-end fibers onformed to ACI requirements). Test results indiate that the normalized shear stress at failure is substantially inreased for all SFRC speimens as a result of fiber inlusion. In spite of the various effetive depths of the speimens, the disrepany in normalized shear strengths was relatively marginal. Also, even though the speimens were tested at a quite high age ranging from 502 to 1005 days and exposed to weather, no degradation was observed in terms of their shear behavior. 1. INTRODUCTION A large body of researh onduted on shear behavior of reinfored onrete (RC) slender beams with no shear reinforement has demonstrated the existene of size effet on ultimate shear stress [1-4], in whih shear strength (in terms of stress) dereases as the beam depth inreases. Therefore, the test results from rather small beams are not appliable to large-sale 375

2 High Performane Fiber Reinfored Cement Composites (HPFRCC7), beams. Size effet on ultimate shear stress for plain onrete is explained by the inreased inlined rak spaing when the slender beam beomes deeper. If the strain in onrete between two onseutive raks is negleted, the average width of the inlined raks an be approximately represented by the produt of the average rak spaing and the strain of the reinforement. Therefore, at a given longitudinal bar strain, an inrease in rak spaing auses wider raks, thereby reduing aggregate interlok apaity in resisting the shear [2]. Based on this assumption, any fator ausing an inrease of either rak spaing or tensile strain in longitudinal reinforement an lead to an inrease in rak width and subsequent redution of the aggregate interlok apaity, thus exaerbating the size effet. This an happen when using reinforement with smaller modulus of elastiity or an insuffiient reinforement ratio [5-7]. On the other hand, any fator whih enhanes aggregate interlok suh as using larger aggregate size or plaing layers of longitudinal reinforement along the depth of the beam or using stirrups an minimize the size effet [4, 5]. It is well-established that addition of disrete steel fibers into onrete an onsiderably inrease the shear strength. [8-14]. While using steel fiber-reinfored onrete (SFRC) to replae onventional mild steel shear reinforement has been permitted by design ode [15], it is unlear if size effet is also a onern in SRFC slender beams. This onern is refleted in the ACI ode where the maximum beam overall depth, h, is not allowed to be more than 610 mm (24 in.). Nevertheless, some researhers have suggested that size effet on ultimate shear stress shall be less serious for SFRC beams [16]. Information olleted in Figure 1 illustrates the relation between normalized shear strength and effetive depth for the SFRC beams with 0.75% fiber volume fration [14, 17]. Note that the steel fiber volume fration of 0.75% is the minimum amount of steel fiber speified by [15] used to replae the onventional minimum shear reinforement (stirrup). As notied, all the test beams had an effetive depth ranging from 178 mm (7 in.) to 572 mm (22.5 in.) and an average strength of approximately 033. f MPa ( 40. f psi) with no obvious size effet. Additional researh is needed to investigate the size effet in deeper SFRC beams with depths greater than 610 mm (24 in.). Effetive depth, d (mm) Normalized Shear stress (, psi) Members with a/d 2,. % Normalized Shear stress (, MPa) Effetive depth, d (in) Figure 1: Normalized shear stress at failure versus beam effetive depth, V f = 0.75% 376

3 High Performane Fiber Reinfored Cement Composites (HPFRCC7), 2. EXPERIMENT PROGRAM 2.1. Speimen desription and reinforement detailing A total of 10 beams inluding four pairs of SFRC and one pair of 457 mm (18 in.) deep RC beams were tested. To redue the potential unertainty of the test results, the beams in eah pair were idential. For all the speimens, shear span to effetive depth ratio, a/d, longitudinal reinforement ratio, ρ, steel fiber volume fration, V f, fiber type, and onrete ompressive strength were held onstant. For SFRC beams, overall depth was onsidered as the only variable parameter ranging from 457 mm (18 in.) to 1220 mm (48 in.). Steel fibers used in this researh were hooked-end fibers (l/d =67, l = 51 mm (2.0 in.), d = 0.76 mm (0.03 in.), f t = 1096 MPa (159 ksi)) onforming to ASTM A820. The fiber ontent was fixed at 0.75% by volume, whih is the minimum amount as speified by ACI 318 building ode [15]. The design ompressive strength of onrete was 42 MPa (6000 psi) in ompliane with the maximum allowable ompressive strength for SFRC [15]. Table 1 lists the design properties of beams used in this experimental program. Table 1: Design properties of the speimens Speimen Width mm (in.) Overall depth (h) mm (in.) Effetive depth (d) mm (in.) a/d ρ (%) V f (%) Targeted f MPa (psi) Measured f MPa (psi) SFRC18a,b 152 (6) 457 (18) 394 (15.5) (6000) 39 (5707) SFRC24a,b 203 (8) 610 (24) 541 (21.3) (6000) 50 (7210) SFRC36a,b 254 (10) 915 (36) 813 (32) (6000) 50 (7210) SFRC48a,b 305 (12) 1220 (48) 1118 (44) (6000) 50 (7210) RC18a,b 152 (6) 457 (18) 394 (15.5) (6000) 38 (5514) Prior researh has shown that the shear strength (in terms of stress) of RC beams is not a funtion of beam width [18, 19]. The width of eah speimen pair was determined by the following required tasks: 1) to ensure that required longitudinal steel bars an be aommodated with a proper over thikness; 2) to ensure that the shear apaity of speimens (loading) does not exeed the apaity of the equipment and setup; 3) to minimize the respetive width and onsequent redued weight to ease transportation and disposal. It is well known that for plain onrete beams without shear reinforement, the shear strengths vary with the shear span-to-depth ratios [20, 21]. The diret strut between the loading and support has great influene on the shear strength when a span to effetive depth ratio (a/d) is approximately less than 3.0. In this study, a/d ratio was seleted to be 3.5 to minimize the effet from diret strut. Suffiient flexural reinforement was provided to ensure that the failure is governed by shear rather than flexure. The amount of the longitudinal reinforement was alulated aording to highest shear apaity ( 0.5 f MPa ( 6.0 f psi)) reported for SFRC beams with 0.75% V f [17]. Self-weight of speimen was also taken into onsideration. Geometry and reinforement details of the RC and SFRC beams are shown in Figure 2. To ensure that shear failure would our in the instrumented span, the other span was reinfored by shear reinforement as shown in Figure 2. Mehanial terminators (headed bars) were employed at the end of the longitudinal bars to alleviate the ongestion exept for 377

4 High Performane Fiber Reinfored Cement Composites (HPFRCC7), the 457 mm (18 in.) deep RC and SFRC beams in whih the longitudinal bars were bent 90 at the ends to provide the anhorage. (a) (b) () (d) Figure 2: Geometry and reinforement details of the large-sale RC and SFRC beams; (a) RC18; (b) SFRC18; () SFRC24; (d) SFRC36; (e) SFRC48 (in. = 25.4 mm) 2.2. Mixture ompositions and material properties Table 2 gives the SFRC mix proportions with a target maximum ompressive strength of 42 MPa (6000 psi) aording to [15]. Table 2: Mix proportions by weight Type of Mix Cement (Type I) Fly Ash (Class C) Sand Coarse Aggregate 10 mm (3/8") Water [1] Steel Fiber Total Weight SFRC [2] 4.77 RC [1]: W/CM = 0.3; [2]: V f = 0.75% 2.3. Test setup and instrumentation SFRC speimens were loaded by a onentrated fore at the mid-span through a 2891 kn (650 kips) hydrauli ylinder. For the RC beams, however, the load was applied at one third (e) 378

High Performane Fiber Reinfored Cement Composites (HPFRCC7), of the span length (Figure 2a).

Then, loads were monotonially inreased and paused at a few loadings to trae the raks and take photos.

For safety purposes, lateral supports were provided for speimens with depths of 1220 mm (48 in.

For eah test, a total of three bearing plates were used at the supports and loading point.

To provide a uniform interfae ontat, a layer of non-shrink grout was used between the onrete and bearing

A shemati view of the test setup, instrumentations, and loading onfiguration is depited in Figure 3a.

In eah speimen, two pairs of strain gauges were mounted on the bottom layer reinforing bars at the

Three linear variable differential transformers (LVDTs) were employed to measure the defletions under the

During of the tests, the applied load was measured by a load ell.

5 High Performane Fiber Reinfored Cement Composites (HPFRCC7), of the span length (Figure 2a). In eah test, the beam was initially loaded until the first visible flexural rak. Then, loads were monotonially inreased and paused at a few loadings to trae the raks and take photos. The proess ontinued until failure. For safety purposes, lateral supports were provided for speimens with depths of 1220 mm (48 in.) and 915 mm (36 in.). The lateral supports did not ontat the speimens. For eah test, a total of three bearing plates were used at the supports and loading point. Dimensions of the bearing plate are illustrated in Figure 3a. To provide a uniform interfae ontat, a layer of non-shrink grout was used between the onrete and bearing plate at the loading point. A shemati view of the test setup, instrumentations, and loading onfiguration is depited in Figure 3a. Speimen SFRC48b (overall depth = 1220 mm) is shown in Figure 3b. In eah speimen, two pairs of strain gauges were mounted on the bottom layer reinforing bars at the loation shown in Figure 3a. Three linear variable differential transformers (LVDTs) were employed to measure the defletions under the loading point and the settlement of eah support. During of the tests, the applied load was measured by a load ell. Testing was arried out 502 to 1005 days after asting. The speimens were exposed to weather during this period. (a) (b) Figure 3: (a) shemati views of the test setup, loading onfiguration, and instrumentations (1 in. = 25.4 mm); (b) speimen SFRC48b. 3. DISCUSSION OF TEST RESULTS The speimens initially raked in flexure near the mid-span. Contrary to the fast propagation of flexural raks in RC beams (RC18a & RC18b), the presene of steel fibers in SFRC beams onsiderably slowed down the rak propagation, speifially in the deeper SFRC speimens with 915 mm (36 in.) and 1220 mm (48 in.) overall depth. The first diagonal 379

High Performane Fiber Reinfored Cement Composites (HPFRCC7), rak appeared either in the form of multiple web-shear raks with a nearly 45 slope, or flexural-shear raks extended from initial flexural

Inreasing the external load aused progressive development of many new shear raks distributed aross the shear span of SFRC beams until the failure.

At higher loads, a series of small inlined raks started ourring along the very top layer of the longitudinal bars when the inlined raks beame wider (Figure 4).

This observation was only notied in SFRC speimens and an be attributed to the effetiveness of fibers in enhaning the tensile strength and bond of onrete surrounding the reinforing bars.

They either formed from a branh of existing diagonal raks, or extended from existing web-shear raks. In other words, they ourred at late stage of loading after multiple raks had developed.

6 High Performane Fiber Reinfored Cement Composites (HPFRCC7), rak appeared either in the form of multiple web-shear raks with a nearly 45 slope, or flexural-shear raks extended from initial flexural raks. Exept for SFRC18b and SFRC24a, all of the SFRC speimens developed web-shear raks. Inreasing the external load aused progressive development of many new shear raks distributed aross the shear span of SFRC beams until the failure. During this proess, the propagation of the existing raks in SFRC speimens appeared to be slow and stable toward both the ompression zone and loading point as well as toward the longitudinal bars. At higher loads, a series of small inlined raks started ourring along the very top layer of the longitudinal bars when the inlined raks beame wider (Figure 4). Development of these small inlined raks indiates the involvement of dowel ation in resisting shear fore. This observation was only notied in SFRC speimens and an be attributed to the effetiveness of fibers in enhaning the tensile strength and bond of onrete surrounding the reinforing bars. In all SFRC beams, the ritial shear raks that lead to ultimate shear failure were not the first few shear raks reorded. They either formed from a branh of existing diagonal raks, or extended from existing web-shear raks. In other words, they ourred at late stage of loading after multiple raks had developed. Eventually, failure was found to be triggered by the breakdown of dowel ation. It was observed that if the bottom of the ritial shear rak was away from the support, dowel ation was exhausted by development of the splitting rak along the longitudinal bars (Figure 4b). In the other ase, the inlined rak extended all the way through the onrete over in the tension zone, whih led to kinking of the longitudinal bars and destrution of dowel resistane (Figure 4a). Sometimes, due to the high dowel fore, the onrete fratured before the kinking ourred, as an be seen in SFRC36b and SFRC48b, (Figure 4 andfigure 4d). (a) (b) () (d) Figure 4: Craking pattern and failure modes: (a) SFRC18b; (b) SFRC24a; () SFRC36b; (d) SFRC48b The ultimate shear strength for the RC and SFRC speimens and the average of shear strengths for eah pair of SFRC and RC beams with the same effetive depth are plotted in terms of f versus effetive depth in Figure 5. Figure 5a learly indiates that the shear strength of onrete beams were greatly inreased by the addition of steel fibers when omparing SFRC18a and SFRC18b ( 0.47 f MPa ( f psi)) with RC18 ( 0.21 f MPa f psi)). In this partiular ase, the shear strength inreased about 125%. In addition, ( 380

7 High Performane Fiber Reinfored Cement Composites (HPFRCC7), for the tested beams with the effetive depth varying from 394 mm (15.5 in.) to 1118 mm (44 in.), the average of shear strength of all SFRC speimens was about ( f MPa ( 5. 4 f psi)), with no obvious size effet. Effetive depth, d (in.) Effetive depth, d (in.), SFRC18 (394, 0.47) SFRC24 (541, 0.42) RC18 (394, 0.21) SFRC36 (813, 0.48) SFRC48 (1118, 0.43),, SFRC18b (394, 0.52) SFRC24b (541, 0.49) SFRC18a (394, 0.43) SFRC24a RC18a (541, 0.34) (394, 0.22) RC18b (394, 0.20) SFRC36b (813, 0.48) SFRC36a (813, 0.47) SFRC48a (1118, 0.44) SFRC48b (1118, 0.42), Effetive depth, d (mm) Effetive depth, d (mm) Figure 5: Normalized shear strength vs. effetive depth: (a) average of normalized shear strength for eah pair of speimens with the same effetive depth; (b) normalized shear strength for eah RC and SFRC beam 4. CONCLUSIONS While the speimens were exposed to weather and tested after a long period of time from the asting to testing date (502 to 1005 days), the shear behavior of the speimens was satisfatory. Test results obtained from this study indiated no evident size effet on ultimate shear stress of SFRC beams with a depth of up to 1220 mm (48 in.). It should be noted that a few reent studies did show size effet in SFRC beams [22, 23]. Potential auses resulted in the different findings between the urrent and other studies and are disussed in other future publiation by the authors. ACKNOWLEDGEMENTS Conrete and formwork materials used in this investigation were provided by Mr. Vartan Babakhanian at Hanson Pipe & Preast, Grand Prairie, TX. The SFRC speimens were ast at Hanson Pipe & Preast by their workers. Steel fibers used in this investigation were provided by Maaferri. Dr. Gustavo Parra-Montesinos at the University of Wisonsin-Madison reviewed the design of the speimens. Eah person and ompany represented here offered essential help that is gratefully appreiated. REFERENCES [1] Kani, G.N.J. (1967) 'How Safe are Our Large Reinfored Conrete Beams?' Journal of the Amerian Conrete Institute, Proeedings, Vol. 64, No. 3, pp [2] Shioya, T., Iguro, M., Nojiri, Y., Akiyama, H., and Okada, T. (1990) 'Shear Strength of Large Reinfored Conrete Beams' Frature Mehanis: Appliation to Conrete," SP 118, Amerian Conrete Institute, Detroit, 309 pp. 381

8 High Performane Fiber Reinfored Cement Composites (HPFRCC7), [3] Collins, M. P., and Kuhma, D. 'How Safe Are Our Large, Lightly-Reinfored Conrete Beams, Slabs and Footings?' ACI Strutural Journal, V. 96, No. 4, July-Aug. 1999, pp [4] Sherwood, E. G., 'One-Way Shear Behavior of Large, Lightly-Reinfored Conrete Beams and Slabs,' PhD thesis, Department of Civil Engineering, University of Toronto, Toronto, ON, Canada, 2008, 547 pp. [5] Walraven, J.C. (1981) 'Fundamental Analysis of Aggregate Interlok,' Journal of the Strutural Division, ASCE, Vol. 107, No. 11, pp [6] Vehio, F.J. and Collins, M.P. (1986) 'The Modified Compression Field Theory for Reinfored Conrete Elements Subjeted to Shear,' ACI Journal, Vol. 83, No. 2, pp [7] Khaja, M. N., and Sherwood, E. G. (2013). 'Does the shear strength of reinfored onrete beams and slabs depend upon the flexural reinforement ratio or the reinforement strain?' Can. J. Civ. Eng., 40(11), [8] Swamy, R.N., and Bahia, H. M. (1985) 'Effetiveness of Steel Fibers as Shear Reinforement,' Conrete International: Design & Constrution, V. 7, No.3, pp [9] Mansur, M. A.; Ong, K. C. G.; and Paramasivam, P., 'Shear Strength of Fibrous Conrete Beams without Stirrups,' Journal of Strutural Engineering, ASCE, V. 112, No.9, Sept. 1986, pp [10] Narayanan, R., and Darwish, I. Y. S. (1987). 'Use of Steel Fibers as Shear Reinforement.' ACI Strutural Journal, 84(3), [11] Ashour, S. A., Hasanain, G. S., and Wafa, F. F. (1992). 'Shear Behavior of High- Strength Fiber Reinfored Conrete Beams.' ACI Strutural Journal, 89(2), [12] Swamy, R.N., Jones, R., and Chiam, A. T. P. (1993), 'Influene of Steel Fibers on the Shear Resistane of Lightweight Conrete I-Beams,' ACI Strutural Journal, V. 90, No.1, pp [13] Kwak, Y.-K., Eberhard, M. O., Kim, W.-S., and Kim, J. (2002). 'Shear Strength of Steel Fiberreinfored Conrete Beams without Stirrups.' ACI Strutural Journal, 99(4), [14] Dinh, H. H. (2009). 'Shear behavior of steel fiber reinfored onrete beams without stirrup reinforement.' Dotoral Dissertation, Dept. of Civil and Environmental Engineering, Univ. of Mihigan, Ann Arbor, MI, 285. [15] ACI Committee 318 (2014). Building Code Requirements for Strutural Conrete and Commentary, Amerian Conrete Institute, Farmington Hills, MI. [16] Dinh, H. H., Parra-Montesinos, G. J., and Wight, J. K. (2010). 'Shear behavior of steel fiber reinfored onrete beams without stirrup reinforement.' ACI Strut. J., 107(5), [17] Parra-Montesinos, G. J. (2006). 'Shear Strength of Beams with Deformed Steel Fibers.' Conrete International, 28(11), [18] Kani, M.W., Huggins, M.W. and Wittkopp, R.R. (1979) 'Kani on Shear in Reinfored Conrete,' University of Toronto Press, Toronto, Canada, 225pp. [19] Lubell, A.; Sherwood, T.; Bentz, E. C.; and Collins, M. P., 'Safe Shear Design of Large, Wide Beams,' Conrete International, V. 26, No. 1, Jan. 2004, pp [20] ASCE-ACI Task Committee 426, 'The Shear Strength of Reinfored Conrete Members,' Journal of the Strutural Division, Proeedings of the Amerian Soiety of Civil Engineers, V.99, No. ST6, June, 1973, pp [21] Wight, J. K., and MaGregor, J. G., Reinfored Conrete Mehanis and Design, 6 th Ed., Upper Saddle River, NH, Prentie Hall, [22] Shoaib, A., Lubell, A. S. and Bindiganavile, V. S. (2014). 'Size Effet in Shear for Steel Fiber- Reinfored Conrete Members without Stirrups.' ACI Strut. J., 111(5), [23] Minelli, F., Conforti, A., Cuena, E., & Plizzari, G. (2014). 'Are steel fibres able to mitigate or eliminate size effet in shear?' Materials and strutures, 47(3),

Suggested Changes to NZS3101:2006 with Amendments 1 and 2

1 Suggested Changes to NZS3101:2006 with Amendments 1 and 2 Rihard Fenwik and Dene Cook Introdution NZS 3101: 2006 Conrete Strutures Standard is a design ode published by Standards New Zealand. Sine the