Shear Strength of Steel Fiber Reinforced Prestressed Concrete Beams Authors: Jae-Sung Cho, doctoral student, University of Texas at Arlington, Arlington, TX, 76019, jae.cho@mavs.uta.edu Joe Lundy, Director of Structural Product Design, Hanson Pipe & Precast, Inc., Grand Prairie, TX, 75050, Joe.Lundy@hanson.biz Shih-Ho Chao, Assistant Professor, University of Texas at Arlington, Arlington, TX, 76019 shchao@uta.edu

ABSTRACT

Due to the increasing evidence from previous research results, the 2008 ACI Building Code allows engineers to use steel fiber reinforced concrete (SFRC) to replace the conventional shear reinforcement (i.e. steel stirrups) even if the design shear force is greater than half of the concrete shear strength. Though the new ACI provisions, marked a significant transfer from research to practice, beams constructed of steel fiber reinforced concrete are required to have a minimum amount of steel fibers of 0.75% in volume (100 pounds per cubic yards) and compressive strength not greater than 6 ksi. The ACI provisions are primarily formulated on experimental studies on non-prestressed concrete beams and majority of them had a cylinder compressive strength less than 6 ksi. However, in a prestressed concrete beam, the beneficial effect from prestressing forces could further relax the minimum required fiber volume fraction thus make the use of SFRC more economical. Further, concrete compressive strengths much higher than 6 ksi are commonly used in prestressed concrete beams to reduce the creep related issues as well as to provide larger loading capacity. Therefore, the current ACI requirements will hinder the use of SFRC in structures with prestressed concrete members made of high strength concrete.

This paper presents preliminary shear test results of large scale prestressed concrete beams constructed of steel fiber reinforced concrete.

INTRODUCTION

Shear failure in plain concrete members is brittle in nature and consequently predisposes structures to sudden collapse without any advance warning. One measure to protect concrete members from brittle shear failure under excessive loads is to use fiber reinforced concrete (FRC). Numerous research [e.g. Narayanan and Darwish, 1987; Adebar et al., 1997; Casanova and Rossi, 1999; Kwak et al., 2002; Cucchiara et al., 2004] has been conducted on the shear behavior of FRC over the past decades and the general conclusion is that, with proper mixture design and with fibers selected for the appropriate material properties, FRC is capable of considerably increasing performance in terms of shear strength and ductility when compared to plain concrete [ACI Committee 544, 2002].

Despite the increasing evidence from previous research results, the American Concrete Institute did not allow steel fiber reinforced concrete (SFRC) as an alternative for conventional

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shear reinforcement (i.e. steel stirrups) until the 2008 ACI Building Code [Section 11.4.6, ACI 318-08, 2008]: “……when 0.5 c u cv v v , steel fibers can be used to replace the minimum shear reinforcement for flexural members (prestressed and nonprestressed) constructed of steel fiber-reinforced concrete with cf compressive strength not exceeding 6 ksi, depth not greater than 24 inches, and shear stress vu not greater than 2 cf ”. A steel fiber volume fraction of 0.75% (i.e., 100 lb steel fibers per cubic yard concrete) has been recommended as the minimum amount that needs to be used [Section 5.6.6.2 ACI 318-08, 2008].

It should be noted that although this ACI provision applies to prestressed concrete, a major structural material used in a significant array of structural applications, all prior investigation results that lead to this new provision were based on experimental tests on nonprestressed beams [Parra-Montesinos, 2006]. Due to the enhanced concrete shear strength coming from prestress, a minimum amount fiber volume less than 0.75% is highly probable for prestressed members. This in turn reduces the initial material costs and will provide an incentive for practicing engineers to use this type of material. In addition, while complete replacement of transverse shear reinforcement by steel fibers in typical reinforced concrete beams may not be the common case, (i.e. u cv v in typical reinforced concrete member), total replacement by steel fibers is very likely in most common design problems involving prestressed beams. This is due to fact that the minimum amount shear reinforcement generally prevails across the spectrum of typical prestressed beams ( 0.5 c u cv v v ) [Naaman, 2004]. The elimination of stirrups will further reduce the labor and construction costs.

Experimental results on the shear performance of steel fibers reinforced prestressed concrete beams are very limited, and the majority of them are on small specimens (with a depth about 10 inches) [e.g. Padmarajaiah and Ramaswamy, 2001; Tan et al., 1995; Thomas and Ramaswamy, 2006]. As a consequence, the objective of this preliminary study is to conduct shear tests on large scale steel fiber reinforced prestressed concrete (SFRPC) beams, to investigate the efficacy and benefits mentioned above. The test results will define the contribution of steel fibers to the shear resistance of SFRPC beams as well as verify and extend the use of the 2008 ACI provisions. EXPERIMENTAL PROGRAM

Two large scale prestressed beams (one with plain concrete, and the other with 0.75% steel hooked fibers) with a height of 24 inches and average compressive strength of 9.2 ksi were designed as shown in Figure 1(a). It is noted that concrete compressive strengths at this level are commonly used in prestressed concrete beams to reduce the creep related issues as well as to provide larger loading capacity.

Both beams had the same geometry, where the longer span of the specimen was to be reinforced by stirrups in order to insure that the shear failure occurs at the shear span under investigation. The shear span to depth ratio was 3.0 (a = 63 in., dp = 21 in.). A total of six 0.5-inch diameter prestressing strands (ASTM A416, Grade 270, stress relieved) were used in each beam. Initial prestressing of 189 ksi was applied to each strand, which in turn gave an average initial prestress of 380 psi in the beams. Additional nonprestressed mild steel (Grade 60) was used to prevent premature flexural failure. This led to a total longitudinal reinforcement ratio of 1.6%. The concrete mixtures, with an average aggregate size of 1/2 inch, were prepared and cast by professional workers at local precaster. The steel fibers used had hooked ends, aspect ratio of 80 (length = 2.4 in. and diameter = 0.03 in.), and a tensile strength of 152 ksi.

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Experimental tests were conducted approximately six months after the beams were cast. Load was monotonically applied through a hydraulic cylinder mounted on a steel reaction frame (Figure 1(b)) in 5 kip increments up to the first visible cracking then in 10-20 kip increments up to failure.

(a)

(b)

FIGURE 1 – (a) GEOMETRY OF THE LARGE-SCALE SFRPC BEAM TESTED IN THE PRELIMINARY STUDY (UNIT: INCH); SHEAR SPAN TO DEPTH RATIO = 3.0; (b) TEST SETUP

EXPERIMENTAL RESULTS A comparison of the total load and shear stress (in terms of cf ) versus displacement results

between conventional prestressed concrete (PC) and steel fiber reinforced prestressed concrete

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(SFRPC) beams is shown in Figure 2. It is seen that the shear strength of the PC beam was considerably enhanced by addition of steel fibers. The ultimate shear strength of the SFRPC beam (5.6 cf ) is 2.2 times of the PC beam ( 2.5 cf ). Figure 2 also shows a higher ductility in shear of the SFRPC beam compared to the plain concrete PC beam. It is noted that the performance of SFRPC in shear was promising: a shear stress capacity of 5.6 cf and an average concrete compressive strength, cf , of 9.2 ksi, both of which are significantly greater than the limitation given by the ACI code. It is observed in the preliminary study that the load leading to first cracking in the SFRPC beam is approximately two times of that in conventional PC beam, thus indicating a better serviceability performance can be expected by using SFRC (see Figures 3 and 4).

0 -0.5 -1 -1.5 -2 -2.5

Displacement under Loading Point (in.)

0

50

100

150

200

250

300

350

LoadP(kips)

SFRPC ( 0 .75%)

PC (Control)

0

1

2

3

4

5

6

ShearStress

First Cracking

Vf =

Note: Shear Force V = 0.65P

cf

FIGURE 2 – LOAD AND SHEAR STRESS VERSUS DISPLACEMENT RESPONSES OF CONVENTIONAL PC AND SFRPC BEAMS Shear Behavior of PC Beam

Figure 3 shows the behavior of the test PC beam from first cracking up to failure. Figure 3(a) indicates that the first visible flexural cracking occurred at a load of 85 kips (corresponding shear force is 55.3 kips), which is slightly higher than the expected cracking load of 76 kips calculated according to a modulus of rupture of 7.5 cf and effective prestress fpe = 150 ksi. The initiation of primary flexure-shear was noticed at a load of 110 kips (shear force = 71.5 kips), as shown in Figure 3(b).

It should be noted that the expected shear strength Vci, given in ACI (ACI Committee 318, 2008) Equation (11-10) and shown in below, is corresponding to the formation of the primary flexure-shear crack rather than the ultimate strength [ACI Committee 318, 1965].

max

0.6 i creci c w p d

V MV f b d V

M (1)

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where bw is beam width; dp is the distance from extreme compression fiber to centroid of prestressing steel (= 21 in.); Vd is shear force at section (at the section the loading was applied) due to unfactored dead load; Vi is factored shear force at section due to externally applied loads occurring simultaneously with Mmax; Mcre is the moment causing flexural cracking at section due to externally applied loads, ( / )(6 )t c pe dI y f f f ; Mmax is the maximum factored moment at section due to externally applied loads; yt is the distance from centroidal axis of gross section, neglecting reinforcement, to tension face.

Vci calculated based on (1) is 67.1 kips which is slightly less than the observed shear force (71.5 kips, or shear stress = 1.9 cf ). The percentage of longitudinal reinforcement ratio (1.6%) used could have resulted in the higher shear force at which the primary flexure-shear crack initiated [e.g. Hawkins and Kuchma, 2007].

It is also observed from Figure 3(b) that, due to the vertical compressive stress resulted from the applied load, the primary flexure-shear crack deviated from its original path in the vicinity of Point A and became flatter. Upon increased loading, this primary flexure-shear crack kept extending beyond the loading point until crushing of the compression zone. The ultimate shear force was 95.7 kips, which corresponding to a shear strength of 2.5 cf (see Figure 3(c)).

Figure 3(d) shows that a substantial crack slip resulted from the crushing of concrete in the reduced compression zone. Shear Behavior of SFRPC Beam

Figure 4 shows the behavior of SFRPC beam from first cracking load up to the loading right before failure. As can be seen in Figure 4(a), compared to the PC beam, the first cracking was significantly postponed due to the presence of steel fibers. While the primary flexure-shear crack in SFRPC beam has an angle close to that of the PC beam, it deviated from the original path (Point B in Figure 4(b)) much earlier than the one in the PC beam. This is due to the fact that, higher stresses were required for the crack initiation and propagation in SFRPC beam, therefore the principle stress state at Point B when the crack was approaching in the SFRPC beam is close to that at Point A in the PC beam.

This premature deviation of primary flexure-shear crack led to a much larger compression zone at the end of the crack in SFRPC beam, thus being able to engage arch action as the primary shear transfer mechanism. It is noted that, as shown in the test, arch action cannot be engaged for the conventional PC beam having a large shear span to depth ratio such as the one used in this study [Fenwick and Paulay, 1968]. The larger compression zone, together with the higher compressive strains at failure of SFRC materials [ACI Committee 544, 2002; Bencardino et al., 2008], led to a concrete arch with greater strength and ductility under compression (see Figure 4(c)), as demonstrated in the load versus displacement response shown in Figure 2.

The failure occurred at a load of 327 kips (shear force = 212.6 kips, or shear stress =5.6 cf ) due to instability of the crushed compression zone and a major splitting crack along the arch, as shown in Figure 5(a). By comparing Figure 4(b) with Figure 5(a), it can be clearly seen that shear failure was not caused by the primary flexure-shear crack but a new crack induced by the failure of a compression strut. Figure 5(b) shows a close-up of the crushed compression zone, which led to a significant slip of the shear crack. Contrary to PC beam (see Figure 3(d)), it can also be seen that SFRPC beam exhibited much ductile behavior at the crushed compression zone.

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(a)

(b)

(c)

(d)

FIGURE 3 – FIRST CRACKING AND ULTIMATE SHEAR FAILURE OF PC BEAMS (NO SHEAR REINFORCEMENT AT SHEAR SPAN)

Width of Compression Zone

Point where the primary flexure-shear crack deviates

A

PC at first cracking (P = 85 kips; V = 55.3 kips)

PC at ultimate (P = 147 kips; V = 95.7 kips)

Initiation of primary flexure-shear crack (P = 110 kips; V = 71.5 kips)

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(a)

(b)

(c)

FIGURE 4 – FIRST CRACKING AND CRACK DISTRIBUTION AT IMMINENT SHEAR FAILURE OF SFRPC BEAMS (0.75% STEEL HOOKED FIBER; NO SHEAR REINFORCEMENT AT SHEAR SPAN)

Point where the primary flexure-shear crack deviates B

SFRPC at first cracking (P = 170 kips; V = 110.5 kips)

SFRPC at imminent failure (P = 300 kips; V = 195 kips)

Cracking of compression zone

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(a)

(b)

FIGURE 5 – ULTIMATE FAILURE BEHAVIOR OF SFRPC BEAMS (0.75% STEEL HOOKED FIBER; NO SHEAR REINFORCEMENT AT SHEAR SPAN) SUMMARY AND CONCLUSIONS

An investigation on the shear behavior of large scale steel fiber reinforced prestressed concrete beam with moderate initial prestressing stress (380 psi) was conducted in this study. The minimum amount of steel fiber (0.75% by volume) required by ACI 318-08 was used in the beam, which had a concrete compressive strength (9.2 ksi) considerably higher than the allowed value (6 ksi) specified in the ACI code. No shear reinforcement (stirrups) was used in the shear span. A control specimen, with plain concrete and no shear reinforcement in the shear span was also constructed for comparison purposes. The tests were performed under monotonic loading applied at the location led to a shear span to depth ratio of 3.0 for both beams.

Experimental results showed that SFRPC beam had significantly better performance, in terms of shear strength (5.6 cf ) and ductility, even with a high concrete compressive strength. Further examination indicated that the enhanced behavior in shear of SFRPC beam was due to the activation of arch action as the primary shear resisting mechanism. The attained high shear strength also suggests a minimum amount fiber volume less than 0.75% is probable for replacing conventional shear reinforcement in prestressed members.

The preliminary results are most encouraging and further investigation on the SFRPC performance is on-going at the University of Texas at Arlington.

Width of Compression Zone (End of Arch)

Arch action and corresponding splitting crack leads to failure

Primary flexure-shear crack

SFRPC at failure (P = 327 kips; V= 212.6 kips)

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ACKNOWLEDGEMENT Materials and specimens tested in this investigation were prepared and constructed by Hanson Pipe & Precast at Grand Prairie, Texas. Their help is gratefully acknowledged.

REFERENCES

[1] ACI Committee 318, “Commentary on Building Code Requirements for Reinforced Concrete (ACI 318-63),” Report of ACI Committee 318, Standard Building Code, Publication SP-10, American Concrete Institute, Detroit. 1965.

[2] ACI Committee 318, “Building Code Requirements for Reinforced Concrete and Commentary (ACI 318-08/ACI 318R-08),” American Concrete Institute, Detroit. 2008.

[3] ACI Committee 544, “State of the Art Report on Fiber Reinforced Concrete,” Report 544-1R-96 (Reapproved 2002),” ACI Committee 544, Fiber Reinforced Concrete, 2002.

[4] Adebar, P., Mindess, S., St.-Pierre, D., and Olund, B., “Shear Tests of Fiber Concrete Beams without Stirrups,” ACI Structural Journal, V. 94, No. 1, Jam.-Feb., 1997, pp. 68-76.

[5] Bencardino, F., Rizzuti, L, Spadea, G., and Swamy, R. N., “Stress-Strain Behavior of Steel Fiber-Reinforced Concrete in Compression,” Journal of Materials in Civil Engineering, ASCE, Vol. 20, No. 3, March, 2008, pp. 255-263.

[6] Casanova, P. and Rossi, P., “High-Strength Concrete Beams Submitted to Shear: Steel Fibers versus Stirrups,” Structural Applications of Fiber Reinforced Concrete, SP-182, American Concrete Institute, Farmington Hills, MI, 1999, pp. 53-68.

[7] Cucchiara, C., Mendola, L. L., and Papia, M., “Effectiveness of Stirrups and Steel Fibres as Shear Reinforcement,” Cement and Concrete Composites, V. 26, No. 7, Oct., 2004, pp. 777-786.

[8] Fenwick, R. C. and Paulay, T., “Mechanisms of Shear Resistance of Concrete Beams,” Journal of the Structural Division, ASCE, Vol. 94, No. ST10, 1968, pp. 2235-2350.

[9] Hawkins, N. M., and Kuchma, D. A., “Application of LRFD Bridge Design Specifications to High-Strength Structural Concrete: Shear Provisions,” NCHRP Report 579, Transportation Research Board, Washington, D. C., 2007.

[10] Kwak, Y.-K, Eberhard, M. O., Kim, W.-S., and Kim, J., “Shear Strength of Steel Fiber-Reinforced Concrete Beams Without Stirrups,” ACI Structural Journal, V. 99, No. 4, July-Aug., 2002, pp. 530-538.

[11] Naaman, A. E., Prestressed Concrete Analysis and Design—Fundamentals, Second Edition, Techno Press 3000, 2004, 1072 pp.

[12] Narayanan R. and Darwish, I. Y. S., “Use of Steel Fibers as Shear Reinforcement,” ACI Structural Journal, V. 84, No. 3, May-June, 1987, pp. 216-227.

[13] Padmarajaiah, S. K. and Ramaswamy, A., “Behavior of Fiber-Reinforced Prestressed and Reinforced High-Strength Concrete Beams Subjected to Shear,” ACI Structural Journal, V. 98, No. 5, Sep.-Oct., 2001. pp. 752-761.

[14] Parra-Montesinos, G., “Shear Strength of Beams with Deformed Steel Fibers,” Concrete International, V. 28, No. 11, Nov., 2006, pp. 57-66.

[15] Tan, K. H., Paramasivam, P., and Murugappan, K., “Steel Fiber as Shear Reinforcement in Partially Prestressed Beams,” ACI Structural Journal, V. 92, No. 6, Nov.-Dec., 1995, pp. 643-652.

[16] Thomas. J. and Ramaswamy, A., “Shear Strength of Prestressed Concrete T-Beams with Steel Fibers over Partial/Full Depth,” ACI Structural Journal, V. 103, No. 3, May-June, 2006, pp. 427-435.

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