SHEAR STRENGTH OF BEAMS REINFORCED WITH SYNTHETIC MACRO ...demo.webdefy.com/rilem-new/wp-content/uploads/2016/10/c2827746e61... · SHEAR STRENGTH OF BEAMS REINFORCED WITH SYNTHETIC

BEFIB2012 – Fibre reinforced concrete

Joaquim Barros et al. (Eds)

© UM, Guimarães, 2012

SHEAR STRENGTH OF BEAMS REINFORCED WITH SYNTHETIC MACRO-FIBERS AND STIRRUPS

Salah Altoubat*, Yazdanbakhsh Ardavan

1 and Klaus-Alexander Rieder

2

* Associate Professor, Dep. of Civil and Environmental Eng., University of Sharjah

United Arab Emirates, email:[email protected]

1 Graduate Student, University of Sharjah, UAE

2

Principal Scientist, Grace Bauprodukte GmbH, Germany

Keywords: synthetic fibres, shear resistance, synergy, large scale testing.

Summary: This paper presents and discusses experimental results on shear strength and ductility of RC beams. The combined effect of macro synthetic fibers and stirrups on the shear strength and ductility of the RC beams is specifically examined. The experimental program incorporated testing of 16 large beams under center-point monotonic loading. The beams were tested with a shear span to depth ratio of 2.3 and 3.5. The beam cross-section was 390 × 230 mm, and the amount and spacing of the stirrups were chosen to comply with the ACI-318-05 requirements for minimum shear reinforcement. Synthetic macro fibers were added at a volume fraction of 0.5%. The results showed that the addition of macro synthetic fibers significantly improved the shear strength and ductility of the RC beams and modified its cracking and failure behavior in a similar pattern as that with conventional minimum ACI shear reinforcement. The macro synthetic fibers improved the shear strength of the slender and short RC by 14% and 18%. Similarly, the addition of minimum ACI 318 stirrups increased the shear strength of the slender and short beams by 19%.The results also showed that the use of both stirrups and macro synthetic fibers at volume fraction of 0.50% improved shear strength of slender and short beams by 69% and 42%, respectively. These improvements showed that the combined use of the synthetic macro fibers and minimum stirrups has significant synergistic effects on shear strength. The results suggest that the synergy depends not only on the type of fibers and extent of reinforcement but also on the slenderness of the beam.

1 INTRODUCTION

Research over the past three decades has clearly established that fibers can be used to boost the shear capacity of concrete and to improve the shear crack distribution, and therefore are capable of replacing, some of the vertical stirrups in reinforced concrete structural members [1-4]. Furthermore, fibers provide a more confined concrete around the web reinforcement, and thus, a synergistic effect for shear-resistance in RC members is expected. This implies that when fibers and stirrups are combined, the net enhancement in shear strength will be more than the sum of individual contribution of fibers and/or stirrups. The combined use of fibers and shear reinforcement could reduce the need for heavy shear reinforcement imposed by the current design specification of RC members. This helps to reduce the problems associated with congestion of shear reinforcement, particularly at critical sections such as beam column junctions.

The synergistic effect due to hybrid use of fibers and stirrups was first studied by Swamy and Bahia [5] through testing a number of medium size T-beams. The results showed that the addition of 0.8% of steel fibers with small amount of stirrups produced flexural yielding, extensive ductility, and large deflections of beams with heavy flexural steel reinforcement of 2 to 4 percent. Criage [6] tested

BEFIB2012: Salah Altoubat, Ardavan Yazdanbakhsh and Klaus-Alexander Rieder.

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two beams reinforced with steel fibers and stirrups and concluded that the combination of stirrups and steel fibers resulted in slow and controlled cracking, better distribution of tensile cracks, and reduced the penetration of shear cracks into the compression zone.

Sarhat and Abdul-Ahad [7] tested reinforced concrete T beams with steel fibers at different volume fractions (0.5 to 1.5%) and transverse shear reinforcement ratios of 0.18% and 0.35%. The test results showed that steel fibers are more efficient in increasing ultimate shear strength and ductility when used with small amounts of stirrups, and this efficiency will decrease when using large amounts of stirrups. The tests also demonstrated that the contributions of steel fibers and stirrups are not cumulative and there are limits for an optimum combination between them, which lead to higher shear strength and ductility. Similar observation was found by Rusenbusch [8] who tested rectangular reinforced concrete beams with steel fibers and stirrups under center-point loading. The average synergy ratio was also higher for beams with smaller transverse reinforcement ratio.

Watanabe et al. [9] examined the synergy between steel fibers and shear reinforcement and tested reinforced concrete beams reinfroced with different volume fraction of steel fibers (0.3% to 1.0%) and different shear reinforcing steel ratios (0.12% to 0.3%). The results showed that the ratio of the shear reinforcing steel affects the level of synergy and an optimum combination between the volume of fibers and shear reinforcing steel ratio do exists.

Unlike that for steel fibers only limited number of studies looked at the synthetic fibers with stirrups. Majdzadeh et al [10] tested 8 beams to study the combined effect of stirrups and three types of fibers (synthetic and steel) added at a rate of 0.5% by volume. The beams had flexural reinforcement ratio of 2.62% and conventional shear reinforcement (stirrup) ratio of 0.28%. The results showed that the type of fiber plays a major role on the value of synergy ratio; and therefore, it is important to further study the synergistic effect between stirrups and synthetic fibers especially that synthetic macro fibers have been increasingly used for structural applications.

This study is part of a comprehensive experimental program conducted at the University of Sharjah (UOS), which focused on the shear behavior of beams reinforced with macro-synthetic fiber [11]. The paper presents results from large scale testing of sixteen beams under monotonic center-point loading in a simply supported configuration. The beams were reinforced with macro synthetic fibers and/or minimum ACI 318 stirrups. Beams with hybrid fiber-stirrups reinforcement as well as beams with no shear reinforcement were also tested. Load-deflection measurement, failure and cracking behavior, load-strain measurements and the shear capacity of the beams are presented and discussed in this paper.

2 EXPERIMENTAL PROGRAM

2.1 Materials

The main components of the macro synthetic fibers used in this study are polypropylene and polyethylene. The fiber’s nominal length is 40 mm and has an aspect ratio of 90 and a specific gravity of approximately 0.92. The fiber has a rectangular cross section with an average width of 1.4 mm and average thickness of 0.105 mm. The average tensile capacity of the fiber is 620 MPa with a modulus of elasticity of 9.5 GPa. The fiber dosage used in this testing program was 0.5 % by volume, which corresponds to 4.6 kg/m

3 of the fibers. This addition rate of macro synthetic fiber is typically and

commonly used in the concrete industry since it can be easily mixed in conventional concrete mixes and provides a reasonable flexural residual strength when tested according to the ASTM C1609-07 [12].

The concrete mix proportions and properties used in casting the sixteen beams are provided in Table 1. The final water to cement ratio was 0.47. The proportion of coarse to fine aggregate was targeted at 50:50 in order to maintain workability and to have sufficient paste for coating the fibers. The coarse aggregate used in the mix was Gabro gravel with a maximum size aggregate of 20 mm and a specific gravity of 3.1. The fine aggregate constituents were natural washed sand with a specific gravity of 2.6 and dune sand with specific gravity of 2.63. The 28-day compressive strength


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values measured according to ASTM C39 and flexural strength values measured according to ASTM C78 are summarized in Table 1. The flexural and compressive strength results did not vary significantly between the mixes. The average values of the equivalent flexural strength, fe,3, measured according to JSCE-1984 [13] at 3 mm (0.12 in.) deflection were also reported (based on testing six standard beam specimens), which can be obtained from bending of beams under third-point loading with a support span of 450 mm (18 in.). The equivalent flexural strength, fe,3, is calculated by inserting the average load into the formula for the modulus of rupture. The average load is equal to the area under the load versus deflection curve (also called toughness) measured up to a beam deflection of 3

mm (0.12 in.) divided by 3 mm (0.12 in.). The fe,3 value is directly proportional to the 150

150T value defined in the ASTM C1609-07 [12] flexural beam test using a 150 mm by 150 mm by 550 mm (6 in. by 6 in.

by 22 in.) beam. The fe,3 value as well as the 150

150T value are directly proportional to the area under the

load-deflection curve up to a central beam deflection of 3 mm (0.12 in.) (for a support span of 450 mm (18 in.)).

Table 1 : Mix proportions and average properties of concrete

Material Vf =

0.0% Vf =

0.50%

Coarse Aggregate, kg/m3 943 943

Fine Aggregate, kg/m3 942 942

Cement, kg/m3 380 380

Water, kg/m3 195 195

Superplasticizer Daracem 205, kg/m3 2.15 4.55

Water to Cement Ratio 0.47 0.47

Slump, mm 100 100

Cylinder Compressive Strength, MPa 42 42

Flexural Strength, MPa 5.8 5.8

Equivalent Flexural Strength (fe,3), MPa

0.15 2

2.2 Large Beam Testing

The Sixteen full-scale concrete beams were designed, instrumented and tested in displacement control mode under a monotonic three-point loading system in a simply supported configuration. Slender and short beams with shear-span to depth ratio a/d of 3.5 and 2.3 were tested in this study (further information on the difference in behavior between short and slender beams can be found in references 4 and 11). The cross section of the beam was 230 mm x 390 mm. The ACI 318 design code was adopted for the design of the beams and determining the amount of flexural reinforcement such that shear failure would occur. The beams were reinforced with three longitudinal reinforcing bars

having diameter of 32 mm (steel ratio

= 3.18 %). The high ratio of flexural steel was chosen to force the beam to fail in shear and thus the synergestic effect of fibers on shear behavior can be tested. Figure 1 provides details of the beam layout. The large beams and the companion lab specimens, such as the compressive strength cylinders, and the flexural toughness beams were all cast in one day in a modern pre-cast factory. All beams were covered with plastic sheeting for 21 days and the lab specimens were cured in water for 28 days.


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Figure 1: Schematic outline of the location and orientation of (a) embedded strain gauges, (b)

external strain gauges, and (c) details of the beam cross-section. Table 2 presents the details of the large beams constructed and tested in this study. Two groups of

beams one short and the other is long (slender) were cast; each group consists of four sets of beams and each set consist of two identical beams (duplicate) labeled ‘a’ and ‘b’ . The four sets in each group consists of one control pair without any shear reinforcement labeled as control; one set with minimum ACI conventional shear reinforcement; one set with 0.5% of synthetic macro fibers as a shear reinforcement; and the last one includes hybrid shear reinforcement with 0.5% of fibers and minimum ACI steel stirrups. Duplicate beams were tested to enhance the reliability of the results. The identical beam (duplicate) of each pair are labeled ‘a’ and ‘b’. The average test results of the two beams in each set are reported in this paper.

Table 2: Details of the large scale beams tested in the study

Beam type Qty h,

mm

d,

mm

b,

mm

length,

m

Span,

m a/d ρ

Vf ,

%

Minimum

Stirrups

L-C 2 390 330 230 2.7 2.31 3.5 0.0318 0.00 No

L-S 2 390 330 230 2.7 2.31 3.5 0.0318 0.00 Yes

L-F 2 390 330 230 2.7 2.31 3.5 0.0318 0.50 No

L-SF 2 390 330 230 2.7 2.31 3.5 0.0318 0.50 Yes

Sh-C 2 390 330 230 1.9 1.5 2.3 0.0318 0.00 No

Sh-S 2 390 330 230 1.9 1.5 2.3 0.0318 0.00 Yes

Sh-F 2 390 330 230 1.9 1.5 2.3 0.0318 0.50 No

Sh-SF 2 390 330 230 1.9 1.5 2.3 0.0318 0.50 Yes

The average cylinder compressive strength of the concrete used in this study was around 42 MPa.

The beams were labeled to indicate the type of beam (short or slender), the type of reinforcement (C: control, S: minimum stirrups, F: fibers, SF: hybrid fiber and stirrups) The letter ‘L’ denotes long or slender (a/d =3.5) while ‘Sh’ means short (a/d =2.3). For example the beam labeled as L-F stands for a long (slender) beam with fibers as shear reinforcement. The large beams and the companion lab specimens, including the 100 mm by 200 mm cylinders for compressive strength measurement and the 150 mm by 150 mm by 550 mm beams for flexural toughness measurement were all cast in one day in a modern pre-cast factory.


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2.3 Beam Instrumentation and Testing

The beams were instrumented with embedded and external strain gauges to monitor the strain field in the concrete and in the reinforcing bars at different stages of loading. Embedded concrete strain gauges were installed in the shear span of all beams and aligned at 45 degrees; the potential direction for a diagonal shear crack. In addition, strain gauges were attached to the longitudinal reinforcing bars at the section of maximum bending moment to monitor the state of stress in the reinforcing bars. Figure 1 shows schematically the locations of the strain gauges. Gauges 1-6, 8 and 11 measured tensile strains and gauges 7, 9 and 10 measured compressive strains. Global deflection of the beams was measured with linear voltage displacement transformers (LVDTs) at three points along the beam span; at midspan (center) and at the quarter points of the span. Results from the strain and deflection measurements were used to monitor the strain fields and to explain the cracking pattern and overall structural response of the beams.

Figure 2: Test setup and the arrangement of LVDTs.

Testing of the beams began 28 days after casting and was completed over a period of three days. The test set up consisted of a simply supported loading configuration with roller supports to prevent restraint to axial elongation (Figure 2). The beams were loaded at midspan (center point) and tested in displacement control mode using a hydraulic actuator with a capacity of 500 kN. The parameters measured and recorded during the monotonic testing were beam deflections, strains in the concrete at different locations, strains in reinforcing flexural steel bars, and the applied load. The cracking pattern was also observed during testing. The load levels corresponding to initiation of flexural tensile and diagonal shear cracking as well as the ultimate shear capacity were determined for the beams using the load deflection and strain measurement data with the aid of visual observation. The formation of the first diagonal shear crack was associated with a sudden reduction in load carrying capacity of the RC beam. The maximum load carried by the RC beam before failure was used to calculate the ultimate shear capacity of the beam.

The load corresponding to the formation of the first diagonal shear crack is defined as the first shear cracking strength and the collapse load at which shear failure occurred is defined as the ultimate shear strength of the reinforced concrete beam. The first diagonal shear crack was associated with a sudden reduction in load carrying capacity of the beam, significant changes in the strain data and the diagonal crack becoming visible. The ultimate shear capacity marked the collapse of the beam and a complete loss of load carrying capacity.


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4 RESULTS AND DISCUSSIONS

4.1 Load-Deflection Responses

Slender Beams: The load versus mid-span deflection response of the long (slender) beams tested can be seen in Figure 3. The response of the beams is similar up to the load where the first diagonal shear crack occurs. At this point, the control beams exhibited sudden shear failure followed by a complete loss of their load carrying capacity. The beams reinforced with shear reinforcement behaved differently and continued to resist loads after the formation of the first diagonal shear crack: The concrete beams reinforced with 0.5 % synthetic macro fibers exhibited a slight reduction of the load when the first diagonal crack formed, and then continued to resist higher loads until shear failure occurred. The concrete beams reinforced with minimum ACI shear reinforcement exhibited similar first shear cracking strength and ultimate strength as that for the beams with 0.5% fibers, but it sustained the ultimate load for a greater range of deflection. The beams with hybrid shear reinforcement (fibers and ACI minimum steel) showed a distinguished global and cracking behavior as indicated by the load deflection behavior in Figure 3. The loads corresponding to the first diagonal crack and the ultimate shear capacity were significantly greater than that for the control and for the beams reinforced with either fibers or ACI minimum steel. The hybrid beams sustained a higher ultimate load over a high range of deflection indicating the significant improvement in the shear strength and ductility of the RC beams. The wide plateau in the load deflection curve in Figure 3 for the hybrid beams reflects the multiple shear cracks developed in the beams before failure and showed the improvement in ductility.

Figure 3: Load deflection curves of slender beams

Table 3 presents a summary of the average loads corresponding to the formation of first diagonal

shear crack and ultimate load capacity of the tested beams. This table also includes the percent increase of the load carrying capacity of the beams with shear reinforcement relative to the corresponding control RC beams. The results show that the addition of 0.5% of macro-synthetic fibers to the concrete beams increased the first diagonal cracking load and the ultimate load similar to that with the minimum shear reinforcement (14% and 19%). The results also show that the combined addition of fibers and minimum steel increased the ultimate shear strength by 69% which is much greater than the cumulative addition of the individual improvement. This suggests that a significant

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synergistic effect on the shear behavior exists when synthetic macro fibers and minimum steel are used in the RC beams.

The shape of the load deflection curves of the beams with hybrid shear reinforcement relative to the control beams (Figure 3) demonstrates the improved global structural response that discrete macro-synthetic fibers add to RC beams when used in combination with conventional shear reinforcement (steel). The benefit can be immediately seen in the first diagonal cracking load, the ultimate load carrying capacity, and the ductility of the beams before failure. Furthermore, the toughness, which is proportional to the area under the load deflection curve, is another indicator of the benefit of macro-synthetic fibers.

Table 3 : Average Loads at first diagonal crack and ultimate capacity of slender beams

Beam First diagonal crack load, kN

Ultimate load, kN

Percent Increase in ultimate load

L-C 233 233 -

L-F 265 265 14

L-S 268 278 19

L-SF 317 394 69

Short Beams: The load versus mid-span deflection curves of the short beams are presented in

Figure 4. As for the long beams, the load versus deflection response of all short beams were similar up to the load at which the first diagonal shear crack was formed in the control beams. For control beams, the load did not increase beyond the first diagonal cracking load, and thus it marked their ultimate capacity. The addition of 0.5% of macro-synthetic fibers or the ACI minimum shear reinforcement increased the first diagonal cracking and the ultimate load relative to the control beams in a similar manner. The effect of macro synthetic fibers on ultimate strength and ductility is more pronounced in the tested short beams than that for long beams. The short beams with hybrid shear reinforcement exhibited a significant improvement in ultimate shear capacity and ductility of the RC beam as reflected in the load deflection behavior. The hybrid beams showed much greater ultimate load carrying capacity and sustained the ultimate load for a greater range of deflection before collapse.

Table 4 presents a summary of the average loads corresponding to the formation of first diagonal shear crack and ultimate load capacity of the tested short beams as well as the percentage increase in the load carrying capacity of the beams with shear reinforcement relative to the corresponding control RC beams. The results show that the addition of 0.5% of macro-synthetic fibers to the concrete beams increased the ultimate load capacity similar to that with the minimum shear reinforcement (18% and 19%). The results also show that the combined addition of fibers and minimum steel increased the ultimate shear strength by 42% which is nearly similar to the cumulative addition of the individual improvement. This suggests that the synergistic effect on the shear behavior does exist when synthetic macro fibers and minimum steel are used in the RC beams, but it is less pronounced in short beams than that for long beams (42% versus 69% in long beams). Apparently, the synergistic effect is influenced by the type of beam in addition to its size and the amount of reinforcement used in combination with fibers.


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Figure 4: Load deflection curves of short beams

Table 4 : Average Loads at first diagonal crack and ultimate capacity of short beams

Beam First diagonal crack load, kN

Ultimate load, kN

Percent Increase in ultimate load

Sh-C 270 270 -

Sh-F 285 318 18

Sh-S 308 322 19

sh-SF -- 384 42

4.2 Cracking and Failure Mode

Slender Beams: Cracking pattern and sequence were carefully monitored and mapped during testing. Figure 5a-d presents pictures of cracking pattern and sequence for slender beams. During the test of slender beams, it was observed that flexural cracks started at the mid-span and spread out to the shear span, where the flexural cracks - with increasing load - began to incline as diagonal shear cracks. The number of flexural cracks and the inclination of the diagonal shear crack that lead to failure characterize the cracking pattern of the tested RC beams. The control RC beams failed with a single and steep diagonal shear crack as can be seen in Figure 5a. Moreover, the control beams had fewer flexural cracks before the formation of the first diagonal crack, which marked the failure of the beams. Unlike the control RC beams, the beams reinforced with either macro synthetic fibers (0.5%) or minimum ACI stirrups developed multiple flexural and diagonal shear cracks before failure occurred as can be seen in 5b and c. The formation of the first diagonal cracking in these beams did not mark the failure of the beams. They continued to resist higher loads and developed more diagonal cracking

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before failure, which occurred in a gradual and stable manner. Furthermore, the primary diagonal crack that leads to failure of these beams was flatter relative to that of the control beam, and extended further toward the support. The cracking pattern of the beams reinforced with 0.5% macro synthetic fibers is similar to that with minimum ACI steel stirrups as can be seen in Figure 5b and c.

Figure 5: cracking pattern of slender beams- a) control, b) synthetic fiber, c) minimum ACI stirrups,

and d) hybrid reinforcement (synthetic fiber and stirrups) The beams reinforced with hybrid reinforcement (macro synthetic fibers and ACI minimum stirrups)

exhibited distinguished cracking behavior among all beams. The beams exhibited significant multiple flexural and shear cracks relative to other beams. Multiple major diagonal cracks in the two shear spans (both sides of the beams) occurred which suggests that hybrid reinforcement enhanced the global homogeneity of the RC beams. The extent of flexural cracking is much more in these beams relative to other beams due to the synergistic effect between fibers and conventional steel stirrups. The major diagonal cracks extends further toward the support suggesting that appreciable arch action is occurring in these beams [15] and thus increased the shear strength of the beam. The synergy is apparent through the multiple major flexural and shear cracks in shear spans. The creation of significant multiple major flexural and shear cracks in these beams contributed to the increase of the ductility as reflected in the load deflection curves for the RC beams relative to other beams reinforced with either macro synthetic fibers or stirrups alone.

Short Beams: Figure 6a-d shows the cracking pattern and sequence for short beams. The short control RC beams Figure 6a developed single web-shear crack that lead to a sudden and brittle shear failure. Unlike the control RC beams, the beams reinforced with 0.5% of macro synthetic fibers developed more web shear cracks with small number of flexural cracks before failure as can be seen in Figure 6b. The beams reinforced with minimum ACI stirrups showed better flexural and shear crack

a) control

b) 0.5% synthetic fibers

c) Minimum stirrups

d) Hybrid fiber and stirrups


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distribution (Figure 6c). The addition of macro-synthetic fibers or minimum ACI stirrups improved the cracking distribution with signs of a shift in the failure mode from web shear cracking to flexural shear cracking where the shear crack initiated as a flexural crack and then start to incline as a shear crack developed. This shift in mode of failure is clearly exhibited by the beams with hybrid reinforcement. The beams reinforced with ACI minimum stirrups and macro synthetic fibers showed multiple shear and flexural cracks in both shear spans and the shear cracks were all initiated as flexural cracks and then inclined as shear cracks. This suggests that the hybrid reinforcement changed the mode of failure of short beams from web-shear cracking to flexural shear cracking. This can be attributed to the fact that macro-synthetic fibers and stirrups provided a synergistic effect that increased the shear strength of the beam to a level that was sufficient to mobilize flexural cracking prior to shear failure.

Figure 6: cracking pattern of short beams- a) control, b) synthetic fiber, c) minimum ACI stirrups and d) hybrid reinforcement (synthetic fiber and stirrups)

The synergetic effect of hybrid reinforcement on changing the mode of failure was also indicated by

the strain data measured in this study. The strain of the flexural steel in the long beams is shown in Figure 7. The control beams as well as the beams with either fibers or steel showed that the flexural reinforcing bars were not yielded at ultimate indicated that shear failure occurred prior to yielding of the flexural reinforcement. In Contrast, the beams reinforced with hybrid reinforcement (fibers and steel) exhibited yielding of the flexural reinforcement prior to failure. This suggests that hybrid reinforcement changed the mode of failure from shear failure to a combined flexural and shear failure mode.

(a)

(b)

(c)

(d)


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Figure 7: Strain in the flexural steel in long beams

5 SUMMARY AND CONCLUSIONS

Structural testing of large-scale beams under center point loading was conducted to determine the effect of a newly developed synthetic macro fiber on the shear strength of longitudinally reinforced concrete beams. The combined effect of conventional shear reinforcement and synthetic fibers were specifically examined. Slender and short beams with shear-span to depth ratio a/d of 3.5 and 2.3 were tested in this study. The cross section of the beam was 230 mm x 390 mm. Four sets of beams were tested; the control set without fibers, one set with macro synthetic fibers at volume fraction of 0.5%, one set with minimum ACI-318 stirrups and the last one with hybrid reinforcement of minimum stirrups and synthetic fibers. Two identical beams were tested in each set and the average response is reported in this study. The beams were instrumented with embedded and external strain gauges to monitor the strain field, which were used to interpret the cracking patterns of the beams and to explain the differences between the control beams and the beams with synthetic macro fibers.

The results showed that the addition of macro synthetic fibers significantly improved the shear strength and ductility of the RC beams and modified its cracking and failure behavior. The results also showed that the use of both stirrups and macro synthetic fibers at volume fraction of 0.50% improved shear strength of slender and short beams by 69% and 42%, respectively. These improvements were significant and showed that the combined use of macro synthetic fibers and stirrups has synergistic effects on shear strength. The macro synthetic fibers improved the shear strength of the slender and short RC by 14% and 18%. Similarly, the addition of minimum ACI 318 stirrups increased the shear strength of the slender and short beams by 19%. The results suggest that synergistic effect was more pronounced in long beams than in short beams. Fibers and minimum stirrups became twice as effective as when used alone in long beams. This suggests that the synergy depends not only on the type of fibers and extent of reinforcement but also on the slenderness of the beam.

The load versus deflection curves showed that the beams reinforced with either macro synthetic fibers or ACI minimum stirrups are more ductile relative to the control. Significant improvement in the load deflection behavior was observed when hybrid reinforcement is used, which suggests that the

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synergy on ductility of the RC beams is quite remarkable. The cracking behavior of the beams with hybrid reinforcement was characterized by significant multiple cracking, flatter and narrower cracks relative to all other beams. The hybrid reinforcement changed the mode of failure of short beams from web shear cracking to flexural shear cracking.

ACKNOWLEDGMENT

Support to this project was provided in part by the College of Graduate Studies at the University of Sharjah, Juma Almajid Company in Dubai; and W.R. Grace, Cambridge, MA, USA. The authors would like to acknowledge support provided for this project.

REFERENCES

[1] Kwak, Y.K., et al., “Shear Strength of Steel Fiber Reinforced Concrete Beams without Stirrups”, ACI Structural Journal, V. 99, No. 4, 2002, pp. 530-538.

[2] S. Li, V., R. Ward, and A.M. Hamza, “Steel and Synthetic Fibers as Shear Reinforcement”, ACI Materials Journal, V. 89, No. 5, 1992, pp. 499-508.

[3] Parra-Montesinos, G.J., “Shear Strength of Beams with Deformed Steel Fibers”, ACI Concrete International, V. 28, No. 11, 2006, pp. 57-67.

[4] Altoubat, S. A., Yazdanbakhsh, A., Rieder, K.-A., “Shear Behavior of Macro-Synthetic Fiber Reinforced Concrete Beams without Stirrups”, ACI Materials Journal, Vol. 106, No.4, 2009, pp. 381-389

[5] Swamy, R.N. and H.M. Bahia, “Effectiveness of Steel Fibers as Shear Reinforcement”, ACI Concrete International, V. 7 N. 3, 1985, pp. 35-40.

[6] Criage, R. J. (1984). "Structural Applications of Reinforced Fiber Concrete." Concrete International Design and Construction, 6(12), 28-32.

[7] Sarhat, S. R., and Abdul-Ahad, R. B. (2006). "The Combined Use of Steel Fibers and Stirrups as Shear Reinforcement in Reinforced Concrete Beams." ACI Special Publication, SP-235-18, 269-282.

[8] Rosenbusch, J. (2002). "Subtask 4.2, Brite Euram Project, Contract Nr. BRPR-CT98-0813." Final Report.

[9] Watanabe, Ken; Toshihide Kimura; Junichiro Niwa, "Synergetic effect of steel fibers and shear-reinforcing bars on the shear-resistance mechanisms of RC linear members." Construction and Building Materials Journal, Volume 24, 2010, pp. 2369-2375.

[10] Majdzadeh, F., M. Soleimani, and N. Banthia, “Shear Strength of Reinforced Concrete Beams with a Fiber Concrete Matrix”, Canadian Journal of Civil Engineering, V. 33, 2006, pp. 726-734.

[11] Yazdanbakhsh, A., “Shear Behavior of Synthetic Fiber Reinforced Concrete Beams”, Master Thesis in the Department of Civil and Environmental Engineering, University of Sharjah, Sharjah, UAE, 2008, pp. 125.

[12] ASTM C 1609-07, “Standard Test Method for Flexural Performance of Fiber-Reinforced Concrete (Using Beam With Third-Point Loading),” ASTM International, 2007, West Conshohocken, Pa.

[13] JSCE-SF4, ”Methods of Tests for Flexural Strength and Flexural Toughness of Steel Fiber Reinforced Concrete”, Japan Society of Civil Engineers, Concrete Library International, No. 3, Part III-2, 1984, pp. 58-61.

[14] Fenwick, R.C. and T. Paulay, “Mechanisms of Shear Resistance of Concrete Beams”, Journal of Structural Division, ASCE, Proceedings, V. 94, ST10, 1968, pp. 2325-2350

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