Performance of Fiber Reinforced Concrete Beams with and without Stirrups

Transcription

1 Volume 4, Issue 1, 2012 Performance of Fiber Reinforced Concrete Beams with and without Stirrups Travis Hockenberry, Structural Engineer, WorleyParsons Group Maria M. Lopez, Associate Professor, Dept. of Civil and Env. Engineering, Penn State University, Abstract The use of Fiber Reinforced Concrete (FRC) in non-structural applications such as slabs on grade and pavements has become widely accepted by the design and construction industries. Structural use of FRC, however, has not received a similar acceptance rate in spite of the large research effort conducted in the past 50 years. This study evaluates the performance of fiber reinforced concrete beams with and without steel stirrups. In particular, the interaction between conventional stirrups and hooked steel fiber reinforcement as well as the size effect of the specimens tested is investigated. Experimental results are compared to the new ACI provisions for FRC use. Keywords: concrete, shear, size effect, steel fiber, stirrups Introduction The addition of short fibers to concrete or cement-based matrices enhances the structural performance of these composite materials. Enhanced properties include shear strength, tensile strength, compressive strength, toughness, ductility, and durability among others [1]. Fiber reinforcement has been shown to be effective as shear reinforcement when used to supplement or replace conventional steel stirrups in reinforced concrete (RC) beams [2]. The use of Fiber Reinforced Concrete (FRC) could allow for greater spacing between conventional stirrups thus alleviating reinforcement congestion; it could also eliminate the need for conventional stirrups in areas where minimum reinforcement is needed. The American Concrete Institute, in its 2008 Building Code Requirements for Structural Concrete [3], accepts hooked steel fibers as a reinforcing material for structural concrete. Even though its use is limited to shear reinforcement in beams subjected to small service loads, its inclusion in a building code requirement document opens the door for the acceptance of FRC as a structural material. This paper presents results of an experimental study aimed at evaluating key factors that affect the shear strength of hooked steel FRC beams with and without stirrups. In particular, the interaction between conventional stirrups and hooked steel fiber reinforcement as well as the size effect of the specimen tested are investigated. The results from the experimental study are then compared with the new ACI design provisions regarding FRC use. Experimental Study A total of 27 concrete specimens were fabricated and tested at the Civil Infrastructure Testing and Evaluation laboratory at Penn State University. These specimens had variations in geometry, fiber volume fraction, fiber geometry, and stirrup spacing. Table 1 shows the matrix of the specimens details. Four different sizes of specimens were investigated; specimen heights were (A) mm, (B) mm, (C) mm, and (D) mm respectively. All beam specimens had a constant width equal to 254 mm. The shear span-to-depth ratio was kept constant (2.5) for all beams. These beams are considered as short beams, where the shear capacity controls their behavior [4]. Conventional reinforcement consisted of longitudinal steel rebar (metric notation shown in Table 1) and double leg steel stirrups. The longitudinal reinforcement ratio, ρ, was selected as 1.0% for all specimen sizes. Stirrups, when used, were spaced at d/2, 2/3d and d (effective depth). Hooked steel fibers with two different aspect ratios (L/D) were used (80, 65) with a tensile strength of 1100 MPa (data provided by manufacturer). Two different fiber volume fractions (V f) were also evaluated (0.5% and 1%). Details of specimen fabrication can be found in [5]. Specimen Identification The specimen identification scheme consists of a 5-part code, α-β-γ-δ-ε, where α = specimen size A, B, C or D β = NF for no fiber; F for fiber γ = 0.5 for 0.5% fiber volume fraction; 1.0 for 1%V f δ = 65 for fiber aspect ratio (L/D=65); 80 for L/D=80 1

2 ε = NS for no stirrups; S for stirrups Three specimens were tested for each set of conditions investigated as listed below. Table 1. Matrix of tested specimens Size Specimen Identification d (mm) Flexural Reinforcement Area 1 Transverse Spacing Fiber volume fraction V f (%) Fiber L/D A A-F NS 457 B B-NF-NS 229 B B-NF-S 229 B B-F S 229 B B-F S 229 B B-F S 229 B B-F S 229 C C-F NS 152 D D-F NS 76 1 Note: METRIC bar sizes are listed (M) 3#22M bars (1161 mm 2 ) 3#13M bars (387 mm 2 ) 3#10M bars (213 mm 2 ) No No - d/2 = /3d = d = /3d = d = mm No No Test Setup Beam specimens were tested by applying a single concentrated load from a hydraulic actuator at midspan under displacement control (three-point bending configuration, as shown in Figure 1) at a rate of 0.3 mm/min. A self-reacting frame was used to contain the reaction beam. Load-displacement data from the actuator was recorded, along with additional midspan specimen deformation data from two LVDTs. Strain gages were placed in the longitudinal rebar as well as stirrups, strain data was also continuously monitored during testing. Load, displacement and deformation were recorded at a rate of 5 Hz. Load, P Hinge Support Beam Specimen Roller Support LVDT Reaction Beam Figure 1. Three-point bending test set-up In addition to the flexure tests, compressive and split tensile tests were conducted following ASTM guidelines. The concrete mixture with no fibers (control specimens) had an average compressive strength of 29 MPa and a tensile strength of 2.58 MPa. Compressive strengths for the fiber reinforced 2

3 concrete mixtures fluctuated slightly depending on the fiber content, but in all cases exceeded the control specimens with a maximum increase of 8%. The split tensile strength of these mixtures, in contrast, was shown to increase dramatically with the addition of fiber. Increases of between 40% and 95% were found. Table 2 presents the material properties of all concrete mixtures at the time the beam specimens were tested (average of 3 specimens is reported). The batch identification scheme follows the first 4 letters code used for specimen identification (α-β-γ-δ) described previously. Table 2. Material properties of the concrete mixtures investigated Concrete mixture description Compressive strength f c (MPa) Split Tensile strength f t (MPa) B-NF B-F B-F B-F B-F C-F NS D-F NS A-F Failure Modes All beam specimens failed as expected, by propagation of a major shear crack that extended from one of the supports to the point load. Figure 2 shows typical crack patterns observed on all specimens. The dotted line corresponds to the primary failure crack. It can be seen that for the specimens with fibers, there was a greater distribution of cracks throughout the span. In the case of the specimens with 1% fiber volume fraction and stirrups, the major crack progressed from a flexural crack to a flexural-shear crack. During the testing of these specimens, it was noted that specimens with fibers had a more ductile failure mode than control specimens. Overall, it was important to confirm that all beams had shear failures so the true shear contribution of fibers and stirrups could be evaluated. Effect of Fiber Volume Fraction Figure 3 shows representative curves of the load-deflection behavior of all the size B specimens. It can be observed that the peak load and corresponding midspan deflection increases with an increase in the fiber content. It can also be seen that for the same volume fraction as the fiber aspect ratio (L/D) increases, the peak load also increases although its effect appears to be less significant. For the specimens with 1%V f, peak loads are similar for both aspect ratios. 3

4 (a) B-NF-NS specimen (b) B-NF-S specimen (c) B-F S specimen (d) B-F S specimen (e) B-F S specimen (f) B-F S specimen (g) D-F NS specimen (h) C-F NS specimen (i) A-F NS specimen Figure 2. Crack patterns of tested beams 4

5 Figure 3. Load-midspan deflection curves for Size B specimens Combined Effect of Fibers and Stirrup Spacing Because the size B beam specimens had different fiber volume fraction and stirrup spacing, it is important to note that their combined effect is what is reflected in the curves shown in Figure 3. From a design point of view, it is important to note that the shear capacity of the control specimen NF-S (no fibers and stirrups at d/2) was achieved and exceeded by the specimens with combinations of fibers and larger stirrup spacing. Figure 4 shows this combined effect. All size B specimens are represented in this Figure. It is interesting to observe that for specimens with 1%Vf and a stirrup spacing of d this increasing trend slows down indicating the possibility of a practical limit on how much stirrup spacing can be increased even with an increasing fiber volume fraction. There is certainly an interaction between these two parameters. More importantly, these results indicate the possibility that an FRC beam specimen with no stirrups could have the same shear capacity as a control beam specimen (no fibers and with stirrups). Size Effect In order to evaluate the influence of the specimen geometry on the shear capacity of fiber reinforced beams, three series (A, C, D) of specimens with different effective depths were tested. All specimens had a 1% fiber volume fraction, L/D = 80 and no stirrups. All had the same shear span-depth ratio (2.5). Figure 5 shows the influence of the effective depth on the shear strength of the FRC beams. It can be observed that the ultimate shear stress of the specimen decreases as the effective depth increases. The ultimate shear stress v u is calculated as the ratio between the peak load and the effective cross section of the specimen (bxd). The percentage decrease from one size to the next is 41.3% from size D to size C and 11.5% from size C to size A. Although size B specimens (B-F S) have the additional effect of the stirrups (spaced at d), these results are included as an additional reference point. It can be inferred that these B specimens agree well with the trend shown by the other specimen sizes. This indicates that there is a size effect present in FRC beams with and without stirrups. Contrary to some previous studies, the ductile behavior on the structural response of these tested specimens does not seem to diminish the size effect in the shear performance of FRC beams. 5

6 180 Shear Failure Load (kn) Control Series Spacing=114.3 mm (4.5 in), d/2 No Stirrups Spacing=152.4 mm (6 in), 2/3d Spacing=228.6 mm (9 in), d Control L/D=80 L/D= Fiber Volume Fraction (%) Figure 4. Effect of fiber volume fraction and stirrup spacing in the beam shear capacity ACI Shear Design Provisions The American Concrete Institute, in its 2008 Building Code requirements for Structural Concrete [3], accepts discontinuous deformed steel fibers as acceptable reinforcement for shear under specific conditions. Only hooked steel fibers with aspect ratios (L/D) between 50 and 100 are accepted. Fiber dosage is limited to no less than 100 lb/cubic yard; deformation characteristics are defined based on ASTM C1609 test procedure. The 1%Vf fiber reinforced concrete mixture used in this study meets these criteria. ACI shear provisions limit the use of FRC in beams subjected to small shear loads. The new shear design provision is intended for beams with design load values smaller than the expected shear capacity of the concrete cross section. In previous editions of the building code requirements, if this difference is less than 50%, these beams would be required to have some minimum shear reinforcement. In the 2008 building code edition, beams made of fiber reinforced concrete meeting the specifications shown above, are allowed. Results shown in Figure 5 indicated that the shear capacity provided by these FRC beams is larger than the limit set by ACI (shown as a dotted line) by at least 1.5 times. It must be pointed out, however, that larger size of specimens, allowed under ACI provisions, could have lower shear capacities than the ones reported in this study. 6

7 8000 Shear Stress, vu (kpa) A mm C mm B mm D mm 254 mm 254 mm 254 mm 254 mm ACI Shear Capacity Limit for FRC Type D Type C Type A Type B Effective Depth, d (mm) Conclusions Figure 5. Size Effect on FRC Beams with 1% V f This study evaluates key factors that affect the shear strength of hooked steel fiber reinforced concrete beams with and without steel stirrups. The following conclusions are drawn: All beam specimens failed in shear thus proving that the true contribution of fibers and stirrups was assessed by the experimental program. As expected, an increase in the fiber volume fraction ratio was shown to increase the shear capacity of each specimen tested. It was also observed that for the same volume fraction, as the fiber aspect ratio (L/D) increases, the shear capacity also increased, however its effect was less significant. The shear capacity of the control specimens (no fibers and stirrups at d/2) was achieved and exceeded by the specimens with combinations of fibers and larger stirrup spacing. It was inferred that this increasing trend may reach a plateau due to the possibility of a practical limit on how much stirrup spacing can be increased even with an increasing fiber volume fraction. It is very likely that a properly designed FRC beam with no stirrups can have the same shear capacity as a control specimen. The four different sizes of specimen tested showed that there is a size effect present in FRC beams with and without stirrups. All FRC beams tested had a shear capacity at least 1.5 times larger than the limit set by ACI building code provisions. It must be pointed out, however, that larger size of specimens, allowed under these ACI provisions, could have lower shear capacities than the ones reported in this study. References 1. ACI Committee 544. State-of-the-Art Report on Fiber Reinforced Concrete. American Concrete Institute, Detroit, MI (Reapproved 2002). 2. Lim, D. H. and Oh, B. H. Experimental and Theoretical Investigation on the Shear of Steel Fiber Reinforced Concrete Beams, Engineering Structures, Vol. 21, ACI Committee 318. Building Code Requirements for Structural Concrete (ACI ) and Commentary, an ACI Standard. American Concrete Institute, Detroit, MI., January MacGregor, J.G. and Wight, J.K. Reinforced Concrete: Mechanics and Design, New Jersey: Pearson Prentice Hall, Hockenberry, T. Evaluation of Shear Capacity of hooked Steel Fiber Reinforced Concrete Beams with Stirrups, M.S Thesis. The Pennsylvania State University, August