LARGE SCALE TENSILE TESTS OF HIGH PERFORMANCE FIBER REINFORCED CEMENT COMPOSITES

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1 LARGE SCALE TENSILE TESTS OF HIGH PERFORMANCE FIBER REINFORCED CEMENT COMPOSITES Shih-Ho Chao (1), Wen-Cheng Liao (2), Thanasak Wongtanakitcharoen (3), and Antoine E. Naaman (4) (1) Post-Doctoral Research Fellow, University of Michigan, Ann Arbor, USA (2) Ph.D Student, University of Michigan, Ann Arbor, USA (3) Department of Highways, Thailand (4) Professor, University of Michigan, Ann Arbor, USA Abstract The macro-scale properties of high performance fiber reinforced cement composites (HPFRCC) depend on their stress-strain characterization under tension; hence direct tensile tests are essential for determining the fundamental tensile behavior of HPFRC composites. Most tensile tests for obtaining the tensile response of FRC composites are carried out on relatively small-size specimens which do not account for more realistic fiber distribution and content variability in full scale structural applications. Moreover, they do not incorporate the tension stiffening effect due to the presence of continuous reinforcement in real structural concrete elements. In this study, long prismatic specimens of dimensions mm reinforced with one unstressed prestressing steel tendon along their longitudinal axis were tested in tension under monotonic load. The tensile load was applied to the prestressing tendon and strains in the tendon (inside and outside the matrix) as well as in the FRC material along the specimen were recorded. The advantage of using a prestressing steel tendon is that a strain as high as 0.9% can be applied while the tendon remains linear elastic, thus allowing a stable environment for loading-unloading and for measurements of crack width and spacing. In this study, the stress-strain curves of HPFRC composites (here a self-consolidating concrete mixture is used) obtained from long-prism tests were compared to curves obtained from small scale direct tensile tests of dog-bone shaped specimens without continuous reinforcement. It was observed that the onset of damage localization following peak stress is significantly delayed in the presence of continuous reinforcement. The strain capacity of an HPFRC composite was also considerably enhanced due to the presence of the reinforcing steel strand. 1. INTRODUCTION HPFRCCs are characterized by their strain-hardening response under tension accompanied by multiple cracking; generally, a direct tensile test is the best way to determine the fundamental tensile behavior of HPFRC composites, which is essential in design and Page 0

2 modelling. Currently, however, most tensile tests for obtaining the tensile response of FRC composites are carried out on relatively small-size specimens that do not account for more realistic variability in fiber distribution in full scale structural elements, and possible scale effects. Moreover, they do not incorporate the tension stiffening effect due to the presence of continuous reinforcement in a real structural concrete element. In this study, large scale HPFRCC tensile prisms reinforced with an unstressed prestressing steel strand along their longitudinal axis were tested in tension under monotonic load and their response was compared to that of typical tensile dog-bone shaped specimens. 2. EXPERIMENTAL PROGRAM 2.1 Materials The HPFRCC used in this study is one of a series of self-consolidating high performance fiber reinforced concrete (SCHPFRC) developed at the University of Michigan (Liao et al., 2006) for seismic applications, as part of a NSF-NEES project. This mixture has a maximum coarse aggregate size of 12.7 mm and 1.5% volume fraction of high strength steel hooked fibers. Average 28-day compressive strength based on mm cylinders is approximately 50 MPa. Details of matrix composition and fiber properties are given in Tables 1 and 2, respectively. Figure 1 shows the steel fiber type used in this study. Note that in order to obtain a self-consolidating mixture, a strict mixing procedure (involving mixing steps and mixing time) must be followed. The reinforcement used is an unstressed prestressing steel tendon (seven-wire strand), having a nominal diameter of 12.7 mm and ultimate tensile strength of 1860 MPa. Table 1: Relative composition of concrete mixture by weight and compressive strength Cement Fly Coarse Sand** (Type III) Ash* Aggregate Super- Water VMA Steel f c plasticizer Fiber (MPa) *Type C ; **Flint Sand ASTM 50-70; Maximum Size of 12.7 mm ; Viscosity Modifying Agent Table 2: Properties of fiber used in this study Fiber Diameter, Length, Density, Tensile Strength, Elastic Modulus, Type (mm) (mm) (g/cc) (MPa) (GPa) Hooked Figure 1: Steel hooked fiber used in this study Page 1

3 2.2 Specimen Geometry and Test Setup for Large Scale Tensile Test Details of the specimen geometry, test setup, and instrumentation are shown in Figure 2. The long prismatic specimen has a cross-sectional dimension of mm. It has been established that the presence of continuous reinforcement helps concrete to carry tension between cracks through transfer of bond forces. This in turn results in better control on member stiffness, deformation, and crack widths in RC members as compared to plain concrete members (e.g. Fields and Bischoff, 2004). In this study, an unstressed prestressing steel strand was placed at the centroid of the specimen to simulate the presence of reinforcing steel in HPFRCCs. The advantage of using a prestressing steel tendon is that a strain as high as 0.9% can be applied while the tendon remains linear elastic (Naaman, 2004, Chapter 12), thus allowing a stable environment for loading-unloading and for measurements of crack width and spacing at every loading step. Moreover, tests conducted by Chao et al. (2006) have shown that HPFRC composites lead to a much higher bond strength between a seven-wire strand and surrounding matrix than plain concrete (as high as three times), thus ensuring the tension stiffening effect in the test specimens. The specimen was placed in a prestressing bed and supported by a few steel strips which allowed the specimen to move easily on its bed support during tension. The strand going through the specimen was attached at each end by a prestressing chuck. The tensile load was applied monotonically to the strand through a hydraulic jack and recorded by a pair of load cells at both ends of the prestressing bed (Figure 2a). Five zones in the middle of the specimen were selected to record the strains in the concrete through the use of linear variable differential transformers (LVDTs), as shown in Figure 2b (Zones 1 thru 5). The gauge length of each zone was 250 mm. Elongation of the entire specimen was also monitored by two LVDTs attached to the ends of the specimen. Strains in the strand (inside the matrix) were measured by strain gauges attached at pre-designated locations corresponding to the middle points of the five zones (Zones 1 thru 5) mentioned above. Strain gauges were also mounted on the strand outside the matrix (Figure 2b) in order to obtain the stress-strain curve of the bare strand. The experimental procedure of the large scale tensile test is shown in Figure Specimen Geometry and Test Setup for Small Scale Tensile Test For comparison purposes, a small scale tensile test was also carried out by using dog-bone shaped specimens as illustrated in Figure 4. These specimens have a cross-sectional dimension of mm, therefore leading to a somewhat two-dimensional distribution of fibers (Note the fiber length is 30 mm). Same matrix (Table 1) was used for the small tensile specimens but without continuous reinforcement. The applied load was monitored by the load cell of the testing machine and elongation was recorded by a pair of LVDTs attached to the specimen, with a gauge length about 175 mm. It is noted that this type of direct tensile test has been extensively used previously to obtain tensile stress-strain responses in FRCCs with great success (e.g. Sujivorakul and Naaman, 2003; Chandrangsu and Naaman, 2003). Page 2

4 (a) 64 mm (b) Figure 2: (a) Geometry of large scale tensile specimen and test setup; (b) Instrumentation (a) (b) Figure 3 : Photos illustrating the experimental procedure: (a) Prestressing bed and application of tensile load; (b) Typical zones for measurements Page 3

5 (c) Figure 3 (continued): (c) Tracing of cracks and measrument of crack width Tension LVDT Tension Figure 4: Geometry of small scale specimen and test setup layout 3. EXPERIMENTAL RESULTS 3.1 Calculation of Stress in HPFRCC The stress in the fiber concrete for the large scale tensile test was calculated by: σ = ( F E ε )/ ( A A ) c s s t s (1) Page 4

6 where σ c is the tensile stress in fiber concrete (MPa); F is the total force measured by load cell (kn); Es is the elastic modulus of strand (MPa) ; ε s is the strain in strand measured by strain gauge (mm/mm); A t is the gross cross-sectional area of the specimen (= 4860 mm 2 ) ; As is the nominal cross-sectional area of a 12.7 mm seven-wire strand (= 100 mm 2 ). It was mentioned previously that a prestressing steel strand remains linear elastic when the strain reaches as high as 0.9%. This was indeed the case in the strand used in this study, as indicated by the stress-strain relation obtained based on strain gauges mounted outside the matrix; the curve was linear with an elastic modulus of 206 GPa. Since the specimen was able to move freely during testing with minor frictional force, the force measured by the load cell ( F ) can be taken as constant along the specimen and used for Zones 1 thru 5. The force sustained by the fiber concrete was calculated by the difference between F and force in the strand, Esε s. The average tensile stress was then obtained by dividing the force difference using the net concrete area, At As. The tensile strains in the fiber concrete were obtained by dividing the elongation (measured through LVDTs) by the gauge length of each zone (= 250 mm). 3.2 Stress-Strain Response Obtained from Large Scale Specimen Typical tensile load-elongation responses of the composite and bare strand in Zone 3 are shown in Figure 5a. Stress-strain response of the HPFRCC material were obtained based on Equation 1 and plotted in Figure 5b, along with an envelope curve. The unloading loops were the result of softening of the hydraulic jack during crack measurement and photographing. Figure 5 shows that the HPFRCC used in this study exhibited tensile strain-hardening behavior up to 0.7% composite strain, along with extensive multiple cracking as shown in Figure 8b. The stress-strain curve is generally very stable without any sudden degradation in strength. This can be attributed to the presence of the longitudinal reinforcement, which was able to redistribute tensile stress through bond when cracks occurred. Figure 6 shows the relation beween average concrete tensile strain versus average crack number (within the range of 250 mm) and average crack width based on the measurements from Zones 1 thru 5. It was observed that generally there were one or two primary cracks in each zone, which have wider width than the remainder cracks. The width of a primary crack was measured by an 8X magnifier with a minimum reticle scale of 0.05 mm. Although the width of primary cracks increased with the concrete strains, no localization occurred. Additional secondary cracks can still develop and their average crack width was less than 50 μ m, even when maximum loading condition (matrix tensile strain of 0.7%) was reached. The residual width of the primary cracks after unloading was generally from 0.05 mm to 0.2 mm. It is noted that the yield strain and strain at onset of strain-hardening of a typical Grade 420 M deformed reinforcing bar are approximately 0.2% and 0.6%, respectively. This signifies that the HPFRCC used in this study can still sustain load with no degradation even when a deformed reinforcing bar has yielded and strain-hardened (Naaman and Reinhardt, 2006). This is essential for RC elements, especially when subjected to large inelastic deformation, to prevent earlier degradation due to concrete softening. Page 5

7 (a) Figure 5: Typical tensile load-elongation and stress-strain responses of HPFRCC obtained from large scale tensile test (b) (a) Figure 6: Average crack number and width versus average concrete tensile strain in the large scale tensile specimen (b) 3.3 Comparison between Results from Large and Small Scale Specimens Figure 7 compares the tensile stress-strain responses of small and large scale specimens using the same HPFRC composite. Two observations can be made: 1) The tensile strength (or peak stress) of the small scale specimen is higher than that of the large scale specimen. This can be attributed to scale effects and to the possible two-dimensional versus three-dimensional fiber orientations in each specimen, respectively. Everything else being equal, the tensile capacity of a fiber reinforced cement composite is affected by the fiber orientation. This can be accounted for by a Page 6

8 bridging efficiency factor, which defines the amount of fibers bridging across a crack with respect to fiber orientation effect. Generally, the 3-D random distribution leads to the lowest bridging efficiency due to loss of fiber bridging when oriented at high angles with respect to the tensile stress direction. Krenchel (1964) derived analytically efficiency ratios for 1-D: 2-D: 3-D fiber distribution leading to numerical values of 1, 0.636, and 0.5, respectively. This translates into a composite tensile capacity ratio of 2-D/3-D = 1.27, which generally agrees with the observation shown in Figure 7. 2) The onset of damage localization as a result of fiber pullout at peak stress is significantly delayed in the presence of continuous reinforcement. Indeed, the tensile strain up to the peak strength in the large scale specimen is more than two times that of the small scale specimen. The smaller strain in the small specimen possibly resulted from the fact that smaller specimens are more sensitive to defects such as non-uniformly distributed fibers and coarse aggregates. In addition, without continuous reinforcement, crack extension is more likely to become unstable during stressing. It is seen from this study that a small scale tensile specimen with no continuous reinforcement can slightly overestimate the tensile strength and overly underestimate the strain capacity in a conventionally reinforced full scale structural element. Figure 7: Comparison of stress-strain responses between small and large scale specimens using SCHPFRC mixture (Liao et al., 2006) Page 7

9 3.4 Crack Distribution Figure 8 shows the crack distributions in the small and large scale specimens. As can be seen, the specimen with no continuous reinforcement developed a smaller number of cracks before damage localization started; the gauge length was 175 mm. On the other hand, the large scale specimen with continuous reinforcement developed extensive multiple cracks and no significant damage localization was observed up to about 0.8% strain which was the limit of the test set-up. The average visible crack spacing at crack saturation was approximately 10 mm. 50 mm (a) Small scale tensile specimen (dog-bone shaped; gauge length = 175 mm) 76 mm (b) Large scale tensile specimen Figure 8: Crack distributions in smaller and large scale tensile specimens at the end of tests 4. CONCLUSIONS 1. Direct tensile stress-strain curves of strain-hardening FRC composites are needed for material characterization in structural modelling and applications. However, such curves are very sensitive to scale effects and may be significantly influenced by the presence of conventional reinforcement (such as reinforcing bars or prestressing strands). 2. The use of a large scale tensile specimen allowing three-dimensional fiber distribution and incorporating the presence of continuous reinforcement lead to a stress-strain response more realistic for use in structural elements than a pure tensile test. 3. The procedure described here whereas a long tensile prism is strained through a single concentric prestressing strand is recommended as a reliable method of testing. The Page 8

10 advantage of using a prestressing steel tendon as continuous reinforcement is that a strain as high as 0.9% can be applied incrementally while the tendon remains linear elastic, thus allowing a stable environment for loading-unloading and for measurements of crack width and spacing. For higher strain capacity, a carbon bar or a glass bar could also be used, although their bond properties may be different from those of steel bars or strands. 4. The peak tensile strength obtained from small scale specimens was generally higher than that of larger scale specimen. 5. The tensile strain at the onset of damage localization in the large scale specimens where reinforcing strand was used was about twice that observed in the small scale specimens where no reinforcement was used. This leads to a significantly better crack development as well as a significant increase in energy absorption capacity. ACKOWLEDGEMENTS The research described herein was sponsored by the National Science Foundation under Grant No. CMS and by the University of Michigan. Their support is gratefully acknowledged. The opinions expressed in this paper are those of the authors and do not necessarily reflect the views of the sponsor. REFERENCES [1] Liao, W.-C., Chao, S.-H., Park, S.-Y. and Naaman, A. E., 'Self-Consolidating High Performance Fiber Reinforced Concrete (SCHPFRC) Preliminary Investigation', Report No. UMCEE 06-02, Department of Civil and Environmental Engineering, University of Michigan, Ann Arbor, MI, [2] Fields, K. and Bischoff, P. H., 'Tension Stiffening and Cracking of High-Strength Reinforced Concrete Tension Members', ACI Structural Journal, 101 (4), July-August, (2004) [3] Naaman, A. E., 'Prestressed Concrete Analysis and Design Fundamentals', 2nd Edn (Techno Press 3000, 2004), Ann Arbor, Michigan, 1072 pp. [4] Chao, S.-H., Naaman, A. E. and Parra-Montesinos, G. J., 'Bond Behavior of Strands Embedded in Fiber Reinforced Cementitious Composites', PCI Journal, 51 (6), November-December. (2006), Precast/Prestressed Concrete Institute, [5] Sujivorakul, C., and Naaman, A.E., 'Tensile Response of HPFRC Composites Using Twisted Polygonal Steel Fibers', in Innovations in Fiber-Reinforced Concrete for Value, N. Banthia, M. Criswell, P. Tatnall, and K. Folliard, Editors, ACI Special Publication, SP216, American Concrete Institute, 2003, pp [6] Chandrangsu, K., and Naaman, A.E., 'Comparison of Tensile and Bending Response of Three High Performance Fiber Reinforced Cement Composites', in High Performance Fiber Reinforced Cement Composites (HPFRCC-4), A.E. Naaman and H.W. Reinhardt, Editors, RILEM Publications, Pro. 30, June 2003, pp [7] Naaman, A.E., and Reinhardt, H.W., 'Proposed Classification of FRC Composites Based on their Tensile Response', Materials and Structures, 39, page , Also, Proceeding of symposium honoring S. Mindess, N. Banthia, Editor, University of British Columbia, Canada, August Electronic proceedings, 13 pages. [8] Krenchel, H., 'Fiber Reinforcement', Akademisk Forlag, Copenhagen, Denmark, Engl. Translation (1964). Page 9