FIBER REINFORCED ROUND PANELS SUBJECTED TO IMPACT LOADING

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1 FIBER REINFORCED ROUND PANELS SUBJECTED TO IMPACT LOADING Sidney Mindess, Nemkumar Banthia and Hanfeng Xu University of British Columbia, Vancouver, B.C., Canada ABSTRACT A modified version of ASTM C 155 was used to study the impact behavior of fibre reinforced concrete. Specimens, in the form of circular plates, with a diameter of 65 mm and a thickness of 6 mm were prepared using plain high strength concrete (HSC) with a compressive strength of about 8 MPa, steel fiber reinforced HSC, and synthetic fiber reinforced HSC. Some panels were also reinforced with welded wire mesh. The panels were supported on three symmetrically located points, and were subjected to impact loading at their centers using a large, instrumented drop weigh impact machine. It was found that the failure modes under impact loading were similar to those under static loading, though the peak loads were higher under impact. It was found that the steel fibers led to a higher increase in toughness than the synthetic fibers. For the panels that contained the wire mesh, the fracture behavior was strongly dependent on the exact position of the mesh (i.e., whether the impacting hammer was located over a mesh opening, or over the intersection of two wires). INTRODUCTION The role of fibers in concrete is to bridge across matrix cracks as they develop when the concrete is stressed, and so to provide some post-cracking ductility to the material. If the fibers develop sufficient bond with the matrix, the crack widths will remain small, and the fiber reinforced concrete (FRC) will be able to withstand significant stresses in the strain-softening stage. In most applications, with relatively low rates of fiber addition (less than 1.% by volume), the fibers are not there to provide additional strength, though modest increases in strength may occur in some cases; their primary purpose is to impart toughness to the material. However, while the concept of toughness, or energy required to fracture a specimen, is relatively easy to understand (it is often discussed in terms of some function of the area under the load vs. deflection curve for FRC), toughness has proved to be an elusive property to define or measure in an unambiguous way. Over the past 2 years, many different test methods have been proposed to characterize the toughness of FRC, and a number of these have been adopted as standards by various organizations. They include: ASTM C118, Standard Test Method for Flexural Toughness and First-Crack Strength of Fiber-Reinforced Concrete (Using Beam with Third-Point Loading). This

2 standard was first published in 1984, but has been substantially revised since then, with a greater emphasis on determining residual strengths. ASTM C1399, Test Method for Obtaining Average Residual-Strength of Fiber- Reinforced Concrete, first published in ASTM C155, Standard Test Method for Flexural Toughness of Fiber reinforced Concrete (Using Centrally Loaded Round Panel), first published in 22. EFNARC, European Specification for Sprayed Concrete, contains a description of a plate test using a rectangular plate. Japan Society of Civil Engineers SF-4, Method of Test for Flexural Strength and Flexural Toughness of Fiber Reinforced Concrete, published in Japan Society of Civil Engineers SF-5, Method of Test for Compressive Strength and Compressive Toughness of SFRC, published in Japan Society of Civil Engineers SF-6, Method of Test for Shear Strength and Shear Toughness of SFRC, published in It is not the intent of this paper to provide a comparison of these methods; suffice it to say that it is not possible to correlate the values obtained by these different procedures (1). They measure different properties in different ways, and may often lead to quite different comparative rankings of the relative effectiveness of a given suite of fibers. All of these methods (and other methods not listed here) were developed to evaluate FRC for static loading conditions. Most cannot be modified to deal with impact loading, which is the subject of the present investigation. However, of the available tests, it appears that ASTM C155 is the most suitable procedure for evaluating the toughness of FRC under impact loading, and so a somewhat modified form of this test was adopted for the present study. ASTM C155 The centrally loaded round panel test (2) was first proposed by Bernard (3-6). The test specimen is a circular plate, with a diameter of 8 mm and a thickness of 75 mm. The specimen is supported on three symmetrically placed pivoted supports located on a 75 mm diameter circle, and is point loaded at the center. This is shown schematically in Fig. 1. The resulting load vs. center-point deflection curve may then be used to determine the energy absorbed by the specimen out to any specified deflection. (One disadvantage of this test method is that the specimen itself weighs about 85-9 kg, which makes it rather awkward to handle.) Although there is now considerable experience with this method under static loading, there appears to be no current information on the use of this specimen configuration for impact loading; this is the subject of the present research. In the work reported here, due to limitations of the size of the impact machine, a somewhat scaled down version of the standard specimen was used, though maintaining approximately the same ratio of dimensions as above. The specimen size adopted had a diameter of 635 mm and a thickness of 6 mm; these specimens weighed about 45 kg. The three supports were located on a 596 mm diameter circle. Specimens of fiber reinforced concrete, some also reinforced with welded wire steel mesh, were tested both statically and under impact loading.

3 Fig. 1. Schematic view of the centrally loaded round panel test (ASTM C 155). EXPERIMENTAL PROCEDURES For the tests reported here, six different types of specimens round panel specimens were cast, accompanied by standard cylinders for compressive strength determinations. The basic concrete mix proportions were: CSA Type 1 (ASTM Type I) Portland cement: 453 kg/m 3 silica fume: 44 kg/m 3 1 mm max. size coarse aggregate: 153 kg/m 3 fine aggregate: 796 kg/m 3 water: 139 kg/m 3 superplasticizer: 2.72 l/m 3 air entraining agent:.38 l/m 3 water/(cement + silica fume) ratio: day compressive strength: 76.7 MPa All of the panel specimens were reinforced with a single layer of welded wire mesh located at mid-thickness of the specimens, and were cast in molds as shown in Fig. 2. The wires had a diameter of 4.8 mm, with a square spacing (center-to-center) of 12 mm. For the two sets of specimens without fibers, two different mesh configurations were used. The first configuration (Type E) was such that the hemispherical striking tup contacted the specimen in the center of one of the 1 x 1 mm squares of the mesh. The second (Type C) was such that the tup contacted the specimen at one of the wire mesh intersections. The four sets of specimens containing fibers all used the latter configuration. Both static and impact tests were carried out on all six types of specimens. Two types of fibers were used, each at.5% and 1.% by volume of the concrete. The hooked end steel fibers had a length of 5 mm, and a diameter of.72 mm. The synthetic fibers consisted of a blend of polypropylene and polyethylene; they too were 5 mm long, but with an irregular cross-sectional shape.

4 Fig. 2. Photograph of mold for round panels, showing the welded wire mesh reinforcement. The instrumented drop weight impact machine used here has been described in detail elsewhere (6). It is capable of dropping a 575 kg mass onto the target specimen from heights of up to 2.5 meters. The test setup for impact loading is shown in more detail in Fig. 3. RESULTS AND DISCUSSION Static Tests The average load vs. center-point deflection curves for the various specimens loaded statically are shown in Fig. 4. (Each curve represents the average of three specimens). The average energy absorption values for these specimens are given in Table 1. Note that according to ASTM C155, the energy absorption of the specimen is calculated at a center-point deflection of 4 mm. However, because of the smaller size of the specimens tested here, the energy absorption was determined at a center-point deflection of only 3 mm. From these data, it may be seen that, without fibers, the exact position of the welded wire mesh with respect to the striking tup did not seem to make much difference, though Mesh E did perform better at higher deflections. Clearly, the addition of fibers improved the

5 Drop weight hammer Hammer guide column Piston with bolt load cell Panel specimen Panel supports Accelerometers Laser transducer a) b) Fig. 3. Impact testing of round panel specimens 35 Applied Load (kn) Mesh C+PPN1. Mesh C+ SF1. Mesh C+PPN.5 Mesh C+SF.5 Mesh C Mesh E Fig. 4. Average load vs. center-point deflection curves for centrally loaded round panels under static loading energy absorption of the specimens. However, under static loading, there was no clear differentiation between the two fiber types. Table 1. Energy absorption of centrally loaded round panels at a center-point deflection of 3 mm.

6 Mesh Reinforcement Pattern Impact tests Fibre Reinforcement Energy Absorption at 3 mm (Joules) Type E Type C Type C.5% steel 574 Type C 1.% steel 715 Type C.5% synthetic 61 Type C 1.% synthetic 691 Drop weight impact tests on companion specimens to those described above were carried out using a drop height of 12 mm, giving an impact velocity of about 1.5 m/s. Fig. 5 shows a comparison of the impact load vs. deflection curves for the two mesh arrangements, without any fibers. It may be seen that Mesh E (point of impact between the wires) gave both a higher peak load and greater energy absorption than Mesh C (point of impact at the intersection of two wires). This may be because with Mesh C, the crack always ran preferentially along one of the wires, leading to debonding and fracture of some of the perpendicular wires, as shown in Fig. 6. This did not occur with the Mesh E specimens Drop height 12 mm 4 35 Load (kn) Mesh C Mesh E Fig. 5. Effect of mesh orientation on behavior under impact loading.

7 Fig. 6. Failure of a round panel with Mesh C reinforcement under impact (without fibers) Fig. 7 shows the effect of fiber reinforcement on the impact behavior of the round panels reinforced with Mesh C. As expected, the fibers make the system considerably tougher, and the test clearly distinguishes the different behaviors of the two different fiber types and volumes For instance, the bridging action of the steel fibers may be seen in Fig. 8, which again also shows a crack running along one of the wires. The ability of the round panel test to distinguish between fiber types under impact loading is shown in Fig. 9, for a drop height of 2 mm. Clearly, the two fiber types behave quite differently. Up to a deflection of about 2 mm, the steel fibers provided a higher residual load carrying capacity, and consequently greater energy absorption. Beyond about 2 mm, however, the synthetic fibers became more effective, displaying a greater load carrying capacity for large deflections. The differences between the two fibers may be more clearly seen in Fig. 1, which shows the energy absorption (or toughness) of panels reinforced with the two types of fibers from the same 2 mm drop height. At a deflection of about 45 mm, the synthetic fiber specimens overtake the steel fiber specimens.

8 45 4 Drop height 12 mm Load (kn) Mesh C + SF 1.% Mesh C + PPN.5 Mesh C Fig. 7. Effect of fiber additions on impact behavior of wire Mesh C reinforced round panels Fig. 8. Failure of a round panel with Mesh C reinforcement and.5% steel fibers under impact loading From a drop height of 12 mm, while the other specimens were severely damaged, the continuous welded wire mesh with 1.% steel fibers did hold the round panels together and led to relatively small deflections. Fig. 11 shows the residual static load carrying capacity of these damaged panels. These three panels were all nominally the same, containing 1.% steel fibers, and reinforced with Mesh C. It may be seen that the three specimens clearly suffered quite different degrees of damage during the impact test, not in terms of their peak loads, but in terms of the quite different areas (i.e., energy absorption capacities) under the load deflection

9 curves. This suggests that considerable care must be taken in interpreting impact data from the round panel test. 7 6 Impact Test (Drop height 2 mm) 5 Load (kn) Mesh C + PPN.5 Mesh C + SF Fig. 9. Effect of fiber type on the impact behavior of round panels containing Type C Mesh Toughness (kj) Mesh C+.5SF Mesh C+PPN.5 Impact test (drop height 2 mm) Fig. 1. Energy absorption (toughness) of round panels under impact loading

10 25 Static test after impact (12 mm impact) 2 Load (kn) Fig. 11. Residual static load vs. deflection curves of round panels reinforced with Mesh C and containing 1.% steel fibers. CONCLUSIONS 1. The combination of fibers and welded wire mesh clearly improves the load carrying capacity of round panel specimens under both static and impact loading. 2. The precise orientation of the wire mesh with respect to the point of impact appears to have relatively little effect on the results. 3. The round panel test appeared to discriminate amongst different fiber types and volumes better under impact loading than under static loading. 4. The different degrees of damage undergone by nominally identical specimens under impact loading suggests that the results of such tests must be analyzed with considerable care. ACKNOWLEDGMENTS The authors would like to thank undergraduate Civil Engineering students Andrew Chand, His- Yung Chen, Ravinder Grewal, Palwinder Litt and Sudip Talukdar, who carried out much of the experimental work as part of the course requirements for Civil 321 Laboratory Project in Civil Engineering Materials. REFERENCES 1. Banthia, N. and Mindess, S., Toughness Characterization of Fiber-Reinforced Concrete: Which Standard to Use? Journal of Testing and Evaluation, American Society for Testing and Materials, West Conshohocken, PA. 32 (2) (in press, March 24). 2. ASTM C 155, Standard Test Method for Flexural Toughness of Fiber-Reinforced concrete (Using Centrally-Loaded Round Panel), American Society for Testing and Materials, West Conshohocken, PA, Bernard, E. S., Behaviour of Round Steel Fiber Reinforced Concrete Panels under Point Loads, Materials and Structures, RILEM, 33 (2) Bernard, E. S., Correlations in the Behavior of Fiber Reinforced Shotcrete Beam and Panel Specimens, Materials and Structures, RILEM, 35 (22) Bernard, S., Release of New ASTM Round Panel Test, Shotcrete, 5 (2) (23) Bernard, E. S., Correlations in the Behaviour of Fibre Reinforced Shotcrete Beam and Panel Specimens, Materials and Structures, RILEM, 35 (22) N. Banthia, S. Mindess, A, Bentur and M. Pigeon, Impact Testing of Concrete Using a Drop Weight Impact Machine. Experimental Mechanics, 29 (2) (1989)