PENETRATION RESISTANCE OF HYBRID FIBRE REINFORCED CONCRETE UNDER LOW VELOCITY IMPACT LOADING

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1 Congrès annuel de la Société canadienne de génie civil Annual Conference of the Canadian Society for Civil Engineering Montréal, Québec, Canada 5-8 juin 2002 / June 5-8, 2002 PENETRATION RESISTANCE OF HYBRID FIBRE REINFORCED CONCRETE UNDER LOW VELOCITY IMPACT LOADING P. Sukontasukkul A, S. Mindess B and N. Banthia B A Department of Civil Engineering, King Mongkut Institute of Technology-North Bangkok, Thailand B Department of Civil Engineering, University of British Columbia, Vancouver, Canada ABSTRACT: Small circular concrete plates of fibre reinforced concrete (FRC) with a diameter of 100 mm and thickness of 20 mm were subjected to low velocity impact, leading to penetration of the plates. Three kinds of concrete were used: plain, single fibre FRC and hybrid fibre FRC. For the FRC, macro fibres (hooked end steel) and micro fibres (carbon and polypropylene) were used. For the single fibre FRCs, 1% by volume (V f ) of each of the three fibre types was investigated. For the hybrid FRC (HyFRC), combinations of steel-polypropylene and steel-carbon at V f = 0.5% V f each were used. It was found that at this proportion, the performance of the hybrid FRC was not as good as that of the V f = 1.0% steel FRC. 1. INTRODUCTION Concrete may sometime be required to withstand dynamic loads due to impact, or explosion. These dynamic loads can vary widely in both impact velocity and intensity. In the case of impact loading leading to penetration, the specimen response is usually dominated by the local response of the small zone at the contact area (Mindess and Yan 1993). However, concrete is a brittle material; in order to improve its performance under impact, fibres are often used. Fibres are available in different shapes, dimensions and materials. In particular, a distinction may be made between macro and micro fibres using this specific surface areas (Banthia 1992); a micro fibre is defined as one with a specific surface area in excess of 200 cm 2 /g. Currently, the use of hybrid fibre system is under investigation. In a hybrid fibre system, two or more kinds of fibres are combined to produce a composite, which utilizes the benefits of each individual fibre. There are two main categories of hybrid FRC s (Banthia N, and Nandakumar 2001). 1) Fibres of different sizes and/or shapes mixed together to achieve better packing and stability; 2) Fibres of about the same dimensions, but with different elastic moduli mixed together to provide better toughness over a wide range of crack openings. In this study, the first type of hybrid system was investigated for penetration resistance. Two types of fibres, macro and micro were mixed together at a combined volume fraction of 1%. The failure pattern, load response, peak load and energy absorption were determined

2 2. TESTING PROGRAM Specimens in the form of cylinders of dimensions 100 x 200 mm (diameter x length) were prepared using normal CSA Type 10 (ASTM Type 1) cement, river run sand, and coarse aggregate with a maximum size of 10 mm. Mix designs with 28-day compressive strength of about 40 MPa were used. For the fibre reinforced specimens, a macro steel fibre (hooked end) and two different micro fibre types (polypropylene and carbon) as described in Table 1 were used at total volume fraction of 1%. The fibre combinations are shown in Table 2. Table 1. Physical Properties of Fibres Type Material Shape Density Crossection Kg/m 3 Shape Diameter Macro fibre Hooked end (HE) Steel 7800 Circular 0.50 mm Micro fibres Pitch-based carbon Ψ (CR) Carbon Straight 1900 Circular 9 to 11 micron Stealth (PE) Polypropylene Straight 900 Circular 6 Denier* *Weight of a 9000 m long fibre in grams The concrete was mixed using a pan type mixer, placed in plastic forms, and vibrated on a vibrating table before being covered with polyethylene sheets. After 24 hours, the specimens were demoulded and transferred to storage in a water tank for 30 days. Prior to testing, each cylinder was cut into disks with a thickness of 20 mm. An instrumented, drop-weight impact apparatus designed and constructed in the Department of Civil Engineering, University of British Columbia was used for the impact tests. The machine has the capacity of dropping a mass of about 11 kg from heights of up to 2000 mm on to the target specimen (Fig.1). The striking tup, mounted with a bolt load cell (Fig. 2) was connected to the drop hammer. Specimens were placed on a 65 mm-diameter circular support anvil (Fig. 2), which provided simple support around the edges. The hammer was dropped from a height of 100 mm, to provide an impact velocity of 1.4 m/s and impact energy of J. Two types of fibre reinforced concrete were investigated: single fibre FRC and hybrid fibre FRC (HyFRC). For the hybrid FRC, the fibre combinations are given in Table 2. Table 2 Fibre Combinations Designation Fibre Type Vf No. of Specimen Plain PLN Single Fibre FRC HE-FRC HE 1% 3 PE-FRC PE 1% 3 CR-FRC CR 1% 3 Hybrid Fibre FRC HP-HyFRC HE+PE 0.5% each 3 HC-HyFRC HE+CR 0.5% each 3 Total number of specimen 18 Ψ The pitch-based carbon fibre is a by-product from petroleum and coal-tar pitches. It is inexpensive and has a high modulus to strength ratio, a low coefficient of thermal expansion, and high thermal and electrical conductivity.

Striker (Penetrator) 100 5 Specimen 100 110 20 65 65 Support

3 Hoist Guide rails Hammer Penetrator Target Controller Data acquisition system Figure 1 Impact Testing Machine Striker (Penetrator) Specimen Support (Side view) Support (Top view) Fig. 2 Striking Tup and Support Anvil

4 3. RESULTS AND DISCUSSION 3.1 Failure Pattern and Load-Deflection Response In general, there are two main types of failure possible in penetration tests: global (or flexural) and localized failures (Fig. 3). Global or flexural failure occurs when the specimen is fractured into several pieces. Localized failure occurs when the failure is limited to the contact area (between striker and target); it may be divided into four subcategories: penetration, perforation, spalling, and scabbing. Penetration means that the penetrator is able to penetrate into the specimen surface without creating severe damage. Perforation occurs when the striker exits through the specimen with a residual velocity. Fragments of the specimen falling off the top or bottom surfaces of specimen are defined as spalling and scabbing, respectively. The occurrence of each failure mode is strongly dependent on the test setup, specimen geometry and material, and impact velocity and energy. Quite often, a mixed mode of failure can be observed. Global Local Penetration Perforation Flexure Scabbing Spalling Fig. 3 Typical failure patterns of concrete subjected to penetration tests In this study, the types of failures observed were flexural and mixed mode (between flexure and perforation). The flexural type of failure could be divided into brittle and ductile failures. In brittle flexural failure (Fig. 4), the specimen was totally fractured into several pieces. This occurred mostly in plain and single micro-carbon fibre reinforced concrete (CR-FRC). For plain concrete, the flexural failure was obviously due to its brittleness. However, in the case of CR-FRC, since carbon fibres are themselves quite brittle (low elongation, and high strength and elastic modulus), fibres fracture probably occurred almost simultaneously with the matrix cracking.

$Thus, they can help to increase the strength before fibre fracture occurs but not the post-peak response, as seen in the load-deflection responses of both plain concrete and CR-FRC (Fig. 5).$

5 (a) (b) Fig. 4 Brittle Flexure Failure of (a) Plain Concrete and (b) Carbon Single FRC Carbon fibres are effective in bridging across cracks only within a small range of bond-slip. Thus, they can help to increase the strength before fibre fracture occurs but not the post-peak response, as seen in the load-deflection responses of both plain concrete and CR-FRC (Fig. 5). Both responses were brittle; higher strength of CR-FRC was due to the effect of fibre bridging. In ductile flexural failure (Fig. 6), even though the specimens were fractured into several pieces, those pieces were still held together by the fibres. This type of failure was found commonly in hooked end (HE) FRC, and in both hybrid FRCs (HyFRC): HE + poly (HP) and HE + carbon (HC) Load (kn) Plain concrete 1% Carbon FRC 1% Polypropylene FRC Displacement (mm) Fig. 5 Brittle Response of Plain, CR-FRC and PE-FRC

(a) (b) (c) Fig. 6 Ductile Flexure Failure of (a) Steel, (b) Hybrid Steel-Carbon and (c) Hybrid Steel-Polypropylene FRC The load-deflection responses of HE-FRC and both HyFRCs are given in Fig. 7.

6 (a) (b) (c) Fig. 6 Ductile Flexure Failure of (a) Steel, (b) Hybrid Steel-Carbon and (c) Hybrid Steel-Polypropylene FRC The load-deflection responses of HE-FRC and both HyFRCs are given in Fig. 7. The responses of the HE- FRC and the Hybrid FRCs were more ductile than those of plain concrete and CR-FRC, as indicated by the longer post-peak (descending) response. The better post-peak response of HE-FRC was clearly due to the high strength and elastic properties of the steel fibres, which, by bridging across the cracks, allowed the cracked concrete to carry the applied load. For HyFRC, it appeared that the post-peak responses of the hybrid FRC s were dominated by the performance of the macro steel fibres. This was indicated by the decrease in the post-peak response of both hybrid-frcs with decreasing macro steel fibre content. However, we may see that each type of micro fibre also contributed to the post-peak response, though in different ways. While the Carbon-Hooked end hybrid FRC (HC-HyFRC) contained only half the amount of steel fibre (compared to HE-FRC), an increase in the peak load of the CR-HyFRC was observed because of the high strength and highly effective fibre bridging prior to the peak of carbon fibres. However, after most of the micro-fibres were fractured at the peak load, the post-peak response was governed by the steel fibres alone. Since the FRC at this point effectively contained only 0.5% steel fibres, the post-peak response decreased at a more rapid rate than for the other two FRCs (Fig. 7). For the hybrid steel-polypropylene (HP-HyFRC), because of the low strength and elastic modulus of the polypropylene fibres, the polypropylene fibres become effective mostly at larger crack openings (Bindiganaville and Banthia 2001). They did not contribute particularly to strength and, as a result, a lower peak load (similar to plain concrete) was found. However, beyond the peak load, the polypropylene fibres were mostly still not fractured, and thus continued their bridging effect, leading to a better post-peak response (in between HE-FRC and HC-HyFRC, Fig.7).

Load (kn) 4.00 3.50 3.00 2.50 2.00 1.50 1.00 0.50-1% Hooked End Hybrid 0.5%Hooked End + 0.5%Carbon Hybrid 0.5%Hooked End + 0.5% Polypropylene - 1.00 2.00 3.00 4.00 5.00 Displacement (mm) Fig.

8), the specimen failed primarily due to flexure, but accompanied by perforation at the center of the specimen. This failure mode was found generally with the micro polypropylene fibres (HP-FRC).

7 Load (kn) % Hooked End Hybrid 0.5%Hooked End + 0.5%Carbon Hybrid 0.5%Hooked End + 0.5% Polypropylene Displacement (mm) Fig. 7 Ductile Response of HE FRC, HP-HFRC and HC-HFRC In the mixed mode between perforation and flexure (Fig. 8), the specimen failed primarily due to flexure, but accompanied by perforation at the center of the specimen. This failure mode was found generally with the micro polypropylene fibres (HP-FRC). Polypropylene fibres exhibit a large elongation to failure, as well as a low elastic modulus and strength, and are much more ductile than carbon fibres. When subjected to load, after the first cracked associated with the matrix strength, most of the fibres were not fractured because of their low elastic modulus. They were able to hold the cracked pieces together and were also able to elongate a little further to allow the penetrator to perforate the specimen without separating the specimen into several pieces. The response of 1%PE-FRC was given in Fig. 5. Compared to plain concrete and CR-FRC, it had a better post-peak response due to its high bridging effectiveness at large deformations. Fig. 8 Mixed Flexure-Perforation Failure Mode of Polypropylene FRC

8 3.2 Peak Load and Energy Absorption The peak load and energy absorption of each specimen type are given in Table 3. (Note that the energy absorption of each specimen was calculated up to the end of impact event). The peak loads of plain concrete (2.13kN) and PE-FRC (2.16 kn) are the lowest. The polypropylene fibres, because of their low strength, did not contribute much to the peak load. However, they did increase the energy absorption of the concrete significantly (six times larger than that of plain concrete). The peak load was improved significantly in HE-FRC, CR-FRC and HC-HyFRC (3.43, 3.77, and 3.60 kn, respectively). Both steel and carbon, because of their high strength and elastic modulus, were able to improve the load carrying capacity in concrete significantly. However, the energy absorption of CR-FRC was quite low, since most carbon fibres were fractured at the peak load. Table 3 Peak Load and Energy Absorption Concrete Type Average Load (kn) Average Energy (J) Peak load/weight (N/kg) Energy/Weight (milli-joule/kg) Plain HE-FRC CR-FRC PE-FRC HP-HyFRC HC-HyFRC Note: weight per cubic metre, plain concrete = 2350 kg, HE-FRC = 2428 kg, PE-FRC = 2359 kg, CR-FRC = 2369 kg, HP-HyFRC = 2394, and HC-HyFRC = 2399kg. For this particular mix, whether one looks at the peak load, the energy absorption, the peak load/weight ratio or the energy/weight ratio, the hybrid system did not perform any better than the hooked end steel fibre system; they were slightly worse. Based on these results, they offer no particular advantages for this type of loading. Thus, hybrid fibre FRC system should not be assured always to give better performance than well-designed single fibre systems. 4. CONCLUSIONS 1. The penetration resistance of FRC under impact loading is much better than that of plain concrete. 2. All plain concrete and FRC specimens in this study failed in a flexural mode, except for the PE- FRC, in which mixed mode failure was observed. It must be noted, however, that different failure pattern can be obtained with different testing configurations. 3. Hooked end FRC performed the best among the FRCs at equal fibre volumes. 4. The hybrid fibre systems, with the combination and proportions used in this study, did not show any significant improvement in resisting impact loading. ACKNOWLEDGEMENT Authors would like to thank Christopher Choi, a grade-eleven student from Killarney Senior Secondary School, Vancouver, for his assistance during the experiments. REFERENCES Mindess S, and Yan C. Perforation of Plain and Fibre Reinforced Concretes Subjected to Low-Velocity Impact Loading. Cement and Concrete Research 1993;(23): Banthia N. Strength and Toughness of Micro-Fiber Reinforced Cement-based Materials. Proceedings CSCE Annual Conference. Quebec City: 1992

9 Banthia N, and Nandakumar N. Crack Growth Resistance of Hybrid Fiber Reinforced Cement Composites. Cement and Concrete Composites 2001: 1-7. Bindiganaville V, and Banthia N. Polymer and Steel Fibre-Reinforced Cementitious Composites under Impact Loading-Part 1: Bond-Slip. ACI Materials Journal 2001; 98 (1),

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