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Phản ứng của sợi thép bê tông cốt thép chịu AIR
RESPONSE OF STEEL FIBER REINFORCED CONCRETE SUBJECTED TO AIR BLAST LOADING Mohammed Alias Yusof(1), Norazman Mohamad Nor(1), Ariffin Ismail(2), Muhamad Fauzi Muhamad Zain(3), Risby Mohd Sohaimi(1), Ng Choy Peng(1) (1)Faculty of Engineering, Universiti Pertahanan Nasional Malaysia, Kem Sungai Besi, 57000 Kuala Lumpur Email: alias@upnm.edu.my (2)Faculty of Defence Management and Studies, Universiti Pertahanan Nasional Malaysia, Kem Sungai Besi, 57000 Kuala Lumpur (3)Faculty of Engineering and Built Environment, Universiti Kebangsaan Malaysia, 43600 Bangi, Selangor ABSTRACT This paper investigates the behaviour of concrete panel subjected to air blast loading. The test component include normal reinforced concrete (NRC) and also steel fibre reinforced concrete (SFRC) with a fibre content of 1.5 % in volume percentages. Field blast test were conducted using 1kg of plastic explosive (PE4) at standoff distance of 0.6 meter. Failure modes of each specimen were investigated and are presented in this paper. The experimental results indicate that incorporation of steel fibres in concrete have significantly improved the blast resistance of concrete as compared to plain reinforced concrete. Key word : Steel fiber concrete, Air blast loading, normal concrete. INTRODUCTION Recent terrorist attack on Moscow Metro Train (2010), Marriott Hotel in Jakarta (2009), and Mumbai (2008) had shown that the destruction of the hotels and other facilities such as public transport and military base has become target of the terrorist [1-3]. In most of the cases, the terrorist used explosives located in the vehicle at a close distance from the target. The explosion creates intense shock waves which propagate outward at supersonic velocity accompanied by released of heat and light that induced pressure on the structural members and causes significant damaged to the structure and loss of life. There are a number of methods to stop the terrorist attack. One of the methods is gathering information on the terrorist and stop the attack before it take place, another way is protect building from damaged is by incorporating blast resistance design and also retrofitting of the existing structure [4]. This area of research is currently receiving more attention from many structural engineers in which they began to consider blast loading and also blast resistance materials in their design in order to protect important building and structures from such attacks. Concrete is one of the most widely used construction material for structures because it possesses considerable mass per unit cost compared to other construction materials, and has excellent fire-resistance and is able to absorb large amounts of energy [5] However, one of the disadvantages of concrete it have a very low resistance againts tensile stress and also posses low ductility . From previous research it was found that mechanical properties of the concrete can be inproved by adding fibers into the mix. Otter & Naaman (1987) reported that the addition of fiber in the concrete have significant effect on strenght and toughness of concrete.[6] The test results of Nagarkar et. al (1988) indicated that the compressive, split tensile and flexural strenght of concrete incresed by the addition of fiber. [7]. The objective of this reseach is to investigate the response of Steel fiber reinforced concrete (SFRC) subjected to air blast loading. In this study two type of concrete specimens, which are normal reinforce concrete (NRC) and also steel fiber reinforced concrete (SFRC) were tested using charge weight of 1 kg explosive at a standoff distance of 0.6 meter to determine their response to air blast loading. After each test, the failure mode of the specimen was recorded and examined. The details are presented in this paper. BLAST WAVE PHENOMENA An explosion is as a large-scale, rapid, and sudden release of energy. As the explosive detonates, it creates a shock wave that travels at supersonic velocity and at the same time the pressure around the area increases. The shock wave expands until it hits the test structure and is reflected upwards. As the shock wave hits the test structure, pressure decreases rapidly. As a result of the sudden decrease in pressure, a vacuum were created around the blast area which drags pressure on the surface of the test structure and also the surrounding area. The vacuum creates high suction winds that carry fragments and debris far from the blast source. This blast also creates missiles ground shock, and cratering at the surrounding area depending on the capacity of the explosive. [8]. The blast phenomenon is as shown in the Fig 1. Fig 1: Typical blast wave profile [8] EXPERIMENTAL PROGRAM The concrete specimen used for the experiment is Normal Reinforced Concrete (NRC) and Steel fiber reinforced concrete panel (SFRC) as shown in Fig 2. Fig 2. Typical Plan and section for the panel The specimen was reinforced with 10 mm diameter steel at 200 mm centre-to-centre in both ways. The specimen measures at 600 mm × 600 mm and a thickness of 100 mm . Ordinary Portland cement, crushed stone coarse aggregate having maximum size of 10mm and also river sand were used. The steel fiber reinforced concrete, mix incorporated volume fraction of 1.5 % of hooked-end steel fibers which is made of mild carbon steel as shown in Fig 3. Fig. 3. Hooked end steel fiber The mix consists of the proportion of cement: water: aggregate: sand was 357:160: 997:534: kg/m3. In the production of concrete, firstly the aggregate and sand was put into the mixer and mixed for a few minutes and then cement and water were added into the mix. Finally the fibres were added in small amounts to avoid fibre balling and to produce the concrete with uniform material consistency and good workability. The freshly mix steel fibre reinforced concrete was placed in two equal layers into mould to cast a standard 150mm × 150mm × 150mm cube and 150mm × 300mm cylinder concrete specimen for a compressive strength test and a split tensile test and also into a 100mm ×100mm ×500 mm beam mould for a flexure strength test. Each layer was consolidated using a vibrating table. At the end of 24 hours after consolidating, the specimen was removed from the mould and cured in water for 28 days. Finally the NRC and SFRC were poured separately into the panel mould of 600 mm × 600 mm × 100 mm. The NRC and SFRC panels were later removed from the mould after consolidation and cured with wet gunny sacks for 28 days before the field blast test. Table 1 shows the results of the average compressive strength tests, flexural strength tests and split tensile strength tests for both the NRC and SFRC specimens. Table 1 : Test result for NRC and also SFRC Average strength NRC SFRC Compressive 31 38.5 Flexural (MOR) 3.5 6.0 Split Tensile 3.0 4.5 FIELD BLAST TEST PROGRAM A field blast test was conducted at a disclosed military facility. In the test two steel I beam support of a length 800 mm each of was fabricated to support the specimen. The supports were fixed to the existing heavy reinforced concrete structure located at the blasting site. The concrete panel was then fixed to the steel support frame as shown in Fig 4. The explosive is suspended in mid air at 600mm stand-off distance towards the panel. The blast test set up is shown in Fig. 5. 600mmve600mmve600mmve ExplosiveExplosiveExplosive Test panelTest panelTest panel Fig 4. Steel support frame Fig 5. Test set up RESULTS AND DISCUSSION At the end of each test, the failure pattern of specimen were recorded. A set of photographs of specimens is presented in Fig 6. (a) Back face of NRC (b) Back face of SFRC Fig. 6. Failure pattern of NRC and SFRC Specimen NRC:This specimen failed in shear mode. Two large cracks developed on the back face of the specimen. A number of smaller cracks were also observed on both the front and rear faces.This is because the normal reinforced concrete have a very low resistance againts tensile stress and also low ductility Specimen SFRC: Basically there is minor damaged on back face of this specimen.The specimen shows high resistance to blast loading. This is because of by incorporating the fibers into concrete restrain the initiation and propagation of crack by the bridging effect of steel fibers The bridging effect observed in the panel is shown in Fig 7. Steel Fiber bridge crackSteel Fiber bridge crackSteel Fiber bridge crack Fig 7 : Bridging action of steel fiber on concrete . CONCLUSION Field blast tests have been conducted on the normal fiber reinforced concrete panel (NRC) and also steel fibre reinforced concrete (SFRC) panel The experimental result indicates that incorporation of steel fibres in concrete has significantly improved the blast resistance of concrete as compared to normal (NRC). ACKNOWLEDGEMENTS The authors wish to gratefully acknowledge the support Science and Technology Research for Defence (STRIDE) for providing support and and test fasilities during the feild test program. REFERENCES [1] Peters A.Train Bombing in Moscow Kill Dosen, Available from http://www.wsws.org/ articles/2010/mar2010/russ-m30.shtml (Accessed on 30 March 2010). [2] Moestafa B.Marriot Hotels in Jakarta Hit by Bombing, Available from http:// www.bloomberg.com/apps/news?pid=newsarchive&sid=a_SusXvRLY.0 (Accessed on 17 July 2009) [3] Milli A.Taj hotel attack in Mumbai. Available from http://www.nowpublic.com/world/taj- hotel-attack-mumbai-puts-nowpublic-worldwide-spotlight(Accessed on 26 November 2008) [4] Buchan P.A. Blast resistance of FRP composites subject to blast loading . J Composites Part B 38 (2007) 509-522. [5] Mohammadi Y. Impact resistance of steel fibrous concrete . J Construction building materials 23 (2009) 183-189. [6] Otter DE, Naaman AE.Strain rate effect on compressive properties of fiber reinforced concrete. In: Proc of the int Symp on Fiber Reinforced Concrete, December 16-19, Madras, India,1987, 2.225-35 [7] Nagarkar P K ,Tambe SK. Study of fiber reinforced concrete. In : Proc of Int Symp of fiber reinforced concrete, December 16-19, Madras India, 1987, 2.130 [8] Ngo. T, P. Mendis . Blast loading and effect on structures’. EJSE Specials issue . Loading on structure (2007). . RESPONSE OF STEEL FIBER REINFORCED CONCRETE SUBJECTED TO AIR BLAST LOADING Mohammed Alias Yusof(1), Norazman Mohamad Nor(1), Ariffin. ABSTRACT This paper investigates the behaviour of concrete panel subjected to air blast loading. The test component include normal reinforced concrete (NRC)