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Protective components made of steel fiber reinforced concrete under contact detonation
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Available online at www.sciencedirect.com Procedia Procedia Engineering Engineering 00 00 (2017) (2017) 000–000 000–000
ScienceDirect
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Procedia Engineering 199 (2017) 2525–2530
X International Conference on Structural Dynamics, EURODYN 2017
Protective components made of steel fiber reinforced concrete under contact detonation Tobias Zircheraa, Manfred Keuseraa, Albert Burbachbb, Steffen Lehmanncc aUniversity of German Armed Forces, Chair for concrete construction, Werner-Heisenberg-Weg 39, 85577 Neubiberg University of German Armed Forces, Chair for concrete construction, Werner-Heisenberg-Weg 39, 85577 Neubiberg 52), Oberjettenberg, Oberjettenberg, 83458 83458 Schneizlreuth Schneizlreuth Bundeswehr Technical Technical Center Center for for Protective Protective and and Special Special Technologies Technologies (WTD (WTD 52), cc Federal Infrastructure, Environmental Environmental Protection Protection and and Services Services Section Section Infra Infra II II 2, 2, 53123 Federal Office Office of of Bundeswehr Bundeswehr Infrastructure, 53123 Bonn Bonn a
bb Bundeswehr
Abstract Abstract As the German German Armed Armed Forces Forces we we investigate investigate steel steel fiber fiber concrete concrete for for use use in in As aa part part of of aa research research project project at at the the University University of of the protective structures. structures. protective At the the chair chair for for concrete concrete construction construction of of the the University University of of German German Armed Armed Forces Forces aa literature literature study study on on protective protective concrete concrete At components under under gunfire gunfire and and contact contact detonation detonation was was conducted. conducted. In In aa first first short short experimental experimental study study tests tests were were performed performed in in components order to obtain data for numerical modeling and for supplementary test setups. order to obtain data for numerical modeling and for supplementary test setups. In the the main main research research project project different different mixtures mixtures of of steel steel fiber fiber concrete concrete were were produced produced and and investigated investigated under under gunfire gunfire and and contact contact In detonation. For For the the concrete concrete local local raw raw material material from from the the area area of of mission mission was was used used as as aggregates aggregates and and cement. cement. Plates Plates for for the the contact contact detonation. detonation with with different different thicknesses, thicknesses, varying varying fiber fiber geometries geometries and and reinforcement reinforcement systems systems were were produced. produced. The The plates plates for for contact contact detonation detonation were loaded with 850 g PETN explosive at the test facility of the German Armed Forces Technical Center for detonation were loaded with 850 g PETN explosive at the test facility of the German Armed Forces Technical Center for Protective and and Special Special Technologies. Technologies. Protective During the the contact contact detonation detonation aa pressure pressure wave wave runs runs through through the the plate plate and and is is reflected reflected as as aa tensile tensile wave wave at at the the free free surface, surface, During opposite to to the the loaded loaded surface. surface. This This tensile tensile wave wave generates generates aa damage damage crater crater on on the the rear rear side side of of the the plate. plate. The The volume volume as as well well as as opposite the surface of the crater is determined by Impact-Echo tests, by 3D-scans and by measurements of the crater geometry. the surface of the crater is determined by Impact-Echo tests, by 3D-scans and by measurements of the crater geometry. One target target of of this this project project is is to to work work out out aa semi semi empirical empirical formula formula for for the the determination determination of of the the crater crater volume volume and and its its surface surface in in One dependence on on the the main main input input parameters parameters in in order order to to provide provide aa mobile mobile application application for for the the soldiers soldiers in in charge charge of of the the infrastructure infrastructure dependence during aa mission. mission. The The input input parameters parameters for for this this formula formula include include amongst amongst others others the the compressive compressive strength strength ffck during the bending bending tensile tensile ck,, the , the fiber geometry and the fiber content. strength f ct strength fct, the fiber geometry and the fiber content. This of the the contact contact detonation detonation tests. tests. Furthermore, Furthermore, aa mathematical mathematical approach approach for for numerical numerical This publication publication focuses focuses on on the the conduction conduction of analysis analysis is is presented. presented. © 2017 The Authors. Published by Elsevier Ltd. © The Authors. Authors. Published Published by by Elsevier Elsevier Ltd. Ltd. © 2017 2017 The Peer-review under responsibility of the organizing committee of 2017. Peer-review organizing committee committee of of EURODYN EURODYN Peer-review under under responsibility responsibility of of the the organizing EURODYN 2017. 2017. Keywords: "Steel fiber fiber reinforced reinforced concret; concret; protective protective components; empiric formula" formula" Keywords: "Steel components; conatact conatact detonation; detonation; highly highly dynamic dynamic loading; loading; semi semi empiric
1877-7058 1877-7058 © © 2017 2017 The The Authors. Authors. Published Published by by Elsevier Elsevier Ltd. Ltd. Peer-review Peer-review under under responsibility responsibility of of the the organizing organizing committee committee of of EURODYN EURODYN 2017. 2017.
1877-7058 © 2017 The Authors. Published by Elsevier Ltd. Peer-review under responsibility of the organizing committee of EURODYN 2017. 10.1016/j.proeng.2017.09.434
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Tobias Zircher et al. / Procedia Engineering 199 (2017) 2525–2530 "Tobias Zircher" / Procedia Engineering 00 (2017) 000–000
1. Introduction Military facilities of the German Armed Forces in out of area missions are under the permanent threat of terrorist attacks. Lehmann et al. [5] described the advantages of the adding of steel fibers to the concrete mixture used for protective structures. In this research project steel fiber concrete using low strength standard concrete mixtures (C25/30) with different aggregate types were produced in order to simulate concrete made of local material being available in military operational areas. Plates for the contact detonation testing, with different reinforcing systems, different thicknesses and varying fiber geometries were produced. An extensive experimental program is currently ongoing at the University of German Armed Forces in collaboration with the Bundeswehr Technical Center for Protective and Special Technologies (WTD 52) [2] and the Federal Office of Bundeswehr Infrastructure, Environmental Protection and Services Section Infra II 2. In the case of contact detonations, the explosive is located directly on the test specimen and the applied loads are usually destructive. In the area around the explosive the concrete is destroyed by high compressive stresses and shear stresses. The pressure wave from the detonation runs through the plate and it is reflected as a tensile wave at the unloaded surface opposite to the loaded surface. When the tensile stress exceeds the tensile strength of the concrete, debris is hurled out of the rear side of the plate [3] (Fig. 1). These fragments present a serious danger for people and equipment in the protected area. The adding of steel fibers significantly reduces the debris flow.
Fig. 1. Schematic system of contact detonation
This paper is focused on the implementation of the contact detonation tests and the first results. Furthermore, a mathematical approach for a numerical analysis is presented. The tests were performed at the site of WTD 52 in cooperation with the University of German Armed Forces. The influence of the concrete composition on the bending tensile strength, the compressive strength and the contact detonation resistance are in the focus of the investigation in this research project. The medium cube compressive strength (edge length 15 cm 15 cm 15 cm) was intended not to exceed a value of 30 N/mm². The first concept for the concrete mixture included the use of cement with a strength of 42.5, plasticizer and flue ash. In the modified mixtures the flue ash and the plasticizer were replaced by water and cement, because transporting plasticizer and flue ash into military operational areas might be expensive even not possible. The investigation comprises of small scale test specimens for the determination of the concrete compressive strength and the bending tensile strength, plates with a dimension of 2.0 m 2.0 m 0.20 m and 2.0 m 2.0 m 0.30 m for contact detonation and smaller plate test specimens for gun-shot tests. 2. Test program and setup for the contact detonation testing The test plates for the contact detonation were produced on the site of the University of German Armed Forces in Munich. The steel fiber content was defined to be 1.0 Vol.-%. The concrete was supplied as ready-mixed concrete and the steel fibers were already added in the concrete plant. 15 different batches were produced with different
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concrete mixtures, steel fiber types, reinforcement systems and plate thicknesses. Of each series, three plates were produced to allow for the possibility of a statistical test evaluation (c.f. Table 1). Ingredients concrete mixture Cement II/A-M(V-LL)42,5N Cement II/A-M(V-LL)32,5N Flue ash Aggregate 0/4 NfGK Aggregate 0/8 NgGK Aggregate 8/16 NgGK Water superplasticizer Water/cement ratio Steel fiber KH DE35/0,55 N Steel fiber KH DE60/0,9 N Steel fiber Dramix 4D-65/60BG Consistency Plate thickness
Table 1. Concrete Composition Mix 1 – 5 Mix 1 Mix 2-3 330 kg/m³ ----400 kg/m³ 70 kg/m³ --893 kg/m³ 770 kg/m³ 132 kg/m³ 224 kg/m³ 629 kg/m³ 609 kg/m³ 224 kg/m³ 250 kg/m³ 1,32 kg/m³ --0,65 0,625 78,5 kg/m³ (Mix 1.3) 78,5 kg/m³ (Mix 2.2) 78,5 kg/m³ (Mix 1.2) 78,5 kg/m³ (Mix 2.3, 3.2) --78,5 kg/m³ (Mix 2.4) F4 F4 30 cm 30 cm
Mix 4-5 --400 kg/m³ --770 kg/m³ 224 kg/m³ 609 kg/m³ 250 kg/m³ --0,625 78,5 kg/m³ (Mix 4.2, 5.2) 78,5 kg/m³ (Mix 4.3) 78,5 kg/m³ (Mix 4.4) F4 20 cm
Three different steel fiber types (c.f. Table 1) produced by Krampe Harex GmbH & Co. KG with the manufacturer’s name KH DE35/0,55 N (length: 35 mm, diameter 0,50 mm, N = Steel, tensile strength = 1250 N/mm²) and KH DE60/0,9 N (length: 60 mm, diameter 0,90 mm, N = Steel, tensile strength = 1150 N/mm²) and Dramix 4D-65/60BG (length: 60,5 mm, diameter 0,90 mm, steel tensile strength = 1600 N/mm²) have been used. The medium cube concrete compressive strength was in the range of 30-35 N/mm². The contact detonation tests were carried out at the test facility on the site of the WTD 52. The plates are oriented vertically in the blasting cavern, (c.f. Fig. 2 (a)) and are subjected to the explosive. a
b
Fig. 2. (a) Test set up with installed plate; (b) explosive charge
The explosive charge consists of 850 g of PETN explosive, which is placed in a plastic tube with a height of 62 mm and an inner diameter of 103.6 mm on the center of the loaded surface (c.f. Fig. 2 (b)). After the contact detonation, a photo documentation is conducted. Subsequently, the aim was to determine the exact damage distribution, crater geometry and crack pattern on the unloaded surface. For this matter the impact-echo method was used which is a non-destructive testing method generally used for the detection of imperfections in reinforced concrete constructions or for example to measure the thickness of the inner lining of a tunnel or for the verification of foundation piles. The results of this measurements were published in [8].
Tobias Zircher et al. / Procedia Engineering 199 (2017) 2525–2530 "Tobias Zircher" / Procedia Engineering 00 (2017) 000–000
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a
b
c
Fig. 3. (a) Schematic drawing blast cavern; (b) measuring grid in the cavern with debris; (c) contact measuring sensors to detect an impact of debris at the back wall of the cavern
During the detonation, the debris flight is recorded at the side of the unloaded surface of the plate by two high speed cameras. The speed of the secondary debris can be calculated from the image rates and the distance from the fragments. Furthermore, measuring sensors are installed at the end of the blasting cavern, (c.f. Fig. 3 (c)) which document whether there is an impact of debris on their back wall and also allow a calculation of the speed of the debris from the detonation. The debris fragments were collected, measured and weighed after the tests, (c.f. Fig. 3 (b)). 3. First experimental results The first contact detonation tests have shown that the debris flight on the unloaded surface on the plates with steel fibers could be significantly reduced in comparison to the plates without steel fibers (c.f. Fig. 4 (a-c)). For the plates without steel fibers, the amount of debrided parts was up to 50 kg. For the plates with steel fibers, the amount of debrided parts was between 0.70 kg and 3.1 kg. The results were almost identical for all fiber types used. a b c
Fig. 4. Unloaded surface of a plate after contact detonation: (a) without steel fibers; (b) with Dramix 4D-65/60BG; (c) with KH DE60/0,9 N
After evaluating the debris flight and carrying out the 3D scanning, the plates were sawn in the centerline of the crater and the exact course of the damage crater became apparent (c.f. Fig. 5 (a-b)). The sectional images showed that the size of the damage center of the plates is not influenced by the steel fibers since the concrete failure is mostly dependent on the tensile strength. However adding steel fibers to the concrete mixture reduces debris flight in a great manner. a b
Fig. 5. (a) Sectional image from a plate without steel fibers; (b) and with steel fibers
Tobias Zircher et al. / Procedia Engineering 199 (2017) 2525–2530 Tobias Zircher / Procedia Engineering 00 (2017) 000–000
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4. Design formula based on the theory of wave propagation Based on the experimental data at hand and by support of further experiments we intend to use the method of ‘Design of Experiments’. By the help of a full factorial model (set up from taking the actual experiments as design points) we will gather a semi-empirical formula which takes into account the concrete tensile strength, the type of steel fibers, the plate thickness and the amount of the explosive charge. This semi-empirical formula will be enhanced and validated by a numerical study which solves the wave-equation:
2u 2 G G 2u x
(1)
2v G G 2v 2 y
(2)
2w 2 G G 2 w z
(3)
Where = Lamé-constant, G = shear modulus, ² = laplace-operator, defined as:
2
2 2 2 x 2 y 2 z 2
(4)
The equations (1), (2), (3) are difficult to interpret. The displacement field can be described by the superposition of a vector field (distortional) and a scalar field (Lamé-potentials, dilatational) with the Helmholtz-approach:
1 •• 0 c 2p
i
(5)
1 •• i 0 cs2
(6)
The solution of equations (1) to (3) thus is split into a pressure wave part (c.f. (5)) and a shear wave part (c.f. (6)), which are characterized by the assigned pressure wave velocity cp and the shear wave velocity cs. The velocities of propagation are:
cp
cs
E 2G 1 (1 )(1 2 )
(7)
G
(8)
where = Poisson's ratio, E = Young’s modulus,
= density.
Tobias Zircher et al. / Procedia Engineering 199 (2017) 2525–2530 "Tobias Zircher" / Procedia Engineering 00 (2017) 000–000
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5. Numerical simulation Based on the results from the contact detonation tests, fluid dynamical analysis for the propagation in concrete are carried out by using the hydrocode Ansys Autodyn. The simulations are carried in order to perform a parameter study in which the plate thickness, the reinforcement system and the amount of explosive are varied. With the help of these studies further boundary conditions and parameters for the semi empirical formula will be investigated. As there are a lot of details in a hydrocode simulation which may potentially effect the solution such as time-step-size or the variability in the material parameters, this procedure is not a simple one. In the context of this paper we can only highlight some of these responsibilities: I. II. III.
investigate for a load model which represents the contact detonation load history properly, investigate for a material model of the concrete failure, which takes into account the rate dependence of the tensile strength fct and taking into account uncertainties in terms of the variability of the material parameters (Poisson's ratio , Young’s modulus E, density and concrete tensile strength fct).
This is of great importance, as on the impact side the whole wave characteristics strongly depend on the first three parameters (, E, ) whereas on the other hand the variability of the concrete tensile strength fct massively influences the damage characteristics. We expect that a model, which solves the three challenges, will describe the volume and the surface of the crater properly within the bounds of statistical uncertainty in comparison to the conducted experiments. 6. Conclusion It was apparent from the first contact detonation tests that the amount of debrided parts on the unloaded surface can be significantly reduced by the addition of steel fibers into the concrete mixture. The different geometries of the steel fibers had no significant influence on the reduction of the debris which was almost identical in all experiments. In the next step of the project, our work will be focused on numerical simulation and on the development of a semi empirical formula for the determination of the crater volume and its surface in dependence of the main input parameters in order to provide a mobile application for the soldiers who are in charge of the infrastructure during a mission. 7. Acknowledgements The authors thank the Federal Office of Bundeswehr Infrastructure, Environmental Protection and Services Section Infra II 2 for their support and the Bundeswehr Technical Center for Protective and Special Technologies (WTD 52) for providing their experimental facilities for this research. 8. References [1] Braml T., 2010, Structural reliability assessment of reinforced concrete bridges on the basis of the results from a bridge inspection Disstertation, University of German Armed Forces Munich, Chair for concrete construction, 2010 [2] Burbach A., 2015, Fiber reinforcement for conventional reinforced concrete elements; decrease of weapon effects by usage of steel fibers, 3rd International conference on Protective Structures (ICPS3) pp. 65-71, Newcastle, Australia [3] Fuchs M., Keuser M., Schuler H., Thoma K., 2007, Fiber Reinforced Concrete under High Dynamic Loading, Beton und Stahlbetonbau 102, Heft 11, pp. 759-769 [4] JCSS – Risk Assessment in Engineering, Probabilistic Model Code, Joint Committee on Structural Safety, 2008 [5] Lehmann S., Walz M., Heckersbruch A., 2015, Fiber reinforcement for reinforcing conventional armoring in reinforced concrete elements exposed to highly dynamic loads, 3rd International conference on Protective Structures (ICPS3) pp. 274-281, Newcastle, Australia [6] Studer J. A., Laue J., Koller M. G., 2007, Bodendynamik, Grundlagen, Kennziffern, Probleme und Lösungsansätze, 3., völlig neu bearbeitete Auflage, Springer, Berlin Heidelberg New York [7] Schulz T., Non-destructive testing of concrete structures with and without steel fiber reinforcement under highly dynamical stress and implementation of a finite element model, Master thesis, Chair of Concrete and Masonry Structures, Technical University of Munich, 2016 [8] Zircher T., Keuser M., Schulz T., Burbach A., 2017, Non-destructive testing of protective components after contact detonation, fib symposium, Maastricht, accepted paper
ScienceDirect
Available online at www.sciencedirect.com Procedia Procedia Engineering Engineering 00 00 (2017) (2017) 000–000 000–000
ScienceDirect
www.elsevier.com/locate/procedia www.elsevier.com/locate/procedia
Procedia Engineering 199 (2017) 2525–2530
X International Conference on Structural Dynamics, EURODYN 2017
Protective components made of steel fiber reinforced concrete under contact detonation Tobias Zircheraa, Manfred Keuseraa, Albert Burbachbb, Steffen Lehmanncc aUniversity of German Armed Forces, Chair for concrete construction, Werner-Heisenberg-Weg 39, 85577 Neubiberg University of German Armed Forces, Chair for concrete construction, Werner-Heisenberg-Weg 39, 85577 Neubiberg 52), Oberjettenberg, Oberjettenberg, 83458 83458 Schneizlreuth Schneizlreuth Bundeswehr Technical Technical Center Center for for Protective Protective and and Special Special Technologies Technologies (WTD (WTD 52), cc Federal Infrastructure, Environmental Environmental Protection Protection and and Services Services Section Section Infra Infra II II 2, 2, 53123 Federal Office Office of of Bundeswehr Bundeswehr Infrastructure, 53123 Bonn Bonn a
bb Bundeswehr
Abstract Abstract As the German German Armed Armed Forces Forces we we investigate investigate steel steel fiber fiber concrete concrete for for use use in in As aa part part of of aa research research project project at at the the University University of of the protective structures. structures. protective At the the chair chair for for concrete concrete construction construction of of the the University University of of German German Armed Armed Forces Forces aa literature literature study study on on protective protective concrete concrete At components under under gunfire gunfire and and contact contact detonation detonation was was conducted. conducted. In In aa first first short short experimental experimental study study tests tests were were performed performed in in components order to obtain data for numerical modeling and for supplementary test setups. order to obtain data for numerical modeling and for supplementary test setups. In the the main main research research project project different different mixtures mixtures of of steel steel fiber fiber concrete concrete were were produced produced and and investigated investigated under under gunfire gunfire and and contact contact In detonation. For For the the concrete concrete local local raw raw material material from from the the area area of of mission mission was was used used as as aggregates aggregates and and cement. cement. Plates Plates for for the the contact contact detonation. detonation with with different different thicknesses, thicknesses, varying varying fiber fiber geometries geometries and and reinforcement reinforcement systems systems were were produced. produced. The The plates plates for for contact contact detonation detonation were loaded with 850 g PETN explosive at the test facility of the German Armed Forces Technical Center for detonation were loaded with 850 g PETN explosive at the test facility of the German Armed Forces Technical Center for Protective and and Special Special Technologies. Technologies. Protective During the the contact contact detonation detonation aa pressure pressure wave wave runs runs through through the the plate plate and and is is reflected reflected as as aa tensile tensile wave wave at at the the free free surface, surface, During opposite to to the the loaded loaded surface. surface. This This tensile tensile wave wave generates generates aa damage damage crater crater on on the the rear rear side side of of the the plate. plate. The The volume volume as as well well as as opposite the surface of the crater is determined by Impact-Echo tests, by 3D-scans and by measurements of the crater geometry. the surface of the crater is determined by Impact-Echo tests, by 3D-scans and by measurements of the crater geometry. One target target of of this this project project is is to to work work out out aa semi semi empirical empirical formula formula for for the the determination determination of of the the crater crater volume volume and and its its surface surface in in One dependence on on the the main main input input parameters parameters in in order order to to provide provide aa mobile mobile application application for for the the soldiers soldiers in in charge charge of of the the infrastructure infrastructure dependence during aa mission. mission. The The input input parameters parameters for for this this formula formula include include amongst amongst others others the the compressive compressive strength strength ffck during the bending bending tensile tensile ck,, the , the fiber geometry and the fiber content. strength f ct strength fct, the fiber geometry and the fiber content. This of the the contact contact detonation detonation tests. tests. Furthermore, Furthermore, aa mathematical mathematical approach approach for for numerical numerical This publication publication focuses focuses on on the the conduction conduction of analysis analysis is is presented. presented. © 2017 The Authors. Published by Elsevier Ltd. © The Authors. Authors. Published Published by by Elsevier Elsevier Ltd. Ltd. © 2017 2017 The Peer-review under responsibility of the organizing committee of 2017. Peer-review organizing committee committee of of EURODYN EURODYN Peer-review under under responsibility responsibility of of the the organizing EURODYN 2017. 2017. Keywords: "Steel fiber fiber reinforced reinforced concret; concret; protective protective components; empiric formula" formula" Keywords: "Steel components; conatact conatact detonation; detonation; highly highly dynamic dynamic loading; loading; semi semi empiric
1877-7058 1877-7058 © © 2017 2017 The The Authors. Authors. Published Published by by Elsevier Elsevier Ltd. Ltd. Peer-review Peer-review under under responsibility responsibility of of the the organizing organizing committee committee of of EURODYN EURODYN 2017. 2017.
1877-7058 © 2017 The Authors. Published by Elsevier Ltd. Peer-review under responsibility of the organizing committee of EURODYN 2017. 10.1016/j.proeng.2017.09.434
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Tobias Zircher et al. / Procedia Engineering 199 (2017) 2525–2530 "Tobias Zircher" / Procedia Engineering 00 (2017) 000–000
1. Introduction Military facilities of the German Armed Forces in out of area missions are under the permanent threat of terrorist attacks. Lehmann et al. [5] described the advantages of the adding of steel fibers to the concrete mixture used for protective structures. In this research project steel fiber concrete using low strength standard concrete mixtures (C25/30) with different aggregate types were produced in order to simulate concrete made of local material being available in military operational areas. Plates for the contact detonation testing, with different reinforcing systems, different thicknesses and varying fiber geometries were produced. An extensive experimental program is currently ongoing at the University of German Armed Forces in collaboration with the Bundeswehr Technical Center for Protective and Special Technologies (WTD 52) [2] and the Federal Office of Bundeswehr Infrastructure, Environmental Protection and Services Section Infra II 2. In the case of contact detonations, the explosive is located directly on the test specimen and the applied loads are usually destructive. In the area around the explosive the concrete is destroyed by high compressive stresses and shear stresses. The pressure wave from the detonation runs through the plate and it is reflected as a tensile wave at the unloaded surface opposite to the loaded surface. When the tensile stress exceeds the tensile strength of the concrete, debris is hurled out of the rear side of the plate [3] (Fig. 1). These fragments present a serious danger for people and equipment in the protected area. The adding of steel fibers significantly reduces the debris flow.
Fig. 1. Schematic system of contact detonation
This paper is focused on the implementation of the contact detonation tests and the first results. Furthermore, a mathematical approach for a numerical analysis is presented. The tests were performed at the site of WTD 52 in cooperation with the University of German Armed Forces. The influence of the concrete composition on the bending tensile strength, the compressive strength and the contact detonation resistance are in the focus of the investigation in this research project. The medium cube compressive strength (edge length 15 cm 15 cm 15 cm) was intended not to exceed a value of 30 N/mm². The first concept for the concrete mixture included the use of cement with a strength of 42.5, plasticizer and flue ash. In the modified mixtures the flue ash and the plasticizer were replaced by water and cement, because transporting plasticizer and flue ash into military operational areas might be expensive even not possible. The investigation comprises of small scale test specimens for the determination of the concrete compressive strength and the bending tensile strength, plates with a dimension of 2.0 m 2.0 m 0.20 m and 2.0 m 2.0 m 0.30 m for contact detonation and smaller plate test specimens for gun-shot tests. 2. Test program and setup for the contact detonation testing The test plates for the contact detonation were produced on the site of the University of German Armed Forces in Munich. The steel fiber content was defined to be 1.0 Vol.-%. The concrete was supplied as ready-mixed concrete and the steel fibers were already added in the concrete plant. 15 different batches were produced with different
Tobias Zircher et al. / Procedia Engineering 199 (2017) 2525–2530 Tobias Zircher / Procedia Engineering 00 (2017) 000–000
2527 3
concrete mixtures, steel fiber types, reinforcement systems and plate thicknesses. Of each series, three plates were produced to allow for the possibility of a statistical test evaluation (c.f. Table 1). Ingredients concrete mixture Cement II/A-M(V-LL)42,5N Cement II/A-M(V-LL)32,5N Flue ash Aggregate 0/4 NfGK Aggregate 0/8 NgGK Aggregate 8/16 NgGK Water superplasticizer Water/cement ratio Steel fiber KH DE35/0,55 N Steel fiber KH DE60/0,9 N Steel fiber Dramix 4D-65/60BG Consistency Plate thickness
Table 1. Concrete Composition Mix 1 – 5 Mix 1 Mix 2-3 330 kg/m³ ----400 kg/m³ 70 kg/m³ --893 kg/m³ 770 kg/m³ 132 kg/m³ 224 kg/m³ 629 kg/m³ 609 kg/m³ 224 kg/m³ 250 kg/m³ 1,32 kg/m³ --0,65 0,625 78,5 kg/m³ (Mix 1.3) 78,5 kg/m³ (Mix 2.2) 78,5 kg/m³ (Mix 1.2) 78,5 kg/m³ (Mix 2.3, 3.2) --78,5 kg/m³ (Mix 2.4) F4 F4 30 cm 30 cm
Mix 4-5 --400 kg/m³ --770 kg/m³ 224 kg/m³ 609 kg/m³ 250 kg/m³ --0,625 78,5 kg/m³ (Mix 4.2, 5.2) 78,5 kg/m³ (Mix 4.3) 78,5 kg/m³ (Mix 4.4) F4 20 cm
Three different steel fiber types (c.f. Table 1) produced by Krampe Harex GmbH & Co. KG with the manufacturer’s name KH DE35/0,55 N (length: 35 mm, diameter 0,50 mm, N = Steel, tensile strength = 1250 N/mm²) and KH DE60/0,9 N (length: 60 mm, diameter 0,90 mm, N = Steel, tensile strength = 1150 N/mm²) and Dramix 4D-65/60BG (length: 60,5 mm, diameter 0,90 mm, steel tensile strength = 1600 N/mm²) have been used. The medium cube concrete compressive strength was in the range of 30-35 N/mm². The contact detonation tests were carried out at the test facility on the site of the WTD 52. The plates are oriented vertically in the blasting cavern, (c.f. Fig. 2 (a)) and are subjected to the explosive. a
b
Fig. 2. (a) Test set up with installed plate; (b) explosive charge
The explosive charge consists of 850 g of PETN explosive, which is placed in a plastic tube with a height of 62 mm and an inner diameter of 103.6 mm on the center of the loaded surface (c.f. Fig. 2 (b)). After the contact detonation, a photo documentation is conducted. Subsequently, the aim was to determine the exact damage distribution, crater geometry and crack pattern on the unloaded surface. For this matter the impact-echo method was used which is a non-destructive testing method generally used for the detection of imperfections in reinforced concrete constructions or for example to measure the thickness of the inner lining of a tunnel or for the verification of foundation piles. The results of this measurements were published in [8].
Tobias Zircher et al. / Procedia Engineering 199 (2017) 2525–2530 "Tobias Zircher" / Procedia Engineering 00 (2017) 000–000
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a
b
c
Fig. 3. (a) Schematic drawing blast cavern; (b) measuring grid in the cavern with debris; (c) contact measuring sensors to detect an impact of debris at the back wall of the cavern
During the detonation, the debris flight is recorded at the side of the unloaded surface of the plate by two high speed cameras. The speed of the secondary debris can be calculated from the image rates and the distance from the fragments. Furthermore, measuring sensors are installed at the end of the blasting cavern, (c.f. Fig. 3 (c)) which document whether there is an impact of debris on their back wall and also allow a calculation of the speed of the debris from the detonation. The debris fragments were collected, measured and weighed after the tests, (c.f. Fig. 3 (b)). 3. First experimental results The first contact detonation tests have shown that the debris flight on the unloaded surface on the plates with steel fibers could be significantly reduced in comparison to the plates without steel fibers (c.f. Fig. 4 (a-c)). For the plates without steel fibers, the amount of debrided parts was up to 50 kg. For the plates with steel fibers, the amount of debrided parts was between 0.70 kg and 3.1 kg. The results were almost identical for all fiber types used. a b c
Fig. 4. Unloaded surface of a plate after contact detonation: (a) without steel fibers; (b) with Dramix 4D-65/60BG; (c) with KH DE60/0,9 N
After evaluating the debris flight and carrying out the 3D scanning, the plates were sawn in the centerline of the crater and the exact course of the damage crater became apparent (c.f. Fig. 5 (a-b)). The sectional images showed that the size of the damage center of the plates is not influenced by the steel fibers since the concrete failure is mostly dependent on the tensile strength. However adding steel fibers to the concrete mixture reduces debris flight in a great manner. a b
Fig. 5. (a) Sectional image from a plate without steel fibers; (b) and with steel fibers
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4. Design formula based on the theory of wave propagation Based on the experimental data at hand and by support of further experiments we intend to use the method of ‘Design of Experiments’. By the help of a full factorial model (set up from taking the actual experiments as design points) we will gather a semi-empirical formula which takes into account the concrete tensile strength, the type of steel fibers, the plate thickness and the amount of the explosive charge. This semi-empirical formula will be enhanced and validated by a numerical study which solves the wave-equation:
2u 2 G G 2u x
(1)
2v G G 2v 2 y
(2)
2w 2 G G 2 w z
(3)
Where = Lamé-constant, G = shear modulus, ² = laplace-operator, defined as:
2
2 2 2 x 2 y 2 z 2
(4)
The equations (1), (2), (3) are difficult to interpret. The displacement field can be described by the superposition of a vector field (distortional) and a scalar field (Lamé-potentials, dilatational) with the Helmholtz-approach:
1 •• 0 c 2p
i
(5)
1 •• i 0 cs2
(6)
The solution of equations (1) to (3) thus is split into a pressure wave part (c.f. (5)) and a shear wave part (c.f. (6)), which are characterized by the assigned pressure wave velocity cp and the shear wave velocity cs. The velocities of propagation are:
cp
cs
E 2G 1 (1 )(1 2 )
(7)
G
(8)
where = Poisson's ratio, E = Young’s modulus,
= density.
Tobias Zircher et al. / Procedia Engineering 199 (2017) 2525–2530 "Tobias Zircher" / Procedia Engineering 00 (2017) 000–000
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5. Numerical simulation Based on the results from the contact detonation tests, fluid dynamical analysis for the propagation in concrete are carried out by using the hydrocode Ansys Autodyn. The simulations are carried in order to perform a parameter study in which the plate thickness, the reinforcement system and the amount of explosive are varied. With the help of these studies further boundary conditions and parameters for the semi empirical formula will be investigated. As there are a lot of details in a hydrocode simulation which may potentially effect the solution such as time-step-size or the variability in the material parameters, this procedure is not a simple one. In the context of this paper we can only highlight some of these responsibilities: I. II. III.
investigate for a load model which represents the contact detonation load history properly, investigate for a material model of the concrete failure, which takes into account the rate dependence of the tensile strength fct and taking into account uncertainties in terms of the variability of the material parameters (Poisson's ratio , Young’s modulus E, density and concrete tensile strength fct).
This is of great importance, as on the impact side the whole wave characteristics strongly depend on the first three parameters (, E, ) whereas on the other hand the variability of the concrete tensile strength fct massively influences the damage characteristics. We expect that a model, which solves the three challenges, will describe the volume and the surface of the crater properly within the bounds of statistical uncertainty in comparison to the conducted experiments. 6. Conclusion It was apparent from the first contact detonation tests that the amount of debrided parts on the unloaded surface can be significantly reduced by the addition of steel fibers into the concrete mixture. The different geometries of the steel fibers had no significant influence on the reduction of the debris which was almost identical in all experiments. In the next step of the project, our work will be focused on numerical simulation and on the development of a semi empirical formula for the determination of the crater volume and its surface in dependence of the main input parameters in order to provide a mobile application for the soldiers who are in charge of the infrastructure during a mission. 7. Acknowledgements The authors thank the Federal Office of Bundeswehr Infrastructure, Environmental Protection and Services Section Infra II 2 for their support and the Bundeswehr Technical Center for Protective and Special Technologies (WTD 52) for providing their experimental facilities for this research. 8. References [1] Braml T., 2010, Structural reliability assessment of reinforced concrete bridges on the basis of the results from a bridge inspection Disstertation, University of German Armed Forces Munich, Chair for concrete construction, 2010 [2] Burbach A., 2015, Fiber reinforcement for conventional reinforced concrete elements; decrease of weapon effects by usage of steel fibers, 3rd International conference on Protective Structures (ICPS3) pp. 65-71, Newcastle, Australia [3] Fuchs M., Keuser M., Schuler H., Thoma K., 2007, Fiber Reinforced Concrete under High Dynamic Loading, Beton und Stahlbetonbau 102, Heft 11, pp. 759-769 [4] JCSS – Risk Assessment in Engineering, Probabilistic Model Code, Joint Committee on Structural Safety, 2008 [5] Lehmann S., Walz M., Heckersbruch A., 2015, Fiber reinforcement for reinforcing conventional armoring in reinforced concrete elements exposed to highly dynamic loads, 3rd International conference on Protective Structures (ICPS3) pp. 274-281, Newcastle, Australia [6] Studer J. A., Laue J., Koller M. G., 2007, Bodendynamik, Grundlagen, Kennziffern, Probleme und Lösungsansätze, 3., völlig neu bearbeitete Auflage, Springer, Berlin Heidelberg New York [7] Schulz T., Non-destructive testing of concrete structures with and without steel fiber reinforcement under highly dynamical stress and implementation of a finite element model, Master thesis, Chair of Concrete and Masonry Structures, Technical University of Munich, 2016 [8] Zircher T., Keuser M., Schulz T., Burbach A., 2017, Non-destructive testing of protective components after contact detonation, fib symposium, Maastricht, accepted paper