FIBRE REINFORCED HIGH STRENGTH CONCRETE WITH CELLULAR STEEL SANDWICH COMPOSITE PANEL FOR BLAST AND IMPACT MITIGATION ABRAHAM CHRISTIAN NATIONAL UNIVERSITY OF SINGAPORE 2015 FIBRE REINFORCED HIGH STRENGTH CONCRETE WITH CELLULAR STEEL SANDWICH COMPOSITE PANEL FOR BLAST AND IMPACT MITIGATION ABRAHAM CHRISTIAN (B.Eng.(Hons.), Diponegoro University) A THESIS SUBMITTED FOR THE DEGREE OF DOCTOR OF PHILOSOPHY DEPARTMENT OF CIVIL AND ENVIRONMENTAL ENGINEERING NATIONAL UNIVERSITY OF SINGAPORE 2015 Declaration Page DECLARATION I hereby declare that this thesis is my original work, and I have written it in its entirety. I have duly acknowledged all the sources of information which have been used in the thesis. This thesis has also not been submitted for any degree in any university previously. Abraham Christian February 2015 i Acknowledgements All praise to God, only by His grace this study can be completed. The author wishes to express his sincere gratitude to his supervisors, Assoc. Prof. Ong Khim Chye, Gary for his personal commitment, patience, interesting discussion, invaluable guidance and constructive advices throughout the course of this study. The author’s heartfelt appreciation is dedicated to Dr. Lado R. Chandra, Dr. Patria Kusumaningrum and Dr. Satadru Das Adhikary for their contributions and continuous supports. Sincere thanks are also extended to the DSTA for providing the funding required for blast experimental testing. The kind assistance from all the staff members of the NUS Concrete and Structural Engineering Laboratory is deeply appreciated. Special thanks go to Mr. Koh, Mr. Ang, Mr. Ishak, Mr. Stanley and Mr. Kamsan for their continuous support during experimental phase of the study. Finally, special thanks and loves go to my parents, brother, sister and friends for the moral supports and constant love. Thank you for making this study possible and may God bless all of you. ii Table of Contents Declaration Page i Acknowledgements ii Table of Contents . iii Summary viii List of Tables . x List of Figures . xi List of Symbols xviii Chapter Introduction Background of the study . Literature review . Composite panel as an energy absorber . Metal sandwich panels Steel-concrete-steel (SCS) sandwich panels 12 Fibre composite panels . 14 Steel stud composite panel . 18 Multi-material layered composite panel . 19 Observations arising from the literature review . 20 Objectives and scope of study . 22 iii Thesis structure . 24 Chapter Proposed Composite Panel 26 Design criteria 26 Proposed composite panel design 29 Constituent composite sections 30 Steel sandwich core structure design 32 Steel sandwich fabrication techniques 35 Connection details for handling and installation 37 Finalized panel design . 37 RC specimen fabrication for performance comparison 38 Material usage and cost comparison 39 Fabrication process . 40 Chapter Finite Element Analysis . 43 Introduction . 43 General parameters of EASP FE model . 44 Mesh convergence study . 44 Development of EASP model . 45 Material Models 47 Strain rate effect 59 Contact model . 63 Hourglass . 64 Bulk viscosity for dynamic loading case 65 iv Erosion criteria for solid elements 66 Other LS-Dyna keyword . 66 Validation of FE model 66 Numerical study on the EASP energy absorption . 68 Simulation of three point static bending of EASP and RC panels 72 Simulation of EASP specimens subjected to impact loading . 74 Simulation of EASP specimens subjected to blast loading 78 Blast wave loading from cylindrical charges . 79 Summary of finite element analyse of EASP specimens . 83 Chapter Static Performance of EASP . 84 Introduction . 84 Composite panel under static loading . 84 Specimen details . 85 RC specimen fabrication for performance comparison 85 Moment capacity analysis . 86 Testing setup and instrumentation . 92 Results and discussion . 93 Load deflection response 93 Failure mode and cracking behaviour of the concrete layer 95 Steel plate strain data . 98 Effect of concrete strength and steel fibre incorporation 100 Comparison to analytical and numerical result . 101 v Conclusions 108 Chapter Impact Performance of EASP . 110 Introduction . 110 Impact studies on reinforced concrete and steel-concrete composite panel 111 Failure modes 113 Testing method . 115 Specimens 115 Test set-up . 117 Results and discussion . 119 Crack propagation and final crack pattern 120 Damage in the concrete and steel-sandwich layers 127 Displacement-time history 132 Strain measurement on the steel sandwich interface and distal plates . 138 Comparison with numerical results 142 EASP with full core configuration 142 EASP with hollow core configuration 145 Discussion 149 EASP with various concrete materials . 149 EASP full core versus hollow core . 150 EASP versus RC 151 Conclusions 151 Chapter EASP Blast Resistance 153 vi Introduction . 153 Air blast wave in explosions 153 Composite structure subjected to blast loading . 160 Testing method . 163 Specimens 163 Experimental set-up 164 Results and discussion . 165 Pressure histories 168 Displacement-time history 174 Damage assessment 175 Conclusions 177 Chapter Conclusions and Recommendations 179 Review on completed research . 179 Conclusion summary . 179 Recommendations for future studies . 183 References 185 Appendices . 190 Publications 191 vii Summary The study investigated the performance of concrete-steel composite panels subjected to static and dynamic loading. The panel consists of fibre-reinforced high strength concrete overlaid on top of a specially configured steel sandwich, which function to dissipate the imparted kinetic energy. The core structure of the steel sandwich resists mostly tensile forces while absorbing the energy through plastic buckling. The novel composite panel was assigned the name Energy Absorption Sandwich Panel or EASP. The intention was to use it as a secondary protection or sacrificial cladding panel in existing buildings to shield critical structural components against blast. Experimental testing was done in three phases; three point static bending tests, lowvelocity drop-weight impact tests and close-in cylinder TNT explosion tests. Parametric studies were carried out utilizing two types of steel sandwich core structure (bended and straight type) as well as various concrete materials: normal strength concrete (NSC) 60 MPa, high strength concrete (HSC) 110 MPa and fibre reinforced high strength concrete (FRHSC) 110 MPa. Ordinary reinforced concrete panels of the same geometric dimensions and tension capacity cast with HSC and FRHSC were also included for benchmarking. The RC panel was on average about 33% heavier than the EASP. The response of the EASP against static bending, impact and blast loading was obtained through experimental tests. The performance assessed by observing the damage to the constituent concrete and steel sandwich layers, comparing the maximum and residual deformation and observing the failure modes of the different types of panels. It was found that under static bending tests, the maximum flexural resistance did not differ much although the EASP could achieve much higher ductility vis-à-vis the high strength viii of the blast, the design adopted with respect to the concrete material and type of the core structure used, support configuration, shape and size etc., different responses are expected. FIGURE 6-18 FAILURE MODE OF THE EASP1-C110F SPECIMEN The damage to the concrete layer for the EASP1-C110F specimen can be seen in Figure 6-18. Buckling of the core extended around the center of the span. As shown in Figure 6-7 the concrete layer delaminated fully on one side of the panel with end plate bend out of shape. The shear connector used was not sufficient to maintain the composite action between the concrete and the steel sandwich layer. On the other side of the span, there was also delamination of the concrete layer, without detachment and less deformation of the end plate. Although the panel experienced severe deformation, it was not breached, and very little concrete spalling was observed. The fibre reinforced concrete present in the concrete layer seemed to have minimized the propagation of cracks while keeping the crushed concrete portions intact, hence minimizing concrete spalling and fragmentation. The steel sandwich layer seemed to have absorbed the blast energy through plastic buckling, it experienced severe core compression; the steel rebar attached to the end plate was sheared off and the side steel plate connection separated cleanly from the side cover plate and was bend out of shape due to the large bending deformation. The global failure 176 mode observed was one of bending failure. The proposed EASP performed rather well even with such a severe blast load. FIGURE 6-19 EASP1-C110 SPECIMEN BLAST TEST RESULT The EASP1-C110 specimen failed in shear. The failure may be due to the brittleness of high strength concrete. The experimental result corresponds well with the numerical prediction (see Figure 6-20). FIGURE 6-20 EASP1-C110 NUMERICAL MODEL The mode of failure seems similar to that reported by Low and Hao (2002) associated with slabs subjected to high blast load amplitude. They suggested that RC slabs tend also to fail in the direct shear mode if it is relatively stiffer and with a smaller span length. Conclusions In the third phase of the present study, two EASP1 specimens were subjected to close-in blast loading. The EASP core type specimens with C110 and C110F concrete as the 177 concrete layer were subjected to cylindrical shaped, kg TNT explosion at a SoD of meter. EASP1-C110F specimen was tested with a perpendicular charge configuration resulting in severe damage as well as large deflections. The present findings suggest that the actual blast pressure generated may be more than ten times that of a spherical charge of the same mass. Despite this the specimen was able to withstand the blast with minimum concrete fragmentation of the concrete layer. It may indicate that the proposed panel is feasible to be used as sacrificial cladding panel subjected to close-in blast loading. The blast test of EASP1-C110 specimen was done with equal kg cylindrical TNT with m SoD, but the orientation was changed into parallel charge placement. It was found that the uniform blast pressure generated was similar to 0.85 times the equivalent spherical shaped charge. The failure mode of the specimen was predominantly in shear. The FE model developed was calibrated with the experimental results wherever possible. The numerical model of EASP1-C110F and EASP1-C110 specimen was able to predict the experimental residual displacement with good accuracy. 178 Chapter Conclusions and Recommendations Review on completed research A composite panel (EASP) comprising a fibre reinforced high strength concrete receptor layer cast on top of a steel sandwich layer was proposed to be utilized as blast and impact mitigation panel based on some predetermined design criteria. Three phase study involving three-point static bending tests, low-velocity drop weight impact tests and closein blast loading tests on the proposed EASP and RC specimens have been completed. Parametric studies involved two types of steel sandwich core structures (type bended cores and type straight cores), three cementitious mix designs of NSC (C60), HSC (C110) and FRHSC (C110F). RC panels cast with similar HSC and FRHSC mixtures were also fabricated for benchmarking purposes. Finite element (FE) modelling using the LS-Dyna software was done for each of the loading cases. The results and conclusions are summarized. Conclusion summary This study has examined the performance and the feasibility of the proposed EASP for use as blast mitigation precast panels. The EASP was designed to have higher performance than ordinary reinforced concrete panels with the fabrication cost kept as low as possible. Hence, in the development process, fabrication of the steel sandwich layer utilized a slot joints system that enables conventional welding. In addition, the top concrete layer can be cast using conventional concrete casting process as per conventional precast panels. The design also integrates a set of shear connector plates protruding from the steel sandwich core into the concrete layer to hold the mesh reinforcement bars in place before casting. 179 RC specimens were also fabricated of the same geometric dimensions and tension steel capacity. Based on the total weight of the specimens fabricated, there was an approximately 33% reduction in self weight in the case of EASP compared with the RC panels. The reduced mass is an advantage when such panels are used as secondary protective panels. In finite element modelling of the EASP, concrete is modelled using MAT 72R3 as H8 solid elements with different element formulation (Type for static and blast loading simulation and Type -1 for impact simulation). Material parameters of the concrete were acquired from available experimental data. Hourglass control Type for blast and impact, Type for static simulation are applied in the model with fine-tuned hourglass coefficients to stabilize the reduced element integration and to acquire accurate crack growth and propagation simulation by the FE model. Steel plates are modelled as solid and shell structural elements with the MAT3 Plastic Kinematic material model. The CowperSymonds relation scale up the yield stress based on the strain rate experienced was used to model the dynamic increase factor of the steel material. Reinforcement bars are modelled using MAT3 Plastic Kinematic as beam-truss elements that only can resist axial loads. Erosion is introduced to all the EASP and RC panel FE models using threshold strains to prevent negative volume errors due to the presence of highly distorted elements and to properly capture the inelastic behaviour of the specimens. The connections between the concrete layer and the steel sandwich layer makes use of two types of contact models, simply joined node and surface to surface contact to account for the different connection properties with the appropriate static or dynamic friction coefficients as necessary. The FE simulation results obtained was able to represent the experimental results with acceptable accuracy. From the calibrated FE model, further study of EASP with different core structure, thickness, width, length and optimization of the concrete-steel sandwich layer 180 thickness can be done to obtain design guidelines useful for the EASP implementation in real structure. However, due to time constraint, this may be done for future study. The three-point static bending tests conducted as the first part of experimental test investigated the performance and benchmarked the proposed EASP against conventional RC panels. Parametric study was carried out, utilizing two types of steel sandwich core structure (type and type 2) and three types of concrete material. The primary modes of global failure were flexural. The cellular steel sandwich layer utilized in the EASP specimens increased the ductility of the panels tested while maintaining composite action. It was found that the EASP2-C110 specimen outperform the RC-C110 specimen in terms of ultimate strength and ductility. EASP2-C110F specimen performed similarly to the RCC110F specimen based on results of the FE simulation. The EASP type specimens exhibited higher bending resistance than similar EASP type specimens, especially when FRHSC is used. Overall, all the EASP specimens tested could achieve much higher ductility compared to the RC panels. The drop weight impact test forming the second part of the study investigated the performance of proposed EASP against impact loading. The loading used were an 800 kg steel hemispherical projectile with 2.5 m of free fall height. The impact velocity and impact energy calculated were 6.26 m/s and 15.68 kilo Joules respectively. It was found that the EASP and performances surpass that of the ordinary reinforced concrete panel. The EASP showed better performance than EASP in terms of maximum and residual displacement. Comparing NSC and HSC concrete used in the EASP concrete layer, the specimens used HSC exhibited more damage in the concrete layer but with less residual deflection. This can be attributed to more energy absorbed by concrete layer through cracking. 181 Type H specimens with a hollow core structure in the central zone below the projectile impact location was tested to observe the effects of more localized damage at the impact zone. It was observed that the EASP ’H’ specimens exhibited punching shear damage in the concrete layer, produced with a lower impact velocity than the EASP specimens with conventional core configuration. Although the concrete layer was fully breached with more extensive and severe cracking in the case of the EASP ‘H’ specimens cast with C60 and C110 concrete in the concrete layer, the panel itself was not breached. These results suggest that the proposed panel was able to resist breaching under impact of 800 kg mass at a velocity of 5.94 m/s. This finding is significant because the EASP uses less material but exhibits better performance, vis-à-vis the control RC specimens. In the third phase of the present study, two EASP1 specimens were subjected to close-in blast loading. The EASP core type specimens cast using C110 and C110F in the concrete receptor layer were each subjected to a cylindrical shaped, kg TNT explosion at a SoD of meter. The EASP1-C110F specimen was tested with a perpendicular charge configuration resulting in severe damage as well as large deflections. The present findings suggest that the actual blast pressure generated may be more than ten times that of a spherical charge of the same mass. Despite this the specimen was able to withstand the blast with minimum concrete fragmentation of the concrete layer. It suggests that the proposed panel is feasible for use as a sacrificial cladding panel subjected to close-in blast loading. The blast test of EASP1-C110 specimen also involved a kg cylindrical TNT at m SoD. However, the orientation was changed into parallel charge configuration. It was found that the uniform blast pressure generated was similar to 0.85 times that of an equivalent spherical shaped charge. The failure mode of the specimen was predominantly in shear. Based on the static and dynamic response of the proposed EASP, combining a receptor layer cast with fibre reinforced high strength concrete on top of a cellular steel sandwich 182 layer is very efficient in absorbing close-in blast and low velocity-large mass impact energy. The EASP is feasible to be used as blast and impact mitigation panel due to better performance when subjected to drop weight impact and close-in blast loading, better efficiency in material usage, and low cost fabrication design employed. The present study serves as a preliminary study in multi-layer composite panel that give a novel way in utilizing ordinary material with greater efficiency. The EASP design may provide better alternative that can lead to sustainable approach when designing structural protection for ordinary, existing structures. However, the current design development of the EASP is still not the optimum design as no parametric study has been done on the thickness and core dimension. Further efficiency may be achieved by balancing the steel sandwich and concrete layer thickness as well as putting more core in the steel sandwich layer. Further research in connection designs and various dimension parameters must be done in order to complete the EASP as blast and impact mitigation system. Recommendations for future studies 1. The bond-slip characteristic between the concrete and the steel sandwich layer requires further investigation to come up with more accurate analytical and numerical models to predict the EASP response. 2. Further research can be carried out to obtain the relationship between panel performance and types of shear connector, other core structure configuration, different panel dimensions as well as concrete and steel sandwich layer depth optimization for given loading arrangements. Calibrated numerical model can be employed for future parametric studies to suit the varied requirements such blast and impact mitigation panels when used in practice. 183 3. Improvement of the panel connection designs is necessary in order to prevent premature shear failure under dynamic loading, especially if the span of the panel is longer and support conditions change depending on how they are deployed as sacrificial protection panels in actual building. 4. 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(listed in Procedia Engineering Journal, DOI: 10.1016/j.proeng.2014.12.174) 191 [...]... review Composite panel as an energy absorber The subsequent section review some studies conducted involving several types of composite panel structures that have been used as blast and impact protection panels or armour They include metal air sandwich panels or cellular metal panels, steel- concretesteel sandwich panels, fibre composite panels, steel stud composite panels and multi-layer composite blast panels... predict the panels’ response against static and dynamic loading with good accuracy The combination of fibre reinforced high strength concrete and steel sandwich structure demonstrated good potential for use as blast and impact mitigation protective panels with better weight-performance ratios and enhanced energy absorption properties Keywords: steel -concrete composite, drop-weight impact, blast loading,... concrete types and sandwich cores were able to withstand 800kg drop weight impact loading at an impact velocity of 6.26 m/s The similar high strength reinforced concrete panel failed with full projectile penetration These demonstrated that EASP performed better and possess higher energy absorption capacity compared with the RC panel Last but not least, when subjected to close-in TNT blast loading,... increase factor for tensile strength DIFf ' c Dynamic increase factor for compressive strength DIFE Dynamic increase factor for elastic modulus Etan Plastic tangent modulus Fc Compression concrete force xviii Fsc Compression steel force Fst Tension steel force f 'c Uniaxial compressive strength of concrete f co Reference strength of 10 MPa f cd Design strength of concrete f cm Mean strength of concrete f... Energy Absorption Sandwich Panel EIFS External insulation and finish system xx ELFORM Element Formulation EMP Electro-Magnetic Pulse SS EN Singapore Standard - European Norm EOS Equation of state FE Finite element FIP Fédération Internationale de la Précontrainte FRHSC Fibre reinforced – high strength concrete FRP Fibre Reinforced Polymer GFRP Glass Fibre Reinforced Polymer HG Hourglass HRWA High range water-reducing.. .reinforced concrete panel Under the drop weight impact tests, the EASP exhibited improved resistance with reduced damage compared with the RC panels The better performance of EASP with FRHSC concrete material is characterized by less residual and maximum deflection, reduced concrete fragmentation and large reduction in concrete crack propagation The EASP in general cast using various concrete. .. contributing to the overall panel strength and can be formed using various core configurations The combination of the core structure with outer skins creates a panel with superior stiffness and lightweight characteristics The sandwich skin plates are designed to withstand bending and axial stresses whereas the cores are designed to resist transverse shear stress FIGURE 1-4 SANDWICH PANEL TYPICAL GEOMETRY... materials for use as the core and skin plates as well as core structure configuration makes the sandwich panel fully customizable to meet the desired design requirements Until recently, the majority of sandwich panels have been designed with outer skins of fibre -reinforced polymer composites or thin sheet metals sandwiching a structural foam, timber or elastomeric cores The development of all-metal sandwich. .. FRHSC (BrightHub-Eng., 2011) 15 Figure 1-11 Steel stud wall construction (before and after blast) 18 Figure 2-1 Early Design of Energy Absorption Sandwich Panel (EASP) 29 Figure 2-2 Basic principle of the proposed panel 30 Figure 2-3 (a) Typical Structure of SCS; (b) Cellular Sandwich Panel 33 Figure 2-4 Six Types of Steel Sandwich Core Configuration 33 Figure 2-5 Groups... metal sandwich panels: pyramidal truss, square honeycomb and folded panels The plates were clamped along their sides and subjected to a uniform impulsive load They found that all three types of sandwich plates were capable of sustaining a larger blast when compared with the corresponding solid plates of equal mass These seem to indicate that there is huge potential for the use of all metal sandwich panel . FIBRE REINFORCED HIGH STRENGTH CONCRETE WITH CELLULAR STEEL SANDWICH COMPOSITE PANEL FOR BLAST AND IMPACT MITIGATION ABRAHAM CHRISTIAN . NATIONAL UNIVERSITY OF SINGAPORE 2015 FIBRE REINFORCED HIGH STRENGTH CONCRETE WITH CELLULAR STEEL SANDWICH COMPOSITE PANEL FOR BLAST AND IMPACT MITIGATION ABRAHAM CHRISTIAN (B.Eng.(Hons.),. Literature review 7 Composite panel as an energy absorber 7 Metal sandwich panels 7 Steel -concrete- steel (SCS) sandwich panels 12 Fibre composite panels 14 Steel stud composite panel 18 Multi-material