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CONTACT CHARGE BLAST PERFORMANCE OF FIBER REINFORCED AND POLYUREA COATED CONCRETE VEHICLE BARRIERS
ABSTRACT
Among the measures used to protect personnel from terrorist attacks are massive concrete vehicle barriers which form a perimeter around the protected establishment. Typical concrete barriers can be brittle and will break apart easily when exposed to a close-in blast. Fiber reinforced cementitious composites (FRCCs) can be used to overcome this drawback since fibers in a concrete mix carry tensile forces that keep fragments from breaking off. Another promising solution is a polymer coating, such as polyurea, which can introduce added ductility and contain fragments of concrete from flying away. Full scale barriers of traditional concrete, two fiber reinforced concretes, two fiber volumes of high performance fiber reinforced cementitious composite, and polyurea coated concrete were tested under contact charge blast loads to assess their performance against a known threat. The fiber reinforced concrete barriers performed much better than their fiberless counterparts with only 7-18% of material being ejected from the barrier compared to 41% in the control barrier. The 0.5 mm (0.02”) polyurea coating only slightly improved the performance of the barriers with 36% of material being ejected. An analytical model using the finite element code LS-DYNA was found to be able to predict crater dimensions and relative damage with reasonable accuracy.
TABLE OF CONTENTS
LIST OF FIGURES ………………………………………………………………………………………..vi
LIST OF TABLES………………………………………………………………………………………….ix
ACKNOWLEDGEMENTS……………………………………………………………………………..x
Chapter 1 Introduction……………………………………………………………………………………1
1.1 Problem Statement………………………………………………………………………………4
1.2 Objectives ………………………………………………………………………………………….4
1.3 Scope…………………………………………………………………………………………………5
Chapter 2 Background ……………………………………………………………………………………6
2.1 Blast Barriers ……………………………………………………………………………………..6
2.2 Blast Loads ………………………………………………………………………………………..8
2.3 Blast Resistant Design (TM5-1300 1990)………………………………………………10
2.3.1 Loads ……………………………………………………………………………………….11
2.3.2 Flexural Capacity……………………………………………………………………….12
2.3.3 Shear Capacity…………………………………………………………………………..14
2.4 Fiber Reinforced Concrete (FRC) …………………………………………………………15
2.4.1 Lab Testing of CFRC …………………………………………………………………21
2.4.2 Blast Testing of CFRC ……………………………………………………………….25
2.5 High Performance Fiber Reinforced Cementitious Composite (HPFRCC)…28
2.6 Polyurea Coatings……………………………………………………………………………….32
2.7 Fiber Reinforced Concrete Modeling…………………………………………………….36
2.7.1 Micromechanical Approach ………………………………………………………..36 2.7.2 Phenomenological Approach (Lemaitre and Chaboche 1990)………….40
2.7.3 LS-DYNA Material Models………………………………………………………..44
2.7.4 Strain Rate Effect ………………………………………………………………………49
2.8 Polyurea Modeling………………………………………………………………………………52
Chapter 3 Blast Test Procedure………………………………………………………………………..54
3.1 Barrier Fabrication………………………………………………………………………………54
3.2 Blast Testing………………………………………………………………………………………57
Chapter 4 Blast Test Results……………………………………………………………………………61
4.1 Barrier Damage…………………………………………………………………………………..61 4.2 Debris Fields………………………………………………………………………………………68
4.3 Discussion………………………………………………………………………………………….70 Chapter 5 Modeling Procedure………………………………………………………………………..72
5.1 Model Description ………………………………………………………………………………72
5.2 Loading……………………………………………………………………………………………..74
5.3 Material Models………………………………………………………………………………….75
Chapter 6 Modeling Results…………………………………………………………………………….78
6.1 Barrier Damage…………………………………………………………………………………..78
6.2 Validation ………………………………………………………………………………………….84
6.2.1 Damage Comparison ………………………………………………………………….84
6.2.2 Pressure History…………………………………………………………………………86
6.2.3 High Speed Video………………………………………………………………………87
6.3 Discussion………………………………………………………………………………………….89
Chapter 7 Conclusions……………………………………………………………………………………93
References……………………………………………………………………………………………………..99
Appendix A Barrier Drawings…………………………………………………………………………103
Appendix B LS-DYNA Material Input Cards ……………………………………………………105
B.1 Traditional 45 MPa (6500 psi) Concrete………………………………………………..105 B.2 45 MPa (6500 psi) Carbon Fiber Reinforced Concrete (CFRC) ……………….105 B.3 ASTM A615M Grade 420 Reinforcing Steel …………………………………………106
B.4 Polyurea ……………………………………………………………………………………………106
Appendix C Debris Field Mapping…………………………………………………………………..107
Chapter 1
Introduction
In recent years, the threat of global terrorism has heightened the need for perimeter security around critical establishments. Installations such as embassies, military bases, and critical infrastructure rely on a strong perimeter to prevent terrorist attacks from causing widespread destruction in the form of structural damage and human casualties. A critical installation, either at home or abroad, is often protected from vehicle entry with massive concrete vehicle barriers. These are designed to limit the damage caused by threats such as vehicle-born improvised explosive devices (VBIEDs) by increasing the standoff distance from the center of the blast to the intended target.
However, as the threat of terrorism continues to evolve, so must the technology used to defend against it. An initial attack on the perimeter defenses can be done with a human carried explosive detonated in contact with a barrier, in some cases causing a breach in the perimeter. A vehicle containing explosives is then free to pass through the breach and attack its target. In addition to the secure perimeter being breached, any attack on a barrier can cause flying secondary debris that can harm personnel and damage buildings and critical infrastructure.
Concrete has traditionally been used in many protective applications due to its high mass per unit cost, allowing large barriers to be constructed economically. However the drawback of concrete as a building material lies in its brittle nature. Concrete’s inability to carry large tensile stresses means the material is prone to spalling, the ejection of material from the front face, and scabbing, the loss of material from the back face (Bangash and Bangash 2006). Even concrete reinforced with steel can exhibit spalling and scabbing since the member will only behave in a ductile manner when it responds as a unit. Extreme loadings such as blasts cause localized effects which prevent the structure from behaving as a unit and allow for unreinforced areas to be ejected.
One means of overcoming this behavior in concrete is to use a mix containing one or more types of short discontinuous fibers. Randomly dispersed and oriented fibers in a concrete mix have been shown to increase impact strength and fractures toughness (ACI 544 1996), improve tensile strength and ductility (Akihama et al. 1984), and better resist spalling (Lan et al. 2005). These fibers can be made of steel, glass, synthetic polymers, or natural materials. Much research has been carried out on steel fiber reinforced concrete (Luo et al. 2000, Lan et al. 2005, Agardh 1997, Song et al. 2005) and with some types of synthetic fibers (ACI 544 1996, Maalej et al. 2005) under dynamic loading. This research has been carried out mostly with fibers around 25 mm (1”) in length or less. Longer fibers typically have not been studied due to problems with balling of the fibers during mixing (ACI 544 1996).
A coating was developed for carbon fiber yarn allowing fibers of lengths up to 75 mm (3”) to be incorporated into a mix by increasing the initial fiber rigidity (Musselman 2007). This coating has also been applied to individual nylon fibers that are also cut to 75 mm (3”) in length. After the rigid fibers are mixed into the concrete, the coating dissolves and cement paste surrounds the individual fibers before hardening. With correct mix proportioning, mixes of carbon (CFRC) or nylon (NFRC) fiber reinforced concrete can achieve good workability and fiber dispersion. The greater fiber lengths cause more energy to be released during fiber pullout, resulting in greater post peak ductility of the composite. Concrete mixes of carbon or nylon fibers have been successfully implemented with fiber volumes of 2.5% for members without reinforcement and 1.5% for members containing reinforcement (Musselman 2007).
Greater volumes of fibers in a concrete mix will typically cause the material to exhibit greater tensile strength and ductility since there are more fibers bridging each crack. The difficulty arises in incorporating the fibers into a workable mix. Since the large amount of fibers requires more cement paste to coat them, these mixes often contain no course aggregate. Mixes have also been engineered to achieve an optimal balance between high modulus fibers (e.g. carbon and steel), which improve strength, and low modulus fibers (e.g. polypropylene and polyethylene), which improve ultimate strain
(Maalej et al. 2005). These concrete mixes are sometimes called High Performance Fiber Reinforced Cementitious Composites, or HPFRCCs, when they exhibit properties superior to normal fiber reinforced cementitious composites, such as strain hardening in uniaxial tension (Naaman and Reinhardt 2005).
Other measures can be taken to mitigate spalling and increase strength in barriers. Recent full scale blast tests of polyurea coated masonry walls demonstrated the polymer’s ability to introduce ductility and contain secondary fragments in the system (Davidson et al. 2005). Polyurea can be sprayed onto concrete in layers or cast on the surface until the desired thickness is reached. Though polymeric coatings do not actually improve the spall resistance of the concrete itself, they have the potential to contain fragments of concrete that break off during a blast so they will not become a flying hazard. The polymer’s bond to the concrete also allows for composite action and the system as a whole can behave in a more ductile manner.
1.1 Problem Statement
This study will determine if traditional concrete, carbon and nylon fiber reinforced concrete, high performance fiber reinforced cementitious composite, and polyurea coated concrete vehicle barriers are successful in preventing vehicle entry and minimizing secondary debris after being attacked by a known in-theater explosive threat.
1.2 Objectives
The primary objective of this study is to find the suitability of traditional concrete, fiber reinforced concrete, high performance fiber reinforced cementitious composite, and polyurea coated concrete vehicle barriers in preventing vehicle entry after an explosive is detonated in contact with the barrier. The secondary objective is to compare the debris fields created from the blasts to understand which barrier types are more effective in minimizing the threat of secondary fragments. The tertiary objective is to assess the accuracy of an analytical model used to predict the behavior of several different barrier types (traditional concrete, carbon fiber reinforced concrete, and polyurea coated concrete) under contact charge blast loading.
1.3 Scope
The scope of this research includes full scale blast tests of traditional concrete, carbon and nylon fiber reinforced concrete, two fiber volumes of high performance fiber reinforced cementitious composite, and polyurea coated concrete vehicle barriers. The barriers were placed in a chain and a charge of C-4 explosive was detonated at the center in contact with the barrier. Due to the extreme heat and pressure generated by such a blast, the only instrumentation that was running during the test was a free field pressure gauge and a high speed video camera. Post blast data was documented with digital photographs, permanent deflection measurements, and debris field mapping of all fragments of diameter 125 mm (5”) and larger.
An analytical study was also carried out for the materials where sufficient laboratory data existed to adequately define the parameters of the material model. These included traditional concrete, carbon fiber reinforced concrete, and polyurea coated concrete. The analytical models were developed with the Lagrangian finite element code LS-DYNA (LSTC 2007) using the concrete material model “Continuous Surface Cap Model” (Murray 2007b). An element erosion algorithm was invoked and the model was validated against experimental data by comparing the damage observed from the specimens with that observed from the models. Recommendations for modeling concrete vehicle barriers under blast loads were also made so future barrier designs may not have to be tested experimentally to be proven adequate for service.
