Structural Concrete Innovations: A Focus on Blast Resistance Hershey Lodge Preconference Symposium 17 March 2008

Blast Overview  Blast can effect structure in multiple way –Air blast –Drag –Ground shock –Primary and secondary fragmentation –Fire

Blast Loading  Air blast design can be governed by max pressure, impulse, or combination –Function of size of explosive, standoff distance, and structure

Air Blast Loads  Properties of the air blast load a function of the: –Size and shape of explosive –Distance to explosive –Orientation of specimen –Type of blast l Free air burst l Ground burst l Contained burst

Scaled Distance  Convert explosive to equivalent weight of TNT  Determine scaled distance using Z = D / W^(1/3) where Z = scaled distance W= equivalent TNT weight D = distance between specimen and explosive  Use figures in references (TM5-1300): “Structures to Resist the Effects of Accidental Explosions” –determine the expected peak pressure and impulse for determined scaled distance

Scaled Distance Figure 2-7 TM5-1300

Types of Cross Sections  TM5-1300: 3 types of cross sections –Type I: l Concrete is sufficient to resist compressive component of moment l Cover remains undamaged –Type II: l Concrete is no longer effective at resisting moment l Equal top and bottom reinforcement l Cover remains in tact l Single leg stirrups used to resist shear –Type III: l Equal top and bottom reinforcement l Cover disengages l Lacing used to resist shear

Example Type II Cross-Section

Motivation for Innovation in Blast Resistant Concrete  Increased demand for impact and blast-resistant building materials  Need for practical, constructible options  Need for reduction in secondary fragmentation

Innovation  Long (3”) fibers –Increased bond with concrete matrix –Length provides crack bridging, spalling resistance, increased ductility, energy absorption (through long-fiber pull-out)  Coated “tape” –Mix retains workability (no balling, etc) –Can be used with aggregate  Potentially economical –Carbon fiber yarn is waste product from the aerospace industry  No special mixers required –Lightweight “additive” reinforcement –Precast or cast-in-place  Molds to any shape

Experimental Program  Mix design development –Workability  Static flexural strength –Small and large scale –Ductility  Impact testing –Small beams –Panels  Blast Testing  Finite Element Modeling

Experimental Program  Mix design development –1.5% to 2.5% fiber content (by volume) –Various admixture combinations –Pozzolans (interground SF + GGBFS)

Preliminary Testing  Mixture Design –Avoid balling –Increase workability –Increase fines and cement in mixture  Preliminary Static Tests –6” X 6” X 18” beams loaded at third points –Flexural Strength = 2112 psi

Slab Strips  4” X 12” X 10’ slab strips loaded at midspan  Specimens: –2 control specimens with reinforcing mesh –2 fiber reinforced concrete specimens –2 fiber reinforced concrete specimens with mesh  Used to obtain load vs. deflection plot  Useful for obtaining toughness

Slab Strip Results Compressive Strength (psi) Tensile Stress (psi) Toughness (lbs-in) Average Plane + mesh Average Fiber Average Fiber + mesh

Impact Test Setup  15 ft maximum drop height  50# weight  Panels 2’x2’x2”

Impact Testing: Panels Drop Height at failure

Impact Testing: Panels Drop Height at first cracking (top side)

Impact Testing: Panels (No Steel Reinforcement)  Fiber addition controlled spalling  Failure in fiber specimens along weak plane due to fiber orientation Plain panel Fiber panel

Impact Testing: Panels (Steel Reinforcement)  Fiber panel with steel reinforcement did not fail after repeated blows at top drop height Plain panelFiber panel

Blast Testing  6’ x 6’ x 6.5”  Heavily reinforced (as per TM5-1300) –resist shear failure at supports –evaluate comparison of materials under full blast design –Identical reinforcement in all specimens  Clear cover ¾” to ties

Test Setup  Slabs were simply supported on all four sides  Restraint provided along two sides to prevent rebound

Test Setup  TNT suspended at desired height  Pressure gages record reflected pressure and incident pressure

Hit 1: 75# at 6’ (scaled range 1.4) Extensive cracking, some spalling A few hairline cracks Standard Concrete SafeTcrete

Hit 2: 75# at 3.2’ (scaled range 0.76) Standard Concrete SafeTcrete

Hit 2: 75# at 3.2’ (scaled range 0.76) Standard Concrete SafeTcrete Some concrete loss due to pop out where reinforcement buckled (3/4” cover) Concrete rubble within steel cage

Hit 2: 75# at 3.2’ (scaled range 0.76) Standard Concrete SafeTcrete

Summary of Impact & Blast Testing  Much improved workability and dispersion of coated tape fibers  Increased ductility over plain concrete and further improved combined with standard reinforcement  Significantly increased flexural strength under both static and impact loads  Complete control of spalling in panels under impact load  Excellent performance in blast testing

Potential  Low cost fiber alternative  Applications requiring impact and blast resistance –Protective cladding panels –Structural components: columns, walls –Barriers –Bridge piers  May be used as a replacement for, or in combination with standard reinforcement depending on application

Material Properties  Stress-strain curves for material in both compression and tension needed for modeling –Compression: standard 6” diameter cylinders –Tension: dogbone specimens will be utilized –Varied load rates and fiber orientation

Tensile Properties  New test method for tension in fiber concrete –Difficulties with direct tension –Size-effect with long- fibers  Dogbone specimens 32” high, 8” neck width, 16” top width

Concrete Dogbone  Mechanical anchorages were used to load specimen  Anchorage consisted of 5/8”, 125 ksi threaded prestressing rod  LVDTs for displacement  Failure occurred in desired region

Tensile Properties  Increase in energy dissipation  Testing will determine if cracking stress is affected by the addition of fibers

Finite Element Modeling  Material model developed from testing  Comparison to field blast test and instrumented impact testing  Loading –CONWEP (built into LS Dyna) –Gas dynamics model (Lyle Long, AE) –Field data

Current Work  Continued model refinement –Material model –Incorporation of fracture mechanics –Contact charges  Application specific testing –Durability –Reinforcement and fiber content variations  Specification development

Barrier Application Testing  Use of fibers & polyurea for barriers –Large volume of concrete with small reinforcement percentage –Reduction in secondary fragmentation needed

Wall Testing: Spec Development

Questions? Hershey Lodge Preconference Symposium 17 March 2008