Bklast-FAA Hershey Schokker

8/2/2019 Bklast-FAA Hershey Schokker

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Structural Concrete Innovations:

A Focus on Blast Resistance

Hershey LodgePreconference Symposium17 March 2008


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Blast Overview

Blast can effect structure in multipleway

Air blast

Drag

Ground shock

Primary and secondary fragmentation

Fire


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Blast Loading

Air blast design can be governed by maxpressure, impulse, or combination

Function of size of explosive, standoff distance, and

structure


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Air Blast Loads

Properties of the air blast load afunction of the:

Size and shape of explosive Distance to explosive

Orientation of specimen

Type of blast Free air burst

Ground burst

Contained burst


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Scaled Distance

Convert explosive to equivalent weight ofTNT

Determine scaled distance usingZ = D / W^(1/3)

where Z = scaled distanceW= equivalent TNT weightD = distance between specimenand explosive

Use figures in references (TM5-1300):Structures to Resist the Effects ofAccidental Explosions determine the expected peak pressure and

impulse for determined scaled distance


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ScaledDistance

Figure 2-7TM5-1300


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Types of Cross Sections TM5-1300: 3 types of cross sections

Type I:

Concrete is sufficient to resist compressive component ofmoment

Cover remains undamaged Type II:

Concrete is no longer effective at resisting moment

Equal top and bottom reinforcement

Cover remains in tact Single leg stirrups used to resist shear

Type III:

Equal top and bottom reinforcement

Cover disengages

Lacing used to resist shear


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Example Type II Cross-Section


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Motivation for Innovation inBlast Resistant Concrete

Increased demand for impact andblast-resistant building materials

Need for practical, constructibleoptions

Need for reduction in secondary

fragmentation


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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 aerospaceindustry

No special mixers required Lightweight additive reinforcement Precast or cast-in-place

Molds to any shape


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Experimental Program

Mix design development Workability

Static flexural strength

Small and large scale

Ductility

Impact testing

Small beams

Panels Blast Testing

Finite Element Modeling


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Experimental Program

Mix design development 1.5% to 2.5% fiber content (by volume)

Various admixture combinations

Pozzolans (interground SF + GGBFS)


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Preliminary Testing

Mixture Design Avoid balling

Increase workability

Increase fines andcement in mixture

Preliminary StaticTests 6 X 6 X 18 beams

loaded at third points

Flexural Strength = 2112psi

1595

21121887

0

500

1000

1500

2000

2500

B1-2.5 T1-2.5 T2-2.5

FlexuralStress(psi)


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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


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Slab Strip Results

Force vs Displacement for Slab Strips

0

0.5

1

1.5

2

2.5

0 1 2 3

Displacement (in)

Force

(K)

Plane 1

Plane 2

Fiber 1

Fiber 2

Fiber + Mesh 1

Fiber + Mesh 2

CompressiveStrength (psi)

TensileStress (psi)

Toughness(lbs-in)

Average Plane + mesh 6151 750 186

Average Fiber 6652 1904 1834

Average Fiber + mesh 6619 2116 2619


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Impact Test Setup

15 ft maximum dropheight

50# weight

Panels 2x2x2


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Impact Testing: PanelsDrop Height at failure

0

20

40

60

80

100

120

140

160

180

1 2 3

D

ropHeight(in

)

Plain FiberWire Mesh

7blows

9blows

7blows

7blows


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Impact Testing: PanelsDrop Height at first cracking (top side)

0

20

40

60

80

100

120

140

160

180

1 2 3

D

ropHeight(in)

Plain FiberWire Mesh


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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


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Impact Testing: Panels(Steel Reinforcement)

Fiber panel with steel reinforcement didnot fail after repeated blows at top dropheight

Plain panel Fiber panel


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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 inall specimens

Clear cover to

ties


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Test Setup

Slabs were simplysupported on all

four sides Restraint provided

along two sides toprevent rebound


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Test Setup

TNT suspended atdesired height

Pressure gagesrecord reflectedpressure andincident pressure


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Hit 1: 75# at 6 (scaled range 1.4)

Extensive cracking, some spallingA few hairline cracks

Standard Concrete SafeTcrete


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Hit 2: 75# at 3.2 (scaled range 0.76)



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Some concrete loss due to pop out wherereinforcement buckled (3/4 cover)

Concrete rubble within steel cage


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Standard Concrete

SafeTcrete


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Summary of Impact &Blast Testing

Much improved workability and dispersionof coated tape fibers

Increased ductility over plain concrete andfurther improved combined with standardreinforcement

Significantly increased flexural strengthunder both static and impact loads

Complete control of spalling in panelsunder impact load

Excellent performance in blast testing


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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 incombination with standard reinforcementdepending on application


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Material Properties

Stress-strain curves for material inboth compression and tension needed

for modeling Compression: standard 6 diametercylinders

Tension: dogbone specimens will be

utilized Varied load rates and fiber orientation


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Tensile Properties

New test method fortension in fiber concrete

Difficulties with directtension

Size-effect with long-fibers

Dogbone specimens

32 high, 8 neck width,16 top width


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Concrete Dogbone

Mechanical anchorageswere used to loadspecimen

Anchorage consisted of

5/8, 125 ksi threadedprestressing rod

LVDTs for displacement

Failure occurred indesired region


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Tensile Properties

Increase in energy

dissipation Testing will

determine ifcracking stress isaffected by theaddition of fibers


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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


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Current Work

Continued model refinement Material model

Incorporation of fracture mechanics Contact charges

Application specific testing Durability

Reinforcement and fiber contentvariations

Specification development


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Barrier Application Testing

Use of fibers & polyurea for barriers

Large volume of concrete with smallreinforcement percentage

Reduction in secondary fragmentationneeded


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Wall Testing:Spec Development


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Questions?

Hershey LodgePreconference Symposium17 March 2008

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Bklast-FAA Hershey Schokker