Structural Concrete Innovations:Structural Concrete Innovations:

A Focus on Blast ResistanceA Focus on Blast Resistance

Hershey LodgeHershey Lodge

Preconference SymposiumPreconference Symposium17 March 200817 March 2008

Blast OverviewBlast Overview

Blast can effect structure in multiple Blast can effect structure in multiple wayway– Air blastAir blast– DragDrag– Ground shockGround shock– Primary and secondary fragmentationPrimary and secondary fragmentation– FireFire

Blast LoadingBlast Loading

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

structurestructure

Air Blast LoadsAir Blast Loads

Properties of the air blast load a Properties of the air blast load a function of the:function of the:– Size and shape of explosiveSize and shape of explosive– Distance to explosiveDistance to explosive– Orientation of specimenOrientation of specimen– Type of blastType of blast

Free air burstFree air burst Ground burstGround burst Contained burst Contained burst

Scaled DistanceScaled Distance

Convert explosive to equivalent weight of Convert explosive to equivalent weight of TNTTNT

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

where where Z = scaled distanceZ = scaled distanceW= equivalent TNT weightW= equivalent TNT weightD = distance between specimen D = distance between specimen and explosiveand explosive

Use figures in references (TM5-1300): Use figures in references (TM5-1300): “Structures to Resist the Effects of “Structures to Resist the Effects of Accidental Explosions”Accidental Explosions”– determine the expected peak pressure and determine the expected peak pressure and

impulse for determined scaled distanceimpulse for determined scaled distance

Scaled Scaled DistanceDistance

Figure 2-7Figure 2-7 TM5-1300 TM5-1300

Types of Cross SectionsTypes of Cross Sections TM5-1300: 3 types of cross sectionsTM5-1300: 3 types of cross sections

– Type I: Type I: Concrete is sufficient to resist compressive component of Concrete is sufficient to resist compressive component of

momentmoment Cover remains undamagedCover remains undamaged

– Type II: Type II: Concrete is no longer effective at resisting momentConcrete is no longer effective at resisting moment Equal top and bottom reinforcementEqual top and bottom reinforcement Cover remains in tactCover remains in tact Single leg stirrups used to resist shearSingle leg stirrups used to resist shear

– Type III:Type III: Equal top and bottom reinforcementEqual top and bottom reinforcement Cover disengagesCover disengages Lacing used to resist shearLacing used to resist shear

Example Type II Cross-SectionExample Type II Cross-Section

Motivation for Innovation in Motivation for Innovation in Blast Resistant ConcreteBlast Resistant Concrete

Increased demand for impact and Increased demand for impact and blast-resistant building materialsblast-resistant building materials

Need for practical, constructible Need for practical, constructible optionsoptions

Need for reduction in secondary Need for reduction in secondary fragmentationfragmentation

InnovationInnovation

Long (3”) fibers Long (3”) fibers – Increased bond with concrete matrixIncreased bond with concrete matrix– Length provides crack bridging, spalling resistance, Length provides crack bridging, spalling resistance,

increased ductility, energy absorption (through long-fiber increased ductility, energy absorption (through long-fiber pull-out)pull-out)

Coated “tape” Coated “tape” – Mix retains workability (no balling, etc)Mix retains workability (no balling, etc)– Can be used with aggregateCan be used with aggregate

Potentially economicalPotentially economical– Carbon fiber yarn is waste product from the aerospace Carbon fiber yarn is waste product from the aerospace

industryindustry No special mixers requiredNo special mixers required

– Lightweight “additive” reinforcementLightweight “additive” reinforcement– Precast or cast-in-placePrecast or cast-in-place

Molds to any shapeMolds to any shape

Experimental ProgramExperimental Program

Mix design developmentMix design development– WorkabilityWorkability

Static flexural strengthStatic flexural strength– Small and large scaleSmall and large scale– DuctilityDuctility

Impact testingImpact testing– Small beamsSmall beams– PanelsPanels

Blast TestingBlast Testing Finite Element ModelingFinite Element Modeling

Experimental ProgramExperimental Program Mix design developmentMix design development

– 1.5% to 2.5% fiber content (by volume)1.5% to 2.5% fiber content (by volume)– Various admixture combinations Various admixture combinations – Pozzolans (interground SF + GGBFS)Pozzolans (interground SF + GGBFS)

Preliminary TestingPreliminary Testing

Mixture DesignMixture Design– Avoid ballingAvoid balling– Increase workabilityIncrease workability– Increase fines and Increase fines and

cement in mixturecement in mixture Preliminary Static TestsPreliminary Static Tests

– 6” X 6” X 18” beams 6” X 6” X 18” beams loaded at third pointsloaded at third points

– Flexural Strength = 2112 Flexural Strength = 2112 psipsi

1595

21121887

0

500

1000

1500

2000

2500

B1-2.5 T1-2.5 T2-2.5

Fle

xura

l Str

ess

(psi

)

Slab StripsSlab Strips 4” X 12” X 10’ slab strips loaded at midspan4” X 12” X 10’ slab strips loaded at midspan Specimens:Specimens:

– 2 control specimens with reinforcing mesh2 control specimens with reinforcing mesh– 2 fiber reinforced concrete specimens2 fiber reinforced concrete specimens– 2 fiber reinforced concrete specimens with mesh2 fiber reinforced concrete specimens with mesh

Used to obtain load vs. deflection plotUsed to obtain load vs. deflection plot Useful for obtaining toughnessUseful for obtaining toughness

Slab Strip ResultsSlab Strip Results

Force vs Displacement for Slab Strips

0

0.5

1

1.5

2

2.5

0 1 2 3

Displacement (in)

For

ce (

K)

Plane 1Plane 2Fiber 1Fiber 2Fiber + Mesh 1Fiber + Mesh 2

Compressive Compressive Strength (psi)Strength (psi)

Tensile Tensile Stress (psi)Stress (psi)

Toughness Toughness (lbs-in)(lbs-in)

Average Plane + meshAverage Plane + mesh 61516151 750750 186186

Average FiberAverage Fiber 66526652 19041904 18341834

Average Fiber + meshAverage Fiber + mesh 66196619 21162116 26192619

Impact Test SetupImpact Test Setup

15 ft maximum drop 15 ft maximum drop heightheight

50# weight50# weight Panels 2’x2’x2”Panels 2’x2’x2”

Impact Testing: PanelsImpact Testing: PanelsDrop Height at failure

0

20

40

60

80

100

120

140

160

180

1 2 3

Dro

p H

eigh

t (in

)

Plain FiberWire Mesh

7 b

low

s

9 b

low

s

7 b

low

s

7 b

low

s

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

0

20

40

60

80

100

120

140

160

180

1 2 3

Dro

p H

eigh

t (in

)

Plain FiberWire Mesh

Impact Testing: PanelsImpact Testing: Panels(No Steel Reinforcement)(No Steel Reinforcement)

Fiber addition controlled spallingFiber addition controlled spalling Failure in fiber specimens along weak Failure in fiber specimens along weak

plane due to fiber orientationplane due to fiber orientation

Plain panel Fiber panel

Impact Testing: PanelsImpact Testing: Panels(Steel Reinforcement)(Steel Reinforcement)

Fiber panel with steel reinforcement did Fiber panel with steel reinforcement did not fail after repeated blows at top drop not fail after repeated blows at top drop heightheight

Plain panel Fiber panel

Blast TestingBlast Testing 6’ x 6’ x 6.5”6’ x 6’ x 6.5” Heavily reinforced (as per TM5-1300)Heavily reinforced (as per TM5-1300)

– resist shear failure at supports resist shear failure at supports – evaluate comparison of materials under evaluate comparison of materials under

full blast designfull blast design– Identical Identical

reinforcement in reinforcement in all specimensall specimens

Clear cover ¾” to Clear cover ¾” to tiesties

Test SetupTest Setup

Slabs were simply Slabs were simply supported on all supported on all four sidesfour sides

Restraint provided Restraint provided along two sides to along two sides to prevent reboundprevent rebound

Test SetupTest Setup

TNT suspended at TNT suspended at desired heightdesired height

Pressure gages Pressure gages record reflected record reflected pressure and pressure and incident pressureincident pressure

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

Extensive cracking, some spallingA few hairline cracks

Standard Concrete SafeTcrete

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

Standard Concrete SafeTcrete

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

Standard Concrete

SafeTcrete

Summary of Impact & Summary of Impact & Blast TestingBlast Testing

Much improved workability and dispersion Much improved workability and dispersion of coated tape fibersof coated tape fibers

Increased ductility over plain concrete and Increased ductility over plain concrete and further improved combined with standard further improved combined with standard reinforcement reinforcement

Significantly increased flexural strength Significantly increased flexural strength under both static and impact loadsunder both static and impact loads

Complete control of spalling in panels Complete control of spalling in panels under impact loadunder impact load

Excellent performance in blast testingExcellent performance in blast testing

PotentialPotential Low cost fiber alternativeLow cost fiber alternative Applications requiring impact and blast Applications requiring impact and blast

resistanceresistance– Protective cladding panelsProtective cladding panels– Structural components: columns, wallsStructural components: columns, walls– BarriersBarriers– Bridge piersBridge piers

May be used as a replacement for, or in May be used as a replacement for, or in combination with standard reinforcement combination with standard reinforcement depending on applicationdepending on application

Material PropertiesMaterial Properties

Stress-strain curves for material in Stress-strain curves for material in both compression and tension needed both compression and tension needed for modelingfor modeling– Compression: standard 6” diameter Compression: standard 6” diameter

cylinders cylinders – Tension: dogbone specimens will be Tension: dogbone specimens will be

utilizedutilized– Varied load rates and fiber orientationVaried load rates and fiber orientation

Tensile PropertiesTensile Properties

New test method for New test method for tension in fiber concretetension in fiber concrete– Difficulties with direct Difficulties with direct

tensiontension– Size-effect with long-Size-effect with long-

fibersfibers

Dogbone specimens Dogbone specimens 32” high, 8” neck width, 32” high, 8” neck width, 16” top width 16” top width

Concrete DogboneConcrete Dogbone Mechanical anchorages Mechanical anchorages

were used to load were used to load specimenspecimen

Anchorage consisted of Anchorage consisted of 5/8”, 125 ksi threaded 5/8”, 125 ksi threaded prestressing rodprestressing rod

LVDTs for displacement LVDTs for displacement Failure occurred in Failure occurred in

desired regiondesired region

Tensile PropertiesTensile Properties Increase in energy Increase in energy

dissipationdissipation Testing will Testing will

determine if determine if cracking stress is cracking stress is affected by the affected by the addition of fibersaddition of fibers

Finite Element ModelingFinite Element Modeling

Material model developed from testingMaterial model developed from testing Comparison to field blast test and Comparison to field blast test and

instrumented impact testinginstrumented impact testing Loading Loading

– CONWEP (built into LS Dyna)CONWEP (built into LS Dyna)– Gas dynamics model (Lyle Long, AE)Gas dynamics model (Lyle Long, AE)– Field dataField data

Current WorkCurrent Work

Continued model refinement Continued model refinement – Material modelMaterial model– Incorporation of fracture mechanicsIncorporation of fracture mechanics– Contact chargesContact charges

Application specific testingApplication specific testing– DurabilityDurability– Reinforcement and fiber content Reinforcement and fiber content

variationsvariations Specification developmentSpecification development

Barrier Application TestingBarrier Application Testing

Use of fibers & polyurea for barriersUse of fibers & polyurea for barriers– Large volume of concrete with small Large volume of concrete with small

reinforcement percentagereinforcement percentage– Reduction in secondary fragmentation Reduction in secondary fragmentation

neededneeded

Wall Testing: Wall Testing: Spec DevelopmentSpec Development

Questions?Questions?

Hershey LodgeHershey Lodge

Preconference SymposiumPreconference Symposium17 March 200817 March 2008