Structural Concrete Innovations:

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25 Νοε 2013 (πριν από 3 χρόνια και 11 μήνες)

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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


Free air burst


Ground burst


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:


Concrete is sufficient to resist compressive component of
moment


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

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

1595
2112
1887
0
500
1000
1500
2000
2500
B1-2.5
T1-2.5
T2-2.5
Flexural Stress (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

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
Compressive
Strength (psi)

Tensile
Stress (psi)

Toughness
(lbs
-
in)

Average Plane + mesh

6151

750

186

Average Fiber

6652

1904

1834

Average Fiber + mesh

6619

2116

2619

Impact Test Setup


15 ft maximum drop
height


50# weight


Panels 2’x2’x2”



Impact Testing: Panels

Drop Height at failure

0
20
40
60
80
100
120
140
160
180
1
2
3
Drop Height (in)
Plain
Fiber
Wire Mesh
7 blows
9 blows
7 blows
7 blows
Impact Testing: Panels

Drop Height at first cracking (top side)

0
20
40
60
80
100
120
140
160
180
1
2
3
Drop Height (in)
Plain
Fiber
Wire Mesh
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 panel

Fiber 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