Post on 16-Jul-2015
transcript
Fibers as Reinforcing for Concrete Some insights into where they fit now -
and where they might in the future
January 27, 2015
NEPCA
1
Tim Durning
Agenda
Introduction
Fundamentals of reinforcing
Quick primer on mechanics of materials
Where fibers can be an effective replacement
for rebar
Some rules of thumb for mainstream fibers
Some examples of fiber reinforced precast
elements
Introductory Thoughts
Almost all concrete is reinforced – for the last 30 years, growing
acceptance of fibers to perform this function for certain situations The idea of discrete fibers as reinforcing in a brittle media goes back to ancient times
Innovation to develop enhanced fiber configurations and new fiber
materials continues to accelerate
Fibers are vastly under-utilized relative to their utility and economics,
but that will change in time, and there are some good reasons why it
lags (codes , balance of risk – reward, switching costs)
I will give a little exposure to “exotic” (non-mainstream) products and
uses, but try to spend more time on fundamentals of more traditional
fiber use in concrete
Apologize up front that some of my pictures and illustrations show CIP
concrete. Ask you to overlook that – the same principles apply – and I
will show quite a few precast pictures as well
Macro and Micro Fibers
Length: 0.8 to 50 mm
Diameter: < 0.3 mm
- Reduce/prevent plastic
shrinkage cracking
Polypropylene, glass, nylon, cellulose…
Micro-Fibers
Length: 25 to 65 mm
Diameter: 0.3 to 1.3 mm
Control crack width
Limited Structural reinforcing
Steel, Synthetic, glass, Carbon…
Macro-Fibers
Mono
filamentFibrillatedSynthetic Steel
Fundamentals of reinforcing
Most (but not all) concrete reinforcing provides it’s function once a
crack has formed – it then continues to provide resistance to further
opening of that crack.
Primarily due to relatively low volumes of fiber and/or low stiffness
relative to concrete
The crack bridging properties of any single re-bar or fiber are functions
of cross sectional properties (cross-sectional area, ultimate stress and
axial modulus of elasticity), the anchorage into the concrete on either
side of the crack, and the effective length of the bar or fiber over which
it resists the crack opening
With rebar the code forces proper detailing such that there is
always enough embedment beyond critical crack zones such that
the full tensile capacity of the bar can be counted on to resist crack
opening once yield strain is reached
With fibers some will be ideally positioned across a crack
(perpendicular to crack, crack bisects fiber near the middle), and
some will not – so not all fibers are not fully effective across any
crack face)
W. R. Grace - Confidential
Concrete Reinforcing Basics:
Microfibers, and some Macrofibers – reinforcing plastic concrete
Traditional market for synthetic
fibers. Rebar only holds cracks
together. Fibers actually prevent
plastic cracking
From “Means Graphic Construction
Standards” Published by R S Means Inc
Substantial skepticism regarding fibers
Still prevalent attitude: plastic can never replace steel
Saw this in code discussions around fibers as an
alternative to WWM for minimum steel in CMD
Misconception #1 and #2:
1. Always show equivalence in flexure, that doesn’t
demonstrate equivalence to hold concrete in bulk tension
to resist crack growth
2. Even if the strength across the crack is equivalent the
stiffness will not be – they will be like rubber bands
allowing very wide crack widths
W. R. Grace - Confidential
Concrete Reinforcing Basics:
Need for Reinforcing
Co
mp
res
sio
n
Te
ns
ion
4000
400
Concrete strength (typ)
PC RC
PC
Deflection
0.25%
0.50%
BIC SynMF – 1.0%
Rebar – 0.2%
Rebar – 0.5%
Load
From “Concrete Repair and Maintenance
Illustrated” by Peter H Emmons
5-1 disadvantage, fibers to rebar
at outer edge of beam in flexure
Dispelling Misconception #1
h 2/3*h
0.1 h
0.5 h 0.45 h
0.50 h
0.1 h
0.9 h
C1
T1
C2
T2 = AsFy
M
eq
act
eq = 6*M / (b*h^2)
M = As*Fy *b * 0.45h
C3
T3
T3 = 0.9 *T2M
act = 0.37 * eq
Total tensile resistance = T2
Total tensile resistance = T3/0.9 = T2
A fiber reinforced section with equivalent flexural strength to same sized section
conventionally reinforced at mid-depth DOES also have the same full tensile capacity
From RILEM design Manuals
The International Union of
Laboratories and Experts in
Construction Materials,
Systems and Structures
(RILEM, from the name in
French) was founded in June
1947, with the aim to promote
scientific cooperation in the
area of construction materials
and structures.
W. R. Grace - Confidential
Concrete Reinforcing Basics:
Need for Reinforcing
Co
mp
res
sio
n
Te
ns
ion
4000
400
Concrete strength (typ)
PC RC
PC
Deflection
0.25%
0.50%
BIC MF – 1.0%
Rebar – 0.2%
Rebar – 0.5%
Load
From “Concrete Repair and Maintenance
Illustrated” by Peter H Emmons
5-1 disadvantage, fibers to rebar
at outer edge of beam in flexure
W. R. Grace - Confidential
Fibers as Reinforcing:Minimum reinforcing
(keep cracks tight)
From “Means Graphic Construction
Standards” Published by R S Means Inc
Primary current MacroFiber market –
non structural slabs
Minimum reinforcing
For Composite Metal Deck
now accepted in Bldg Codes
PC
Deflection
0.25%
0.50%
BIC MF – 1.0%
Rebar/WWF – 0.67%
Rebar/WWF – 1.3%
Load
Rebar Volume advantage only 1.5 to 1
How can “plastic” fibers match the stiffness of
Steel Rebar in holding cracks tight ?
Axial Stiffness = PL/AE
Load Length
Area Modulus of Elasticity
Example of fibers alternative to WWM in CMD for minimum temp and shrinkage reinf.
WWM Syn Macrofibers
Effective Length
Mod of Elast
Area
4 to 6 inches
29 E6 psi
0.075%
0.25 – 0.5 inches
1.4 E6 psi
= 0.25% * 50% = 0.125%
16X more stiff
20X less stiff
1.67X more stiff
Stiffness of fibers = WWM stiffness * (16 * 1.67)/20 = 1.33 X stiffer than WWM
Validated by Testing
Concrete Reinforcing Basics: Shear Reinforcing
From “Concrete Repair and Maintenance
Illustrated” by Peter H Emmons
Relative to Shear steel, synthetic
fibers are at a 3-1 disadvantage on a
volume basis – in critical zone
Critical zone for Shear
W. R. Grace - Confidential
Concrete Reinforcing Basics: Confinement Steel
From “Means Graphic Construction
Standards” Published by R S Means Inc
W. R. Grace - Confidential
Reinforcement Spectrum
Plastic $200mm
Min Reinf $4- 6B
Shear $2- 3B
Confinement $1-1.5B
Flexural $12-15B
Compressive $2-3B
Perf
orm
an
ce
Codes as Barrier
Current Syn
Macrofibers
Microfiber
BIC Steel &
Next Gen
??
NA
EU
ACI 318 allows steel fibers for min shear reinforcing
IBC allows fibers for min steel in CMD construction
EU design stds for incorporation
into all aspects of reinforcing
Resistance to Impact Loadingprecast pump chamber covers
57% increase in energy absorption
Mesh reinforced Syn Macrofiber
28 impacts
4617 Joules
37 impacts
6477 Joules
Design methods to incorporate fibers will continue to advance – as will building code acceptance Lead by Europe – with an initial emphasis on steel fibers
Geographic spread of standards can be facilitated if based on sound science – which they appear to be
Expansion to include materials other than steel is even easier to accomplish once performance parameters are set for fiber reinforced concrete
Labor rates, labor availability and cost of safety will continue to drive up the installed cost of reinforcing bar in developed economies
Trends
Examples of Fiber use in Precast
Steps
Septic
tanks
ManholesInsulated
wall panels
Parking
curbs
Pipes
Marine Environments
FRC Dolosses
and breakwater
protection
Piles and Piers
“Fiber-reinforced concrete in precast concrete applications: Research leads to innovative products”
Nemkumar Banthia, Vivek Bindiganavile, John Jones, and Jeff Novak. PCI Journal Summer 2012
Closing Thoughts
Ignore fibers at your peril
Trust blindly at your peril
Fiber materials and configurations continue to
advance
Innovative uses continue to advance
Design practices and building codes continue to
advance – but very slowly
Fibers are not THE answer – just part of the answer.
Generally underutilized – occasionally overhyped
and oversold