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

4

Concrete reinforcing

All different shapes, size and materials

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

Fibers Reinforcing Hardened Concrete

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

Testing Fiber Reinforced Concrete

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

W. R. Grace - Confidential

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

Precast Tunnel Sections

Precast Rail Ties

Sound Barriers – other precast wall sections

Burial Vaults

Precast - Utility Cabinet:

GFRC Permanent Beam Forms-Nominal

thickness ½”

GFRC Permanent Beam Forms

GFRC Pipeline Trench

Marine Environments

FRC Dolosses

and breakwater

protection

Piles and Piers

Blackpool, England Sea Defense Project

“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

Sometimes “over-sold”

Sometimes “under-used”

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