8/12/2019 Structural Steel Fibre Reinforced Construction

1/2

During the last 15 years, the structuraluse of steel fibres as the only principalreinforcing in concrete has beendeveloped and widely used.

Such novel techniques, where steel fibres completelyreplace all traditional reinforcing bars and fabric, havebeen used repeatedly in the following applications involv-

ing plane structural members:

Pile-supported industrial slabs where the ground does

not provide any bearing capacity (TAB-Structuralwith span-to-depth ratios of up to 22).

Ground-bearing general rafts as general foundations

under condominiums, schools, hospitals, office tow-

ers, clad-rack warehouses, shopping malls and tanks

(TAB-Raft).

Cast in-situ free suspended elevated slabs in residen-

tial and commercial applications (TAB-Slab, span-to-

depth ratio of up to 35).

Second phase layers on top of concrete planks and

void formers.

Composite floors on steel decking (TAB-deck with up

to two hour fire rating).

Retaining walls.

Bridge slabs.

The steel-fibre-only reinforced concrete in these appli-

cations, with dosage rates of 30100kg/m3 depending on

the application and type of fibre used, is able to resist the

moments, negative and positive, together with the shear,

shrinkage and punching stresses, so that all reinforcing

steel is omitted. Continuity and starter bars are generally

needed, as in traditional concrete.

The site conditions require the steel-fibre-reinforced

concrete to be fully pumpable and flowing in such a way

that it does not require any mechanical vibration.

Matrix saturation

An essential feature is the need to saturate the concretematrix with steel fibres in such a way that the steel fibre

spacing in all directions is equal to at least the maximum

aggregate size. The fibre length should be 2.5 to 3 times the

size of aggregate so that the fibre can overlap and bridge

fully all large aggregate particles.

The 3D-fibre spacing, s, is a function of the fibre diame-

ter, d, and dosage rate, Vm , as follows:

s = 122 d/Vm

For example:

50kg/m3 of TABIX+1/60, s =122 1/50 = 17.25mm100kg/m3 of TABIX1.3/50, s = 122 1.3/100 =

15.86mm.

The correct steel fibres for these structural uses are of

round cross-section (BS EN 14889-1, Group I(1)), strong,

ductile, rather stiff (ie, difficult to bend between three fin-

gers of your hand) and should provide an anchoring shape

so that they offer high pull-out loading. Undulated TABIX,

HE hooked ends and Twincone (conical button ends),

which provide total anchorage are the shapes that are most

commonly used in our structural applications.

Examples of pull-out loadings of individual steel fibres

are as follows:

1mm/60mm (L/d = 60),1500MPa,TABIX+ 800N

1mm/50mm (L/d=50), 1500MPa, HE+ 600N

1mm/54mm (L/d=54), 1000MPa,Twincone 700N

1.3mm/50mm (L/d = 38),900MPa,TABIX13/50 800N

Stiff fibres that do not bend easily also create less fric-

tion inside the plastic concrete matrix, so that there is lessslump loss. Consequently, a stiffer fibre has less tendency

to show at the surface of the concrete.

To achieve the workability that is needed on the job site,

the larger 1.3mm-diameter fibres are generally used, up to

the very high dosage rates of 80 and 100kg/m3, while the

1mm-diameter fibres are limited to a dosage rate of

50kg/m3.

Smaller-diameter fibres are not used although they sat-

urate the concrete at lower dosage rates, as low as 25kg/m3

for a 0.60mm diameter fibre. Their very low pull-out load

(125N) together with a loss of workability at higher

dosages, makes it extremely difficult to use them.

Mix designSelecting the correct mix design is essential to ensure that

the concrete can be satisfactorily transported, pumped

(using a minimum 125mm-diameter line), placed and fin-

ished. Hence the purchase order of concrete shall include

at least the following:

the mix design together with the aggregate grading

the cement content and the maximum water/cement

ratio of no more than 0.55 (preferably 0.50); a super-

plasticiser is needed to supply a flowing concrete

the slump prior to any addition of superplasticiser and

the strength class.

The aggregate grading should be continuous, with

20mm maximum aggregate size together with an increasedcontent of 12mm maximum size to ensure that the steel

fibres can fit between the aggregate particles. The

gravel/sand ratio should be 0.9:1.0, with at least 475

500kg/m3 fines smaller than 200 (including 320350kg/m3

of cementitious material).

CONCRETE SEPTEMBER 2007 23

FIBRES

Structural steel-fibre-reinforced

concrete construction

XAVIER DESTRE, ARCELOR MITTAL

Figure 1 left: Installationof a TAB-Structural pile-supported ground floorslab in a Bauhaus

warehouse in Ingelsta,Sweden. The slab is300mm-thick requiring45kg/m3 dosage rate ofTABIX+1/60 over a pilegrid of 3.5m 3.5m toallow for a uniformlydistributed load of50kN/m2. Fibre blastmachines were on-siteto introduce the steelfibres into the concrete,

which was laid at a rateof 500m2 to 1650m2 per

day.

Figure 2: TAB-Raft usedas a general foundation(4000m2) slab under anoffice building inCouillet, Belgium. This

raft is 400mm-thickrequiring 50kg/m3

HE+1/50 fibres using aconcrete pump. Columnstarter bars were tiedonto steel fabric.

(Photos:ArcelorMittal.)

8/12/2019 Structural Steel Fibre Reinforced Construction

2/2

Steel fibre mixingUsing the above-listed fibres, various methods are possi-

ble. In the UK, very few concrete plants can mix the fibres

into the central mixer. Most often the fibres are therefore

introduced at a rate of 1 minute per cubic metre into the

ready-mixed concrete truck revolving at maximum speed

by using blast machines or being loaded onto mobile con-

veyor belts or the aggregate belts used to load the ready-

mixed trucks. TABIX+1/60 and Twincone are blown into the ready-

mixed truck using proprietary blast machines pro-

vided by Arcelor Mittal UK and other services

companies.

HE+1/50 and TABIX1.3/50 steel fibres can be intro-

duced using a conveyor belt.

TABIX1.3/50 can also be loaded in the truck mixer

prior to dumping any of the constituent materials into

it.

Design criterionBasically, there is almost always a steel-fibre-only rein-

forced concrete solution for plane elements when,working

under unfactored-service loadings together with their ownweight, the concrete stresses in flexure and shear are less

than 5N/mm2 and 1.5N/mm2 respectively, the latter calcu-

lated at mid-depth with a 45 angle of distribution of

stresses. Currently, steel fibres do not replace the principal

reinforcing bars in beams and columns.

For suspended slabs and rafts, the above design rules

are derived from no less than five full-scale tests (including

several fire tests) conducted using the TAB-Deck system

(steel fibres used in composite metal deck construction)

together with 15 years experience in numerous countries in

Europe and the Americas involving several million cubic

metres of concrete. As outlined in Concrete Society

Technical Report 63, Guidance for the design of steel-

fibre-reinforced concrete(2), Arcelor Mittals methods arepart of the design assisted by testing route.

AdvantagesThere are numerous advantages resulting from the use of a

pre-reinforced concrete that is readily and easily placed or

pumped into the forms:

higher-quality concrete elements as there are no more

mistakes related to the installation of steel and no

unexpected variations in the effective depth

design and technical assistance by Arcelor Mittal

appointed structural engineers

reduced and controlled concrete shrinkage

slabs and rafts with flat soffits,without drop panels

anti-progressive-collapse reinforcement as in NorthAmerican Standards is included in all TAB-Slabs,

ensuring that no catastrophic collapse occurs if a col-

umn fails

TAB-Slabs are flat slabs and do not need any beams

or pedestals and so are quite easy to cast on-site when

non-rectangular floors are required, resulting in

greater design freedom

material savings due to a reduction in thickness in

most cases is feasible

material savings due to the replacement of two layers

of reinforcement and stirrups as they are completely

omitted

labour savings as cutting, bending and placing of steel

are no longer needed labour savings during installation as the concrete is

self-levelling and does not need poker vibration; it is

easily consolidated in the formwork

the concrete has a more professional appearance as it

is completely smooth against smooth formwork

project schedules benefit as the critical path task of

installing and tying traditional reinforcement is omit-

ted; it is commonplace to see several weeks saved on

large projects

crane savings as there is no handling of steel

better job site management as there is no need to store

and handle cut and bent steel reinforcement

better personnel safety; none of the risks attributable

to traditional steel a better environment with less crane noise, cutting

and bending of steel and no noise from concrete

vibrators

the concrete is easily pumped.

SEPTEMBER 2007 CONCRETE

FIBRES

24

References:

1. BRITISH STANDARDS INSTITUTION. BS EN 14889-1:

Fibres for concrete. Steel fibres. Definitions,

specifications and conformity. BSI, 2006.2. CONCRETE SOCIETY. Guidance for the design of steel-

fibre-reinforced concrete, Technical Report 63. TheConcrete Society, Camberley, 2007.

Figure 7: A 150mm-thick TAB-Deck composite floor onsteel decking with 30kg/m3 HE1/50 steel fibres at abuilding site in Thanet, UK.

Figure 6: A completed building in Talinn, Estonia, whichhad five 230mm-thick TAB-Slab floors with a 7.5m spanand the raft foundation, all with a fibre dosage of80kg/m3.

Figure 3 and 4: TAB-Raft was used for the

350mm-thick, 2000m2ground slab underneath

a condominiumbuilding. Figure 3

shows the installationof 80kg/m3

TABIX1.3/50 includingthe column footing

thickening. The raftsurface is mirror

finished, as shown inFigure 4, in order to

comply with therequirements for a

parking garage.

Figure 5: A typicalinstallation of a TAB-Slab steel-fibre-only

free suspendedelevated slab 180mm-thick, using 100kg/m3

TABIX1.3/50. Anti-progressive-collapse

reinforcement isprovided from column

to column.