1. STEEL FIBRE REINFORCED CONCRETE (SFRC)
SLABS ON GRADE
This presentation is broken up into several sections:-
It outlines how fibres work in concrete, the importance of good fibre distribution on crack control, the importance of specifying minimum fibre numbers and hence dosages to achieve crack control and advises on a suitable specification clause.
It introduces the way in which fibre reinforcement provides flexural moment capacity compared to conventional reinforcement and introduces a suitable specification clause for ensuring performance.
It introduces the specific case of fibre reinforced slabs on grade, showing how slabs can be designed for the increased load carrying capacity that fibres provide and discusses the extent of the increase and the impact this has on material costs.
It advises on the technical support available to designers.
Author: Royce Ratcliffe

2. FIBRES REINFORCE CONCRETE
Fibres – like all reinforcement - have their greatest effect after cracking develops.
The role of reinforcement in concrete, irrespective of the reinforcing material used, is to carry load that the concrete matrix, by itself, would be incapable of carrying.
In terms of tensile and flexural stresses this essentially comes down to maintaining the ability to carry tensile load across a region of cracked concrete.
Fibres are small discreet elements that achieve this role at an earlier stage of cracking than conventional bars or mesh I.e. fibres are effective with micro-cracks, whereas bars or mesh only control macro-cracks.

3. X-Ray Of SFRC
A good distribution of fibres means that developing cracks are quickly cut off and prevented from developing into full blown macrocracks.
The appearance of a visible macrocrack in fibre concrete means the section is overloaded and the fibres are either yielding or pulling out.
Compare this to conventionally reinforced concrete, where visible cracking means the reinforcement is carrying load and being effective.

4. COMPARING FIBRES TO CONVENTIONAL REINFORCEMENT
This section determines the ultimate moment capacity of SFRC to that of conventionally reinforced concrete.

5. REINFORCED CONCRETE STRESS BLOCK
ACTUAL
MODEL Ft Fc
Ft = Fc d fc
The moment capacity of conventionally reinforced concrete is determined from the distance between the lines of action of the compressive stress in the concrete and the tensile stress in the reinforcement.
The actual concrete stress block is approximated for this calculation. Ft
Moment Capacity = Ft x d

6. FIBRE CONCRETE STRESS BLOCK
ACTUAL fc ft
MODEL
fe (from beam test)
EQUIVALENT
f’t Ft Fc
0.5D D
0.9D
In fibre reinforced concrete the tensile stress block is more complicated so the model used is of an uncracked concrete section with the tensile stress value being equal to the average residual stress determined on slide 17.
Moment Capacity = f’tb.0.9D0.5D
= fe bD2/6
 f’t = 1/6/0.5/0.9 = 0.37fe

7. DIRECT TENSION
DIRECT TENSION FROM REINFORCEMENT BRIDGING CRACKS IN FLOOR SLABS WORKS TO KEEP THE CRACK NARROW, THEREBY MAINTAINING AGGREGATE INTERLOCK AND HENCE LOAD TRANSFER.
NECESSARY AT SAW CUTS AND INTERNAL CRACKS TO MAINTAIN A HIGH LEVEL OF LOAD CARRYING CAPACITY IN THE SLAB.
MESH – Ft = As . fy
FIBRE – Ft = 0.37feBD

8. TESTING THE REINFORCING PROPERTIES OF FIBRES
This section deals with quantifying the load carrying capacity of fibre reinforced concrete in flexure.

9. Beam Testing
Failure of beam with Steel Fibres

10. ESTABLISHING REINFORCING PROPERTIES FOR SFRC
International test methods:-
1. BEAM TESTS: Several variations on the same theme dependent on the country of origin. I.e. a beam of prismatic cross section is loaded at 1/3rd points with the deflection being at a controlled rate.
Span/3
Width
Height P
There are standardised test methods from Japan, America and a number of European countries that specify the test method for determining the flexural load carrying capacity or TOUGHNESS of fibre reinforced concrete.
The most commonly accepted method is a third point loaded beam test, similar to a concrete flexural strength test but with the deflection rate being controlled while the load is measured.

11. ESTABLISHING REINFORCING PROPERTIES FOR SFRC
International test methods:-
1. BEAM TESTS: Several variations on the same theme dependent on the country of origin. I.e. a beam of prismatic cross section is loaded at 1/3rd points with the deflection being at a controlled rate. P fe
Stress
Once the concrete cracks at the so called “first crack” load, typically taken as very close to the modulus of rupture or flexural tensile strength, any residual strength is solely due to the fibres bridging the crack, which is a function of the fibre anchorage, tensile strength and dosage(fibre count/spacing).
Height
Width
Span/3
Span/3
Span/3

12. TYPICAL BEAM TEST RESULTS
First Crack
P or f
P or f
Concrete Property
A typical result up to first crack, taken as the point where the load deflection line deviates from a straight line. Up to first crack the fibres have little if any affect and the line is determined by the properties of the matrix. 1 2 3
Deflection (mm)

13. TYPICAL BEAM TEST RESULTS
Fibre/Matrix Property
First Crack
Strain Hardening
P or f
Strain Softening
After cracking any residual strength is given by the fibres, with the quality of the anchorage also being related to the strength of the matrix.
The residual load carrying capacity provided after cracking is determined by the physical properties and dosage of the fibres.
An increasing residual load is described as strain hardening behaviour with a decreasing residual load giving strain softening behaviour.
Fibre dosages in ground slabs typically provide strain softening behaviour. 1 2 3
Deflection (mm)

20. Square Panel

21. Efnarc panel test European standard EN 14488 - 5
The punching
flexion test is an ideal test
to check the SFRS behaviour: 1)
A shotcrete tunnel ling behaves like
a slab 2)
The hyperstatic test conditions allow
load redistribution 3)
The test can be carried out with
mesh reinforcement
This test was introduced in 1989 by the French Railway Authority, prior to being accepted and promoted by EFNARC then finally becoming a Euronorm (EN) in 2006.

23. EFNARC SQUARE (INDETERMINATE) PANEL TEST
100 x 100 P
500 x 500
600 x 600 P
Deflection

24. EFNARC SQUARE (INDETERMINATE) PANEL TEST
100 x 100 P
500 x 500
600 x 600 P
High early toughness reinforcement
Low toughness reinforcement
Deflection

25. 80 J
400 J
800 J
1250 J

26. 1.0 vol %
9.1kg/m3
0.5 vol %
4.55kg/m3
0.5 vol %
4.55kg/m3
1.0 vol %
9.1kg/m3

27. POLYPROPYLENE
1.0 vol %
9.1kg/m3
1250 J
0.5 vol %
40kg/m3
STEEL

28. LOAD CARRYING CAPACITY
HOW FIBRES INCREASE
LOAD CARRYING CAPACITY
This section deals with the design approach given in Appendix F of Technical Report 34 from the Concrete Society (UK), The approach is specific to fibre concrete.

29. FULL SCALE TESTING 3000 3000 100 x 100 150 k = .035Nmm3
Any theory has to be proved in practice and this test set up was investigated in Europe to determine what increase, if any, could be provided in the load carrying capacity of a slab on ground, by steel fibre reinforcement.
150
k = .035Nmm3

30. THEORETICAL SLAB RESPONSE
fft P P fe
fft
Load
Deflection
P(fft)

31. THEORETICAL SLAB RESPONSE
fft P fe fe
Load
fft
Deflection

32. THEORETICAL SLAB RESPONSE
Load
Ultimate Limit State
Material Factor
Load Factor
Serviceability Limit State
Deflection

33. PERFORMANCE VERSUS TOUGHNESS
Load
Increasing Toughness(fe)
Ultimate
Ultimate
Ultimate
Plain Concrete
Deflection

34. NEW
PARADIGM
LOAD CARRYING CAPACITY FOR FLOOR SLABS IS A FUNCTION OF FLEXURAL STRENGTH
& TOUGHNESS
This approach represents a new paradigm in the design of slabs on grade.

35. ACTUAL RESULTS Beam Results 6 P1(kN) 180 PUlt(kN) 200 Plain Concrete
RC 60/60
30kg/m3 6 fe
240
340
fe = 3.48N/mm2
These results show how an increase in the residual strength provided in the beam test translates into an increase in both the cracking load” P1”, taken as the load when a crack appeared at the sides of the panel, and the maximum or ultimate load the panel could support.
The highest ultimate load could not be established due to the limit of the loading rig being reached.
Note how an increase in fibre aspect ratio alone translates into a higher performance in both the laboratory and field.
RC 80/60
30kg/m3 6 fe
290
>345
fe = 4.79N/mm2

36. Creep occurs in the visco-elastic phase between Tglass & Tmelting
Creep is the term used to describe the tendency of a material to move or to deform permanently to relieve stresses. Material deformation occurs as a result of long term exposure to levels of stress that are below the yield or ultimate strength of the material. Creep is more severe in materials that are subjected to heat for long periods and near melting point.
Polymeric Materials
Creep occurs in the visco-elastic phase between Tglass & Tmelting
Polypropylene
Tglass = -100C
Tmelting = C

37. CREEP

39. THANK YOU!