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

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.

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.

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

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

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

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

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

Beam Testing Failure of beam with Steel Fibres

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.

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

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. .05-0.1 1 2 3 Deflection (mm)

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. .05-0.1 1 2 3 Deflection (mm)

Square Panel

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.

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

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

80 J 400 J 800 J 1250 J

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

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

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), 1994. The approach is specific to fibre concrete.

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

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

THEORETICAL SLAB RESPONSE fft P fe fe Load fft Deflection

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

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

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.

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

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 = 170-1800C

CREEP

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