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SFRC: Practical Considerations andCommercial Feasibility

Abstract: During the last three decades SFRC was considered a new technology for Construction Industry. However this tech-nology has found high acceptance among today’s Construction industry. Currently, steel fibers are used mainly in Industrial flooring, Tunneling and Pavements etc.

Construction Time and durability are the main factors among the various advantages which help SFRC to command its superiority over other methods.

In our country lot has been written or published about SFRC, but we are not using this technology as it is being used in other countries there is a definite and detail approach on how to design Fiber concrete and achieve a homogeneous dispersion of Steel fibres. Steel fibre geometry and grading of concrete play a very important in role in practicalities of SFRC.

Following article talks about various aspects of Steel Fi-bre reinforced Concrete Viz. Design Methods, Design of SFRC Floor based on lose berg’s yield line Model Selection Criteria, Mix Design and other practical considerations and commercial feasibility.

Definition

Steel fibre reinforced concrete is defined as a concrete, containing discontinuous discrete steel fibres. Steel fibres are incorporated in Concrete to improve its Crack resistance, Duc-tility, Energy absorption and impact resistance characteristics. Properly designed and dosed SFRC can reduce or even contain cracking, a common cause for concern in plain concrete.

Scope

Concrete composition, admixtures, placing and curing play another evident role but here focus will be on design Principals and Methods a sample design of SFRC Industrial floors using Drapro and selection Criteria of Steel Fibre.

Design Methods

SFRC necessarily behaves very different as that of plain concrete. The performance of SFRC varies when compared

in post crack stage. Conventional methods do not necessar-ily consider post crack behaviour of concrete. Design method based on Lose bergs yield line model considers post crack strength of concrete in a right manner hence it is till date the best method to design SFRC as Shown in Table 1 and Picture 1

CONCRETE: SFRC

Sr Design Methods Applicability Why Test results Limitation/Economy

1Elastic – Elastic (Westerguardard or FEM )

Applicable but not suitable

Post crack behaviour and sys-tem properties are not taken in

to account

Far from reality (Actual Test

results)

Rather very safe Hence not economical

2 Elastic –Plastic

Applicable and closer to more accurate Plas-

tic- Plastic

Post crack behaviour Proper-ties are taken into account to

some extent related toFlexural Strength

Closer to Reality

Fibres do not increase flex-ural Strength of the section

within the section but increase load bearing capac-

ity of the system

3 Plastic- Plastic Applicable and Suitable

Considers Ductility of steel fibre reinforced concrete and both material as well as system

properties in account

Closer to actual results

Generally economical as compared with Plain or Rebar

reinforced concrete

Table No 1 Comparison of various design methods

Picture 1 contains a comparison of real scale test results and the results of back-calculation according to the different design approaches. It demonstrates the importance of taking the right design approach for elastically supported steel fiber reinforced concrete slabs. As a simple guideline, the results of elastic-elastic calculation can never be more economic than those of a plastic-plastic calculation providing same material properties and level of safety. The elastic-plastic approach is in the range of plastic-plastic approach.

Design of an Industrial Floor

Industrial floors are generally subjected to Loads such as point load, UDL and Wheel Load. In Interest of explaining load effects certain loads and sub base values are assumed to ar-rive at Flexural Stress and corresponding dosage. Other as-sumptions such as Temperature, Joint distance, loading factor can be made available on request.

- Input –loads- Point Loads

Picture 1: comparison of real scale test results and results of back-calculation

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Above figure (Picture 2) illustrates Point loads arising from Rack loads, Stacking Area, Lines Etc.

We need to design a floor which is efficient of taking these loads at various locations such as joint of panels, centre of pan-els etc.

Anticipated Location of Load

Input Sub base

Sub base plays an important role in Floor. Generally fol-lowing sub base (Picture 6) is seen in industries. To analyze the effect of sub base on floor design, it is necessary to arrive at equivalent E modulus or CBR value of the sub base.

If there are more than 2 layer of sub-base defined the equivalent E-modulus of the ground is calculated using the formula below

Picture 2: Point Loads

Picture 3: Various location of loads

Wheel Loads

Wheel loads are loads coming form Moving Equipments like Fork Lift , The diagram gives details of Loads arising out of a 6 ton Capacity Fork Lift having a tire pressure of 1.5 N/mm ^2 ( Picture 4)

UDL

Above Figure (Picture 5) illustrates UDL of 5 Ton /M ^2

Picture 4: Wheel loads

ResultAs it is not known beforehand which yield will occur first,

we have to consider all possible load combinations. After con-sidering various load combinations and locations following maximum moments (Table 2) are foreseen.

Assumptions / Design Criteria

Steel Fibres

Selection Criteria

The most important aspects controlling the performance of steel fibres in concrete are as follows

- Tensile Strength on the wire( > 1225 Mpa)- Aspect ratio- Geometrical shape

Higher aspect ratio (Picture 8) always gives better perfor-mance of the SFRC with respect to flexural strength, impact resistance, toughness, ductility, crack resistance etc.

Unfortunately, the higher the aspect ratio and volume con-centration of the fibre, the more difficult the concrete becomes to mix, convey and Pour. Thus there are practical limits to the

Ultimate Limit State Serviceability Limit State

Bending moments (kNm)

Loads (m+m’)max 5.67 kNm (m+m’)max 4.02 kNm

Shrinkage Ms 1.41 kNm

Temperature MT> 1.84 kNm

Settlement Mw 0.00 kNm

Floor thickness 120 mm

Required SF concrete flexural stress 1.09 N/mm2 0.86 N/mm2

Table No 2 Result

Picture 5: UDL

CONCRETE: SFRC

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Ultimate Limit State: for a dosage of 15 kg/m3 Dramix RC 80/60-BN.

Serviceability Limit State: for a dosage of 15 kg/m3 Dramix RC 80/60-BN

bres present no difficulty in mixing. They are added as an extra aggregate and require no special equipment to be added to the mix, whether dry mix or wet mix. The hooked ends improve the bond and anchorage of the Dramix steel fibres in the concrete/shotcrete and increase the reinforcing efficiency and ductility. Hooked ends are proved to be best as compared to any other shape of fibres. Bekaert has done extensive research on same copies of which can be made available on request.

Fibre Dosage

This is one of the most important elements in SFRC. As discussed earlier fibre performance clearly depends upon pa-rameters like tensile strength, Aspect Ratio, Anchorage. The dosage of fibres for a certain performance varies as per type of fibre used .This can be established by making a proper design followed by field test. Following table gives comparison of vari-ous types of fibres in terms of dosage.

Comparison with Alternatives

A conventional pavement with 200 mm Thk with single Mesh can be replaced by a 120 mm Thk (SFRC) pavement with following combinations.

Although unit cost of lower aspect ratio (45) fibre is less, due to high dosage ( 31.5 ) Kg) per M ̂ 3 cost of SFRC becomes very high as compared to that of SFRC with lower dosage ( 15 kg ) of High Aspect ratio ( 80 ) Fibres.

Ultimate Limit State Serviceability Limit State

Concrete design stress 1.45 N/mm2 2.18 N/mm2

Dramix®

Type RC 80/60-BN Type RC 80/60-BN

Dosage 15 kg/m3 Dosage 15 kg/m3

1.14 N/mm2 1.39 N/mm2

SF Ductility (%) 41.08 50.00

E k value : 3000.00 N/mm2

Concrete compressive strength, f ck :

C20/25

For ultimate limit state, the gov-erning load case is :Four wheels in a rectangle - Saw Cut

5.67 kNm

For serviceability limit state, the governing load case is :Four wheels in a rectangle - Saw Cut

7.28 kNm

Temperature differential between top and Bottom of the slab

28 °C

Coefficient of friction (µ) between slab and sub base :

0.50

Dramix ® Solution

Floor thickness : 120 mm

Dosage : 15 kg/m3

Fibre type : RC 80/60-BN

Re,3 value : 41.08 %

Equivalent flexural strength (Ffct,eq,150) :

1.52 N/mm2

Max joint spacing : 4000 mm * 4000 mm

Table No 4 Governing case & proposals

amount of single fibres, which can be added to SFRC, with the amount varying with the different geometrical characteristics of the several fibre types. Loose steel fibres with a high 1/d as-pect ratio, which is essential for good reinforcement, are diffi-cult to add to the concrete and to spread evenly in the mixture.

BEKAERT has glued (Picture 9) the loose fibres together with water-soluble glue into bundles of 30-50 fibres to facilitate handling of the Dramix steel fibres. The individual Dramix steel fibres have the necessary high 1/d aspect ratio, but as they are glued together in compact bundles, they have approximately the same size as the other aggregates. Glued Dramix steel fi-

Picture 7: Dramix® steel fibres

Picture 8: Aspect Ratios Picture 9: Glued Dramix® steel fibres

Fibre Type Type Len-gth

Diam-eter

Aspect Ratio( L/D)

Dosage perM ^ 3 *

mm mm Length/Diameter

Kg

RL 45/50 Loose 50 1.05 48 31.5

RC 65/60 Glued 60 0.9 67 20

RC 80/60 BN Glued 60 0.75 80 15

Table No 5 comparisons of various types of fibres

CONCRETE: SFRC

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Practical considerations

Steel fibre reinforced concrete is better concrete as com-pared to RCC in certain applications. To make this technology practically possible it is very much necessary to give impor-tance to fibre geometry, Concrete consistency, gradation Etc. What we want is concrete with right mix and Homogeneous dispersion of steel fibres (As below)

tion. What fibres want is concrete with enough paste around the aggregates.

Case I –Practical Project at Coimbatore Given factsMix Design

Picture 10: Fresh Concrete Picture 11: X-ray image of SFRC

Fibre Geometry

Length of the fibre should be more than sum total two Ag-gregate sizes (Picture 12). At the same time fibre length should not exceed 2/3rd of the inner dia of the conveying system (Pic-ture 13).

Here first factor is related to interlocking of two aggregates whereas second factor is related to workability of concrete through the pumping system.

Picture 12: (Minimum length of fibre)

Picture 13: (Maximum length of fibre)

Mix Design for Hansen/Shapporji Project As on 11.2.8 Reference PSG COLLAGE REPORT P/SM/T &CON/LN1309/2007/34D DATED 22.01.08

Description

Grade of Concrete M30

Required Slump 40-80

Type Of Cement OPC 43 GRADE

Grading of Sand Zone II

Maximum Size of Coarse aggregate 20

Specific Gravity

Cement 3.15

Sand 2.67

Coarse Aggregate 2.69

60 to 40 ratio of 20 and 12.5 Dia Aggregate

Bulk Density KG/M ^3

Cement 1440

Sand 1570

20 MM Coarse Aggregate 1542

12.5 MM Coarse Aggregate 1565

Water Absorption ( %)

Sand

Coarse Aggregate 0.41

Target Mean Strength ( N/MM ^2) 38.25 Mpa

Standard Deviation = 5.0 Mpa

Water Cement Ratio 0.4

Water content per m ^3 of concrete ( kg) 144

Sand as percenatge of total aggregate by Absolute volume

35

Entrapped Air as % of Volume of Concrete 2

Cement Content per M ^3 of concrete (kg) 360

Sand per M ^3 of Conccrete (KG) 674.4

Coarse Aggregate per m ^3 of Concrete (KG)

1261.9

(20 MM AND 12.5 mm In ratio of 60.40)

Admixture ( kg) 1.44

Mix Proportion by Weight

C, S ,CA ( 20 MM) ,CA(12.5MM) ,W 1:1.873,2.103,1.402. ,0.4

C= CEMENT, S = SAND, CA COARSE AGREGATE, W = WATER

Quantities of Materials( KG) Per M ^3 OF CONCRETE

Cement 360

Sand 674.4

Coarse Aggregate ( 20 MM) 757.14

Coarse Aggregate ( 12.5 MM) 504.6

Water 144

In order to have more networking of fibres it is suggested to have fibres with highest available L/D Ratio or least available diameter which finally gives more fibres per kilo (Picture 13)

Concrete Consistency and Gradation In addition to selection of appropriate fibres it is very much

necessary to have consistent concrete with continues grada-

Picture 14: Network of fibres Picture 15: Sieve curves

CONCRETE: SFRC

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Steel Fibres

Type 1

Length : 60 MMDiameter : 0.9 MMFormation : Glued Anchorage : Hooked End (Dramix)Tensile Strength : > 1000 N/MM ̂ 2Dosage : 30 KG/ M^3

Type 2

Length : 60 MMDiameter : 0.75 MMFormation : Glued Anchorage : Hooked End (Dramix)Tensile Strength : > 1000 N/MM ̂ 2Dosage : 20 KG/ M^3

In order to create more paste in existing formulae of con-crete following suggestions were made to job site.1. Depending on availability pl. add either of following (30-50

Kg per M ̂ 3, Fine sand <= .125mm, Fly Ash, 3. GGBS)2. Start from W/C Ratio of 0.5 and take trials up to 0.463. Increase cement content to 380-400 KG ( Trail and error)4. Increase slump to minimum 80 and maximum 120 ( Trial

and Error)It was difficult to get fine sand of required fineness so it

was decided to increase 20-40 KG of existing fine grade sand (ZONE II).

Six Samples of various combinations were checked for fi-bre dispersion as follows.

Case II – Commercial as Per Annexure I

Conclusion

Although proper design and economics is important for the project it is very much necessary to engineer the concrete to suit the selected fibre geometry. Concrete consistency and gradation should be different for every mix and should depend on the type of fibre as suggested by manufacturer.

Steel fibre reinforced Industrial floors can be designed us-ing Lose berg’s Yield line model. At www.bekaert.com/building one can register to get a free design of Steel fibre Industrial floors based on the inputs provided.

Steel fibres being an essential part of this design should be selected very carefully as discussed in the paper. More em-phasis should be given on total cost impact than per unit cost as mentioned in the Annexure II

Admixture 1.44

Confirmatory Test Result

7 days Compressive Strength 33.7

Expected 28 Days Compressive Strength 50.5

Workability

Slump 62

Required fiber content

Actual as per Sieve Test

Variation in % Slump

Grams Grams % MM

1060 974 8.11% Collapse

1060 1041 1.79% 80

1060 891 15.94% 80

706 729 -3.26% 130

706 570 19.26% 130

706 635 10.06% 170

Average 8.65%

Table No 7 Results of washout test

No balls were observed during the mix W/C Ratio maintained was 0.48/0.49Further improvements at the time of actual project can be

as follows.

1. Make fine sand available and reduce cement content2. Reduce water cement ratio to 0.463. Maintain slump in the range of 80-1204. If possible increase mixer speed to 18 RPM

References

1. Gerhard Vitt Design –Presentation at Malenovice approach for Dramix In-dustrial floors

2. Beckett D, Humphreys J The Thames Polytechnic , Dart ford : Compara-tive tests on Plain , Fabric Reinforced and Steel Fibre reinforced Concrete Ground Slabs ,

3. Lose berg A : Design Methods for structurally Reinforced Concrete Pave-ments , Sweden, 1961

4. Thooft H : Dramix Steel Fibre Industrial floor Design in accordance with the Concrete Society TR34

5. Practical guide to the installation of Dramix Steel fibre concrete floors.6. Ganesh P. Chaudhari , Design of SFRC Industrial floor Indian Concrete In-

stitute , Seminar on Flooring and Foundations7. Ganesh P. Chaudhari, Design of Durable SFRC Industrial Floor, Interna-

tional conference of “Sustainable Concrete Construction “ACI, 8-10 Febru-ary, Rantagiri, India. w

Parameter Acceptance Criteria Significance Remark

Tensile Strength

Rm nom = 1225

N/MM^2

Higher tensile strength , Better

performance

Anchorage Hooked end Better Anchorage

Hooked end gives Better anchorage as compared

with other forms of anchorage such as Flat or

corrugated

Length ( MM) 60

Length of Fibre should at least

cover three major aggregates

Diameter ( MM) 0.75

Lesser the diameter , more number of

fibres per kg

More fibre gives more Length , More surface

Area/Volume , Better Cor-rosion Resistance

Aspect Ratio ( L/D) 80

Higher aspect ratio leads to better

performance

Length Per KG 280 Meter

More length per KG gives optimum

results

Formation Glued fibreGlued fibre ensures

better dispersion and no fibre balling

Tolerance ± 7.5 AvgCloser Tolerance leads to designed

performance

No Of Fibres Per KG 4600

More fibres more network , More

ductility

Standards

CE-label system 1 according

EN 14889-1

Table No 8 Annexure II Selection Criteria for Steel fibres

CONCRETE: SFRC