92 The Masterbuilder - December 2013 • www.masterbuilder.co.in

Use of SFRC in Industrial Flooring

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, Ductility, 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 necessarily 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

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

Ganesh Chaudhari General Manager Building Products, Bekaert India

During the last three decades SFRC was considered a new technology for construction Industry. However this technology 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 Fibre 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.

Sr Design Methods Applicability Why Test results Limitation/Economy

1Elastic – Elastic (Westerguard ard or FEM )

Applicable but not suitablePost crack behaviour and system prop-erties are not taken in to account

Far from real-ity (Actual Test results)

Rather very safe Hence not eco-nomical

2 Elastic –PlasticApplicable and closer to more accurate Plastic- Plastic

Post crack behaviour Properties are taken into account to some extent related to Flexural Strength

Closer to Reality

Fibres do not increase flexural Strength of the section within the section but increase load bearing capacity of the system

3 Plastic- Plastic Applicable and SuitableConsiders 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

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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.

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

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

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 arrive at Flexural Stress and corresponding dosage. Other assumptions such as Temperature, Joint distance, loading factor can be made available on request.

- Input –loads - Point Loads

Picture 2: Point Loads

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 panels etc.

Anticipated Location of Load

Above Figure (Picture 3) illustrates various locations of loads as discussed in above paragraph.

Wheel Loads

Wheel loads are loads coming form Moving Equipments like

Picture 3: Various location of loads

Picture 4: Wheel loads

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

Input Sub base

Sub base plays an important role in Floor. Generally following sub base (Figure v) 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

Result

As it is not known beforehand which yield will occur first, we have to consider all possible load combinations. After

Picture 5: UDL

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considering various load combinations and locations following maximum moments (Table 2) are foreseen.

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

Picture 6: Input Sub Base

1

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

ffct~eq,150 1.14 N/mm2 ffct~eq,450 1.39 N/mm2

SF Ductility (%)

41.08 50.00

Table No 3 Materials

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

Assumptions / Design Criteria

E k value : 3000.00 N/mm2

Concrete compressive strength, f ck : C20/25

For ultimate limit state, the governing 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

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

Picture 7: Dramix® steel fibres

Picture 8: Aspect Ratios

Unfortunately, the higher the aspect ratio and volume concentration of the fibre, the more difficult the concrete becomes to mix, convey and Pour. Thus there are practical limits to the 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 aspect ratio, which is essential for good reinforcement, are difficult 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 fibres present no difficulty in mixing. They are added as an extra aggregate and require no special equipment to be

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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 parameters 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 various 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.

Practical considerations

Steel fibre reinforced concrete is better concrete as compared to RCC in certain applications. To make this technology practically possible it is very much necessary to

give importance to fibre geometry, Concrete consistency, gradation Etc. What we want is concrete with right mix and Homogeneous dispersion of steel fibres (As below)

Fibre Geometry

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

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

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

Case I –Practical

Project at Coimbatore Given facts Mix DesignSteel Fibres

Type 1

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

Picture 9: Glued Dramix® fibres

Sr Fibre Type Type Length Diameter Aspect Ratio ( L/D) Dosage per M ^ 3 *

mm mm Length/Diameter Kg

1 RL 45/50 Loose 50 1.05 48 31.5

2 RC 65/60 Glued 60 0.9 67 20

3 RC 80/60 BN Glued 60 0.75 80 15

*Results valid only for Dramix FibresTable No 5 comparisons of various types of fibres

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

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Type 2

Length : 60 MM Diameter : 0.75 MM Formation : Glued Anchorage : Hooked End (Dramix)

Picture 12: LMin (Minimum length of fibre)

Picture 13: LMIN(Maximum length of fibre)

Picture 14: Network of fibres

Picture 15: Sieve curves

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 1.9

Coarse Aggregate 0.41

Target Mean Strength ( N/MM ^2)

Standard Deviation = 5.0 Mpa 38.25 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

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

Admixture 1.44

Confirmatory Test Result

7 days Compressive Strength 33.7

Expected 28 Days Compressive Strength 50.5

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

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Tensile Strength : > 1000 N/MM ^2 Dosage : 20 KG/ M^3

In order to create more paste in existing formulae of concrete 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 0.46 3. 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 fibre dispersion as follows.

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

1. Make fine sand available and reduce cement content 2. Reduce water cement ratio to 0.46 3. Maintain slump in the range of 80-120 4. If possible increase mixer speed to 18 RPM

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 using 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 emphasis should be given on total cost impact than per unit cost as mentioned in the Annexure II

References

1. Gerhard Vitt Design –Presentation at Malenovice approach for Dramix Industrial floors

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

3. Lose berg A : Design Methods for structurally Reinforced Concrete Pavements , 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

Institute , Seminar on Flooring and Foundations 7. Ganesh P. Chaudhari, Design of Durable SFRC Industrial Floor,

International conference of “Sustainable Concrete Construction “ACI, 8-10 February, Rantagiri, India.

Workability

Slump 62

Table No 6 Mix design

Sr Required fiber content

Actual as per Sieve

TestVariation in % Slump

Grams Grams % MM

1 1060 974 8.11% Collapse

2 1060 1041 1.79% 80

3 1060 891 15.94% 80

4 706 729 -3.26% 130

5 706 570 19.26% 130

6 706 635 10.06% 170

Average 8.65%Table No 7 Results of washout test

Sr Param-eter

Accep-tance

CriteriaSignificance Remark

1Tensile

Strength

Rm nom = 1225

N/MM^2

Higher tensile strength , Better performance

2 AnchorageHooked

endBetter Anchorage

Hooked end gives Better anchorage as compared

with other forms of anchorage such as Flat

or corrugated

3Length ( MM)

60Length of Fibre should at least cover three major aggregates

4Diameter ( MM)

0.75Lesser the diameter , more

number of fibres per kg

More fibre gives more Length , More surface Area/Volume , Better Corrosion Resistance

5Aspect Ratio (L/D)

80Higher aspect ratio leads to

better performance

6Length Per

KG280 Meter

More length per KG gives optimum results

7 FormationGlued fibre

Glued fibre ensures better dispersion and no fibre

balling

8 Tolerance± 7.5 Avg

Closer Tolerance leads to designed performance

9No Of

fibres Per KG

4600More fibres more network ,

More ductility

10 Standards

CE-label system 1 accord-ing EN

14889-1

Table No 8 Annexure II Selection Criteria for Steel fibres

Industrial Flooring