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Transcript of Use of SFRC in Industrial Flooring - The Masterbuilder ... · Use of SFRC in Industrial Flooring...
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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www.masterbuilder.co.in • The Masterbuilder - December 2013 93
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
Industrial Flooring
94 The Masterbuilder - December 2013 • www.masterbuilder.co.in
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
Industrial Flooring
96 The Masterbuilder - December 2013 • www.masterbuilder.co.in
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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www.masterbuilder.co.in • The Masterbuilder - December 2013 97
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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98 The Masterbuilder - December 2013 • www.masterbuilder.co.in
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