Fibre-reinforced self-compacting concrete

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1 Fibre-reinforced self-compacting concrete Stephan Zeranka Stellenbosch University February 2013 Durban, Port Elizabeth, Cape Town & Johannesburg 1

2 My background BEng (Civil) 2010 The impact of concrete rheology on the mechanical properties of steel fibre-reinforced concrete MScEng/PhD (Structural): Characterising the shear behaviour of reinforced steel fibre concrete 2

3 Fibre-reinforced concrete Introduction: What is FRC? Composite material which consists of: cement, aggregates and discrete discontinuous short fibres 3

Fibre-reinforced concrete Introduction: The origin of FRC Fibrous materials used since ancient times Reduce cracking, improve toughness and strength of brittle building materials Ancient

4 Fibre-reinforced concrete Introduction: The origin of FRC Fibrous materials used since ancient times Reduce cracking, improve toughness and strength of brittle building materials Ancient civilisations of West Asia, Africa and South America used straw fibres used to reinforce adobe bricks Horsehair to reinforce masonry mortar and plaster Over the past 100 years, the use of fibres as reinforcement in concrete has been growing Adobe bricks 4

5 Fibre-reinforced concrete Introduction: Types of fibres: Basic fibre categories include: steel, glass, synthetic (organic polymers) and natural fibres Steel fibre is the most common fibre type in the building industry; plastic, glass and carbon fibres contribute to a smaller part of the market Fibres have different properties resulting in different material and structural behaviour and therefore have a variety of structural and non-structural applications 5

Fibre-reinforced concrete Introduction: Types of fibres, continued: Fibre type Appearance Typical applications Glass Glass reinforced cement (GRC),

6 Fibre-reinforced concrete Introduction: Types of fibres, continued: Fibre type Appearance Typical applications Glass Glass reinforced cement (GRC), nonstructural uses, flat sheet and pipe applications, precast products, agriculture, architectural cladding and components, small containers etc. 6

Fibre-reinforced concrete Introduction: Types of fibres, continued: Fibre type Appearance Typical applications Steel Pumped concrete, shotcrete applications: ground support, rock slope stabilisation,

7 Fibre-reinforced concrete Introduction: Types of fibres, continued: Fibre type Appearance Typical applications Steel Pumped concrete, shotcrete applications: ground support, rock slope stabilisation, tunnelling and repairs, slurry-infiltrated fibre concrete, supplementary reinforcement in concrete, flat slabs on grade, where placement of reinforcing bars is difficult e.g. hydraulic structures (dams, spillways), large industrial slabs, tunnel linings, and bridge decks etc. 7

8 Fibre-reinforced concrete Introduction: Types of fibres, continued: Fibre type Appearance Typical applications Synthetic fibres e.g. acrylic, aramid, carbon, nylon, polyester, polyethylene, polypropylene, polyvinyl alcohol Plastic shrinkage and bleeding control, slabs on grade, floor slabs, stay-in-place forms in multi-storey buildings etc. Polyester fibres 8

9 Fibre-reinforced concrete Introduction: Types of fibres, continued: Fibre type Appearance Typical applications Natural fibres e.g. Unprocessed: coconut, sisal, sugarcane, bamboo, jute, flax, wood and vegetable fibres; Processed: wood cellulose Sisal fibres Low-cost, low-energy, availability, developing countries; used primarily in the African construction industry for concrete pipes, tanks and houses; less durable than other fibre types, prone to degradation, natural fibre coatings have been developed, further research on durability under severe environmental conditions is needed for implementation in develop countries 9

10 Fibre-reinforced concrete Material enhancement and the mechanisms involved: Material enhancement through fibre-reinforcement: Unreinforced concrete is brittle and does not have significant post-cracking capacity or ductility Provision of reliable post-fracture capacity and toughness are the most notable material enhancements achieved by fibre reinforcement Other benefits which vary depending on a number of factors include: Enhanced tensile-, flexural- and shear-strength Impact and abrasion resistance Greater ductility of failure Fatigue resistance (dynamic loading, seismic effects etc.) Reduction of spalling due to thermal shock and thermal gradients (maintain structural integrity at ultimate loads) Shrinkage, expansion, creep, thermal characteristics (fire resistance, freezethaw action) Improved long-term serviceability of structure, maintenance of strength and integrity (improved durability) 10

11 Fibre-reinforced concrete Material enhancement and the mechanisms involved Primary fibre/matrix energy absorbing mechanisms that contribute to the enhanced post-fracture capacity and toughness are: Fibre rupture Fibre pull-out Fibre-bridging and Fibre/matrix de-bonding Effectively changing the material behaviour from brittle to pseudo-plastic Extent and distinctiveness of material enhancement varies considerably depending on the composite type: Composite fracture at matrix cracking (inadequate reinforcement) Strain/deflection softening (localised cracking) Strain/deflection hardening (multiple cracking) 11

12 Fibre-reinforced concrete Energy-absorbing fibre-matrix mechanisms (Zollo, 1997) 12

13 Fibre-reinforced concrete Possible load-deflection responses for a FRC member subject to flexural loading (Naaman et al. 2007) 13

14 Fibre-reinforced concrete Material enhancement and the mechanisms involved: Factors which influence the effectiveness of fibre-reinforcement: Fibre type i.e. fibre- aspect ratio, length, profile (straight or crimped), end anchorage (hooking, teeing or end enlargement), strength, modulus Fibre content, distribution and orientation (more on this later) Matrix strength and fibre-matrix interface bond (more on this later) These factors are influenced by the mix constituents and the mixing and consolidating procedures (more on this later) 14

15 Fibre-reinforced concrete Composite design considerations: Requirements for conventionally placed SFRC applications: Adequate workability to allow placement, consolidation and finishing with minimum effort (similar to conventional concrete) Minimum segregation (higher potential for FRC) and bleeding In addition achieve uniform fibre distribution Balling of fibres must be avoided (function of maximum size and overall gradation of the aggregate fraction, fibre aspect ratio, fibre volume fraction, fibre shape, and method of fibre introduction into mixture) Degree of consolidation influences strength and other hardened-state material properties (as it does for plain concrete) Effect of the aggregate size on the fibre distribution [Grünewald, 2004 after: Johnston, 1996] 15

16 Fibre-reinforced concrete Composite design considerations: Fibre addition results in a loss of workability (reduced slump) Vibration required to increase density, decrease air void content and improve bond with reinforcement Factors which influence the properties of freshly mixed SFRC: Fibre aspect ratio Fibre geometry Fibre volume fraction Matrix proportions (cement paste and aggregate fraction) Compromise between hardened properties and workability 16

17 Fibre-reinforced concrete Composite design considerations: Large fibre aspect ratios and lengths negatively impact workability Techniques for retaining high pull-out resistance while reducing fibre aspect ratio: enlarged or hooked ends, roughened surface texture, crimped, wave-like profiles i.e. mechanical anchorage and surface roughness SFRC mixtures typically characterised by: Higher powder/cement content Higher fine aggregate content (and in some instances an optimised aggregate grading distribution); smaller maximum aggregate particle size (< 10 mm recommended) Conventional admixtures and pozzolans for air entrainment, water reduction, workability, and shrinkage control 17

18 Fibre-reinforced self-compacting concrete (FRSCC) To summarise, the fresh and hardened state performance of FRC is influenced by the: Fibre type, Fibre content, Fibre dispersion, Fibre orientation and the fibre/matrix bond. These factors are influenced by the mix constituents and the mixing and consolidating procedures. The superior workability of self-compacting concrete (SCC) can be used to improve the uniform dispersion and effective utilisation of fibres, which is necessary for the wider and reliable structural use of FRC 18

19 Fibre-reinforced self-compacting concrete (FRSCC) Note: FRSCC is a tailor-made type of concrete Facilitates the production process Ease of manufacture: Mould, cast and finish No placement of bar reinforcement or vibration Workability of FRSCC is an improvement over conventional FRC Combined benefits of SCC in the fresh state and properties of FRC in the hardened state New possible fields of application 19

20 Fibre-reinforced self-compacting concrete (FRSCC) Effect of fibres (steel) on the behaviour of concrete in the fresh state: As with conventional FRC, fibres affect the characteristics of SCC in the fresh state and the key characteristics of SCC have to be quantified: Filling ability (slump-flow, flow-time, v-funnel, mortar funnel, etc.) Segregation resistance and (wash-out test) Passing ability (slump-flow with J-ring, L-box or U-box not applicable for FRSCC) Fibre-reinforcement influences these characteristics in the following ways: Reduction of slump-flow Yield value, plastic viscosity (resistance to flow) and bar spacing required to avoid blocking increase compared to plain SCC Greater potential for segregation 20

21 Fibre-reinforced self-compacting concrete (FRSCC) Main parameters affecting the characteristics of FRSCC in the fresh state: Mixture composition, Aggregate size and distribution Paste properties and content Applied fibre type (shape, stiffness, surface properties and anchorage) and quantity of fibres Production process: mixing process and compaction technique 21

22 Fibre-reinforced self-compacting concrete (FRSCC) Mixture composition of FRC, compromise between requirements on the fresh and hardened state. Key requirements (Grünewald & Walraven, 2001 cited by Grünewald, 2004): Slump-flow > 600 mm No segregation of fibres (visual inspection of fibre distribution in mixer) Homogeneous distribution of SCC: Uniform distribution of fibres, aggregates and cement paste Circular flow spread No clustering of fibres 22

Fibre-reinforced self-compacting concrete (FRSCC) Visual indicators (Grünewald, 2004) with regard to flow pattern which indicate that the maximum fibre content for SCC has been surpassed: A:

23 Fibre-reinforced self-compacting concrete (FRSCC) Visual indicators (Grünewald, 2004) with regard to flow pattern which indicate that the maximum fibre content for SCC has been surpassed: A: Obstruction of free flow mixture too cohesive, reduction of filling ability, high content of entrapped air B: Clustering of fibres and/or aggregates unstable mixture and segregation of fibres or fibre dosage exceeded, large slump-flow C: Clustering and obstruction of free flow combination of A and B A B C 23

24 Fibre-reinforced self-compacting concrete (FRSCC) Optimisation of FRSCC mix design (especially for higher steel fibre contents): Optimise packing density of granular skeleton Limit coarse aggregate content (meet demands of passing ability and performance in the hardened state Ensure adequate paste content and viscosity, counteract segregation and fulfil demand on filling ability Viscosity modifying admixtures to enhance stability of fibres in mixture 24

25 Fibre-reinforced self-compacting concrete (FRSCC) Influence of concrete rheology on the mechanical properties of FRC (FRC vs. FRSCC): Composite behaviour depends on pull-out behaviour of fibres, as well as their orientation and distribution over the crack surface Production process and fresh-state properties (rheology) of concrete affect the above-mentioned Orientation and distribution of fibres affected by flow and bond behaviour of steel fibres, which in turn affect the performance and variability of material behaviour Fibres tend to orientate in flow direction of FRSCC (SCC has lower yield stress, fibres have greater mobility within cement matrix; stiff concrete with higher yield stress has the risk of forming fibre balls and consequently a homogeneous distribution of fibres cannot be achieved) Benefit: Enhanced flexural performance over FRC due to pronounced orientation of fibres in the direction of principal stresses 25

26 Fibre-reinforced self-compacting concrete (FRSCC) Orientation of fibres influenced by: Fibre geometry Concrete rheology i.e. flowability Interaction effects (fibres-aggregates-formwork) The production method (casting and compaction techniques) Note: Preferential fibre alignment due to enhanced rheological properties of SCC may be an advantage or disadvantage depending on the application and whether it can be controlled Assumption of randomly 3D-oriented fibres is difficult to enforce, even for very stiff concrete, since it has to be vibrated, which causes the fibres to orientate into specific planes i.e. FRC is not a homogeneous material An orientation number is often used to describe the effect of various parameters on the fibre alignment This information is essential if the test results are to be translated into structural performance 26

27 Fibre-reinforced self-compacting Comparing SFRSCC and SFRC: concrete (FRSCC) Flexural performance of FRSCC much greater (significantly higher fracture energy) and the variation of the results much lower (Grünewald, 2004), (Boulekbache et al. 2010) Test results of three-point bending tests SCFRC (Grünewald, 2004) versus SFRC (Kooiman, 1998) Load vs. deflection curves for FRSCC-FROC- FRHSC (Boulekbache et al. 2010) 27

28 Fibre-reinforced self-compacting concrete (FRSCC) Characterising the material behaviour of FRSCC: Study orientation of steel fibres and single fibre pull-out behaviour and what we find is: Orientation numbers for FRSCC were higher than for FRC (higher orientation numbers for longer fibres) In most cases, SCC resulted in higher pull-out forces compared to conventional concrete Microstructure of matrix, distribution and orientation of fibres different in SCC and conventional concrete In FRC, entrapped air and neighbouring fibres affect performance of fibres in FRC more than in FRSCC 28

29 Fibre-reinforced self-compacting concrete (FRSCC) Effect of production process on the orientation and distribution of fibres: Production process has a significant influence on the characteristics of FRSCC in the hardened state Actual orientation and distribution of fibres in structure is important to design structural elements with FRSCC There is a need to integrate material characteristics, production processes and structure itself The main stages of the production process influencing fibre orientation: mixing, casting method, dynamic effects (external vibration and flow), formwork geometry and wall effects In order to fully utilize the benefits of fibre reinforcement, the development and implementation of a standardised production method for SFRC is required. Note: Production methods will vary for different applications and trials are required to establish these methods 29

methods Non-destructive methods: X-ray photographs Computer tomography Visualisation of

30 Fibre-reinforced self-compacting concrete (FRSCC) Methods of determining fibre-orientation (orientation number): Destructive methods: Image analysis and counting methods Non-destructive methods: X-ray photographs Computer tomography Visualisation of steel fibre alignment and distribution in a FRSCC specimen using computer tomography (Zeranka, 2012) 30

31 Fibre-reinforced self-compacting concrete (FRSCC) Some structural applications of FRSCC investigated: A: Precast pre-stressed thin-webbed roof elements (Ferrara et al. 2012) B: Sheet piles (Grunewald, 2004) C: Tunnel segments (Grunewald, 2004) C A B 31

32 Fibre-reinforced self-compacting concrete (FRSCC) Conclusions\Summary: Challenges for further application of SFRC: Production methods, reproducibility from lab to site and quality assurance Adequate design methods and standardised testing methods The method for the determination of fibre-orientation and distribution must be faster and less cost intensive (additional tool for design of FRC) The cost of FRSCC can be much higher than conventional reinforced concrete, however incentives are given: Conventional bar reinforcement can be partially or totally replaced by fibres Economical competitive solutions with FRSCC can be achieved, provided that aspects of the production process are optimised for this particular type of building material: Shape of structural elements The production process Storage and transport Labour costs etc. Higher demands on working conditions, intensified research and an increasing number of applications will promote FRSCC even further 32

33 Oceanographic Park Valencia, Spain Shotcrete thin shell structure Combined reinforcement solution: End-hooked steel fibres and mesh Concrete thickness: 6 12 cm 33

34 References ACI 544.1R-96, 1996, Report on fiber reinforced concrete, Michigan: American Concrete Institute: Farmington Hills Boulekbache B., Hamrat M., Chemrouk M., Amziane S., Flowability of fibre-reinforced concrete and its effect on the mechanical properties of the material, Construction and Building Materials 24, 2010, pp Ferrara L., Park Y., Shah S. P., A method for mix design of fiber-reinforced self-compacting concrete, Cement and Concrete Research, 37, 2007, pp Ferrara L., Bamonte P., Caverzan A., Musa A., Sanal I., A comprehensive methodology to test the performance of steel fibre reinforced self-compacting concrete (SFR-SCC), Construction and Building Materials 37, 2012, pp Fulton s concrete technology 9 th Edition, Fibre-reinforced concrete, Midrand: Cement & Concrete Institute, pp Grünewald S., Performance-based design of self-compacting fibre-reinforced concrete, PhD Thesis, 2004 Naaman A. E., Fischer G., Krstulovic-Opara N., Measurement of tensile properties of fiber reinforced concrete: draft submitted to ACI Committee 544, HPFRCC5, Mainz, Germany, July, Zollo, R. F., Fiber-reinforced concrete: an overview after 30 years of development. Cement and Concrete Composites, Volume 19, pp

Fiber Reinforced Concrete (FRC)

Progress in Fiber Reinforced Concrete (FRC) Concrete is relatively brittle, and its tensile strength is typically only about one tenths of its compressive strength. Regular concrete is therefore normally