As a rule of thumb, small fibres tend to be used where control of crack propagation is the most important design consideration. High fibre count (number of fibres per kg) permits finer distribution of steel fibre reinforcement throughout the matrix ' and consequently, greater crack control during drying process. On the other hand, because they exhibit better matrix anchorage at high deformations and large crack widths, longer, heavily deformed fibres afford better post-crack "strength". However, unlike shorter fibres, the dramatically reduced fibre count of longer product yields correspondingly less control of initial crack propagation.
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When steel fibres are added to mortar, Portland cement concrete or refractory concrete, the flexural strength of the composite is increased from 25% to 100% - depending on the proportion of fibres added and the mix design. Steel fibre technology actually transforms a brittle material into a more ductile one. Catastrophic failure of concrete is virtually eliminated because the fibres continue supporting the load after cracking occurs. And while measured rates of improvement vary, Steel fibre reinforced concrete exhibits higher post-crack flexural strength, better crack resistance, improved fatigue strength, higher resistance to spalling, and higher first crack strength, Figure 2 shows concrete flexural strengths when reinforced at various fibre proportions. Additionally, deformed fibres provide a positive mechanical bond within the concrete matrix to resist pull-out. Steel fibres are available in lengths from 38 mm to 50 mm and aspect ratios between 40 and 60. The fibres are manufactured either deformed or hook end, and conform to ASTM A-820.
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Conventional practice usually concentrates welded wire fabric reinforcement within a single plane of a floor slab. Fabric does very little to reinforced the outer zones, which is why spalling is common at the joints and edges. The primary function of welded wire fabric is to hold the floor slab together after the first small hairline cracks have propagate to larger fractures. This serves to maintain some degree of "structural integrity". Conventional wisdom's approach to floor slabs is to maintain "material integrity" through SFRC mix designs. This integrity is accomplished by:
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Industrial Ground Floor Slabs ' Warehouses, Factories, Aircraft Hangers, Roads, Bridge Decks, Parking Areas, Runways, Aprons and Taxiways, Commercial and Residential Slabs, Piling, Shotcrete, Tunnels, Dams and stabilisation.
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Steel fibre reinforced concrete is a castable or sprayable composite material of hydraulic cements, fine, or fine and coarse aggregates with discrete steel fibres of rectangular cross-section randomly dispersed throughout the matrix. Steel fibres strengthen concrete by resisting tensile cracking. Fibre reinforced concrete has a higher flexural strength than that of unreinforced concrete and concrete reinforced with welded wire fabric. But unlike conventional reinforcement ' which strengthens in one or possibly two directions ' Steel fibres reinforce iso tropically, greatly improving the concrete's resistance to cracking, fragmentation, spalling and fatigue. When an unreinforced concrete beam is stressed by bending, its deflection increases in proportion with the load to a point at which failure occurs and the beam breaks apart. This is shown in Figure 1. Note that the unreinforced beam fails at point A and a deflection of B. A Steel fibre reinforced beam will sustain a greater load before the fist crack occurs (point C). It will also undergo considerably more deflection before the beam breaks apart (point D). The increased deflection from point B to point D represents the toughness imparted by fibre reinforcement. The load at which the first crack occurs is called the "first crack strength". The first crack strength is generally proportional to the amount of fibre in the mix and the concrete mix design.
Two theories have been proposed to explain the strengthening mechanism. The first proposes that as the spacing between individual fibres become closer, the fibres are better able to arrest the propagation of micro cracks in the matrix. The second theory holds that the strengthening mechanism of fibre reinforcement relates to the bond between the fibres and the cement. It has been shown that micro cracking of the cement matrix occurs at very small loads. Steel fibres, then service as small reinforcing bars extending across the cracks. So as long as the bond between the fibres and cement matrix remains intact the Steel fibres can carry the tensile load. The surface area of the fibre is also a factor in bond strength. Bond strength can also be enhanced with the use of deformed fibres, which are available in a variety of sizes.
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The proportions of Steel fibres in mix designs usually range from 0.2% to 2.0% (15 to 150 kg/m3 ) of the composite's volume. Key factors to consider largely depend on the application under consideration and/or the physical properties desired in the finished project. Mix designs with fibre proportions above 60kg/m3 are usually adjusted to accommodate the presence of millions of steel fibre reinforcing elements. The adjustments are an increase in the cement factor, a reduction in the top size of the coarse aggregate and the addition of a super plasticiser. Prototype testing is recommended to determine the optimum design for each application.
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Fibre-reinforced concrete is a composite material comprised of traditional concrete and steel fibres (look picture below).
Normal unreinforced concrete is brittle with a low to not existing tensile strength and strain capacity.
Steel fibres increase durability and ductility of the concrete mix as well as decrease installation and labour cost.
Slender structures such as CMG Headquarters in Beijing could be achieved.
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The concept of using fibres as reinforcement is not new. Fibres have been used as reinforcement since ancient times. Historically, horsehair was used in mortar and straw in mudbricks.
In the s, asbestos fibres were used in concrete. In the s, the concept of composite materials came into being and fibre-reinforced concrete was one of the topics of interest. Once the health risks associated with asbestos were discovered, there was a need to find a replacement for the substance in concrete and other building materials.
"There is strong evidence of asbestos leading cancers of the lung, larynx and ovaries," comments Ruban Selvanayagam of property renovation / buying company from the UK.
By the s, steel, glass (GFRC), and synthetic (such as polypropylene) fibres were used in concrete. Research into new fibre-reinforced concretes continues today.
Courtesy of Tyler Ley
' The increased load-bearing capacity of concrete
' Reduction of concrete slab thickness
' Load capacity is not diminished by concrete cracks (crack control)
' Increased durability
' Low maintenance costs ' extended service life
' Improved flexural properties
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' Reduced absorption of water, chemicals, etc.
' Can be used on the fast track schedule.
' Easier positioning of joints (fewer joints required)
' Reduced site labour for managing steel reinforcement
' Reduced project costs ' ensures economical designs
' Increased impact and abrasion resistance
' Even distribution of fibres throughout the concrete (concrete tensile strength can be specified)
' Tougher surface with fewer bleed holes (improved concrete quality expected)
' Savings will be greater for heavier crack control systems
Slender structural members loaded axially in compression will experience buckling. A relatively slender compression member (e.g. a column) may deflect laterally and fail by bending rather than failing by direct compression. The behaviour can be demonstrated by...
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' No requirement for heavy lifts of rebar and labour requirements. Reinforcement is incorporated in the mix.
' Corrosion-free surface finish.
' Reduces permeability of concrete (because micro-cracks are controlled).
' No deformation of corner castings.
' No Eurocode Standards yet addressed for steel fibre reinforcement Design processes. However individual National Annex of some countries may provide some guidelines regards design suggestions.
' More expensive than traditional rebar. Can't be used in heavy loadings situations ' rebar is preferred.
' May require manufacturer license for batching this type of concrete mixes.
' Labour workers may require training.
- Steel decks.
- Pile-supported floors.
- Power-station floor slabs - The opportunity of pre-fabricated slabs manufactured at the factory and brought on-site for installation.
- Use with rebar reinforcement increasing strength using less rebar.
- Flat pavements.
- Existing columns strength reinforcement.
- Tunnel linings reinforcement.
- Potential Pile material.
Dramix® steel fiber concrete reinforcement
Sources: Wikipedia, www.bekaert.com ,
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