Steel Fibres - Advantages And Disadvantages

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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Properties of reinforcement

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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Steel fibre reinforced concrete (SFRC) Floor slabs.

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:

• Increasing the initial first crack strength.
• Large numbers of fibres intercepting the micro-cracks and preventing propagation by controlling tensile strength.
• Unlike rebar and welded wire fabric, fibres are dispersed throughout the slab to reinforce isotropically, so there is no weak plane for a crack to follow.
• Increases in flexural strength can make it possible to use a thinner slab and eliminate the cumbersome welded wire fabric.
• Whether it is for lighter duty commercial service or for heavy manufacturing, SFRC slabs are capable of withstanding any load. The only variable is the addition rate of fibre, which could be as low as 12.5kg/m3 to as high as 100 kg/m3.

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How they save time and money

COMPLETELY ELIMINATE STEEL FABRIC REINFORCEMENT, SAVING ON BOTH MATERIALS AND LABOUR

• Reduce slab thickness giving savings in concrete and placement costs.
• Possibilities of wider joint spacing. Save on joint forming costs and joint maintenance
• Simplicity of construction. Simpler joints and no more errors in steel fabric positioning
• Increase speed of construction. Save time and reduce costs.

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Technical and user benefits

• Significantly reduced risk of cracking.
• Reduced spalling joint edges.
• Stronger joints.
• High impact resistance.
• Greater fatigue endurance.
• Reduced maintenance costs.
• Longer useful working life

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Typical areas of applications include

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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Improved strength and durability

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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Product mix designs

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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Advantages

• Reinforcing concrete with Steel fibres results in durable concrete with a high flexural and fatigue flexural strength, improved abrasion, spalling and impact resistance.
• The elimination of conventional reinforcement, and in some cases the reduction in section thickness can contribute to some significant productivity improvements. Steel fibres can deliver significant cost savings, together with reduced material volume, more rapid construction and reduced labour costs.
• The random distribution of Steel fibres in concrete ensures that crack free stress accommodation occurs throughout the concrete. Thus micro cracks are intercepted before they develop and impair the performance of the concrete.
• Steel fibres are a far more economical design alternative.

Disadvantages

• Steel fibres will not float on the surface of a properly finished slab, however, rain damaged slabs allow both aggregate and fibres to be exposed and will present as aesthetically poor whilst maintaining structural soundness.
• Fibres are capable of substituting reinforcement in all structural elements (including primary reinforcement), however, within each element there will be a point where the fibre alternative&#;s cost saving and design economies are diminished.
• Strict control of concrete wastage must be monitored in order to keep it at a minimum. Wasted concrete means wasted fibres.

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The most common way is to use dry shake hardeners on the top to suppress the steel fibers near the surface. In most cases when dry shake hardeners are used; some fibers still exist on top after finishing, and the problem gets even worse if you mechanically grind and polish the surface; as you will remove the cement paste which hides the fibers. Typically, just 3-4 kg/m2 (0,6-0,8 lb/sqft) of dry shake hardener is applied on top and that is not enough if the surface is going to be polished, as part of the dry shake will get mixed up with base concrete, resulting in just ~1mm layer on top. In many cases grinding and polishing dry shake with only 3-4kg /m2 (0,6-0,8 lb/sqft), you will grind through the dry shake, or you will exposed the fiber which is in the dry shake layer.

The only way you can grind and polish with ZERO fiber exposure is to apply dry shake hardeners in 2-4 layers and using minimum 6 kg/m2 (1,2 lb/sqft) of dry shake hardener or even 10-12 kg/m2 (2-2,5 lb/sqft) with lighter colors. Applying more layers than just one or applying a thicker layer has always been a risk, as dry shake hardeners need water from base concrete to hydrate and for bonding. Thicker or more layers will have a much higher water demand; many contractors avoid applying dry shakes because of a high risk of delamination.

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