FIBRE REINFORCED CONCRETE
What is fibre reinforced concrete?
Fibre reinforced concrete is concrete reinforced with discreet short fibres. The concrete as such is referred to as a composite material.
What types of fibres are available for concrete reinforcement?
Fibres range in both shape, length and material. Macro synthetic fibres and steel fibres are commonly seen in the marketplace. Further information on what constitutes a structural fibre for concrete reinforcement can be found in BS EN 14889 2006 Part 1 & 2
- Part 1 Steel fibres. Definitions, specifications and conformity.
- Part 2 Polymer fibres. Definitions, specification and conformity.
How do fibres reinforce concrete?
Fibres are able to reinforce concrete by bonding to the cement similar to the way in which aggregate does. The fibre’s shape and geometry help this bonding process.
How do concrete fibres work in hardened concrete?
Concrete fibres are able to produce Post-crack Residual Strength unlike plain concrete.
Put simply Post-crack Residual Strength is the measure of the reinforcing fibres ability to absorb energy by holding the cracked concrete together.
How do concrete fibres add toughness to hardened concrete?
Toughness as defined by ASTM C-1018, is the measure of fibrous concrete’s ability to sustain load after first crack. This is also sometimes referred to as ductility. Additionally, ASTM C-1116 states toughness achieved in any mixture is primarily a function of the type, length, and amount of fibres employed…”. Plain concrete has no toughness. Fibrous reinforced concrete’s toughness can be measured using either a beam or plate test.
The key elements determining a fibre’s Post-Crack Residual Strength are the material it is manufactured from, its physical configuration which influences its bond to concrete and the dosage of fibres used. This is usually described in kilos per cubic metre (kgs/m3) or pounds per yard (lbs/yd) Quality concrete fibres provide excellent Post Crack Residual Strength or toughness.
MACRO SYNTHETIC FIBRES
What are macro synthetic fibres?
Macro synthetic fibres are fibres made from plastic (mostly polypropylene) usually by extruding plastic pellets to a yarn which is then cut into short lengths. Special manufacturing techniques have allowed these fibres to be manufactured to an extremely high performance so that they can be used as reinforcement in structural concrete applications.
A more precise definition of polymer fibres for concrete can be found in BS EN 14889 -2006 Part 2 Polymer fibres Definitions specifications and conformity. http://shop.bsigroup.com/
Macro fibres have been used to reinforce concrete of all types for the last 25 years and can replace old technologies like steel fibre which has durability concerns.
Are all macro synthetic fibres the same?
No. Manufacturing techniques vary between products and thus fibres offered by different manufacturers will look different, have different technical characteristics and perform differently in concrete. The Association recommends that customers evaluate a particular fibre on its merits by testing prior for a specific application.
Are micro plastic fibres the same as macro synthetic fibres?
No. These fibres are used for different non-structural applications such as to reduce the formation of plastic-shrinkage and settlements cracks at the concrete’s surface and anti-spalling where a concrete element is subjected to a high intensity fire. The only similarity between the two products is that they are manufactured from the same raw material polypropylene. A further definition of micro plastic fibres can be found in BS EN 14889 Part 2. . http://shop.bsigroup.com/
What is macro synthetic fibre reinforced concrete (MSFC)?
MSFC is a composite material made of hydraulic cements, water, fine and
coarse aggregate, and a dispersion of discontinuous fibres. It may also contain pozzolans and admixtures commonly used with conventional concrete. All admixtures meeting ASTM specifications for use in concrete are suitable for use in MSFC.
What is the function of macro synthetic fibres in concrete and how do they perform in concrete?
Unlike welded wire reinforcement or rebar, which is specifically located in a single plane, macro synthetic fibres are distributed uniformly throughout the concrete matrix. The primary function of macro synthetic fibres is to modify micro and macro cracking. By intercepting cracks at their origin, macro synthetic fibres inhibit crack growth. For this reason, MSFC can be used to replace welded wire reinforcement or rebar which is used to control temperature or shrinkage cracks.
What are the benefits of using macro synthetic fibres in place of steel fibres and/or wire welded fabric (wwf)?
Although steel fibres and wwf have been used in recent years MSFs offer significant advantages over steel not least of which is because they do not corrode. Many concrete structures will often crack after construction as concrete is a brittle material and this allows moisture to attack the reinforcement and thus corrode the reinforcement. As MSF are manufactured from polypropylene this cannot happen and so can give the asset a much longer life therefore significantly reducing the long-term cost of an asset.
Other advantages include faster and lighter to handle for the same performance and premixed into the concrete so there is no need for site handing.
TESTING FIBRE REINFORCED CONCRETE
How can I test to determine the performance of different fibres in concrete?
This is an important question as there are many types of fibres on the market with different characteristics and quality. Testing of fibres is done by using either a beam or plate test to achieve a residual energy performance for a certain dosage.
What is ductility and post-cracking behaviour?
Ductility, toughness and residual energy are all descriptions for the post-cracking behaviour of fibre reinforced concrete. As above this can be measured by either a beam or panel test.
TIPS ON USING MSFC
What are some examples of structural applications?
Macro synthetic fibres have been used in a wide variety of concrete applications including reinforcement for shotcrete in underground construction, poured in place concrete for secondary linings and slabs on grade and precast applications.
Refer to our gallery for more information on uses.
When can macro synthetic fibres be added to the concrete?
MSF can be added to the concrete either at the batching plant or at the project site. To get the best of what we call a premixed reinforcement it is better for them to be mixed at the batching plant in a controlled environment.
Can MSFC be pumped?
Pumping has been used to transport MSFC on many projects. In general a mixture that will pump satisfactorily without fibres will pump with fibres. In pumping MSFC, the chute from the readymix truck should be 200 mm above the grate on the hopper. This will prevent the fibre from bridging the gaps in the grate and ensure a steady flow of concrete to the pump. Note that the grate should never be removed from the hopper.
Can I use macro synthetic fibres in self compacting concrete (SCC)?
Yes MSF can be used in a self-compacting concrete. There are many MSF being used for SCC poured in place or for precast elements.
What is fibre balling and how can I avoid this?
Generally the longer and thinner a fibre is the more chance of it balling there is. Today many manufacturers undertake rigorous testing to ensure that the fibre is suitable for the market by ensuring a low balling possibility and they can also assist in the dosing process so as to minimise any balls.
Collation of some of the thinner fibres also prevents balling when they are mixed into the concrete.
How can I guarantee that fibres will be distributed uniformly in the concrete?
Using a quality fibre that is well used in the construction industry will help to ensure that the fibre will be distributed in the concrete. Generally a fibre that is probably added to the concrete when it is batched will allow the fibres to be evenly distributed throughout the concrete.
Does the addition of macro synthetic fibres influence the workability of concrete?
The addition of any type of fibre to a concrete mix will have an affect on its workability but this is dependent on the quantity of fibre and the type of fibre used. Mostly where a mix is used for pumping the affect will be minimal but prior testing should be undertaken to gauge the affect where required.
How can I determine the fibre dosage rate on fresh or on hardened concrete?
The fibre dosage rate is defined by a project specification in order to meet a required performance level. This performance level might be as required in a plate test (mostly used for tunnelling shotcrete applications and defined as Joules) or a beam test (mostly used for civil applications such as precast items and defined as residual strength MPa). Testing is then undertaken to see what fibre dosage rate of a particular fibre is required to meet that performance level.
What are the benefits of high strength premium fibres?
Using a high strength premium fibre will ensure that the long term benefits of its performance are retained. It is more likely that a lower dosage of fibre will be needed compared to a lower quality fibre and thus this will help to reduce overall costs such as concrete admixtures.
Can macro synthetic fibres replace traditional reinforcement in any application and if not, why not?
No. MSF cannot replace traditional reinforcement in any application. Any replacement has to be well designed and approved by an engineer.
What is the difference between loose and collated fibres?
Loose fibres are usually delivered in bags whereas collated or pucked fibres are wrapped in a dissolvable wrapping. Collated fibres can be used in an automatic dosing machine. Loose fibres are able to distribute throughout the concrete immediately whereas the collated fibres disburse in the concrete after their wrapping has dissolved or broken down.
TIPS ON USING MSFC FOR FLOORING APPLICATIONS
Can concrete fibres be used in slabs on grade as an alternate to welded wire fabric?
Yes, prior to using concrete fibres as an alternate to WWF in slabs on grade, one needs to understand the purpose of welded wire fabric. As stated by Armand H. Gustaferro, PE, in his article How to Plan and Specify Concrete Floors on Grade, „There seems to be a great deal of confusion about the purpose of reinforcement in floors on grade“. The small amount of reinforcing steel used in most floors on grade is insufficient to compensate for non-uniform soil conditions or to help distribute loads to the subbase. The amount of reinforcement is not sufficient to compensate for inadequate slab thickness or to prevent curling.
The sole purpose of reinforcement in floors on grade is to minimize the widths of random cracks that might occur. To be effective, the reinforcement must be located near the top of the slab rather than the bottom. Such reinforcement, which is usually in the form of welded wire fabric or mats, should be located 30 to 50 mm. below the surface of the slab. This type of steel reinforcement is known as secondary reinforcement. Concrete fibre reinforcement provides a superior form of secondary reinforcement to typical welded wire or mats. Control of plastic settlement and shrinkage crack formation, uniform bleeding, increased abrasion, impact and shatter resistance, reduced permeability, added toughness, increased freeze-thaw durability, multidimensional distribution and labour savings are realised with the use of concrete fibre reinforcement. Properly placed welded wire fabric’s only purpose is to hold the inevitable cracks together. Concrete fibres can also bridge random cracks.
Can shake-on-toppings or liquid hardeners be applied over a MSFC floor?
Both can be used on a MSFC floor. Shake-on-toppings can act as a fibre suppressor and limit the number of fibres on the surface. If liquid hardeners are used the floor should be moist cured for 7 days and allowed to air dry per manufacturer’s recommendations before installing the liquid hardener.
What depth should saw cuts be in a MSF floor?
Contraction joints can be installed using either a wet saw or an early entry saw.
For wet saws the depth should be ¼ the depth for slabs less than 125mm thick. For slabs over 125mm thick or slabs with over 6 kgs/m3 of concrete, the saw cuts should be ⅓ the depth. For early entry saws, the manufacturer’s recommendation should be followed.
Do concrete fibres float or sink in concrete?
No, this is a misconception with absolutely no validity. Concrete is a composite material composed of rock, sand, cement, water and appropriate admixtures. Floating or sinking is based on the different specific gravities of materials. The important specific gravity is the specific gravity of concrete, which is greater than 2.0, the viscosity of concrete makes it difficult for either floating or sinking. However, specific gravity does play a role in the application rate of fibre reinforced concrete. The higher the specific gravity of a fibre, the lower the volume contribution is per kilo of fibres used.