HOW DO YOU KNOW WHICH FIBER IS THE BEST FOR EACH CONCRETE SLAB?

The first thing we must know is:

Micro fibre (fibres up to 20mm in length) and macro fibre (fibres from 21mm in length) do not compete with each other.
Each one has a totally different function in a concrete slab or screed.
The micro fibre cannot do the function of the macro fibre and the macro fibre cannot do the function of the micro fibre.
If we make a mistake in choosing the type of fibre in our project, our money will be totally wasted and the slab will not get ANY BENEFIT.
If we apply the wrong fibre, worse than the previous point, our slab won’t be built properly because it lacks the fibre it needed.

Traditionally, when the stresses resulting from the design were small compared to the bending stress that the concrete can withstand (in the case of a 25MPa concrete (C25) bending stress limit will be around 3.3 MPa – 3.4 MPa), a steel mesh was placed in the upper third of the slab. This mesh does not help to increase the flexural strength of the slab as is commonly believed. Its task is to absorb the stresses of the plastic shrinkage of the concrete, ensuring that the unavoidable micro-cracks in the concrete remain as micro-cracks and do not become larger (it immobilizes the concrete of the top layer of the slab).

The micro fibre is necessary in EVERY SLAB since it replaces the steel mesh placed in the upper third part of the slab to prevent cracks due to the plastic shrinkage of concrete. The microfiber works from the first minute of its addition, penetrating the microcracks that occur in the concrete due to plastic shrinkage at the very beginning of the curing process. Micro fibre increases concrete’s density, reduces its porosity, decreases the penetration of air, water, carbonation, and its capillary absorption, thus improves the quality of concrete. Because of all these improvements, Fibratec V12-AM increases concrete’s flexural strength. In summary, it PREVENTS CRACKING due to plastic shrinkage in concrete (much better than a steel mesh) and increases its useful life.
The macro fibres only work after a crack has occurred at the base of the slab due to an overload. Thanks to their large size, they sew this kind of big cracks, preventing the growth of them and the piece from collapsing. The micro fibre is always necessary, but the macro fibre is not in 95% of the cases. (The macro fibres replace the steel mesh situated at the lower part of a double mesh). Therefore, we need to be able to detect when macro fibres are necessary to save money.

FIBRATEC offers the most suitable micro fibres that are being manufactured for concrete slabs. Fibratec V12-AM alkali resistant (AR) micro glass fibres (13mm long) are manufactured with the highest content of zirconia in the market.

Unlike the polypropylene (synthetic) micro fibres, Fibratec V12-AM can replace the steel mesh placed on the upper third area of a concrete slab and do not leave any trace on the surface.

When designing a pavement, we must first calculate the stresses that will be created in that pavement due to loads and other circumstances. To do this, we will use Westergaard’s stress calculations. FIBRATEC has developed an online stress calculation that already incorporates the reinforcement information that our V12-AM microfiber is able to provide. Thus, anyone can calculate these stresses in just 15 seconds and determine whether the slab to be built can be made using only V12-AM (AR glass microfibers) or not. If the resulting stress exceeds the flexural stress limit of the concrete to be used, we should use macro fibres together with micro fibres.

The Westergaard stress calculation shows us whether the loads applied will cause “structural cracks” in the bottom of a slab or not. This will determine the need for using only micro fibres or both micro and macro fibres.

Our 13mm FIBRATEC V12-AM micro fibre made in Japan with the highest zirconia content in the world provides benefits that a steel mesh placed on the upper third part of a slab cannot provide. (See detailed information of V12-AM)

After Westergaard stress calculation has been made, we may find that there are going to be stresses that exceed the concrete’s bending limits. Traditionally, this would lead us to place a double steel mesh (double electro-welded mesh). The lower mesh placed at a distance between 3 and 5 cm from the base would be in charge of holding the cracks that appear at the base of the slab due to overloading. This mesh placed in the lower third part of the slab thickness would prevent the collapse of the slab.

Instead of placing this second mesh on the bottom of the concrete slab, many times macro fibres can be used to replace it. (Steel fibres or macro synthetic fibres)

AFTER HAVING USED OUR ONLINE CALCULATION, YOU MAY ENCOUNTER ONE OF TWO POSSIBLE SITUATIONS:

  • SITUATION A: The concrete slab is viable using only V12-AM micro fiber
  • SITUATION B: The concrete slab requires structural reinforcement with macro fibres or a second steel mesh placed in the bottom third part area of the slab.

SITUATION A (Your slab can support loads only with the use of V12-AM in dosages ranging from 1 to 3 kg / m3:

Once you have checked the feasibility of the slab using only our V12-AM micro fibres manufactured in Japan, you can request a detailed calculation from our technical department for free. This calculation includes factors such as rotational static loads, daily repetition and speed of dynamic loads, stresses produced by the concrete curing process, stress created from the concrete’s own weight, stress due to distances of cut joints, stress relief from the placement of a polyethylene sheet on the ground and other potential factors that may influence the stresses that the concrete will have to withstand. We include safety factors and deliver you a report that justifies that there is no need for using any other reinforcement, only Fibratec V12-AM is enough.

You can request this report sending an email to: tecnico@fibratec.eu

SITUACTION B (Your slab cannot work with only AR micro glass fibers):

After having compared your slab’s load requirements to the loads that our micro fibres can deliver in that given thickness using our online calculator, you might find that you need a bigger reinforcement. This means that, if you don’t use macro fibres or a steel mesh in the bottom of your slab you are going to have cracks at the bottom of the slab due to “excess of point loads”. Another option you may take is increasing the slab thickness until Westergaard calculation shows a thickness that don’t require this kind of structural reinforcement and you only add Fibratec V12-AM micro fibres.

PAY ATTENTION TO THIS FAKE SUGGESTION:

The following frequently used statement is totally incorrect: “Instead of placing 2 kg of micro glass fibre, it is better to place 2 kg of macro glass fibre (long) because it is structural”. This statement is misleading since 2 kg of micro glass fibre does an exceptional job of preventing micro cracking as explained at the beginning of this article. However, in order to do this job, hundreds of millions small fibres are needed. Even the addition of 15kg of long (macro) glass fibres would not be able to provide the equivalent benefits of 2kg of 13mm long V12-AM micro glass fibres.

Going back to the example in the previous paragraph, if we were to add 2 kg of macro glass fibre or macro steel fibre, WE WOULD NOT BE DELIVERING ANY BENEFIT TO OUR CONCRETE SLAB because they would not provide “crack prevention” improvements nor would they provide post-cracking “structural reinforcement” due to VERY FEW FIBERS. A TR34 structural reinforcement with macro fibres calculation must be performed in order to know the correct dosage for a specific slab project. Normally, the minimum dosages required for this kind of reinforcements, depending on the type of fibres are around 20kg in the case of steel fibres, between 2,5 and 4 kg in the case of macro synthetic fibres and 10 kg in the case of macro glass fibres.

LET’S REMEMBER:

MACRO FIBERS (whether glass, steel or synthetic) ACT ESSENTIALLY AFTER THE CRACK HAS OCCURRED. THEIR JOB IS “STOPPING EXISTING CRACKS AT THE BOTTOM OF THE CONCRETE SLAB”. (V12-AM microfibers act throughout the life of the slab to prevent micro cracks).
MACRO FIBERS DO NOT PREVENT MICRO-CRACKING UNLESS THEY ARE DOSED IN HUGE QUANTITIES, WHICH WOULD BE ECONOMICALLY UNFEASIBLE. (V12-AM micro fibres are perfect for this job with only 2kg dosage).
MACRO FIBERS HAVE ALMOST NO INFLUENCE ON THE FIRST CRACKING IN BENDING TEST. (V12-AM micro fibres increase the first breaking strength of concrete because micro fibres improve concrete’s quality eliminating most of the micro cracks).

You can calculate yourself the load that a slab can support with our 13mm long V12-AM micro AR glass fibre using our online calculation and if you need a detailed calculation for approval of project management, don’t hesitate to request it sending an email to: tecnico@fibratec.eu

AFTER HAVING USED OUR ONLINE CALCULATION, YOU MAY ENCOUNTER ONE OF TWO POSSIBLE SITUATIONS:

  • SITUATION A: The concrete slab is viable using only V12-AM micro fiber
  • SITUATION B: The concrete slab requires structural reinforcement with BARCHIP macro fibers in order to “provide residual resistance to stop the propagation of cracks up towards the surface and collapse.

SITUATION A (Your slab can support loads only with the use of V12-AM in dosages ranging from 1 to 3 kg / m3:

You compare the loads that your slab will have to withstand with the loads that our online calculation shows. If the online calculation indicates that the slab with V12-AM micro fibers can withstand higher loads than the slab you need to build requires, than the use of only V12-AM micro fibers is feasible.

Once you have checked the feasibility of the slab using our V12-AM micro fibers manufactured in Japan, you can request a detailed calculation from our technical department for free. This calculation includes factors such as rotational static loads, daily repetition and speed of dynamic loads, stresses produced by the concrete curing process, stress created from the concrete’s own weight, stress due to distances of cut joints, stress relief from the placement of a polyethylene sheet on the ground and other potential factors that may influence the stresses that the concrete will have to withstand. We include safety factors and deliver you a report that justifies the proposed fiber dosage for that given slab.

You can request this report sending an email to: tecnico@fibratec.eu

SITUACTION B (Your slab cannot work with only AR micro glass fibers):

After having compared your slab’s load requirements to the loads that our micro fibers can deliver in that given thickness using our online calculator, you might find that you need a bigger reinforcement. This means that, if you don’t use macro fibers, you are going to have cracks at the bottom of the slab due to “excess of point loads”. At this point, one of your options would be to increase the thickness of the slab. If you do not have the possibility of increasing the thickness of the slab, you already know that YOUR SLAB WILL CRACK AT THE BOTTOM.

Don’t panic because there is a solution. In this case you will have to use our macro synthetic Barchip fibers.  These fibers will act once the crack occurs at the bottom of the slab stopping their propagation. This type of reinforcement is called STRUCTURAL REINFORCEMENT. The calculation method to determine the dosage for structural reinforcement is TR34.

Although most macro fiber manufacturers prefer describing “long fibers”  as “structural fibers”, we prefer to just say MACRO FIBERS because in order to deliver “structural reinforcement”, you need to add certain minimum quantities of fiber. Otherwise you won’t reinforce structurally the concrete slab and the fiber will not perform as “structural fiber function”.

In order to state that a certain fiber has been able to reinforce structurally a concrete slab, it has to accomplish some conditions when testes in a beam test that is described in our web page. This is very important since you than understand that adding 5 kg of macro glass fibers (36mm long) to structurally reinforce a slab is not possible. According to the norms, in order to accomplish the “structural reinforcement” requirements you would need to add minimum 10kg/m3 of macro glass fiber to the concrete (C25). Adding 5 kg of macro glass fiber would literally mean throwing money down the drain.

The following frequently used statement is totally incorrect: “Instead of placing 2 kg of micro glass fiber, it is better to place 2 kg of macro glass fiber (long) because it is structural”. This statement is misleading since 2 kg of micro glass fiber does an exceptional job of preventing micro cracking as explained at the beginning of this article. However, in order to do this job, hundreds of millions small fibers are needed. Even the addition of 15kg of long (macro) glass fibers would not be able to provide the equivalent benefits of 2kg of 13mm long V12-AM micro glass fibers.

Going back to the example in the previous paragraph, if we were to add 2 kg of macro glass fiber or macro steel fiber, WE WOULD NOT BE DELIVERING ANY BENEFIT TO OUR CONCRETE SLAB because they would not provide “crack prevention” improvements nor would they provide post-cracking “structural reinforcement” due to VERY FEW FIBERS.