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Ferrocement Educational Forum  |  Ferrocement Discussions  |  Projects  |  Topic: LFC methods « previous next »
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W. P. R. D. Weerasinghe
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« on: March 09, 2016, 02:51:39 AM »



I would like to learn about the structural design theory about the LFC.

Mr. Paul Sarnstrom could you pls. help me only for the St. Design on the above subject

This refers to your comments "if you are interested in learning more about the LFC method and how it may be used please let me know" on 30'-0" FC foot bridge

Thank you

Weerasinghe W. P. R. D
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Paul Sarnstrom
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« Reply #1 on: March 11, 2016, 11:35:04 AM »

Hello Mr. Weerasinghe,

You have asked about structural design theory for LFC and specifically as it relates to a 30' long foot bridge.

Reinforced concrete is most efficient in relatively thick sections, i.e. a minimum thickness of 3" or 4" on up. Concrete functions most efficiently when used in applications where the loads are primarily compressive loads. FC and LFC are defined as a composite is most effectively used in tensile load applications.

While both FC and LFC are a composite material, LFC is also a laminated material, i.e. LFC = Laminated Ferrocement. The method of production is similar to that used in the Fiberglass industry. In LFC a thin layer of mortar is placed on a form or mold and then a layer of mesh is imbedded in the mortar. More mortar and more mesh is added in successive layers until the desired number of layers of reinforcement is reached. LFC is best accomplished with flat mesh rather than mesh that comes from a roll. Rolled meshes can be used but they must be flattened first as any irregularities will tend to spring back after they have been embedded in the thin layer of mortar. The thin layer of mortar allows the mesh to be completely encapsulated and also allows the next layer of mesh to seat against the previous layer of mesh. Because the layers of mesh are touching each other the shear load is transmitted through the layers of mesh in the laminate without the need for any cross ties which eliminates much of the labor in traditional FC.

So, how to employ LFC in a 30' foot bridge? Three different possibilities come to mind.

1. The foot bridge can be designed and constructed in the same way as the 55' LFC boats were produced at Fibersteel. While the hull and deck were relatively thin sections the whole boat was very strong because of bulkheads being placed every 2' throughout the length of the boat. Some of the bulkheads were solid with hatches [doorways] through them to create cabins, the galley and other areas that needed to be separated from each other. The rest of the bulkheads, the majority of them were only ribs that ran the circumference below decks, so from the keel, up the hull, to the inside or underside of the deck. These bulkhead ribs made up the majority of the bulkheads used in the boat. There is also an element that is commonly used in construction where high strength and low weight are required. This element that is used in the construction field is a box beam, in other words a hollow beam that has rib-type bulkheads spaced evenly throughout its length. The bottom and sides of the foot bridge can be laminated as a single unit, the precast bulkheads would then be inserted and grouted into place. Once the bulkheads or ribs have been fixed the top section can be laminated to the assembly forming a complete, box beam foot bridge.

2. The bridge can be designed with two, narrower, arched box beam sections with one section on each side of the bridge. The deck is attached to the two, slightly arched box beam sections. In this case the two arched beams can be placed in position first and then the decking applied. In this example the decking can be more LFC, wood or any other material.

3. The bridge can be designed using a combination of RC and LFC. In this case the LFC structure or LFC components are used as forms for the RC. You could make thinner and lighter arched ribs [see example 2] and these can be filled with concrete. In this example the LFC is a form that is left in place. The LFC serves to strengthen the RC by reinforcing the surface with an overall increase in strength of approximately 15%-20% compared to the RC alone. Since you are using LFC as formwork in this case the two, LFC, arched rib forms can be joined with cross LFC sections of formwork so that the whole unit can then be cast with concrete as a single unit. The decking of your choice would then be applied to this framework.

There are other possible strategies as well for using LFC for the 30' foot bridge. Remember that with LFC individual elements or pieces can be made separately and then joined to make the complete structure.

I hope this helps and if you have further questions or need suggestions please let me know.

regards,
Paul
« Last Edit: October 16, 2016, 10:32:51 AM by Paul Sarnstrom » Logged

Paul Sarnstrom
Paul Sarnstrom
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« Reply #2 on: April 19, 2016, 04:35:21 PM »

Hello Mr. Weerasinghe,

I am curious to know if you have practiced the LFC method as I have described it to you and if so what have you been learning?

Please respond so that we nay continue this discussion.

regards,
Paul
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Paul Sarnstrom
Janoahsh
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« Reply #3 on: December 21, 2016, 03:13:17 PM »

Thank you for keeping this forum going and researching for so many years. 
I would like to know how you are joining flat pieces of LFC.  Do you leave uncoated reinforcement around the edges that can tie into other pieces and additional strips to overlap the coated portions?  How long can the panels cure before you mortar them together?
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Paul Sarnstrom
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« Reply #4 on: December 22, 2016, 11:01:54 AM »

Hi Jano,

It's good to hear from you once again. You've been around since the beginning of the group!

Your question about joining flat pieces of LFC is very relevant to the flexibility of the LFC process.

One of the basic rules in FC work, indeed In any cementitious work that uses reinforcing, is that the reinforcing be as continuous as possible. If it is not possible to make the reinforcement continuous then the edges and ends of the mesh and any other reinforcement like rebar should be left protruding and overlapped with the next section of reinforcement. The minimum, recommended overlap is four mesh openings. Leave at least four openings on the mesh protruding from the already finished section of LFC and overlap that with four mesh openings of the new section that you are adding. In traditional FC these overlapped mesh layers would be tied together with wire to provide a mechanical connection between the mesh layers.

As I said this overlap rule of at least four mesh openings is used with traditional FC as well as LFC. When using this method one issue becomes readily apparent. If I have four layers of mesh in one section of FC, four layers of mesh is another section of FC and I then join these sections with the overlap method, I create an area with eight layers of mesh. The results are a bond area twice as thick as the rest of the structure which can be a problem.

With the LFC method the thickness issue can be eliminated. As you suggested some areas of mesh are left bare and the edges to be joined are butted against each other, then a strip of mesh that overlaps the joint is embedded in mortar, joining together the two pieces of LFC at the same thickness. The process is repeated for the desired number of layers. The essential point here is to plan the length of the mesh layers in the panels to be joined at a later date. By thinking about and carefully planning the mesh layers and how they may overlap I have been able to make 'C', 'H', 'L', 'I' and 'h' shaped beam sections that transfer the stresses between the layers very efficiently.

My mentor Martin Iorns, discoverer and patent holder for the LFC process stated that providing the above principle is followed, LFC can be added to at any time even years later from the initial construction. Please note that the mortar used in LFC work is an acrylic latex modified mortar. Acrylic latex is also used a s bonder between new and old sections. When using acrylic latex as a bonding agent the following should be considered. When acrylic latex is dry it acts as a release agent. Latex molds are commonly used in plaster casting and with other cementitious materials. When using the latex as a bonder be sure it is still moist or damp when the new mortar is applied. If you have painted or sprayed an area with the latex material and it has dried before you have applied new mortar then paint or spray a little more latex on that area and proceed with mortar and mesh.

I hope this answers your questions and if I can be of further assistance just ask.

Paul
« Last Edit: December 23, 2016, 05:47:07 PM by Paul Sarnstrom » Logged

Paul Sarnstrom
Janoahsh
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« Reply #5 on: January 01, 2017, 09:50:23 PM »

helps very much.  Thank you Paul
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