Issues with Ferrocement

Dr. Nabil Taha has over 27 years of structural engineering experience. Prior to opening his own engineering firm in Oregon in 1997, he was a Professor of Engineering at Northern Montana State University and at Oregon Institute of Technology. He has structural expertise in a wide range of building systems and can answer questions related to virtually any common building method. His focus is on green design and he is always willing to trying something new. Dr. Taha is dedicated to future sustainability through innovation; he creates solutions for beautiful sustainable and safe structures by melding old and new technologies. He loves a good challenge. He is Licensed in twenty three states and can design buildings and/or consult to assist with structural permitting in these states as well as internationally. As a prior College Professor, Dr. Taha is a teacher at heart. He loves to share his knowledge and offers educational seminars and trainings for the do-it-yourselfers and professionals alike. Dr. Taha's goal is to continue to grow and provide knowledge and services for those trying to make their dream project a reality. No project is too big or too small. For information about Dr. Nabil Taha and his engineering firm see www.structure1.com

Questions and Answers

Q: I'm wanting to use fly ash as a cement admixture in a ferrocement water tank, but rumors exist that it may not be drinking-water safe because of heavy metal contaminations. Do you have info or can you refer me somewhere else?

A (Paul Sarnstrom): I have heard that any heavy metals present in concrete, where some of the Portland cement has been replaced by fly ash, are locked up in the concrete and that it is not a problem but as I remember this reference was not specifically to water tanks. I think that the heavy metals in fly ash will differ significantly both in types and quantities in different samples depending on the type of coal burned and the completeness and cleanliness of the combustion process for a particular sample.

A (Kelly): I did a little search on this topic and came up with the following summary from this study: http://greenwood.cr.usgs.gov/energy/factshts/163-97/FS-163-97.html Radioactive elements in coal and fly ash should not be sources of alarm. The vast majority of coal and the majority of fly ash are not significantly enriched in radioactive elements, or in associated radioactivity, compared to common soils or rocks. This observation provides a useful geologic perspective for addressing societal concerns regarding possible radiation and radon hazard.
      The location and form of radioactive elements in fly ash determine the availability of elements for leaching during ash utilization or disposal. Existing measurements of uranium distribution in fly ash particles indicate a uniform distribution of uranium throughout the glassy particles. The apparent absence of abundant, surface-bound, relatively available uranium suggests that the rate of release of uranium is dominantly controlled by the relatively slow dissolution of host ash particles.
      Previous studies of dissolved radio elements in the environment, and existing knowledge of the chemical properties of uranium and radium can be used to predict the most important chemical controls, such as pH, on solubility of uranium and radium when fly ash interacts with water. Limited measurements of dissolved uranium and radium in water leachates of fly ash and in natural water from some ash disposal sites indicate that dissolved concentrations of these radioactive elements are below levels of human health concern.

Another website quotes L. Max Scott, CHP, " Anything extracted from the earth's crust usually contains trace amounts of naturally occurring radioactive material-namely uranium, thorium, radium, their radioactive decay products, and potassium. The amount of radioactive material in fly ash may vary by a factor of more than 10, depending on the source of the coal. Generally coal mined in the western states is lower in radioactivity than eastern state coal. The radioactivity tends to stay with the fly ash and because of weight lost during burning, on a weight basis, the amount of radioactivity increases in fly ash. Since brick, concrete blocks, and concrete are made from material extracted from the earth, they also contain naturally occurring radioactive material. The concentrations are generally comparable to that of coal.

The scientific community has not reached a definitive conclusion regarding the health effects of very low radiation exposure. Some feel there is an effect, others feel there is not an effect. Generally one can state that the health effects of living in a house made of fly ash block would be similar to that resulting from living in a house made of brick or concrete block. I don't think that you can find fly ash or much of anything else that is absolutely free of radioactive materials. Even wood contains trace amounts of radioactive carbon."

Several other sites seem to confirm these findings, so based on this, I would not be overly concerned with potential radiation from the use of fly ash in building products.

C: I believe that in the US you have products that change the structure of cement products ie, concrete blocks,concrete slabs in fact any product that contains cement, so that they become very water proofed up to 4". One product I know of is used by NASA on the concrete launching pads. No fabric to tear or hope that it was installed properly or damaged by poor back filling methods.

R (Paul Sarnstrom): I have no idea exactly what he is talking about. Sodium silicate [waterglass] can be applied to the surface of concrete to waterproof it, I am not sure it would penetrate 4". There are of course other topical treatments that claim to waterproof concrete but again I don't think any of them would penetrate more than 1/2"=1" let alone 4".

Concrete block is a different creature altogether. It uses a less Portland cement in the mix. Just enough Portland and sand to stick the aggregate together. Blocks are usually more porous than poured or placed concrete.

There are also additive that go into concrete as it's being mixed that can aid in waterproofing. Another possibility is to add 20% - 30% fly ash or other pozzolan as a partial replacement for the Portland cement. The particle size of fly ash is much smaller than Portland so you get a better gradation of particle size leading to fewer and smaller voids.

One of the best ways to insure waterproof characteristics in concrete or mortar is to keep the water/cement ratio low. If the water is kept to .35-.4 parts to 1 part cement by weight, it is generally considered waterproof. Such a mix is also very stiff and may require the use of additives and or vibration to properly place it.

C: I know about the mixture used in the concrete mix; the Trojan masonry sealer works in a different way. It is a Polyester Polymer which penetrates the concrete to seal, it doesn't sit on the surface of the concrete product. One of our construction companies uses this product to seal the inside of very large planter boxes, it stops water seeping out onto the carpet /tiles etc, it saves this company millions of dollars in damages.               

Out here, we excavate perhaps differently than you do when building basements. Here we excavate about 3' larger than the basement and the abutments are sloped at about 45-55 degrees, after the slab/footings are poured and the walls are built a trench is dug around and about 18'' out and 18" deep from the slab/footings, a 4" drainage pipe is placed into the trench and covered with 1/2" screenings, the walls and footings are then sealed , by what ever means the contractor uses,  the back fill is with screenings and then earth which slopes away from the wall, at the top of the excavated slope a spoon drain is put in. Any water will now follow the excavated slope to drainage trench below the footings/slab, and will with all things being equal keep water away from the wall, and it's protective coating! A web address www.enviropacific.com will give you lots of information on the Trojan products. I feel that it is better than bitumenous membranes, and other such products which can be damaged.

C: I designed a new way to make ferrocement water jars (pot-shaped cisterns common in Thailand).  My method is lighter, faster, uses less material and is lower cost.  The secret?  It uses a completed jar as a form.  So instead of spending lots of time making a form and using lots of materials to make it strong, you just wrap burlap bags and thin wire around a jar.  And it should produce perfectly symmetrical pots each time.  I plan to work with a local company to test it out.

Q: I have 2 Quonset-style greenhouse frames. One is 16' x 20 ' and the other is 20' x 40'. I was interested in your thoughts of converting these to permanent housing.  My idea is to erect the frames. Then cover them with 6x6 driveway mesh (as in flying concrete) then cover the mesh with fiberglass screen (as in Latex Concrete Habitat) then impregnate the mesh and screen with latex cement layered about 1/4". Then when this cures build up the thickness to the dimensions listed in latex concrete habitat. After that cures then I will add a 18" to 24" layer of recycled styrofoam (I already have) and cover the styrofoam with a layer of latex cement. The bows that make up the frame are driven up to 30" in the ground. They are galvanized of course! Latex concrete when used with fiberglass screen has incredible strength, so the cement does not have to be very thick to be strong. So the weight is less.  What are your thoughts on this?

A: (Kelly) It seems to me that his would work well. It is similar to ferrocement and Monolithic dome construction. Also similar to what my friend did in Colorado applying the concrete over the mesh by hand. The inner concrete will be good thermal mass and it will be well insulated and protected by the other layers.

Q: I have an enquiry regarding using ferrocement as a roofing material. My plan for the ferrocement roof build up is: Cover roof with OSB to form shuttering. Then attach 3 Layers of mesh using staples tightly to the OSB shuttering. This will form a capping structure around the roof. Then I will trowel 2:1 standard grade silica sand and ordinary Portland cement through all three layers finishing a couple of millimeters above the top layer of mesh. I will then spray the surface with an acrylic resin to cure, seal and protect the ferrocement membrane to form an impervious skin. Would this work and has anyone got experience of this technique being used before?

A: (Paul Sarnstrom) I would not staple or attach mesh directly to OSB or any other wooden structure for two reasons.

1. Wood has a different co-efficient of thermal expansion and contraction than the FC has. If the FC is tightly held to the wood this will lead to cracking. Even micro cracks will allow corrosion of the mesh and eventual failure. I have seen a FC roof where one layer of expanded metal lath was laid directly on tar paper [roofer's felt] and very occasionally stapled through the tar paper to the decking. Mortar was spread and a second layer of expanded metal was then pressed into the mortar. In this case the very light, occasional stapling of the first layer of mesh did not firmly hold the FC mesh to the wooden substrate and the whole FC roof could move independently of the substrate.

2. Stapling 3 layers of mesh to any surface will severely inhibit the ability of mortar to penetrate and encapsulate the mesh. Proper penetration and encapsulation of the mesh which is vital for successful FC.

Proper curing, finishing and protecting the surface of the FC will of course lead to the best longevity.

I initially thought the same as you.  However, a flat roofing system in the UK called RoofKrete is backed by the following patent:   espacenet.com This patent is just FC and I'm wondering whether I could just install it myself on my garage roof.  Maybe use it as a green roof. Also I would use a trowel to push the mortar through the mesh until its completely impregnated. What do you think?

Yes, I've read about the roofcrete product/technique before. One caveat to bear in mind, simply because something has a patent does not guarantee success. I think you can do this yourself, however be certain to use a good quality mortar. Get some FC practice with a test piece or pieces before moving on to a roof. Get familiar with the materials and tools you will use and actually practice the procedures that you will use in the roof. Be certain that what you envisage will work well in practice.

Having said that proper FC procedures need to be followed. Be certain that all mesh and reinforcing is thoroughly encapsulated with mortar. Be certain that all applied mortar is never allowed to dry out. Keep covered and damp throughout the curing time of a minimum 10 days. Temperatures influence curing rate. Conventional RC curing procedures can be followed for the most part. Ideally once mortar application has begun it is carried through to the finish without stopping. Cold joints can be handled if they are planned for ahead of time.

Q: I am wondering if ferrocement could be a good way to support the heavy load of a sod roof.

A: (Kelly) Ferrocement, when shaped like a dome or other convex curved shapes, can be extremely strong and resist a considerable amount of weight on top of it...so yes I do think this could work.

My next question is about permeability. I know they build boat hulls this way but I wonder if you need to coat the ferrocement roof with tar or add a layer of plastic.

Ferrocement, being highly rich in cement, is naturally moisture resistant, but as a precaution I would also cover it with plastic and perhaps even use an additive when you mix the cement to make it more waterproof. I have used latex for this purpose.

Q: I have a question about ferrocement mix that I'm spreading on the inside of the dome. Saturday I got half of it done using a mix of 2 shovels portland common, 2 shovels of sand and 1/2 shovel of lime. Seemed to stick pretty good. I keep reading a standard mix is 3 sand to 1 cement. That seems like not enough cement to me to make it the strongest possible? Is my mix better than the 3 to 1 or am I making it too rich by doing 2 to 2? I originally was going to pour a 3 inch slab in sections over the outside of the dome (vault) but now I'm seriously thinking about doing 2 or 3 coats of 1/2 inch cement/sand just like the inside then bury? Any benefit to either way?

A: (Paul Sarnstrom) In my opinion a 1 part sand to 1 part Portland mix is optimal for FC [ferrocement] work. It provides great strength and little porosity. Older FC recipes call for 1 part Portland and 2 or 3 parts sand because of the cost and availability of Portland in some parts of the world.

The addition of 1/2 part of lime will make the mix much stickier and is a great plaster/stucco type mix. Some masons say that the addition of lime contributes a 'self-healing' attribute to any cracking. There is some concern among professionals, myself included, that extra lime in the mix may contribute to or accelerate mesh corrosion when used in structural, FC applications.

I don't know the grade of sand used, i.e. mortar sand or concrete sand, so I cannot comment on any penetration or encapsulation issues of the wire mesh armature. And last but not least I don't know the degree of tension used in applying mesh layers or the spacing of the cross-ties that are used to join mesh layers to each other. These components both affect the ultimate 'strength' of the FC.

Regarding finishing the exterior of the dome: You one mention one option as pouring 3" thick slabs on the outside. My comments are limited as no mention is made of materials, or ratios of materials that would be used to do this. I presume you are referring to using concrete, i.e. sand, cement and aggregate. 3" of concrete would certainly contribute greatly to ultimate strength but a very well designed mix would need to be employed for keeping a water-tight exterior. Given that using slabs is stated there will also be a number of cold joints that must be designed properly and joined in a way that does not permit water ingress.

Plastering the exterior with the same mix used for the interior would give the best join between the interior and exterior layers as you are using the same ratios of components. If using a different mix on the exterior then use neat Portland or an acrylic like Acryl-60 as a bonder. When acrylic latex is used a s bonder it is best done while the layer of acrylic [applied by hand or by spray] is still wet or at least damp. If you've applied acrylic latex as a bonder and it dries before you get to it with the next coat of mortar, then apply some more latex over the dried section and continue applying the fresh mortar.

Whichever method is used for the exterior I recommend some type of coating over the finished dome, especially if it is to be buried. Elastomeric coatings such as high-quality, synthetic stucco is a good choice. A self-adhesive, bituminous product called Bituthane [Bitchuthane ?] is also an alternative for waterproofing.

I would also be certain a properly drained, french-drain system is used, especially in any earth sheltered applications.

Q: I am thinking of building a dome shaped root cellar with a vertical wall and door on the downside of the hill, using ferrocement construction, with maybe 2' backfill on top.

A: Engineers check buildings in a similar way to what doctors/physicians check with your and my body when we visit them. The doctor usually orders blood analysis, blood pressure, x-rays, cat scan etc. to check your body. Similarly, what engineers do to buildings is to analise them using computers. By modeling these buildings/drawings on the computer, engineers obtain results that show the stresses and strains that occur in these buildings. Then, the engineers compare these stresses and strain to see if they are within the allowable limits of the local building code. So, until the building is molded on the computer and we see the results of the analysis, no one can tell if the building can support 2 feet of dirt or not or it is safe or not.

Q: I have been trying to learn more about what is called air-crete where large balloon type liner is blown up ( whatever size home you want) In my case it would be a dome shape. The concrete is made into kind of a foam with air as far as I can tell and it is then sprayed on the inside and outside of the balloon structure, once hardened the exterior is then sprayed with a polyurethane type spray which they claim is very insulating. What are your thoughts on this and would it be warm in cold climates?

A: (Kelly) Monolithic Domes have been using this technology for over two decades, and it does create a very strong and fairly energy efficient home. The concrete thermal mass on the inside is insulated on the outside. Sometimes the balloons are reused on other projects and sometimes they are left in place. The process of construction is highly technical and fairly toxic, so it has some drawbacks. I would not call this technology particularly green.

Q: We are a young(ish) couple from Auckland New Zealand, with our staggering house prices (median price $900,000) and rising interest rates (7%) and after a devastating loss of our baby boy in Jan we are looking to move, and we have found what we think is a pretty amazing property in Northland New Zealand. The only issue is there are only 4-5 dome houses we can find in NZ, and no one is an expert. We think with a bit of hard work it will be a wonderful home, it’s 45-50 years old and has been lived in by the people that own it, but we don’t know where to start with understanding the potential risks of the house.  We are getting the ferrocement tested for asbestos, and understand the power needs updating and a few fixes (all the windows and mostly cosmetic) but worry that if there has been leaks in the exterior that the structural steel may be corroding etc. 

A: (Kelly) Judging by the photos, the house looks to be in pretty good repair. I do know that steel rebar and mesh embedded in ferrocement can oxidize over time and render the structure less secure. It is hard to tell just by the surface appearance. Perhaps there is some place on the house where a probe can be made to determine the condition of the inner support system.

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