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Discussion Starter · #1 · (Edited)
Hello folks,

I am in the early planning stages of a 120 gallon aquarium build. I have been calling around to my local glass and mirror companies but generally they are quoting me a price between 650 and 1000 dollars for 1/2" thick annealed glass. This does not include euro bracing which this type of aquarium would need. Online the cheapest price I found by a lot was glasscages.com which quoted me a price of 365 for the glass (no polishing on the edges) but 250 dollars for shipping (again not including euro bracing).

Then I hit on the idea of taking apart an older used aquarium (or likely aquariums) and re-using the glass since the older ones are made of 1/2" glass. This unfortunately has not worked out; there seems to be a real lack of supply in my area. The few that exist the owners seem to want near as much as a new aquarium would cost.

At last I hit on the idea of making an aquarium out of concrete for 2 of the 5 sides. Specifically the back and the bottom. The remaining 3 sides would be made of glass. Because I plan to use concrete for the back and bottom it would allow me to use pre-made tempered glass panels for the remaining 3 sides. Why pre-made tempered glass panels? Because they are incredibly cheap by comparison. These are panels designed to be used for table tops. I can get a 24x48 panel made of 1/2" tempered glass for around 120$. Each 24" square side costs around 70$. So for about $260 I can get all the glass I need for the aquarium. Concrete will likely run me somewhere between 60$ to 100$ depending on how much I need. Grand total of 360$ not including silicone.

The major negatives to this idea are 1) I haven't seen anyone do this on a small scale. Large scale concrete tanks are everywhere, small scale... not so much. So I would be forging a new path with something that I really don't want to explode in my house. 2) This is not the path to instant gratification, concrete will take 28 days to cure plus additional time waiting for sealants to dry etc. 3) This method requires a lot of research on my part since concrete is far more complex then glass, I need to learn a lot about what kind of concrete is best, and also best methods of preparation. 4) I will need to do test pieces before I can be satisfied this will work which will add additional time to the project. 5) The aesthetic will not be one everyone will enjoy. Personally this is not a negative for me since I am sure I will enjoy it since my theme from before I hit on this idea was to make what I am calling an urban nature aquarium that will include items from both nature and the modern world.

That gets me up to present. What I would love to know from anyone who has built using concrete before, is whether silicone will stick to concrete that has been sealed? My plan is to create a channel in the base and back of the concrete for the glass to sit in, and silicone it in place. This will give me an additional bracing method on the bottom and back of the aquarium. But, will silicone stick to the concrete after I use some method of sealant on it? I have seen large aquariums use epoxy coatings so I was thinking I would do the same. I just want to make sure it will stick.

I am at this point not planning on doing any kind of euro bracing for this aquarium. Instead I would use angle aluminum on the two exposed corners to increase my silicone coverage. That plus tempered glass and the channels I plan to make in the concrete as additional bracing are my plans for holding the tank together. I realize I am forging my own trail here and that no one has done this before (to my knowledge anyway), but if anyone has worked with silicone, glass, and concrete together I would very much appreciate any advice you have.
 

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Hmmm. Interesting idea but only one side concrete seems err. unstable..
most are 3 sides crete one side glass (ignoring bottom ATM..)

Generally doesn't seem like an issue getting glass to stick to it..
https://www.amazon.co.uk/Aquarium-Table-100x100-Waxed-Concrete/dp/B01MAV0H4H

Matter of fact looks like cast concrete panels..
https://www.amazon.co.uk/Zolux-Aqua-Table-100-Concrete/dp/B00JPFI0SA

100 gal:
https://www.plantedtank.net/forums/12-tank-journals/344514-my-100-gal-concrete-tank.html

considering the weight factor.. plywood tank.. ;)
 

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Discussion Starter · #3 ·
Wow! Thank you for the added research! I honestly never considered doing a plywood tank. In my mind I think of them as less permanent structures but that is probably not true. It's something I will look into. I had a hard time visioning the stability issue when I first came up with this idea. What helped me along was remembering the existence of the other 3 glass sides are bracing the concrete as much as the concrete braces the glass.

The fellow you found who did this before unfortunately it seems did not succeed. One thing I want to test is how thick do I need to make the concrete for this to work? Can I for instance get away with half inch thick concrete if I use glass fibers to reinforce it? Concrete doesn't weigh much more than glass for similar volumes. But if I need 2 inches of concrete than this whole plan will fall apart real quick.
 

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@minorhero
Concrete is made of sand and rocks bound together with portland cement. If you take out the rocks, you have mortar which has high bonding strength to similar materials, but it loses it's strength. It is not the cement that gives it it's strength so much as the rocks and sand that form a composite that has very high compressive strength. It resists compression very well. It does not resist shear or bending forces very well at all. I have seen many concrete sidewalks, driveways, paths, and even floors that develop cracks due to shear and bending forces that are 4-6" thick. It would definitely require considerable thickness and reinforcement to be useful as a fish tank. I would be uncomfortable even recommending 2 inches thick. Plywood would be a much better choice. There are many liquid rubber coatings that have been used successfully to seal plywood for use as a fish tank.

The other issue (as you brought up) is the bonding ability of silicone to materials other than glass. I am not a chemist, so this is mostly just what I have read (please forgive the layman's terms). Silicone is mostly silicon and oxygen. Glass is mostly silicon dioxide. The bond of silicone to glass is very strong because the similar materials create a more chemically compatible bond. Most other materials bond with silicone based solely on the adhesive properties provided by the silicone. Silicone has very poor bonding strength to certain surfaces like acrylic and most other plastics. I cannot say for certain whether silicone will bond well with concrete, wood, or liquid rubber coated wood.

Most plywood tanks are designed with a full frame design made out of plywood with viewing windows adhered to the inside of the frame. This allows the water pressure to press the viewing panels against the plywood frame. This prevents water pressure from pulling the glass away from a dissimilar surface.

There are other adhesives that have different properties with different materials that could be a better choice. Polyurethane caulk comes to mind because of it's very high adhesion to most materials. I do not honestly know how well it bonds to glass though.

.
 

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Discussion Starter · #5 ·
Thank you aguescape for the feedback!

I was also a bit skeptical of concrete being used in this way (or mortar mix or any other concrete like product) but I started doing some research and basically what I came up with is that technology for concrete has come a long way. The stuff that gets poured for driveways and sidewalks is a very different animal then what I will be using.

Compressive strength is all fine and good, but I am definitely more interested in flexion strength for my concrete. This is apparently measured by making a beam of concrete and then applying pressure in the middle of the beam where it is not supported. Concrete I will be using will have at least 800 psi of flexion strength.

Glass fiber reinforcement apparently adds even more strength. The downside there is that the fibers poke through the casting. However, I will almost certainly be covering those up so its not a big deal. I really need to give this a test in real life to see if it works.

I have also found that silicone to raw concrete is a very bad idea. I will definitely need to seal it or at least "prime" it (whatever priming is for a silicone) before I can apply a bead of silicone.
 

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If you have the time and put in the effort I'm sure this is possible. Ofcourse like you said a higher psi concrete would be best, so that's a must.
The suggestions of the other two guys are valid, I'm sure there is a sense of concern and no one wants to see a tragic ending to this. However, if you are going to go forward with it, and I hope you do, I for one will be extremely interested to follow along.
As for help with how the build would work, I cannot give any advice other than what you have already said.
The channels to set the glass into are a must in my eyes as I think it may have a two fold outcome. 1 better strength and 2 the ability to run a bead of silicone or some adhesive along the glass and concrete, below the concrete surface level.
If we take into account that the pressure of the water will be holding said adhesive in place then an initial first seal is more important than anything.
I'm sure this was of no help, but honestly i wish you the best of luck!
 

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Discussion Starter · #7 ·
Thank you for the support Jamo!

Reading into primers a bit it looks like SS4044P is the name of a primer that will bond to concrete and work with RTV108. Not sure about SCS1200 yet, and also not sure which I will use for this build if I proceed. And unfortionately still not sure if I should seal concrete before applying silicone or should I wait?

If you google glass reinforced concrete videos on youtube you come up with some pretty cool stuff people are building. This fellow made a a coffee table that looks to my eye to about 5 or 6 feet long and about 3/4" thick:


So my plan as of right now will involve doing a real world test. The strength of glass seems pretty consistent and there are calculators online to figure out how much a glass shelf can hold before it will break. My real world test will involve making a concrete board at various thicknesses (one will be .5" and one will be 1") and then set them up on supports and step on them. I can position things so that I would reposition my supports so that I can see if my concrete will hold me at points where a glass shelf would hold me, and at points where a glass shelf would not hold me. If concrete is at least as strong as glass then I'm good to go.
 

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I build a number of things from concrete and there is one item that makes me shy away from a tank. Check with concrete contractors and many cities and you will find that they do NOT offer warranty against cracking. It can be done but it is almost always a risk and the thinner, the more likely to crack. Part of the process is in drying it very carefully but that is not something that I can really guarantee. In construction, like floors, we simply pour it way thick and hope as we do lots and lots of that work and it is just part of the expense but we still can spot lots of concrete which has cracked. With enough rebar and thickness as well as proper cure, it can be done but they also put expansion joints in all highways and bridges and note the cracks built into sidewalks to give it a place to crack.
I build hypertufa pots and get about a 10% loss, even though I do a number of them. I do not recommend a tank as a learning project.
 

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How about you build a test tank? Place it outside as a means fo prove your work. Everyone here will say to not do this project, as concrete is such a difficult material to trust, but I commend a try. It would be cool if you could test a 10 gal or similar. Allows you to test the treatments etc you place on the concrete and methods for glass.
 

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I love the idea.... I have quite a bit of research into concrete: Concrete floors, concrete fire pits, concrete furniture, concrete countertops, concrete sculptures....
I wouldnt trust it with aquarium unless it had significantly high level of fiberglass/polymer and it would be INSANELY heavy (your concrete would end up potentially 6 inches thick!). The mixing and then curing process is the most important and cant be rushed. After that you would want pretty heavy wet sanding (which really needs to be done in the shop) as concrete is very rough and porus- which will look like crap pretty quick with algae, etc. Then of course a sealer (porus)... and then waiting for all the VOC's to vacate before adding water. Even then, 6 months later it could very easily crack.
 

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You already know you have to seal the concrete.
Why concrete? That tank would be a bear to move if ever needed.
Dow 795 sicks to rocks, slate, glass. Disney uses it for their tanks/water features.
The problem I see is the weakness of only having the bottom and back in concrete.

What about a plywood tank?
Lighter.
Go over it with fiberglass and resin.

Bump:
There's two types of concrete: Concrete that has cracks, and concrete that's going to going to have cracks!
Concrete tanks are fairly abundant. Down here the farms use concrete burial vaults as grow out tanks. Heck I almost bought 10 myself. (until I go the trucking estimate for the 40 mile move)

Joey Mullen did build his 2000 out of it. There are ways to avoid cracking.
 

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Discussion Starter · #14 ·
Sorry guys I haven't responded sooner! To address the concerns I am definitely going to do some tests before I try the build. My concern is not whether the concrete can support a truck but whether the concrete is at least as strong as glass. Additionally I will not be building the concrete portion of this tank thicker then 1 inche. My goal is to make the tank 1/2" thick. I am not sure if that will work yet (hence my testing). My thickness is not totally made up since GFRC concrete is commonly made into 1/4" thicknesses.

Concrete used to make sidewalks, basements, and other bits of general construction is a very different animal then the concrete I plan to use. It would be like comparing balsa wood to oak. Both are wood and come from trees but they are vastly different. Lots of people are making furniture from GFRC concrete and they do not have the shrinkage and cracking issues of other types of concrete. Additionally, most construction concretes crack either due to freezing and thawing issues, or changes in the grade they are resting on. Obviously those are not concerns for an indoor aquarium.

Either way, step one is to make some concrete boards and test their flexion strength in 1/2" and 1" thicknesses. I will not be going thicker then that because of weight issues as well as cost. The whole purpose of this build is to make it cheaper then glass. Concrete also doesn't weigh substantially more in volume then glass so the tank should still come in just over 200lbs dry weight if made from 1/2" concrete.

If I am satisfied with the my concrete board test I will proceed with the build, but I won't be attaching any glass to the concrete till at least 28 days after casting to give it time to fully cure. That should solve any shrinkage issues as well as cracking problems. If its going to crack due to strain it will do so once I fill it which of course will be done outside before I move it inside.
 

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Can be done and certainly feel free to try but I give some thought to projects of this type that I think about. If it is that easy, why do we not see it done more often?
Are you the first bold person to want to build a large tank cheaply?
Are you the first to think of concrete as a material for tanks?
Are you sure that you will be better than the others who have tried?
If so, throw down the money and go for it!
There may just be a "better mousetrap" in your future!
 

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Discussion Starter · #16 ·
Can be done and certainly feel free to try but I give some thought to projects of this type that I think about. If it is that easy, why do we not see it done more often?
Are you the first bold person to want to build a large tank cheaply?
Are you the first to think of concrete as a material for tanks?
Are you sure that you will be better than the others who have tried?
If so, throw down the money and go for it!
There may just be a "better mousetrap" in your future!
We probably haven't seen it done on a small scale (as discussed its done all the time on large scale) because:
1) Most people prefer the look of glass, you have to really want at least one side to look like concrete before this is even something worth pursuing
2) Even assuming this works perfectly, the build will be significantly more difficult then a glass tank build since you need to make a mold, mix the concrete, pour it into the mold, let it set, sand the concrete, seal the concrete, and let it cure all before you ever get to assemble it
3) This build is cost effective (assuming it works) only because I can buy premade tempered glass panels at a fraction of what custom cut annealed glass panels would cost, if it were not for that one issue there would be little to no cost savings
4) GFRC concrete has really only become popular in the public eye in the last 10 or so years, so there would have been less public awareness of this even being possible before that
5) With all of that taken into account we also would only hear about it if someone posted their build and or experience on a popular aquarium forum

So that is probably why we haven't seen more of these builds. Then again the whole thing could fail spectacularly at several different points. So maybe that is why? I really can't claim any degree of certainty in success at this point in time. I can say that I am very interested in people's experiences who have worked with gfrc concrete to create furniture or other similar objects and also interested in anyone's experience attaching glass to concrete. If folks haven't done either, I admit I approach their recommendations with a large grain of salt.

I can promise the people here only that 1) I plan to make some test boards and assuming that works to make this aquarium as laid out, and 2) That success or failure I will post my results.
 

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Discussion Starter · #17 ·
Hello folks,

Just finished pouring the concrete so I thought I would post a small update.

I looked locally for "GFRC" specific blends of concrete mix and simply could not find one. I can buy these online and have them shipped, but shipping is expensive. Specifically it costs between 35 and 40 dollars per bag of concrete to have it shipped. This effectively doubles the price of GFRC specific mixes to 70-80 dollars per bag. I decided that since a primary purpose of this exercise is to save money, I simply would not be able to use GFRC specific mixes. Instead I would need to use a different mix. This boiled down to the big box hardware stores. I chose Home Depot and bought a single 50 lb bag of Rapid Set Concrete Leveler. I chose this mix because it advertises 1150 psi of flexion strength. To give some perspective RTV 108 advertises 400 psi tensile strength. It is also a mix designed to be extremely loose since it is supposed to be "self leveling". I then poured 3 test pieces that are each 3 inches wide and 48 inches long.

The first test piece is a half inch thick and is only Rapid Set Concrete Leveler with nothing added and mixed to package directions.

The second test piece was supposed to be a half inch thick but I think ended up slightly thicker. It also has added to it 1/2 inch glass fibers. 4% by weight.

The third test piece is 1 inch thick and also has added to it the glass fibers.

Here is a picture of all 3 pieces.



My forms are super rough and also warped since I didn't have a flat surface to work from. I am not stressed about either since this is just to give me a rough estimate of strength. I frankly expect these test pieces to be significantly stronger then a similar piece of tempered glass. If they are I will proceed with this build. Next up is a lot of waiting. It takes 28 days to cure fully. Tomorrow I can take them out of the form and frankly they will have achieved around 70% of their strength by that time. I may get impatient and test them before the 28 days we shall see.
 

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It also has added to it 1/2 inch glass fibers. 4% by weight.
right glass?????
https://www.amazon.com/Fishstone-1-...t=&hvlocphy=9019191&hvtargid=pla-571671552715

sorry.. need to ask.
https://www.concretenetwork.com/glass-fiber-reinforced-concrete/
The problem with using glass fibers as reinforcement for concrete is that glass breaks down in an alkaline environment--and there's almost nothing more alkaline than concrete. You may have heard of concrete being damaged by alkali-silica reactivity (ASR) when there is reactive silica in the aggregate. Glass is primarily silica. The original GFRC in the 1940s rapidly lost strength as the glass was destroyed by the alkaline environment. In the 1970s alkali-resistant (AR) glass fibers were perfected by Owens-Corning and by Nippon Electric Glass (NEG) leading to a rapid increase in applications.
http://www.buddyrhodes.com/understanding-ar-glass-fibers-in-artisan-concrete/
 
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