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Old 10/17/2018, 07:25 AM   #1
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Join Date: Oct 2014
Posts: 108
Concrete Tank Build Ideas and Suggestions

Hi Guys and Gals,

A friend of mine is moving house and wishes to install a concrete tank. We have considered other options as well and this option is most feasible. We have very poor quality of plywood here so the cost to acquire marine grade ply is not justified for a plywood build.

I do apologise in advance for the metric sizes but will try and convert the measurements to the best of my ability.

The rough dimensions will be 3.6m x 1m x 1m high. So 11ft x 3.2ft x 3.2 ft. I have perused a few concrete tank builds so have a fair understanding of how they are constructed but need assistance with some aspects which I wish you guys can provide input on.

At this stage we are most concerned with the structural integrity of the shell. and the glass thickness and other aspects regarding the filtration and such will be tackled later.

I will break down the different aspects and the options available to get the best possible solution or suggestions.

The Concrete Shell:

We have 2 options here.

1) A plywood form that will allow us to cast the concrete with rebar in place. The rebar will be 10mm / 0.39inch tied into a grid that ties the side walls to the base. the wall thickness will be 150mm/ 6inch. The front wall holding the glass will be slightly thicker to incorporate the thickness of the glass recess so that the thinnest point will still be 150mm/ 6 inch.

This seems to be the norm on wall thickness but not sure if this is overdesigned. Thinner walls may be cheaper but may be difficult to vibrate the concrete.

I noted on some pours that due to the weight of the concrete it pushes out into the base and difficult to control so still looking at options here to prevent this. Can we pour the base and wait a few hours to set a little to prevent pour out before we continue with pouring the side walls?

2) A double brick cavity wall built up from the base of the tank. So basically we cast the base with the rebar in place. The upright rebar will be in the centre of the brick work so basically an outer skin and inner skin. The cavity will then be poured with concrete to tie to the base. This option is better as it will allow us time to work on the build and probably cheaper as we wont need to build the entire form work.

The only issue here is the portion above the glass. To circumvent the need for a stressed lentil and a possible weak point the perimeter of the tank will be poured so the top edge of the glass will rest on solid cast concrete. This will be poured simultaneously with the cavity so they cure at the same time and tie the top to the sides. The overflow pipes and pluming will be set into this concrete as well. Once done the internal surface will be rendered so we have a smooth surface to apply the epoxy and fibreglass.

The Glass

Given the height of the tank at 1m /3.2ft we are opting for a 20mm/ 0.78 inch low iron glass. the actual pane will be set 150mm/6inch from the base and top and 150mm/ 6inch from the sides as well. Is this sufficient meat or should I opt for a larger lip. Given that this is a rough estimated sizes the width and depth could change marginally. with the glass thickness chosen what will be the maximum water depth internally I could safely go to, still getting a good safety factor.

Another option will be to get 2x10mm or 12mm laminated low iron glass. This will still give us the thickness but unsure if they perform the same as a single thick pane. Which will be a better option?

The recess for the glass for seating the silicone will be 100mm is this sufficient or should we opt for a larger surface area? the glass will be spaced away from the concrete with rubber spaces to ensure a uniform seal and even distributed pressure. What spacers are suggested.

To seal the glass to the lip a generous amount of silicone will be used to seat the glass in place whilst the spacers will ensure an even seam.

Is marine silicone sufficient for this or are there special silicones for this purpose.

Should we opt to go for an acrylic window, what thickness will be required for this build and will the same silicone be used. I noted some guys use the Dow Corning Brand but none is available locally so may consider importing. Please could you give me the exact product I should use if we opt for an acrylic viewing pane.

With either method chosen for the shell will I require a steel frame to seat the glass in?

The fibre-glassing and epoxy

I note on plywood builds the suggested number of quotes is 3 or more. Does this mean three layer of fibreglass and epoxy or one layer of fibreglass with 3 quotes of resin applied when the epoxy is still green? How many layering's of fibreglass is recommended?

Which fibreglass material is best suited for this type? chopped strand matting or E Cloth (for surfboards). Do chop strand matting come in different weights like with E Cloth? What weight matting is best for this type of construction if they do come in weights (oz. usually)?

With the epoxy resin do I tint the final quote with a suitable tint or will I need to apply another gel coat layer like they do on boats?

I know there are a good few questions being asked, but I will really appreciate the help if you possess it and take the time to respond, and perhaps this information will help others who chose to take on this endeavour.

Thank you kindly.

Last edited by Mohammed; 10/17/2018 at 07:29 AM. Reason: Added info.
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Old 10/17/2018, 10:33 AM   #2
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Join Date: Sep 2013
Location: Grand Rapids MI area
Posts: 732 is a great thread about building a tank bigger than yours. I am 99% sure he used concrete, but not 100% sure on that. He has some great info though that could be beneficial to you.

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Old 10/17/2018, 02:22 PM   #3
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Thanks, awesome build has some kewl ideas and answers some questions, it was plywood though. Pity it was taken down 6 months later.

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Old 10/18/2018, 09:49 AM   #4
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Location: Michigan
Posts: 1,679
Check out King of DIY on youtube. He's got a 2,000 gallon tank made of concreate. 8" walls I believe. Acrylic for the viewing panels.

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Old 10/19/2018, 05:40 AM   #5
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Thanks will give it a look.

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