Idea for Faux sand bed. Envirotex Lite high gloss finish.

I think it probably would lefty - I think in this case since he added really cold water it might have contracted very rapidly - but still not sure entirely what happened.
 
Lefty said:
Does the resin shrink as it cools?
Click to expand...
All resins shrink somewhat in the curing process, but epoxy is known as the one that shrinks the least. I'd be surprised if shrinkage is the most significant issue at play.

Here is something from a technical page on epoxy (I don't know which specific product hdtvguy used, and all epoxies have slightly different properties):

"The reactivity of Epoxy and Polyurethane compounds will depend on the type of hardener employed and the chemistry involved. All reacting (curing) materials generate heat as a by-product of the reaction. The amount of heat generated will depend on the chemistry involved and can reach very high temperatures. for example; epoxy systems formulated for thin film bonding applications can generate enough energy to self-ignite in large mass castings.

The un-dissipated heat generated as a by-product of the curing process will intern speed the reaction further until the material solidifies.

Pot life is defined as the period of time, commencing from the time the resin and hardener are mixed together, the mixture remains pourable in its intended application. As a rule of thumb, the faster the reaction the higher the exotherm.

The slower the reaction the better.

Exotherm is defined as the increase in temperature above the cure temperature due to the energy released by the reaction. Excessive exotherm can damage components especially in encapsulating electronic circuits. If the resin and hardener are heated to lower the mixed viscosity, the resultant greater reactivity will cause the exotherm (heat) to be generated in a shorter period of time. The ultimate temperature will be much higher as there is less time for the mix to dissipate the internal heat being generated.

High reactivity hardeners used in large mass casting can result in "runaway exotherm" because the heat being generated can not be dissipated at a sufficient rates from the center of the mass. In extreme cases, the temperature in the center of the mass can reach extremes to the point where it actually chars or even explodes.

The lower the exotherm the better. Use low reactivity hardeners for large mass castings and faster reactivity hardeners for small mass casting or thin film applications.

Shrinkage is the reduction in volume or linear dimensions as a result of cure. Excessive shrinkage will result in damage to embedded components and residual built in stresses in the casting. Built in stresses make the finished casting prone to cracking.

As a rule, un-filled products shrink more than those containing fillers and slow reactivity materials will shrink less than high reactivity systems. In most cases, the higher the filler content the lower the shrinkage and the better the thermal conductivity of the casting.

The lower the shrinkage the better.

The ability to withstand thermal shock is generally a function of flexibility. The more flexible the cured system the better its ability to withstand thermal shock. This can be a problem if the potted component is required to operate at elevated temperatures because many flexibilized casting systems are not well suited to high temperature operation. The most suitable casting systems are those that possess a good combination of toughness and flexibility."

It is important to note that commercial epoxy products come in many flavours for specific applications. Marine and industrial epoxies come in different speeds, viscosities and have different flexibility properties. There are also a multitude of fillers used that can change the final properties, like microballoons, fibers, etc.

Sand is, unto itself, a filler in the epoxy mix. As long as the sand is dry, it should add to the desirable properties of the epoxy and should minimize shrinkage, but not to the degree of specialized additives.

I still don't know that the epoxy mix was the smoking gun with hdtvguy's failure (how long did it take to kick?), but I think it would be safe to say that there may be less margin of error when dealing with large tanks and deeper FSBs. Until others step up and assume the risk in other larger tanks, I think the safest approach is to cast drop in parts or to cast in as shallow a layer as possible.

My $0.02
 
I used the same Epoxy as in this thread ( Envirotech Lite Epoxy) I still think it was caused by the extreme cold water from the tap that caused this. In the pictures posted you can tell by the cracks. That they are stress related from the hard cure epoxy sand mix and the glass just could not take the contraction.

I had to get the the tank out of the house it just made me sick looking at it!
 
I figured epoxy would work well. I used PVC glue on mine...about the time this thread started. It's held up fairly well, but next time, I'll be trying a boatbuilders' epoxy or something similar. It should resist UV better (even though bulbs are shielded, there's still UV there). The bottom does get covered in coralline and other things and won't stay the pristine white it looks originally. I think it look more natural, though and actually takes on a resemblance to the live rock.

I personally wouldn't ever seal the bed right to the bottom (for reasons we've seen here). I fixed my current bed to HDPE and will do the same in the future. It's not worth the risk to me...
 
RonSF- I tried to make it as thin as possible, around 1/8-3/16"

CamBarr- yes, Famowood epoxy (got that at HD), was mixed in with cc, let it dry slightly (so that top layer does not mix or disturb with lower layer). Mix the next batch of fine sand/epoxy & spread. Then sprinkle a layer of fine sand on top of all that & let dry. Next day vacuum excess off.

G-money- you said it, not worth the risk!!! You will sleep better if you add that bottom layer!
 
Did you guys who have done this find the epoxy still smells after the required cure time? I'm thinking maybe the sand in the mix and over top of the epoxy is slowing down the cure. I'm not sure I want to add water yet if it's still giving off fumes.

Jay
 
Mine smelled, well after a few days worth of curing. Some fresh water seems to take care of that, leave some FW in your tank for a while & remove. Repeat a few times.
 
Well, I filled the 240 for the first time since doing the Faux bottom. I let the bottom cure for full week, but vacuumed out the excess sand after the recommended 72 hour cure time. While I was filling it I listened closely to see if I could hear the epoxy cracking. After hdtvguy's disaster I decided to use room temperature water. I heard no cracking and had no problems at all.
It looks great. Now I just have to wait 4 weeks or so for the aragacrete islands to cure.
I'd like to thank hdtvguy for posting the problem he had. If he hadn't, i probably would have used cold water too and things might not have faired so well.
Jay
 
Here's my 240 with Envirotex Lite Faux sand bed, Great Stuff Foam backgrounds and Aragacrete islands with PVC built in to accept framework for aquascaping.

11682fauxbed.jpg


Jay
 
Cambarr,
I'm planning on corals encrusting on it. The foam is about 2 to 4" thick depending on location. I put quite a few 3/4" dia holes in it all over the place to accomodate frags.

Jay
 
docjay, can you give us a link to the foam that you used.
Also, does the foam meet the glass back there, or is there plenty of more room for you to put stull like powerheads...etc. Are the pipes the only support structure?

More pics would be awesome.
 
The Foam is called Great Stuff. I got it at Home Depot. It took 8-9 cans to do the job. The backgrounds are flush up against the back of the tank siliconed in place. Also I put the backgrounds in first then poured the epoxy bottom so the epoxy binds the backgrounds to the bottom of the tank too.
In the middle of the background you can see apipe extending downward into the background. It actually extends into an upsidedown pocket that has an Oceans Motions Revolution in it. When activated it extends downward out of the pocket spraying water into the tank halfway down the water column. When deactivated it pulls back up into the pocket out of sight.

I'll post some more pics later.

Jay
 
Hmmm. I'm considering this for my 20g nano, as I'm tired of my CC and I don't want to use a traditional sandbed because I transport my tank 4 times a year.

So you guys have had good luck with the High Gloss stuff from Home Depot? I may try that.
 
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