Artificial reef 1

[globalsatria]

We want to make artificial reefs (for tourism and conservation) using cemented oyster shells covered with a layer of concrete. Can anyone give me an estimate or better give me a reference of how long the concrete might last in tropical setting with exposure to swell? The cement layer would ideally be no more than 50cm and as thin as possible.
Thank you
Marie

 

[civilperson]

Cement and concrete are two different things. What is proposed, shells as aggregate in a concrete mix? Other concrete mixes in under water environments last two hundred years or more, less if subject to splash zone forces.

 

[Prepakt1]

To cover your cemented shells you need a fabriform concrete mat probably with a basketweave to reduce damage from impacts.
500 mm or 20 " is about right. We have used that thickness in the ocean in various locations. Its difficult to pump a mat any thicker.If you want thicker you have to layer the mat. For that thickness mat needs to be nylon.Its about twice the strength of polyethylene and will take higher pump pressures. Polys also degrade in sunlight and float. Nylon is easier to work with as its denser than water.

How long it will last in the surf zone is always a question. Depends on the force of the next big storm ,totally unpredictable. You can anchor the mat into the seabed rather than depending on concrete weight alone. Plate anchors , screw anchors etc can be integrated into the mat construction. It all depends on how much you want to spend. In the ocean everything eventually gives, you can only do your best and hope it functions as requested for a while.Nylon mat will give you a recreation friendly surface nothing too sharp.
Various reefs have been built around the world some sucessfully sometimes total failures. Generally the heavier the material the more stable the reef. Most are made from different weights of rocks built up in layers 3 ton rocks or heavier.
You can also use old ships in an excavated trench as a base anchor and build up over them in layers.

Intrusion Prepakt /marineconcrete.com

 

[csd72]

Marie,

This issue is far more complex than you realise.

If it is underwater, how is the concrete applied to the reef? What is the available equipment in the local area?

Is the reef exposed at high tide or is it permanently wet?

Will it be abraded by moving sand e.t.c. e.t.c.

I think you will find that most artificial reefs are precast in a factory and then dropped into place. This gives a far more durable concrete due to the controlled environments and would also be cheaper than what you are proposing.

Precast units also have places for fish to hide/breed that a concrete apron would not.

If you are still determined to go along this route then I would advise you to put another post in the concrete forum as this is largely an issue of an appropriate concrete mix.

 

[DaveVikingPE]

You may want to contact this person:

http://www.fit.edu/faculty/profiles/profile.html?value=62