Concrete as a deck covering material

Discussion in 'Boatbuilding' started by iWill, Sep 25, 2009.

  1. iWill
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    iWill Junior Member

    As I've mentioned in other threads I'm helping fix up a 50' Horstman Trimaran. The owner has been in the concrete and engineered stone business for many years so he ended up using it as a deck material over top plywood. I originally winced at the thought but recently I've come to think that maybe it might not be such a bad idea after all. As you can imagine, in the process of working its been getting a fair amount of use and abuse and I'm surprised with how well its holding up. I'm also surprised by how thin it is, only about 1/8” in most places. Its been on there for a couple years he's told me and it appears to be holding up and it hasn't even had its final sealing coat on it yet.

    I'm wondering if anybody has any thoughts on this. Whether it's a good or bad idea in general or special treatments that would make it better for a marine application.

    Will
     
  2. boat fan
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    boat fan Senior Member

    Heavy for a deck . Even that thin.( raised c of g )

    Different expansion to underlying plywood.( separation )

    Different modulus of elasticity / flex.

    Strong possibility of cracking once the boat is subjected to stresses in a seaway.(How do you repair that ?)

    If cracking does occur , how do you stop rainwater from entering and rotting out the underlying plywood ?

    What happens when you put this boat in a travel lift ?

    Difficulty in moving deck fittings.

    IMHO :confused: I don`t believe I would recommend this .....I could be wrong of course , but I would not do that to my boat.
     
  3. Itchy&Scratchy
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    Itchy&Scratchy Senior Member

    Actually, your friend might have something there.

    I can see it now,,,,, a beautiful tropical island, the wind blowing gently through the trees. A few lovely white crusiers bob gently at anchor- as my eye sweeps over the silver bleached teak decks dotted here and there with gleaming polished stainless winches, I gaze in awe as my eye follows the natural progression down the beautiful sheerline towards simple yet elegant cockpit area. I glance further aft in anticipation to where my eye will certainly find an elegant pair of davits from which hang an equally elegant tender waiting to transport the tanned bevy of beauties to an evening of enchantment but alas...................................

    in its place a JCB cement mixer,complete with trowel, shovel and .......

    Oh forget it.:p

    J
     
    Last edited: Sep 29, 2009
  4. rwatson
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    rwatson Senior Member

    The big problem is uniformity - "1/8" thick in some places". will mean 1/2" thick in others. Laying concrete isnt a precise science, and you cant fair it easily once cured.

    You will get a lot more value if you add appropriate plastisers and waterproofing agents.

    It might be worth investigating the "self levelling" concrete products they use for the final layer of warehouse floors. They are a lot tougher, and a lot more chemical and waterproof than plain cement.

    Without some sort of supporting matrix (aluminium mesh, coarse woven glass fibres), all concrete will crack and separate, especially with something as motile as plywood under it.
     
  5. gonzo
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    gonzo Senior Member

    I've seen concrete layed over deck of old ships. It makes up for the tourist traffic wear and seals the leaks
     
  6. masalai
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    masalai masalai

    Concrete over steel, gives a "derelict" a few more years of life before drowning all the passengers in some third world area - Too F&%#ing dangerous and unreliable (see my gallery for an image of "the Atibemo") a death trap waiting to sink - even the rats abandoned her in spite of a bountiful supply of copra on board... Do not be associated with such ill advised process - If this guy was such an expert he would KNOW that there are different coefficient of thermal expansion and a lack of bonding wood to concrete... AND on a multi-hull, weight is a penalty that negates any advantage in that hull form... just nuckin futs as Mr Spooner would have said... http://www.boatdesign.net/gallery/showphoto.php?photo=15612&cat=500 this one was held together by multiple layers of paint over the concrete on the decks . . . Yikes!
     
  7. hoytedow
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    hoytedow Wood Butcher

    At least the concrete will make it last longer as a fishing reef.
     
  8. masalai
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    masalai masalai

    Do they really need another reef inside the marina, or just blocking the channel out?
     
  9. gonzo
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    gonzo Senior Member

    Several US Navy ships, now floating museums, have cement over their decks.
    Masalai: If cement and steel couldn't be together in a structure, every large building, bridge and dam in the world would colapse. At least do a little reading before critizicing.
     
  10. masalai
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    masalai masalai

    I said nothing against concrete and steel as it is used in a boat building method called "ferro-cement" where the concrete is applied as plaster, forcing it through 8 plus layers of "chicken wire", plus arc-mesh and reo-bars from both sides of the structure... difficult to get insurance as build quality is variable and was often amateur built...

    Concrete laid over the deck of a steel working boat adds weight where it was never considered in the design, has nothing to bond with (being usually laid over a painted &/or rusting deck and there is no reinforcing to stabilise and reinforce the "layup") and the flexing of a working hull only speeds up hidden deterioration... Just think about things a little... "Cement" is the grayish powder derived from limestone/coral, Concrete is the 'mix' that is applied in making walls, floors, pavement, bricks, tiles and other applications,,, Concrete can be set with a partially acceptable coefficient of expansion to steel - hence its use, effective and otherwise, in many forms of structure...
     
  11. gonzo
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    gonzo Senior Member

    Fish holds are routinely covered with cement and last for years.
     
  12. Petros
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    Petros Senior Member

    concrete would make a good deck material for very large vessels. I do not see how it can be made to stay attached at only 1/8" thick, unless it is mixed with epoxy or something similar.

    I had consulted on a large navy floating service dock with concrete decks. They were steel floating barge-like work platforms used to lift subs out of the water for servicing them. The service vehicles drove over the concrete surfaces along side the subs. The navy had the idea of using 3/4 thick concrete to armor the working surface and to make it less slippery. IT crumbled away after less than a year, the metal deck expanded at a different rate, and flexed when the trucks drove over it. The contractor called me up with help on redesigning the deck. I added a metal lath (wire mesh) that gets welded to the deck and included some concrete ad-mixtures to prevent water intrusion, and increased the thickness of the concrete to 1-1/2". It has held up well so far.

    I have often thought of using more weather stable materials on sailboats, no one would ever put fiberglass, or varnished wood, on the roof of a house and expect it to hold up very long. But we do it with boats. If concrete can be made to stay attached, and use ad-mixtures to make it water proof and to lighten it, it would make a great deck and even cabin top material, especially on larger vessels. It will not rust and it will not break down in the sun. How about copper for the cabin tops? Or using that synthetic Trex decking instead of teak. It is a bit heavy, but it should hold up well.
     
  13. TollyWally
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    TollyWally Senior Member

    "synthetic Trex decking instead of teak. It is a bit heavy, but it should hold up well."

    Big time thermal movement. It's used for crab decks etc. but it's no substitute for teak or any other kind of laid decking.
     

  14. boat fan
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    boat fan Senior Member


    This is a MULTIHULL gonzo. 50 ft Horstman Tri . Who in their right mind would weigh a multihull down that much , especially high up at deck level ?

    And ....like I said before .... the slightest crack......leaks ..... ( and please don`t tell me it won`t happen either ) .......how will you get rainwater out from under it ?

    I would run a mile .......

    Each to his own I guess.
     
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