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Thread: OT: Concrete Mix ratios for plaster coat ?

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    Spud's Avatar
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    Default OT: Concrete Mix ratios for plaster coat ?

    Cement : Aggregate : Water ratios.
    For plastering over a brick wall.

    This is for a bathroom brick wall where a door frame used to reside. Frame is being replaced. Concrete plaster coat was damaged while removing old frame, so new plaster coat of about 1/2 " is going to be applied , over the now exposed brick & remanants of the old cement plaster coat.

    I got a nearly full bag of regular Portland Cement. So wondering what Mix ratios I should use for the plaster coat? And how coarse or fine should the aggregate be? I've been reading online and learnt that the lower the water ratio, the stronger the concrete.

    Do I need to instal some kind of metal lathe / mesh, cause it (concrete plaster coat) is only going to be an inch thick at most?

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    Like so many things, "it depends". Depends on the absorbtion rate of the brick, depends on whether the brick base is unpainted - going over old plaster? Depends on what the finish was if it will stay. Depends if it's two coat or three coat work you are planning. With all due respect from someone who would perform surgery on himself rather than farm it out, were that possible,it might be good to farm this one out to a stucco guy. Be real sure they cure it out properly - keeping it moist for a few days, or it will spiderweb.

    Below is from Masterspec.

    Base-Coat Mixes: Single base coats for two-coat plasterwork as follows:

    1. Portland Cement Mix: For cementitious material, mix 1 part portland cement and 0 to 3/4 part lime. Use 2-1/2 to 4 parts aggregate per part of cementitious material.
    2. Portland and Masonry Cement Mix: For cementitious material, mix 1 part portland cement and 1 part masonry cement. Use 2-1/2 to 4 parts aggregate per part of cementitious material.
    3. Plastic Cement Mix: Use 1 part plastic cement and 2-1/2 to 4 parts aggregate.

    D. Base-Coat Mixes: Single base coats for two-coat plasterwork as follows:

    1. Portland Cement Mix: For cementitious material, mix 1 part portland cement and 3/4 to 1-1/2 parts lime. Use 2-1/2 to 4 parts aggregate per part of cementitious material.
    2. Masonry Cement Mix: Use 1 part masonry cement and 2-1/2 to 4 parts aggregate.
    3. Plastic Cement Mix: Use 1 part plastic cement and 2-1/2 to 4 parts aggregate.

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    How old is your cement? Open cement doesn't keep well, and for the price vs. aggravation, I'd buy new material.

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    Quote Originally Posted by Spud View Post
    I've been reading online and learnt that the lower the water ratio, the stronger the concrete.
    Correct, but the workability really decreases. Lots of plants will run a water to cement ratio of around .29 to get strength quicker but you have to add superplasticizer to get any ability to work it but you certainly aren't going to mix that with a hand tool. Cement is cheap, pick up some new stuff. Working with old/junk cement is an exercise in futility.

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    Quote Originally Posted by Rudd View Post
    Like so many things, "it depends". Depends on the absorbtion rate of the brick, depends on whether the brick base is unpainted - going over old plaster? Depends on what the finish was if it will stay. Depends if it's two coat or three coat work you are planning. With all due respect from someone who would perform surgery on himself rather than farm it out, were that possible,it might be good to farm this one out to a stucco guy. Be real sure they cure it out properly - keeping it moist for a few days, or it will spiderweb.
    Can I use regular Portland Cement without the Lime?
    Since I am adding only 0.5 inches thinck plaster coat, can I apply it in 1 coat?
    What water to cement ratio?
    The brick is unpainted, and I removed loose plaster with a metal wire brush which also gave it a rough surface finish, the better for gripping new plaster. Ofcourse if I keep wire brushing, the old cement plaster coat would keep coming off.

    ---
    gbent & rbent

    The cement is 2 months old and was stored in a dry dark place. The bag was opened but the open end was rolled up.
    If I get new cement, should it be regular Portland Cement or Portland Lime Cement? Do I need lime or will plain cement be fine?

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    i used to do architectural concrete and we wouldn't touch regular portland cement unless the bags were still warm from the plant. check out the stuff at Outdoor - not in english but the red tab is "products" so just play around with it....even the black concrete items there started out as a high grade white cement. and even though it all looks like simple castings it took years of experience to get those results. plaster and stucco is no different, all the info you find on the web is worthless without lots of experience.

    if you want a new hobby then by all means start experimenting with different aggregates, plasticizers, water reducers, additives, ......if you just want the job done then look around for building/remodeling sites in your neighborhood and offer the plasterers a six pack and some pocket money to help you out....everybody likes to make a bit of cash on the side after hours.
    Mark Rand likes this.

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    A plasterer won't want to even drive over for a tiny job like that..... I know, I've tried with somewhat bigger jobs, and had to do them myself.

    I'd use coarse sand for aggregate, nothing bigger than that, definitely NO gravel.

    Lime can improve workability and stickiness.... Places like Home Dept have it, but only in big bags.

    Wet the brick down 3 times before you start. Let it dry (soak in, really) to where the surface hasn't got droplets on it, but brick is still damp. Brick is porous and will suck the water right out of the stucco mix, giving you a dry uncured layer next to the brick... NOT what you want. You want the brick to be "neutral" as far as water content. not wet and not dry. Either one will make the stucco peel off.

    A half inch is probably do-able if you mix the material stiff. It really depends on the amount it is going to "slump" If too wet you'll never get a half inch to stay. For plastering I have not gone much over 3/8 inch for the "scratch coat" that keys to the lath.

    if you get nervous about it, just cut strips of drywall and use them... glue to brick. You'll have to pound off all the old stucco, and fill in the gaps, but if you are covering with a door frame, it will just be filler and not seen.

    Water mix ratio.......... mix for stiffness, not for a fixed ratio. it will be fine that way. Job #1 is it has to stay up while it cures......don't get hung up on weights or volumes.

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    Quote Originally Posted by JST View Post
    A half inch is probably do-able if you mix the material stiff. It really depends on the amount it is going to "slump" If too wet you'll never get a half inch to stay. For plastering I have not gone much over 3/8 inch for the "scratch coat" that keys to the lath.
    Slight change of plans; going to apply between 3/4 to 1 inch of cement plaster coat. So do I need to affix a metal lath? Do I apply 1 coat or 2 ?


    Can I use 2 part epoxy adhesive to affix the metal lath to the brick and old cement coat? If I drive in nails, it will most likely just break/chip the brick and old plaster coat.

    Do I need to clean the brick and old plaster coat with any sort of chemical to prepare it to accept the new cement coat? Can I apply the new coat without a bonding agent?


    The below website's ratio recommendation sound about right?

    "Usually the average thickness of the first coat of plaster is 12 mm on brick masonry or ashlar masonry and 23 mm on rubble masonry. In case of concrete masonry this thickness varies from 9 to 15 mm depending upon the nature of work. For the first coat cement plaster with mix-proportions as 1 : 3 or 4 (1 cement : 3 or 4 sand) is generally used. "



    http://www.theconstructioncivil.com/...lastering.html

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    If this is just a few inches wide of trim around the door.. I would put a screw or nail every 6 inches or so and wrap some wire from point to point. we have dry heat in summer here no idea about how it will cure in hot humid conditions. It should be keep damp for several days as it cures. do not paint it for 30 days or the paint will burn off in a few years. If this is a whole wall consider color coat stucco so you never have to paint it.
    Bill D.
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    For epoxy to concrete we use Pro-Poxy 300 Fast, its a structural epoxy for anchor bolts and assorted items. I've never done anything of the nature that you are doing, but a 1 part cement, 3 or 4 part sand mix sounds heavy on the sand. Our mortar mixes (for prestress concrete) are right around a 1 part cement, 2.5 part sand. I wouldn't doubt the heavy on the sand part though, the thickness of the coating they recommend sounds appropriate I think.

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    Why not cement/tile backer board? Cut to size and screw into place. Tile or plaster over the top.
    John

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    That thick, two coats for sure. probably three coats.

    I use three for finish plastering...... first coat thick, scratched up a bit when part cured to give some keying. Second coat thinner, "brown coat", a bit less sand, scratching optional, usually left as-is after rodding. Final coat white finish plaster, about 1/8" should be left for it. Total is about 3/4 inch, usually.

    That's not what you are doing, I gather.....

    As for adhesion, IF you wet the brick, and there is any roughness to help, it should hold on fine.... think of the bits you couldn't get off. The brick pores, if not too filled up, should hold the stucco..... if you think it is filled, you can acid wash the brick, but neutralize it well after. I've never done that..... we have a hollow tile liner with plaster keys built-in on the outside brick walls.

    As a couple of us have said, plasterboard or concrete board as a filler, with a finish of mortar or "setting type" drywall mud (really gypsum plaster with cure retarders) will be functional, and a whole lot faster and less hassle. Quite a few plasterers around here will do that instead of teh multi-coat, it is done faster and most people are happy with it.

    If you want, either the nails and wire, OR metal lath is a good base, but the lath may not allow enough material through to adhere well.... it will key to the lath, but won't join with the brick wall.

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    So if I use metal lath, can I just 2 part epoxy glue it to the old brick and plaster?

    Mix ratios are by volume right? cause I have read/received conflicting info elsewhere. My hunch is it is by volume ( 90% certain) but no harm in asking.


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    I do my plaster mix by weight, otherwise you get a LOT of sand, it's pretty dense material vs plaster, not so sure about cement. Cement might be closer on a volume basis.

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    rbent is offline Hot Rolled
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    Quote Originally Posted by Spud View Post
    Mix ratios are by volume right? cause I have read/received conflicting info elsewhere. My hunch is it is by volume ( 90% certain) but no harm in asking.
    Weight. Trying to measure volume on cement is like trying to push a rope. Once you get into moisture correction factors on your aggregates volume batching goes out the window.

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    Question from another land...

    Why do you folks use cement based plaster indoors instead of Gypsum based? I can understand cement based renders on the outside of a house, but indoors doesn't seem to need it.


    Expiring minds want to know.


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    Well, I don't..... but some do. And the scratch coats can be a portland cement type, for better strength, I assume.

    if that is a door to the outside, a cement based might be better, although I would clean off more of the old stuff, most likely, both for good adhesion to bricks, and because the old material is unknown so I'd rather have one type only.

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