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kebwi 11-09-2009 10:49 AM

Parge recipe
I'm sorry to be a pest about matters which have already been addressed (on other threads), but I'm find extreme variation in prescriptions for parging mixtures. My current goal, as applies to a discussion of parging, is to fill voids, smooth, and strengthen (against crumbling and decay) some rather dry concrete pours along the edges of my foundation and on the vertical sides of my hearth.

I have found references online to 1:1 sand/portland ratios, but have also been advised to use a 5:1 ratio, which is so utterly divergent from 1:1 that I feel paralyzed on the matter until I get some clarification.

I am also somewhat unclear whether to add any lime to a parging mixture. Why would one add it? Why would one omit it?

On a related -- but not identical -- topic, the same question (about lime) seems to apply to brick mortar. Some recipes include it, some recipes dramatically decrease it, some remove it entirely. I find this confusing. It seems to me that either the chemistry of the substance in question (concrete, mortar, parging mix, etc.) should either need it or not; I don't understand how it can be so ambiguous. Does anyone here understand this precisely enough to clarify the matter? I would truly appreciate it.


RTflorida 11-09-2009 01:38 PM

Re: Parge recipe
I know this does not help. I have done a lot of masonry over the years, but I am FAR from an expert and my projects are always spaced months or even a year or two apart.
The simple facts that I am not an expert who does this for a living, day in and day out and my twisted need for perfection have lead me to NOT try to be an expert. There is a reason the manufacturers sell premix....DIYers like you and I. I know there mixes are far from perfect, BUT, in nearly 30 yrs of projects, I have yet to have a failure and my projects have come out perfect in my eyes. I am guilty of using Heatstop 50, Sakrete premixed bags, fortified thinset, premix stucco, the list goes on. I can't wait for the next project (whatever it may be), I want to try the surface bonding cement that has been highly praised on the forum.
Trying to save a buck or two can actually cost more money, a lot of experimentation time, and frustration. All of the commercially available premixes are perfectly suitable.
I will leave the experimenting and product developement to the experts.

Just my two cents, hope you get some helpful answers (rather than ramblings such as this)


kebwi 11-09-2009 01:44 PM

Re: Parge recipe
I concur. I just like to understand things. I'm not against cheating (heck, I didn't "make" the concrete for my foundation and forms, I used Sakrete...and I still did a horrible job because the Sakrete didn't come with water premixed in :D ), but I still want to understand what the ingredients do and how various ratios alter things chemically. I'm just one of those folks who wants to understand everything; it's who I am.

Thanks. I'll consider just buying mortar for the parging. Seems silly though since I have the ingredients sitting in my basement anyway for the oven, but I'll see what HD has. I need to head that way for a few supplies anyway.

Tscarborough 11-09-2009 02:21 PM

Re: Parge recipe
Standards for mortar are strength standards. As such, there are many ways end up with the desired result. Mortar is not concrete, and the desired physical properties of the two and NOT the same. Stucco (parging) is also NOT concrete, nor is it mortar, although most of the desired properties of mortar and stucco are similar, with a few important differences.

For concrete, the desired physical properties are compressive strength, flexural strength and density (freeze/thaw resistance).

For mortar, the desired physical properties are bond strength, workability, and flexural strength as it relates to drying shrinkage.

For stucco, the desired phyiscal properties are flexural strength is it relates to drying shrinkage, workability, and absorption.

The standard recipe for concrete is 1:2:3 (cement:concrete sand:gravel)
The standard recipe for Mortar is 1:2-1/2 to 3 (cement/lime:masonry sand)
The standard recipe for Stucco is Basecoat 1:3, scratchcoat: 1:2-1/2 to 3, finishcoat varies widely for different effects (cement/lime:torpedo sand)

Concrete ratio is by weight usually, mortar and stucco ratios are by volume of cementious (portland and lime) materials.

kebwi 11-10-2009 01:22 PM

Re: Parge recipe
So, I tried parging a very small section of my foundation last night (along the edge where, due to a dry concrete mixture, there is some crumbling and overall rough texture). I used only portland and sharp silica #30 sand, about 3:1 high on the sand. I used no lime at all, as per Neil2's recommendation (and many other parging references online).

First of all, I added water to get what seemed like a sensible slurry, not too thick, but not runny. I discovered that my mixture (call it what you like, mortar, stucco, I have no idea what it is, it's parging goop) would quickly separate in the bucket, with pools of water gather at the edges. I could stir it back up again, but then it would separate again. Any thoughts on this? Should I use more cement, or add some lime?

At any rate, so I troweled it on the foundation edge in question. It really didn't seem smooth at all, but rather like a very gritty, sandy mud (albeit gray, not brown), and the sand particles seemed to virtually separate in the thin coat instead of remaining in a goo-like plaster. I wasn't trying to build up a layer at all, just to smooth and level the bumps and holes in the concrete. Maybe when I'm going this thin I shouldn't use any sand at all. I don't know.

Anyway, this distinction of the sand grains might suggest that it was too dry, but I can't imagine adding more water since it was separating into puddles in the bucket. That suggests to me that it was saturated and couldn't take up any more water.

I'm confused, I feel like I am not achieving smooth, peanut buttery, creamy mortars or parging mixtures, and I have no idea why. They are always quite gritty. The sand I'm using seems like good stuff. It's extremely clean, pure white sand with sharp cubical edges (which I've read is good because the lime grips it better). I've already posted photos of the sand with size references before and no one has told it looked obviously large or anything like that, so I'm not convinced that is the source of my problem.

I'll take some photos of the parge coat tonight.

Tscarborough 11-10-2009 01:36 PM

Re: Parge recipe
Peanut butter is the consistency not the texture. It will be grainy and sandy. For straight foundation parging, you should dampen the concrete, mix the stucco thoroughly (1:3 is fine for parging), trowel it on 1/4 to 3/8" thick and then let it sit. When it no longer makes a thumbprint, rub it with a sponge float, a piece of burlap, or an old pc of short nap carpet to smooth it off.

kebwi 11-10-2009 02:01 PM

Re: Parge recipe
Just to clarify the concrete was damp when I parged it. It is reassuring to hear you say it should be grainy. Mortar (I understand I'm talking across several topics here) always looks creamy in photos, but maybe they just aren't zoomed in enough. One thing is, I definitely didn't add it anywhere near as thick as you suggested. My goal has been to simply smooth the rough texture and subtle bumps (hidden aggregate), which although considerably rougher than conventional, properly poured and smoothly floated concrete, nevertheless does not suffer from 3/8" texture variation as would be smoothed by your suggestion above. If I had parged the edge that thick, there would be a noticable 3/8" lip at the edge of the foundation where it would rise up and wrap over the edge.

Do you see what I mean about the effect that a thick parge layer would have? That's what I don't understand about making a thick application. Obviously, one could parge the entire surface of the foundation, raising it up 1/4" or so, but, to be honest, my hope was to just fortify the edge and not worry about the interior. I don't really care if it's pretty (I will probably cover it with flagstone or brick or any number of options later). I just want to strengthen the edge against chipping and crumbling.

Relatedly, I need to do something similar to the vertical sides of my hearth, which suffer from varyingly severe roughness. The worst place is a true hole in the side of the hearth, disappointingly deep, that a spider has taken up residence in. That's worst case, that's one place mind you, most of it isn't that bad, but it's still very rough (see the photos in my main thread or my album, in my signature).

Tscarborough 11-10-2009 02:09 PM

Re: Parge recipe
At thicknesses less than 1/4" you are not parging, you are rubbing. As a rule, you will want to add bonding agent when you do this, although everything else is the same, including the finish with a sponge float.

kebwi 11-10-2009 04:12 PM

Re: Parge recipe
My understanding is that Portland cement is a "bonding agent". Since that is already in the recipe, I assume you are referring to something else, right?

Tscarborough 11-10-2009 05:48 PM

Re: Parge recipe
Portland cement is a bonding agent for aggregates, not surfaces. That thin it will tend to flake off and dust and will not provide a proper surface to add a finish coat to.

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