I need some help understanding how to go about replacing my chimney crown. I’ve just finished looking at it and see tons of cracks and gaps. Yuk!
FHB had an article about crowns (aka caps) in #117 that suggests using mortar to build up a crown in a form. But … it doesn’t say what kind of mortar (type S, type N, type ??). Others comment on the web that concrete is the only way to go.
What say you experts? Also, whatever I use, how do I calculate how many bags of mortar (concrete) to buy before I start?
Thanks in advance from a non-mason.
Replies
I'm planning to redo the crown on my chimney soon. I was thinking of using that mortar that has the fiber glass strands in it. I think it will hold up better than anything else out there. I could be wrong though.
Everything that I've read about chimney crowns says NOT to use mortar. Use concrete instead. The crown should extend about 2 to 2 1/2 inches beyond the sides of the chimney so that water does directly run down the sides. Sometimes you can use a pre-cast concrete chimney crown if the size of your crown is comparable to what's available commercially.
http://www.maconline.org/tech/construction/chimney/chimney.html
http://www.brickinfo.org/html/frmset_thnt.htm
Regards,
PK
Use a "sand concrete" mix and put a little latex admix in it, if you have any around. Sand concrete mix is just cement and sand - no gravel.
Try not to feather the outer edge too thin.
"Cement and sand" is the definition of mortar, isn't it? The coarser the sand, the more concrete-like it becomes, I suppose.
I use very coarse sand mortar for laying up stone walls, and finer sand mortar for repointing brick or for tooled joints.
Just curious what you mean.
Allen
Mortar is a mixture of sand, portland cement and lime. Compared to sand for concrete, sand for masonry mortar contains more fines and the largest particles will be smaller than for the concrete sand.
You thought I wouldn't know, didn't you.
I'd think that no lime is needed in the and mix for a cap either, just the portland and the sand. Maybe that is what he was going after for clarification. He's an old lime-kiln guy
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I call stuff concrete when it has some rocks in it. Could be small rocks like gravel. Maybe really small rocks look like sand, i.e. mortar, but somewhere along that road we end up with mortar rather than concrete.
I sorta think that concrete also do not have hydrated lime and has more portland cement than mortar. Am I on the right page? Looks like a bagged mix of concrete (with relatively smaller rocks) is what's being suggested?
Thanks for the help, guys.
More or less right.Concrete is a solid after it cures. Thje addition of water to portland cement begins a chemical reaction where the particles of the portland combine themsellves with the aggreate that it is mixed with.
The aggregate is any clean mineral particle, from the very finest fines used for floor leveling to the stone as large as three inches in some massive structures.The larger stone is generally stronger and withstands loading better than smaller agregates and the smaller sand particels in a mix are there to fill the spaces between larger stone. Seen upo close, the sand fillsspaces between stones while the portland particles fills the spaces between those smaller particlesSo in a load bearing wall or other struture that needs to withstand force, large aggregate is used in mix with smaller. But in a cap like this, that kind of strength is not needed. sand alone in conjunction with the portland is all that is necessary.
Same thing with concrete canoes. it is a thin wall, so no largeaggregate is needed. with concrete, the chemical reaction begins immediately as soon as water is added, so disturbing the mix too much too long weakens it by disturbing many of the miroscopic nbonds that have already begun to lock in.Mortar has a different purpose altogether. It is more of a strutural filler than a permanet bonding agent. Gravity holds the bricks in place. Modern mortar normally has portland in it for good bond strength between particles, but much of the fill is lime, which makes the sand mix slippery and workably smooth. It has a different kind of boind than the chemical bond of the portland. It is the historical bonding agent of mortar. The lime is from the exoskeletons of miro-organisms that once lived under water and have now become limestone.
So think for a minute of most of the little shells you ssee ner water. Clams, oysters, snails...they are all parts shaped like little sution cups. That is essentially the kind of bond that is formed with lime mortar.That is less than a good thing to have in this kind of cap. water washing over and thru lime based mortaar will gradulally but eventually leach the lime out of the mix.so you don't need stone aggregate in the mix for load bering strength
and you don't need lime for workability
You do need plenty of portland for strength to "glue" the sand particles together.
You want only enough water to kick things and to be able to shape the material into the needed formToo much water in the mix mean - on the microscopic level - that water will be filling some of the spaces between the various particles of portland and aggregate that you want to be bonding together. Then later, when the excess water evaporates, there are cracks, both from shrinkage and from lack of full bonding.Mortar is differentiated from conrete in part by the way it is used. Concrete is for bearing and mortar is used for filling space between the strutural parts. Mortar must be more workable than crete.that is the simple versionThere is lots more. there are other kinds of lime and of mortar for special purposes, but that is the basic deal
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I now know what to do ... use a concrete sand mix to construct the crown, building a form like described in FHB #117. Using a latex admix wouldn't hurt either.
I also now understand much better the differences between concrete and mortar. Thank to all for educating me, guys. Now, if it will quit raining I can get going.
"You thought I wouldn't know, didn't you."
I was just curious about your definitions and how you were making the distinction. I generally consider the lime or portland or a combination of the two as the "cement." I work a lot with lime mortars (no portland at all).
BTW, I like Piffin's mortar vs concrete definition:
"Mortar is differentiated from conrete in part by the way it is used. Concrete is for bearing and mortar is used for filling space between the strutural parts. Mortar must be more workable than crete."
Allen
Type "S" and "N" are grades of portland cement (not what you want). For what you need, a premixed bag of mortar, sand, or concrete mix will be perfectly fine. The sand mix would be my first choice although this would not be a good choice for repointing any loose bricks you might find while you are up there (you'd want mortar for that). Most bag mixes come in two sizes, ~40 and 90 lbs. The volume that these bags will make will be clearly stated on the label, and is pretty much the equal to the size of the bag. If you have trouble with the calculation, send along the approximate size of your chimney and someone here will surely help you with the math.
Concrete mix has coarser aggregate, so if this is a fairly narrow cap, sand mix is definitely what you want. If you will be shaping up something that is fairly beefy, like >3 or 4" in section, then the concrete should be ok.
Edited 10/2/2007 9:26 pm ET by RedfordHenry