Type N Mortar failure

Old 08-12-05, 05:41 PM
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Type N Mortar failure


I'm using Max-it Type N mortar. The bag has no instructions whatsoever and in fact it's only "stamped" type N. I checked the Max-it website and they say the sand is already in it and it does feel a little gritty (if it didn't have sand in it I assume it would feel like talcum powder) so I assume I'm using the right product.

Problem is, I'm building a 16" block wall and I put a concrete cap on it. I noticed that some of the mortar that squeezed out of the joints can easily be broken with my hand. This may not be unusual since some 1:3 Portland-sand squeeze out I'm using elsewhere wasn't much harder to break. Also, the mortar does not seem to be very "sticky". I place a block and if I remove the block there is almost no mortar stuck to it. As a test, I kicked one of the caps and it came loose (after 3 days curing). I'm not really worried about the blocks since I filled them up with concrete for good measure but at this point I think I have to remove the caps and do them over.

BTW, the cap that I kicked was on the end of the run so it was not "locked" by caps on either side. One side is open. I kicked some of the other caps that have a cap on either side and they are firm. The fact that the sqeezeout is so crumbly really bothers me though.

The only reason I added the caps was to raise the wall slightly before I add flagstone veneer and bullnose brick caps. I don't want to take any chance whatsoever that the brick caps will fail because of loose caps. Perhaps I should nix the caps and cut a block down to 4" tall (I actually put 2 caps on top of each other), mortar it in as usual and fill it with concrete.

Any idea what's going wrong here? Perhaps Type N has almost no tensile strength whatsoever?

Last edited by AlexH; 08-12-05 at 06:47 PM.
Old 08-12-05, 07:31 PM
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Type N Mortar failure

Since you are using Max-it, I assume you may be near Chicago or in California.

Type N mortar or any concrete has virtually no tensile strength. What you want is bond.

Your Type N mortar may be (just a guess) a mixture of Portland cement, fillers (ground limestone?), pozzolanic materials and additives to give the mortar workability. The other mixture to form Type N is a mixture of Portland cement and lime. Both meet the requirements of ASTM C270 for mortar. I personally prefer the portland/lime blend. Usually both types work well.

A couple of things about your project -

1. High lime mortars are slower to gain strength than a Portland/sand mix. Because of this, you have to be more careful initially.

2. The mortar joints gain a great deal of strength due to the weight of the materials above it. - In other words, the mortar on the bottom of a wall is stronger and has better bond and compressive strength than the same mortar at the top of a wall due to the load applied when the mortar is wet. The lack of weight is why the vertical mortar joints can be more prone to leakage than horizontal joints.

3. Tooling joints also adds to the strength (bond & compressive) and water tighness, but the joints on a cap are tough to tool because you cannot press very hard against them.

One trick I have seen used by old timers using hard, unabsorptive brick for a cap is to just pile some brick on top of the cap and leave it there until the next day. - They claim it helps especially since the caps may be laid in a full mortar bed spreading out the weight even more.

Your mortar may have been fairly fresh when you gave it the kick test. It only gets stronger. The reason the end unit came off too easily was that when you kicked it you also probably twisted it a bit. The other units should be OK. I would suggest replacing the cap and you can try the old timers trick to see if it improves things.

Masonry walls are the weakest and look the worst right after they are built.

Old 08-12-05, 07:44 PM
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I did something similar... had to build up a huge window opening (16' wide) in a block wall with some 4" solids, because the soil was getting too close to the bottom of the opening and digging the landscaping out wasn't an option.

Immediately after laying the blocks, I cleaned off the excess with my trowel and tooled the joint. As it dried, I was very concerned about the strength of the mix, because I had mixed it up in a 5 gallon bucket by hand, and was worried that I didn't mix it right. I was very careful not to add too much water at one time, because I know that can weaken the mix. When I chipped some of the excess cement with my finger, it would come off just like muddy sand, and I was really worried. But I left it alone, and several weeks later when I went back to finish up, it seemed strong as nails.

So I guess what I would say is give it time.
Old 08-12-05, 08:55 PM
The Tuckpointer
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Type N mortar will not fail if mixed the right way, it sounds like you had too much water in it.

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