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MoreCowbell 11-12-2012 09:49 PM

How much can I fudge mortar joint thickness?
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I am an amateur DIY'er and did an amateur screw up. Now I need to figure out my options to recover.:( Here is some backround: I'm plugging away at my crawlspace-to-basement project. My original post describing the project is HERE.

I'll have a "L" shaped retaining wall that runs between 18" and 42" in height. The wall is non-structural (for lack of a better term) as it is positioned just outside the 45 degree slope from the foundation footing that my engineer told me to stay out of. This weekend I poured the footing for the wall. Instead of using my rotating laser to guide the height of the footing, I (like an idiot) used a length of PVC pipe that I could hold against the joists above to gauge the concrete level. That was not such a good plan because I got out my laser out today and realized that the height of the footing the surface varies about 7/8" between the highest and lowest points along its length.

I suppose that I could gradually make adjustments in mortar joint thickness over each course so that the top course is level. However, I'd like to make as big an adjustment as I can at the bottom - the one joint that the floor slab will hide.

So, how much can I fudge the thickness of a mortar joint either above or below the standard 3/8"? I am overbuilding the rebar (verticals are spaced 16"-24"). Does this extra strength allow me more wiggle room with the joint spacing? Or, are there other solutions I am not thinking of? Thanks a lot.

nealtw 11-12-2012 11:43 PM

You could pore a curb to have a level start or do the floor first and build the wall on top of that. I have no idea about spacing.

JoeD 11-13-2012 06:44 AM

Cut the blocks?

mudmixer 11-13-2012 09:12 AM

Lay the block first and then pour the slab over the exposed part of the footing and up to the face of the wall that built before the floor will be poured. Many codes require the slab to abut the basement wall. Since the interior slab is a "floating slab", you just pour up to the wall and do not tie it into the wall.

The thickness of the mortar joint will have virtually no effect on the strength of the wall, especially that huge amount of rebar and grout.

What seems to your concern is more about appearance. and ease of construction.

Make the bed joint (full bed and not face shell bedding) as thin as possible at the high point of the footing (about 1/4" maximum) and make the joint as thick as necessary to keep a level line for the first course of block.

If you have a definite height to meet, spread the "fudging" uniformly over the remaining horizontal joints. There is nothing to say the joints have to be exactly 3/8" and even the ACI 530 codes allow a tolerence in the actual thickness. - The joints above the first course will be under 3/8" and depending on the height of the wall will not be noticeably different in appearance to a normal 3/8".

If the height of the masonry is not needed to be precise, just use 3/8" joints for the second and higher courses. Since this is the normal 3/8" joint all the course heights will be able to be maintained at 8" ( 7-5/8" high block plus 3/8" mortar bed).

Your engineer may require you to tie the top of the wall into something above to keep everything tight.

Fill the cores AFTER laying the block and use a grout mix that is very soupy to insure complete filling and bonding to the rebar. The grout would/should settle as the excess water is pulled out by the block, so rod/probe the filled cores to make sure all settlement has occurred and then top off with a final amount of grout to finish it up in the same day of a few hours. - You can use a fine concrete mix, but it will not be as good as a grout mix that is used on commercial work.


MoreCowbell 11-13-2012 02:58 PM

Mudmixer, that was the answer I was hoping for! The exact height of the wall is not too critical, so I can pad the bottom joint to even things out without causing a problem at the top. :D

I'm just realizing the mismeasured wall footing has created another potential issue with the floor slab. However, I think that proper forum ettiquette dictates that I should start a new thread on that one which I will do shortly. :D

Thanks a lot.

BridgeMan 11-13-2012 10:50 PM

Have you bought the concrete blocks yet? If not, do yourself a big favor, and pour the entire wall using reinforced concrete. Doing so will result in a wall 3 or 4 times stronger than a block wall, and will take a lot less (total) time than laying up and filling a block wall, will.

mudmixer 11-14-2012 04:23 AM

Pouring the wall will entail making form for both sides of the wall, setting them, pouring the concrete, stripping the forms and deciding what to do with them when done. That is assuming you can even get enough concrete into the area to do it in several pours and can tolerate the appearance and cold joints.

The strength is not an issue for this application and the 3 or 4 times strength difference is just simply uninformed baloney! If you need it you can get 7500 psi block and I have seen numerous (50+) partially grouted/reinforced 20 story loadbearing block building constructed with just 6" thick block (no concrete or steel columns) . The engineers that did the design were just well informed on the codes and standards (ACI 530 and IBC).

One of the reasons was minimizing the amount of materials needs and making logistics easier. The is normally no excuse for arbitrarily filling all cells of a block if there is no reinforcement.

The advantage of using block is that the wall can be built as fast as needed instead an all out single pour.


BridgeMan 11-14-2012 10:36 AM


Originally Posted by mudmixer (Post 80012)
Pouring the wall will entail making form for both sides of the wall, setting them, pouring the concrete, stripping the forms and deciding what to do with them when done. . . . . . . .Strength is not an issue for this application and the 3 or 4 times strength difference is just simply uninformed baloney!

Sorry, Dick, but if you think typical block walls are as strong as reinforced concrete walls, you definitely live in a different world than I live in. Resistance to horizontal loads, not vertical, is what I was referencing. I've never seen a properly-constructed reinforced concrete wall blown over by wind, but there have been plenty of block walls (and even solid block fences) that have been. Several of the former with loss of life, which is anything but "simply uninformed baloney" for the parties involved. The strength difference is certainly more of a factor when built with the soupy grout mix you told the OP to use, having a water/cement ratio that's out of sight.

And in the OP's situation (should he decide to pour his wall), there's absolutely no need to build 2 separate forms--just a well-braced, front face form would be needed, pouring "wild" against the natural ground in the rear. I've built several retaining walls that way, first one in Wisconsin in 1970. A neighbor up the street built a slightly lower block wall on his property at the same time, and his had started to crack and displace blocks within a few years of having been built. Mine never showed any signs of failure, even more than 40 years later (went back to look at it last year).

nealtw 11-14-2012 11:06 AM

The side load that a wall can support is directly related to the width of the footing, over building the wall after the footing is in is pointless.

mudmixer 11-14-2012 12:16 PM

Neal -
You are right about the footing and soil conditions. With that reinforced/partially reinforced masonry can be as stong or stronger than normal reinforced concrete.

Bridge -
I live in the same world as you, including the 40 countries I have been to over the last 40 years to observe construction, products, materials and tech masonry design to engineers. I found the international engineers more advanced and willing to learn and apply much quicker than in the U.S.. They understand that a masonry wall is no different than any other wall construction and design parameters are based on the performance and standards for the materials used since it a system and not a bunch of pieces.

The most difficult and frustrating group were the engineers in a masonry design class that I helped to develop 35 years ago and teach numerous sessions every term. The international engineers (especially, Chinese, Central/South American, Russian and European) have no problem with designing and using higher performance walls, while U.S. engineers usually often just dump in grout since they think it does some good, but it really does little to increase the compressive or flexural wall strength. I have seen 8500 psi block and 2500 psi mortar combine the produce a hollow 8" prism of 4800 psi (F'm). This was used in some portions of a multi-story building and other portions used lower strength units used for partially reinforced walls. Most U.S. engineers have never read Appendix 1 of ASTM C270 that recommends using the lowest strength mortar possible to carry the loads and even think mortar has major effect on wall performance. Some engineers have been exposed to some provincial uses of masonry and the units (shapes and strengths) because those areas never have had engineers or contractors that knew enough to establish higher performance materials.

While in Brazil and visiting one project of 15 - 10 story loadbearing block buildings, I asked the engineer what code was used he replied "We use yours, but we use it better" since he had learned masonry design while working in California. They designed for what the loads are and use high tech inspection methods in Brazil to maximize the strength and increase the reliability.

In this specific application, the loads are not that great and normally available strengths and shapes of block are completely acceptable and the real key is the proper construction methods for the situation. Normal block are usually more than 50% higher strength than ASTM standards and it make no sense to use a much higher strength block even though it is only few % higher in cost because there is no real benefit.


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