Advice on my first masonry block project


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Old 01-09-15, 02:56 PM
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Advice on my first masonry block project

First post on this forum, looking for some feedback on a mailbox I plan to build. Iím pretty handy with woodworking but Iíve never worked with concrete before. So Iím hoping I can layout my plan and see if anyone has tips or sees problems.

One main question: Say I pour some concrete one day and let it cure, then pour more on top of it. Will the two adhere just as well as if I'd done one complete pour? Or is there a major loss in strength?

Here's what the finished product will (hopefully) look like.
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I plan to dig a 8-12" diameter hole about 2'6" deep and put 6" of gravel in the bottom and fill the rest with concrete to support a 2" diameter steel pipe. The pipe will run right up the center of the blocks.
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Each block will have a 1" thick concrete filler with a drainage hole in the bottom for the succulents I'll be planting. In the diagram it looks like the succulents will be planted in concrete, but that's just a rocky groundcover.
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I'll stack the blocks and use 3/8" of mortar so they stick together. I'll support the open ends of the stack as I build it up. When the stack is built, I'll fill in the entire center with concrete.
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I want to cast some sort of bolt receptacle into the top block so I can attach the mailbox portion to the base. Not sure what to use for this and would appreciate any tips.
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I'll build a wooden box to go into a polished concrete shell that is 3/4" thick. I'm a little worried that 3/4" will be too thin, but if I go much thicker the thing will weigh a ton! Can anyone confirm that 3/4" will be thick enough for a non load bearing application such as this? I'll build the box to have a 1/32" gap all the way around to allow for seasonal expansion of the wood. Anyone think this is not enough? I have no idea how wood and concrete will differ in their expansion.
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The back of the concrete shell will be open to see the wooden box.
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I'll be doing more reading about specifics of polished concrete for the top, but mostly I'm looking for any big picture things that I might be totally missing. I've never even put in a fence post, so if something seems way overkill or underkill I'd really appreciate any feedback.

Thanks.
 
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Old 01-09-15, 03:10 PM
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Welcome to the forums!

I'd pour the concrete footer and have piece of rebar sticking out of it where it will enter the block's void that you intend to fill with more concrete, then lay the block and after the mortar has dried - insert another piece of rebar and fill with concrete. A form could be used to pour an inch or two of concrete to close off the bottom of the blocks being used for planters.

Have you talked with the permit office to find out if your design is allowed? Some jurisdictions don't allow heavy duty mailbox posts because of the potential for damage to vehicles.
 
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Old 01-09-15, 04:42 PM
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What a cool mailbox idea! I love it. My spin would be to nix the polished concrete mailbox enclosure and make the box my mailbox out of polished stainless steel. Still I think polished concrete would contrast nicely with the CMU texture.

You could even make your CMU mailbox post dry stack. Embed a threaded rod in your footer. Then you could just stack your block like your plan and on top a steel plate with a hole for the threaded rod. Tighten a nut down to hold the whole stack together with no cracking mortar to worry about.
 
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Old 01-09-15, 04:57 PM
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You could even make your CMU mailbox post dry stack. Embed a threaded rod in your footer. Then you could just stack your block like your plan and on top a steel plate with a hole for the threaded rod. Tighten a nut down to hold the whole stack together with no cracking mortar to worry about.
I really like the idea using the threaded rod to hold the blocks together before you fill them with concrete! It makes the entire assembly operation much quicker and as you said avoids having to worry about the mortar cracking out in the future. I don't see any disadvantages doing it that way.

I wonder if you would be better to epoxy a piece of stainless steel with pent-up edges to the bottom of of the blocks with the plants will be planted instead of pouring a thin concrete membrane. If you bend the size up of the stainless you would have plenty of area for the epoxy to bond to.

In response to the OPs original question, subsequent pours of concrete will not bonded to each other as well as if they are poured at the same time. However in this case I don't see that being a major issue.
 
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Old 01-09-15, 05:16 PM
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Thanks marksr. I checked with the post office and they didn't ask about how I was going to secure it in place...so I didn't say anything I live on a tiny side street with virtually no traffic, so I'm not going to worry about that too much I think. But a great tip for sure.

Are you suggesting the steel pipe up the center as well as the rebar? Or replace the idea of the steel pipe with rebar instead?

Thanks for the feedback.
 
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Old 01-09-15, 05:21 PM
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You could even make your CMU mailbox post dry stack. Embed a threaded rod in your footer. Then you could just stack your block like your plan and on top a steel plate with a hole for the threaded rod. Tighten a nut down to hold the whole stack together with no cracking mortar to worry about.

Read more: http://www.doityourself.com/forum/br...#ixzz3ONCCBhYa
Thanks for the tip. I like the sound of this. Are you suggesting one threaded rod that goes all the way into the footer and then up to the very top of the stack. Steel plate on the top to hold it all together? I've never heard of this. That would be good too because it wouldn't weigh 8 million pounds in case a car does accidentally hit it. With one told in this fashion, should I be concerned about it tipping left of right or forward or backward. Half of the blocks are unsupported so they will want to fall away from the center. The top is going to weigh a lot and I don't want it to lean over time. Any thoughts?

Thanks! I also like the idea of the stainless steel top, but one of the objectives is to practice with polished concrete for other projects I might do down the road.
 
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Old 01-09-15, 05:24 PM
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I really like the idea using the threaded rod to hold the blocks together before you fill them with concrete! It makes the entire assembly operation much quicker and as you said avoids having to worry about the mortar cracking out in the future. I don't see any disadvantages doing it that way.

Read more: http://www.doityourself.com/forum/br...#ixzz3ONE5CQuw
Thanks Msradell for the feedback. Do you mean I should do the threaded rod without mortar to hold the blocks together, and then fill the void with concrete?
 
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Old 01-09-15, 06:05 PM
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Thanks Msradell for the feedback. Do you mean I should do the threaded rod without mortar to hold the blocks together, and then fill the void with concrete
Yes, that's what I was thinking. It means you can build the entire stack in one day and you don't have to worry about supporting the overhangs while the mortar dries. In answer to your other question just use one piece of threaded rod from the footer all the way to the top. There's also actually no need for the gravel at the bottom of the hole.
 
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Old 01-09-15, 06:07 PM
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Basically you're building a post tensioned structure. The strength comes from the tension on the threaded rod. Your blocks are sandwiched in between the plate on top and footer on the bottom.

Pour your footer and embed threaded rod (put a large washer and nut on the bottom). Let the concrete footer cure. Then you can use mortar to fine tune the surface if needed to make it level and the elevation you want. Once that cures stack your CMU blocks. Cap with the steel plate and tighten the nut to sandwich everything together with no cement or mortar in your stacked blocks.

Everything can be done in small, manageable projects. Nothing is time critical and you have numerous opportunities to fix boo boo's without it showing.

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As for your worry about the mailbox structure tipping the key is your foundation. Make sure the surface where you start stacking your block is level. Then, your CMU blocks are going in opposite directions. One layer counter balancing the next. If you didn't do the planters in the outer cells the weight of your polished concrete mailbox on top the whole thing would stand dry stacked with no mortar, no rebar, no threaded rod. It's really a very elegant design.
 
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Old 01-09-15, 08:34 PM
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Nice design, but it could use some fine-tuning. One issue could be the stability of the staggered block shape, using just a single threaded rod running up the middle. A single rod with thin steel plate top anchor will allow the blocks to rotate with respect to each other, when the block cantilevers are impacted laterally by modest forces (interpret that to mean 6-yr. olds on tricycles aren't known for their driving skills). Two parallel rods and a heavier top plate would be better. Also, the coefficient of thermal expansion of steel is close to 20% greater than that of concrete, meaning in hot weather there's a good chance that the anchorage system will loosen and allow the blocks to move with respect to each other. Masonry units also have a tendency to absorb and slowly bleed water, meaning rainwater and succulent irrigation water are likely to show prolonged/unsightly staining on the exterior surfaces. Also, you don't want any anchor bolts protruding into the interior of the mail box, as they have the potential for causing injury to the fingertips when mail is retrieved.
 
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Old 01-09-15, 09:19 PM
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I hear what you're saying about rotation and thermal expansion BridgeMan. I do live in earthquake country as well. Would you then suggest the rebar and concrete fill solution? I'll also think about lining the interior of the CMU for the water staining issue. Thanks for the response.
 
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Old 01-09-15, 09:37 PM
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I would definitely go with reinforced concrete cores in the blocks. Two continuous No. 4s, from the footing all the way up to/through the top block. I built a similar sized masonry mailbox pedestal in NM many years ago, except it was composed of mortared, cantilevered slump block instead of CMUs. Never had an issue with cracking of any kind, even though temperature swings of 40+ degrees were common. The most critical element is the footing--don't skimp on it. Mine was a rectangular chunk of concrete, 2' x 2' x 1'-6" (almost 1000 lb. with the rebar in it), and it kept the mailbox upright even when clobbered by a yuppie driving his BMW too fast in the snow. I also illuminated the pedestal's house numbers on both vertical faces, running a conduit branch from the front porch light circuit inside the footing and concrete core.
 
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Old 01-10-15, 07:39 AM
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You're not going to get appreciable thermal expansion differences in a 4' high mailbox post and I'm not talking about dinky little 1/4" threaded rod. Even bridges hundreds of feet long accommodate the different expansion rates of their pre and post tensioned concrete & steel. I'm thinking of something like 3/4" or 1" and a hefty top plate. Tightening the nut stretches the rod so even with thermal expansion there is still thousands of pounds of force holding the stack together.
 
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Old 01-11-15, 10:28 AM
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I like the idea of the rod and metal plate but if I need to put the plate on the top of the stack I don't think there is enough room to recess it into the mailbox. That also messes a little bit with how I would go about attaching the mailbox to the stack.

Are the downsides to mortar and filling the stack that it would take more time and eventually the mortar would crack? I don't mind spending the time and effort and if it's relatively easy to fix the mortar in twenty years, that's probably okay too.

Could I dry stack and fill with concrete so I don't worry about the mortar? Would the concrete stick and hold everything together enough?
 
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Old 01-11-15, 11:45 AM
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Concrete along with rebar should hold the block firmly together.
 
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Old 01-11-15, 12:15 PM
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Due to the manufacturing process, all block have one horizontal surface that is smoother. - They are made on a steel pallet (the block is made upside down compared to the orientation when normally laid). Make sure you lay the block with the smooth up since there might be some slight irregularities on the other horizontal surface.

Also due to manufacturing methods, the face shells are tapered and they are wider on the top as laid and slightly thinner are the bottom. - This could pose a problem with the thin 1" concrete you propose for the bottom to contain the soil. The 1" cast locally could be prone to slide out due to the very slight taper and the fact that the concrete shrinkage of the 1" panel could cause a small crack between the block and the 1" and it could slide out the bottom in time due to the soil and moisture.

You could lay the block with the smoothest side downward ane the 1" should not drop out. - On the better block, the wide mortar bed is wider than the opposite face, since on an 8" block the minimum face shell is 1-1/4" and the width of the very top could be as much as 1-3/4". The wider mortar bed is cast into the block for ease of spreading mortar and makes handling easier. Masons usually prefer the flared out wider bed since makes life easier and additional weight is negligible.

In CA, you may not see much of increase of the mortar bed, but the face shell will still have a slight taper.

It gets a little difficult to explain clearly, since many people that a block has a top & bottom and top and bottom are reversed between manufacturing and and laying. Some masons do not even notice, but the is a difference.

Dick
 
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Old 01-11-15, 04:40 PM
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I don't think you will have trouble with mortar cracking. Millions of buildings are constructed with CMU blocks mortared together so it can handle a mailbox. I think the big issue is your skill and speed working with mortar. Once you mix it up the clock is ticking so you'll have to be prepared to set your blocks and get everything square and level to your liking. You may have to temporarily support the cantilevered ends of your blocks until the mortar cures.

One option would be to mix very small batches of mortar and do one layer/block every couple days. Then after the stack is built you can fill the core with concrete for further strength. It would be slow but you can take your time and get each block nice and straight without the time pressure of mortar that's curing ready or not.
 
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Old 01-12-15, 09:56 AM
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I'm thinking of something like 3/4" or 1" and a hefty top plate.
I'm coming back to the threaded rod idea Pilot Dane. How thick should that top plate be? Do they make pre-cut plates for this task or would I need to get a sheet of some kind of steel and cut it down? I'm thinking about running the bolt into the concrete box and sandwiching the concrete box to the stack with the steel plate. Then I can cover that up with the wooden enclosure somehow. Thoughts?
 
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Old 01-12-15, 10:29 AM
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Your project is totally custom so you won't find anything pre-made for the job. Look up welding or metal fabrication shops in your area. They can cut the steel and put in whatever holes you want. I would not go thinner than 1/4". You also have the option of using stainless steel and aluminum so you won't have to paint it or worry about rust stains.

Have you considered having a custom mailbox made? The base of the mailbox could be the top plate for your stack with the tightening nut inside the box.
 
 

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