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Cast concrete wall caps

| Posted in General Discussion on May 20, 2002 01:22am

Here’s the situation: I have a mortared stone retaining wall in the back of my house, probably original (1976).  The wall is in pretty good shape, but the cap is toast.  At some point in history someone smeared mortar/concrete on top, and it is all cracked and falling apart.  Of all the options I’ve heard of to fix it, I think cast concrete caps would be best.  But, from what I’ve been told, the price would be out of my range.

So I’m thinking, why not cast my own?  I could make 3/4″ plywood forms for 2″ thick slabs, poly the insides of the form, lay in some wire mesh, and go to town.  One catch is that I’m working on a pretty short clock.  I would like to build the forms this week, cast the caps over the holiday, and install them the next week.  I’m looking for somebody to tell me this is a crazy idea, and hopefully answer some questions.

First, considering my time constraints, would it be possible to use Quikrete?  Before anybody says anything about durability, we’re not planning on staying in this house for much longer.  Second, I was planning on just buying bag mortar mix to glue these things on.  Does that sound like a reasonable idea?  Are there other potential problems I’m overlooking?  Mongo, you gave me some great info on casting countertops a while back, I’m hoping you’ll weigh in here.

Thanks, I appreciate any info.

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Replies

  1. rez | May 20, 2002 03:54am | #1

    I'd make sure I made a key for it and planned it in my forms.

  2. HONewbie | May 20, 2002 05:34pm | #2

    Good tip on the keys.  Anyone else have advice?

    1. Rein_ | May 21, 2002 02:51am | #3

      First of all, the quickcrete (or any premix) stuff is not evil. You just might not get the mixture you exactly want.

      Secondly, I would make sure you keep the pour moist as long as possible. The concrete will be much stronger with some good curing time. Even if you move out, its nice to know it will last.

      If you are working fast, you may not have time to wait for a proper cure in the mold. Maybe you can cover the placed caps with plastic for a few days after?

      Are you sure ready cast products are too expensive? I have found them to be quite reasonable compared to the mess and time it would take to make your own.

      The big drawback to ready made products is that they think everybody wants some tacky old fashioned look. When will these suppliers get with 21st century design!

  3. timkline | May 21, 2002 05:21am | #4

    Quickrete bag mix is fine for anything. It should get you at least a 3500 # mix if you don't overadd water. Have you considered forming the caps in place rather than making separate pieces that will have joints ? Have you considered making them any thicker than 2 inches ?  I did radius wall caps on top of the stone walls in front of my house. The walls have a nice curve and I was replacing the old poured in place caps with new ones. Turned out great ! Pouring in place sure would give you a better job if you can do it. And save you a step in the process. If you do decide to install it in pieces, make sure that the joints between the pieces are at least 3/8" wide. The joints should be raked so that they can be caulked and then sanded while still tacky. Mortar in these horizontal joints will crack and allow water to enter and destroy the wall.  The caulk should be a urethane. The sand gives the joint a more natural appearance.

    carpenter in transition

  4. rez | May 21, 2002 06:13am | #5

    Well, if you really want to get carried away, get yourself an electric concrete vibrator. I built a support column around the dead edge of an old double pour foundation to help support the seam. Built a 3 1/2 ft high form around two sides about 3 inches thick, probably a foot wide each way. Anyhow, after I poured it around the rebar and flush to the top of the form, I used the electric vibrator down inside the form, and the top level of the concrete dropped about three inches as the air moved out and the concrete solidified. I use that thing anytime I do concrete now.

  5. User avater
    Mongo | May 21, 2002 06:44am | #6

    Bart,

    Careful, or soon you'll be blowing off the bagged stuff and ordering sand and stone by the truckload.<g>

    I've done plenty of wall caps myself. Varying styles, but the basics are the same as pouring a countertop. If I sent you a copy of that word.doc file on countertops, the methods remain the same. I do prefer a pitched cap...either a beveled top, or a convex top surface...something to shed water.

    The bagged stuff will work fine for wall coping/caps. No need to really worry about entrained air, either.

    Keep the size manageable. A 2" thick slab will run you about 25 pounds per square foot. If I'm placing coping solo, I try to keep the pieces below 225 pounds. Don't kill your back doing this.

    Mortar them in place as if you are laying up cmu. A couple of lines of mortar, one on each side of the top of the wall, then place the coping on the lines of mortar and tap it with a mallet to level it. Run a taught string the length of the wall to help keep all of the coping aligned.

    Even though this is a rush job for you, if you end up with any voids in the top surface, you can use a putty knife and fill them with a contrasting colored mortar mix...or just rub it on with a burlap sack to blend it a bit. Filling the voids will help if you're in a freeze/thaw environment.

    If "regular" concrete coping will be too light in color, you can take the edge off by adding a bit of carbon black to each batch. When you install the coping, alternate pieces from one batch to another to mix any discrepancies in batch coloration.

    I can't imagine anything more fun than firing up the cement mixer and tossing a few tons of concrete around...for a house you'll soon be moving out of! Painful! when you're done, come on over...the beer's on me.<g>

    Mongo

    1. HONewbie | May 21, 2002 02:26pm | #7

      Thanks, Mongo.  If I'm going to be slinging these things around by myself, they would certainly have to be less than 225# (LOL).  My plan is to go 2'x2'x2", except for whatever special sizes I need to come to the ends and corners, and lure some buddies over to help.  So would you cast these face down like you do your countertops?  For those you leave them in mold for quite a while, how quickly could I unmold these?  I did think about casting these in place, but the front face of the wall isn't straight or flush (it's natural stone), so I wasn't sure how I would build that side of the form.

      Okay, another question.  I can figure out how much concrete mix, but how many bags of mortar mix am I talking about?  I may have already said this, but the wall is 2' wide and about 46 linear feet (not all in a line, just to make things interesting).

      Thanks again for all the help.

      1. User avater
        Mongo | May 21, 2002 05:50pm | #8

        I'd definitely cast them inverted.

        Since you're going for an approximate 2' square cap, consider using 3/4" thick sheets of white melamine for the base of the form. The sheets of melamine are slightly larger than sheets of ply, and less expensive than 3/4" ply as well. The melamine will give you a good surface on your slab, free of woodgrain which can sometimes telegraph from the plywood, through poly. You can then put in dividers to section the melamine into eight approximate 24" squares. You can bevel these dividers should you want to give the caps bevels sides.

        Give yourself an "out" when it comes time to unmold the caps. I usually screw the forms together, then I can unscrew the forms to pop the caps out. It keeps you from having to damage or destroy parts of the form to get the caps out, and the screws make it easy to reassemble the form to pour the next batch.

        I'd leave them in the form overnight with the form covered with poly. 24 hours is sufficient, but be careful when popping them out of the mold.

        Materials? For 2' by 46', rounded up to 50' to include a bit of waste, that's 100 sq ft by 2" deep, for about 17 cu ft of concrete. I'm not a Quikrete expert, but I think their small bags (60#) are good for about a half cubic foot and their large bags (80#) are good for about two-thirds of a cubic foot. So, that's either 25 large bags or 37 small bags. Don't bank on those numbers, as I haven't touched a bag of Quikrete in several years.

        Oops. I just reviewed your post to see you had a question about mortar quantity, not concrete quantity. Oh well. For the mortar..."it depends." It depends on the top surface of the wall, if there are voids to fill, etc, etc. Buy 10 bags, see how they go, then use that as a basis to either buy more or return a few.

        1. HONewbie | May 22, 2002 02:24pm | #9

          Okay, I did some pricing last night.  The local guy I found who makes these quoted me $250, assuming I could use all standard sizes and wouldn't require anything special.  On the other hand, I can get all of the  Quikrete, 10 bags of mortar, wire mesh, a new wheelbarrow, and everything else for around $240.  No brainer there.  No luck on the melamine at the local big box, and the place where I usually buy my sheet goods has a $200 minimum.  I have a couple of sheets of 3/4" shop-grade birch I'll probably end up using.  I don't think anyone would see the wood grain from a galloping horse, so I'm not too worried about that.

          I think I'm pretty well set until I get the forms built and start casting.  No doubt I'll be back with more questions come the weekend.

          Thanks for all the info

        2. HONewbie | May 24, 2002 03:14pm | #10

          Another question for you Mongo.  Is the polyurethane enough of a release agent, or do I need to put something else in the form before casting?  The forms are all cut to size and got a first coat of water-based poly last night.  LOML is going to keep coating them today, hopefully they'll have three coats by tonight and be ready to cast into tomorrow morning.

          Since we were just quickly slapping on the poly, it of course has all kinds of bubbles.  In my mind that's not a bad thing for this application, because I don't want a glass smooth surface on the concrete anyway.

          Let me know what you think, and thanks again.

          1. User avater
            Mongo | May 24, 2002 04:44pm | #12

            Bart,

            Polyurethane can work just fine for the molds...but it depends.

            If you're assembling a grid of dividers on a 4' by 8' sheet of ply, with the intent of pouring 8 caps at a time and just "popping" the cured concrete caps out of  the form, it may not work too easily. When making a hard form (screwed together wood form, etc) I find it best to take the form apart to get the caps out of the form. Then I screw it back together again and pour the next batch.

            While it can be done, it's mighty difficult to pop concrete caps out of an inflexible, rigid form, similar to popping ice cubes out of an ice cube tray, without damaging some of the caps...damaged edges, corners, etc may be the result.

            AS long as the water-based poly is well cured, and you remove the caps from the mold after about 24-48 hours, you should be okay. If the polyurethane is not fully cured and the concrete is allowed to really set up in the forms, it can sometimes strip parts of the poly right off the form. Not likely, and not common, but it could happen.

            In the past for a "quick and dirty", I'd simply cover the 4' by8' sheet of ply with a sheet of polyethylene plastic. I then assemble my dividing grid on top of the plastic-covered plywood, screw the dividers in from below, and pour.

            The 3/4" by 2" high dividers can be wrapped in plastic as well (a quick wrap secured by staples into the 3/4" edge) or sealed with polyurethane like you're doing.

            If you're urethaning the assembled form...base ply, as well as 2" high dividing grid...all together...the urethane may seal the dividers to the base, making it difficult to break the form down to remove the cured caps.

            It can work, and likely it will...but you may damage the form or the caps in the process. For a one-time pour that may not be a concern, but I save forms to reuse, which is why I prefer easy form assembly and breakdown using screws.

            For keying the bottom of the caps, I'd come back a bit after the pour, when the concrete has set up a bit, and just score the bottom of the cap (top surface of the pour) with a trowel. Rake the trowel to give a few "V" grooves where the mortar will contact the cap.

            So...

            If you're going to break down (unscrew) the form to remove the cured caps, no, you won't need a release agent. If you're going to try to "pop" the cured caps out fo the form, a la ice cubes out of a tray, then you'd need a release agent...but I don't think it'd help due to the vacuum/suction created between the cap and the form when trying to pop it out.

            For your project, I'd key the bottom of the not quite cured caps with a trowel.

            Did I mis-read your previous post? You were quoted a price of $250 for the caps, but you can do it for $240 so you're going to do it to save $10?

          2. HONewbie | May 24, 2002 06:21pm | #13

            If I bought the caps for $250, I would still have to buy mortar etc. that is all included in my $240 price.  Plus, the $240 includes tools I've been meaning to buy for a while, like a wheelbarrow.  If I bought the caps plus the mortar plus the tools, I'd be closer to $350 or more.  Plus, it'll be cool to say I did it (assuming it works).

            I am doing like you suggested, using a grid on a 4'x8' sheet, and I will disassemble it to get the caps out.  The perimeter walls are 2 3/4" tall and are screwed into the edge of the big sheet, then the internal 2" dividers will be screwed up through the bottom and in through the perimeter.  I am doing all the polyurethaning (sp?) disassembled.  I think I will go ahead and wrap everything in plastic, I like that.  I am planning to keep the mold for re-use, 2'x2' would make some nice side table tops.

            First casting tomorrow, I'll post how it goes.  I figure I can make a pour each morning for the next three days, then start installing on Tuesday.  Wish me luck (God knows I'll need it).

          3. User avater
            Mongo | May 24, 2002 10:00pm | #14

            It sounds like you've got an excellent plan.

            One additional tidbit...give a little extra attention when you screw the form sides into the form bed, or when you screw through the bed and into the 3/4" edge of the 2" ply dividers. Screwing into the 3/4" edge of the plywood may cause the plies to seperate, which would result in little how-ya-doin's in the show sides of each cap. Pre-drilling would help. Again, screwing without a pilot hole may not split and delaminate the edge of the ply, but I'd thought I'd give you a head's up just in case.

            Been there...done that.<g>

          4. HONewbie | May 28, 2002 05:28pm | #15

            Okay, here's the post-mortem from the first cast on Saturday.  First of all, mixing 720 lbs of concrete by hand in a wheelbarrow ####!  I mixed and shovelled, LOML troweled and leveled.  I mixed some of it just plain too dry.  I overestimated how long it would take to set, and underestimated how long it would take me to mix, so we didn't get enough vibrating done.  When I busted them out Sunday morning, there were quite a few voids on the tops and edges.  The really dry ones didn't have enough moisture to even get glossy up against the plastic.  But the long and short of it is that they don't look bad, and they will be usable.

            My plan is to get some Portland cement and a rubber float to fill in the voids.  I'm thinking of mixing a little black in with it, maybe take away some of that "new concrete" look.  Does that sound like it might work?

            I also gave up on my original schedule.  There was no way we could cast and install all these suckers in a week.  But all in all, I'd call the project a success so far.

        3. HONewbie | May 24, 2002 04:04pm | #11

          Okay, I forgot another question.  One of the guys recommended making some kind of key in the caps.  Since I'm casting these inverted, would there be any advantage to imbedding a few nails or something in the bottom to extend down into the mortar, or even down into the wall cavity?

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