Replies

1. 
   philarenewal | Apr 09, 2006 02:02am | #1

   Concrete.

    

   "Let's get crack-a-lackin"  --- Adam Carolla

1. 
   saulgood | Apr 09, 2006 02:14am | #2

   Thanks. What blend would you recommend?

1. 
   philarenewal | Apr 09, 2006 07:06am | #6

   Thanks. What blend would you recommend?

   I like the idea of a cement slurry as a bonding agent.  That's what they used in the "old days."  You can also use a commercial bonding agent product.

   Since you said you plan on painting it, I'd steer clear of portland/lime mixes.  The lime will tend to inhibit a good paint bond.

   I'd go with 1 part masonry cement to 8 parts sand and small aggregate.  It's a slightly rich mix, but maybe even not rich enough.  Make a test batch and see how difficult it is to do final shaping while still "green."  If it doesn't hold together very well, use a bit more masonry cement.  If it's too hard, use a bit less. (of course you won't be using ordinary woodworking tools, but it's not hard to shape and carve green concrete with steel tools (a cold chisel and even chunks of brick can be used to shape and smooth green concrete -- use a chunk of brick or block like sandpaper).

   Any chance of posting a pic?  You'll probably get better ideas from the folks here if everyone can see exactly what you're trying to do. 

   "Let's get crack-a-lackin"  --- Adam Carolla

2. 
   Danno | Apr 09, 2006 02:59am | #3

   For what it's worth--in Mexico we tried repairing concrete window sills that had spalled and chunks missing by nailing (with hardened nails) a board across and filling it with concrete. never worked. What did work for me was to thoroughly wet the area to be patched (clean and remove dust, etc. first) and paint with a cement slurry (wouldn't hurt to use a bonding agent either--we didn't have that), then really work in a cement-rich concrete mix with a trowel, and finally trowel in the rest of the concrete. At that point, I guess you could use a form board as sort of a screed to use to level the patch. Dash the thing with water (I used an old brush and dipped it in a bucket of water and flung it against the curing concrete) as it starts to cure, keeping it always damp for a couple days. That keeps the patch from just dropping out in one chunk when you think you're all done.

3. 
   brownbagg | Apr 09, 2006 03:53am | #4

   if you can find another one, plaster of paris will make a mold
4. 
   Fonzie | Apr 09, 2006 05:09am | #5

   We repaired a chunk out of a inside concrete step using hydrolic cement.

   We drilled and set a couple of cement screws strategically as reinforcement.

   I agree with the principle of first priming the exposed surface with a slurry of the cement you are going to use.

   Since I don't know what the shape is I can't comment on how to recreate the form.

5. 
   SamT | Apr 09, 2006 07:10pm | #7

   Preperation is the most important phase.

   If the wound has been exposed more than a few days, it needs to be scarified to expose a fresh surface. A small electric jack hammer with a buffing (Waffle face) head works good. So will hand held concrete chisels, just take longer. Best if you remove 100% of the weathered surface, but 95% should be OK.

   After scarifying, place a sopping wet bath towel on the surface, and place plastic over it to slow evaporation. Let sit overnight. When you return; If the surface appears damp, fine; If the surface seems dry, rewet it with plenty of water, then do as follows; If the surface has standing water, pat dry.

   ---

   There must be no time break between the following steps!

   Pre mix dry enough 100 grit silica sand and cement at a 4:1 ratio to complete the job twice, plus 1/4  extra. Set aside in two containers.

   Slurry: 1 cup of straight cement should be enough for that sized patch. Add enough water to make a milk or cream consistancy slurry. Brush it on the wound, working it into the surface, flick off any puddles so there is just a very thin damp film over all.

   While stirring, slowly add water to your dry mix  just untill it is uniformly damp. It should appear crumbly, not wet. Check by firmly squeezing a ball in your fist. When you open your fist, the ball should not fall apart, if it does, add a little water. If it doesn't, drop it about 6"-10". Your hand should be dry and the ball should break, not squish. If neither are true, add a little of the extra mix to absorb the excess water. Think of building sand castles at the beach. You need ZERO slumping.

   Placing the mud. It's hard to accurately describe the technique verbally, but I'm hoping my words paint a good picture. And I'll try not to use a thousand of 'em (|:>)

   Using a metal trowel, make a dam and place a bunch of mud in place, then pound it down with a rubber mallet. Rake the pounded surface with a hair comb or stiff brush, then add more mud. Pound it down, rake, and repeat till you've built it up past the desired finish surfaces.

   Wait 30 minutes while you; Clean your tools; smoke 'em if ya got 'em; very lightly mist the new mud IF it dries to the point of discoloration, (a paler shade of grey.)

   Using automotive body repair shure-form files, or, if you're good, a 4" grinder with 30 grit sanding disk, trim the mud to the desired shape. Hand sand out the shure-form lines with a dry sponge or 100 sandpaper. Use a light touch. If you get too deep a gouge, mix a little of that extra mix with a little more water than before, think room temp butter or fresh sour cream, and fill and smooth with a butter knife, tablespoon, or artists' paint knife. Whatever.

   Wait another 30 minutes, using your mist bottle as needed, then wrap the repair with a sopping wet towel and plastic. This time do a good job with the plastic. Repeat as needed to keep the towel wet for 7 days.

   ---

   It will help if you take the time on the second day to allow the surface to dry just to the point that you can prime it with latex primer. Let the primer set up (dry to touch) as directed by the manufacturer, then re-towel.

    

   SamT
1. 
   woodway | Apr 09, 2006 08:30pm | #8

   Sounds like a good idea...very clear instructions and when the need arises again, I think I'll give your method a try. I was going to suggest something similar, wet surface first and use a little wetter mix of pure nonshrink hydraulic grout. Playing around with some extra stuff the other day and I found that after letting it dry for about 45 minutes I could almost mold this stuff like modeling clay. I first mixed small amount by hand in a bucket with minimum amount of water to yield a very stiff mix then applied. When first applied, my batch would very slowly flow on it's own but after short waiting period it stopped flowing and became solid enough to mold with a metal trowel.

2. 
   Danno | Apr 09, 2006 11:09pm | #9

   Excellent reply! I totally missed that this was ornamental and had details to be molded into it!

3. 
   saulgood | Apr 10, 2006 04:54am | #10

   Thanks all for the great info. Sorry I can't post pics philarenwal, but I think I'm getting more diverse answers this way.

   SamT, excellent words on concrete repair -I'm saving this thead in my files. The thing is, the missing piece has a level of detail that will have to be cast, so any shaping I do will be secondary (blending and such). I'm thinking I need a pourable mix that can be cast in place and vibrated, then smoothed a little when green. It seems like a slurry would make a better transition than the bonding agent I've used in the past, which I've heard compared to white glue (although it does seem to work). Also, despite all the good advice, I don't trust myself not to use steel reinforcement to rod the old to the new. As for the mold, Fonzie (if that IS your real name ;^) I'll be taking a negative from another peice, like brownbagg suggested. Because the detail would require at least two separate rigid molds (probably more), I'm planning to instead try a brushable product that builds up to make a rubbery, one piece negative that can be peeled off and reused. The trick will be to make a "mother mold" to support this during the pour, and I was thinking about using expanding foam for this. Back to the mix, though. Should I be thinking of adding anything to lighten, reinforce or control shrinking and cracking of the poured material?

1. 
   SamT | Apr 10, 2006 05:22am | #11

   Saul,

   If you can centerline your mold, cast a pin in the bottom.

   Drill a 3/4" hole in the 'wound', fill with epoxy after you figger out how to hold the replacement piece in place, then dry pack the gap with the previously mentioned mix.

    SamT

1. 
   chrispy | Apr 11, 2006 05:34pm | #12

   I think an Abocrete type product(I see there adds in my Fine homebuilding Mag) might be just the thing for your situation.