I was attempting to patch a portion of my basement slab that heaved. I used regular Sakrete high strength concrete and I think either didn't use enough water, or the aggregate was too big for the shallow depth I was filling (2") resulting in voids on the surface. My plan is to let it harden, apply adhesive and top with sand mix. Will this work? Is there a better way to clean this up that doesn't involve a jackhammer?
Comments
No_Pomegranate2580 t1_j6flb3q wrote
I would ask on one of the concrete subs.
md9918 OP t1_j6g878i wrote
Thanks-- I posted over there on your suggestion
Kerwinkle t1_j6hq8tn wrote
Looks like you didn't mix it well enough. Form the volcano, pour a little water in and mix it by working the walls of the volcano into the wet part. You add another bit of the water and work it in until you finish all the water. After that you work the mix for a few extra minutes working it through the full depth, repeat mixing and stabbing with your shovel or trowel. When you think you are done you still lift the mix from the bottom. And fold it upon itself to make sure you didn't leave some dry mix under all. And you mix and stab the mix some more for another 10 minutes.
When you place the mix you have to tamp it in and vibrate it. Make all the bubbles come out allow the gravel to settle into the hole. Them, without overdoing the vibration, the cement sand grout will rise and that is what you float smooth.
As to repairs, if it has set then wirebrush clean while the concrete is weak to expose the agregate. Then you can try a mortar patch. Ideally as thick as possible and with no thin feathered edges as those will just spall as it dries. Close to vertical edges to the patches you are putting in. Hopefully there's something else over the floor to protect it and reduce the risk of the repair popping out.
Mist with water while curing per the instructions. Drying out too much leads to cracking from shrinkage.
hostile_washbowl t1_j6g5jm8 wrote
Did you follow the instructions on the amount of water to use?
md9918 OP t1_j6g9dub wrote
I did-- if my measurement was off, it wasn't by much. I had this happen before when doing some under-slab plumbing repairs for a new shower, but chalked it up to not troweling it much. I guess from a liability standpoint they'd rather have people mix it too stiff and retain the strength of the concrete, than too loose, and have it be brittle and break.
hostile_washbowl t1_j6gas99 wrote
If you mix it too stiff or too loose is when you run into strength issues. It looks like you only use about half the water needed.
Sorry but I’m gonna have to blame the user on this one.
Banana_Ram_You t1_j6gm0a2 wrote
I don't know anything about concrete, but given the existence of the post, and the tray of mix looking like sand... I can only concur
KRed75 t1_j6h1jbz wrote
That looks about right with the water. The less water the stronger the concrete will become when fully cured. It shouldn't have any dry mix, however.
hostile_washbowl t1_j6h1lwv wrote
If it has dry mix then it doesnt have the right amount of water
Anyways looks like they added water to the mix versus adding the mix to the water which results in a bad blend
KRed75 t1_j6h99um wrote
It's mixed correctly in the photo. OP just didn't work the surface enough with a trowel or float to smooth it out.
Here's how properly mixed concrete should look which is what I'm seeing in OPs photos. If it's visibly wet it has too much water.
hostile_washbowl t1_j6i22ev wrote
Yep. Based on that video, OP used too little water. The dry mix is not fully saturated (or ‘damp’).
Engatsu t1_j6j8x4t wrote
So don't add water to concrete mix for max strength... Gotcha
KRed75 t1_j6ke9m9 wrote
Not sure how you got that out of my post seeing as I said It shouldn't have any dry mix.
jimmio92 t1_j6gt1wr wrote
I find it's best not to read the instructions at all. If you've ever seen concrete pours done on TV, you know the consistency you want -- partially melted ice cream made of rock. Add a little water at a time until you're there, stirring, folding, and kneading the whole time with a shovel. Shovel it in the cleaned out hole, smooth the surface, bingo.
Mix more than you think you need -- worst case scenario, you got yourself a boat anchor, but you had enough to finish the job and that's what matters.
What you're at now is "hammer and chisel it all out and start again" -- make sure to use eye protection and gloves.
KRed75 t1_j6h28s7 wrote
Nope. That would be way too much water. The least amount of water the better. The reason you see it so watered down is because it's easier to work with but it's wrong and makes for a very weak concrete that will crack and crumble in a few years. They make plasticizer additives that once added will make what looks like an almost dry mix look and act like it has 2 times too much water.
Check out this youtube video where they take a super dry mix, add a tiny bit of plasticizer and within a few seconds of mixing, it looks like someone dumped a gallon of water in it.
syco54645 t1_j6jjntg wrote
That is incredible. Thanks for sharing!
KRed75 t1_j6khee7 wrote
Plasticizer in concrete is like magic as the video shows. By the downvotes I've been getting, It's clear that there's a huge misconception that concrete should have so much water that it's liquid to do its job properly which is completely false. The more water, the weaker concrete becomes. It's harder to work when there's not a lot of water though which is why one should use plasticizer instead of adding more water.
Powder plasticizer is best because it has a much longer shelf life. liquid plasticizer has a very short shelf life. If one buys it from a big box store, one should check the expiration date because it won't work if it's much older than that expiration date. The liquid plasticizer at most big box stores is long ago expired.
I used to own a construction business and to keep cost down, I bought a concrete business and a drywall business. Actually made more money off those doing jobs for other companies than I did building houses but I'm an expert in everything concrete. Most companies would water down their mix and add too much aggregate and sand to save money. That's wrong and that's fraud. We did it correctly and would use plasticizer if doing jobs that required nice finish work such as counter tops or concrete walls that were to be the visible finished product. Lots of vibration is needed as well to eliminate voids.
md9918 OP t1_j6iaq00 wrote
Thanks to all who responded. I called Sakrete and they recommended their "Top N Bond," it's a polymer-modified concrete patch that's supposed to be good from thicknesses of 1/2" to featheredge. They say I can put it down within 24 hours of the initial pour, or after the final cure in a month. I'm going to put it down now in the hopes of a better bond.
WISteven t1_j6g2r2y wrote
Adhesive?
NerfHerderEarl t1_j6gpk2g wrote
Yes, typically called a bonding agent. It's a good idea when adding new concrete to old.
tackle_bones t1_j6i8u75 wrote
If you want, you can get a small plastic tub of the vinyl resin enhanced patching concrete (less money; for thickness of 1/4” or less) or the patching concrete (more money; for thickness of 1/4 to 2”) and patch the holes… These types of concrete do not have aggregate. You could probably just remove the aggregate from a bit of your high strength concrete and see if that works. I’d probably mix this second “coat” a little more wet than you have here… it will smooth out easier.
KRed75 t1_j6h1e4s wrote
It's fine. You can add a little more concrete to it as start troweling it until the surface becomes smooth. If it has already started setting up, Just mix up a slurry with the concrete mix but remove the large stones. Then work the slurry it in to the voids and trowel smooth.
trekkerscout t1_j6fjwml wrote
That mix was way too dry. It won't set properly without adequate hydration.