CONCRETE – SLAB – fixing the trenches we so carefully cut
Peace descends on Costa Mesa Sur Mer. Shell-shocked neighbors emerge. The #90 jackhammer has gone back to the rental yard, again. The Brute stands in the corner. The footings are blown thru in six places. The inspector has signed off on the outside drains and water supply and “Gomez” is backfilling and tamping all those trenches.
Our slab is a mass of 12″ wide trenches and neat saw cuts. Next week, after interior pipe and drains, they will begin to think about having all that filled in and fixed. Then we can frame the four bathrooms. (4 BA, 2 BR)
My suggestion of good tamping and colorful throw rugs was rejected out of hand, so to Breaktime I come.
Who has some neat methods of repairing trenches in slabs???
Vapor barrier – not there any more. Replace with ???
How best to reenforce the repairs – dowels? rebar, wire mesh?
Of interest: K thinks what they did back in ’58 was to pour the footings and a 2″ thick rat slab. That was coated in tar as a bond breaker, then the upper slab was poured over it. All this stuff was full strength and went inch by inch.
The ToolBear
“Never met a man who couldn’t teach me something.” Anon.
Replies
pictures.
However, if one designs to construct a dwelling-house, it behooves him to exercise a little Yankee shrewdness, lest after all he find himself in a workhouse, a labyrinth without a clue, a museum, an almshouse, a prison, or a splendid mausoleum instead.
Parolee # 53804
Did you jackhammer the trenches or sawcut them?
What is the finished floor going to be?
Do you have frost footings? Perimeter drains?
What is the worst soil moisture you can expect?
Where in SoCal? In a previous life I lived all over SoCal at one time or another.
SamT
Praise the Corporation, for the Corporations' highest concern is the well being of the public.
Edited 4/28/2007 8:50 am by SamT
He said they saw cut them, then jack hammered, but just 12" wide.
Depending on slab thickness. I would likley dowel the edges, since they are tamping in the fill. My preferance is to backfill with crushed stone or sand, then add a strip of 6 mil poly before pouring. The poly would depend on the local soil/water conditions than just the need to replace the vb.
Dave
"Our slab is a mass of 12" wide trenches and neat saw cuts"
Are you sure that's what that means?
I can see where it means sawcut expansion joints and rough trenches.
But, whaddaino?SamT
Praise the Corporation, for the Corporations' highest concern is the well being of the public.
You are probably right. Sometimes I follow the first mental image that pops up when reading a post. Maybe toolbear will give us more info this evening.
Dave
Reinforcement
#4 bar dowels cut 8" long or so placed about 4" into the slab edges across from each other so the stubs can be tied with an additional piece laid across them overlapping each side. Do this about every foot or so the length of the trenches. Than lay 1 or 2 long pieces the length of the trenches also dowled into the ends of the trench in the slab.
Vapor barrier
Depends on the area, it's tough to achieve in a trenched in slab area as small as 12" wide but if you are really dedicated you can place a barrier of choice in the bottom skirted by a 1/2" wide strip of water stop stuck to the edge of existing slab. I've done this in municipal waste water treatment plant tank bottom repairs. You can always just lay plastic tight to the edges but your only stopping moisture in the middle of the new crete and water will find the saw cut anyway because it's the path of least resistance so without the water stop it's pointless. Their are also newer latex infused concrete that are less water permeable but again is the issue of the saw cut being the real point of access.
Bonding
I would roughen the saw cut edges to create a better bonding surface and slightly undermine the existing slab so mud flows under it locking the slabs together kind of like a key way. You can also simply take a grinder and make some horizontal scores in the existing slab edges, a small chipping hammer to roughen it or even beat on it with a hammer a little. It's just that the saw cut is so smooth because the saw blade actually polishes the concrete it cuts like a grinder leaving little for the new crete to grab on to. Make sure to shop vac all dust and loose debris now or blow out with an air hose. Than at a minimum wet the slab edges before pouring, or make a slurry mix to rub on, better would be a commercial made bonding agent.
Compaction
Add water to the soil as you tamp it, just enough that the water pools for a split second and disappears and pummble it like crazy. Personally I never really trust compaction in an excavation of an existing slabs soil, there is always the problem of the soil loosening under the existing portion adjacent to the hole and there is no good way to compact soil vertically. Your absolute best method to not worry about compaction problems is to use a concrete mix of flowable fill.
Additional tip
Always, always, always finish your new concrete a 1/16" to 3/16" lower than the existing to allow for slab curl. It's a million times easier to shim framing that will sit on it or to feather finish a floor for flooring than to grind out uneven ridges.
A lot of this may seem excessive but it's based on my experience in commercial building specs and I prefer to over build than fix shoty work.
Many thanks for the infomative trench-fixing protocol.
Vapor barrier
Depends on the area, it's tough to achieve in a trenched in slab area as small as 12" wide but if you are really dedicated you can place a barrier of choice in the bottom skirted by a 1/2" wide strip of water stop stuck to the edge of existing slab.
@@@ Water stop? = Peel N Stick?
Bonding
@@ We can easily grind a bit or chip.
Compaction
Your absolute best method to not worry about compaction problems is to use a concrete mix of flowable fill.
@@@ Love to see it. Might.
Additional tip
Always, always, always finish your new concrete a 1/16" to 3/16" lower than the existing to allow for slab curl. It's a million times easier to shim framing that will sit on it or to feather finish a floor for flooring than to grind out uneven ridges.
@@@ Will do. Have no desire to drag our grinder over and run it around. We can float as needed when doing the tiles.
A lot of this may seem excessive but it's based on my experience in commercial building specs and I prefer to over build than fix shoty work.
The ToolBear
"Never met a man who couldn't teach me something." Anon.
Well, your only mistake was sawcutting the trenches.
Given the complete situation you've described, ncproperties <!----><!----> has pretty much covered it. The only thing I will add is that after you chip the saw polished sides, hose 'em off good, then, immediately before the pour, wet them down, or paint them with adhesive Admix. I wouldn't worry too much about vapor barriors in Costa Mesa. Good drainage there.
Been through Costa Mesa many times.
OC. I know the man who founded OC choppers. Maybe after he's dead, I'll tell some stories. I don't want any "Agents" on my tail now, so the stories will have to wait.
Well, here's a short one about coincidences. He was a tail gunner in the war and was the man who shot down the founder of Hondas' fighter plane. So it has always been that Harley can take Honda.SamT
Praise the Corporation, for the Corporations' highest concern is the well being of the public.
Vapor barrier
Depends on the area, it's tough to achieve in a trenched in slab area as small as 12" wide but if you are really dedicated you can place a barrier of choice in the bottom skirted by a 1/2" wide strip of water stop stuck to the edge of existing slab. <!----><!----><!---->
@@@ Water stop? = Peel N Stick?<!----><!---->
Yes, sort of. Typically a brushed on primer, let sit and change color than peel stick and press. The following is not a brand I’ve used but was the first I found in a quick search online to better explain. <!----><!---->
RX-101WATERSTOP-RX¯ is a flexible strip concrete construction joint waterstop that provides a positive seal by expanding upon contact with water. WATERSTOP-RX is an active bentonite/butyl rubber-based waterstop that is designed to replace conventional passive PVC dumbbell waterstops, thus eliminating the requirement of split-forming and product seam welding.<!----><!---->
Compaction<!----><!---->
Your absolute best method to not worry about compaction problems is to use a concrete mix of flow able fill.<!----><!---->
@@@ Love to see it. Might.<!----><!---->
http://www.fhwa.dot.gov/pavement/recycling/fach05.cfm<!----><!---->
“Flowable fill mixtures make up a class of engineering materials having characteristics and uses that overlap those of a broad range of traditional materials including compacted soil, soil-cement, and concrete. Consequently, flowable fills are proportioned, mixed, and delivered in a form that resembles a very workable concrete; and they provide for an in-place product that is equivalent to a high-quality compacted soil without the use of compaction equipment and related labor.”<!----><!---->
I love seeing it fresh as it’s easy to work with for just your situation, I’ve used it commonly in commercial projects like your’s for slab replacement when the trench may contain underground mechanical running through it to be well supported or incased, in “duct bank” pours to encase fiber optic or high voltage lines, or restore undermined footings and structures because as it’s name “flow able fill” it flows into every void and fills solid. The mix contains more of a pea stone aggregate. I hate to see it accompanied with a jackhammer. You can put a 90lbs. air hammer to it all day and never get chunks bigger than your fist to brake off at a time. Unlike with demo of regular concrete that you can feel where it will separate and create shear points by the placement of your hammer bit pattern to get nice chunks, flow able fill only crumbles where it wants to and that’s all it does is crumble or flake off like shale.<!----><!---->
Your right on with all the other points I made to follow so I won’t wish you luck, you won’t need it. <!----><!---->
"I wouldn't worry too much about vapor barriors in Costa Mesa. Good drainage there."
My 5 years spent down the beach in San Juan Capo taught me different.
After lifting the foundation of the house and having to remove and replace the garage slab, I learnt that I certainly wish I had VB'ed that slab.
After everything was all said and done, I found I could NOT put down an epoxy floor on the garage floor due to excessive moisure wicking up resulting from the migration of irrigation water from the beds along the sides of the garage. Although I had accounted for substantial drainage capacity, the clay soil held onto every bit of moisture it could. Even put a foot of compacted sand under the slab with good drainage out and down to the street.
or you could fill it up with concrete and forget about it
Maybe toolbear will give us more info this evening.
Der Bear has attempted to attach a photo of where the trenches meet in CM.
Nice saw-cut trench sides. The ToolBear
"I am still learning." Goya
Photo, try #2The ToolBear
"Never met a man who couldn't teach me something." Anon.
Did you jackhammer the trenches or sawcut them?
sawcut trenches
jack thru the footings
What is the finished floor going to be?
assorted tile
Do you have frost footings? Perimeter drains?
no, no
What is the worst soil moisture you can expect?
winter of 2004 was nasty
soil is clay - alluvial fill from the Santa Ana River
Where in SoCal? In a previous life I lived all over SoCal at one time or
Costa Mesa sur Mer (the sur Mer never stuck <g>) in the O.C.The ToolBear
"Never met a man who couldn't teach me something." Anon.
Wet the sides of the existing saw cut slab. The new concrete will bond better and not shrink or crack.
Wrap and insulate any copper water pipe.
K thinks what they did back in '58 was to pour the footings and a 2" thick rat slab. That was coated in tar as a bond breaker, then the upper slab was poured over it. All this stuff was full strength and went inch by inch.
Instead of overkill, why don't you do the same as your predecessor?
Over excavate under the slab by 4 inches and undermine by the same amount. Fill to the underside of the existing slab with high slump concrete and let set.
Spray or brush on foundation dampproofing only on the new concrete and not the exposed sides of the existing slab.
Pour the remaiderwith low slump and give it a steel finish even with the existing concrete. Don't worry about curling, not enough to worry about.
Done
Gabe
Over excavate under the slab by 4 inches and undermine by the same amount. Fill to the underside of the existing slab with high slump concrete and let set.
Interesting approach. That's what I like about Breaktime - so many ideas.
Let me send these ideas up the food chain for decision.
With this approach, do we eliminate all the doweling and such. BTW - there was no rebar or mesh found in the slab. Two sticks of bar in the footings. The ToolBear
"Never met a man who couldn't teach me something." Anon.
If you didn't have rebar in the original, why would you need it in the patch. It's got nowhere to go. Just make sure that the substrate is well compacted and leave it at that.
Gabe
89202.17 in reply to 89202.16
If you didn't have rebar in the original
He sawcut the trenches. He'll have to dowel the new mud to the existing.SamT
Praise the Corporation, for the Corporations' highest concern is the well being of the public.
He sawcut the trenches. He'll have to dowel the new mud to the existing
Why?
This isn't new construction. The existing has finished settling. Nothing is going nowhere.
Gabe
The existing has finished settling
That's why.
No, there's more to it than just the new settling without the existing. The existing slab has probably settled all it was going to as a unit. Now it is in many seperate pieces, any of which may do a little more individual settling or even shift away from each other.
Doweling helps return the slab to a monolithic unit again. Freshly broken surfaces have so much microsurface area that an excellent bond is likely as long as the new edges were prepped properly and the new mud is well vibrated near the edges
But, SoCal is earthquake country. I would be doweling even broken edges 16'OC, and angling the holes in alternate directions.SamT
Praise the Corporation, for the Corporations' highest concern is the well being of the public.
"SoCal is earthquake country."
You're right in that there are a number of fault lines along that coast. The Sunday OC Register has a map of them along with each and every wiggle of the past week.
As the geologist told me upon initial inspection of my property,
"Quakes are the least of your problem. Since this land resulted from an uplift of the sea bottom, it is primarily clay. You MUST control surface water - get it off the property as soon as possible and do NOT let it soak in. Your concrete will get spider cracks all over otherwise. And footings are only required to go down 12" here."
The OC Register used to be my favorite news rag. Thanks for reminding me, I just added it to my favorites again, (new box.)
I'll have to read it a while and see if the quality is still up there.SamT
Praise the Corporation, for the Corporations' highest concern is the well being of the public.
No, there's more to it than just the new settling without the existing. The existing slab has probably settled all it was going to as a unit. Now it is in many seperate pieces, any of which may do a little more individual settling or even shift away from each other. The slab is simply a surface structure. The substrate has settled over the years and is now stable, regardless of the number of components that make up the surface structure. If you compact it to 98% on a modified proctor, it will behave as would undisturbed and is therefore as stable. Nothing is going to shift away from anything.
Doweling helps return the slab to a monolithic unit again. Freshly broken surfaces have so much microsurface area that an excellent bond is likely as long as the new edges were prepped properly and the new mud is well vibrated near the edges Nothing and I mean nothing will return the slab to a monolithic unit again. Doweling is used to prevent one section from lifting or sinking from another. It does not hold them together and it's not designed to.
But, SoCal is earthquake country. I would be doweling even broken edges 16'OC, and angling the holes in alternate directions. On a clay base, as indicated before, moisture fluctuations are far more detrimental to the integrity of the slab than being on a fault line. We commonly build on clay and it can be a challenge. Surface water control is as important and consistency in the moisture level inside the footprint.
Sometimes we tend to overdesign and therefore introduce problems where they don't exist.
Gabe
I agree with Gabe. Just wet the old slab a the new cement will stick better
okSamT
Praise the Corporation, for the Corporations' highest concern is the well being of the public.
Glad you haven't totally abandoned us.
Did you see the JLC Brats thread?
Dave
Didn't mean to give anyone the impression that I had abandonned the board, it's just that some times during projects, I tend to get a little busy.
Haven't checked out that post yet but I'll have a look.
Gabe
Senior inspector sez: Dowels and epoxy on 16" centers. Boss hoped for 30" OC.
We are looking at this thing and thinking: Plate tectonics. The kitchen plate will start to subduct under the bathroom plate. Dowel it good or get faulting.
NOW - HOW TO DRILL all those holes in a trench 12" wide. My rotohammer is about 24" long. That's a serious diagonal. Anyone make an air hammer?
Right angle hammer?The ToolBear
"Never met a man who couldn't teach me something." Anon.
Ahh, good point I've encountered this a lot and it didn't cross my mind when I gave you my long set of directions for this repair. All you have to do is rather than putting your dowels in perfectly perpendicular to the slab edge is angle them enough that your drill will fit in the trench. Now the trick for this is you will have to start your hole perpendicular to the slab edge angled downward with your hammer above the opposite side of the trench slab and drill about an inch inward, this is just to create a pigeon hole to hold the tip of the bit from glancing off the surface when you go to drill the actual hole at a sharp angle from within the trench. Now the bit at first will still want to wander and pull around a little and bind to try and continue in the direction of your pigeon hole but just be strong and it will quickly smooth out and start drilling normally.
A second method that can be used is to shoot your holes in straight like you would have if you had the room, but angle them downward. If you do this set your holes deeper from the top of concrete on the edge and make the holes maybe 2" deep/longer also. Than use longer dowels and stick in as usual when done stand on the dowel sticking out above the floor surface to bend down into the pour. Making these dowel holes deeper and lower will help to prevent popping the edges of the concrete on the top when you bend the dowels down.
Thanks for the tip.
I suspect that we will have many opportunities to experiment next week with the new Hilit and my Makita SDS+ - which seems to be about as fast as a 60# hammer in cutting through footings. I've dug out my tie wire and tools for the rebar work.
There are opportunities for volunteers here... The ToolBear
"Never met a man who couldn't teach me something." Anon.
I been reading this post, you could compact sub, pin and dowel, use 5000 lbs mix or just fill in up with sackcrete and get the same results, somewhere somebody need to decide their not building a nuclear plant.
I been reading this post, you could compact sub, pin and dowel, use 5000 lbs mix or just fill in up with sackcrete and get the same results, somewhere somebody need to decide their not building a nuclear plant.
Would have liked to toss out the clay, backfill with flowable fill and go from there.The ToolBear
"Never met a man who couldn't teach me something." Anon.
yes, or sand. I mean if it does settle, it be maybe a 1/8 to 1/4 inch pour a $5 bag of self leveling epoxy grout and be done.
It's california. Their outhouses got more engineering than Alaborida's nuclear facilities.SamT
Praise the Corporation, for the Corporations' highest concern is the well being of the public.