Slab Breakdown - (Footer Process)

3422 Views 18 Replies 9 Participants Last post by jomama45, Dec 8, 2012

tjbingha

Discussion Starter · Dec 6, 2012 (Edited by Moderator)

Like many posts, our house has a slight sag due to notching of main beam, settlement, overspanning, etc. We have had the structural engineer in to help us with our beam calcs and how to fix the beam problem.

I need to dig out two footers in my basement (his preliminary estimate was 18 x 18 footer but will have the official size shortly) and currently have an old slab basement, prob. poured over the original dirt slab in the 50's/60's maybe.

This is prob. only an inch or two thick at the most. My question is when actually digging up this concrete slab, what is the best/easiest/most efficient way to dig out this footer and breakdown the slab? I have ample space (about 5'5 from slab to main beam) and am looking for best tools to use, techniques, etc. to dig this up. I just want to make sure the correct process is used for my own peace of mind and ease of procedure.

Thanks guys for your advice.

Tom

1 - 19 of 19 Posts

joecaption

· #2 · Dec 6, 2012

I'd use a ciruler saw with a masonery blade to cut it out in a square, then make two more cuts to break it up in sections. A few wacks with a sludge hammer should crack it enough to pry out.
Then use a flat shovel and or a post hole digger to clean it out.
Going to need a 5 gal. bucket to haul out the extra material.

Reactions: 1

hand drive

· #3 · Dec 6, 2012

As Joe mentioned, a circular saw with a masonry blade and to help with dust in a confined place I keep a water hose on site to keep the area wet down while cutting,( two guys help with this process, one to cut, the other to man the hose) just a little water around the blade of the saw nothing much. Also, 24 x24" wide by 12" deep footing will cover most any application.

Bondo

· #4 · Dec 6, 2012

Ayuh,... I'd just skip the saw, 'n bust out the existin' concrete with a pick-ax...

Shovel out enough dirt to the size of the new footer, payin' attention to keepin' the bottom flat, 'n level, 'n the sides sharp, 'n square...

Then mix up some mud, 'n back-fill it flush to the floor with concrete....

I also agree with hand drive, 2'x2'x1' will cover most any weight yer puttin' on it...

AtlanticWBConst.

· #5 · Dec 6, 2012

TIP: This is what we use to keep the dust down, when cutting concrete (with a diamond bladed concrete saw) - works great: http://www.homedepot.com/h_d1/N-5yc...&langId=-1&keyword=pump+sprayer&storeId=10051

Also, you can rent a concrete cutting saw, if the slab is thicker, or tougher, than you expected.

joecaption

· #6 · Dec 7, 2012

Just beating it out will leave a sloppy looking hole.

jomama45

· #7 · Dec 7, 2012

Pay close attention to what the engineer draws, and prepare to dig a slightly deeper hole than you expect, and to pour the concrete in two different pours. Typically, the post/column should be encased within the concrete floor section, so a column pad is poured 3-5" low, the column is placed/adjusted/plumbed, and the second concrete is poured around the column.

jagans

· #8 · Dec 7, 2012

an inch or two???

This is another one of those posts without enough information.

First of all, Concrete was cheap in the 50's. Are you sure there is only one or two inches of it?

Second, you are defiinatley going to need a diamond blade to cut concrete that old. Concrete gets harder over time. First cure is 28 days and then it hardens year after year.

It is not clear whether you are removing the whole slab or just putting footers in for posts to carry the main girder.

I dont know what jomama is talking about, but you need a given amount of thickness and width in the concrete under your post to properly distribute the load imposed by your new column on its base plate. I guess he means an overpour to lock the column in a given position, but this can be done with anchor bolts through the base plate during the pour, which should be galvanized, and set on bituminous felt. I have seen a lot of steel columns that rotted out on the bottom that were set like he describes.

Your structural engineer should have given you all of the specs and drawings.

Working with concrete is heavy work. You will need a shale bar for prying, and maybe even a jack hammer from the local rental. The electric ones work pretty good, but make sure you get the right sized cord. And a good Sledge hammer and gloves. Not sure what a Sludge hammer is :laughing:

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oh'mike

· #9 · Dec 7, 2012

Just a note on shimming your post----the steel shim plates go under the posts--never sandwiched between the top of the column and the beam---

The shims are then buried in the slab---

stadry

· #10 · Dec 7, 2012

our minimum is 2.0 x 2.0 x 1.0 w/3 #4 rebar at 4" & the other 3 @ 8" - both mats at 90*,,, you had an engineer who had 'eyes' on it - just seems small to us - don't go crazy on the wtr when using bagg'd conc mix - compressive strengths ARE as printed on the bag IF you don't add excessive wtr :thumbsup:.

we use a 4" grinders, diamond blades, & 2# hammer rather than ' ciruler saw ' :yes: w/masonary blade ' - cheaper blade, & easier to handle grinder - short handle shovel,,, everyone agrees on the 5gal bkts, tho :thumbup:

technique on the shovel should take about 2min :laughing:

jomama45

· #11 · Dec 7, 2012

jagans said:
This is another one of those posts without enough information.

First of all, Concrete was cheap in the 50's. Are you sure there is only one or two inches of it?

Second, you are defiinatley going to need a diamond blade to cut concrete that old. Concrete gets harder over time. First cure is 28 days and then it hardens year after year.

Too many people get hung up on "28 days" when it comes to concrete. Some concrete will hit it's design strength within the first day, and some may take 2 months to do it.

It is not clear whether you are removing the whole slab or just putting footers in for posts to carry the main girder.

I dont know what jomama is talking about, but you need a given amount of thickness and width in the concrete under your post to properly distribute the load imposed by your new column on its base plate. I guess he means an overpour to lock the column in a given position, but this can be done with anchor bolts through the base plate during the pour, which should be galvanized, and set on bituminous felt. I have seen a lot of steel columns that rotted out on the bottom that were set like he describes.

We've used adjustable columns here for decades, and I've yet to see one installed the way I described rot out. The hollow tube never makes contact with the concrete, only the 1.5" diameter adjustment screw and the cast iron bottom plate, leaving the tube above the floor. I suppose if you use a non-adjustable lally column, it could certainly rot out.

Your structural engineer should have given you all of the specs and drawings.

Working with concrete is heavy work. You will need a shale bar for prying, and maybe even a jack hammer from the local rental. The electric ones work pretty good, but make sure you get the right sized cord. And a good Sledge hammer and gloves. Not sure what a Sludge hammer is :laughing:

Or it could be sand below the floor, who knows........

stadry

· #12 · Dec 7, 2012

have to disagree w/the ' cheaper in the 50s ' comment,,, very little was less expensive back then,,, many items we have today were't even dreams then, either,,, what HAS changed is the value of our $$$ - eg, when i 1st married in '65, store brand bread was 5 loaves = $ 1 - today its over $ 2 EACH loaf,,, what happened ? the loaf is still the same size, right ? :huh: its our $$$ that's gotten smaller,,, we are more efficient placing the same amount of conc - we have smaller crews - we work faster - AND we deliver better finished products.

let's not forget where the REAL blame should be placed - even today, our $$$ are under attack to the point many just give up & close their businesses :thumbsup: it just ain't worth it no more :furious:

tjbingha

Discussion Starter · #13 · Dec 7, 2012

Thanks guys for the reply. I apoligize that not enough "detail" was provided but I really just wanted to get some basic tips on how to cut the slab, not necessary strength of concrete, size, building methods, econcomics lesson, etc. I really enjoy trolling on this forum b/c no matter what the original post always digresses and goes off on a bunch of different tangents, which can be very informational and at times comical. It is a very cool forum indeed. And, itsreallyconc, if you really want to know what is going on with your money, I am a CPA and would be glad to assist you, for a small fee of course.

jagans

· #14 · Dec 7, 2012

Lack of Information

"Like many posts, our house has a slight sag due to notching of main beam, settlement, overspanning, etc. We have had the structural engineer in to help us with our beam calcs and how to fix the beam problem"

Well which is it, or is it all of the above? If you are adding lolly columns, then I guess we have to assume that the span between columns is currently too great. I find it very hard to believe that a licensed structural engineer would not be very specific on what size footers you need, what size bearing plate, and how to anchor and reinforce everything.

Like I said in a previous post, working with concrete is usually tough work, and you need the right tools to work with it. You can cut out a square with a circular saw and a diamond blade, but you can only go about 2.5 inches deep with a saw or a right angle grinder. In many cases the best approach is to drill a series of holes in a line with a serious hammer drill (Hilti) and beat the heck out of the concrete with a lump hammer or sledge. You dont want a John Henry 10 pounder if you are not used to it, but buy a hammer made in the USA like a Stanley. They understand balance, unlike the Chinese. The key to breaking concrete is to get the bearing surface out from under it, so if you can get a large enough hole in it to get a shop vac under there to remove the dirt, your efforts will be well rewarded. Loosen up the dirt with a crowbar, screwdriver, piece of rebar, or whatever, then vacuum out the loose dirt. Concrete has tremendous compressive strength, but lousy tensile or flexural strength. That is why you will want your rebar located near the bottom of your footers, so it is in tension. about one third up.

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tjbingha

Discussion Starter · #15 · Dec 7, 2012

Thanks Jagans. It is all of the above. The previous ownners notched the main beam, the main support column (masonry) settled, and the beam is overspanned. The structural engineer is giving me plans for what size footer, new beam (for sistering), and which lolly columns to use. I just wanted to be prepared for when I get the plans to just go ahead right away with the footers, as that will be as most people mentioned heavy work, time consuming and laborious. All are very good tips and I am pretty pumped to try my luck with some of the procedures explained.

jagans

· #16 · Dec 7, 2012

To Jomama

OK. Normally mixed concrete (1-2-3) without any additives achieves good working strength in about 28 days. Remember the nuclear power plant that collapsed killing all those men because they rushed it?

I have never seen an adjustable lolly column installed with the threaded end down, but I guess you could do it, if you didnt mind cracking your shins on the jacking bar. LOL When I was doing this type of work, I only used the adjustable type for jacking floors into position I used the concrete filled 4 inch steel pipe ones for permanent installation. That gives you the compressive strength of the concrete, and the tensile strength of the steel.

Funny how people do things differently in different areas, or by how they learn.

stadry

· #17 · Dec 7, 2012

jagans, i doubt if filling a lalley column w/conc adds an appreciable increase in its ability to support static loads,,, i always understood it was required for fire code, no ?

no one knows when concrete cures - we can measure testing time ( 24hrs, 7d, & 28d ) & those periods have become accepted as a testing norm,,, we can also set compressive/tensile/flexural goals - when testing reveals the goals have been met, we're satisfied - but conc contines to cure, cure, & cure more ad nauseum

jagans

· #18 · Dec 7, 2012

Column

Not sure if it is for fire or not, good question, but the area of a 4 inch circle is around 12.5 inches. even if the concrete is 2200PSI ( which is really low, 3400 is typical) we are talking about an ability to bear 27,500 lbs of static load per column, correct?

jomama45

· #19 · Dec 8, 2012

I've always been under the impression that lally columns were filled with concrete for fire rating as well. Simplifying the concrete compressive strength by area is unreliable as well, as the entire unit of steel column with concrete fill would have to be tested as a complete assembly to give an accurate reading. Most 3.5" steel columns rate in the 13K-18K# area for load, and there's no way adding concrete inside of them will come even remotely close to doubling that. Not to mention, the addition of concrete inside of a steel column is typically a recipe for early deterioration of the steel......

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