Bad Concrete Removal 2

[sswan60]

Assuming that coring confirms a low 28 day break, we are planning on removing a section of a concrete wall we are building. Wall is 18" wide and the section to be removed is 9'x30'. Section is defined by a construction joint along the bottom with steel reinforcement in the upper wall lap slicing with steel protruding from the lower pour. Steel is #8's at 6" on one face and #6's at 12" on the other.

My question is, are there guidelines on the size of jack hammers or other demolition equipment to use in removing the concrete from the lower steel so as not to cause damage to the lower pour's concrete and steel? For example, use a 15 lb hammer within 1 ft of the construction joint? My fear is that they would use a large back hoe mounted hammer right up to the construction joint resulting in unseen cracks in the lower pour.

Any suggestions or references would be appreciated.

 

[rowe]

I don't have much experience beyond a standard note that we use in rehab projects. Where a portion of concrete is still adequate, we usually do not prescribe the method of removal of the deteriorated concrete (we leave it to the contractor). But we include some verbage that in effect tells the contractor to take care not to damage the good concrete and the rebar to remain - or the contractor will have to repair/replace what he damages at no additional cost to the owner.

If you must specify a method, I would think about saw cutting to within some distance (although 18" is pretty thick to saw cut) and then specify removal by pneumatic chisel for the remainder (but still keep the standard note above).

good luck

 

[RDK]

I always get a laugh out of engineers who think that they have done their jobs by simply telling the contractor that it’s his responsibility.

You are an engineer.

You are supposed to be a professional.

You are supposed to be in responsible charge of the project.

Stop abdicating your responsibility and tell the contractor exactly what you want. Be willing to pay for this method and be willing to take professional responsibility for the results of your work.

That’s why you went to school and that’s what you get paid for.

Earn your fees.

That does not mean that you cannot be open to suggestions of alternate work methods. There is a lot of practical knowledge available from the trades. Learn to work with them to produce the end product that everyone can be proud of.

Another suggestion that will work for the specific problem would be to use a diamond rope saw. This consists of a wire with industrial diamond cutters along its length. The wire saw travels in a loop and is capable of making some very precise cuts. (Within 30-40 mm of line). Simply cut the wall away in pieces sized to me manageable with the cranes and other equipment available to you. You can then lay the wall down and use it as a sidewalk.



Rick Kitson MBA P.Eng

Construction Project Management
From conception to completion

http://www.kitsonengineering.com

 

[rowe]

RDK,

I always get a laugh when engineers think they have all the answers.

When there are significant or specific problems with the design and construction that does require a specific technique, sure, go ahead and specify a method (or two!) if it's appropriate.

Many structures are designed without knowing who the contractor will be. But when a hundred different contractors may bid on a project after the plans are completed, and they each have different resources and preferred methods for various contruction (or demolition) techniques, why not let them figure out a way that suits them best. Although they may not know how to calculate a moment diagram, most contractors have enough field experience to complete a construction project that they have been qualified to bid upon.

How many concrete walls have I cut out? none.
How many concrete walls has an average contractor cut out? Tens, or hundreds, or maybe even thousands.

Who's more qualified?

 

[DTGT2002]

Now how about a little coordination. No one want to put there hands on a problem, but someone has to. Finger pointing never got me anywhere.

However, far to many jobs these days seem to operate in a very hostile environment. Somebody thinks somebody is out to screw him. Force coordination. But I don't think you have a place to specify a precise demo method. Throw in your verbage "Removal must not disturb surround satisfactory concrete, etc." Submit demolition plan for review. If you have a bad working relationship with the contractor, I don't know how to help you.

I too would suggest the diamond rope saw. It is far more expensive than a chipping hammer, but will be faster, likely safer, amazingly precise and will likely make everyone feel better about the repair. You will still need to do the final chipping by hammer of some sort.

I believe we took down a 42"x42" column that was poured 5" off center earlier this year, two cuts, about $10000 a cut. Came out cheaper than the labor projection and didn't shut down the area of the job.

Good luck.

We as engineers cannot be responsible for all means and methods, but we must try to understand the options, and cannot expect a contractor to do what we are thinking but not saying.


Daniel

 

[JAE]

ACI has addressed this question (how big of hammer) in a number of publications dealing with concrete repair. Generally you will find a 30lb hammer being mentioned as a maximum to use so as to avoid micro cracking or fracture cracking adjacaent concrete.

Sawcutting to provide a straighter joint can be used as well but keep if fairly shallow as the true position of the reinforcing may be a bit closer to the face than you might anticipate and you wouldn't want to cut any bars.

 

[dik]

How much off is the strength, and is the strength necessary, judging from the level of reinforcing plus the compression reinforcing, strength may be an issue.

How will the wall fail, and is it really necessary to remove?

Failing that, you may want to cut the rebar to enable a splice and to minimize the effect on the balance of the wall. A heavy reinforced 18" wall is pretty 'tough' and cutting the rebar should allow it to be removed with a hoeram (backhoe jackhammer, in these locales). To get the splice length with the larger bars will require that the balance of the concrete be removed by less agressive means.

 

[Zambo]

what about hydro demolition? Many advantages, a saw cut to determine the edge would also be useful.

Zambo

 

[BigH]

I suppose that I many times try to sit back and breathe a little before I start trying to say we will do this or that. SSWAN60 indicates that he is going to demolish - he seems to want to demolish. Now the questions I'd like to have an answer to is what was the 28-d strength? What was the specified strength? What strength does he really need? How will he interpret the results of the core. If the core strengths are less than the specified strength (that the 28-day strength should have obtained), is he going to order the demolition? Or, will he accept the concrete if the average of three cores (of requisite diameter and length) is >85% of the specified value with no single core being <75%?

If one reads the AASHTO bridge spec, they have an interesting philosophy, I believe. They will review the results and also will carry out computations to see if the low strength is still acceptable for the servicebility (spelling???) of the structure. If, when all is said and done, the strength is adequate for the needs of the structure, why will you demolish? Give the contractor the choice (as he didn't reach the 28-day required strength) of removing or taking a payment reduction, or . . .

Several things are to be remembered too - the strength of the concrete cylinders taken have no absolute relationship to the strength of the concrete in the structure. The curing was different, etc.

Neville wrote a good paper about less than specified strengths and cores, etc. Sorry, at present, I do not have the reference but will try to find out.

So, I suggest SSWAN60 - et. al. - smoke a good Havana, wait until your core strengths are known, rationally evaluate the core results AND the needs of the structure (rather than just the payment requirement) and take the plunge accordingly.

Anyway - that's my thoughts at the moment.

 

[structuresguy]

I agree both with BigH / diK and with Zambo. Is it really necessary to take the wall down? Is it necessary to take it down right now? Have you considered waiting for a 90 day strength? You may be surpised what you find at 90 days.

I once had to order the removal of a 36'x36'x3' mat foundation on concrete piles because of low strength. in that case we had specified 3000 psi, the contractor had ordered 4000 psi to be sure, and we got strengths well under 2000 psi, some as low as 1200-1400 psi. It was no fun at all telling the contractor to take it out. Lots of arguing over who was at fault. Plus they had to be very careful in removing so as not to damage the 15 piles it was sitting on. Basically got a 6' diamater diamond saw, cut it up into cubes, and plucked the cubes out. then had light weight jackhammers carefully break out around the dowels from the piles. Turns out bad batch of cement was to blame. In our case it was required as a 100' precast concrete control tower was sitting on the mat.

As for hydro demolition, if cracking is a major concern, this is the way to go. They use it on bridge decks all the time. Dont know for sure if 18&quot; wall would be a problem or not. Probably depends on local hydro-demo contractor capabilities. I know they use it to remove deteriorated concrete from tunnel and mine wall all the time as well.

Good luck.

 

[BigH]

Structuresguy: thanks for your case history. These are always most welcome.

 

[sswan60]

Thanks for the replies.

First, I don't want to discuss whether or not the concrete needs to be removed. I appreciate all the comments on not being too hasty in removing concrete. I agree, but for this thread let's just assume there is no alternative.

Contractor is very experienced and we will work with them. We'll have them submit a plan for approval. I'll review the ACI publications &quot;Concrete Repair Manual&quot; and &quot;Removal & Reuse of Hardened Concrete.&quot; Does anyone know of other resources?

 

[dik]

sswan60:
Because of the mass, thickness and heavy reinforcing, by removal of the concrete the balance of the wall is put into some jeopardy. It is valid to review the reasons to demolish if necessary; part of engineering is trying to determine if something will work or why it works when it shouldn't.

Can you advise the forum, just how far 'out of whack' the strengths are and the necessity for the higher strength? or the necessity for the concrete removal?

 

[RiBeneke]

Another factor that may need consideration is that lower strength concrete, or concrete with excessive mix water (causing lower strength) and particularly poorly cured concrete, will have poorer long-term durability.
Depending on the location, durability may also be an issue in the decision-making.

 

[DTGT2002]

Keep in mind, additional shoring may be required to support the balance of the wall/structure when the section is removed. If the wall was a crucial element in any other system to the building, it must be temporarily replaced.

I can't think of any viable references off hand. Best of luck. Keep us posted.

Daniel

 

[CRAIGSTANLEY]

Tired workers operating a jackhammer tend to not be very accurate...and it takes forever to bust concrete out with a jack hammer...and damage to surrounding surfaces is bound to occur

I remember 10 yards of concrete being busted out of a bridge deck ....took a 10 man crew 10 days....bent rebar...bent shelves... and holes in the deck panels...

How about High Pressure Water.....check this option out....some really good new technology out there using water

Craig

 

[sswan60]

Here is a follow up to close out this thread.

Two sets of cores were drilled and tested. Average strengths were 4000 psi and 3600 psi and the lowest individual break was 3400 psi. Since the averages are greater than 85% of the specified strength and any one core does not fall below 75% we are confident that the in place concrete is greater than 4000 psi and we are accepting the concrete.

Original 28 day break was 2600 psi.

Thanks for the input on concrete removal. Just happy we did not have to use it.

 

[dik]

Great to have closure! Thanks for the added info...