Honeycomb in concrete footings

[work2play]

Hello,

I am trying to get opinions or good references for determining how bad a problem there is. A contractor poured concrete pad and strip footings. Formwork was not good and the footings bowed out on the sides. Once the forms were removed, there are locations that are honeycombed. Any good references or issues I should look for to determine a good course of action? The building is a prefab metal building supported by piers and pad footings. Thank you.

 

[DaveAtkins]

I am not familiar with any guide for this. When I encounter this, I use my engineering judgment--is the honeycombing below the neutral axis of the footing? Only the concrete above the neutral axis is needed for the compression block for flexure. How bad is the honeycombing? Is it bad enough to cause problems with resisting shear? If you can be comfortable the footing will still function properly, accept the honeycombing.

I have seen this more often than I would like, which is why I think low water-cement ratio is a BAD thing, especially for footings. Footings don't get vibrated, and if the concrete mix is too stiff, this is what you get.

DaveAtkins

 

[Zambo]

DaveAtkins,

which location are you in? Is it a remote part of Africa? "Footings don't get vibrated" is rather a scary statement.

In general I recommend having honeycombing broken out back to sound concrete. Repair can either be by recasting or using a repair compound.

It is useful to keep a record of unacceptable work, maybe you can trace it back to a particular supervisor or labour gang.

 

[kslee1000]

I wouldn't accept, nor reject a footing without knowing the exact extent of defects. It can be a localized edge defect caused by improper vibration, or loosing mixing water (the concrete mix is too wet usually) at the dislocated form works. Or it can be the concrete mix possess problems - keep adding water to maintain flo-ability thus reduces strength globally. I will have less problem to accept honeycombs on side face of wall or pedestal, which can be repaired as suggested without much adverse consequence.

Collecting the testing result, record and mapping the defects (add'l tests maybe required), then talk to the EOR should be the proper way to handle this.

 

[DaveAtkins]

Zambo,

No need for sarcasm. Here in Wisconsin, on smaller footings, my understanding is that vibration is not done.

And if you are going to repair honeycombing, "breaking out to sound concrete" is improper. A concrete patch should never be feathered, so the area with the honeycombing should be cut out deep enough so a proper patch can be cast.

DaveAtkins

 

[civeng80]

Im from Australia !

Footings including pads and strip footings are normally not vibrated (just well spaded) and I have poured so many of these that its no joke. Never had an absolute issue with them. I do agree that engineering judgement is needed on honeycombed concrete and where forms fail, it throws alot of doubt on the competence of the contractor.

 

[concretemasonry]

The repair could be worse than the current situation.

What kind of load (magnitude and type)is on the strip footing? What kind of structure? It sounds like you have vertical columns and a wall between may have very little load on the footing, depending on the aspect ratio of the wall and connections/continuity.

If the load is not great, the footing could be vastly overdesigned dur to standard details.

On something like a simple residential footing, it is often not needed if have good soil, but the prescriptive codes require a footing, but not always reinforced, because the load is distributed and there is no flexure.

Dick

 

[Qshake]

Zambo - perhaps you are new to the industry or from a different location (read other world).

However small building projects like this do not normally have budgets for QA/QC inspectors every hour, every day. Similarly the class of builder for this work is also lacking.

Engineers are not required, unless contractually obligated to oversee the construction of the work, and it is often best that they not set foot on the project thanks to a very litigous society.

If you are new, perhaps this is an eye opener and will be informative. If you are experienced, we'd rather not be subject to you sarcasm.

Regards,
Qshake

Eng-Tips Forums:Real Solutions for Real Problems Really Quick.

 

[Zambo]

Apologies for the sarcasm.

I think the opinions must indicate an industry difference. I usually work on international standard infrastructure projects - including in Africa.

We always ensure all structural concrete is compacted. This can either be by vibrating or by specifiying a self-compacting mix. One of the points is that the design would assume the concrete monolithic and not honeycombed.

We do not however compact lean concrete which is being used to form a level area for the later casting of structural members.

I think having reread the posts that the industry being discussed by most of the posters is residential, this is a field I have not worked in.

The original post referred to strip and pad footings for a prefab building. As my experience is more in the line of bridges, elevated roads and underground structures I cannot judge whether honeycombing is accepted for prefab building substructures.

 

[Zambo]

Back to the original post. The first step is to determine how serious the problem is:

1. are the pad and strip footings reinforced concrete?
2. if they are RC is the rebar now visible?
3. are there any anchor bolts cast into the pad footings?
4. if there are anchor bolts is it possible that the honeycombing extends to a zone around the anchor bolts?

If the concrete is not reinforced and anchor bolts are not present, or not affected, then I agree that there may be a less extensive rectification possible rather than my earlier advice to break back to sound concrete and then recast or repair.

If rebar is visible, or the cover to the rebar is sufficiently reduced to affect the durability I think there is no option other than an effective repair.

If the honeycombing could affect a zone around anchor bolts then further investigation should be carried out. Anchor bolts are often at maximum loading during erection of the building and poor quality concrete could cause a failure.

 

[work2play]

Thank you for all the responses. My computer has been down until today and this is my first chance to get back online. The strip footings are lightly loaded. The building frame is supported by the pad footings. They are oversized for uplift. The rebar in the footings is not exposed and has proper cover. After thinking on it further (all weekend unfortunately), I am not discovering a good reason to reject the footings. The honeycombing is at the edges that blew out, poor workmanship. They did not do a good job on this and it raises quality issues down the line. The same contractor will be doing the piers and floor slab. A preconstruction meeting prior to the slab and pier construction is already scheduled to reiterate what is expected. Thanks again for the responses, this site is most helpful.

 

[hokie66]

An issue which often occurs with unconsolidated concrete in footings is plastic settlement cracking. If you have top reinforcement (you indicate that the pads are designed for uplift), poor compaction will often result in cracking along the bars as the concrete settles. This could be a durability problem.