indentify the defect 2

[concreteworld]

Dears

please analyze the defect encountered in the water tank wall. concrete is flowable 500mm poured at a hieght about 3.5m using mobile pump.

Kindly explain type od defect, REASONS AND REPAIR

THANKS

 

[dik]

It's called 'honeycombing'. Do an internet search for concrete defects, honeycombing, manner of repair...

Dik

 

[concreteworld]

Yes its a honeycomb but weather it is due to concrete or bad workmanship? concrete was cohesive and did not bleed. bcos of improper compaction the lower part of the wall is showing segregation and also the the concrete was placed from a height of 4 mtrs fall.

 

[JAE]

May not be due to concrete itself but usually honeycombing occurs because of a combination of a stiff mix (which is usually a good thing) and a lack of proper vibration of the concrete to avoid these kinds of air pockets and lack of cohesiveness in the mix.

 

[dik]

Or the aggregate 'pinging' off the face reinforcing... I figgured I'd give him a pointer in where to look...

Dik

 

[hokie66]

I don't have any experience with "flowable" concrete, but suspect that too much reliance on the concrete compacting itself without adequate vibratory consolidation is the reason. There have been a few similar posts on the site in recent times, and they seem to be mostly related to unreasonable expectations of "self-compacting" concrete.

Anyone building water tanks should know better. Now concrete repair is required, i.e. chipping out the defective areas and using concrete repair mortars. Then probably an appropriate membrane.

 

[Ron]

hokie66, dik and JAE are correct...this is underconsolidated concrete, also known as "honeycombing" as noted. It is due to lack of appropriate vibration (consolidation) and results in voids that reduce the cross-section of the concrete, inducing cracks and can cause structural failure if design loads are achieved...usually not, but can happen.

If only aesthetic, it can be patched with surface patching mortar. If structural, epoxy injection with additional forming is required.

 

[dik]

and, the problem is likely caused by improper concrete placement.

Dik

 

[Ron]

dik..yup!

 

[hokie66]

Leaky formwork is another cause of honeycombing, but in the pictures attached, the problem is so widespread that total inattention to vibration is the likely culprit.

 

[dik]

as per Ron...

Hokie...yup

Repair, although generally structurally OK can be problematic from an appearance vantage.

Dik

 

[hokie66]

I would generally agree that repair will solve the problem, but this is a water tank. Repairs are highly likely to be incomplete enough so that the water can find its way to the reinforcement in some locations. I think repair, yes, but then install an effective membrane on the water side.

 

[KateP]

ad. "Flowable" concrete:
In my experience in concrete works I use ASCC (Almost Self Compating/Consolidating Concrete), when I have flowing concrete in spec. The "Almost" means that I need to intruduce (no too much but some) vibration to my mix-ready concrete, which is flowing concrete. It works, but all depends on workmanship and concrete recept / technology etc.

ad. repair technology:
I use SIKA system to solve this kind of problem with concrete

 

[concreteworld]

The height from which the concrete was dropped in the wall is about 3-4metres which itself may cause segregation and possibly honeycombing. also there is change in color of concrete in some areas..why is this for? Is it due to non-uniform w/c ratio in each truck
Or due to rough and absorbent formwork?
Please shed some more light based on the enclosed pic


Thanks

 

[dik]

Discolouration is too irregular likely for it to be formwork. What type of quality control do they have at the concrete supply? How experienced is the concrete contractor?

Dik

 

[hokie66]

Hard to say how that light coloured slug of concrete got there, but the picture just goes further in indicating lack of consolidation as the issue.

 

[MrHershey]

Segregation due to dropping 3-4 meters is likely not due to the drop, but due to lack of vibration. Freefall height during concrete placement has not been shown to result in significant segregation provided adequate vibration is provided. See link below:

Link

In the June 2001 issue of ACI’s magazine, Concrete International, an article by Suprenant cites several references summarizing the effects of free fall. He concludes that free fall of concrete directly over reinforcing bar or at high slumps doesn’t cause segregation or reduce compressive strength, but restricting free-fall height does decrease concrete production rates and increase owner costs.

 

[KateP]

ad. discolouration
It could be made by worn out (sometimes dirty) formwork or by not this same ready-mix concrete in each supply or by more watery concrete. In my opinion, my experience.

I would control framework, pouring proccess, and workship's work during pouring, and concrete recept as well.

 

[Ron]

Difference in color appears to be a significant difference in the placed concrete....different slump, different w/c....overall poor batch consistency and poor quality control.

For a water-retaining structure, as hokie66 noted, this one repair and a membrane or coating. I would also be concerned about internal voids. Test for them.