R.C Base & column with low unacceptable concrete strength

[prostruct]

Being design engineer for a 3 storey R.C building, I requested concrete cores to be tested for base and column upon suspecting column quality from ground to first floor level. Results were surprising, getting only 50% of specified strength. 2 floors have been built supported by this colum & base.

What procedure can I use for remedial work?

 

[BigH]

First of - can you please give us details such as what the specified strength was to be? Also, were your cores tested as per ASTM test methods - i.e., core preparation before testing? What was the cylinder strength of the concrete - from normal QC testing - and strengths at various dates after casting? Were there any voids in the QC cylinders (or cubes?) - or the cores?? How many cores did you take; should take three for one set. To be acceptable average of the three are to be 85% of the specified strength and no single core less than 75%; how many days after concrete placement were they tested. Did the cores cut into or across any deformed rebar? Why did you ask that cores be taken anyway . . . were the QC tests low? Did you use fly ash? Sorry for all the questions - and somewhat disjointed - but the more data you can give, the better replies you can expect.

 

[prostruct]

Specified strength 25MPa. Cores were tested as per SABS Codes(Similar to british standard) No fly ash used.

28 day strenth for the project are fine but concrete for structural elements in question might not have been sampled. I suspect concrete not ment for structural elements was just used by General construction staff without consulting their supervisors.

Horizontal cracks developed on the column, concrete is weak, you can cheap it easily using 20mm reinforcement bar. Contractor's staff are not well skilled in building works.

Requesting cores to be tested was just a formality to to justify need for remedial work.

 

[hokie66]

Why was such low strength concrete on the job? Intended for blinding layer under footings? If so, I think this concrete has to be removed and replaced. Big job.

 

[prostruct]

The issue to be addressed is construction sequence/procedure to be adopted for removal of affected column.
Should we remove column first lift only (affected part) or remove column full lenth casted ( up to second floor level.

 

[asixth]

How where the columns formed. I had a contractor put polystyrene inside a circular column form with the intent of creating a square column. They removed the forms to find that the polystyrene had moved inside the form and boney concrete resulted towards the base.

If possible, can you post a photo.

hokie

I have been using 25MPa concrete for footing beams and 10MPa concrete for blinding layers.

 

[hokie66]

prostruct,
I see no reason to remove concrete other than the low strength portion.

As to the procedure, the hard part is supporting the structure above without overstressing it. In order to remove the footing and column, your ground supports and props will need to be a substantial distance away from the column, yet the beams supporting the structure will need to be close to the column to avoid loading it in an unintended manner. I would preload the supporting beams with jacks to take the full dead load before chipping away the defective concrete.

asixth,
Yes, that is common practice, and I was suggesting the 10 MPa found its way into the footing and column in question.

 

[BigH]

Have you run an analysis with the "low" values to see if the design is still safe?? Might be an idea before you go out and start playing wrecking ball. What will the strength of the concrete be, in say another couple of months; does this improve your safety factor? Have you tried to take more cores? Was the steel installed correctly and all you have to worry about is the concrete? Have you taken into account the confinement on the strength of the concrete.

 

[hokie66]

For existing old buildings, BigH's approach is often used if structural elements are judged to have sufficient durability as well as strength for another design life period. But for a building under construction, the building should comply with current practice. The codes I am aware of have a lower limit on concrete strength in order for the code provisions to even be applicable.