Guide for Obtaining Cores and Interpreting Compressive Strength Result 4

[shakeelahsan]

We have to go about obtaining cores from RCC Slabs. I'd request to guide me for obtaining cores and interpreting compressive strength results.

 

[Ron]

I'm not sure where you are located, but in the US, the cores would be taken in accordance with ASTM C42 and the evaluation of the concrete would be done in accordance with ACI 318.

In general, if the compressive strength of the cores achieves at least 85 percent of the design compressive strength, the concrete is considered acceptable.

 

[BigH]

Add to Ron's note - "and no single core shall be lower than 75% of the design compressive strength." Neville had an article in Concrete International (I think) about coring and evaluation of cores. I'll try to find it a bit later.

 

[Ron]

thanks, BigH...you're correct as usual!!

 

[shakeelahsan]

Thanks for your replies.

I am located in Pakistan, and we follow ACI Codes, and ASTM.

Our client requires a witness representation from us as Consultants, while the Material Testing Laboratory Team, takes out the cores for testing. I have recommended at least 4"x4" Cube cores for Slabs.

The objective being repair and renovation works recommendations to existing RCC Slabs, in which concrete is corroded due to penetration of salt water and rain water, Steel reinforcement is also rusted. Could you recommend in this scenario.

A picture is attached for your review and further recommendations.


Engr. Shakeel Ahsan
Director Engineering
CMEC - Pakistan

 

[concretemasonry]

If you are testing for compressive strength, you will have to do some research and use corrections factors to compare cubes with cylinders that have a h/w (or h/t) ratio of 2:1, especially if the cubes are true cubes.

Observation of the "cracking" or removing the bottoms of the samples from the concrete mass is important. This could create some microscopic cracking that can reduce the compressive strength of the samples.

If you are testing samples for durability, it is more acceptable to use sawed samples, but the orientation during testing could possibly be a factor.

Dick

 

[ali07]

Hi Shakeel,
This structure need to be demolished, rather repair. Reinforcement appeared to be almost finished.
If you will take cores, you are doing more
worse to the structure than thinking of good.

 

[shakeelahsan]

BigH,

I await the article by Neville in Concrete International about coring and evaluation of cores.

Ali,
The last resort would be demolishing the structure, but this is a workshop area, with lots of machines installed underneath the roofs to be treated or demolished, which makes it difficult for us to just let it go.

We need to find a solution to it. Can you make any suggestions.

Engr. Shakeel Ahsan
Director Engineering
CMEC - Pakistan

 

[Ron]

Shakeel...from your photo, that concrete is in pretty bad condition. Concrete doesn't corrode. What you are seeing is likely spalling as a result of exfoliation of the corroding rebar. Unless you have another mat of steel in that slab, you're in danger of collapse, if the rebar has corroded to the extent that it appears.

You have a few of choices...

1. Sandblast the existing surface, add a mat of rebar and shotcrete the exposed underside.

2. Suspend a mat from the existing slab, form below, and pump high strength grout through core holes in the slab to create a new thickened slab section.

3. Demolish the slab section and dowel back into existing beams, form and re-pour the slab.

4. Consider a carbon fiber overlay (underlay) after sandblasting.

Good luck.

 

[dik]

Coring for compression strength will do little except determine the use of improper materials, either specified or supplied.

The cores can be used to help determine the degree of the chloride intrusion. The amount of cement content can also be determined. This will be a measure of the durability.

Unless the surface above is used for parking, I've not seen this degree of corrosion with normal RCC.

Except for areas of shear, the amount of reinforcing steel appears to be small, and with the corrosion even less. Concrete strength has little impact on flexural strength for lightly reinforced sections. Compressive strength, however, can be a measure of the durability.

The slab is in need of major remediation as noted above and this work should be undertaken ASAP. The existing condition is unsafe.

Dik

 

[kslee1000]

Ron:

On top of the new reinf mat, would you consider to insert vertical dowels at specified spacing to force the new and existing concrete stay together by mechanical means, should the bond fails locally for wharever reasons?
Or you feel it is an over-kill.

 

[Ron]

kslee1000...doweling should be done. My recommendation for suspending the mat from the existing concrete would be done with dowels.

 

[kslee1000]

Ron:

Thanks for clarification. I have similar situation at hand as well (almost identical). I will follow the procedures outlined.

 

[ali07]

Shakeel,
Option 1,2 and 4 as suggested by Ron are ideal. But in Pakistan, it may cost you highly expensive which you can't afford. Pump concrete is not that common there. Carbon fibre need a smooth surface for epoxy bonding, which you don't have there.

By the way is that workshop is in service, it is highly unsafe to be work under that structure. Machines are not worthy than the humans.

 

[BigH]

Paper by Neville titled "Core Tests: Easy to Perform, not Easty to Interpret", Concrete International, November 2001. You should be able to find it in Pakistan - big consulting firm, university, etc.

 

[shakeelahsan]

Thanks to you all for replies.

Indeed it is spalling of concrete. Adding another mat of steel in that slab, might be a solution, but do you think this can be achieved through introducing Structural Steel section, like channels supported on existing beams and perlins at short spans welded to channels to make it a support structure for existing slab.

Would that be a right idea to go for?




Engr. Shakeel Ahsan
Director Engineering
CMEC - Pakistan

 

[Ron]

shakeel...you could do that, but you have cast in place beams and you will have to establish connections from the steel to the concrete, which might be difficult depending on the steel pattern in the beams. Further, the steel section lengths will have to be controlled tightly to prevent fastener overload in bending.

While it can be done, forming and placing concrete with a mat or using shotcrete (assuming you have local structural shotcrete expertise) would be a bit easier.

While the chloride intrusion doesn't hurt the concrete, it does damage the steel as has happened. Whatever course you take, be sure to treat the existing concrete to mitigate the chloride issues.

 

[dik]

What is the condition of the existing concrete beams?

Dik

 

[kslee1000]

Concrete spall like this usually occurs in a humid area. Provide better ventilation, or crate just air flow may helps. I think the shotcrete is the way to go. Examing the reinforcing carefully to get an idea of rate/amount of deteriation. To my opinion, if the remaining concrete still has adequate strength, you may just need to make up the steel lost from corrosion. As Ron pointed out, structural steel framing action might work, but it adds dead weight, and after all, you will still have to clean and cover the defective concrete.

When you examine the existion reinf steel, keep an eye on suspecious cracks (both top and bottom, and slab and beams). Deep penetrating cracks might indicate shear failure, which speel trouble for the survival of this slab.

 

[shakeelahsan]

Existing beams appear in much better condition, with no visible cracks or spalling of concrete.

Please see attached picture for a better idea of physical condition of beams.

Engr. Shakeel Ahsan
Director Engineering
CMEC - Pakistan