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Guide for Obtaining Cores and Interpreting Compressive Strength Result
4

Guide for Obtaining Cores and Interpreting Compressive Strength Result

Guide for Obtaining Cores and Interpreting Compressive Strength Result

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

 

RE: Guide for Obtaining Cores and Interpreting Compressive Strength Result

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.

RE: Guide for Obtaining Cores and Interpreting Compressive Strength Result

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.

RE: Guide for Obtaining Cores and Interpreting Compressive Strength Result

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

RE: Guide for Obtaining Cores and Interpreting Compressive Strength Result

(OP)
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

RE: Guide for Obtaining Cores and Interpreting Compressive Strength Result

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

RE: Guide for Obtaining Cores and Interpreting Compressive Strength Result

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.
 

RE: Guide for Obtaining Cores and Interpreting Compressive Strength Result

(OP)
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

RE: Guide for Obtaining Cores and Interpreting Compressive Strength Result

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.

RE: Guide for Obtaining Cores and Interpreting Compressive Strength Result

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

RE: Guide for Obtaining Cores and Interpreting Compressive Strength Result

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.

RE: Guide for Obtaining Cores and Interpreting Compressive Strength Result

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

 

RE: Guide for Obtaining Cores and Interpreting Compressive Strength Result

Ron:

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

RE: Guide for Obtaining Cores and Interpreting Compressive Strength Result

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.

RE: Guide for Obtaining Cores and Interpreting Compressive Strength Result

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.
cheers

RE: Guide for Obtaining Cores and Interpreting Compressive Strength Result

(OP)
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

RE: Guide for Obtaining Cores and Interpreting Compressive Strength Result

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.

RE: Guide for Obtaining Cores and Interpreting Compressive Strength Result

What is the condition of the existing concrete beams?

Dik

RE: Guide for Obtaining Cores and Interpreting Compressive Strength Result

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.

RE: Guide for Obtaining Cores and Interpreting Compressive Strength Result

(OP)
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

RE: Guide for Obtaining Cores and Interpreting Compressive Strength Result

Yes. The beam looks good from pic. However, it looks like the concrete cover wasn't adequate to begin with. Anyway, if the exposed area coluldn't be fixed locally, knock out the remaining covers and re build as suggested with adequate concrete clear cover.

RE: Guide for Obtaining Cores and Interpreting Compressive Strength Result

The visual examination of the core is important as well. You can download

Petrographic Methods of Examining Hardened Concrete: A Petrographic Manual; Publication No. FHWA-HRT-04-150
July 2006

at

http://www.fhwa.dot.gov/pavement/pccp/pubs/04150/

It's a large file 26 MB

RE: Guide for Obtaining Cores and Interpreting Compressive Strength Result

Shakeel,

Have you measured the thickness of the existing roof slab?  Also the cover that was provided originally.  And lastly, can you describe the reinforcement?  It seems to be some type of fabric or ladder reinforcement.

Operations like sandblasting, shotcreting etc. would probably require removal of the equipment below.  Because the beams/joists appear to be in good condition, I would try to find a way to work from above.  If you have the details of the original structure, you could analyze the joists to see if they can carry a new slab on top to span the short distance between the joists.  You would still have to find a way to stabilize the spalling, which can't be doing either the equipment or the operators any good.

This spalling is a vivid example of failure to provide or maintain an adequate waterproofing membrane.

RE: Guide for Obtaining Cores and Interpreting Compressive Strength Result

(OP)
Thickness of slab is 6" and clear cover to reinforcement steel is 0.75".

Could you please explain how sandblasting to this slab would be done, the procedure I may recommend to my client, if approved?

 

Engr. Shakeel Ahsan
Director Engineering
CMEC - Pakistan

RE: Guide for Obtaining Cores and Interpreting Compressive Strength Result

Sandblasting may be done in several ways...

1. You can use actual sand...usually for this application a 20-30 sandblasting sand is used.  Lots of cleanup and protection of equipment below is required.

2. You can use an abrasive grit, such as carborundum or other abrasive grit,like sand...same cleanup required.

3. You can use dry ice. Almost not cleanup other than the blast residue from the slab.  


 

RE: Guide for Obtaining Cores and Interpreting Compressive Strength Result

4. Hydroblasting with proper pressure... dry ice seems to take forever...

Dik

RE: Guide for Obtaining Cores and Interpreting Compressive Strength Result

Ron & Dik:

I am interested in learning more on sand/hydro-blasting, any good reference material?

RE: Guide for Obtaining Cores and Interpreting Compressive Strength Result

I have used hydroblasting on a couple of projects to remove poor concrete and with a reasonable operator it works very well.  Like using dry ice, it can be used in environmentally sensitive areas with fewer problems.  With higher pressures, hydroblasting can remove sound 40 MPa concrete.

I don't have any manuals, except for a couple of 'advertising publications, but a search of the net should bring up a pile of information.  A matter of discussing this with someone that undertakes this work.  It's been around for a decade, maybe more, but I first used it about 10 years back.

Sandblasting may be the least costly and is more common.

Dik

RE: Guide for Obtaining Cores and Interpreting Compressive Strength Result

What is the slab surface above used for?  Parking? and why the large concentration of salt? and what about the top level of reinforcing to accommodate continuity of the slab?

Thanks, bridgebuster, for the very good link.  More reading.

Dik

RE: Guide for Obtaining Cores and Interpreting Compressive Strength Result

From Wiki... and there's more.

From Wikipedia, the free encyclopedia
Jump to: navigation, search

Hydrodemolition (also known as hydro demolition, hydroblasting, hydro blasting, hydromilling, waterblasting, and waterjetting) is a concrete removal technique which utilizes high-pressure water to remove deteriorated and sound concrete as well as asphalt and grout. This process provides an excellent bonding surface for repair material and new coating applications. First developed in Europe in the 1970s, this technology has become widely accepted for concrete removal and surface preparation throughout Europe and North America.

Hydrodemolition is not used as much for demolition as it is for surface restoration and protection projects. When concrete has deteriorated or the reinforcing steel has begun to corrode, it is necessary to remove any unsound concrete and reinforcing bars (rebar) in order to replace it with new concrete and maintain the integrity of the structure. This process has also been used to remove sound concrete that is not compromised in any way. This may be done to install a preventive cathodic protection system, or to remove concrete in structures in which vibration is a concern. Unlike jackhammers, hydrodemolition does not produce vibrations throughout a structure and therefore does not introduce micro fractures.

Dik

RE: Guide for Obtaining Cores and Interpreting Compressive Strength Result

Dik:

Very informative. Thanks.

Which method you prefer for deteriated concrete removal, and what are the essence of considerations for each application?

RE: Guide for Obtaining Cores and Interpreting Compressive Strength Result

dik...good idea on the hydroblasting...just wasn't sure if the equipment below could take the water.  You're right, dry ice is slow, but the lack of cleanup is often worth it.

RE: Guide for Obtaining Cores and Interpreting Compressive Strength Result

I figured that the stuff would have to be protected anyway and it was just offered as another alternative.

Hydroblasting seems to remove loose material that regular sandblasting doesn't seem to get at... in particular with corrosion byproducts; they may find the slab and rebar is in worse condition than the photos show.

kslee:
For corrosion related work, I like hydroblasting if you have a good applicator.  If you have an overzealous cowboy (my apologies to any Texans in the forum) he can do some serious damage if he's not careful.  Regular sandblasting is more common and generally less costly.  For environmentally sensitive stuff I think dry ice is maybe a little better.  In support of Ron's comment, the less aggressive nature of it generally makes containment of the byproducts more containable.

Dik

RE: Guide for Obtaining Cores and Interpreting Compressive Strength Result

(OP)
What is the unit cost for Hydro blasting, Sand blasting and Dry Ice?

You can give the unit cost for your area, I'd just like to interpret it for Pakistan.

 

RE: Guide for Obtaining Cores and Interpreting Compressive Strength Result

Any estimate would just be a WAG and not likely applicable for your environs.  Once you have the equipment, the work is labour intensive and costs would not likely be applicable.

Maybe someone more local can help.

Dik

RE: Guide for Obtaining Cores and Interpreting Compressive Strength Result

(OP)
Dear All,

I have these results for Concrete Cores from laboratory now.

I'd request comments from all of you.

Sample L/d   Correction Load    Strength Corrected Strength
No.    Ratio Factor      N      (N/mm2)  Strength  (lb/in2)
                              
1      1.25  0.94       139250   17.72   16.63      2416
2      1.25  0.94       138360   17.61   16.55      2400

1      0.70  0.74        98913   12.59   9.32       1351
2      0.70  0.74       112640   14.34   10.61      1538

1      1.00  0.87        85607   10.90   9.43       1374
2      0.75  0.80       115980   14.76   11.81      1712

1      1.25  0.94       138450   17.62   16.56      2402
2      1.00  0.87       137840   17.54   16.26      2213

1      0.75  0.80       123150   15.67   12.54      1818
2      0.75  0.80       123811   15.73   12.59      1825

1      0.87  0.82       137315   17.48   14.33      2078
2      1.00  0.87       139260   17.72   15.42      2236

I need recommendations on existing condition of concrete.

Tests on three samples of steel reinforcement resulted Yield Strength of 490000 Psi, 42909 Psi & 50708 Psi and Ultimate strength of 69997 Psi, 60440 Psi & 71421 Psi respectively. Elongation was 12%, 10% & 11% respectively.

What are your recommendations?

 

Engr. Shakeel Ahsan
Director Engineering
CMEC - Pakistan

RE: Guide for Obtaining Cores and Interpreting Compressive Strength Result

I would not consider any of these results as structural grade concrete.  Concrete strength is a good indicator of durability, and it is apparent from the core results that the concrete is of very poor quality.  Demolition and reconstruction is looking like the answer.  

RE: Guide for Obtaining Cores and Interpreting Compressive Strength Result

I would suggest that reconstruction be considered; the material is not durable.  What is the area above the slab used for and what type of salt exposure did it have?

On a positive note, and to re-iterate, unless shear is an issue, the strength of the concrete has little effect on the flexural strength for the amount of reinforcing shown in the photos.

Dik

RE: Guide for Obtaining Cores and Interpreting Compressive Strength Result

It may be possible to provide new structure to encapsulate and support the existing structure.  This can be done with a mesh to support it on new light steel beams/channels spanning between the existing beams.

RE: Guide for Obtaining Cores and Interpreting Compressive Strength Result

(OP)
These are single storey structures, with no roof water/ heat proofing at all.

During my inspection visit to site, it was revealed that the structures never had any roof drainage system, leaving the area exposed to rains and extensive sun light.

Temperature in this area goes up to 50C and water table changes during seasons, water is saline, and perhaps the atmospheric conditions over 35 to 40 years time span have resulted this damage to the structure.

 

Engr. Shakeel Ahsan
Director Engineering
CMEC - Pakistan

RE: Guide for Obtaining Cores and Interpreting Compressive Strength Result

Considering the poor quality of the concrete, it is amazing that the structure has lasted so long.  

RE: Guide for Obtaining Cores and Interpreting Compressive Strength Result

I believe it is important to determine the cause of the deterioration before considering rehabilitation or replacement. The deterioration looks significant.

If chlorides are suspected, what would be their source, i.e. chloride laden water or calcium chloride in the original concrete mix, deicing salts used to melt snow and ice in winter climates or other? The photographs show deterioration on the underside ... what is the condition of the top surface? If the top surface of the concrete is not deteriorated than it is a possibility that chlorides in the original concrete mix combined with moisture has led to this deterioration on the underside. Chloride profiles over various slab depths would provide indicators.

Has carbonation of the structural slab been considered? I have encountered this type of deterioration a couple of times where machinery/furnace fumes were not exhausted properly. Carbonation typically occurs over long periods of time.

I have worked on several projects using hydrodemolition equipment and it works really well under the right circumstances. I have only worked on the top surface of slabs with hydrodemolition equipment although I understand there is equipment that will do vertical and soffit surfaces. The equipment is specialized and expensive.

Given the low compressive strength results for the concrete and the possibility the concrete is contaminated with chlorides, the choice of saving the slab would likely be a short term solution, deterioration will continue or even advance at a more rapid rate in the concrete you leave. Gather as much information as possible before considering the rehabilitation, total replacement or constructing a new slab below the existing slab options. The testing mentioned above will cost very little compared to the construction costs being contemplated. Concrete mix design, waterproofing, etc.. all need to be considered.

RE: Guide for Obtaining Cores and Interpreting Compressive Strength Result

Is it located near a sea/ocean?  Saline groundwater? may not have an impact on above grade structure.  Rainwater may not be saline, either.

Dik

RE: Guide for Obtaining Cores and Interpreting Compressive Strength Result

Snow is not a problem in that region (SE Asia?). Mix of high heat, salty air (if by the ocean), humidity is likely the culprit. You may nedd to take a few samples from different areas to evaluate the extent of deterioration to make the call - demo, or repair.  

RE: Guide for Obtaining Cores and Interpreting Compressive Strength Result

I agree that this concrete is not capable of structural rehabilitation.

SkiisAndBikes brings up a good point about calcium chloride in the mix; however, if this is in a temperate climate, that's not likely...in any case, the source of the chloride needs to be determined in order to accommodate a repair scheme.

At this point, the beams would be suspect as well, since they are likely of similar concrete.

It looks like reconstruction is in order.

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