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
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
RE: Guide for Obtaining Cores and Interpreting Compressive Strength Result
RE: Guide for Obtaining Cores and Interpreting Compressive Strength Result
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
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
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
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
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
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
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
RE: Guide for Obtaining Cores and Interpreting Compressive Strength Result
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
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
RE: Guide for Obtaining Cores and Interpreting Compressive Strength Result
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
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
Dik
RE: Guide for Obtaining Cores and Interpreting Compressive Strength Result
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
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
RE: Guide for Obtaining Cores and Interpreting Compressive Strength Result
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
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
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
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
Dik
RE: Guide for Obtaining Cores and Interpreting Compressive Strength Result
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 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
Thanks, bridgebuster, for the very good link. More reading.
Dik
RE: Guide for Obtaining Cores and Interpreting Compressive Strength Result
From Wikipedia, the free encyclopedia
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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
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
RE: Guide for Obtaining Cores and Interpreting Compressive Strength Result
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
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
Maybe someone more local can help.
Dik
RE: Guide for Obtaining Cores and Interpreting Compressive Strength Result
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
RE: Guide for Obtaining Cores and Interpreting Compressive Strength Result
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
RE: Guide for Obtaining Cores and Interpreting Compressive Strength Result
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
RE: Guide for Obtaining Cores and Interpreting Compressive Strength Result
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
Dik
RE: Guide for Obtaining Cores and Interpreting Compressive Strength Result
RE: Guide for Obtaining Cores and Interpreting Compressive Strength Result
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.