Questionable Concrete Strength 2

[msquared48]

Has anyone ever heard or seen the strength of a concrete pour going from 1450 psi at 28 days to over 4000 psi at one year? Seems like one value or the other is phony to me...

Mike McCann, PE, SE (WA)

 

[dcarr82775]

Nope, that unpossible. The only time I have seen things like that is when the initial set of cylinders was damaged in some way (freezing) and later cores broke just fine

 

[JAE]

the initial set of cylinders was damaged in some way

My thoughts too.

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[Ron]

What is the mix design? Any chance there is ground, granulated blast furnace slag in the cement and if so how much. The numbers are unusual and would agree that the likelihood of damaged specimens is there. What was the 7-day break?

 

[msquared48]

That is information I will find out tomorrow when I meet with the owner. From what he said, there is a lot stuff (as in four letter word) in the mix here, and just not in the concrete. This is going to get interesting.

Mike McCann, PE, SE (WA)

 

[darthsoilsguy2]

if a real 1450 @ 28, there would be notable damage when the forms were stripped. whenever i've seen sub-1500 concrete breaks, the only purpose the set was finished was for the contractor to back charge the plant for the work they already ripped out, and nobody needed a testing firm to let them know they had crap.

i hope it is something really dumb.... like 6" cylinder math used on 4" cylinders

 

[msquared48]

Went out to the site and there a lot of issues to deal with. I will refrain front quantifying much until I get the drawings and legal documentation to review.

However, have you ever heard of all thread for holddowns being able to be backed out of a foundation wall with a socket wrench? To be able to do that to means little to no bond to the all thread to me. This is torsional shear, but is also related to tensile shear from seismic or wind. So... greatly reduced capacity for any holddowns that employed threaded rods.

Mike McCann, PE, SE (WA)

 

[BigH]

My two bits. Had a low strength concrete mix (18 MPa) required at 90 days. Used crushed sand from quarry - peridotite and contained significant fines content - 15% or so. As we were more critical of density than strength, we had no problems with the high fines content. Cementitious mix was 50% Type 1 Portland Cement and 50% fly ash. Our 7 day strength average was 50% of the 28 day strength (same batch). The 90 day strength was 200% of the 28 day strength. I have some 365 day test values and will have to hunt them up (will post later). The remarkable thing was that the peridotite dust was pozzolanic - so, in fact, we had additional "pozzolan" other than the fly ash (note - when we washed out the fines and used clean peridotite sand - the 90 day strength was 1.5x the 28 day test value). The point is, perhaps the fines were pozzolanic and you had a mix with a lot of fly ash?? This was a large project and we had a plethora of test results.

 

[shazi1]

More strength gain often can be seen with mix design with GGBS or fly ash.. more percentage of ggbs/ fly ash results in prolong hydration and reduction of hydrooxide resulting in higher strength in longer duration. Sampling deficiencies or damage for cube tested in earlier duration may be other possible reason for such discrepancies.

 

[msquared48]

Apparently the contractor added 55 gallons to a 7.5 yard mix with a spec'd w/C ratio of .45. No air entraining although it was specified in the design.the calculated w/C ratio is 0.7. Picture of concrete flowing down the chute about 1" deep - like water.

Mike McCann, PE, SE (WA)

 

[Ron]

Yup, Mike....that'll do it!

 

[concretemasonry]

This morning, I had breakfast with a fellow engineer that worked for 30 years in major local testing lab (4 or 5 small labs) and a central lab for doing many different very large scale testing in addition to routine testing of soils, concrete and hot-box thermal testing.

He happened to mention that they saw variations in fly ash cylinder tests. They attributed some of variations for early testing may have come from the slow rate of curing for fly ash concrete and the handling of "young" samples that were very fragile and easy to damage with minor errors, while cores taken from the same projects yielded much more reliable (less "scatter") and more predictable.

This anecdotal comment was based on using a common Type F (not C) fly ash and there was little GGBS in their 3 state area.

Dick

Engineer and international traveler interested in construction techniques, problems and proper design.

 

[msquared48]

Also found out that the contractor poured a 6" X 4' high stem wall for a 30 X 40 foot garage from only one corner, and the concrete self-leveled all around. I think this one is only going to get better. I think I am going mining in that corner and on one of the other sides.

Mike McCann, PE, SE (WA)

 

[msquared48]

Question:

Other than concrete coring which is destructive, is there a non-destructive way to specifically test for aggregate distribution in a pour?

Mike McCann, PE, SE (WA)

 

[shazi1]

you can do ultrasonic pulse velocity test

 

[Ron]

Pulse velocity will not check for segregation. You can use it to find voids if your grid it tight enough; however, with the self-leveling comment....I doubt they have consolidation issues, just potential segregation.

The only NDT method I know of for segregation would be radiography.....expensive, time consuming, requires shut down of other work around it. Check with Olsen Engineering in Colorado....they have a few whizbang NDT devices that might pick up segregation.

 

[msquared48]

Thanks Ron. I will run this past the owner.

Mike McCann, PE, SE (WA)

 

[msquared48]

Well, I have engaged a friend of mine too for this one who is a former concrete contractor and mixer truck driver. This should be interesting...

Mike McCann, PE, SE (WA)

 

[shazi1]

Dear Mr. Ron, thanks for correcting me.
Just want to know, as UPV is to check uniformity of concrete cant it interpret segregation of concrete or radiography is only option?

As per Qatar Specs.
For upv
The use of data from nondestructive testing devices, such
as impact hammers or probes (ASTM C 805, ASTM C 803), ultrasonic equipment
(ASTM C 597), and pull-out devices (ASTM C 900) may be useful in determining the
uniformity and quality of the in-place shotcrete. These tests, however, may not provide
reliable values for compressive strength.

For radigraphy
Gamma rays and high energy X-rays, which illustrate by radiographs the concrete defects:
The testing shall be carried out in accordance with the requirements of BS 1881-205 or
equivalent.

 

[Ron]

shazi1....the small difference in pulse velocity between segregated and non-segregated aggregate sections in concrete would not allow determination with sufficient accuracy to make it worthwhile. To give you an example, let's consider that the velocity through good, consolidated concrete with normal aggregate distribution is 12000 feet per second. If we check an area that is 1 foot away in two directions and we get 12500 fps in one and 11500 fps in the other, which one would be segregated? We don't know because we don't have a correlation with cores taken from the exact locations and a visual confirmation of segregation.

Conversely, if we are looking at voids or underconsolidation in the concrete, we might find that in one location we get 12000 fps and in another location we get 9000 fps. From that we can clearly conclude that there is some reason for such a large reduction in velocity, most likely related to the sound path having to bounce around the voids, thus creating a longer sound path and a lower observed velocity.

Pulse velocity and other ultrasonic methods are generally considered secondary methods of testing. They do not yield direct results, but must be correlated to direct results such as the compressive strength.

Radiography is a direct, visual method whereby the aggregate can actually be seen and its distribution identified.