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Beam Breaks falling @ 28days
3

Beam Breaks falling @ 28days

Beam Breaks falling @ 28days

(OP)
Just broks some 28 day breaks and all of them across the board fell from 12-20% from the 14day break.  We are using good crushed stone from MO with wash sand Type I cement and 16% fly ash.  Nothing special about the mix, looking for 750@ 28 days.  All of the mix's had the strength at 14 days, but fell from grace after that.  Human error,lab?  Some feedback would be interesting.  

RE: Beam Breaks falling @ 28days

You might call the lab and ask to discuss this with them.  I would suggest that the same batch of concrete will not loose strength like this unless the test was run in error (applied pressure too fast, caps not put on right, etc.), or the cylinders in question were cured differently.

RE: Beam Breaks falling @ 28days

Doh - I see that these are beam breaks (now that I read the post carefully) and not cylinders....my mistake....but perhaps the concept I suggested is still valid - talk to the lab and find out whether the procedure can or cannot be ruled out as a cause.

RE: Beam Breaks falling @ 28days

Several reasons could explain the apparent loss in strength. But let's assume the specimens came from the same mix(with no additive that causes degradation) and were tested by the same compression machine (with no calibration in between) then, it could be related to how the specimens were cured. The ASTM method is C 78 and the strength result obtained is very sensitive to how the specimens are cured.

RE: Beam Breaks falling @ 28days

It doesn't make sense that the specimens would lose compressive strength over time. As long as Portland cement is exposed to moisture it will undergo hydration. The moisture in the air is often enough to bring cylinder samples to 75% f'c at 7 days, provided the mix was correct. Therefore, I don't think its a curing problem. I think there is some human error involved in the breaking process. Check with the lab and the technicians.

RE: Beam Breaks falling @ 28days

Actually the discussion was on flexural strength testing. ASTM fabrication and curing standard practice is C31 and testing method is C78.

In the case of flexural strength beams, the need for proper curing is paramount or results will be erratic.

Proper curing procedures are not always followed by techs; some wrongly assume they have to be cured just like compressive strength cylinders. 24 plus or minus 4 h prior to testing, the beams are required to be stored in water saturated with Ca(OH)2 at about 73 deg F. Furthermore, drying of surfaces must be prevented between removal from water storage and completion of test. Check ASTM C 31 Sec 9.2.3.2 and Note 7 which states "Relatively small amounts of surface drying of flexural specimens can induce tensile stresses in the extreme fibers that will markedly reduce the indicated flexural strength".

So, it is possible the specimens for 14 days were better cured than the 28 day specimens resulting in higher test values for the former.

RE: Beam Breaks falling @ 28days

(OP)
Thanks everyone, I am on the same page as you all.  I have seen the beams and the tank they were in, and half of the beam was in water and half was out of water.  Also the beams were stacked up before being tested, not taken from the tank to the machine.  We are addressing the problem, thanks to everyone, this forum is really a good resource for people like me.  

RE: Beam Breaks falling @ 28days

2
Carefully check the beams for very small shrinkage cracks and for stress raisers at the corners from stripping the beams from the molds.  

Other points are good also.

RE: Beam Breaks falling @ 28days

Were all the beams taken from the same transit mixer?  Or did the size of the beams force you to take from two or three separate mixers.  I ask this as I have seen this in normal cylinder testing, but we found that the specimens of each "sample" were not from the same batch (we were using 0.25m3 portable mixers).  Perhaps . . .  Curing, though, has a marked influence.  One point, though, is that as bad as the curing might have been at the lab, how was the curing in the field?????

RE: Beam Breaks falling @ 28days

(OP)
The beams were covered with plastic sheet with wet burlap on top, placed in the cure tank 24 hrs. Burlap did not touch fresh pcc.  The samples were part of the initial Mix Design, from the same batch. I ran 7 mix designs and all, but 1 fell from the 14 day break, the one that did not was a very high strength with 8sack mix for 1 day breaks to be used on spall work.

RE: Beam Breaks falling @ 28days

7515325...you imply that the specimens were not submerged, just covered with wet burlap, isolated by plastic sheeting.

A few questions...

1.  How long were they in the molds before stripping?
2.  What were the temperature and relative humidity under the sheeting?
3.  Were the edges rubbed with a stone prior to testing?
4.  Were they tested with tension on one of the side faces rather than the top or bottom?
5.  Did you drill cores from the respective failed flexural specimens to check the compressive strength and/or splitting tensile strength of the concrete?
6.  What type of molds did you use....plastic or steel?
7.  Did you use a beam breaker or an adapter for a compressive testing machine?
8.  What was your strain rate?
9.  Have you done any microscopy on the remaining pieces, particularly near the failure plane?


The concrete didn't lose strength, unless the initial curing temperature was significantly higher than the temperature at subsequent days.  A temperature and relative humidity change during the curing process can affect the strength results.  The appearance of strength loss is a testing anomaly that needs to be determined.  

RE: Beam Breaks falling @ 28days

The appearance of strength loss is a testing anomaly that needs to be determined---Ron

Great points Ron. I could not have stated it any better.

What 7515325 experienced is something that I personally experienced a long time ago which is why the influence of curing on flexural strength test results immediately came to mind.

In our situation, the lab manager had personally supervised early strength breaks..from the tank to the testing machine. On this particular day, the 28 day breaks were handled by an impatient tech who had never performed this test before by himself and was not authorized to do so. The 28 day breaks came out lower and everyone panicked.

Fortunately we had companion compressive strength cylinders and some splitting tensile strength cylinders (for research purposes) which had gained strength with age. Also previous 28 day flexural strength breaks from different batches of the same mix design were higher than earlier breaks. This led us to the conclusion that there was obviously a problem with testing techique.

Upon further investigation we determined the tech had set the machine at a significantly higher strain rate and had allowed the surface of the flexural strength specimens to dry out before they were tested. With proper testing procedures implemented, other sets of 28 day flexural strength breaks from different batches were higher than earlier breaks.

It's great that ACI is now offering a Concrete Strength Technician Certification Program http://www.concrete.org/certification/Cert_pgminfo.asp?pgm=Concrete+Strength+Testing+Technician

RE: Beam Breaks falling @ 28days

(OP)
1.  How long were they in the molds before stripping? 24 hr
2.  What were the temperature and relative humidity under the sheeting? Do not know
3.  Were the edges rubbed with a stone prior to testing? Yes
4.  Were they tested with tension on one of the side faces rather than the top or bottom? YES
5.  Did you drill cores from the respective failed flexural specimens to check the compressive strength and/or splitting tensile strength of the concrete? NO
6.  What type of molds did you use....plastic or steel?Plastic and Steel
7.  Did you use a beam breaker or an adapter for a compressive testing machine? Adapter
8.  What was your strain rate? Not sure
9.  Have you done any microscopy on the remaining pieces, particularly near the failure plane? NO
 To day I broke some 7 day breaks, that I took over the curing and the lab CQing. Went to the lab and made sure the bbeams went from the tank to the machine, prepped per astm.  Same mix, 1/2% less air than other failing beams. Same pcc temp and  out side temp as failing beams. Today beams were 118lbs higher on average some higher than that than the other beams.  Improper curing and lab prep is my conclusion.  I will let you all know about the 28 day breaks.

RE: Beam Breaks falling @ 28days

henri2  writes, “…the 28 day breaks were handled by an impatient tech who had never performed this test before by himself and was not authorized to do so. The 28 day breaks came out lower and everyone panicked.”

I'm going to have to give you a "WHAT THE?!" for the part about “an impatient tech who had never performed this test before”

ACI-318 - 5.6.1 — … Qualified laboratory technicians shall perform all required laboratory tests.

How convenient to blame the tech at this late stage.  Just a thought here, but had YOU properly trained and then properly supervised this tech (loose cannon that he is), maybe you wouldn’t be looking at erroneous test results.

RE: Beam Breaks falling @ 28days

boffintech,

This happened over two decades ago. At the time I was relatively new in the company and not in the position to dictate who received training. Besides I was learning the ropes myself and some of the techs were very helpful. BTW, I do agree that structured on the job training of techs is a good idea.

As for the "impatient" tech, I got to know him better after that. Found out he was a pretty smart guy who loved challenges.

RE: Beam Breaks falling @ 28days

to add to Ron's questions, what was the percentage of aggregate that was fractured in the fracture plane?  did they change aggregate?  (bank or river run aggregate will give lower flex strength)  

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