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
RE: Beam Breaks falling @ 28days
RE: Beam Breaks falling @ 28days
RE: Beam Breaks falling @ 28days
RE: Beam Breaks falling @ 28days
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
RE: Beam Breaks falling @ 28days
Other points are good also.
RE: Beam Breaks falling @ 28days
RE: Beam Breaks falling @ 28days
RE: Beam Breaks falling @ 28days
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
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:
RE: Beam Breaks falling @ 28days
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
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
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