Beam Breaks falling @ 28days

[drfads]

I have the same problem,the beams break before the 28 days period without the required flex strenght.I have checked the plant, the lab techs.
But last time I saw that the mix was loaded into dump trucks,forget mix trucks.....

I thought to myself maybe that could be the reason..Please advise.

 

[Ron]

Pavement concrete is often loaded into dump trucks...not transit mixers. That only means that the concrete is mixed in a central batch plant prior to transportation. That's probably not the reason for your low beam breaks.

There are many reasons that beams can break below the flexural strength value (modulus of rupture). You say you've checked the technicians, but since you were not aware that concrete is placed as you described, are you aware of the many variables that you need to check on the technicians, the molds, the handling, the curing, the test preparation, and the testing in order properly evaluate whether they are doing a proper job?

Take a couple of the broken flexural specimens. Core the remaining ends and do both compressive strength and splitting tensile strength on the specimens. If the results are low, then look toward a concrete issue. If the results are OK, then look toward a sampling and testing issue.

 

[drfads]

Reply to Ron (Structural)

Again,I have ckhecked to place where the techs store the beams. Someone suggested that the calibration on the machines might be the reason. But,The lab is clean...certified and has been operating for yonks... the lab just passed its inspection too. so nothing regards to the lab, people, the mix or anything of that nature. One of my senior engineer suggests that the job site is too far from the plant, hence the water ratio probably declines and hence the situation.Please advise further.

 

[cvg]

you say nothing wrong with the mix - how do you know? What if extra water was added in the transit mixer? This is sometimes done when the job is too far from the plant. What if the batch plant scales were not working properly (on the day that the concrete was produced)?

 

[henri2]

Check this thread

http://www.eng-tips.com/viewthread.cfm?qid=139537

 

[drfads]

Reply to cvg (Civil/Environme)

My point is that there exists no transit mixer at all,these guys use dump trucks to transfer the concrete mix from the batch plant to the site.As for my information we have our inspectors at the plant to make sure the process is done according to the specs.

 

[drfads]

Reply to henri2 (Materials)

Thats then link which made me join these thing. I initially checked it out and did not get much from there, anyways thanks again

 

[cvg]

original mix design could be at fault - if the tests were fouled up. But at this point, it hardly matters. you have cured concrete on the job that must be either accepted or rejected. core the pavement and test it to see if it meets your requirements.

 

[Ron]

Just because the lab is certified and it's clean, doesn't mean they did the test right, handled, or prepared the specimens right.

Beams are very sensitive to handling and preparation. For instance, if the beams were allowed to dry out just prior to testing, the microcracking that can occur can also cause early cracks to develop at the corners, thus lowering the modulus of rupture.

If the technicians didn't "rub" the corners of the beam prior to testing to remove the rough edges from demolding, you can get a stress raiser and lower the apparent strength.

If the specimen wasn't tested on its side, the roughness of the top can affect the results.

If the strain rate during testing was too fast, the results will be off.

If the specimens were not cured at the right temperature, the results will be off.

Testing beams is not like testing cylinders. It requires a greater level of care and results can be all over the place.

 

[dik]

Have you asked the tekkie for a possible explanation? Can you test one of the beams in compression as a bit of a marker. Do you have a copy of the mix design that you can post?

Dik

 

[henri2]

Was there a mix design submittal? Did it indicate compliance with flexural strength, curing and other relevant spec requirements? Was the mix design based on the materials used for the mixture that was actually used?

 

[drfads]

Yes,there a mix design submittal and it did indicate compliance with flexural strength, curing and other relevant spec requirements.
And yes, the mix design was based on the materials used for the mixture that was actually used for.
So this is real frustrating mystery which has no ends or clues, Nevertheless, efforts are still persistent.

 

[Ron]

I reiterate my previous point...test the leftover beam pieces in compression and see how the comparison runs. If the compressive strength is on par with the expectations of your mix design and water-cement ratio, then you probably have a flexural specimen testing problem, not a concrete problem.