Question about concrete strength 4

[comav]

Im currently working on a post tentioned bridge project. It seems that most of our cylinders thus far have failed. We are using AAA concrete which should be breaking at a minimum of 5000 lb to suit the DOT but it seems that most are breaking at 4200 to 4700 and some as low as 3500 lbs. At this point even the cores which were tested broke less than 4500 lbs. My question is, What could cause such a decline in strength ? I wish i had the mix design on hand to disclose but i dont. I was just looking for some basic conclusions ... Thanks in advance ... Jack.

 

[jhardy1]

Can you tell us where the bridge is? I would like to keep well away from it!

Seriously, there are all sorts of reasons why concrete doesn't come up to strength. I assume the cylinders have been made and stored in accordance with standard requirements etc, and are of the appropriate age.

If so, you really need access to the mix design and batch certificates, so that you can check all materials are correct (appropriate source of aggregates, appropriate proportions of fine and coarse aggregate, adequate cement content, water/cement ratio is OK, use of admixtures, etc, etc).

Also, what temperature were the cylinders stored at? Concrete curing can be much slower at lower temperatures.

If you can't get satisfactory evidence that everything is OK with the concrete, you would need to seriously look at condemning the concrete, and definitely talk to the designer, because this is a significant shortfall of strength, and could give rise to serious structural issues in a post-tensioned structure, especially at the anchorages.

 

[JAE]

You say "thus far" - at what age were the cylinders tested?

 

[comav]

All the cylinders that have been tested so far were done so at 28 days,I say "thus far" because all of the cylinders havent been tested yet. Due to the pour sequence there are several days and in some instances weeks between pours.
It seems that the cylinder handling was done correctly and they were stored accordingly.Also,the batch sheet shows that it was mixed according to design.
What i dont understand is this, How can there be such an inconsistancy between the concrete one day and the concrete the next ? Everyone seems to think it may be the materials used but if that were the case ALL of the cylinders would fail but as it stands now one may pass and then one may fail...

At this point it seems that it will be coming down, We are just waiting for the higher ups to give the word.

 

[JedClampett]

IHOP, these kind of problems can be usually traced to one simple reason. Possibly the cement got damp in transit. We've had cases where the ratios of fly ash and cement were reversed. Once you tell the contractor he's going to have to rip things out you can bet he's going to get to the bottom of the problem so he can figure who he's going to backcharge.
I don't think the variations are that unusual. They're more noticable when you're looking at low results.

 

[tincan]

Most plants design their mixes on a yield/strength/% basis.
a yield of 27 cf/cy at the low slump, design strength at 28 days with the high slump, and with everything passing spec at the first standard deviation from the bell shaped curve. Sometimes you get the bad batches on the wrong end of the curve. However for you to have this many falling below spec at one time is statistically improbable. I would look first to the batch plant and check the mix proportions/weights. Sometimes the scales get off and you get a bad mix. Second, if this is a transit mix, check for water added after the trucks left the plant. Also check delivery times. A lot of things can go wrong.

Best

 

[bylar]

We had a similar problem in Illinois many years ago. The mix was speified by the Architect incluing where the stone and sand was to come from. After exhausing many other avenues we had a Professor from Chicago do a complete study at a hugh expense and found that the rock could not withstand the required PSI for the concrete.

 

[hydrodude]

Don't know where you are at but here in South Texas you almost have to carry a baseball bat to keep the crews from talking the driver into adding water to the mix as soon as the truck shows up or doing on there own when no one is looking.

 

[Ron]

There are many reasons for low strength and for slow strength gain. Hopefully you have the latter. You mentioned that the cores were around 4500 psi for a 5000 psi design mix. Assuming consistency of this result, that would be acceptable according to ACI 318 (achieving 85 percent of design strength in cores).

One common reason for slow strength gain is the used of ground granulated blast furnace slag cement as part of the cementitious material. At percentages greater than about 30 percent of the cement, retarded strength gain will likely result. Similarly, fly ash added to the mix will retard the strength gain if used to substitute for part of the cement. Both are allowable, but you have to know the repercussions.

The first move is to obtain the mix designs and determine if either GBFSC or fly ash was used. The look at the field sampling reports and see if water was added. If there were no limits placed on the water cement ratio, ultimate strength can be achieved to the detriment of slow strength gain and reduced durability.

Consistency of the delivered mix should be monitored by a competent testing laboratory. Each truck should be checked for slump and air content, and appropriate samples taken at reasonable intervals for compressive or flexural strength testing. Variations in mix consistency can be spotted immediately, with resulting correction also done at that time. Inconsistency in the delivered mix is usually a quality control problem of the supplier...often a result of improper moisture control of the aggregates or improper batching procedures.

 

[kitko]

I would think that a slow steady curve in strength gain would be desirable for this application to prevent premature cracking. Have any tests been conducted beyond 28 days?

 

[MichealA]

As a matter of course, from experience I would also have the laboratory procedures and testing machines correlated with another firm (not necessarily with this job's important cylinders--test 7 day cylinders on "other" concrete). In high strength concrete, have previously seen incorrect cylinder capping material used on 50MPa concrete, which caused the capping material to fail prematurely and adversely affect results. The capping compound is available in different grades. Also ensure the lab is not "recycling" the compound back into the pot--any small pieces of concrete will negatively affect the results of the next compression test that uses the recycled product. Good luck.

 

[dirtdoctor]

Anybody been reading the construction newsletters out lately?? If so, you have to notice that we have a very high demand for cement now - it's gotten so bad in some areas of the SE U.S., that cement suppliers have been rationing their product. Structural engineers can vouch for steel shortages (and much higher prices) and can sympathize with the concrete guys - blame it all on the very active Chinese construction market and a weak US dollar.

The first thing to happen in response to shortages is the concrete suppliers skimp on the cement, or the cement producers begin pulling product from the clinker too soon and send out 'hot' loads. I'm not accusing your supplier of doing this now, but it has happened a lot in past periods of high demand. But, the first thing I would do is check out the cement used on the project - see if it has varied in fineness and/or set time, particularly over the past 4-6 months - your state DOT should be randomly testing cement samples. Another quick check can be done at the plant's aggregate piles - check to see if stone is dirty. Dirty stone has a much greater effect on high strength mixes than more frequently specified mixes.

In the testing business, if you know your equipment and procedures are in compliance, it's a good market for concrete testing right now. We all can expect to see concrete problems as long as cement shortages exist. It's just the nature of the industry.

 

[BigH]

On the low breaks - did you ever take photos and take a really hard look at the cylinders? While not likely for so many low values, casting of cylinders is also important and if not done correctly . . . As for the cores, usually the standard (Canadian Standards Association, for example) say that the average of three cores is to be 85% of the specified strength with no single core less than 75%.

 

[discopants]

as a concrete supplier I can honestly say that every cube failure I've ever been involved in has always been traced back to the site and poor testing and especially curing. The cubes are usually the responsibility of the lowest ranked man on site who hasn't been trained. They traditionally get left to bake in the sun before being sent to the cheapest possible testing lab and when they fail everyone looks for very complicated reasons why. Usually the answer lies much closer to home.

Hope that is the reason, if your supplier is any good he'll open his doors and have nothing to hide.

 

[concreteguru]

In my experience, with numerous DOT's, they usually have a list of approved suppliers which would include cement, admixtures, aggregates and concrete. The materials are tested well in advance and testing is usually done by AASHTO approved labs. Minimum Cement contents are generally higher than what would be needed to produce the required design strength. So if the mix designs are all predetermined to have the potential to meet or exceed the specifications, what has changed? Delivery tickets will give you some clue as to what was put into the truck. Having a batching plant inspector at the plant to check batching and perform slump and air tests will also give you better control. Look at the methods used to sample, fabricate and initial cure cylinders/cubes on-site. If all this is as it should be, try casting and sending comparative samples to multiple laboratories to see if the variation is normal. If that checks out, go back to raw materials and see if they are as they were originally submitted. Did the cement source change? Require cement mill reports to be provided as they become available. Check for blaine (fineness) and for conformance to ASTM. A change in cement source can provide wide strength variations especially if supplementary cementitious materials are being used. With the cement shortages we have recently experienced, many readymix producers feel fortunate to have a source of Portland period.

Good Luck with your quest. Hope this helps, Fred

Fred J. Croen, RSM
Engius, LLC
Boston, MA
www: engius.com

 

[techmaximus]

As a concrete testing lab I can honestly say that every cube failure I've ever been involved in has always been traced back to the concrete supplier and poor batching and poor material handling. The batching and material handling duties are usually the responsibility of the lowest ranked men at the plant who haven't been properly trained. These, usually unsupervised, guys traditionally mix loads in dilapidated trucks that have not been properly washed out from the previous load, and have worn out or caked up blades that are incapable of properly mixing the load. These guys rarely do the required moisture content checks of the aggregates prior to batching, and often confuse flyash with cement. Usually the answer lies at the batch plant.

The National Ready Mixed Concrete Association (301-587-1400) puts out a Quality Control Manual. Section 3, Certification of Ready Mixed Concrete Production Facilities, has an excellent check list for doing batch plant inspections. I’d suggest getting a copy of this document and performing a complete point-by-point batch plant inspection at your earliest opportunity. Additionally, inspect each truck that the supplier wants to use and only approve the latest model trucks with the cleanest drums. I have this doc in pdf format but I don't know how I could get it to you.

Techmaximus

 

[concreteguru]

In Contrast with techmaximus, I have seen plenty of low strengths that were attributed to poor sampling technique or poor initial curing or early cylinder handling. One can avoid the marginal quality readymixed concrete suppliers by specifying that the concrete supplier must ship from a NRMCA Certified Production facility or they must provide a statistical analysis of recent test data representative of a like quality of concrete, This certified analysis should be signed and stamped by a PE. Any concrete supplier that cannot comply should not be considered for use on the project. The same level of care should be adopted when selecting a testing company. Make sure the people they will use are competent and well trained, hold an ACI Field Technnician Level 1 as a minimum and that they have the tools to do the job properly. There are many reputable concrete suppliers and testing companies out there. Don't be confused with low bids from marginal players that will in the end, cost you more time and money.



Fred J. Croen, RSM
Engius, LLC
Boston, MA
www: engius.com

 

[7515325]

I just left a lab where the beams in the cure tank were only half full of water with no lime. The beams they made yesterday were in the back waiting to be put in the tanks tomorrow! I proceeded to strip the beams help log and place in the tank, add proper lime and fill vats with water. Not all labs are the same....oh by the way did I tell you some of those beams were mine and all of them made 750@14 days and then all failed at 28days. They said they did not have time to log them in......................