Low Break - High Compression 2

[SiPaul]

An interesting case...

The contractor elects to construct everything from 4500 psi concrete. this is higher than most of required compressive strengths.

The cylinders break below 4500 (say 3500 for argument although most were higher). The design only called for 3500 so there shouldn't be a problem with strength. In fact we could probably live with 3000 with the ACI 5.6.4 500 psi provision.

Some have argued that there is a problem because the concrete supplier said he was giving us 4500 psi. This is true but if the concrete is good 3500 concrete I wouldn't be concerned. Note that the contractor is paying for the higher strength concrete at his expense.

Any thoughts..

 

[Ron]

The lower than anticipated strength from the 4500 psi mix indicates a mix problem. It could further indicate a durability problem.

If your application does not require high durability and the structural engineer of record accepts the concrete strength, then it probably is not an issue.

If I were doing the reporting as the testing laboratory, I would still report it as a "low break".

 

[SiPaul]

Thanks Ron!
What would you describe as a durability problem?
That is, if the mix was in fact low but acceptable as a 3500 psi concrete the concrete should have the same durability as a standard 3500 psi mix.


It could be that the w/c ratio was off in the mix which produced the low break but the structural integrity should be OK.

 

[Ron]

SiPaul...the w/c ratio is exactly my point. When it is high enough to lower the strength of the designed mix, it produces a fair amount of latent water at the surface (bleed water). This makes the near-surface concrete water/cement ratio very high, leading to crazing, cracking, scaling, and other durability problems.

 

[SiPaul]

I see your point!

I suppose there is no way to determine from the compression test alone if the w/c cement ratio is off(i.e. to produce the conditions you described) or if it is simply a lower strength concrete.

I will probably require an extended maintenance period on this one(maybe 3 years). That should provide us with engough time to monitor the situation.

 

[Ron]

SiPaul...you can do a petrographic examination of a core and determine most of those parameters related to durability. It isn't cheap though....about $1000 per core.

 

[SiPaul]

Is the petrographic examination only done on core samples?

It occured to me that if the w/c was off and there wasn't enough water to hydrate the cement this may cause the problems. But there would be a low slump. The slumps seemed to be right on.
After reviewing the lab results I noticed that there were low breaks for various suppliers. This to me raises questions about the testing.

 

[Ron]

SiPaul...yes, petrography is usually done on cores as you are looking for "in-place" conditions. While it could be done on a reserve site-cast cylinder specimen, the results could be different as the construction technique imparts some differences to the concrete.

Good point on the testing. If the results are consistently low across several suppliers, but from the same testing lab, I would do a quick audit of the laboratory's curing and testing facility. Lower than standard curing temperature (their moisture room or curing tank temperatures below the ASTM standard)will cause lower apparent strength.

Most laboratories are willing to let others audit their practices. If they are not, switch labs.

 

[SiPaul]

Very helpful info Ron!

I have never audited a laboratory but may start here. They have assured us that everyting is OK but I am a bit wary. I noticed that some of the breaks were vayring with the weather conditions.

I was told they put the samples in a lime bath. I didn't think about the temperature.

I know that the Ready Mix Association in their CIP series indicates that most times a low break is attributable to the testing. Could be in this case.

Further, I have not heard about problems with the curing at the site or any other indication of faulty concrete, i.e., visible signs such as cracking, scaling, etc.

The lab noted that some of the cylinders had aggregate separating from the cement. This could be a low w/c ratio and not all of the cement hydrated. They also indicated the type of breaks columar,etc? Do you know where one could find more information about the significance of the break pattern?

Again, Great information!

 

[Ron]

SiPaul...thanks. A few things to look for when you audit the lab...

They are using "horse trough" curing tanks (lime saturated water bath), so temperature control is a bit trickier than with a fog room. In the tanks, they need the capability to heat and cool. This can be done with recirculation of water and direct immersion heating, along with a stable laboratory temperature.

Ready mix supplier ALWAYS blame the testing lab! Sometimes they're right, but reputable testing laboratories spend a lot of time, effort, and money to be right. Their reputation depends on it, so most try to do it right. Any of them can have a problem though, so it's good to check.

If the lab noted that some of the cylinders had aggregate separating from the cement paste, you need to find out whether they meant that in the context of "honeycomb" (underconsolidation) or if they meant that the aggregate was contaminated and the past didn't stick to it. Either can cause low breaks but they have different causes (underconsolidation could be either a testing technique problem or it could be a mix segregation problem)(contaminated aggregate is a supplier's problem).

Let us know how it goes.

Ron

 

[SiPaul]

Maybe I am looking for too much information but do you know where to look for information on interpretting the types of fracture (i.e., columnar, shear, cone, etc)?

This may be significant. I reviewed the ASTM for testing and it appears that field curing is permissible. I take this to mean that they can simply leave the cylinders at the site. I agree that they may not have had adequate temp. control if they transported to a lab and cured in the lime. I would expect the temperature to be high and get a low 28 day break. Actually the 7 days were not as low as I expected for a low 28 day. The ASTM mentions that a high temperature can lead to a high 7 day and a low 28 day break.

They noted columnar breaks but these had high strengths.

I'll keep you posted. (so to speak)

 

[Ron]

The type of break is related to the stress distribution in the concrete specimen. It is a function of the loading rate, the aggregate distribution and size, the aggregate-paste bond, and (significantly) the fabrication technique of the technician making the specimen.

Columnar breaks are often caused by improper capping of the specimen resulting in stress concentration at the top of the specimen during loading. Check this when you audit the lab.

The ideal break pattern for consistency is a cone-shear. This indicates a more triaxial load distribution in the specimen.

Field curing is permissible under ASTM C31; however, it should be used with caution. As noted, high initial field temperatures will increase the initial strength and retard the ultimate strength.

Remember also that the concrete specimens are intended to check the MIX DESIGN, not the in-place concrete. It is sometimes a stretch to compare the in-place concrete strength to the standard-made, lab-cured specimens. The intent is to check to see if you got what you bought, not if you achieved strength in place. While field curing under proper conditions might get you closer to this value and can be better for form release purposes, it does not truly reflect the in-place condition even then as there are quite a few variables that do not compare between cylinder specimens and in-place masses of concrete (for example; the surface areas are different, the mass heat gain is different, the curing conditions are different, and the consolidation procedures are different).