Durability and Compressive Strength, Low Tests 3

[Boiler106]

With respect to ACI 318-14, our compressive strength tests for a retaining wall fall marginally below the minimum f’c requirements for durability given in Table 19.3.2.1 for Exposure Class F2, but our analysis indicates the lower concrete strengths are acceptable for the applied forces. The difference in value is 200 psi, however, the arithmetic mean of the tests does not equal or exceed f’c.

Does the concrete need to meet the strength requirements of 26.12.3.1 Compliance Requirements and 26.12.4 Investigation of low strength-test results for durability even though its adequate for the applied forces? Or are there relaxed requirements for durability alone?

If not, are there other solutions such as applying xypex?

 

[KootK]

Perhaps discuss it with the owner but 200 psi is likely to be on the order of a 5% deficiency. I'd be inclined to call that good enough. I'd be fine with that even for strength purposes.

 

[Boiler106]

yes, we're definitely fine with it for strength purposes, no doubt. however small the difference is, by the letter of the code, it doesn't meet chapter 19 requirements.

we're thinking that the strength requirements are driven by the w/cm requirement, which has already been satisfied. maybe that's our way out.

 

[KootK]

To be clear, I meant to say that I also consider 5% to be good enough for durability purposes as well. That stuff is even less precise than the strength numbers. If 95% durability isn't good enough, then I would argue that the original design wasn't good enough in the first place. You're building a retaining wall after all, not a piano.

If you can't skate past this based on professional judgement, perhaps you can have a sample taken to try to ascertain whether or not stuff like target permeability numbers have been met etc. Something like xypex would surely help but, if this is a numbers thing, then you'd need a way to quantify whether or not its addition would put you over the top. Off of the top of my head, I can't say wheter or not xypex publishes such data.

 

[phamENG]

If you're going to accept it as is, KootK's rounding philosophy is probably more palatable finding a "way out."

Haven't read it, but this report from ACI looks like it may be of use for you.

Strength is important, but so is durability - durability establishes how long the concrete will have said strength. Skimp there, and your concrete structure could drop from a 50-75 service life to a 25-30 year. Yikes.

(EDIT: for clarification, I'm not saying a 5% decrease in strength will cut service life in half. Just illustrating that durability is important.)

 

[dik]

I think Koot's approach to the slightly low concrete strength is on the mark. There will be a minor effect on the durability, but likely negligible. As far as strength goes, you are likely not close to the shear capacity so the reduction is also negligible, based on the root of f'c. Unless you have a high percentage of reinforcing, flexural strength is not an issue with concrete strength. For small amounts of reinforcing, concrete strength has little effect on flexural strength. I wouldn't lose any sleep over it. Good report, pham... It's just a matter of 'making the client' happy.

-----*****-----
So strange to see the singularity approaching while the entire planet is rapidly turning into a hellscape. -John Coates

-Dik

 

[Boiler106]

yes, of course, thats not the issue. i hope i was clear that the wall is structurally adequate at the lower value. however, we're not big on buying retaining walls that don't meet building code requirements.

 

[dik]

I would suggest that it meets building code requirments. This is where Ron should step in. I seem to recall the ACI has a means of determining in situ strength. Take cores if required and test and see how it is. If the tests are OK, then the owner pays for the testing and if not, the contractor does... and take if from there... if it matters that much.

-----*****-----
So strange to see the singularity approaching while the entire planet is rapidly turning into a hellscape. -John Coates

-Dik

 

[phamENG]

we're not big on buying retaining walls that don't meet building code requirements.

Certainly a reasonable stance to take.

This is what I would do if a) I wanted to be conservative and ensure my back side was covered beyond a doubt and b) I have an intelligent client capable of making informed decisions (homeowner - no, State DOT - yes):

Inform the client of the deficiency and explain to them that it does not meet the stated requirements. The fact that they batched, delivered, and cured concrete that doesn't meet your specifications isn't your fault (is it?). It may be your problem now, but it isn't your fault....unless you accept it. When you inform them, give them your initial opinion and lay out the options. You can rip it out and start over (big legal issue), accept it as is (potential impact on service life), or somebody could pay you to do additional analysis backed up by additional testing to determine what the actual impact on durability might be to allow a more informed choice.

If you want to CYA and it's for a homeowner or similarly ignorant client (don't take this as being mean...it's just the truth...I'm ignorant of all sorts of things, it's why we have experts in important fields like ours)...just skip to the 'this is too low, you need to pay me to evaluate it further' and let them hold it back from the concrete sub if they so desire.

 

[Enable]

Strength is but a poor to moderate correlate of the actual characteristics that impact durability (such as permeability). As long as it's the correct exposure class you will be fine.

If you need something more authoritative point the inquiring minds to the relevant section of your code that allows field cores to be performed, and if in-situ average strength is > 85% of specified you are good to go as if the cylinders never failed in the first place. If people still balk at that have the lab also analyze it for the relevant durability characteristics directly such as air content. In Canada our governing code would be CSA A23.1 but I don't know where the similar out would be in ACI (excerpt from RMCAO industry best practice guideline below)

 

[dik]

Thanks, enable...

-----*****-----
So strange to see the singularity approaching while the entire planet is rapidly turning into a hellscape. -John Coates

-Dik

 

[dik]

I'm not so sure...

-----*****-----
So strange to see the singularity approaching while the entire planet is rapidly turning into a hellscape. -John Coates

-Dik

 

[phamENG]

I'm not so sure...

Do you regularly reimburse clients for construction costs when a contractor fails to meet your specs? I know I don't...

 

[KootK]

however, we're not big on buying retaining walls that don't meet building code requirements.

I also question the reasonableness of such a hardline stance from this perspective:

1) Every project in the real world requires some compromise. No owner really "gets what they paid for". A practical structural engineer has to choose his or her battles. And, in my mind, 5% on a loosely correlated durability parameter isn't the hill to die on. At the least, not unless you're being backed by a hardass DOT etc who will do the bad copping for you.

2) The building code requirements are themselves rooted in considerable engineering judgement. It's not has though they constitute a hard, binary line between when something is satisfactory and when it's crap. As such, applying some reasonable engineering judgement to a judgment informed code requirement is surely not too far off reservation.

3) Most codes and standards have an "out" clause tucked away someplace allowing engineers to "use alternate rational methods" etc. Basically, you can do whatever you want if there's a rational basis for it and you're qualified to make the call. In my opinion, pointing at a statistically negligible deficiency and saying that it's statistically negligible falls into this category.

 

[dik]

Koot answered the question, better than I could have... see my earlier comment that the construction met code. Too often an owner will want to extract an extra pound of flesh for 'unreasonable' reasons.

-----*****-----
So strange to see the singularity approaching while the entire planet is rapidly turning into a hellscape. -John Coates

-Dik

 

[phamENG]

I guess we have different interpretations of what the OP meant. If it meets the requirements, then by all means, approve it. But if it doesn't meet the durability requirements (the real ones, as I spoke of before and Enable elaborated on, not just a rough association to f'c), then I would not approve it.

 

[KootK]

In my experience, these things usually go like this:

KootK: dear client, this thing is 5% less good than I wanted it to be.

Client: dear contractor, please do XYZ to make this thing as good as KootK wanted it to be.

Contrator: dear client, $1B dollars! And we don't think this needs fixing anyhow. Some engineer we know said so.

Client: dear KootK, with this 5% shortfall, how much less than 50 yrs will this structure last?

KootK: dear Client, I have no bloody idea. Not much less.

Client: dear Everybody, let's jut move on.

Once in a blue moon, I'll see an owner ask for some $$$ in exchange for the added durability risk. Most often, the owner will just accept some contractor good will in exchange for letting it go. It's tough to get through any sizeable project with out some screwup being at least partially your fault. Having a little good will in the bank is often handy.

 

[Settingsun]

My concern would be whether additional water was added on site. Water:cement ratio has a much larger impact on durability than on strength. 5% may not be just 5%.

Remember that they've missed the target strength by more than they've missed f'c.

 

[JoelTXCive]

Boiler - What was the required compressive strength that the contractor is 200psi short? Do you have any extra cylinders where you can do a 56 day break?

That's such a small low-break that I think the concrete will make it. If it was 4000psi concrete and you came in at 3800 on day 28; then my spreadsheet says you will make it on day 40.

 

[JoelTXCive]

I have had more low-breaks in the past 6 months than I have had in the past 6 years. These low breaks are from different contractors and different suppliers.

Common excuses and items that come up are.......

For a 5% shortage, the concrete supplier is probably going to question the testing equipment's accuracy and ask how long it has been since the equipment was calibrated. Then they are going to ask if an ACI certified technician took the cylinders and was operating the testing rig.

If that doesn't get them off the hook; they will start talking about the curing method of the cylinders.

If that doesn't work, they will offer to core a fresh sample or do some field testing of the compressive strength.