High Slump for SOG

[fsuenginerd]

Hello all,

I've got a project with a specified slump of 3"-5" for the SOG, but the field measured slump was over 6". I think this is a case where it was easier for them to ask for forgiveness than permission because I wasn't made aware of this until after they poured the slab.

At any rate, the compressive strength is fine so my first inclination was to let it slide, however I'm a little concerned about durability. The project is in Florida so freeze/thaw cycles aren't a major factor, but the excess water could lead to higher shrinkage and increased cracking. The slab will be used for parking and remain visible, so substantial cracking would be noticed.

What do you all think? Is the extra inch of slump enough to worry about, or would I be safe to let it go with a stern warning to get it right next time?

If it makes a difference, I suspect that the higher slump is due to a higher w/c ratio rather than a larger water reducing admixture volume because the compressive strength is also lower than they're getting from the other batches.

Thanks for the advice,
Nick

 

[jayrod12]

Added water is not ok in my eyes. However, I'm generally of the mindset that if strength (and all other requirements like air) meet what I need then I'm ok.

As you pointed out, there is the possibility for increased shrinkage cracking due to the higher w/c ratio. If increased shrinkage cracking is noted, then it would be the contractor to foot the bill since they likely added the water on site. Any reputable redi-mix plant would provide exactly what was in the spec but once the concrete gets to site it's out of their hands.

 

[DaveAtkins]

Could you recommend to the owner that he make a "deal" with the contractor that for the first year, if there is unacceptable cracking, the contractor must repair the cracks free of charge?

DaveAtkins

 

[Guastavino]

I'd note it and advise the owner (assuming I'm working for them) that there could be negative repercussions down the road such as cracking, etc. But the repercussion of making the contractor tear things out are much higher because they will find a way to make the owner pay for it through change orders, and ultimately, I think that would be kind of crazy. It's a SOG, so it's not a life safety issue, it's just a serviceability issue.

If I was working for the contractor, I would advise them in writing of the possible outcomes so that when the owner asks later, you have something to show them that you warned the contractor. As engineers, I don't think we are required to be nit-picky about serviceability issues for a SOG. What I mean by that is that if you're working for the contractor, I wouldn't feel obligated to go around them and tell the owner. If it was life safety issue and the contractor wasn't listening, to the owner I would go.

 

[CANPRO]

The measured slump doesn't always tell the whole story either. A truck could have been stuck in traffic or delayed for whatever reason for a couple hours - driver keeps putting the water to the mix and gets to site with a 3"-4" slump. Another truck could have went directly to site and added some water to help the finishers out bringing the slump to 6". The truck in the first scenario could potentially have added more water to the mix and resulted in a lower slump. Common belief among the truck drivers is that the water evaporates out of the mix on the way to site, which justifies them keeping the water flowing. Also keep in mind that a typical slump test is only valid up to a certain number - I don't recall off hand but I believe it is less than 5". I know this doesn't really answer your question, just something else to consider.

 

[Ron]

First of all, the slump test is valid to whatever slump is measured, whether 2" or 10", assuming it was done correctly.

Next....you need to determine if the slump was a result of added water or did the ready-mix supplier use a water-reducing admixture that increased the slump without an increase in water. This is why it is important to have a mix design submitted prior to concrete placement and to have a pre-placement conference on site.

A 6-inch slump is not necessarily bad; however, I do agree with you that if the slump was a result of water addition, then durability can be compromised. For exposed slabs on grade, good control over the concrete mix design, supply, placement and curing are important. Proper finishing and control jointing must also be controlled.

When a transit mixer loses slump during travel to the jobsite, it is a good indication that the coarse aggregate was not in a saturated, surface-dry condition when the concrete was batched. If the moisture content of the aggregate is below SSD, then the concrete will lose slump during transit. If the moisture content of the aggregate is above SSD, then the slump will likely be higher than specified.

You can check the delivery tickets of the supplier to determine if water was added to the truck at the site. They tend to keep track of this because it relieves them of responsibility if the strength doesn't come up. If water was added at the site and it is not allowed by your specification, then some level of compensation is necessary....either an extended warranty or replacement.

 

[fsuenginerd]

Thanks for the advice everyone, I appreciate your help.

Since it's not a life safety issue, I agree that it's probably not worth it to have them tear it up and repour; that'd be overkill. I'll respond with something to the effect of it's structurally acceptable (on account of it making strength), but durability may be compromised and cracking, etc. could be an issue later on.

I'm a little hesitant to start asking for more data than I already have because I don't want to give them the impression that I've reviewed and approved the specifics and therefore am taking a share of the responsibility for any serviceability issues that might come up later. Basically if I say that X gallons of additional water is fine, then the unspoken implication is that X+1 gallons may not be fine. I don't want them coming back in two years with "what's with the cracking, Nick said everything's OK."

I've got a copy of their mix design and the test data, but the "water added" field is blank. That mix does have a water reducer, but the slump's fine on other batches of the same mix so I can't see them accidentally adding more to that batch. I'm confident that water was added at some point since the strength is about 1000 psi lower than all the other batches, but I don't think it's worth the time and trouble to prove it since the answer's still the same.

Thanks again for all of your help.

Nick

 

[Ron]

Nick...a short lesson in construction defects and litigation....

Compromise by the design professional increases his/her liability. You need to raise the issue of durability. If you are shot down by others, so be it....you've met your standard of care. One truck was tested and shown to be out of the specification. My guess is that you didn't sample or test each truck.....what about the others. You mentioned that this was the only one "out of spec". What about the ones that were not tested. How do you know?

I don't get your hesitation to ask for more data. That doesn't protect you. If there's a problem later, you'll get asked why you didn't ask for more data! Don't put yourself in that position. It is poorly defensible.

If you ask for data, evaluate the data and discount the data....that's your prerogative under "engineering judgement". If you don't ask for the data and don't evaluation, that's negligence.....much worse!

Further, life safety is not the only test for rejection of non-compliance. Remember...in Florida as with many other states, you have a statutory obligation to protect the health, safety and welfare of the public. Durability deficiencies can be all three of those, depending on the severity.