SOG Compressive Strength

[marinaman]

I've got a project that's essentially a big two story house. It has a first level slab-on-grade.

I called for a 4000 psi cast-in-place slab-on-grade. I called for 4000 psi concrete as I know that 4000 psi concrete requires less water, and ultimately gives a better end product with less shrinkage cracking and a more durable surface.

When my test reports came back, at 7 days, I was only getting 1,000 psi breaks on my cylinders. We rebound tested the slab to find the areas of relative weak concrete, then cored the slab and tested the cores in accordance with ACI. Tests came back at 14 days as being 1,100, 1,200, and 1,250 psi concrete.

Structurally, I don't believe there's a problem, as the stud bearing walls do bear on the slab, but, I have placed a CMU foundation wall below all bearing walls that ultimately bears on a continuous 24" wide x 12" thick concrete footing. The only thing the slab has to do is transfer the vertical force directly thru from the wall above to the CMU below....and I know the concrete will not be crushed.

But what about serviceability? I'm sure we're more likely to see shrinkage cracking...but what about hazing/crazing of the concrete. What effect may this have on finishes?

FYI, I did require a high quality vapor barrier beneath the slab as well as 4" granular (#57 stone) fill.

Does anyone see a potential problem with the low compressive strength concrete I'm overlooking?

Thanks in advance for your opinion!!

 

[msquared48]

I would be more concerned as to why this occcurred. The difference is rather large.

Have you had problems with other areas of the foundation pour?

Mike McCann
MMC Engineering
Motto: KISS
Motivation: Don't ask

 

[Ron]

First of all, why do you think that 4000 psi concrete has less water? Not true. Further, why would you think it has less shrinkage and cracking...also not true. Most of those parameters you've noted as being related to the compressive strength are actually moreso related to the coarse aggregate size. Do a little research into concrete technology.

As for your strength results...those are not even close to what they should be for what was specified. This is rejectable concrete in any sense of the imagination. Your application is not one where durability is a big deal, since I doubt you have traffic or abrasion; however, water intrusion and environmental durability might be issues....bottom line, when you have concrete this far off your specs, don't accept it...you take on more liability than you should and subject your client to a potential issue with durability and performance....why do it?

Since we don't know the reason for the low strength, it is difficult to assess the effect on durability. Is there a greater potential for more shrinkage...yes...is there a greater potential for crazing/surface durability issues ....yes.

 

[hokie66]

In addition to the quality issues Ron has addressed, your client hasn't gotten what he paid for. If he, on your advice, chooses not to require removal of the slab, he should receive a credit.

 

[mfrad]

Marinaman,
Was the slab cast in cold weather and did not get properly cured? Were the cylinders properly made and initially placed in a storage box 60 deg to 80 deg, then transported to the lab within 48 hours, then cured at 73 deg. If the cylinders were not properly made they may have microcracks and will never get to f'c. did the mix have flyash, that also causes low strength gain. the cold weather reduces strength gain and it takes longer than 28 days to get f'c. wait 56 days and core again to test strength.

 

[marinaman]

To be more specific to Ron's comments, by requiring a 4000 psi mix, the water/cement ratio is lower. Yes, the water content, in terms of pounds, is the same, but the cementitious content is higher, resulting in a lower water/cement ratio, and ultimately, resulting in fewer shrinkage cracking.....typically. I understand what is happening with regards to the water/cement ratio, and, I understand what happens with regards to aggregate size. This data was reviewed and approved within the submitted concrete mix design shop drawings. My apologize for incorrectly stating the water content.

In an ideal world, yes, the concrete should be rejected if its this low of compressive strength, but, one can not simply reject something without thinking it through, which is what I'm trying to do at the moment. I need to weigh the pros and cons of keeping it or rejecting it. I have concrete, now, that has low breaks....that is the current condition. Given the current condition, I'm trying to weigh my options. To me, this seems to be the prudent method of moving forward. I may ultimately reject the concrete, but, I may have to make some judgments regarding its current location and condition and deal with it.

Mfrad, the concrete was cast under cold conditions. It is my understanding that it was protected, but I was not at the site when it was cast.

The concrete in question is located within roughly 1/4 of a 3,000 square foot pour. By the time the break data got to this office, the building was framed. So, the question becomes, if I try and reject this concrete as Ron suggested, is one going to tell the owner that he has to have his contractor tear down the entire two story framing, take up the concrete, repour the concrete, and then reframe the building, because the concrete breaks came in at 14 days at 1300 psi.....which appears to have no impact on the structure itself?....and, set construction back by a month or more?

I'm trying to see if I can work with what I've got. Since it appears there's no structural implication, I'm wondering if I can (or need to) require some sort of topping to this slab to create a more durable surface (at the GC's expense).

Yes, I also agree the owner is due a credit for this concrete out of spec also.

 

[dik]

By out of spec, I would be thinking of 80% of spec'd strength, not 30%.

The low strength greatly reduces tensile and shear capacity, factors that can affect the overall behaviour of the concrete. Also as noted, the surface characteristics are greatly diminished, leading to deterioration.

The flexural tensile strength is greatly diminished, assuming that you have small percentages of reinforcing. The reinforced strength is only slightly diminished (subject to the earlier caveat).

You would be further ahead to have it removed...

Dik

 

[hawkaz]

Remove and replace would be the best option. If the owner doesn't want to go this route, spell out the risks (in writing) and have the owner sign off on them. Usually, once they see the risk (and understand that they are assuming it)- they change their mind and do the right thing.

 

[Ron]

Durability is directly related to compressive strength. With strengths as low as you are showing, I would expect a significant compromise in durability.

While most of the time that comes into play for slabs with abrasion or traffic on them, keep in mind that rigid floor coverings (tile/wood) need durable concrete as well. With high water content and low strength, you can expect long term shrinkage...which will affect the performance of tile or other rigid flooring.

 

[dik]

Ron:
I didn't think that there was enough cement in he mix to cause shrinkage problems <G>...

 

[mfrad]

If I were you, I would put the contractor on notice that the concrete is suspect and that it will be rejected and that he should investigate the cause of the low strength concrete at his own cost. He should retain an expert and obtain cores for petrographic analysis to determine approximate water cement ratio, cement content, durability, scaling resistance, etc and additional cores for testing at later stage. (It is possible that the concrete was placed under 23 deg and remained at low temperature and did not reach maturity (refer to ACI 306 cold weather concrete) and that testing of cores taken at a date later than 28 days may give the 4000 psi specified.)

 

[a2mfk]

I agree with hawkaz realistic approach, though I also see where everyone else is coming from in an IDEAL world (which doesn't exist).

Has the plant mix engineer or the plant commented on this yet?

The last time I checked a concrete SOG is not a structural member, it mainly keeps our feet clean. If you can legally absolve yourself from all liability, it becomes a matter of cosmetic finishes. Cracks can be sealed before putting down the flooring. How many have formed to date? That is usually the last thing to go in, so the owner could make some judgment calls on the flooring depending on the amount of cracking in the slab (ie careful with tile).

Ron- I understand what you are saying, but what if the flooring is laminate, wood, carpet, etc. where the condition of the slab can be pretty poor in terms of cracks and it really doesn't affect the flooring? If the contractor waits a couple of months or more and then seals all the cracks before putting down the flooring, what are we worried about here? Moisture intrusion through the slab and cracks that form after the flooring is put down? (I know Ron and I both do forensic work and so we get to look at a lot of problems with finishes and moisture intrusion...)

I do inspections all of the time where people pull their carpet back in their 10 year old house and are freaked out by all the cracks that have most likely been there since it was built...