lots of low breaks on 6K psi mix 4

[boffintech]

I was making a table to show a client the performance of a particular problematic mix (low and slow at 28 days). When I sorted the table by slump I thought I would see all the high 28 day breaks align with the low slumps; however, the reverse appeared more likely.

1) Can low 28 day breaks be caused by not having enough initial water for early cement hydration and thus early strength?

2) Or is the slag + flyash volume to great to get 6K reliably at 28 days?

3) Or are the intricacies of concrete engineering far complex for even a super-tech to understand?

THE MIX
cement 380
slag 260
flyash 110
57 stone 1450
89 stone 250
sand 1205
water 283

bag factor 7.98
unit wt 145.9
W/C+P = .38
max slump 4"

THE PERFORMANCE

Slump 28 Day Average Strength 56 Day Average Strength

(or date due)

7.50 6580
7.25 4570 6280
6.50 6270
6.25 4660 12-01-06
6.00 6015
6.00 4835 12-04-06
6.00 6390
6.00 6700
6.00 6395
5.50 4665 5150
5.50 11-27-06
5.00 5105 6950
5.00 6095
5.00 4445 6150
5.00 5360 7060
5.00 5415 11-23-06
5.00 11-29-06
5.00 12-04-06
4.75 5390 6070
4.50 4750 12-06-06
4.50 4935 12-13-06
4.50 5920 12-20-06
4.50 11-23-06
4.00 4120 6020
4.00 4870 6610
4.00 6025
4.00 4860 11-30-06
4.00 5280 12-14-06
3.00 4660 6510

 

[dik]

With proper quality control, slump and strrength can be independent... based on the mix design. From the scatter of data, the plant appears to lack quality control of the mix.

None of your slumps would indicate insufficient water for delayed hydration.

Dik

 

[henri2]

Boffintech,

Under laboratory conditions, where the materials used are the same, and trial mixtures are performed by the same team, one would expect a somewhat direct correlation between slump and water content....and also decreasing strength for higher slump/water content and vice-versa.

In the field, a lot more variables come into play...there is less control...consequently, you must not be surprised at your observations (relationship between slump and strength).

Water used in the mix serves two purposes, namely: hydration and lubrication. The W/(C+P+S) ratio is more than enough for hydration. Check Section 9.1 of your ICC Concrete Manual.

The mix design for your job has an MSA of 1 inch. In my neck of the woods, this mix would struggle to satisfy f'c of 6000 psi at 28 days on a consistent basis...and may not even make it at 56 days. A 3/4 MSA mix would perform better. Refer to Section 3.11 and Figure 3-8 of the Concrete Manual for commentary.

Some questions for you

1. What sort of admixture was used...HRWR? HRWRs are essential for mixes with f'c>5000 psi at 28 days

2. Was the mix design based on results from trial batches or from field test results (statistical approach)?

3. Assuming the mix design was developed by either field results or trail mixtures....were the materials used for the actual batch from the same source the mix design was based on?

4. Was there any batch plant inspection? IMO, this should be done when f'c>5000 psi

5. Was the curing of cylinders done per C31 Sec 9.2.2?

 

[Ron]

boffintech...it appears that your slump is produced through a HRWRA, not water, so the strength relationship is muddled.

You have two contributors to slow strength gain....slag and fly ash. Both of those will retard the strength gain as compared to portland cement. If you happened to get a combo cement (Type I/II) that is fairly common in the SE, you have a third potential (though slight)retarder.

If your mix were all portland type I cement, you would have no trouble reaching 6000 psi.

I also agree with henri2's comments and questions.

...and your question 3? You know better!

 

[dik]

Ron: Re item 3... I had a problem about 25 years ago that baffled me (I occasionally get baffled)... Concrete wasn't coming up to strength... cause was 'old' cement powder being used... it appears that at random, cement powder that had been processed in the previous spring was used in some of the batches. In the spring, because of the higher demand, the cement clinker wasn't ground as fine... hence, a slower strength gain. I scrathed my head a few times before I started asking some questions... data just didn't make any sense for the 'identical' batches being used.

Dik

 

[boffintech]

Interesting thing, the concrete company made lots of companion cylinders but positively refuses to give up the test results. The concrete company performed lots of coring but refused to notify the owner or the inspector when the coring was to be performed. The “cores” (no chain of custody) were then sent off to a lab to be tested and (to no one’s surprise) they were all greater than 6K psi. They probably have a block of 8K psi concrete back at the yard from which all the cores came from, but that’s a whole different thread.

1) Admixtures used =

30 oz of LRLW from The Euclid Chemical Co
8 oz of 341 from The Euclid Chemical Co

2. The mix design was based on results from (alleged) field test results.

3. Were the materials used for the actual batch from the same source the mix design was based on?

Same “source”? Maybe from the same source, even so, are we ever really sure if the materials are the same? Am i correct to assume that the speed at which we go through these materials in the SE US precludes the materials from being substantially the same?

4. Was there any batch plant inspection?

No. I offered at the onset of the job. I’ll offer again. We have another entire level of 6K psi columns after the next elevated slab pour so an opportunity and an apparent target rich environment are available. I have a plant inspection check list at the ready.

5. Was the curing of cylinders done per C31 Sec 9.2.2?

The ASTM C31 requirement for concrete mixtures with a specified strength of 6000 psi or greater is that the Initial Curing (a period up to 48 hr) temperature shall be between 68 and 78°F. The contractor provided water tubs for Initial Curing; the freshly molded cylinders were placed in water baths and the water covered the tops of the cylinders. On the day following molding, the cylinders were placed in Final Curing at a certified lab. All concrete testing was performed by ACI certified techs.

The concrete company claims that the concrete with its slag and flyash content produces cylinders that are TOO DELICATE to be picked up after only 24+-4 hours of Initial Curing. They claim we goofed up all the tests by not waiting two days to pick up the cylinders.

 

[dik]

Tinkering with the mix designs on the following web:

http://www.union.edu/PUBLIC/CERDEPT/FACULTY/GHALY/CONCRETE/general.htm

and others, the mix proportions are roughly in the same ballpark. The water cement ratio appears to be a tad high but the test cylinders appear be OK. From the scatter to the data, it appears that their quality control is no he** (heck?).You might want to check the following with what local practice dictates...

Regarding Item 1, the contractor may be able to undertake coring at his own cost without confirming this with the owner (not normal). Is there anything in the specifications for testing that requires the contractor to provide test results? A supplier to the contractor, however, shouldn't and the contractor should provide all test results from his supplier.

Item 2:
This is not unusual. Concrete plants often use a computerized mix design that is verified by field testing.

Item 3:
Generally they are; see my little anecdote to Ron.

Item 4:
It was offered? Often with projects the specification states that the plant can be inspected (reviewed?).

Item 5:
If the matter is serious, then you have to confirm that all testing was undertaken in accordance with...

General comments:

I'm not overly familiar with ACI code requirements and passing or failing of quality control tests. You might want to discuss with your testing agency that testing was completed in strict accordance with ACI/ASTM tests including the handling and storage of samples and confirm that the maaterial fails ACI requirements.

Assuming that the issue is serious enough, then you might want to put the concrete contractor/suppier on notice that they have failed to comply with the requirements of ACI, or whatever. You should confirm that the specifications or drawing notes stipulate it is a requirement that they have to comply with some standard. The location of this requirement should be clearly noted in the letter informing them that they are not in compliance.

I don't know what remedies are available in your jurisdiction, but you might have them provide an engineering report confirming that what has been supplied, albeit outside specification, is adequate for purpose and that there are no problems with serviceability. You should note that acceptance of this report shall have the effect that the material supplied is suitable; the report shall be unqualified. Check with the owner to confirm this is acceptable; this should be in writing. Not reasonable to keep the contractor/supplier on the hook after they have produced an engineering report. Careful to spell out the requirements for acceptance.

Contractually, in these environs, the only manner in which payment can be witheld is for the engineer of record to outright reject the work as being in non-compliance. I don't know if you want to get that serious. If so, then the next progress draw should reflect this non-compliance.

The problem with testing is that it is often difficult to undertake physical testing and have it fail. The high slumps may indicate that there will be additional shrinkage, there may be dusting as a consequence of bleed water issues, there may be curling, etc. serviceability items that cannot be immediately tested for.

Dik

 

[dik]

forgot to ask if the concrete supplier was advised earlier that his material was outside specification.

dik

 

[Qshake]

Looking at the compressive strengths in the list, it appears that this work would not meet the ACI requirements for acceptable concrete....see section 5.

Is this in place or just a trial mix at this time?

Regards,
Qshake

Eng-Tips Forums:Real Solutions for Real Problems Really Quick.

 

[Ron]

Boffintech...the concrete company has somewhat of a point about disturbance. If they know their materials are going to retard than badly, then they know what their 24 and 48 hour strengths are...though they're not likely to tell you.

Interesting thing about disturbance of a concrete mass as large as a test cylinder is that there is so much cement in this mix that autogenous "healing" will occur quickly and the long term strength should not be greatly affected. While they will point out the disturbance, you can counter with the autogenous healing argument and that such issues are not germane provided the cylinders were not abused such as by dropping. Cylinders can be carefully moved in the time period you noted without adverse affect. That problem usually occurs with mortar, not with concrete.

Dik...I've seen the same occur with cement demand being high.

Qshake...good to hear from you. Hope things are going well.

 

[henri2]

Boffintech, here are some more of my thoughts on questions I raised...presented in the same sequence I originally posed them

1. I checked the Euclid link below for concrete admixtures

http://www.euclidchemical.com/product_catalog.asp?pselect=4&gpselect1#1

Depending on the dosage, the 341 could be used as a Type A water reducer or Type F HRWR

http://www.euclidchemical.com/fileshare/ProductFiles/techdata/TDPlas341.pdf

for 380 lb cement, 8 oz/cu.yd works out as 8/3.8 = 2.1 fl.oz/100#cem..and if dosage is based on total cementitious material, it works out as 8/7.5 = 1.1 fl.oz/100# C+P+S. Note that the tech sheet recommends 2 to 3 fl.oz/100#C for Type A admix.

I was unable to find anything on the LRLW. Is it supposed to be a super-P? It is common to batch the concrete at the plant with a Type A admix...and then add the super-P (Type F) at the jobsite...to a relatively stiff mix with slump of around 2 inches. Type G super-Ps are added at the plant. Where was the LRLW added?

2. I take it that the mix design was developed by the ready mix company....no? Personally, I prefer mixes with f'c>5000 psi to be developed by labs who have experience with mix designs...and are familiar with the concrete materials to be used on the project.

This mix must be adjusted if they intend to use it for future pours. Qshake is right; it does not comply with ACI 318 Section 5.6.3.3 for a test age of 28 days...and struggles to comply at 56 days. Moving averages are fine at 56 days but the second 56 day result (5150 psi) is low...and not in compliance. Individual test results must not be under 0.9 f’c or 5400 psi in this case.

3. Even when materials are from the same source (aggregates same pit, cement...same type and producer...admix same company..etc) there will be some variation. However, when the source is changed, we must not be suprised when results do not conform to expectations.

4. Please do advise your company that they recommend batch plant inspection when f'c>5000 psi.

I presume the MSA for the column mix will be smaller than #57 aggregate. If that is the case, you should expect the mix to easily satisfy the requirements for f'c = 6000 psi. If you do not mind, please post the mix design for the columns.

As a general rule, smaller MSA mixes will have smaller flaws and thus will be stronger.

5. Unless a lot of retarder was used which significantly increased the final set time, I see nothing wring with the initial curing procedure. ASTM C 31 Sec 10.1 stipulates the specimens shall not be transported to the lab until at least 8 hours after final set.

 

[boffintech]

dik, the concrete company gets copies of the test reports on a weekly basis or daily if a low break

qshake, it's in place, not a trial

ACI318 5.1.1 Concrete shall be produced to minimize the frequency of strengths below fc.....hmmmmm...

The concrete producer wants us, the lab, to pay for coring based on their assertion of us handling the cylinders too early, but with the track record produced by this mix I had to ask if I were on Candid Camera, I mean seriously, that's a joke.

 

[dik]

Can the work be rejected as being out of spec? pending the concrete producer providing an engineered report. Can you confirm with the testing lab that the sampling was done correctly? What is the down side of the low concrete test reports?

Dik

 

[boffintech]

1. So by your calculations the 341 was under dosed? The LRLW as a super-P….well it wasn’t added at the job site. I don’t have that page of the submittal with me but I can look it up next week.

2. As far as I know the mix was developed by the concrete producer.

It has been pointed out to the concrete producer by the EOR that the mix does not meet the requirements of ACI318 5.6.3.3. You guessed it: they aren’t happy, plus their coring bill is way way up.

3. I’ll keep that in mind.

4. I’ll keep that in mind too.

“I presume the MSA for the column mix will be smaller than #57 aggregate. If that is the case, you should expect the mix to easily satisfy the requirements for f'c = 6000 psi. If you do not mind, please post the mix design for the columns.”

Sorry for not being clearer in my earlier post, but this 6K psi mix IS the column mix. The framed elevated slab above is 5K psi. The EOR has sent word down that the framed elevated slab will not be placed until all of the low breaks have been cleared up. Thus we wait….for core breaks…

5. No retarder was used. The mix is just low and slow.

 

[boffintech]

dik, sorry for not being clearer in my earlier post; I am the testing lab. The lab is certified, the techs are certified, I supervised much of the testing and sampling myself, which by the way was flawless. Also, the downside to the low breaks is that the EOR has sent word down that the framed elevated slab above will not be placed until all of the low breaks have been cleared up. Thus we wait…for core breaks…he also mentioned something about remedial measures even if all the cores break good. I wonder what he has in mind for the contractor.

 

[dik]

Sorry... I didn't understand... changes the direction of the work...

I would suggest that coring be done immediately for testing. The claims of delay could far outweigh the costs for coring and testing. I assume at this point you have put the EOR and the concrete supplier on notice that work was undertaken in full compliance with the recognized standards. In addition, I would put the EOR on notice that they would be billed for any and all additional testing and that delay of the project could be considered as negligence and would be at the harm of others. They have to be billed to show to the courts that you considered the work to additional to the project and not a consequence of your 'negligence' (not saying you have any). You might wish to consult with your council to determine that by undertaking the additional work you are not putting yourself in a position of liability.

dik

 

[dik]

Another thought... the EOR should be directing this... you should offer him the services...

Ron et al... feel free to jump in...

Dik

 

[Ron]

dik...I agree. As a testing laboratory, boffintech is in a position to be shot at because of the low breaks. They usually blame the lab first. I agree that he should be pro-active and go on record with such statements. The fact that the ready-mix supplier is hesitant to provide test results is a good indication they are not telling the whole story.

Core results should shed some light on the issue, but it still sounds like a mix design/mix control issue, nothing more.

 

[Qshake]

Ron - likewise, it's good to see that your still posting some of the best advice ever.

I hope all is well and that you're enjoying the holiday season (is that possible nowadays?!)

Best,
Qshake

Regards,
Qshake

Eng-Tips Forums:Real Solutions for Real Problems Really Quick.

 

[dik]

The scatter of the test results is a real good indicator of lack of quality control at the plant. Even if all the results were above min spec... with a scatter like the one shown, a standard deviation approach could pull the entire test results to below minimum. You might want to talk to a statistician.

I think it's imperative that the testing lab put *everyone* on notice that the samples were handled properly as well all testing was undertaken to the strictest standards.

Ron's comment about autogeneous 'healing', although correct, shouldn't be relied on for defending inappropriate handling. Best to start from a strong position that all was done correctly and that the scatter is a result of poor quality control.

The test results from cores taken by the concrete supplier may indicate poor quality control, too. Not that the cores are all high, but that there is a good scatter to the results. The number of coring samples may be too small to be conclusive.

The EOR should direct the location of the cores. With relatively high strength concrete for the columns, plus the generally reduced size and the amount of reinforcing, it may be difficult to take cores. I don't know how the supplier took cores without input from the EOR, but coring columns is the trickiest. It could be dangerous for the supplier to take cores in column elements. I don't know if the cores taken cut reinforcing steel. If they did and if OK by the EOR, then the EOR may have provided too much reinforcing.

As Ron noted, the reluctance of the supplier to provide data may be indicative of low values. Core locations should be determined near mid height, near centre of continuous columns and parallel to axis of minimum moment to avoid regions of high flexural stress. It is sometimes necessary to check for shear; compression can always be an issue. Patching may require use of an epoxy bonding agent and it may be necessary to use a rebar locator to avoid reinforcing steel. The location of the supplier's cores and manner of repair should be reviewed by the EOR.

Anything missed?

Dik