Using Batch Tickets to verify w/c ratio??? (need HELP!) 2

[fogeyville]

On a current project, there was reason to suspect that the concrete supplier had changed the mix without approval. He has taken the initiative to submit batch tickets from the pours in question for our review. For example, for a 7 yd. batch, the ticket lists the following amounts:

Course aggregate: 13,480 lb. (2.0% moisture)
Sand: 8,660 lb. (3.9% moisture)
Cement: 3,380 lb.
Fly Ash: 1,040 lb.
Water: 139 Gallons
(air entrainment and water reducing admixtures also listed)
The batch ticket lists the W/C ratio as 0.41999.

The original mix design for 1 yd. of concrete is as follows:

Course aggregate: 1,885 lb.
Sand: 1,230 lb.
Cement: 480 lb.
Fly Ash: 120 lb.
Water: 250 lb.
W/C: 0.42

In trying to verify the water content, I'm uncertain how to account for the moisture in the aggregate. The aggregate used weighs more that what was in the mix design. If I assume that the additional weight is ALL water weight, and I add that to the 139 gallons of water, and then divide that by the cement+flyash, I get a W/C ratio of 0.33. (By the way, I realize that there is too much flyash in the actual concrete, but that's another battle...)

Am I way off here, or is this a problem?

I've never had to verify a mix in this fashion, and it's been a long time since I learned volumetrics in my CE Materials class. I need to be certain because this could turn into a very ugly situation if we falsely raise a red flag in an already tense situation.

Also, what is the accepted tolerance for being over or under the mix design's W/C ratio?

I would appreciate any help you could give me.

 

[PSlem]

If I take 2% of coarse and 3.9% of sand and 139 gallons @ 8.34#/gal I get 252.4# of water/CY. 252.4 x 7/(3380+1040) = .40

 

[Ron]

The aggregate moisture contents mean nothing with regard to water cement ratio unless you know the absorption of the aggregate. Mix designs are based on a saturated, surface-dry condition of the aggregate. That means that they have absorbed all the water they are going to get but have a dry-appearing surface.

If the moisture contents given are excess moisture, then you can compute their effect on the w-c ratio. Assuming that, PSlem is correct...the w-c ratio is 0.40. It appears they have compensated for the moisture in the aggregate and reduced the amount of direct water addition to keep the w-c ratio in check.

 

[concretemasonry]

It seems like a conflict between idealized lab conditions and the real world of using concrete properly and being able to document the real weights and conditions since surface dry aggregate is rarely available unless the the extra cost of preparing the aggregates is specified in the documents and specification. - Also, the extra digits beyond the decimal point cloud the issue and do not create any real quality controls.

The situation could also create a conflict with the slump requirements specified, unless extra water must be added on site and the truck meters are monitored and recorded.

If it is a single large pour (over 1000 yards) special handling and advance testing of aggregates can be done and the conditions controlled. If it is just a smaller pour on a large project, the specifications, testing and controls can be difficult and and costly to administrate.

 

[dik]

and further to Ron's comment... a common effect of different moisture contents of the aggregates is reflected in the slump delivered to site.

 

[dik]

Just grabbed my calculator and the first w/c ratio would be approx 0.31... not 0.41999

The site ticket should list the design batch number...

Dik

 

[conceng]

dik, The batch weights listed on tickets like fogeyville listed are almost always total weight of material added to the truck. The free moisture on the aggregates is included in this weight and must be accounted for to determine the correct w/c ratio.

Without the absorption properties of the aggregates and know exactly what the moisture # given is (total moisture vs. free moisture), we can't accurately determine the w/c ratio. You should be able to find the absorption values on the original aggregate test reports typically included in a mix design submittal package. Most batch computer systems calculate the w/c ratio incorrectly (computer geek vs. concrete geek) but it only makes a minor difference. The aggregate and moisture data entered in the computer must also be correct.

Anyway, if we assume the numbers given are just the free moisture, and we calculate the quantity incorrectly as most batch plants do, we get 37.8 lbs/yd^3 free water on the rock, 46.4 lbs/yd^3 free water on the sand plus 19.9 gallons or 165.5 lbs/yd^3 add water to get 258.3 lbs/yd^3 total water / yd^3. 258.3/631 = 0.409. Pretty dog gone close to what they and PSlem got.

Greg

 

[fogeyville]

I appreciate all of your responses.

dik: The site ticket lists the batch number, but this was a case of the superintendent telling two different inspectors on separate occasions that his newly-poured walls looked better (less honeycombing) than the previously-poured walls because he ordered the mix to be changed to have less course aggregate and more fine aggregate. After the architect reminded him that he was not supposed to pour any concrete without an approved mix design, their story changed to "we have only poured from the approved mix. Any statement otherwise was a miscommunication." In other words, when they submitted these batch tickets, I wasn't necessarily putting much faith in their authenticity. But I thought that they could be of value if there was something inherently incorrect about what they submitted.

If I am to take the moisture values listed on the batch ticket at face value (and I assume I must), then it looks like PSIem and conceng and the ready-mix supplier are accurate with the w/c ratio.

But here's a mildly interesting tidbit: The total weight of sand used was 0.6% more than the mix design value, but if the moisture is 3.9%, doesn't this mean that they could be as much as 3.3% low on the amount of sand batched? On other batch tickets, the total weight of sand was 1.8% too low, which could put them 5.7% too low in the mix. I'm not trying to catch them on a technicality here (2% tolerance), I'm just using this apparent anomoly to question their moisture value. They have had significant honeycombing and rock pockets in columns and walls, and I'm questioning the workability of this mix once the admixtures wear-off and possibly rapid slump loss occurs.

By the way, the batch tickets also list "Pozzolith 322N" (water reducer) which was NOT included in the original mix design. This is in addition to the "Polyheed 1025" water reducer that was included in the original mix design.

Is this unapproved admixture worth making a fuss over?

 

[hokie66]

I won't try to comment on the mix design, but the honeycombing is more likely due to insufficient consolidation effort than to workability issues.

 

[Zambo]

fogeyville,

what "rock pockets"? is this a different defect from the honeycombing? Sounds like its meant to be a segregation issue, I wouldn't expect such a defect in a modern concrete with admixtures and a proper mix design. Who determines the nature of the defects?

 

[fogeyville]

Zambo,

There seems to be a fine line between honeycombing and rock pockets based on the ACI definitions:

http://www.concrete.org/Technical/CCT/FlashHelp/ACI_Terminology.htm

It's almost a "you know it if you see it" thing. In cases where there are significant voids, we are calling it honeycombing. In places where it looks almost like compacted gravel, we are calling it rock pockets.

The Owner's testing agent (with our assistance)is determining the nature.

I agree with your statement about a proper mix design, which is what caused me to start this thread.

Will you also weigh-in on their use of two water reducing admixtures and the fact that one of them wasn't included in the original mix design?

 

[Zambo]

I wouldn't like to "weigh-in" on such a case without being able to see the defects, or the concrete at the time of placement.

It sounds as if there have been concrete placement and compaction problems on your project. I suspect that all of the defects could be described as honeycombing and "rocket pockets" could be a diversion.

Did the contractor use a tremie tube or pump tube to place the concrete? Was there space in the rebar cage/forms to place such a tube? Is the rebar congested? Probably the contractor will blame the design for the difficulties in placement and compaction.

 

[fogeyville]

Zambo:

The problems are intermittent. When the consolidation problems are evident, it is usually in the bottom of the walls/columns. But more than half of the walls appear to be well consolidated at the bottom. Now the Owner is concerned there may be consolidation problems which are hidden from view.

The typical walls and columns are not congested. There were only three small columns that I would consider congested, but they look the best of all!

They have been using a bucket and drop chute. They tried pumping once or twice, but with no measurable improvement.

As far as "weighing in", I was really just wanting your opinion of the idea that a second water reducer was added without approval. I've been fortunate in my career to have worked on mostly SUCCESSFUL concrete projects, which means I've never before been asked to scrutinize the concrete practices so closely. I'm sure that many contractors make unapproved adjustments to the mix quite frequently. But when the finishes look good and the cylinders break high, there's never any reason to ask questions.

 

[hokie66]

Have you discounted leaky formwork as a cause of the honeycombing at the bottom?

 

[conceng]

If you divide the actual batched quantities by 1+(mc/100) you should get the saturated surface-dry aggregate weights of the aggregates.
The following are what your tolerances should look like on a per-yard basis.
+0.2% Rock 1888
-3.2% Sand 1191
+0.6% Cement 482.9
+23.8% Fly Ash 148.6

Yes the sand is a bit out of tolerance, but the fly ash is way over. Extra fly ash should help with consolidation issues. Being that far off, it looks as though they may have added the ash to aid in resolving their consolidation/finish issues.

I would recommend taking some aggregate gradations to make sure you don't have a severely gap graded gradation issue causing segregation.

Are they using both water reducers at the same time? I find it odd that they would be using the 322n water reducer with the 1025 mid-range. It is normally recommended to use a normal type A water reducer (322n) when using a high-range water reducer, but the benefits are typically not seen with a mid-range. What kind of slumps are you getting? I've never worked with the Polyheed 1025, but there may be a possible admixture incompatibility that may lead to segregation. It is doubtful, but it should be looked into, I've been wrong before.

hokie66 brings up a very good point. Could it be drain-down or loss of paste through the formwork rather than a segregation or consolidation issue? The more they vibrate it, the worse it will get.

Greg

 

[fogeyville]

conceng: I spoke with an industry representative who said the use of both of these products is fine. The product data submitted for the 1025 says it is also meets the requirements for Type F High-range water reducers. They are getting 8" slumps, which should be easy enough to place.

hokie66: leaky formwork is definitely an issue at one particular case where the honeycombing occurs in a perfect 2" strip full-height at a tee-wall intersection, but the leaking seems to be less prevalent elsewhere. Your previous post mentioned consolidation as the likely culprit. That was our first, second and third thoughts, but since the G.C. insisted that they have #1: placed this concrete the same way they have always done it and then later #2:changed placing methods, vibrating equipment and personnel, and since we had these batch tickets submitted to us, we thought we'd see if something obvious stood out about the mix.

Well, thanks in part to all of you guy's help, it's apparent that nothing seems too out of the ordinary with this mix. And even if workability after slump loss was an issue, it's still the G.C.'s responsibility to reject it instead of trying to place it, or to order a different mix.

I think the G.C. is going to need to take another hard look at their means and methods. For their sake I hope they'll figure it out before the first elevated slab pour, because removing walls and columns will get much more difficult after that.

 

[dik]

conceng: sorry... don't know what I was thinkin' about...
all others: please forget that I made that silly statement...

Dik