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]

Error: Cores should be taken parallel to axis of max stress. In that manner minimum highly stressed concrete is removed.

Sorry, Dik

 

[boffintech]

henri2, on that LRLW, they must mean EUCON LW Type A water-reducer. The mix rate is 3-10 oz per 100 lb (196-652 ml per 100 kg) of cement. I verify that with the submittal. I don't remember anything bing added to the mix at the project site.

 

[BigH]

We recently had problems with the cement - contractor was supposed to use Portland Type I (mix design based on it) and ended up using Portland Pozzalan - got his design strengths but at 56 days, not 28. As the testing laboratory - are you acting as Quality Control? or Quality Assurance? I am not sure if QC reports through the line of the contractor or through the line of the EOR. If the later, the contractor should inform the EOR of the non-compliance and propose remedial actions. If through the EOR, then the EOR should be given the test results including the moving three-test average, etc. and if the concrete was in non-compliance, then measures should be taken immediately to re-evaluate the mix; the EOR should have issued a non-compliance report to the contractor (did he?). The contractor (to which the concrete manufacturer reports) would then propose the course of action to be taken - e.g., cores, etc. The EOR would either reject or accept the course of action and you go from there. It seems that there is a bit of muddled lines of communication and hierarchy on responsibility.

 

[Dinosaur]

I haven't been in the mix proportioning business for many years, but a quick glance at the thread and a good read of the first five or six responses leads me to three observations:

The cement content appears high. A 7-bag mix @ w/c=0.35-0.40 can get 6000psi with good aggregates.

My opinion is that with a good mix design, good mixing with a proper dose of HRWR will give you the workability and strength you need.

Who does the acceptance testing on this project? You said the max slump was 4 inches per spec, but 70% (maybe more) of your results show more slump.

Because of water reducers and high-range water reducers, strength and slump are totally independent variables. However, hitting a consistent slump is still a good indication of good QC and not hitting the slump (and air?) is an example of poor QC.

Good Luck

 

[henri2]

Dinosaur, for 7 bag mix at w/c ratio of 0.40, the water content is 7 x 94 x 0.40 = 263.2 = 31.6 gals/cu.yd. For the aggregates used in this mix, the consistency of the fresh concrete may be too stiff to attain a 4 inch slump.

According to the product data sheet and dosage rate information provided by Boffintech, both of these admixtures are classified as Type A admixtures. I wonder why this was done..any ideas? For mixes with f'c of 6000 psi, I have seen combinations of Type A water-reducer and Type F or G super-plasticizing admixture....with slumps in the 7-9 inch range. This higher slump is an indication of satisfactory deflocculation of cement particles, creating larger surface area for meaningful hydration, strength gain etc.

The 4 inch maximum slump requirement for columns precludes the effective use of super-plasticizers. In any event, as you have pointed out, during placement operations, slumps were not in compliance for the most part.

 

[Dinosaur]

henri2,

I agree a 7 bag mix with w/c 0.40 will be too stiff without admixtures. If I didn't make it clear earlier, I would use and advocate the use of water reducers and super plasticizers for concrete in this strength range. My other comment about the maximum slump being 4 inches is just an observation that the spec is not well prepared considering that you will want to use HRWR to get workability. One of the more important points I want to emphasize is that with the admixtures available today, slump and strength are independent variables. Engineers that work with concrete field problems and specifications need to understand that maximum slump is a whole new ball game than it was in the 1970s. My organization is trying to work out the details of proportioning for SCC.

 

[dik]

Dinosaur and Henry2:

Notwithstanding the use of the admixtures, is there a rough guide as to slump and water content?

Dik

 

[henri2]

dik, as you are aware, when admixtures are not used, the TAW (total allowable water) to attain a given slump will depend inter-alia on the characteristics and proportions of the materials used for batching.

The maximum size aggregate (MSA) and percent of sand used (and FM), play a major role in determining the TAW...aggregate grading also plays a role while cement content plays a minor role.

Smaller MSA mixes require more sand and paste, resulting in a larger surface area of aggregates to be coated with cement paste, consequently requiring more water.

Nonetheless, the link you provided in the ACI Code forum, which is reproduced below, is a good starting point, if you do not have specific water demand data for the aggregate source used in the mix.

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

The QC department of a ready mix company should have a water demand chart based on aggregates they use. This chart which is developed with data obtained from numerous trial mixtures will provide a more realistic TAW based on MSA, coarse aggregate factor, and percent of sand in the mix. To perform an effective review of a mix design, this chart should be utilized. Unfortunately, some ready mix companies will not share this information….while others do not have it.

Based on data in the union.edu link, if you take the 1 inch/#57 agg(25 mm) MSA mix as an example, for approximately every 3 percent increase in water content (about 1 gal/cu.yd), the slump increases by 1 inch and vice-versa...consistent with the industry rule of thumb for that aggregate size. Also the TAW for a 1 inch MSA mix with slump of 3-4 inches is about 39 gal/cu.yd. For the aggregates I have dealt with, this is in the ball park..my experience has been 37-39 gal/cu.yd. For the 3/8 inch aggregate (9.5 mm) the chart indicates about 46.1 gal/cu.yd. With the aggregates I am familiar with, the TAW would be about 49-50 gal/cu.yd…about 7% more. All in all, not a bad chart to use if you do not have a specific water demand chart and or data from actual trial mixtures..or field test results.

 

[dik]

Thanks Henri2... I was just wondering if starting from a saturated aggregate if there was a rough 'rule of thumb' for amount of water to slump... For a small range, the 3% change you've noted will have to do in the absense of a water demand chart.

Dik

 

[Ron]

Hey Q! Good to hear from you as well. All is well as I hope it is with you.

Happy Holidays to you also.

Ron

 

[boffintech]

Dinasor, We do the acceptance testing on this project; we are quality assurance. Slumps were something that we went around and around about on the job. The plant is about 1 hour away so it was quite a hassle. We don't have the authority to accept or reject, only to observe and report.

 

[spagetti]

Just some observations from reading this string, Very little attention to air content has been mentioned, with the exception of dinosaurs' comment.
In any strenghth mix air content is directly by slump, low slump mixes exibit lower entrained air than that of a higher slump mix. In our precast plant we generally see a lower air content in tighter mixes and by adjusting the moistures to increase the slump we can also see and increase in air. supposition- therefore a higher dosage rate of entrainer air admix is required to obtain the desired air content.
By looking at your slump variations the slumps you are seeing above 6 inches. would lead me to believe that your actual air is running in the range of 7.5% or higher if your design is for 6%. Having said that, looking at your numbers, they are not reflective of this situation even though they should to some degree be reflective which makes me agree with the previous statements that the batch plant does not have good control of thier mixes.
I don't know where your producer is located, but here ready mix drivers are required to carry a batch sheet with them to the point of delivery and a copy given to the testing company, and because of the large amount of data required on those forms they are generally computer generated using actual wieghts for that particular batch.
plus any wash out water carried on the truck when it left the batch plant and how much was left after the load was placed as well as if and how much of any other admixes may have been added at the point of delivery.
I would start there. pull out those batch sheets and crunch the numbers yourself.
If the slump is to high, segregation could occur, I don't believe this is the case, but, if there is to much course aggregate segregation can happen when the fine and course aggregates seperate during transport, and an hour is a long transport time. this could be part of your problem.
also, (PCI TM-103 sec. 3.1.1 thru 3.1.3) the usual tolerence for slump is +/- 1" unless stated otherwise in a contract though lower than a 3" could be acceptable. It seams to me that any mixes deliveded, that had higher than a 5" slump should have been rejected by the inspector at the job site, even though they may have still been acceptable to meet the 28-day requirements. But I still belive that the culprit is the producer and by crunching the numbers your will find your solution. You should also get with the EOR about getting you own hardened samples and having them tested for air entainment. as well as compresive testing.
If the producer, states that they have performed compresive testing on harding core samples and all samples have passed, then have them provide you , not necessarily with the documentated test results but with a statement from them to the effect and submit it to the EOR and if this is exceptablt to the EOR than that should releive your liability issues.