Concrete oz/cwt 2

[bbadlose]

Hello all ,

Can someone please explain oz/cwt and how that calcs out to total ounces of admixture per yard ?

I have a mix that is approximately 7oz/cwt and 54 oz ?

Been crunching this for a while and stuck

Thanks .....

 

[Ron]

Admixtures are commonly dosed by weight of cement, so the notation you see is ounces per 100 lbs of cement. In your case if you have a dosing of 7 oz/cwt, you have 7 ounces of admixture per 100 lbs of cement. For a common 3000 psi mix having a cement content of around 500 lbs, you would have 7x500/100 or 35 ounces of admixture. If your 54 ounces is the total amount of the dosing, then you would have a cement content of about 770 lbs/cubic yard.


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[bbadlose]

Sorry for the late response but thanks Sir for the reply . Makes sense now .

 

[oldestguy]

Question for designers. My stepson is a driver of a ready-mix truck, a large firm, several trucks.. Has done so for about 10 years of more. As a driver he has to follow the requests of the guys on the job most of the time. A common situation is a job where the mix is to be a certain slump. It may have correct slump at the plant, but on the road tends to use up some of the water and the slump is less than speced. Adding water to get spec slump doesn't bother him. However, as a driver he doesn't agree with a common order. Once the slump tests shows the mix is meeting the spec, the workers many times then order more water for the mix. About the only time he won't add any more is to save enough water in his tank to wash out the drum, etc when it is empty. Even with inspectors on the job they don't seem to care about this adding of water after the test are done, cylinders taken, etc.. It happens at times that the result is a very liquid product. Comments? So far my step son ponders this with no reasonable answer about this.

 

[MotorCity]

Since the original thread appears to have come to a dead end, I will jump on the hijacked thread.

How do they know the slump at the plant is correct? Never heard of measuring slump at the plant.

If additional water is being consumed during transport, the mix designer and/or the plant should be accounting for this and add extra water at the plant accordingly.

If the workers or anyone else is adding water on site without direction from the appropriate parties, that's not acceptable...….especially after the field testing has been completed.

The dirty little secret is that water is probably added on every job without any oversight but everyone looks the other way.

 

[Ron]

OG...kudos to your son for considering the ramifications of post-test water addition.

Slumps are generally not measured by official testing by ready mix suppliers, except when they are developing mix designs in the lab or doing internal QA/QC. The slump at the plant is determined by gut feel by watching or listening to the concrete roll in the drum at low speed. Some transit mixers have electronic slump measurements, but are not likely closely calibrated to slump cone tests.

My suggestion would be to annotate the delivery ticket for any water addition, whether to bring to spec, or post test water addition. Coincidentally, I had this argument with a driver about a month ago when concrete was delivered to my house pool deck addition. I was watching the site glass and noted how much water was added. He did not put anything on the trip ticket. I made him put all the water that was added on the trip ticket. He then argued that some was for bringing to slump. I asked how much they held back at the plant...he didn't know. I asked where his batch printout was....he didn't have one. He couldn't answer any of my questions about the mix.

Your son will protect his employer's liability if he documents these things.


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[dauwerda]

From a structural standpoint the slump isn't really what's important, it's the water/cement ratio that is important. I have gone away from specifying any max slumps as today's admixtures are very good at increasing slump without having any detrimental effects on the overall mix design. I know it has been discussed on this site before, but to me, the only thing a slump test does is test for consistency between trucks/batches. If you only have one truck, what does the slump test really tell you (other than the workability of the concrete)?

 

[TigerGuy]

I remember yelling at the slab crew that their slump was 3 inches over spec. Their response was to signal for more water. As the tester, I had no authority to stop them. Told the superintendent. His response was to wait and see how the breaks came back.

When you add that much excess water to the concrete, you drastically change the water-cement ratio and get to look forward to excessive shrinkage cracking.

I also heard a story of a project where the concrete was ordered at a 9-inch slump. The truck was positioned at the mid-point of one side of the slab, and discharged at maximum speed. The "finishing" crew then placed the concrete with a 2-inch hose, blasting it around with water.

The problem I see is that no project will station somebody near the tester and reject the loads that are out of spec. I've never encountered a building official that refused to issue an occupancy permit for slump or air out of spec, so there are no consequences to doing things wrong.

Just my ranting, thanks for listening