Reading up on concrete water reducer additives, it says it "reduces water by 5% - 10%". I assume this actually means that it reduces the required amount of water? I.e. I would mix in less water, correct?

This would also decrease the amount of shrinkage that occurs during curing, correct?

Maybe this should be its own question, but will it be odd to communicate to a concrete contractor my desire for very low water content and request water reducer be used? Will they get annoyed at me dictating these details? I have read alot of professionals griping about other contractors adding too much water to make the concrete easier to work. I am looking to do a monolithic cast in place pool 16'x32' and plan to do forms+rebar but contract the pour. I am aware these days most pools use shotcrete/gunite which avoid shrinkage cracking due to much lower water content. The gunite guys around here want $40,000 to do a pool so thats out of the question for me. So trying to do a monolithic pour and take every precaution I can against shrinkage cracking. I reallize theres other contributors to cracking but dont want to ramble on.

How can I ensure the concrete contractor mixed in the water reducer and used less water? I know I can do a slump test onsite, but the water reducer acting as a plasticizer will mean that the cincrete with less water+plasticizer will have same slump as concrete with more water and no plasticizer, AFAIK.

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You might want to contract a civil engineer to help keep the concrete people honest. If your engineer provides them with mix specification and makes test cylinders from each load, that should keep them on their toes (even if you don't follow though with testing the cylinders - they don't know if you will or you won't, so they have to assume you will - which they will be far more likely to assume if you have a site engineer and/or technician overseeing the pour and taking samples, rather than you trying to do that yourself.) If you have proper cylinders made by a certified technician and properly cured, you also have the option of hanging onto them and only paying for testing in the event that you have a failure of the pool and need to find out of the concrete was, in fact, up to snuff. Your engineer might also be able to offer you valuable advice that could either save you money or make your project more prone to succeed. They are not free, but they are often quite reasonably affordable, especially when considered as part of an expensive project.

The linked document also mentions measuring air content, which is generally how most "water reducers" work (adding additional air bubbles, which make the concrete flow better without as much water - and they have other, mostly beneficial effects.) So that's one way to verify that the water reducer has been used, rather than more water added. Again, this is probably best done by a technician with the proper tools, hired for the day via your engineer - and would be part of making test cylinders anyway.

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