how to mix concrete with a powdered additive efficiently

Question

I want to mix about 2 cu ft of concrete in a 6 cu ft electric concrete mixer, but before I do that I want to mix in a couple of pounds of dry additive for countertops. I did this once but it was very difficult. The problem that I had was that after mixing the dry ingredients thoroughly I started adding water, but a lot of the dry concrete stayed in the bottom of the mixer and had to be dug out and brought forward by hand which was really hard and slow! I believe that one of the problems was that I should have tried to tilt the mixer forward a little bit while mixing so all the dry mix wouldn't be so low in the bottom.

I'm looking for the best way to do this next time. My idea is to mix all of the dry ingredients thoroughly in the mixer and then empty out most of it in a big tub, tilt the mixer forward a little, put some of the water in the mixer and then start adding the dry (with the mixer on), add more water and then the rest of the dry. I'm trying to avoid the problems I had last time.

Answer 1

I'm doing concrete countertops right now and here's how the supplier of my stuff wants you to mix it to get the most consistent mix:

• Mix the ad-mixture with water until you get a very wet slurry (in my experience almost grey water). This is to make sure there are absolutely no pockets of unmixed fiber.
• Add the sand/portland mix and water, alternating so the entire mix doesn't get dry pockets or sloppy wet
• Keep mixing and adding water until you have cake-batter consistency.

Do this in a bucket or tub with a paddle mixer, not a tumble mixer. The reason for this is because countertop mix is thin enough that you can get a decent whirlpool going in a bucket that you'll never get in a tumble mixer.

I did my island (poured in place) and waterfall edges (poured in the shop and transported) this way and now that they've cured, the joint is the only way you could tell I poured them in different places in different weeks.

Answer 2

I called the company that makes the countertop mix, https://www.concreteexchange.com/how-to-center/ and asked this question. They said that I should mix all of the dry ingredients first, and then start adding water, but they said to cover the mixer with a plastic trash bag and a bungee and then tilt the mixer almost horizontal, forcing all of the dry mix to mix thoroughly with water. I sort of thought that would work, but you have limited time to get everything mixed and poured, so I didn't want to get into a bad situation again!

update: I mixed 2 yards of concrete (3-80lb bags) with about 2lbs of the powdered add-mix dry for about 5 minutes. I think it's a 6yd mixer. I wanted a very accurate amount of water, so I used gallon jugs that I poured from a couple feet higher than the mixer through a funnel and a hose so that I could shoot the water into the back of the mixer while it was turning. We held the mixer horizontal, so the dry mix didn't get stuck in the back of the mixer. I stopped the mixer about every 2 minutes and shoveled some of the dry mix from the back of the mixer (think using a spatula with a kitchen mixer). I did this about 4 times and at that time it became clear that all of the concrete was mixing very nicely. The pour went great. There was no problem mixing the concrete. I would guess that adding the water first would require more shoveling and be dustier.