Step 5: Pouring the Concrete
The day of the pour had finally arrived and to top it all off, our friendly neighborhood Home Depot had rented us a beautiful, brand new, virgin concrete mixer for 50 bucks for the day. That's right, we were the first ones to use this fine machine and Mr. P was anxiously waiting to put it through its paces.
I ended up buying Cheng Pro-Formula Mix off the website because this was my first pour and I didn't want any surprises. The kits come with everything you need, minus bags of Quickrete 5000 that you can buy at any home improvement store. It has the color tinting, support fibers, and water reducers to make the concrete perfect for countertops. I highly recommend this for first timers.
You'll want to make sure to have gloves and masks while you're mixing concrete since it is very dusty and messy. It's also a good idea to have 3 people during the pour so that one person can be cleaning up the mixer and tools while the others are pouring it.
We followed the instructions that came with the kits to mix the concrete and then started rolling wheelbarrows of concrete into the garage. At first we gently put in the concrete with our hands so that we wouldn't disturb the crushed glass. Once the glass was covered, we used a shovel to load it in. Only fill the mold half full before you stop to vibrate the concrete.
You must vibrate the concrete if you want to avoid air bubbles that will create voids in the countertop. There are commercial concrete vibrators, but I found that rapping rubber mallets along the bottom and all around the mold seemed to work best. It really wears you out using the mallets, but you'll soon see air bubbles popping out along the surface so you'll know it's working. Cheng also recommends using an orbital sander with no sandpaper on it to vibrate the concrete. I tried this and it worked ok around the sides of the mold, but didn't do much along the bottom.
Now go ahead and fill the mold with the rest of the concrete making sure to add a bit more than the mold can hold. Use a straight piece of lumber or melamine that is long enough to span the mold to screed the concrete. Start on one side and push the screed back and forth along the sides till you go all the way across the mold for a level finish. Make sure you fill any holes where the concrete might have been low and re-screed it. Go ahead and start vibrating the concrete again. Make sure you've vibrated it as well as possible. It took me about 10 minutes of swinging the mallets before I felt that it was vibrated properly.
Now you wait...and watching it doesn't make it dry any faster.
The ideal curing conditions are in a humid, shaded area between 70 and 90 degrees fahrenheit. I chose to have it cure in my garage since it stays in those conditions through the summer. It's recommended that if the temperature is below 50 degrees you get some sort of heating for proper curing. It's also not good to cure in the sun because it will dry out too quickly and be more likely to crack. I waited 4 days before moving onto the next step. Can you imagine Mr. P's anticipation?
Mr. P Note: "Unbearable!!"