Tips for mixing and casting concrete countertops

Mixing and casting the concrete may seem like the simple part. You’ve made the templates, built the forms and double- and triple-checked them. Now you just dump the ingredients in the mixer, and then dump the concrete in the forms, right? Actually, this is a critical point in creating high-quality concrete countertops.

Here are some tips to make sure your pour goes smoothly and make your life easier in the subsequent steps.

  • Safety first!

All workers should wear dust masks, eye protection and gloves. Designate one person as the Pour Master (or whatever creative name you want to use for him/her). Before turning on the mixer, the Pour Master is responsible for checking around the area to make sure everyone’s hands and limbs are clear of the mixer, then confirming this by yelling, “Clear!”, and waiting for a response from each worker. The Pour Master also checks off on the batch report and the quality checklist.

  • Use a double-checkmark system.

Before you start, print out or write out a batch report that lists every ingredient and exactly how much in each batch. When you measure out the ingredients for each batch, put one checkmark beside that ingredient for each batch. When you add each ingredient to the mixer, put another checkmark beside that ingredient. This applies to bagged mixes as well as from-scratch mixes.

CCI’s Precast Mix Calculator and GFRC Mix Calculator both include a calculator that generates a printable batch report with space for double checkmarks. You just enter how many square feet you’re making at what thickness, the color, and the calculator does the rest.

But regardless of whether you use CCI’s formulas and calculator or your own, be sure to use a batch report. The batch report also serves as a record of exactly how you made the mix. You can keep it in the customer’s file and refer to it if you ever need to duplicate that customer’s color.

  • Make sure everything you need is close at hand.

Pre-measure all of your ingredients in buckets or containers, including your water (especially your water!) and any admixtures you might need (set retarder, superplasticizer, etc.). This applies even for bagged mixes that you “just add water” to. If you have multiple batches, use a set of buckets/containers for each batch. Also make sure you have gloves, scoops, trowels, etc.

  • Pour adjacent slabs from the same batch.

Even if all ingredients are measured to the gram, subtle color differences can result from very slight variations in sand moisture, cement color, aggregate gradation and other factors. If you’re doing a large pour with multiple batches, plan out which slabs will be adjacent in the installed kitchen or project, and use a single batch for adjacent slabs. If adjacent slabs are too big for a single batch, pour the face of all adjacent slabs with one batch, and then fill in the depth with another batch. Stir the edges to prevent casting lines.

  • Add ingredients to the mixer in the correct order.

For a from-scratch precast mix, here’s the best order to add ingredients to the mixer:

  1. Sand, rocks and pigments (dry or liquid)
  2. Water and all admixtures
  3. Fibers
  4. Pozzolans
  5. Add cement gradually

Here’s why:

If you start with the cement in the mixer and add water to it, clumping can occur. Also, mixing everything except cement with the water first ensures that all of the ingredients are fully wetted and dispersed. Then when the cement goes in, it can combine properly with all the other ingredients to make high quality concrete.

Remember, water first, then cement.

If you’re using a bagged mix, you won’t be able to do this. That’s okay. Just follow the manufacturer’s instructions, and measure your water precisely. For the Buddy Rhodes dry polymer mix, I have a preferred mixing order that works very well. Please see this article.

  • Use a quality checklist.

If you are hand packing a stiff mix, you need to make sure to pack the concrete into every corner and edge. If you’re using a fluid mix and hand vibrating, you need to make sure to apply vibration evenly to all edges/surfaces. You need to make sure that the reinforcing is placed at the correct level in the form. You need to screed the back of a stiff mix to make sure it’s level with the forms.

These are just a few items, and you may think it’s easy to remember, but when you’re hustling to get concrete into the forms, it’s easy to miss details. Write down a quality checklist and have the Pour Master double-check it. Doing something like forgetting to screed the back now can result in a lot more work later.

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