All concrete countertops have a basic requirement: a concrete mix that provides the structural, physical, and aesthetic characteristics necessary to make a high-quality countertop that meets the client’s needs and wants.
Aside from ordering concrete from a ready-mix supplier, there are two basic ways to obtain a concrete mix.
- One is by using a commercially available bagged concrete countertop mix.
- The other is to do it yourself, making a from-scratch mix with basic ingredients.
I firmly believe that regardless of which approach you use, it is essential to understand concrete countertop mix design principles and proper concreting techniques. This will make you more successful with any mix.
Keep in mind that “making a high-quality countertop that meets the client’s needs and wants” is about a lot more than which mix you use. For example, sealer is critically important. The client doesn’t know or care which mix you use, as long as the countertop ends up smooth and with no cracks and meets their expectations in terms of staining, scratching and maintenance. Staining, scratching and maintenance are properties of the sealer, not the mix. So you could say that mix doesn’t matter.
I also believe that there is no one right answer for everyone. There are pros and cons to both approaches. Which one you use is ultimately a personal preference based on many factors.
All that said, let’s explore some of the pros and cons of bagged mixes versus from-scratch mixes.
Bagged mixes offer simplicity and convenience as their key feature. Generally all of the necessary ingredients, except pigment, are pre-blended; all that is required is to add the proper amount of water.
Implicit in the offering are the following characteristics:
- The concrete mix is consistent from bag to bag.
- The resulting concrete meets the performance specifications stated.
- Most importantly, the mix itself is appropriate for concrete countertops in general, and specifically for the casting method (cast-in-place versus precast) used.
There are several different concrete countertop mixes on the market. Some come in a single color (e.g. gray cement), while others have gray or white cement bases. Aggregate size, shape, color, and gradation can vary widely. Some bagged mixes have large amounts of coarse aggregate, while others are all-sand mixes with no large aggregate. Some are designed for casting in place, others for GFRC. Some claim to be UHPC (Ultra High Performance Concrete) mixes, although the definition of a true UHPC is quite difficult to meet.
A bag of dry concrete countertop mix contains a variety of ingredients that the manufacturer has chosen for a specific reason. There might a wide range of factors that influence a particular blend, such as the desired compressive strength, economics, the availability or cost of a particular ingredient. Or the manufacturer might use ingredients based on more esoteric criteria, such as the concrete’s in-hand feel and workability.
Regardless of whether the bagged concrete mix is originally designed for the do-it-yourselfer or a professional concrete countertop maker, all bagged mixes share a common characteristic: You don’t really know what’s in the bag, and you have to trust the manufacturer’s instructions.
Ideally, the mix should always yield the same results, but external variables such as temperature can significantly affect your concrete. So having some control over the mix can be important. If you do need to alter the mix – say by adding accelerator on a very cold day – and you don’t know how much cementitious material it contains, you can’t dose properly.
Control, therefore, is one of the main reasons for using a from-scratch mix. Since all of the ingredients are known exactly, accelerators, superplasticizers, pozzolans, defoamers, types of cement, pigments, and decorative aggregates can all be used to tweak the performance and appearance of the mix.
From-scratch mixes are also generally less expensive than bagged mixes. The basic ingredients of concrete can generally be obtained locally and cheaply. If shipping is required, that might increase costs. And keep in mind that material costs may be far less important than labor costs.
If you’re just making a few high-priced sinks and not using a lot of mix, you might not care at all about your material costs. If you are turning out dozens of large projects per month, your material costs may become an important business consideration.
However, from-scratch mixes are less user-friendly than bagged products and require a deeper understanding of mix design. Myriad factors such as mineralogy and aggregate particle shape, size, and gradation can have powerful influences on the fresh and hardened properties of the mix. With so many variables it can be difficult to strike a balance between aesthetics, workability, and physical performance.
Making your own concrete also requires you to source, obtain, and batch all of the ingredients.
- Sourcing all of the ingredients can be challenging depending on where you live.
- Means for precise batching is essential for consistency.
- Weighing out each ingredient takes time. Time is money.
- Weighing out each ingredient requires precision. It introduces more possibilities for making mistakes.
- Storage of raw materials requires space.
If you choose to use a from-scratch mix, here are some tips:
- Use precise scales. See this article.
- Use a system to prevent mistakes in batching and mixing, such as batch reports, a double checkmark system and mix calculators. 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.
- Add ingredients to the mixer in the correct order. See this article.
Mixing up a countertop mix from scratch gives you greater control over its properties, but you need to buy, store, and batch all of the ingredients and proportion them properly. With bagged mixes, you normally just add water. Both approaches are valid, you just need to consider the pros and cons.
To choose the best mix that is right for you and your business, carefully weigh all the pros and cons. You may prefer control over simplicity, or you may simply not be able to obtain a bagged mix in your area. On the flip side, space may be at a premium in your shop for storing materials, or you may want the convenience of just adding water.
Whether the mix is bagged or from-scratch, make sure the mix is appropriate for concrete countertops in general and for the specific casting, forming and finishing methods used (cast in place vs. precast, vibrated or not, water-tight forms or not). Understand the mix’s strengths and weaknesses. And always follow good concrete practices, such as carefully controlling water and moist curing the concrete for as long as possible.
Click here for CCI’s from-scratch mix designs and calculators.