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, and the other is to do it yourself, making a from-scratch mix with basic ingredients. There are pros and cons to both approaches. Which one you use is ultimately a personal preference.
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 is that the concrete mix is consistent from bag to bag, that the resulting concrete meets the performance specifications stated, and, most importantly, that 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. Some even come preblended with pigment. 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 require the addition of polymer admixtures, which are sold along with the dry ingredients.
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 something more esoteric, such as the concrete’s in-hand feel and workability or satisfying certain textural criteria.
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 – you don’t know how much cementitious material it contains, so 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, pigments, and decorative aggregates can all be used to tweak the performance and appearance of the mix. However, from-scratch mixes are less user-friendly than bagged products and require an 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. Variations in ingredients, such as color, moisture content, and availability, all come into play and must be considered. Means for precise batching is essential for consistency, and storage of raw materials requires space.
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.