Concrete Mixes: Bagged vs. From-Scratch
Question:
Besides convenience, are there other advantages to using prepackaged countertop mixes rather than mixing the concrete from scratch?
Answer:
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 features. Generally all of the necessary ingredients, except pigment, are preblended; 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 for the casting method specifically (cast in place versus precast).
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.
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 the specific casting, forming, and finishing methods used. Will the countertop be cast in place or precast, vibrated or not vibrated, or placed in water-tight forms, which will permit use of a wet mix with a higher slump? 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.
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.
Besides convenience, are there other advantages to using prepackaged countertop mixes rather than mixing the concrete from scratch?
Answer:
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 features. Generally all of the necessary ingredients, except pigment, are preblended; 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 for the casting method specifically (cast in place versus precast).
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.
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 the specific casting, forming, and finishing methods used. Will the countertop be cast in place or precast, vibrated or not vibrated, or placed in water-tight forms, which will permit use of a wet mix with a higher slump? 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.
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.
posted by Jeff Girard
at
4:55 PM
4 Comments:
In reading many of these articles, I am more confused than before I started. Why so vague??!! Give some details on aggregate size/materials. Give an ideal w/c ratio. You talk about the importance of these things, but then you really don't tell us anything worthwhile.
By
Anonymous, at 11:48 AM
The blog questions and answers serve to answer questions that often come up about various aspects of making concrete countertops. So instead of providing general information, they focus on specific topics. The Bagged vs. From-Scratch is a good example. That blog article wasn't about what makes a good concrete countertop mix, but what the pros and cons of using a bagged mix are and why someone would want to go that route over making their own.
If you'd like to submit a specific question for the blog, please visit www.concretecountertopinstitute.com for email or phone contact information.
By
Jeff Girard, at 5:51 PM
Jeff, what pre-mixed bag of concrete on the market would you suggest to use, for a counter top.
15' x 25" x 2". Poured from bottom and flipped over after curing 4 days?
By
tony, at 9:29 AM
I have had very good success using EnCounter. It provides good early strength and good shrinkage resistance. Be sure to follow the directions precisely. You'll probably find using 90 ounces of water per bag a good starting point. Of course using superplasticizers will always provide more workability even at the minimum recommended mix water amount.
-Jeff Girard
By
Jeff Girard, at 12:40 PM
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