In a previous post, I discussed the basic types of precast concrete countertop mixers: drum, mortar and vertical shaft. This post explains how to determine size, power source and drum style once you’ve selected the type of mixer.

Size of Mixer

What size mixer you need depends on how much concrete you plan on making at one time. A lot depends on the concrete consistency (fluid versus stiff) and how many people are involved in the process. Other factors include price, storage space and typical batch size.

The smallest mixers available can generally make about 200 pounds of “ordinary” concrete or concrete countertop concrete that has the consistency and makeup similar to conventional concrete. These small mixers are inexpensive and fairly portable. Two hundred pounds of concrete is just about 1-1/3 cubic feet of concrete; at 1-1/2” thick, that amount of concrete makes around 10.5 square feet of countertops.

The largest mixers top out at 12 cubic feet, and can make 1100 pounds, or 7-1/3 cubic feet of concrete in a single batch. These monsters can cover about 58 square feet of countertop at once, which is roughly the size of an average kitchen countertop.

Generally, mixers in the 6 to 9 cubic foot range are what most people select. Within this size range, batch sizes ranging from under 300 pounds to almost 500 pounds are possible. When selecting a mixer size, it’s important to consider the smallest batch as well as the largest batch you’ll be making. Very large mixers may not be able to effectively mix the small batch of concrete that a single small bath vanity needs.

Small shops that have only a few people available during the mixing and casting process may prefer to make smaller, more manageable batches. They may not be able to handle a single, large batch of concrete before it loses workability, while a larger shop that has more hands available could easily and more rapidly deal with a large batch.

For a given drum size, less concrete can efficiently be mixed if the concrete is stiff, while more concrete can efficiently be mixed if the concrete is fluid. Generally speaking, a mixer can handle roughly ¼ to 1/3 of its stated volume when mixing stiff concrete, and about ½ or more of its stated volume when mixing fluid concrete. The type of mixer is also a significant factor here too; drum and vertical shaft mixers can make more concrete for a given drum size than a mortar mixer.

Gasoline or Electric?

Gasoline engines are the most common power source for larger, towable mixers. The benefits are that they are self-contained. No external power supply is needed, so they are not dependent on the job site conditions. The downside is that they are loud, hot, require gasoline, and occasional maintenance. The most significant downside is the exhaust. Gasoline powered mixers should never be used inside, since the engine exhaust can be deadly due to carbon monoxide and carbon dioxide buildup.

Electric motors are much more common on smaller, stationary mixers, but are often available as optional power sources on large, towable mixers. Motors come in a variety of voltage and horsepower ratings. Typically smaller mixers use 110 volt, 1/3 to 1-1/2 horsepower (HP) motors, while larger mixers use 220 volt, 1 to 5 HP motors.

The benefit of 110 volts is that it’s available just about everywhere there is electric service. The downside is that 110 volt electricity draws twice the amperage as 220 volt electricity for the same horsepower motor. Greater amperage requires much larger gauge power cords and higher amperage rating on the outlet. Conversely, 220 volt electricity allows the use of higher horsepower motors, but 220 volt electrical outlets may not be readily available in all locations.

The benefits of electric motors is that there is often no, or very little, maintenance required. The motors are quiet, compact and may only add a few percent more to the purchase price. And they can be used inside.

The disadvantage is that for larger mixers, electric motors are options that need to be ordered; few if any floor models come with electric motors. And some jobsites may not have the power supply required for the mixer.

Electric motors are more efficient and provide more torque than gasoline engines. For an extreme example, a large drum-style mixer capable of mixing about 7.5 cubic feet (or 1100 lbs) of concrete requires only a 2 HP electric motor, but needs a 10 to 11 HP gasoline engine to do the same job. A similar sized vertical shaft mixer would need a 5 HP electric or a 9 HP gasoline engine to power it.

Steel or Plastic Drum?

Steel drums are the most common material and are therefore available in a wider range of mixer sizes and styles than plastic drums are. Steel drums, initially painted, epoxy, or powder coated, eventually wear down to bare metal, which can rust if they are stored wet.

Plastic drums (usually called “poly drums”) don’t corrode, but metal is far less abrasion prone than plastic. A steel drum that’s been kept spotlessly clean may last much longer than a plastic drum, but that depends on the concrete being mixed.

Lastly, bare steel drums, particularly in mortar mixers, can cause “mixer burn”, a phenomenon very similar to concrete that’s burned by a trowel. Essentially, light colored concrete that is very stiff will scrub steel off the drum as its mixing. White concrete that gets burned turns out light gray. Fluid mixes aren’t susceptible because there’s far less scrubbing that occurs.

A steel drum

A plastic (poly) drum

For precast concrete countertops, your mixer is one of the biggest investments you will make at around $2000 and up, so buying just what you need, and not more than you need, is very important.

There are three basic types of concrete mixers available:

• drum mixer
• mortar mixer
• vertical shaft mixer

All of these types come in a variety of sizes. Some can be run either by a gasoline engine or an electric motor. And some drum and many mortar mixers are available with either steel or plastic drums.

Drum Mixers

Drum mixers are what most people think of when they consider a mixer for making concrete. Drum mixers have a round drum with fins or vanes fixed to the inside of the drum. As the drum turns, the concrete is moved and lifted by the fins, and gravity causes the ingredients to tumble and to mix together.

Drum mixers are designed to mix relatively fluid concrete that has significant quantities of large aggregate in it; in other words, basic concrete of a consistency most people are used to and expect to encounter.

Thorough, effective and efficient mixing depends on the tumbling action of the mixer and on the concrete mixture’s physical characteristics to aid in the mixing process. The more fluid the mix, the more effective the mixing action. Typically stiff, very low slump, no slump and all-sand mixes are difficult to mix because they tend to stick to the walls of the drum and not tumble and churn. Often frequent hand scraping is necessary to remove stiff, clumped material that gets packed against the fins.

Mortar Mixers

Mortar mixers, sometimes called stucco mixers, have a horizontal shaft with paddles attached to the shaft. The drum of the mixer is semi-cylindrical and is stationary during mixing. The shaft and paddles rotate to mix the concrete in the drum.

Mortar mixers are designed primarily to mix all-sand mixes, typically used for making stucco and for making brick and block mortar. These mixes tend to be stiff, sticky and completely lack large aggregate. Thorough, effective and efficient mixing depends on the mixing action of the paddles and not so much on the concrete mixture’s physical characteristics. It is for this reason that mortar mixers tend to be more versatile because they can mix both highly fluid and very stiff concrete mixes.

Mortar mixers can usually handle concrete with up to 3/8” aggregate. The downside of mixing larger aggregate is that it significantly increases wear on the drum and on the paddle blades, which often use rubber strips that act like squeegees. Wear is reduced significantly with more fluid mixtures, and is significantly increased with stiff mixtures in mixers with plastic drums.

Vertical Shaft Mixers

Vertical shaft mixers are similar to mortar mixers in that the wide, shallow, circular mixing pan is stationary. The drive shaft rises vertically through the center of the pan, and paddles and scrapers of various configurations move and mix the concrete.

Vertical shaft mixers are less common than the previous two types. However, they make up for this with increased mixing speed and efficiency. Vertical shaft mixers are capable of mixing most types of concrete, both stiff and fluid, but the discharge method may make stiff mixes more difficult to deal with.

With drum and mortar mixers, the entire drum is tilted to discharge the concrete. With vertical shaft mixers, the mixing pan is stationary and a relatively small door in the bottom of the pan is the discharge port. The concrete is discharged by opening the door and letting the mixing paddles scrape the concrete over the hole, letting gravity handle the rest.

It’s best to match the type of mixer to the type of concrete you plan on using when making concrete countertops. If you only work with fluid concrete, then a drum style mixer will meet your needs. However, if you work with all-sand mixes, or very stiff concrete, then a mortar or vertical shaft mixer may be a better choice.

Once you’ve determined the type of mixer, then select the size, power supply, and drum material based on your particular needs. I’ll discuss these factors in another post.