Buying a Concrete Mixer: Key Factors to Consider
Question:
I’ve decided on the type of concrete mixer to buy based on the type of countertop mix I use, but now I have a bunch of other choices to make: How big should the mixer be? Should I get gas or electric? What drum type is better—steel or plastic?
Answer:
You’re on the right path: The first step is to match the type of mixer (drum, mortar, or vertical shaft) to the type of concrete you plan on using when making concrete countertops. Next you should focus on selecting the size, power supply, and drum material based on your particular needs. Here are some pointers.
What size?
That will depend primarily on how much concrete you plan on making at one time and your typical batch size. A lot also depends on the concrete consistency (fluid vs. stiff) and how many people are involved in the process. Other factors include mixer price and the space available for storage.
The smallest mixers available can generally make about 200 pounds of “ordinary” concrete, or a concrete countertop mix that has a consistency and makeup similar to conventional concrete. These small mixers are inexpensive and fairly portable. Two hundred pounds of concrete is equivalent to about 1 1/3 cubic feet of concrete; at 1 ½ inches thick, that amount of concrete yields about 10.5 square feet of countertop.
The largest mixers top out at 12 cubic feet and can make 1,100 pounds, or 7 1/3 cubic feet, of concrete in a single batch. These monsters can mix up enough material for about 58 square feet of countertop at once, which is roughly the amount needed for an average-sized kitchen countertop.
Generally, mixers in the 6- to 9-cubic-foot range are what most people select. Within this range, batch sizes from under 300 pounds to almost 500 pounds are possible. When selecting a mixer size, it’s important to consider both the smallest and the largest batch size you’ll be making. Very large mixers may not be able to effectively mix the small batch of concrete you’ll need for a small bath vanity, for example.
Small shops with 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. A larger shop with more hands available could easily and more rapidly deal with large batch sizes.
For a given drum size, less concrete can be efficiently mixed if the concrete is stiff, while more can be efficiently mixed if the concrete is fluid. Generally, a mixer can handle roughly a quarter to a third of its stated volume when mixing stiff concrete, and about half or more of its stated volume when mixing fluid concrete. The type of mixer is also a significant factor; 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 and require no external power supply, so they are not dependent on jobsite power availability. The downside is that they are loud and hot, require gasoline, and need more frequent maintenance. The most significant downside is the exhaust. Gasoline-powered mixers should never be used indoors, 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 options on large, towable mixers. Motors come in a variety of voltage and horsepower ratings. Typically smaller mixers use 110-volt, 1/3- to 1 ½-hp motors, while larger mixers use 220-volt, 1- to 5-hp motors. The benefit of 110-volt power is that it’s available just about everywhere there is electrical service. The downside is that 110-volt electricity draws twice the amperage as 220-volt electricity for the same motor horsepower. Greater amperage requires larger-gauge power cords and higher amperage ratings 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 are that no or very little maintenance is required and they can be used indoors. The motors are quiet and compact and may only add marginally to the mixer’s purchase price. Because electric motors are more efficient, they also provide more torque than gasoline engines. For an extreme example, a large drum-style mixer capable of mixing about 7.5 cubic feet (or 1,100 pounds) 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 vs. a 9-hp gasoline engine to power it.
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.
Steel or plastic drums?
Steel drums are the most common and are therefore available in a wider range of mixer sizes and styles than plastic drums. Steel drums come painted or epoxy or powder coated, but the paint eventually wears down to bare metal, making the drum vulnerable to rust. Bare steel drums, particularly in mortar mixers, can cause “mixer burn,” a phenomenon very similar to concrete that’s burned by a trowel. Essentially, concrete that is very stiff will scrub steel off the drum as it’s mixing. Light-colored or white concrete that gets burned turns light gray. Fluid mixes aren’t susceptible to mixer burn because far less scrubbing occurs.
Plastic drums (usually called “poly drums”) don’t corrode but are far more abrasion-prone than steel. A steel drum that’s kept spotlessly clean, therefore, may last much longer than a plastic drum, but that depends on the concrete being mixed.
A plastic mixer drum (left) won’t rust or corrode, but it is
less abrasion-resistant than steel (right) and will wear faster
when mixing stiff concretes containing larger aggregate.
I’ve decided on the type of concrete mixer to buy based on the type of countertop mix I use, but now I have a bunch of other choices to make: How big should the mixer be? Should I get gas or electric? What drum type is better—steel or plastic?
Answer:
You’re on the right path: The first step is to match the type of mixer (drum, mortar, or vertical shaft) to the type of concrete you plan on using when making concrete countertops. Next you should focus on selecting the size, power supply, and drum material based on your particular needs. Here are some pointers.
What size?
That will depend primarily on how much concrete you plan on making at one time and your typical batch size. A lot also depends on the concrete consistency (fluid vs. stiff) and how many people are involved in the process. Other factors include mixer price and the space available for storage.
The smallest mixers available can generally make about 200 pounds of “ordinary” concrete, or a concrete countertop mix that has a consistency and makeup similar to conventional concrete. These small mixers are inexpensive and fairly portable. Two hundred pounds of concrete is equivalent to about 1 1/3 cubic feet of concrete; at 1 ½ inches thick, that amount of concrete yields about 10.5 square feet of countertop.
The largest mixers top out at 12 cubic feet and can make 1,100 pounds, or 7 1/3 cubic feet, of concrete in a single batch. These monsters can mix up enough material for about 58 square feet of countertop at once, which is roughly the amount needed for an average-sized kitchen countertop.
Generally, mixers in the 6- to 9-cubic-foot range are what most people select. Within this range, batch sizes from under 300 pounds to almost 500 pounds are possible. When selecting a mixer size, it’s important to consider both the smallest and the largest batch size you’ll be making. Very large mixers may not be able to effectively mix the small batch of concrete you’ll need for a small bath vanity, for example.
Small shops with 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. A larger shop with more hands available could easily and more rapidly deal with large batch sizes.
For a given drum size, less concrete can be efficiently mixed if the concrete is stiff, while more can be efficiently mixed if the concrete is fluid. Generally, a mixer can handle roughly a quarter to a third of its stated volume when mixing stiff concrete, and about half or more of its stated volume when mixing fluid concrete. The type of mixer is also a significant factor; 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 and require no external power supply, so they are not dependent on jobsite power availability. The downside is that they are loud and hot, require gasoline, and need more frequent maintenance. The most significant downside is the exhaust. Gasoline-powered mixers should never be used indoors, 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 options on large, towable mixers. Motors come in a variety of voltage and horsepower ratings. Typically smaller mixers use 110-volt, 1/3- to 1 ½-hp motors, while larger mixers use 220-volt, 1- to 5-hp motors. The benefit of 110-volt power is that it’s available just about everywhere there is electrical service. The downside is that 110-volt electricity draws twice the amperage as 220-volt electricity for the same motor horsepower. Greater amperage requires larger-gauge power cords and higher amperage ratings 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 are that no or very little maintenance is required and they can be used indoors. The motors are quiet and compact and may only add marginally to the mixer’s purchase price. Because electric motors are more efficient, they also provide more torque than gasoline engines. For an extreme example, a large drum-style mixer capable of mixing about 7.5 cubic feet (or 1,100 pounds) 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 vs. a 9-hp gasoline engine to power it.
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.
Steel or plastic drums?
Steel drums are the most common and are therefore available in a wider range of mixer sizes and styles than plastic drums. Steel drums come painted or epoxy or powder coated, but the paint eventually wears down to bare metal, making the drum vulnerable to rust. Bare steel drums, particularly in mortar mixers, can cause “mixer burn,” a phenomenon very similar to concrete that’s burned by a trowel. Essentially, concrete that is very stiff will scrub steel off the drum as it’s mixing. Light-colored or white concrete that gets burned turns light gray. Fluid mixes aren’t susceptible to mixer burn because far less scrubbing occurs.
Plastic drums (usually called “poly drums”) don’t corrode but are far more abrasion-prone than steel. A steel drum that’s kept spotlessly clean, therefore, may last much longer than a plastic drum, but that depends on the concrete being mixed.
A plastic mixer drum (left) won’t rust or corrode, but it is
less abrasion-resistant than steel (right) and will wear faster
when mixing stiff concretes containing larger aggregate.
posted by Jeff Girard
at
1:43 PM
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