Addressing Misconceptions about Countertop Sanitation

Question:
I've been getting the objection from potential clients that concrete countertops are hard to clean or harbor bacteria. What can I say to convince them otherwise?

Answer:
This is a common misconception. When people hear the word "concrete," they picture a material that's porous. Actually, granite and marble face the same issues as concrete in terms of what must be done to protect the surface against liquid absorption and bacteria growth.

First, educate yourself about the issues of sealing, porosity, and antimicrobial properties. There is an excellent article about these issues in granite on the StoneTech Professional website: "Summary of Antimicrobial Study."

When meeting with potential clients, you can allay their concerns about countertop sanitation by:

• -Telling them that the concrete countertop surfaces you produce are completely smooth, with no pits or voids where food or bacteria could collect. Let them feel a sample and see how smooth it is.
• -Recommending procedures for cleaning and maintaining concrete countertops, and providing an owner's guide with these instructions when you install their countertop.
• -Reassuring them that you will seal the concrete and explaining how your sealer works, whether it's a coating that completely seals the surface or a repellant (like those used on granite and marble) that prevents liquids from getting into any pores.

If your potential clients still have concerns after you've explained all this, give them a copy of the article "Are Concrete Countertops Unsanitary?" from the Concrete Connections website. It explains to homeowners and designers why fears about concrete countertops being unsanitary are unfounded.

Concrete countertops are very similar to granite when it comes to cleaning and sanitation issues. Making an analogy to a well-understood and accepted type of countertop material like granite may be reassurance enough for potential clients.

Is Lightweight Concrete Necessary for Countertops?

Question:
Because concrete countertops often must be installed on existing cabinetry, can the weight of the concrete be a problem? Are there advantages to using lightweight concrete?

Answer:
Lightweight concrete mixes are commonly used in the construction industry where weight savings is an important factor. Typical uses include floor, roof, and bridge decks. For most concrete countertops, however, lightweight concrete would not convey any significant advantage over normal-weight concrete for anything but the largest slabs. Factors such as site access, stairs, corners, and general countertop and cabinet configurations often prohibit the use of very large slabs. So in general, the largest practical slabs for countertops are not actually very heavy and therefore do not need lightweight concrete.

To illustrate, normal-weight concrete has a cast density of about 145 pounds per cubic foot (pcf) while lightweight concrete has a cast density of about 115 pcf. A square foot of 1.5-inch-thick concrete weighs about 18 pounds for normal-weight concrete and just under 14.5 pounds for lightweight concrete—a weight savings of less than 4 pounds per square foot. For comparison, a 1.25-inch-thick granite countertop weighs about 17.5 pounds per square foot. For most kitchen and bathroom cabinets, therefore, little or no modifications are necessary to bear the weight of 1.5-inch-thick normal-weight concrete.

If you do run into a situation requiring a lighter-weight slab, most countertop mixes can be "converted" into lightweight mixes by replacing some or all of the normal-weight aggregate with lightweight aggregate. Most lightweight aggregate weighs about one-half to two-thirds the weight of normal aggregate so, on average, 1 pound of gravel can be replaced with a bit more than ½ pound of lightweight aggregate. The volume of aggregate stays the same, but the weight is reduced.

Even though the conversion seems simple, the inclusion of lightweight aggregate in a concrete mix will affect its appearance, performance properties, and workability. Lightweight aggregates are typically dark gray, brown, reddish brown, rust-colored, or even orange (see photo). They weigh less than normal-weight aggregates (crushed limestone, granite, quartz, etc.) due to the porous cellular structure of the individual aggregate particles. Because they have a large amount of internal voids, the aggregate does not polish well. Lightweight aggregate polished with a 3000-grit diamond pad will remain dull because of the open nature of the aggregate (air does not polish).

The compressive strength, elastic modulus, splitting tensile strengths, and other properties of lightweight concrete are significantly affected by the structural and physical properties of the lightweight aggregate used. For countertops, it's important to use aggregates that possess desirable properties such as adequate compressive strength, porosity, appearance, abrasion resistance, and good bonding with the cement paste. For this reason, nonstructural lightweight aggregates such as perlite, vermiculite, and Styrofoam are not considered appropriate for structural concrete, but rather find uses in concrete meant for insulation or as lightweight filler.

Because lightweight aggregates have a cellular structure and are more porous than ordinary crushed stone, they absorb and hold more moisture. Because of this greater porosity, extra care must be exercised when designing the concrete mix and when dosing the mix water. Lightweight aggregates continue to absorb water over time—for hours, days, and even weeks after first being wetted. Concrete that looses mix water to thirsty aggregate during the critical phase when the concrete is setting can exhibit plastic shrinkage, surface crazing, color variation, mottling, and other undesirable and avoidable problems. Therefore, it is very important to calculate and keep track of all of the mix water added. Undisciplined and uncontrolled additions of water will significantly affect the performance, durability, and appearance of the finished concrete.

For more information about the properties and performance of lightweight concrete, check out the following references:

• "Physical Characteristics of Rotary Kiln Expanded Slate Lightweight Aggregate," Carolina Stalite Company (http://www.stalite.com/).
• "Guide for Structural Lightweight Aggregate Concrete," ACI 213R-87, American Concrete Institute (http://www.concrete.org/general/home.asp).
• "Advantages of Structural Lightweight Aggregate Concrete," Expanded Clay, Shale and Slate Institute, (http://www.escsi.org/).
• "Standard Practice for Selecting Proportions for Structural Lightweight Concrete," ACI 211.2-98, American Concrete Institute (http://www.concrete.org/general/home.asp).

Dark gray and orange lightweight aggregate in gray cement.

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.

What Type of Mixer Should I Buy?

Question:
I want to buy a portable mixer for making concrete countertops, but I'm confused by the different types available. Which type works best for countertop mixes?

Answer:
It's best to match the type of mixer to the type of concrete you plan on using when making countertops. There are three basic types of concrete mixers available (see photos): drum, mortar, and vertical shaft. All of these types come in a variety of sizes, and some come with a choice of a gasoline engine or an electric motor. Some drum and many mortar mixers are also available with either steel or plastic drums. Here are basic descriptions of how each mixer type works and what concrete mixtures they are best suited for:

Drum mixers are the most commonly used type and what most people picture when they think of a concrete mixer. They have a round drum with fins or vanes fixed to the inside. As the drum turns, the fins move and lift the concrete and gravity causes the ingredients to tumble and mix together. Drum mixers are designed to mix relatively fluid concrete that contains significant quantities of large aggregate. Thorough, efficient mixing depends on the tumbling action of the mixer and on the concrete mixture's physical characteristics, which can aid in the mixing process. The more fluid the mix, the more effective the mixing action. Typically very stiff low-slump, no-slump, and all-sand mixes are difficult to mix in a drum mixer because they tend to stick to the walls of the drum and not tumble and churn, resulting in the need for frequent hand scraping to remove stiff, clumped material that gets packed against the fins.














Mortar mixers, sometimes called stucco mixers, have a horizontal shaft with paddles attached. The drum of the mixer is semi-cylindrical and remains stationary during mixing. Instead, the shaft and paddles rotate to mix the concrete in the drum. Mortar mixers are designed to mix all-sand mixes, typically used for making stucco or mortar for brick and block. These mixes tend to be stiff and sticky and completely lack large aggregate. Thorough, efficient mixing depends on the mixing action of the paddles and not so much on the concrete mixture's physical characteristics. For this reason, mortar mixers tend to be more versatile than drum mixers because they can mix both highly fluid and very stiff concrete mixes. And they can usually handle concrete with up to 3/8-inch aggregate. The downside is that stiff mixtures and mixes with larger aggregate can significantly increase wear on the drum (especially plastic types) and on the paddle blades, which often use rubber strips that act like squeegees. Wear is reduced significantly with more fluid mixtures.















Vertical-shaft mixers have wide, shallow, circular mixing pans and are similar to mortar mixers in that the pan remains stationary. A drive shaft rises vertically through the center of the pan, and paddles and scrapers of various configurations move and mix the concrete. These mixers are less common than the previous two types, but they offer increased mixing speed and efficiency and are capable of mixing most types of concrete, both stiff and fluid. The drawback is the discharge method, which 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.















In summary, 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.

Working with Self-Consolidating Concrete

Question:
I have been hearing a lot about the advantages of using self-consolidating concrete. What is it, and what are the applications for decorative concrete?

Answer:
Self-consolidating concrete (SCC) is a highly flowable concrete that can spread through and around dense reinforcing under its own weight to adequately fill voids without segregation or excessive bleeding, and without the need for significant vibration. In the decorative concrete arena, it is used to produce castings with a high surface quality, often with few or no pinholes. Less labor, quicker casting times, better surface finish, and increased concrete densities are common reasons for choosing SCC.

Because SCC flows so readily, the flowability is measured in terms of spread instead of slump. The slump flow test (ASTM C 1611) is similar to a standard slump test (ASTM C 143), but instead of measuring a vertical height change, a horizontal spread measurement is made. Typical spread values range from about 20 to 30 inches (see photo).



Self-consolidating concrete is highly flowable and can spread
as much as 30 inches. Note how there is almost no
segregation, bleeding, or variation.

SCC is achieved by designing a mix that has a low yield stress and an increased plastic viscosity (see figure). In other words, the mix should require minimal force to initiate flow yet have adequate cohesion to resist aggregate segregation and excess bleeding. Lowering the yield stress without increasing the viscosity causes segregation and bleeding, both of which result in poor-quality concrete. The yield stress is reduced by using an advanced synthetic high-range water-reducing admixture (HRWR), while the viscosity of the paste is increased by using a viscosity-modifying admixture (VMA) or by increasing the percentage of fines incorporated into the SCC mix design.
















The preferred admixture for reducing yield stress in self-consolidating concrete is a polycarboxylate-based admixture, due to its superior water-reduction capabilities and high-early-strength gains at low dosing rates. This new generation of synthetic admixture has been specially designed to increase the dispersion of the cement particles, which aids in plasticity and strength and can help with pigment dispersion.

However, conventional concrete cannot be transformed into SCC merely by adding the right admixtures. Aggregate shape, sizing, gradation, and cement and water contents all have a powerful influence on the self-consolidating properties. The admixtures and all of the ingredients must be carefully selected and proportioned to preserve the SCC properties. A well-graded aggregate distribution minimizes cement paste content as well as admixture dosage.

When working with SCC, keep the following factors in mind:

• -Self-consolidating concrete has thixotropic characteristics, meaning that in the absence of energy (vibration) the concrete will stiffen on its own. This phenomenon may lead to problems with pour lines between consecutive lifts. These pour lines can be eliminated by simply vibrating for a short period of time, causing the two pours to flow together.
  
• 
  
• -Moist curing is beneficial to SCC because it often has a very low water-cement ratio. Providing a continuous water source to the concrete as it cures will help ensure that the capillary pores are filled and the hydration reaction continues to take place. Increasing coarse aggregate contents will also reduce plastic shrinkage, but again this may affect the fresh properties of the SCC mix.
  
• 
  
• -With concrete countertops,shrinkage-induced curling is always a concern. Drying shrinkage for SCC is very close to that of conventional concrete. However, self-desiccation could cause plastic shrinkage during the first 24 hours. This occurs in very low water-cement-ratio mixes (below 0.40) and is the result of hydrating cement particles consuming the available free water during the hydration process. As the water is consumed, the capillary pores within the concrete partially empty, causing the internal relative humidity to drop considerably. This could lead to bulk shrinkage, resulting in internal microcracking and affecting the overall strength and durability of the product.

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.

The Role of Aggregate in Concrete Countertop Mixes

Aggregate in concrete is a structural filler, but its role is more important than what that simple statement implies. Aggregate occupies most of the volume of the concrete. It is the stuff that the cement paste coats and binds together. The composition, shape, and size of the aggregate all have significant impact on the workability, durability, strength, weight, and shrinkage of the concrete. Aggregate can also influence the appearance of the cast surface, which is an especially important consideration in countertop mixes.

When selecting the most appropriate aggregate for a particular concrete mix, here are the key factors to consider:

Material
Most natural stones and crushed rock are appropriate for use in concrete. Commonly used stones are quartz, basalt, granite, marble, and limestone. Problems arise with soft, reactive, or weak stone or rock. Lightweight aggregates, a topic for another discussion, are also used in concrete.

If the concrete countertop is to be ground with diamond tooling, the aggregate will show. Thus aesthetics also affect the choice of aggregate.


Size
Aggregate size and gradation are the most important factors when selecting aggregate. Aggregate can be large or small, from fist-sized rocks to fine sand. Aggregates larger than 1/4 inch are classified as coarse aggregate, while anything smaller than 1/4 inch is termed fine aggregate. As a general rule, the largest aggregate should be no greater in diameter than one-third the depth of the slab, or one-fifth the smallest dimension of the form. For example, the largest piece of aggregate allowed for a 1 1/2-inch-thick countertop slab is 1/2 inch. Generally coarse aggregate is blended with finer aggregates (such as sand) to fill in the spaces left between the large pieces and to "lock" the larger pieces together. This reduces the amount of cement paste required and decreases the amount of shrinkage that could occur.


Shape
Aggregate shape influences strength, but has more of an immediate impact on the workability of the plastic concrete. Rough, angular particles pack tighter, have more surface area, and have greater interparticle friction than smooth, rounded particles, which reduces workability. Angular particles also require a bit more cement paste to coat them than rounded particles. Therefore, mixes containing them will require a slightly higher cementitious content.


Gradation
In general, coarse aggregates tend to be about 10 times larger than the fine aggregates in concrete, but the range of sizes could be greater than that in certain circumstances. As shown in the figure, there are three typical range categories:

• 
  
• -Well-graded aggregate has a gradation of particle sizes that fairly evenly spans the size from the finest to the coarsest. A slice of a core of well-graded aggregate concrete shows a packed field of many different particle sizes.
• 
  
• -Poorly graded aggregate is characterized by small variations in size. This means that the particles pack together, leaving relatively large voids in the concrete.
• 
  
• -Gap-graded aggregate consists of coarse aggregate particles that are similar in size but significantly different in size from the fine aggregate. A core slice of gap-graded, or skip-grade, concrete shows a field of fine aggregate interspersed with slightly isolated, large aggregate pieces embedded in the fine aggregate.

Poorly graded concretes generally require excessive amounts of cement paste to fill the voids, making them uneconomical. Gap-graded concretes fall in between well-graded and poorly graded in terms of performance and economy. Gap-graded concrete is a viable gradation, but not optimal.

Well-graded aggregates are tricky to proportion. The goal of aggregate proportioning and sizing is to maximize the volume of aggregate in the concrete (and thus minimize the volume of cement paste) while preserving strength, workability, and aesthetics. This balances the proportions of each so there are just enough of each size to fill all the voids, while preserving workability and cast-surface quality.

Mortar Concrete
Concrete made with just fine aggregate (or sand) is known as mortar concrete. Like the mortar used for brick and concrete block construction, which is simply made with mortar cement and sand, mortar concrete has no coarse aggregate in it, so a ground finish will have a fine-grained appearance. Mortar concrete is commonly used in concrete countertop mixes, since the surface finish is so important.

Even with an all-sand mix, aggregate gradation is still an important factor to consider and affects strength, workability, and aesthetics. It is always preferable to have some particle size variation rather than absolute uniformity because the interparticle void volume will be lower than with uniform particle sizes. While it is possible to blend different sands of different sizes together in a fashion similar to graded aggregates, generally only one type of sand is used. Most sand, especially bulk or bank-run sand, already has a particle size distribution that has some variation to it.

In order to achieve adequate workability, the cement paste volume must be high enough to encapsulate all of the aggregate particles and to provide some workability while the concrete is fresh. Therefore, mortar concrete tends to have a high cement content.

Aggregate gradation, whether in a mortar concrete or a traditional concrete mix, involves tradeoffs between strength and workability and is always a delicate balance. Understanding the implications of aggregate gradation is especially important when creating a from-scratch mix and will ultimately help you produce a better concrete countertop.