Concrete Countertops: It’s time to get personal!

If the real estate fallout of the last few years has taught us anything, it is that the old saying ‘your home is your castle’ holds more true today than it ever has. Many of us have realized that the constant need for ‘moving up’ into larger quarters was a hype that had to come to an end, and that we are quite comfortable inside the four walls we currently live in. A little bit of personalization is all it takes to fall in love with your home all over again.

Remodeling your kitchen or bathroom are great ways to update your home’s look and functionality and add the personal touch it deserves. Today kitchens have become rooms where the majority of living occurs, and they are also rooms where money spent on remodeling yields the highest return on investment. Inevitably your quest to beautify your space will bring you to the coordination of cabinets with countertops, and if you are in the market for new countertops, chances are you have visited showrooms filled with granite, marble, engineered quartz, tile, or laminate choices. All those make great options for countertops, and they all have a couple of things in common: They are fabricated in slabs, and you can pick from the suppliers’ standard sample pallets.

If the idea of picking from a standard list of colors to perfectly compliment your selection of cabinets and wall colors sounds less than exciting to you, I would like to introduce you to another material: Concrete!

Concrete is fast gaining in popularity among countertop choices today. Among design options I will illustrate below, the reason for concrete’s gain in popularity is simple: You will not walk through a bone-yard of countertop slabs and pick from a standard list of colors. Concrete countertops are hand-made, custom, by a local craftsman/woman, just for you.

Here are a few ways that concrete countertop craftspeople can add an owner’s personal touch to their homes:

1. Use color

Concrete can be colored through integral pigmentation, surface staining, or dying. Sheer endless colorations and dramatic effects can be created. Whether it is your college team’s color or the perfect ocean blue from your last vacation spot, you may approach a fabricator with a picture, a paint swatch or a fabric sample. Efficient shops will create color samples matched to your swatch within seven to ten days. While white concrete, natural grays, charcoal, or black are always-popular choices, bold colors are possible, and often all it takes to create that ‘wow’-effect in your home.

blue gears orange concrete

 

2. Customize with exposed aggregate or inlays

Terrazzo-look countertops have become popular over the last few years. These are often concrete countertops where glass or stone chips have been added to the mix to add interest. When you use locally fabricated concrete for your countertops you can select the chips you want added. Think river pebbles, recycled glass, stainless steel chips, even mirror glass. Inlays allow for even greater focal points. Those are much larger objects that can be placed inside the countertop form before concrete is poured. Here your fabricator can expose tile, gem stones, and even metal objects.

CounterIntuitive Christian Lotte shell vanity

CounterIntuitive Christian Lotte shower bench

 

3. Add drainboards and trivets

Add functional design right into your countertops through drainboards and trivets. Drainboards can be lowered sections of the countertop, or simply grooves that were cast into the counter. They are areas for fruits and vegetables, or even dishes, to sit and dry. Trivets are usually raised metal rails that are integrated into the concrete. These rails are ideal places to place hot pots. Frequently drainboard grooves or metal trivet rails are simple and straight, but with the versatility of concrete design choices are endless.

Jeff Girard drain grooves

Jeff Girard custom drainboard

 

4. Create depth and thickness

One of concrete’s greatest advantages is that you are not bound to a typical 1 ½” thick slab. Create the illusion of thickness by dropping the front edge 4” if you like. Because concrete is wet when it is cast, it will take on any shape you pour it into. Integral concrete sinks are popular choices and a welcome reprieve from your slab-producers standard undermount sink option. Intricate 3-dimensional design is among the favorite design techniques exclusive to concrete fabrication.

CounterIntuitive Christian Lotte integral sinks

 

5. Don’t be shy

Concrete is quickly and steadily evolving as a countertop material. Among the latest trends are integrated glow-stones, fiber-optics and glass pieces, as well as built-ins into your concrete, such as drawers or wine bottle holders. Your local concrete artisan is a skilled and competent source of information. If you can imagine it, he or she can more than likely fabricate it.

CounterIntuitive Christian Lotte stone vanity

Concrete is no longer the drab material that you walk on. Expert craftsmen have acquired unique talent that has elevated its positioning among high-end interior and exterior finishes while developing a look that remains all its own. When it comes to your remodel, doesn’t your kitchen deserve a personal choice?

The best mix design for concrete countertops

Whether you use a bagged mix specifically designed for concrete countertops, or mix your own, mix design is critical for concrete countertops. Unlike sidewalks or foundations which are slabs on grade, concrete countertops are generally long, slender, thin beams that not only behave very differently structurally from slabs on grade but also have very different aesthetic requirements. For example, color is not an important consideration in structural concrete mix design, but it is in concrete countertop mix design.

Before we get into mix design considerations, note that you can either use a bagged mix specifically designed for concrete countertops, or mix your own from scratch. Our preference at CCI is for mixing your own, since Jeff is an engineer as well as a thrifty Yankee. However, there are pros and cons to both approaches. (More on that in another blog entry.) We do believe that even if you choose to use a bagged mix in your business, you should fully understand how concrete really works, so that you are better able to troubleshoot or adjust for your climate and working conditions.

What you need from a concrete countertop mix:

  • High early strength so you can process and finish faster
  • High flexural strength for greater crack resistance
  • Low shrinkage potential which minimizes curling
precast mix design

 

High Early (Compressive) Strength
Jeff is always saying, “Compressive strength is not as important as you think.” However, high early (compressive) strength is important early on when you need to get the concrete out of the forms, flip it over and start grinding it as soon as possible to get it into your client’s home. Concrete that develops high compressive strength quickly is going to be harder than concrete that develops strength more slowly. This means that the cement paste between the hard sand grains and aggregate will be harder, and the concrete can be ground and polished sooner.

High early strength is accomplished by using a low water to cement ratio, proper pozzolan loading and cement contents higher than construction grade concrete.

High Flexural Strength
Steel reinforcing is still essential, since the flexural strength of concrete is always much, much lower than the compressive strength. For example, the predicted value of flexural strength for ordinary construction concrete that has a very high compressive strength of 12,000 psi is only about 900 psi! But, if the flexural strength of your concrete is as high as possible, it is going to better withstand bending (flexural) forces along with the steel reinforcement, and show less cracking.

High flexural strength is achieved through both mix design and proper reinforcement. Steel reinforcing effectively boosts flexural strength values many times that of unreinforced concrete. GFRC uses a special mix design and high glass fiber loads that create high flexural strength.

Low Shrinkage Potential
Shrinkage can cause either cracking for restrained slabs or curling for unrestrained slabs. Shrinkage occurs when the cement paste dries out. Moisture evaporating from inside the concrete causes strong capillary suction forces in the cement paste that cause it to shrink. If the shrinkage forces are high enough, the concrete cracks. The underlying causes of this can be poor curing practices (allowing the concrete to dry out too soon before it’s strong enough to resist the suction forces), too much mix water, too much cement in the mix, or poor aggregate gradation that requires too much cement paste to achieve good workability.

Curling occurs when one face of a countertop shrinks more than the other side, and the result is that the countertop curls towards the side that shrank more. Curling can occur if one side of the slab remains wet and the other side is dry. Curling is a symptom of shrinkage. Concrete mixes that don’t exhibit significant amounts of shrinkage don’t curl much or at all.

Shrinkage reducing admixtures (SRA’s) are chemicals that reduce the suction forces generated during evaporation. This helps reduce the root cause of cracking and curling: the suction forces in the cement paste.

There are many different styles of concrete countertop mixes:

  • all-sand mixes designed to be stiff and hand packed
  • aggregate-based mixes designed for vibration or cast in place
  • polymer-based mixes that flow like pancake batter
  • GFRC mixes

Regardless of the style of mix, the basic principles discussed above apply.

stiff concrete countertop mix

Stiff mix, all sand

fluid concrete countertop mix

Flowable mix, aggregate based

 

More Details
Want to know more? Both of the above from-scratch mix designs, as well as a handy mix calculator, are included in our Precast Mix Design 101 self-study course. Questions about this article? Submit a comment below. And happy concrete mixing!

precast mix design2

Concrete Countertop Mixes: Stiff versus Fluid, part 2 of 2

This article concludes the discussion of stiff versus fluid concrete countertop mixes, with the final three differences.

Difference #3: Mix Design

Stiff Mixes

Stiff concrete mixes can be either all-sand mixes (no coarse aggregate) or coarse-aggregate based mixes. Generally they are all-sand mixes due to the ease of spreading and packing a fine-grained clay-like concrete. Coarse aggregate makes the mix “chunky”, often making hand-packing difficult or uncomfortable.

All-sand mixes have far more surface area, so require more cement paste. But the fine grained nature of sand inhibits movement due to the increased particle friction generated from many times more contact points. All-sand mixes are typically stiff, zero slump. It is possible to make anall-sand mix fluid with a combination of a powerful superplasticizer and strong vibration, but this is rarely done.

Fluid Mixes

Fluid concrete mixes are generally based on coarse aggregate.  Aggregate based mixes are more easily made fluid because of the large amount of coarse aggregate in the mix. Pound for pound, coarser aggregate has less surface area than fine sand. A mix that has less surface area requires less cement paste to coat the particles, and any excess paste in the mix acts as lubricant, allowing the particles to move past each other more easily.

Difference #4: Forming Techniques

Stiff Mixes

Because of the nature of stiff mixes, watertight forms are not necessary. As such, careful caulking of the seams isn’t required. If the form edges are caulked, it is because a cast rounded edge is desired instead of grinding a rounded edge. Forms should be tight, but the added step of caulking is not necessary.

Fluid Mixes

It’s a different story with fluid concrete. The forms must be watertight with fluid concrete because any leakage will result in discoloration of the concrete and loss of material.

Fluid concrete also requires that reinforcing steel be tied to the sides of the forms, since the steel will sink if simply placed in the form. However, it is very important not to pour the concrete over the steel. Doing so will result in ghosting. To prevent ghosting, either tie the steel after the forms are mostly filled with concrete, or very carefully pour the concrete through the holes between the steel, taking care not to pour onto the steel. Also, if the concrete is to be vibrated, the steel must be tied in place so that it does not move.

Comparison of Forms

Stiff mixes require less form work than fluid mixes when complex or 3 dimensional pieces are being cast. This is especially true for integral sinks cast into countertops. On the other hand ordinary flat countertop slabs use essentially the same forms for both stiff and fluid mixes.

form for fluid concrete countertop mix

Fluid concrete mixes need more complex forming for an integral sink.

form for stiff concrete countertop mix

Stiff mixes require less forming because the concrete is self supporting.

Difference #5: Casting Effort

Stiff Mixes

Stiff mixes take more effort to cast, and if care is not exercised in placement, air will get trapped between the concrete and the form. This results in large, shallow “craters” that often are on the order of 1/16”deep. The resulting surface is ugly and inconsistent in appearance, and it is difficult to repair by grouting.

craters in stiff cast concrete

Fluid Mixes

Fluid mixes are quick and easy to cast, as long as you take care with the reinforcing steel as noted in Difference #4: Forming Techniques.

Conclusion

As you can see, the choice of a stiff mix versus a fluid mix has many implications for all steps of the concrete countertop production process. Understanding the interplay of these implications is important for producing a high-quality concrete countertop.

Concrete Countertop Mixes: Stiff versus Fluid, part 1 of 2

There are several considerations to make when choosing a concrete countertop mix consistency. Some factors include the appearance of the finished piece, the forming and casting methods, and the complexity of the mold geometry.

Other factors that are not so obvious also drive the selection process. These can include the type of mixer that will be used, how the forms are assembled, the shrinkage tendency of the concrete and subsequent vulnerability or resistance to curling and hairline shrinkage cracks.

This article outlines the basic differences between two mix styles, stiff and fluid, in design, look and technique.

There are two basic types of mix consistencies: stiff and fluid.

A stiff mix is a versatile, all-purpose mix that can beadapted to a wide variety of looks, from a “solid” appearance, to a variegated/veined, or a terrazzo look. A stiff mix is typically made using only sand as an aggregate, and it has zero slump.

zero slump concrete countertop mix

A fluid mix is a high slump mix that is more conventional in its ingredients, since it uses both fine and coarse aggregates. While coarse-aggregate based mixes are easy to make fluid, it’s vital that the fluidity is achieved by using superplasticizers and not by adding water. Concrete countertops require high quality concrete for aesthetic and long term performance, and using water to enhance workability instead of superplasticizers results in weak concrete that is prone to shrinkage, curling and cracking. Of course being disciplined with mix water applies to all types of concrete, but fluid concrete is more susceptible to water abuse.

flowable concrete countertop mix

 

Difference #1: The Look

Stiff Mixes

Stiff concrete mixes will always have voids in the cast surface. This is because the cement paste (and the mix as a whole) is so stiff that the entrapped air bubbles cannot escape. Often larger voids are irregularly shaped, and this tends to lend a more organic, stone-like look to the concrete.

pressed concrete stool

small round sink in concrete countertop

copper embedded in concrete countertop

variegated integral sink concrete countertop

The fact that the concrete is stiff and will always have air pockets can be exploited by purposefully introducing voids and patterned fissures. The resulting look resembles the veins in natural stone. Sometimes the voids are left open, but more often they are filled with contrasting cement grout. This highlights the voids and reinforces the resemblance to natural stone.

Fluid Mixes

Fluid concrete creates castings with very high surface quality. Edges are crisp and precise. Surface color and visual texture is smooth and even.

fluid mix pinholes

ramp sink in concrete countertop

Fluid concrete mixes can sometimes have a few small pinholes, or if the casting is done carefully, no pinholes at all. Concrete can be made fluid by using an appropriate mix design and superplasticizer, or astiff mix can be made fluid by vibration. Either way the cast surface will very closely reflect the surface characteristics of the mold. If the concrete was cast against smooth, glossy plastic then the virgin cast surface will be shiny and almost glass-smooth.

Difference #2: Casting Techniques

Stiff Mixes

Stiff concrete is placed by hand, often in small handfuls.The stiff nature of the mix allows the concrete to be packed onto vertical forms and place in thin layers.

hand packing sink form

hand packing in form

Fissured concrete, sometimes called variegated or hand-pressed concrete, is placed so that air gaps between areas of concrete result in fissures once the concrete is demolded.

variegated round table

Fluid Mixes

Fluid concrete is often poured into forms, but it can also be placed by hand.

describe the image

casting fluid concrete countertop mix

 

The next article will explore three more differences between stiff and fluid concrete countertop mixes.

Types of integral pigments used in concrete countertops

Concrete can be colored in a variety of ways, including using acid staining, dyes, colored aggregates, cement and integral pigments. This article explains the various types on integral pigments commonly used in concrete countertops.

Integral pigments are very finely ground inert particles that are mixed into concrete to change its color. Integral pigments can be dry powders or liquids, which really are liquid suspensions of solid particles (much like paint). Pigments are dosed like other admixture ingredients, based on the total cementitous material weight. Dosages can range from 0.1% to 10% (the maximum recommended dosage), but typically the range is narrower, from 0.5% to 5%.

Concrete is a harsh environment for pigments. It is wet, very alkaline and can be exposed to heat and ultraviolet light (from sunlight). This harsh environment can break down some pigments that cannot resist such attacks. This is why it is very important to use pigments designed for use in concrete. A vibrant blue pigment intended for the paint industry may not last long in concrete unless it can completely resist the concrete.

Pigments come in a wide range of colors.

  • Basic iron oxide pigments are the most widely used and are readily available, and they tend to be inexpensive. These are the basic “earth tone” colors: browns, reds, blacks and dirty yellows. Prices generally range from $2 to $6 per pound.
  • Special metal oxides provide other colors: some iron oxides are purply red, blue or yellow; chromium oxide is green, and titanium oxide is bright white. These are generally a bit more expensive, ranging from about $4 to $20 per pound, and true cobalt blue is much more expensive.
  • Synthetic pigments are generally even more expensive but provide the vibrant colors unobtainable from metal oxides. Brilliant violet, pure red and canary yellow are all available. Less expensive versions of expensive natural pigments (like cobalt blue) can make even some colors affordable. Synthetic pigments tend to be more powerful than iron oxides, so less is needed to get the full depth of color. Prices range from $20 to $75 per pound.

Pigments for concrete countertops

Powdered pigments are often sold in bags or cans, and can either be loose or granulated. Granulated pigments are marketed as “dustless” because the fine pigment particles are bound together with a water soluble binder. Granulated pigments are designed for use in mixer trucks where the coarse aggregate and lengthy, vigorous mixing action break the particles up. Some concrete countertop fabricators have noticed inefficient mixing and color streaking because the granules don’t completely break up.

Liquid pigments are solid pigment particles suspended in a liquid. The liquid helps to keep the pigment suspended for the duration of the measuring and dosing. Liquid pigments should be well mixed before dosing, because the pigment particles settle out. Pigment concentrations vary depending on the color or manufacturer.

Lastly, some pigments, especially some liquid and synthetic pigments, have water reducing or even superplasticizing characteristics. And some also have a mild retarding affect. It is important to try out a new pigment to see how it affects the concrete before using it for a paying client’s project.

Last updated by at .