Placing the Concrete Sink

One thing to remember out pouring a counter countertop is that your mold should be properly supported and level. A couple sheets of plywood over some sturdy sawhorses will do the trick. I built a rolling table with leveling adjusters.

Alright, double check that all of the materials are weighed out and ready to go. I have a mixer, but if you don't they are cheap to rent at the local hardware store.

*Important note: The mixture should be large enough for the batch of concrete you want to make. Mine was a little too small and I paid for it with poor mixing. I ended up adding more water and water reducing admixture to get things mixed.
The concrete was placed into the forms by hand being sure to press the first layer in well. That prevents air pockets on what will be the surface. A pad sander without sandpaper works well to vibrate the sides.
Since the concrete did not get mixed well, we had to pick out the clumps. Although you can not see them in the final piece, poor mixing lowers the strength and makes cracking more likely.

The foam knockouts can float up in the concrete. Some scrap wood and weights kept them in place. Concrete countertop mixtures are much stickier than regular sidewalk concrete. This is from the water reducing admixtures and the fibers. While these add strength, you must make sure the concrete is consolidated around all of the corners in the piece otherwise it will not look like what you designed.

After placing the concrete got covered with damp towels to provide moisture during initial hydration.

Then everything got wrapped in plastic and hung out for one day. The longer the concrete stays in the mold, the stronger it will be. One to four days is typical to let it cure before taking off the forms and starting the finishing process.

Concrete Countertop Mixtures

There are several options for actually making the concrete for your concrete countertops.
1) Prepackage countertop mix - Buy all of the ingredients preweighed out, all you do is mix. Buddy Rhodes, Encounter, quikcrete, and regional suppliers all provide these mixtures.
2) Modified packaged mix - Buy regular outside sackcrete dry concrete and add colors and water reducers to make it work for countertops. Cheng Pro-formula is a prepackaged mixture you mix with regular 5000psi quikcrete to make a countertop. His mixture contains water reducing admixture, accelerating admixture, fibers, and color.
3) Your own recipe - With a little knowledge you can make your own.

The important aspects are workability (to get it in the forms), strength (to get it out of the forms and installed), low shrinkage (to prevent unwanted cracks), and low air content (to give a nice surface).

For the sink I developed my own mixture. Since it had very intricate shapes I replaced the coarse aggregate (gravel) with the same volume of sand. So the sink is actually mortar and not concrete, but for most people it is a fine line. Here is the testing summary for about 15 mixes I did. To speed up the process I was placing most of the iterations based on 1-day strength. *Remember concrete gets hard from a chemical reaction process called hydration. This process goes on forever. However, the majority of strength is gained in the first 28-days. Since the goal was to remove forms after 24 hrs, that's when I tested strength.

Testing Summary of 15 mixes:
Mixture 1 was standard Quikcrete proportions with sand instead of gravel, which contains about 540lbs of cement/cubic yard of concrete (pcy). The water-to-cement ratio (w/c) was 0.45. Lower water-to-cement generally means higher strength. I also included a deair-entraining admixture to make sure the surface didn't get holes.
1-day strength 2361 psi (pounds per square inch)
4-day strength 3393 psi
7-day strength 4570 psi (typical concrete is 4000 psi at 28-days so this is pretty good)

Mixture 2 took mixture 1 and increased the cement to 900 pcy, while lowering to w/c 0.30 with a high-range water reducing admixture (HRWR). It takes some special chemicals to lower the w/c while still maintaining workability.
1-day strength 7484 psi
4-day 8457 psi
7-day 10,582 psi (high strength concrete)

Mixture 3 maintained 900 lbs total cementitous, but substituted 15% metakaolin clay for cement. MKC is a fine white processed clay that lowers the permeability and lightens the color. This mixture also included a latex polymer to lower shrinkage and increase tensile strength. The flexural strength was improved by adding polypropylene fibers (think Easter grass) and cellulose fibers (think paper mache).
1-day strength 6391 psi
1-day tensile strength 793 psi

The ultimate mixture had good workability and strength with low permeability and shrinkage. Platinum color was added to give the sink a brighter concrete color.

Sink Inspiration

Every project has to have some inspiration to start. The following are a collection of pictures borrowed from various internet sites (Buddy Rhodes, Cheng) that inspired my design for the seemingly strange looking concrete sink.

This vanity was the model for the one I built under the sink, from Kerf. Our vanity originally started floating from the wall. After handling the 4-inch concrete sink, the resident structural engineer strongly suggested I provide some legs. Our vanity turned out very similar to the far one in the picture, six drawers that will be finished shortly with walnut panels.

A two faucet concrete sink draining to a single drain. Due to space constraints and storage issues, we needed two faucets but could only fit in one drain.

Since the sink will be pretty much all sink and no countertop, this sliding piece over the trough inspired me to try and integrate some removable flat countertop sections.

I originally thought about having solid top pieces, the slotted top in this picture changed my mind. Our drawer fronts will be walnut and the removable top will have several slotted pieces for extra flat space.

Concrete Mold Construction

We love concrete because it perfectly reproduces the form you put it in. However, that means the mold needs to be solid and smooth because grinding hardened concrete is much more difficult than fixing a poorly fitting mold. Below is a concrete design created by raised sections attached to the forms at the Portland Cement Association (PCA) headquarters.

Things to think about when putting together a mold.

1. Mold material should be rigid - When the mold (and whatever you have the mold sitting on) is constructed it needs to support the weight of the fresh concrete. If the selected mold material is not rigid enough there is a risk it may deflect when full of concrete and create a wavy surface. At standard weight concrete weight of 150 lbs/cubic foot and a 1 foot by 1 foot section that is 2 inches thick will weigh 25 lbs.

2. Mold surface should be smooth - Since the concrete will reproduce any imperfections the inside of the mold should be as smooth as possible. This is best done by using a laminated particle board or rigid plastic that have uniform surface texture and smoothness.

3. Mold materials should be easy to work with - Ideally special tools should not be required to create the mold. Particle board or plastic molds can be easily built using a circular saw and drill.

4. Material should be impermeable - Concrete contains water, when wood absorbs water it swells. Egyptians would place a small wood wedge in a stone crack then pour water over the wood. The small piece would swell and crack the large stone pieces. Concrete shrinks when it cures and wood swells when wet so to prevent unwanted cracks, make sure any wood in the mold is sealed. This is especially important if you have a wooden section inside your concrete such as a sink opening.

5. Mold should be designed so it can be disassembled when full of concrete - Remember, the mold will be filled with concrete. Construction of the mold should be done in reverse to allow deconstruction once the concrete is hard.

Commonly used materials include:

- Melamine (laminated particle board available at Home Depot or any other home improvement store),

- Plywood or particle board painted with exterior paint to prevent moisture penetration, or

- Extruded rigid plastic sections fixed to a rigid surface with double-stick tape or screws(available through the Cheng concrete on-line store)

Unless you are building a free-standing piece such as a table, a template will have to be made so the sink or countertop fits in the space. Walls are never straight and this prevents unwanted gaps between the wall and concrete piece. The template can be pretty much anything, plywood, particle board, cardboard, or thick plastic. The template below for the sink was 1/2-inch plywood strips glued together. Be sure to label the front, back, sides, and locations of any knockouts needed for faucets or sinks. The template will be inverted for mold construction, keep track of which direction is the front to prevent misplacing the openings. In this picture the bath vanity will have two sets of faucets with handles and one central drain. Later the drain was moved from the center to the right side.

A little note on design, start with something easy first THEN get more complicated. For this sink I tried incorporating many complicated aspects at once. It turned out well and will provide good learning experience for future project.

This mold construction is a pretty standard technique used by many concrete countertop makers. The Cheng concrete countertop book is a good reference for mold design and construction.

Here is a beginning of mold construction using 3/4-inch melamine particle board. The dimensions are roughly 42-inches long, 23-inches wide, and 4-inches deep. The sides are screwed to the bottom piece and into the adjacent side pieces. This sink bottom will be flat and slope to the single drain now located on the right side. So in this picture the far side will be the front and the near side the back. The piece on the right will be the low point of the sink where the drain is located.
The nearly completed mold. The sink bottom slopes to the right side. Wooden dowels wrapped in clear tape are screwed from the underside to make the space for the faucet and handles. The larger area for the faucet connections was made by wrapping 1/4-inch foam around the dowel and covering with clear tape. Home improvement stores have a wide variety of foam that work well for concrete projects.

Any exposed wood or screw heads get covered with clear tape. Joints and any wood to wood connections get sealed with silicone caulk to prevent moisture soaking into the wood. A dark colored silicone is easy to see on the white melamine.

Notice there is no rebar or wire reinforcement in this particular piece. Concrete is it's biggest volume when fresh and shrinks when it cures. Therefore concrete cracks. Cracking can be controlled by using a good concrete recipe and limiting the size of our pieces. The main purpose of reinforcement in concrete is to provide strength should the concrete crack. So for large concrete pieces or those with cantilevers reinforcement is good, for little pieces with good support reinforcement is not needed.

Concrete 101

Since I am a professor I can't help but include a little education before we start on a project.

Although concrete is the most used material on earth other than water, most people even many of those in the concrete industry do not understand what it is.

- Concrete is a combination of Portland Cement, water, fine aggregate (sand), and coarse aggregate (gravel).

- Portland Cement plus water forms the glue that makes concrete hard through a chemical reaction process called hydration.

This figure shows the approximate volume of each component in concrete, provided by the Portland Cement Association (PCA).

We often include other cementitous materials in our concrete to improve the chemical reaction process. Many times these supplementary cementitious materials (SCMs) are by-products of other processes such as coal, steel, and silicon production. Other SCMS may include certain clays, volcanic ashes, and even residual ash from burning certain plants.

For the purpose of concrete countertops we are concerned with a few aspects important to producing a nice looking project that will lasts a long time.

1. Fresh concrete workability - The fresh concrete must be fluid enough to allow the concrete to fully consolidate into our mold and not have any air gaps or other unwanted voids. Too much water and the concrete will be weak and the gravel may settle out. Too little water and we can't get the concrete to take the shape of the mold. We will achieve a balance using a good recipe (mixture proportions) with help from some concrete chemical admixtures such as water reducers.

2. Strength - It has to be strong enough for us the take off the forms, grind or polish, and transport to the final location. The finished countertop also has to be strong enough for normal uses (i.e. people leaning, little kids walking, big pots of food). Our strength comes from the proper amount of cement and water and the use of certain SCMs that improve the chemistry of the hardened concrete.

3. Permeability - You may not think concrete is permeable, notice a sidewalk when it rains. Afterward the surface may be dry, but the concrete is still dark from water absorbed into the surface. This infiltration of water is generally bad for concrete because it transports bad stuff into the concrete (like salt or toothpaste) and leaches out some of our good stuff (like our color). For countertop purposes we will limit the rate water soaks into our concrete so the surface doesn't look splotchy when someone sets a sweaty glass or toothbrush on the surface. This will be accomplished through our concrete recipe and by the application of a sealer.

4. Other secondary aspects

- Color - Color comes first from our cementitious materials and then from our aggregates through grinding. Typically cement is grey so concrete is grey. If we want to produce a color first the grey must be overcome. However, we can also get white cement to let us make more vibrant colors. If the concrete is ground, aggregates are exposed. If we seed the mold with bright colored stones before placing the concrete then those can be ground and exposed. Color can also be applied by acid stains after the concrete has hardened.

- Weight - Concrete is heavy right? Concrete weight is a combination of the items in the recipe. Regular concrete weighs around 150 lbs/cubic foot because the rocks are heavy. However we can make canoes out of concrete that floats because the aggregate is lightweight. We will make some concrete benches and patio furniture out of lightweight aggregate so it is easy to move around.

Next, the first project a concrete bathroom sink.

Let's Get Rolling

This blog will detail my experiences making concrete items for the home, including concrete countertops, sinks, and other items.

I have a testing lab available along with all of the latest chemical admixtures and concrete components and will post updates on the evolution of my concrete mixtures.
Although I have lots of experience with concrete, my first attempt was a sink for my bathroom remodel.

The upcoming to do list includes:
- a concrete fireplace surround, black dry polished in-place
- a concrete desk for the office, white with 100% crushed glass aggregate
- some patio furniture, hopefully produced with lightweight aggregate

What is pervious concrete? Pervious concrete is concrete designed with interconnected voids so water goes through it without running off. It is used to reduce stormwater produced in urban areas, recharge groundwater, reduce the urban heat island, and creates quieter safer pavements. Typical infiltration rate is 500-2,000 inches per hour. Compared to making this stuff, countertops are a breeze.

If anyone has suggestions on projects or topics they would like to see, let me know.