What is GFRC?

Glass Fiber Reinforced Concrete (gfrc) is just that.: Glass fibers are used to reinforce the concrete in lieu of or in addition to metal.  Generally GFRC project are constructed in a layered approach, similar to the Buddy Rhodes technique.

Most times the fibers are used in the back-up mix. Generally,  you want to have the finished surface free of the fibers so they don’t show in a finished piece.  For some reason GFRC has gotten the reputation that it is only sprayed. Not true. The key element in GFRC is that the fibers are used in the back up coat which is hand placed and not sprayed. The face coalt can be sprayed to produce what we call the ‘machined’ finish, but you cal also press, pack or apply the fiber-free face coat any way you wish. More sophisticated spray systems ($3,000 +) will be able to push out the fibers. Most hopper guns ($125.) you can only push out the fine concrete mix.

How is GFRC different from wet cast?

With a traditional wet cast project you have to over build the mold so the wet concrete will not blow out the mold. Wet cast has large aggregate and is a wet mix. The GFRC method is more of a gradual technique; first the face coat and then a backup fibered mix is laid up by hand. This is very similar to the standard technique pioneered by Buddy Rhodes, and how project made with his Mix are currently constructed. You do not need to “hang” the reinforcement within the mold and pour around it. With Buddy’s methods, whether GFRC or not, you construct the layers. Therefore, with a glass fiber reinforcement you do not have to cast your projects 3-4” thick as with a wet cast. The glass fiber reinforced concrete can be much thinner with makes for larger castings. The glass fibers also give the pieces much more strength and flexibility; this higher strength can broaden your range of design.

One of the BIG differences between wet cast and Buddy’s methods have been the one-day mold time. You cast it today and de-mold it tomorrow. You can also do this with GFRC methods. With wet cast you have to wait three or more days before you can de-mold. The thicknesses of the slabs are greatly reduced from 3-4” to 1-1.5”, reducing the material costs and weight and ease of installation.

General Technique and Recipes for using Glass Fiber Reinforcement in a concrete project

Sometimes when the glass fibers are used in the face or finished surface they will show after polishing or troweling. We like to use the reinforcing glass fibers in the backing of the concrete project.

  • Pressed Finish - This is like a pastry dough which is packed into a mold about 1/2" thick and then backup with the Back-Up Fibered Mix to about 1"- 1 1/2" thick.
  • Hard Trowel Finish - This is when the project is cast right side up using a steel trowel to trowel flat and to add the creamy burnished trowel marks. The Back - Up Fibered mix goes in first holding shy of the top and finished edges. Then a wetter self consolidating mix is poured over the fibered backup and troweld smooth.
  • Wet Cast or Vibrated Finish - This method is made by pouring in the fibered Back-up mix into the mold and vibrated to allow the air bubbles to come to the surface.Use “Back-up mix with fibers receipe”.This finish is lightly polished so as to not reveal the fibers.
  • Spray Finish Method - This is when the concrete is sprayed out of a hopper gun. This is a very thin layer and then it is backed up with the fibered Back-Up Mix, combined total thickness is about 1" - 1 1/2".

Measure by weight rather than volume. Add the water, curing agent, water reducer and pigment to mix bucket. Blend well and fast to disperse any solids. Pour the cry concrete mix and mix well. Slow down mixer speed and add glass fibers. Mix gently without breaking the fibers. As a general rule, add 5% of the concentrated Curing Polymer to the weight of the cement in your mix design. Precautions: Protect from freezing. Read MSDS before use.

Recipes for Face Mixes

I. Pressed Face Mix — Face Down

The pressed technique is made by mixing the concrete dry and clay like. Pack the inside of a clean smooth mold like pasty dough about 1/2” – 3/4” thick. Then a back up mix* with fibers is poured over the existing “pressed”layer. Total thickness is 1” - 1 1/2”.

Recipe for two BRCC Mix bags

16 lbs Water
5 lbs Curing Agent
1 – Can Pigment (2 lbs)

Mix liquids and pigment together and then add:
Two BRCCMix Bags (140 lbs) 70 lbs each bag
Add water if needed for a dry clay like feel.
Pack / press the concrete about 1/2“ thick then back fill with the Back–up mix*. Around sink openings and extra long tops − lay additional reinforcement (Ladur Wire) in the bottom third of the slab.


II. Hard Trowel Finish — Face Up

The Hard Trowel finish is made face up with a hard trowel is the finish shows the hand of the craftsman and the cream that was worked on the surface becomes part of the burnished look. With a face up finish the back-up mix* with AR Glass Fibers is put in first holding shy of the finished top about a 1/2” and any finished edges. A self-consolidating mix is poured over the fibered back up layer and is troweled smooth.


Recipe for two BRCC Mix bags

17 lbs Water
5 lbs Curing Agent
6 OZ Water Reducer
1 Can Pigment (2lbs)

Mix liquids and pigment together and then add:
Two BRCCMix Bags (140 lbs) 70 lbs each bag.

III. Spray Method Face Mix – No Fibers

In order to spray the concrete mix out of a drywall hopper gun you have to drop out the larger stones from the BRCCMix using a 1/16” window screen. Since the spray face coat is very thin – 35 pounds of Mix will go a long way.


Recipe (can be adjusted proportionally by weight)

35 lbs – 1/2 bag BRCCMix (it will yield 28lb after sifting)
4 lbs Water
1.25 lb Curing Agent
1.25 OZ Water Reducer
1/4 lb Pigment

Mix the liquids and color pigment in a five-gallon bucket first. Mix well. Then add the dry concrete powder. Mix for 1 minute or so then allow to rest for 2 to 3 minutes. If needed, add a small amount of water and mix again. Consistancy should be similar to a milkshake. The spray face coat is very thin about 1/4” thick using two 1/8” spray coats.

Spray towards you to blow out any over spray and sand that will get caught in the corners.Then make up the Back-up Mix with fibers and spread over the sprayed layer to a 1” – 1 1/2” thick.



IV. Back - Up Mix with Fibers

The back up mix will generally not be seen in the finished product. This is where the fibers go and where the Scrim can be layered.

Recipe for two BRCC Mix bags

17 lbs Water
5 lbs Curing Agent
6 OZ Water Reducer
1 Can pigment (2lbs)
Mix liquids and Pigment together and then add:
Two BRCCMix Bags (140 lbs) 70 lb each bag.

Mix well.
Then slow down the mixer and slowly add:
2.5lbs AR Glass Fibers

 

 

COLOR COMBINATIONS


Often we get requests for popular and successful color combinations to use with the pressed technique. The first color in each of the following suggestions is the color of the concrete matrix. The color(s) following are for the added paste. Tone on tone combinations were the most popular, especially these:

  • Universe/Universe 
  • Moss/Moss 
  • Coal/Coal 

The colors listed below are popular and the tones have proven to work well for us here at the studio.

  • Bone concrete color with Bone paste infill
  • Bone concrete color with Sand/Bone paste infill
  • Sand Concrete Color with Bone paste infill
  • Sand concrete color with Sand paste infill
  • Moss concrete color with Moss paste infill
  • Moss concrete color with Sand/Mushroom and Ash paste infill
  • Wheat concrete color with Sand/Mushroom and dark Wheat paste infill
  • Universe concrete color with Universe paste infill
  • Ash concrete color with Bone/Coal paste infill
  • Ash concrete color with Coal paste infill
  • Ash concrete color with Ash paste infill
  • Slate concrete color with Sky/Slate paste infill
  • Chocolate concrete color with Chocolate paste infill
  • Straw concrete color with Sand/mushroom and Chocolate paste infill
  • Straw concrete color with Straw paste infill.

 

 

WATER QUANTITY

Your altitude and humidity affect the amount of water needed in Buddy Rhodes Concrete Products mix. The less water you can get away with the better. Buddy has been using 4.5 quarts water for pressed and 5 quarts or less for trowelled in San Francisco. Buddy Rhodes Concrete Products Concrete Mix is a drier dough-like material when wet. Too much water can produce a concrete mix that is too wet and cracking is a possible result. Minimizing the water reduces the possibility of shrinkage and ensures the very best slab. LESS WATER IS BETTER!

 

 

CAST-IN-PLACE REINFORCEMENT

15-20 lbs (2 shovels) of 3/8” pea gravel and 1/4 lb. fibers per bag of concrete mix produces 9,000 psi after 56 days and lessens the shrinkage and warping to which concrete is naturally susceptible. Adjust wire reinforcement when using pea gravel: 4x4 welded wire or Durowall ladder wire will work. PLACE ANY REINFORCEMENT TOWARDS THE LOWER HALF OF THE SLAB FOR OPTIMAL WARP RESISTANCE.

Do not use pea gravel when making the upside-down pressed finish or when working up a vertical wall.

 

 

POUR AND VIBRATE

Material List:

  • Buddy Rhodes Concrete Mix
  • Buddy Rhodes Color
  • Buddy Rhodes Acrylic Additive
  • Fibers
  • Pea Gravel
  • Buddy Rhodes Superplasticizer
  • 2x Vibco Vibrators with mounting plates and variable speed controller www.vibco.com 
  • Model US-450T Vibrator, UMC-1 mounting plate, SPC control box
  • 4” Welded wire remesh, 8 gauge or 6”x6” wire mesh doubled up. 
  • Black GE Silicone II caulk and caulk gun
  • Bucket and mixer

Mold Making:

Prepare your mold as normal but silicone the edges as the mix will liquefy when vibrated. Suspend your welded wire mesh within the mold. If 4” welded wire mesh is not available, use 6” remesh overlapped to create 3” squares.Silicone your corners with GE Silicone II. Tip: Run a bead of silicone in the corners and use the back end of a 3/16” drill bit to get a nice roundover corner.

Mix and Mix Consistency:

  • Sample mix using 2 bags of BRCM
  • 2 Bags BRCM
  • 1 bottle BR Color mix
  • 30lbs pea gravel. (optional)
  • 10oz BR Superplasticizer per two bags of BR concrete mix.
  • 10 quarts liquid, either 50/50 water/acrylic, add more water to get desired consistency, up to 11 quarts
  • 1.5lb of AR Glass Fibers.

Combine the BRCM mix and fibers dry. Start by pouring 10 quarts of 50/50 water/acrylic liquid in the mix. Add the Plasticizer and mix. Continue to add small amounts of liquid until you achieve a ‘Malt 0 Meal’ consistency. Do not go over 11 quarts.Start pouring onto your mold without vibrating, once you have a thin layer covering the surface of your mold start your vibrating at a low frequency. Once you’ve filled the surface adjust your vibrating speed high and low to move the bubbles to the top.

Turn off the vibrator, cut off the suspension wire close to the mesh and fill the void and finish off by trowelling the surface.

Tip: You can check if it’s the right consistency by making a large mound in the bucket and tapping/lightly kicking the side of the bucket, the mix should lay down.  Or have a blank piece of melamine on your vibrating table and place a pile of concrete on the blank and vibrate.  If it flows down, you’ve achieved the right mix consistency.

 

 

MAINTENANCE

Buddy recommends that concrete countertops be sealed with our penetrating sealer, then our satin sealer and finally with our beeswax. Following these three steps will provide the highest level of protection against stains and will keep the concrete surface looking great. It is best to rewax once a month or every two months to maintain a protective finish.

Cleaning agents should be restricted to soap and water, and non-abrasive cleaners, such as diluted Simple Green.

When using knives for food preparation, please cut on a cutting board rather than on the concrete countertop. Knives cut into the sealed surfaces and cause scratched areas that later can attract stains.

Refrain from placing hot pans on the sealed concrete surfaces as they may cause scorch marks.

Avoid keeping items such as wet sponges, terracotta pots or dish drainers directly on the counter as they may darken or stain the finish.

For best results, clean up spills, such as oils and acidic liquids, quickly. Allowing them to linger may cause them to soak into the surface and etch the concrete countertop.

Time and daily usage produces a unique surface patina that enhances the character of the concrete. Hairline cracks sometimes occur but this does not affect the structural integrity of the concrete.

 

 

COLD WEATHER TIPS:

Those of you mixing concrete in cold winters should know that the concrete reaction is temperature sensitive.  Temperatures below 55 degrees will retard or completely stifle the curing process.  That means special care should be taken to maintain a room temperature of approximately 60 degrees and to use room temperature or warmed mix water.   Conversely, cool water can be used in the summer to slow the curing process and increase the time that the concrete is workable. Temperature affects the sealing process too so take heed to the following tips.

Mixing:

 Mix the cement with warm water (above 60 degrees).
Cold water will retard the setting of the cement. In the summer, mix the cement with cold water to extend the workability of the mix so it doesn't set up too fast.

Sealing:

The Satin Sealer will not set or become hard if applied to a cold and damp surface. If need be, place a electric blanket over the projects pieces to warm them up (60-70 degrees) before sealing. The sealer will dust off if applied to a cold surface.

 

 

TOOLS AND MATERIALS:

People often ask me what they will need for their pre-casting shops, and where to get supplies. Here is a list of tools and materials we use, and a few web sites to help you find products.

Web Sites:

Hand-held Concrete Tools in Bon Masonry Tool catalog- http://www.bontool.com

  • Here are tools we use for troweling concrete:
  • Square End Laminated Canvas-Resin Float -  #22-468-B10  14”, #12-688-B10  18”
  • Wood Darby  #82-106-B10   
  • Stainless Steel Swimming Pool Trowel #12-310-B8
  • Midget Trowel #12-194-B9

 

You should be able to find the following supplies at your Buddy Rhodes distributor or at your local contractor supply outlet:

  • Release agent - WR Meadows Durogard II Luan
  • Red Vinyl Tape
  • Melamine sheets 4'x8'
  • Styrofoam 1 ½" thick
  • Diamond tools, 
  • Brick stone, 
  • Mix bucket,
  • Rubber gloves, 
  • Scotch brite pads

Basic shop tools we use include:

  • Drill press, counter sink bit,
  • Chop saw, 
  • Table saw,
  • Level,
  • Wire cutters 
  • Screw gun, screws,
  • Hot glue, 
  • Band saw, 
  • Table sander,
  • Putty knife,
  • Tape measure,
  • Vibrator 


Following these suggestions will provide a beautiful concrete surface for years to come.

 

 

SAFETY TIPS

Making Concrete Art can be dirty, heavy, and often dangerous work. Your and your team’s eyes, ears, back, lungs, fingers, toes, and even the environment are exposed and susceptible to a multitude of assaults. We’ve had our fair share of mistakes, close-calls, and oversights in our shop, so we hope some of these tips will help you stay a little safer as you practice your trade. 

  1. Survey your shop – Is everything in the proper place?  Flammables stowed?  Scrap pieces out of the way?  Heavy tools, projects, and liquids secured? Safety gear (fire extinguishers, safety glasses, boots and gloves) where you’ll need it?
  2. Wear dust masks, gloves, goggles, and a long sleeved t-shirt when casting – Concrete is caustic and corrosive.  It contains extremely fine particles that will infiltrate and irritate.  Your best defense is to cover-up tightly.   
  3. Be wary of anything that spins – There is an incredible amount of energy at work when tools rotate.  Mixers, Polishers, Drill Presses, Table and Chop Saws have an uncanny knack to grab loose clothing, hair and jewelry and instantly pull the wearer right into harms way.  Also, spinning tools can cast off flying debris.  Wear your safety glasses.
  4. Water polishing can leave a wet floor that conducts electricity.  Ground Fault Circuit Interrupters (GFCI) in outlets and on tools can reduce the chance of electric shock.  Also, rubber boots and gloves will offer some protection while keeping your feet and hands dry.  
  5. Gray water from polishing counters and rinsing equipment is, to make a pun, a gray area.  It is likely not good for sewer systems even if nobody notices.  Currently, we attempt to trap and contain our gray water. If we can get it into a bucket, we let it settle for a number of days and then carefully siphon the now clear water into the drain.  The sludge goes into the dumpster.  If the water is on the ground, we let it evaporate, then sweep or shovel the remnants into the waste bin.  Clearly, this is not the perfect solution, and we are working with a filter/tool company to design a small self-contained water recycling system.  Stay tuned.



In a very real way your safety is critical to our business.

Shop ABCs mean Always Be Careful.

Here is our wish for a safe and successful concrete business.