How to eliminate pinholes in concrete

 

How to Eliminate Pinholes in Concrete (Complete Guide)

Pinholes in concrete are one of the most common and frustrating issues in decorative concrete, GFRC, and precast work. These small surface voids can ruin an otherwise perfect piece, especially in high-end applications like countertops, panels, and furniture.

The good news is that pinholes are preventable. With the right mix design, materials, and techniques, you can consistently produce smooth, dense, pinhole-free concrete.

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What Causes Pinholes in Concrete?

Pinholes are typically caused by air trapped at the surface of the mold during casting. Several factors contribute to this:

• Poor mix flow or low workability
• Inadequate vibration or consolidation
• Surface tension preventing air from escaping
• Improper mold release or mold surface
• Incorrect aggregate grading
• Lack of proper admixtures

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1. Improve Mix Flow and Workability

Concrete that doesn’t flow properly will trap air against the mold surface.

To improve flow:

• Use a high-quality plasticizer or superplasticizer
• Avoid adding excess water (this weakens the mix)
• Use well-graded fine aggregates

Using plasticizers such as Optimum or P-120 allows you to increase flow without sacrificing strength.

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2. Use the Right Admixtures

Admixtures play a critical role in reducing pinholes by improving density and reducing surface tension.

• Improve wetting and dispersion
• Reduce trapped air at the mold surface
• Increase overall mix performance

A high-performance system like Super Six Admix is specifically designed to help produce dense, pinhole-free surfaces.

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3. Optimize Your Mix Design

A poorly balanced mix is one of the biggest contributors to pinholes.

Key considerations:

• Use a broad particle size distribution
• Avoid oversized aggregate in thin sections
• Maintain consistent batching

High-performance mixes such as HP50 or Ultracast 360 are engineered for flow, density, and surface quality, helping reduce surface defects.

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4. Proper Mold Preparation

Your mold surface directly affects the finish.

• Use smooth, non-porous mold materials
• Apply mold release evenly
• Avoid contamination or residue

Even the best mix cannot overcome a poorly prepared mold.

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5. Use Proper Placement and Consolidation

Air needs a path to escape.

• Light vibration or tapping helps release trapped air
• Avoid over-vibrating
• For GFRC, ensure proper face coat application

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6. Control Surface Tension

Surface tension is often the hidden cause of pinholes.

Advanced mixes and admixtures reduce surface tension, improve wetting, and allow air to escape more easily—resulting in smoother finishes.

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7. Use High-Performance Materials

Using materials designed for decorative and high-performance concrete makes a major difference:

• KongKrete Acrylic Polymer improves cohesion, strength, and workability
• AR Glass Fiber enhances structure in GFRC applications
• High-performance admixtures improve density and surface finish

When these materials are combined correctly, pinholes are dramatically reduced or eliminated.

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Common Mistakes That Cause Pinholes

• Adding too much water instead of using plasticizers
• Using poorly graded sand
• Skipping admixtures
• Inconsistent mixing
• Poor mold preparation

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How to Get Consistent, Pinhole-Free Results

To consistently eliminate pinholes:

• Use a well-designed high-performance mix
• Incorporate proper admixtures and plasticizers
• Maintain consistent batching and mixing
• Prepare molds carefully

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Final Thoughts

Pinholes are not just a cosmetic issue—they are a sign that your mix or process can be improved. By focusing on flow, mix design, materials, and technique, you can eliminate pinholes and produce higher-quality concrete every time.

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Products That Help Eliminate Pinholes

• Super Six Admix – improves density and surface quality
• Optimum – increases flow without adding water
• P-120 – high-performance superplasticizer
• KongKrete Acrylic Polymer – improves cohesion and strength
• HP50 – consistent high-performance base mix
• Ultracast 360 – advanced mix for dense, detailed castings
• AR Glass Fiber – reinforcement for GFRC systems

• #admixtures
 
• #concrete countertops
 
• #concrete problems
 
• #GFRC
 
• #mix design
 
• #precast concrete

Mastering Cold Weather Concrete Casting

 

Mastering Cold Weather Concrete Casting with SetBoost Pro

As temperatures drop and winter approaches, the challenges of concrete casting become more pronounced. It's crucial to understand that the colder the weather, the longer it takes for concrete to set and cure. In fact, for every 10-degree Fahrenheit drop in temperature, the typical curing time doubles. For instance, concrete that usually takes 2 hours to set at 70 degrees will take 4 hours at 60 degrees. And if the temperature dips to 50 degrees or lower, concrete may become virtually inactive, leading to potential curing issues.

Conquering Cold Weather Concrete Challenges

To tackle these challenges head-on, various strategies can be employed to expedite the setting time:

1. Warm Water: Introducing warm water into the mix can kickstart the concrete's hydration process, aiding in proper curing.

2. Heated Curing Blankets: These provide a controlled environment, maintaining the ideal temperature for curing concrete.

3. Chemical Accelerators: A convenient option is the use of chemical accelerators designed to speed up the curing process.

Introducing SetBoost Pro : Your Cold Weather Concrete Solution

We're thrilled to introduce SetBoost Pro, a game-changing powdered admixture that can significantly reduce concrete setting times. With SetBoost Pro, you can confidently meet project deadlines, even in the most challenging weather conditions.

What Makes SetBoost Pro Stand Out?

Remarkable Speed and Strength: SetBoost Pro can accelerate concrete setting times by up to 50%, while simultaneously enhancing the density and strength of the concrete, ensuring exceptional durability.

No Impact on Color: Composed of a white powder, SetBoost Pro won't alter the color of your concrete, allowing you to maintain your desired aesthetic.

Chloride-Free and Corrosion-Resistant: SetBoost Pro is free from chlorides, making it safe for steel reinforcement. It actively prevents steel corrosion, preserving the integrity of your structures.

Faster Stripping and Reduced Cracking: It expedites mold stripping, saving valuable time, and effectively reduces curling and cracking, resulting in a smoother construction process.

Versatility for All Concrete Types: Whether you're working with standard mixes or high-performance formulations, SetBoost Pro delivers exceptional results.

Ideal for Cold Weather: SetBoost Pro is the ultimate solution for cold weather casting. It not only accelerates setting but also enhances strength gains, making it a go-to choice for challenging environmental conditions.

Conclusion

As the winter season approaches, ensure the success of your concrete projects with SetBoost Pro. Master cold weather casting with this groundbreaking accelerator, and experience faster setting times and enhanced concrete performance. Don't let temperature fluctuations slow you down; embrace SetBoost Pro for a smoother, more efficient construction process.

Discover the power of SetBoost Pro and take your concrete casting to the next level. Contact us today to learn more and keep your projects on track, no matter the weather.

For more information or to purchase: https://concretecountertopsupply.com/setboost-pro-concrete-accelerator/

 

The Basics of Glass Fiber Reinforced Concrete (GFRC)

Glass Fiber Reinforced Concrete (GFRC), also known as Glassfibre Reinforced Concrete (GRC), is a versatile engineered material composed of cement, aggregate, polymer, and fibers. This amalgamation creates a remarkably lightweight yet incredibly robust substance. GFRC is renowned for its exceptional flexural strength, making it an ideal choice for a myriad of interior and exterior applications such as wall cladding, fire pits, countertops, furniture, and more.

 

Understanding GFRC Mix Composition

One of the distinctive features of GFRC is its unique mix composition when compared to traditional concrete. The key components in a GFRC mix include:

1. High Cement Proportion: GFRC mixes typically consist of an equal ratio of cement to sand, which is relatively high compared to conventional concrete mixes.

2. Glass Fiber Reinforcement: Instead of utilizing steel for reinforcement, GFRC employs glass fibers as the primary reinforcement material. These fibers significantly contribute to the material's remarkable strength and durability.

3. Polymer Resin: An acrylic polymer resin is added to the mix to enhance the material's strength, act as a curing aid, and provide numerous other benefits.

4. Additional Components: Many GFRC mixes may also include pozzolans, defoamers, and shrinkage reducers to further enhance their properties.

 

Basic GFRC Mix Design

Here's a breakdown of the basic components in a typical GFRC mix design:

 

·         Cement: 18.5 lbs (8 kg)

·         Alto-Pozz: 4.5 lbs (2 kg)

·         Sand (#30 sieve preferred): 23 lbs (10 kg)

·         Water: 6-6.5 lbs (3-3.5 kg)

·         KongKrete Polymer: 2.3 lbs (1 kg)

·         PVA Fibers 8mm (face coat only): 85 grams

·         ¾” (19mm) AR Glass Fibers (Backer Coats only): 2.6 lbs (1.1 kg)

·         Optimum 380 Superplasticizer: 23-42 mL

·         C-64 Defoamer (optional): 5-10 mL

It's important to note that this information provides only a fundamental understanding of the GFRC mix design process. There are various factors and variables that can influence the final outcome, and therefore, independent practice and testing are crucial.

 

The GFRC Process Overview

The GFRC fabrication process involves several key steps to ensure the material's proper composition and successful casting. Let's delve into the process:

 

A. Prepare Supplies & Equipment

1. Ensure all necessary equipment and tools are set up and ready for use.

2. Prepare the casting forms by cleaning and applying a release agent. Remove any caulk or residue, cover screw heads with tape, and complete other necessary preparations.

3. Cut and prepare any foam, scrim, or inlays that will be included in the project.

 

B. Calculate Recipe & Materials Needed

1. Determine the batches for the face and backer coats based on your project's requirements. Use a mix calculator for accurate measurements.

2. Decide on the total number of batches to be mixed, considering factors like job size, ambient temperature, the type of mixing equipment, and the working time. Avoid mixing more material than can be used within 20 minutes.

 

C. Mixing

1. Begin with the Mist Coat (Face Coat) by mixing one batch using a handheld mixer to create a thin, milkshake-like consistency. Add plasticizer as needed to maintain the desired texture.

2. For the Backer Coat, mix one batch with fibers for the initial backer coat. Dispense only half of the mix into a bucket, ensuring it can be used before it begins to set. Adjust the consistency based on your preference, with thinner mixes being easier to work with and achieving better consolidation.

 

3. Follow these steps for mixing:

·         Add mix water and polymer to the mixing bucket.

·         Dry blend sand, pigment, cement, and pozzolan (PVA fibers for the face mix) in a separate bucket.

·         Add approximately 80% of the dry blended material to the water and mix on low speed until well blended. For backer coats, add AR Glass fibers after thoroughly mixing and blend them in on low speed.

·         Repeat for subsequent and final backer coats.

·         Mix on high speed for 1-2 minutes to shear the mix until it reaches a loose, smooth consistency, similar to a milkshake.

·         Add the remaining dry blended material and mix on low until well blended. Adjust the consistency with plasticizer if necessary.

 

D. Casting

1. Start with the Mist Coat:

·         Fill the hopper gun about halfway.

·         Set the air pressure to around 90 PSI and regulate it at the gun to achieve the desired results (usually around 45 PSI at the gun).

·         Test spray in an area away from the forms to ensure the gun is operating correctly.

·         Begin spraying from a corner, working outward, concentrating on edges first and then open areas. Clear any sand rebound that accumulates and ensure the material looks wet when applied.

·         Do not allow the mist coat to dry; it should remain soft to promote proper bonding.

 

2. Backer Coat Options:

   - You can apply the backer coat with a backer coat gun, maintaining a higher air pressure (100+ PSI) and spraying an even, thin layer over the surface. Use chip brushes and compaction rollers to ensure proper consolidation.

   - Alternatively, you can apply the backer coat by hand, taking care not to push it through the mist coat. Use chip brushes and compaction rollers to achieve the desired thickness and proper consolidation.

3. Make sure to address any corners, edges, and open areas thoroughly. It's crucial to avoid any gaps or air pockets between layers, and reinforcing fibers should be correctly oriented to maximize strength.

 

E. Curing

1. Gently clean excess cement from the forms.

2. Mist the concrete surface with water and cover it with plastic to ensure wet curing for at least 24 hours (or 36 hours in cooler temperatures). The plastic should fully enclose the mold and be secured to retain moisture.

3. Enhance curing by using moving blankets, foam sheets, or other means to maintain heat and minimize gaps between the mold and plastic. It's vital to keep the surface hydrated for the first 24-36 hours.

 

F. Demolding

1. You can demold GFRC as soon as 24-36 hours after casting, provided the concrete is at room temperature. Be cautious when handling the material, as it is still relatively soft. Faster curing can be achieved with accelerators and heat.

2. Once removed from the mold, place the casting on stickers or shims to allow air movement on all sides of the concrete for even curing.

3. Avoid placing any objects on top of fresh concrete, as this could cause "ghosting."

 

G. Slurry Coat (Optional)

1. If necessary, create a slurry coat using the following recipe:

·         Dry Ingredients: Cement (320 grams), Alto-Pozz (80 grams), Microspheres (up to 60 grams).

·         Wet Ingredients: Mix 1 part KongKrete Polymer with 3-4 parts water.

2. Mix only the amount of slurry that you can use within 15 minutes. If it hardens, you can add Optimum plasticizer to extend working time.

3. Dampen the surface lightly and spread the slurry into voids, ensuring it is packed thoroughly. Use hand tools, a grout float, sponge, or any suitable equipment to achieve this.

 

In conclusion, understanding the basics of Glass Fiber Reinforced Concrete (GFRC) is essential for those looking to work with this innovative construction material. By following the process of preparation, mixing, casting, curing, demolding, and optionally applying a slurry coat, you can create a wide range of durable and visually appealing projects, from countertops and wall cladding to furniture and fire pits. Remember that practice, experimentation, and meticulous attention to detail are key to mastering the art of working with GFRC.

 

The Essential Role of Acrylic Polymer in GFRC Concrete

 

In the world of concrete, there exists a versatile and high-performance material known as Glass Fiber Reinforced Concrete (GFRC). This innovative form of concrete combines the strength of traditional concrete with the flexibility and lightweight properties of glass fibers. What makes GFRC truly exceptional, however, is the critical role that acrylic polymer plays in its composition. In this article, we will delve into the importance of acrylic polymer in GFRC concrete and why it is so important in the construction industry.

 

Understanding GFRC Concrete

Before we dive into the significance of acrylic polymer in GFRC, let's first understand what GFRC concrete is and why it's gaining popularity in construction.

GFRC is a composite material made by mixing cement, sand, water, and alkali-resistant glass fibers. The result is a concrete that is not only significantly lighter than traditional concrete but also boasts impressive tensile and flexural strength. This unique combination of properties makes GFRC an ideal choice for a wide range of architectural applications, including façades, countertops, sinks, and more.

 

The Role of Acrylic Polymer

Acrylic polymer, often referred to as a "GFRC admixture," is the secret sauce that elevates GFRC concrete to a superior material for construction. Here's why it's so essential:

Certainly, here's an updated list of the role of acrylic polymer in GFRC concrete with the addition of its impact on curing and concrete hydration:

 

1. Enhanced Flexibility and Durability: Acrylic polymer serves as a bonding agent that effectively bridges the gap between the cement matrix and the glass fibers, creating a robust, flexible matrix that can withstand a wide range of stresses and strains. This results in greater durability, reducing the likelihood of cracking and increasing resilience in challenging environmental conditions.

2. Improved Workability: Acrylic polymer helps maintain the workability of the GFRC mix, allowing for greater creative freedom in shaping and designing architectural elements.

3. Reduced Weight: Acrylic polymer lowers the density of the concrete while maintaining its structural integrity, making GFRC lightweight and ideal for transportation and installation.

4. Enhanced Resistance: Acrylic polymer enhances the resistance of GFRC to alkali, moisture, and chemicals, ensuring that it remains structurally sound and aesthetically pleasing over time, even in harsh environments.

5. Improved Aesthetics: Acrylic polymer provides a smooth surface finish that can be customized with various textures, colors, and patterns, contributing to the material's visual appeal.

6. Elimination of 7-Day Wet Cure: Acrylic polymer helps eliminate the need for the traditional 7-day wet curing process, saving time and resources in construction projects.

7. Enhanced Concrete Hydration: Acrylic polymer aids in the hydration of concrete, promoting the chemical reactions between water and cement that contribute to the concrete's strength and durability.

 

Applications of GFRC with Acrylic Polymer

The combination of GFRC and acrylic polymer opens up a world of possibilities in construction and design. Here are some notable applications:

1. Architectural Façades: GFRC panels with acrylic polymer coatings are a popular choice for cladding building exteriors, as they offer both durability and aesthetic versatility.

2. Countertops and Sinks: The lightweight nature of GFRC makes it a preferred choice for creating custom countertops and sinks that are both stylish and long-lasting.

3. Decorative Elements: GFRC, when reinforced with acrylic polymer, is used to create intricate decorative elements such as moldings, sculptures, and ornamental details on buildings.

 

Conclusion

Acrylic polymer is the unsung hero of GFRC concrete. Its addition to the mix transforms GFRC into a construction material with unparalleled durability, workability, and aesthetic potential. As architects and builders continue to push the boundaries of design, the importance of acrylic polymer in GFRC becomes increasingly evident. This innovative combination is not just changing the face of construction but also allowing for a new era of architectural creativity and durability in the industry.

How to cast a SuperSix Admix bistro table

May GFRC Basics Class

Successful 2 day class last May on GFRC basics. We had a small group that had plenty of hands on time creating an integral sink countertop project, vanity sink, and this great outdoor 30" firebowl with 18" round Fire-burner mounted to a 48" square tabletop.  Our Guest Trainer Dave Magnano, shown in picture at far left, flew in from Conneticut and demonstrated GFRC tips for professionals.

Watch the video from Ask This Old House TV Episode

Peter Somers did a fantastic job showing viewers the correct way to make concrete counter tops in  Episode #11-25 on Ask This Old House which was shown on most PBS stations across America last weekend.

Click here to watch the Episode

Along with the homeowner, Corey Brett, Peter and show host, Kevin O'Connor demonstrated the steps involved in making custom concrete counter tops in a residential kitchen.  Peter used the GFRC spray technique and a custom mix design from scratch to produce a grey toned color and glass aggregate counter top.

This Old House production crew showcased the process involved, being careful not to skip necessary steps in the advanced process. From template to install this episode showed viewers many of the steps that are normally skipped through when filming a short TV episode.

Thank you to Peter, Corey, Kevin and the wonderful production crew and staff at Ask This Old House for spending the time to show viewers that making concrete counter tops takes skill, patience and the correct materials for success.

For more information on the products used in the episode and a from Scratch Mix recipe, visit our website

http://www.concretecountertopsupply.com

Concrete Project featured on Ask This Old House

Ask This Old House episode 

begins airing March 21st

Peter Somers owner of Concrete Poetry uses Fishstone products in a Concrete counter top kitchen project in Massachuttes.  Peter shown with cast and crew designs a concrete countertop to replace wood counter tops in a customer's home.

Peter templates and designs counter top to precast a custom concrete mix in molds on site.





A custom GFRC mix and Hopper Gun is used to Spray the concrete into the molds.

Peter uses a Flex Wet Polisher and Planetary Polisher to produce a smooth and polished surface.

 Peter Somers shown with This Old House host, Kevin O'Connor at Project Taping.

Photos provided by Cory Brett

March Specials & New Products

Hot March Special now available on the 7" Extreme Duty Wet Polisher bundle and $20.00 off the Optimum 380 Super Plasticizer 5 Gallon Bucket..................check it out below.

Click here to see our favorite Mixers

Start the season off right with a new Collomix XO series professional hand mixer. The X06 is the perfect size for your GFRC projects and is easy to use when you have the Mixing assistant. Save your back and let the station work for you. You can preorder these great units which become available to ship in April. Collomix units are European made and built to last.  All models come with a TWO year Manufacturer's warranty.

Peter Somers featured on Ask this Old House using Fishstone Products

We are pleased to announce a long time customer/contractor Peter Somers of Concrete Poetry in Massachusetts was featured on Ask This Old House using Fishstone products. The episode is airing the third week of March with Peter completing a concrete counter-top Kitchen project.

 

Ask This Old House - Episode 1125

Peter has been a longtime customer at Fishstone and successful producer of high quality concrete countertops, furniture, outdoor and commercial projects in the northeast for many years. Shown below in some photos from Peter's work shop in Braintree, MA along with some of his featured projects over the years, Peter has developed a successful business in the New England area as a top notch artist and craftsman. Peter was contacted by a This Old House and a homeowner in the Northeast several months ago to complete a Concrete Countertop Kitchen which would be featured on an episode of Ask This Old House and in turn contacted us to help provide products for the featured project. 

  

Unregistered

February Special

Our February special is out. Receive a FREE 50 LB bag of ALTO-POZZ (value $49.50) when you buy two 5 Gallon pails of Forton VF774 Polymer. On Sale this month, limit of 3 Free ALTO-POZZ per customer.

Concrete Conversations

DIY & Just Getting Started #1

The following is one of the most common questions we are asked by DIY-ers and anyone just getting started with Concrete Counter Tops.

1. What type of equipment & supplies do I need to make concrete countertops?

If this is your first attempt at making a concrete counter top there are some basic tools and supplies necessary to complete a simple designed top. Along with the following lists, a good size work area that is protected from the elements that has running water and electric is a must, along with ideal temperature range of 55-75 degrees.


Tools & Supplies needed for making your mold:
Table Saw 
Drill
Level (4 Foot or longer)
Tape measure
Melamine Board or laminate sheets
Screws ( we like the  1 5/8"  building screws)
Caulk ( we use black, it's easy to see)
Large level work table (big enough to hold your entire mold filled w/ concrete)

Tools & Supplies needed for mixing concrete:
Concrete Vibrator or palm sander
Mixer ( hand held unit for mixing 5 gallon bucket or rent a large mortar mixer)
Scales ( large bench type for heavy ingredients and a gram scale)
Plenty of 5 Gallon or larger buckets for measuring/mixing 
Plastic paint scraper
Rubber gloves
Eye protection
Dust mask

Tools & Supplies needed for Polishing Concrete:
Electric or Pneumatic Polisher or angle grinder (wet polishing is best)
Diamond polishing pads to fit polisher
Diamond hand pads
Apron, Rubber Gloves and Boots for wet polishing
Polishing carts or Saw Horses 

Supplies for Slurry/Grouting and Sealing cured Concrete:
Small plastic bucket/bowls
Bondo Scraper or plastic putty knife
Mixing sticks 
Sponges and applicators for sealers

Our New Website is UP!

Same location www.concretecountertopsupply.com with a new look and many new options for viewing products and information. We would love to hear what you think about the new site. Post a comment here or contact us through the website at our Send Contact Message button.

Thank you Echlin Web Solutions for a fabulous conversion and customization on the Website!

Training Class March 2012

March 2012 Concrete Countertop 5 Day Class

Thanks to all who enjoyed the training that week!

 Students polishing with the Flex LW 1202 and

Flex LCP 1703 VR with DragonFish Diamond pads

 

Sand Mix recipe, Hand Packed mini-counters using

Rapid Set Cement, Forton VF774, Brown & Black Iron Oxide,

 C-Grid Carbon reinforcements & ladur wire.

After hours relaxing around the concrete gas Fire Pit



Fire Pit Workshop June 29th

Our first Fire Pit Workshop was held on June 29th showcasing the brand NEW Concrete Countertop Institute's CRATER Fire Pit.  This modern gas fire pit designed by Jeff Girard is the first in a series of outdoor projects designed for spraying in GFRC.  This first class was offered for free to anyone purchasing the Crater Fire Pit Kit during the month of June.  The next Workshop featuring another new plan from CCI will be held September 16th in Elgin IL, click the following link for more information Fire Pit Workshop

Jeff Girard from the Concrete Countertop Institute



Jeff Girard & Tom Fischer




Crater Fire Pit with Caribbean Mix Fire Glass



Customer Appreciation Lunch & Sealer Demo



May 26, 2011 Sealer Demo & Customer Appreciation Lunch


Thank you to all who came out and enjoyed a day of Sealer demo and discussion.  We truely appreciate everyone taking the time to understand the ins/outs of sealing and process involved with acheiving the best finish through good application.
We featured a demo and had sample sealing of all the Fishstone Brand sealers as well as product gift packs for everyone that came out.  To see a complete listing of the sealers we offer click on Sealers.