Understanding the Difference between Cast Stone and Precast GFRC

It’s important to understand the differences between cast stone and precast GFRC when deciding which product to use in a project. The materials have several similarities and architects and builders often opt for one material even if their application would be better suited for the other.

Nonetheless, while mixing up the applications of these materials isn’t always a recipe for a failed project and an unhappy client, it’s essential to know that they have marked differences not only in terms of their makeup and resistance to dirt, but also strength, appearance, use, and durability.  

What is cast stone?

Cast stone is essentially highly refined precast building stone made using a mix of both coarse and fine aggregates, including natural sands, quartz, granite, limestone, marble, Portland cement, mineral oxide coloring pigments, and calcite. Once the mixture is ready, it’s pushed into sturdy molds to give it a dense texture that resembles that of natural cut stone. It can be cast in a wide range of colors and stone finishes, such as marble, travertine, slate, bluestone, limestone, granite, and brownstone. Cast stone can even work as a substitute for bricks too. Unlike some materials, weathering tends to improve the appearance of cast stone over time, helping it appear more natural.

Cast stone is highly suitable for use as an architectural ornament, trim, or facade. Moreover, it can be reinforced, allowing it to have both the structural advantages of concrete and the aesthetics of natural cut stone. Cast stone is manufactured in two ways: wet cast and dry tamp. There are important differences in the processes, but both of them result in some sort of simulated natural cut stone appearance. For instance, dry tamp cast commonly results in sandstone or limestone. Meanwhile, the wet cast method results in wider possibilities, such as brownstone, granite, slate, and more.

Dry tamp vs. wet cast stone

The biggest difference between wet cast and dry cast stone is water content and the amount of slump (an industry term for how workable and pliable a synthetic stone mix is). Like its name implies, wet cast involves significantly more water than dry tamp, and because of that, wet cast exhibits 3 to 4 inches of slump. For dry tamp cast stone, hardly any water is involved and the concrete mix doesn’t have any slump, meaning it is much harder to shape.

Pros of wet cast stone

When producing wet cast stone significantly less effort is needed for pouring the concrete in the molds. You simply pour it in and then use a concrete vibrating tool and trowel to make sure it evenly sets. In addition, there are not too many restrictions on the mold materials, allowing you to use whatever materials are most time and cost effective. 

Moreover, wet cast stone is more conducive to pre-casting mold treatments, such as texturing additives and retarders. Additionally, it’s possible to achieve different kinds of finishes via post-cure treatments like sandblasting and acid etching too. Wet cast stone can even be poured to have a smooth finish. Furthermore, larger aggregates can be incorporated into wet cast and there’s less of a chance for it to be damaged at the time of demolding.

Wetcast Precast Concrete Panels from Armcon Precast

Cons of wet cast stone

With wet cast stone, you can only use a mold once a day since the wet cast mix needs time to set up and cure. In addition, wet cast stone takes longer to reach its full strength potential and therefore the early strength of the stone is not as high as that of cast stone made with dry tamp.

Wet cast stone needs extra detailing after demolding to make it look neat too. With the dry tamp method, achieving a clean look from the outset is much less of an issue, only minor detailing is generally required. Cast stone made with the dry tamp method also has an inherent limestone look. However, for wet cast stone to have the same look, it must go through surface treatments such as acid etching.

Pros of dry tamp cast stone

Dry tamp cast stone is characterized by a fast de-mold time and its ability to be cast into the same mold multiple times throughout the work day. In addition, the stone doesn’t need to undergo any additional treatment to have it achieve a limestone look.

Moreover, cast stone made via dry tamp looks really clean, natural, and has a much higher early strength than wet cast. Cast stone made using dry tamp is also more conducive to post de-molding repairs than wet cast stone. 

Cons of dry tamp cast stone 

Dry tamp cast stone involves water misting and steam curing, as well as tamping equipment, which can be quite costly to maintain and set up. Also, apart from traditional limestone texture, other surface treatments like mold applied colors or textures are impossible to achieve.

In addition, while much less post de-molding detailing is required, during demolding, an additional step for detailing is needed for dry tamp. Dry tamp is also incapable of producing smooth finished stone.

What is precast GFRC?

When looking for architectural precast concrete, there are two kinds of concrete mixes you’ll come across: conventional concrete and Glass Fiber Reinforced Concrete (GFRC). Which one’s the better choice for your next project? Understanding what exactly GFRC is and its advantages over conventional concrete are important in making this decision. 

Conventional concrete is made roughly the same way today as it was decades ago: a simple mix of sand, pea gravel (or other stone aggregate), Portland cement, and water that can be used to pour formed stone like curbs, sidewalks, and more.  However, when it comes to the architectural precast industry, conventional concrete is not the standard it used to be and is being taken over by GFRC. 

GFRC is essentially a high-performance special concrete mix. As its name suggests, it includes glass fiber in its concrete mix, which gives it enhanced structural properties over conventional concrete, which would generally require some sort of steel rebar reinforcement to achieve an equivalent level of strength. Apart from fiberglass, the mix for GFRC also includes cement, acrylic polymer, fine sand aggregate, and other performance admixtures. While using GFRC for a project can be slightly complicated, the benefits often greatly outweigh the added complexity. 

Cast stone vs. precast GFRC

Precast GFRC and cast stone both have their benefits and drawbacks. The final choice of which one suits your purpose better comes down to these.

Cast stone pros

Cast stone has been in use for quite a long time as a reliable method to replicate natural stone at only a fraction of the cost. Moreover, you can mold cast stone into different shapes and give it a wide range of precise colors. Since it is a manufactured product, its production method can be perfected to produce a quality product consistently.

In addition, cast stone can easily be cut to any suitable length you require. It also provides time-tested freeze-thaw durability, making it relatively strong and a good choice in colder climates. It can also be used as a substitute for both architectural precast concrete and limestone.

Cast stone cons 

Despite all these benefits, cast stone comes with its limitations. While it can be substituted in for precast concrete in situations where it is specified as minimally load-bearing and non-structural, that is not the case when precast concrete is structural. Also, cast stone is a division 4 masonry material. Therefore, the connection methods and sizes need to be within the scope of the masonry contractor’s work.

In addition to that, cast stone made using dry tamp can’t be used for making large panels. Cast stone will also never appear as realistic as natural stone and the color will always be subject to change over time.

Precast GFRC pros

Despite being relatively new in the market, GFRC has several clear benefits. GFRC has high flexural and compressive strength; it doesn’t need any internal steel reinforcement for added strength. Its inherent strength also brings the added advantage of making it more flexible. GFRC can have a thickness as low as ¾ to 1 inch, which can make it the ideal material for making wall panels. It also does not require any additional accommodations.

GFRC also doesn’t weigh a lot and due to its low weight, it puts less stress on structural components. Not only does this make handling easier, but it also makes the material suitable for structures such as tubs, wall panels, floating vanities, floor tiles, and countertops. In addition, it can be used for greater spans without requiring any additional support and with the spans’ enlarged size, there will be fewer seams in the coping, vanities, and countertops. The material’s dense surface also facilitates a low rate of liquid absorption and provides protection from stains.

GFRC also promises excellent color consistency, even over multiple products. By using the sprayed face coat method, you can coat the surface of a large area from just one mix. As a result, there will be no errors from one mixer to the other. GFRC then can be used for floor tiles and wall panels over a bigger space or in a place where there are multiple precast components.

In addition, GFRC has early high strength curing. In other words, it can be handled and placed through both finishing and sealing phases safely and sooner. Therefore, with GFRC, you can enjoy short lead time, freeing up your time to improve the project further. 

Precast GFRC cons

While all these advantages sound impressive, GFRC has its fair share of limitations.

GFRC isn’t ductile, so it can’t undergo changes without breaking. It is also quite expensive. The fiberglass used along with the acrylic copolymer and additives makes the price go up drastically. It is difficult to self-mix as well so you’ll need a contractor not only for mixing but also for pouring it.

While it’s true that GFRC can be highly versatile, if it isn’t poured or applied properly, it can easily fall apart. Moreover, depending on the kind of fiber used, the resulting concrete can be quite heavy. It’s also essential to have the exact fiber amount in a concrete batch. Tests show that even small variations in the fiber can adversely affect the concrete’s strength. 

Lastly, fibers are added to concrete to increase both stiffness and tensile strength and make it perform better. However, corrosion can bring down the performance. 

Typical applications for cast stone

Typically, cast stone is used for decorative purposes. Since it is similar to granite, marble, and natural limestone in detail, it is used as an item for decorative exterior veneers. Another common use is for replicating the appearance of limestone.

As mentioned earlier, it is also used as an architectural facing, ornament, feature, or trim. It can also be used for water tables, bases, copings, window surrounds, quoins, doors, bandings, and sills. In some cases, it can also be used for decorative pieces like balusters, columns, and balls.

Where is GFRC being used?

GFRC is not only being used for restoring old buildings’ facades and renovating exteriors for new buildings. It is also being used for constructing ceilings and walls in buildings. Its use can also be seen in water features and landscaping. Moreover, given its ability to be poured into any cast, it’s suited for sculptures, boulders, and prefabricated rocks. Some landscapers also use it for memorial stones, one-piece waterfalls, and other landscape sculptures.

GFCR is still new, and it isn’t being used to its full extent yet. However, with its lightweight design and durability, it’s also being used for the following purposes:

  • Structures where the core structure of the building is unable to support other cement-based products
  • Structures where structural support is insufficient
  • Structures with complex designs where larger pieces can ease the installation
  • In places where there’s a lack of skilled craftsmen and restriction of installation and rigging equipment
  • For interior applications, transitional areas, and breezeways both outdoors and indoors

Do both materials have options for concealed fastener mounting?

Both of these materials can be mounted with concealed fasteners, but the mounting options available are different.

Cast stone often comes with mounting holes ready to accept anchors such as KEIL undercut anchors, but the manufacturer typically doesn’t supply the anchors or mounts themselves.  For systems similar to this, Monarch’s Under Anchor System can be used in almost any application.  

On the other hand, GFRC typically has anchors cast into the panel during production. Monarch’s Embedded Anchor System for GFRC works with our  LW1250 system and we have options for zinc or nylon inserts available.

Please contact Monarch Metal Fabrication if we can be of assistance with your next project.

Image 1 Credit: “Crystal-like Community Center | Abfaltersbach AT” by Rieder Smart Elements GmbH. Image 2 Credit: “Precast Concrete Stacked Wetcast Panels” by Armcon Precast

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