GFRC panels and quality control during production

A few years ago our customer for the London tunnel project approached me and asked me to prepare a presentation about some of the quality topics in reference to our works package – GFRC panels. I decided to give a short speech about the quality processes that we used in manufacturing of GFRC panels.

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I developed the present post based on that presentation. This article is the first one in a series about our experiences with design, manufacture and installation of GFRC cladding. In this post I will sumarise the quality process from our point of view as one of the glass fiber reinforced concrete manufacturers.

Quality control during GFRC panels production

Manufacturing of GFRC – glass fibre reinforced concrete panels or other GRC elements involves large amount of quality checks at every step of production.

As GRCA member we followed not only the requirements from the project, but also GRCA specification.

Only the best GFRC manufacturers can meet the stringiest specification for glass fibre concrete panels production

Before we started with production of GFRC panels for our construction project in London we had to analyse the needs based on our design and of course based on the client’s demanding architectural specification. Designers, engineers, quality managers, specialist consultants, site managers and project managers were all involved together to make sure we can achieve the high standard of our GFRC panels. Even though we had extensive quality procedures for manufacturing of various types of cladding systems we had to start from scratch with quality checks for GFRC wall panels.

We started from checking all the available standards and specification which were required under the contract. We listed main testing requirements for GFRC as well as material checks. However, we decided to carry out a few more extra checks.

Testing of GFRC in the pre-construction phase

The list below presents a short summary of tests we carried out for Glass Fibre Reinforced Concrete panels:

• GFRC moulds test and checks
• Acoustic tests
• Substructure fixings pull-out test
• Light reflectance test
• Fire test
• Blast test
• Soft body and hard body impact test

Before we conducted any of the above test we had to produce some of GFRC panels and substructure. Prior and during manufacturing we carried out the following tests for GFRC cladding:

• Incoming materials checks
• Raw materials tests
• Sieve tests for aggregates
• Concrete slump test
• Bag and bucket test
• Washout test
• Flexural Bending test
• Dry Density test
• Wet density test
• Water Absorption test

When materials arrived into the factory first step was to check if the supplier provided correct information about material origin and its specification. For cement we checked the right type of Portland cement was used. In this case our mix included both white and grey cement of specific type and ratio.

Sieve test for silica sand in GFRC concrete mix

Next we also carried out the check on some of the GFRC mix ingredients: silica sand and other aggregates. Not only we checked the incoming materials in terms of documents, but also did the sieve tests to check if the sand was the correct size. For this purpose we used specialist sieves with various mesh sizes on them.

We used clean and empty sieves that we weighted on scale and we recorded their weight in notes.

Quality personnel took sand sample randomly from the delivered quantity. They did it directly from the delivery trailer or from the material storage area in the factory. Minimum three 300g samples were taken at random from the sand stack, trailer, batching plant or bag.

Next, we poured the sand sample into the top sieve with the largest mesh size. Each sieve beneath the top one in the column had smaller mesh than the one above.

The column was shaken for approximately 1 minute. After the shaking was completed the material on each sieve was weighed. The weight of the sample of each sieve was then divided by the total weight of sieve and sample together to calculate a percentage retained on each sieve.

It is recommended that for GFRC mix a fine grain silica sand is used, therefore, for the sieve test we used small size mesh.

We analysed the size of the average particle on each sieve to get a cut-off point or specific size range, which we then captured on the mesh. Our team recorded the results and presented them in a table. Its rows showed different sieve sizes and columns displayed weight and percentage of grain of that particular size.

Glass fibre reinforced concrete panels – sieve test frequency is depending on your own overall quality system and project requirements

How often should you do the sieve test? Definitely at least with every new delivery of aggregates. What about if you already have a pile of sand or full silo of it? It depends on a few factors. Mostly it’s about your own quality procedures and assurance that the GFRC materials you are going to produce will be consistent. If you store bulk quantity of different sands for different types of  GFRC mix than it’s best to test more frequently.

Is this test suitable only for silica sand? No, you can test other aggregates in the same manner. GRC panels can vary in their face coat finish depending on the architectural requirements. If your GRC mix includes shiny and darker aggregates you should sieve test these materials as well.

Incoming materials tests during production of GFRC panels

We not only checked silica sand or other aggregates during the manufacturing of glass fibre reinforced concrete panels.  Among other materials we inspected were chemical additives, cast-in brackets, glass fibre roving and moulds for glass fibre reinforced concrete panels. We checked all data sheets and delivery notes for chemical additives against compliance with GRC mix and specification requirements. Our design included stainless steel plates that we casted in glass fiber reinforced concrete panels during spraying. We checked brackets’ dimensions against the manufacturing drawings. Dimensions of these brackets and more importantly casting process were crucial for the team that installed glass fibre reinforced concrete panels on site. With incorrect type of plates or wrong casting these GFRC panels would not fit into the substructure.

Another items that we inspected during the production of GFRC panels were moulds. Damaged mould, incorrect type or size out of tolerance would cause a lot of troubles. If we produced GFRC panels from the damaged mould these panels would be defected as well. We had to check each mould before its use. Dedicated person reviewed the dimensions of mould to make sure these didn’t stretch out of tolerance. Additionally, we inspected mould surface for any even tiny flaws like cracks, dents, blowholes, etc. Some flaws we repaired with our own operatives. However, the mould manufacturer fixed surface inconsistencies or larger damage.

Inspection of glass fibre rovings documentation for GFRC panels manufacture

Glass fibre is essential part of the GRC mix, therefore, its quality is also of great importance.  Only a few worldwide renowned manufactures supply glass fibre in roving or chopped strands to GRC concrete producers. What is the difference between the two? GFRC suppliers use roving glass fibres usually for the sprayed technique. Its a long fibre in a roving form which you can fit into the GRC spraying equipment. Its filament diameter is often smaller than those fibres for poured method.

This specialist machine is able to supply roving fibres and pump the concrete at the same time. Therefore, there is another term for the sprayed GRC method – simultaneous projection. It allows for projecting fibres and concrete at the same time with one machine. When you project glass fibre it runs through a specialist dispenser/gun. It contains a rotating blade which cuts the continuous roving into the shorter glass fibre elements.

Chopped strands is the glass fibre which was already cut to the specific length. You can use it for sprayed and poured method of GFRC panels production. When you use it for sprayed technique you need slightly different spraying equipment. You need to have a small hopper at the end of the gun which will feed the fibres into the spraying machine.

How is glass fibre important to the quality process and manufacturing of GFRC panels?

For high quality GFRC concrete you need to make sure your glass fibre is also of great standard. We used glass fibres with high zirconia content. This amount ranged between 16-19%. Zirconia helps glass fibres to be alkali and acid resistant. Usually this type of glass fibre includes marking ARG – alkali resistant glass fibre. Part of quality process was to source the suppliers and dealers who certified zirconia content in their glass fibre. One of the manufacturers would be Nippon Electric Glass. 

Other properties of glass fibres which we inspected were:

• Young’s modulus of elasticity
• Strand tensile strength
• filament diameter

GFRC panels – LOP & MOR testing

GFRC’s main advantage over standard steel reinforced precast concrete is in its flexural strength for thin elements. While GFRC can achieve the compressive strength similar to the precast concrete elements (approx. 50Mpa) it presents much higher flexural properties.

The most popular use of GFRC is in glass fibre reinforced concrete panels and façade elements. This means that in these cases GFRC is a subject to live and dynamic loads. GFRC flexural and tensile properties allow architects and engineers to design thinner and not so heavy concrete panels.

In order to design the GFRC panels properly it is essential to make sure that these products are of the correct class.

GFRC can be classified according to Glass-Fibre Reinforced Concrete Association (GRCA) into 3 main categories: Class 8, 10 or 18. There are no significant differences between the GFRC classes when it comes to LOP test results. LOP stands for Limit of Proportionality which in other words means bending limit. At this point the base concrete material fails and any further load applied is transmitted into fibres.

The difference in GFRC classes is defined by the results from MOR testing. MOR stands for Modulus of Rupture. In other words, it’s the ultimate breaking point at which the fibre and overall product fail. GFRC class 8 and 10 are usually achieved through premix production method. This means that glass fibres are added as chopped strands to the concrete mix and applied either manually or mechanically through nozzles and pumps.

How to manufacture Glass Fibre Reinforced Concrete Class 18P?

Class 18P is the highest GFRC class according to GRCA. As GFRC supplier we need to achieve certain results in GFRC testing to pass Class 18P mark. 

There are many glass fibre reinforced concrete suppliers who cannot achieve this requirement.

With this type of production glass fibres are supplied in a roving form into the gun type dispenser. There the glass fibres are chopped and sprayed simultaneously with the concrete mix into a mould in thin layers. Each layer needs to be rolled in order to compress the glass fibres and concrete. The rolling process helps in removing air trapped in the concrete. This process has significant impact on the MOR test results therefore it has to be carried out properly with qualified operatives.

Regular LOP and MOR testing is necessary to establish GFRC characteristic values. These values will determine the class of GFRC. GRCA recommends that a minimum of 40 test mean results are analysed in order to establish the characteristic value. Our manufacturing team produced test boards each day before the actual production. These boards were later used for cutting test coupons which were marked or labelled with the production date and coupon orientation.

In order to carry out the test you should use a test house familiar with either ISO EN -5 or GRCA Method of testing Part 3. There is a subtle difference between these two norms. EN -5 requires the same coupons to be tested at 7-days and 28-days cured state. Whereas according to the GRCA Method of testing Part 3 it is only required to test 28-days cured coupons.

How did our GFRC panels coupons performed during the tests?

We have test reports which confirm that our GFRC panels system complies with GRCA requirements for Grade 18P. The mean results for LOP were above 8Mpa and the mean results for MOR were above 21Mpa.

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. 

For more information, please visit Jushui.

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.

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  LW 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.

For more GFRC Constructioninformation, please contact us. We will provide professional answers.