What Is Alkali Resistant Glass Fiber?

Article courtesy of Smooth-On, Inc.

The majority of artisan concrete projects, including concrete countertops, architectural panels, concrete vanities, and concrete furniture pieces often achieve their primary reinforcement using fiber technology. Understanding which fibers are functional as primary reinforcement, and the reasoning behind which fiber to use in a given situation, is helpful in achieving a well reinforced object.

Three of the reinforcing fibers that are the most widely used are Alkali Resistant Glass Fibers, PVA Fibers, and AC50 Acrylic Fibers. There are many other fibers that are available, including nylon and polypropylene, but these are typically used for shrinkage control and secondary reinforcement, not as a primary reinforcement. For now we will focus on the benefits and drawbacks of using Alkali Resistant Glass Fibers.

What Is Alkali Resistant Glass Fiber?

Alkali Resistant (AR) Glass Fiber is glass fiber with added zirconium oxide to help resist attack from alkalinity. This is an important element of these fibers, as concrete is a very alkaline environment. Normal fiberglass (e-glass) degrades in concrete due to the aggressive alkaline environment. AR fibers have been widely used in the concrete industry since the 1970's. Glass fiber allows for thinner and lighter weight concrete. Steel reinforcement is no longer necessary. AR Glass Fiber is the primary reinforcement used in GFRC (Glass Fiber Reinforced Concrete).

AR Fiber is produced in bundles, and held together with a resin called 'sizing'. A bundle will have varying numbers of filaments, depending on the intended application. The bundles work to allow a certain amount of ductility to the concrete, while also producing the necessary tensile strength for many applications. The tensile strength of the glass fiber is greater than that of steel, and the fiber is significantly less 'elastic' than concrete. This means that once the concrete is forced to failure, the glass is still holding the matrix together, providing strength and ductility. Since the glass does not stretch and elongate, the failure of the fiber is sudden and catastrophic, as opposed to the PVA fibers that will stretch prior to failure.


  • Chopped Bundled Fiber is cut into various lengths, and with varying numbers of filaments per bundle. More often than not, when failure occurs, the fibers are more likely to pull out of the concrete matrix rather than break. This means that longer fibers are less prone to failure than are shorter fibers. The number of filaments per bundle has an effect on both the strength and the workability of the concrete mix. When these 2 variables are considered, we believe that a 19mm (3/4") 200 filament fiber bundle is best suited for most artisan applications.
  • Roving is a continuous strand of fiber. This roving is most frequently used with specific equipment that chops the fiber and incorporates the fiber into a concrete slurry during the casting process. Roving is also what is used to produce Scrim.
  • Scrim is a woven fabric of roving. Since fibers are more likely to pull out, than they are to break, having a continuous thread of fiber through the tensile plane will provide greater tensile strength than will fibers alone.

When Should AR Glass Fiber Be Used?


  • The primary benefit of AR Glass Fiber is the strength they provide relative to how they effect the workability of the concrete mix during application. Generally put, glass fibers produce good strength, and are easy to work with.
  • Scrim provides a great way to provide localized strength in specific tensile planes. To understand where these tensile planes occur, read THIS ARTICLE.
  • AR Glass fiber provides a great strength to cost ratio.


  • The primary drawback of AR Glass fiber is visibility in the finished surface. Because the fibers are bundled, the fibers are easily visible in finished surface unless specific measures are taken.
  • Also, AR Glass fiber will breakdown when overmixed.


  • AR Glass fiber is typically used in concrete composites, meaning there is a "face" that doesn't contain fiber, and a structural mix containing the fiber. This is not the case in "SCC" mixes that are properly proportioned. The fibers will lift off the surface of the finished face during casting due to the specific gravity of the fiber in relation to the other constituents of the mix. In SCC applications, only a limited amount of polishing can occur during processing to avoid exposing the fibers.
  • Multiple layers of scrim can be used, with layers of mix between the scrim, to add additional tensile strength.
  • Care should be taken when placing scrim near finished faces in order to avoid "shadowing" of the fiber in the finished faces.
  • Glass should be added later in the mixing process, and should not be overmixed. The fibers should be blended homogeneously into the mix, but continuing mixing beyond that will increase the potential of shredding the fiber.
  • Combinations of glass and pva can be used to increase flexibility and impact resistance.


Dosages rates of glass fibers vary heavily depending on the application and desired outcome. Dosage can range from 1% to 6%. This depends heavily on the mix recipe, mixing process, and application method. In most artisan mixes, a dosage rate around 3% is most appropriate.

Note- All dosage rates are based on the weight of the total mix. (Dry Mix + Liquid) x Dosage Rate

Every situation is different. If you are not certain about whether an application is appropriate, you can share your concerns and receive feedback by filling out this form.

Craftsman Mix and CounterMix - Glass fiber is an ideal reinforcement for this mix. In most cases it will eliminate the need for steel reinforcement, adding benefits not gained with steel, and simplifying the process.

  • Press Finish :: Used in the structural layer. Do not use in the initial press layer. dosage rate- 2-3%
  • Troweled Finish :: Used in the structural layer. Do not use in the final troweled layer. dosage rate- 2-3%
  • Cast Finish :: Glass can be used in 'cast' applications, but should be done with consideration. Often a thin layer without glass is cast into the mold, followed by a structural mix. Working up vertical walls can be tricky with this type of composite. dosage rate- 1-3%

GFRC Blended Mix - "GFRC", it is called that for a reason.

  • Backer Mix :: Ideal fiber for this application- dosage rate- 2-3%
  • SCC :: A good consistency of mix must be achieved to allow the fibers to slightly 'float' off the finished mold surface. dosage rate- 1-3%
  • Sculpting Mixes :: Ideal for use in the structural body of a sculpture, but must be kept away from the finished surface. dosage rate- 1-3%


ECC Blended Mix - There are times that glass fiber is used in ECC mixes through the structural body of the mix. achieving the right mixing protocol and dosage rate to achieve this should be taken into careful consideration.


This FAQ article is offered as a guideline and offers possible solutions to problems encountered during mold making and casting. No warranty is implied and it is up to the end user to determine suitability for any specific application. Always refer to the provided Technical Bulletins (TB) & Safety Data Sheets (SDS) before using any material. A small scale test is suggested to determine suitability of any recommendation before trying on a larger scale for any application.