Fiber Reinforced Concrete

By: Thomas W. Berg

Reprint from The Technical Advisor, June 1993

Since the advent of fiber reinforcing of concrete in the 1940's, a great deal of testing has been conducted on the various fibrous materials to determine the actual characteristics and advantages for each product. This article will examine some of these fibers, their applications and some problems.

Portland cement concrete is considered to be a relatively brittle material. When subjected to tensile stressed, unreinforced concrete will crack and fail. Since the mid 1800's steel reinforcing has been used to overcome this problem. As a composite system, the reinforcing steel is assumed to carry all tensile loads. When fiber reinforcing is added to the concrete mix, it too can add to the tensile loading capacity of the composite system. In fact, research has shown that the ultimate strength of concrete can be increased as much as 5 times by adding fiber reinforcing.

In the American Concrete Institute report titled, "State-of-the-Art Report on Fiber Reinforced Concrete" (ACI 544.1R), the characteristics of fiber reinforced concrete are discussed at length.

Reinforcing fibers will stretch more than concrete under loading. Therefore, the composite system of fiber reinforced concrete is assumed to work as if it were unreinforced until it reaches its "first crack strength." It is from this point that the fiber reinforcing takes over and holds the concrete together.

For fibers reinforcing, the maximum load carrying capacity is controlled by fibers pulling out of the composite because fiber reinforcing does not have a deformed surface like larger steel reinforcing bars. This condition limits performance to a point far less than the yield strength of the fiber itself. This is important because some fibers are more "slippery" than others when used as reinforcing and will affect the toughness of the concrete product in which they are placed.

Toughness is based on the total energy absorbed prior to compete failure.

The main properties influencing toughness and maximum loading of fibre reinforced concrete are;

Type of fibers used.
Volume percent of fiber.
Aspect ratio (the length of a fiber divided by its diameter).
Orientation of the fibers in the matrix.

Materials used in fiber reinforcing including acrylic, asbestos, cotton, glass, nylon, polyester, polyethylene, polyperpylane, rayon, rockwool and steel. Of these, acid resistive glass and steel fibers have received the most attention. Plastic fibers have shown to be of little value in reinforcing concrete until only recently. Natural fibers are subject to alkali attack and are also determined to have little value. Nylon is currently making an appearance in slab-on-grade technology. Most of the test data, however, centers around the use of steel fibers and glass fibers.

Figure 1: Restoration of the walls around the far left window of this Italian Renaissance hotel used GFRC to replace broken terra cotta ornamentation.

The percent of fiber in the concrete mix is based on volume and is expressed as a percent of the mix. Tests ranging from 1.7% to 2.7% are common. When volumes greater than 2% are used, the concrete can be difficult to mix. When concrete is placed by processes other than from a ready mix truck, the fiber volume may be higher. One such example is when shot concrete is employed. Volumes of 2.3% have been successfully used. In some precasting operations using fiber reinforced concrete volume percentages have been used as high as 5%. Generally, if all other properties are equal, concrete strength increases linearly with volume of fiber.

Aspect ratio is simply the length of a fiber divided by its diameter. This property is used to represent the amount of surface area of the fiber against the concrete mix. This aspect ratio is important for another reason. It has been determined that balling of fibers in the mix increases as the aspect ratio increases. An aspect ratio of 100 for steel fibers was found to be optimum.

Orientation of the fibers is generally random, simply because they are not placed one at a time in a straight line. Fibers are either added to the dry cement or sprayed onto a form and covered with the wet concrete mix. Both of these procedures will produce a random pattern of fiber reinforcing. Tests with steel fibers, however, have shown that they can be aligned by using magnets and that the resulting concrete will have an improved ultimate strength. This process is used in fabricating precast beams and columns.

Figure 2: The simple geometry of this high-rise hotel was a prime candidate for GFRC panelization.

Since the 1940's many applications for fiber reinforced concrete have been discovered, and most of them have used either steel fibers or glass fibers. Steel fibers have found their place in paving, rock stabilization and to form the insides of tunnels. Placing methods include direct pouring from a ready mix truck, as in road work and airport runways, and shotcreteing for other civil construction projects, including water runoff and spillways and steep slope control.

Nylon is considered a plastic material, and as such, has not received the growth that steel and glass have. Most plastic fibers are believed to contribute little to the static strength of concrete, however, nylon and polypropylene can increase the bending and impact strength. These fibers are also naturally alkali resistant. New products are being developed by Nycon Inc. for use in concrete slabs on grade such as warehouse and automotive repair facilities. Manufacturers suggest that their fibers can even replace the welded wire fabric in these slabs.

Glass fiber reinforcing has shown the greatest development over the past 25 years. Early glass fibers were not successful because the alkali in portland cement would attack and destroy the fibers. In 1967 Dr. A. J.Majumdar, of the United Kingdom Building Research Establishment, tested the use of zirconia containing glass. It is from his efforts that alkali resistant glass fibers were developed. The Pilkington Brothers, Ltd, of England, took up world wide rights to commercialize this new material under the name CEM-FIL. In 1975 CEM-FIL Corporation was established in Nashville, TN. This corporation began manufacturing AR glass products. Owens-Corning Fiberglass Corporation was also granted license to produce the patented glass product. Today there is world wide competition from over 100 manufacturers of AR glass fiber products.

Figure 3: Stress Strain Chart for Glass Fiber Reinforced Concrete GFRC.

Concrete, reinforced with fiberglass, has been found to perform very well. On the stress/strain chart (Figure 3, left side "A") shows each component (cement matrix and glass fibers) by themselves. (Figure 3, left side "B") shows the composite system. In Region 1, the matrix and fibers act together (as stated previously) behaving essentially elastically. At this stage, there are no visible cracks in the matrix. At the end of Region 1, the matrix cracks and the load at the cracked surface is transferred to the glass fibers. This is called the "First Crack" point. The process continues through Region 2 and very fine, faintly visible cracks appear on the tension face. As more cracking occurs, it reaches a point when it is no longer possible to transfer sufficient load back into the matrix. This is the end of Region 2. In Region 3, the loading is carried only by the glass fibers, and continues this way until failure.

In addition to its structural properties, glass fiber reinforced concrete (GFRC) is often used as an architectural finished product. When this is done, it is very important to control its final appearance and the methods for producing GFRC products are, therefore, critical. the fabricator must be experienced in GFRC construction of the type needed for the specific intended use. For example, GFRC can be fabricated into large building panels of relatively simple geometry (Figure 2) or into very complex ornate shapes simulating terra cotta, on restoration projects (Figure 1). Each condition requires special fabrication techniques to produce the desired finished product.

Figure 4: Early development in GFRC anchoring used direct attachment of the metal frame to the facing concrete.

The methods used to anchor GFRC elements to a building have varied greatly over the years since it was first used. Original technology included a direct method of attachment of the cement slurrey to a steel "cage" of 6" by 16 gauge metal studs (see Figure 4). It was discovered that GFRC moves a lot and may break these direct anchors. Many methods of attachment have been employed including the slip anchor (see Figure 5). Today, each fabricator will use the methods that have worked best for them. Their engineers are comfortable with specific methods of anchoring, both the slurry to the cage, and, in turn, the steel cage to the building frame. It is advisable to use their system and coordinate the building frame with GFRC panel requirements.

Figure 5: Modern GFRC anchor detail with slip connection for panel movement.

Glass fiber reinforced concrete has been used with a great deal of success since its beginning in the 1970's. one example is the Olympic / Four Seasons Hotel in downtown Seattle. GFRC panels were used to simulate terra cotta in the hotel addition and restoration project. At the entrance, interlocking panels of GFRC over 6" x 16 gauge metal stud back-up simulate flat, rectangular terra cotta modules. These panels were erected in sequence and sealed over backer rods at their perimeter joints. The entire surface was then painted to give the wall a look of being made of the small terra-cotta stones.

Ornamental shapes, such as columns, capitals, freezes and lintels, were made by taking molds from the original terra cotta (when available) or building new molds from the architect's detailed design drawings. The GFRC matrix was sprayed into these molds and the steel support frame anchored directly to the facing material by spraying additional GFRC in puddles over frame edges (see Figure 4). Individual elements are supported by steel or masonry building structure and edges sealed over backer rod.

The Olympic / Four Seasons Hotel has been a very visible success for GFRC, however, this is not always the case. Many structures have used GFRC as a substrate for dissimilar facing products such as ceramic tile, thin brick, and other architectural concrete mixes. These composite GFRC wall panels have shown new behaviors that warrant careful study and understanding.

If you have ever painted the surface of a free-standing sheet of plywood or particle board, you might have noticed that, unless you "back paint" the reverse surface, the panel will bow. This bowing is caused by surface tension on the painted surface which, in turn, is caused by the tendency of the paint to shrink as it dries. By painting the plywood panel you have created a "composite structure" of paint and wood, and unless the wood is held securely to a wall, it will bow. This is a rather simplistic example used to illustrate some of the dynamics occurring in composite GFRC panels. When ceramic tile, brick, stone chips or even a thin layer of "Architectural Concrete" are used for building decoration, they can cause results like the paint on the plywood panel. That is, the composite panel may bow or deform under the developed stresses.

This problem is addressed in a paper titled, Aging and Cracking of Composite GFRC Wall Panel Skins on Metal Stud Frames in the United States, by D.W. Pfeifer, E.A. Rogalla and W.J. Nugent of Wiss, Jenny Elstner Associates (WJE), Northbrook, Il. Their study shows that "...when GFRC is composite with conventional face mix concrete, ceramic tile, clay brick or terra cotta, a large potential for out-of-plane bowing occurs due to differential thermal expansion properties [of these facing materials and the backing]. GFRC can develop up to 3 to 4 times the thermal expansion potential when compared to the other [facing] materials."

WJE goes on to address shrinkage of GFRC when applied over decorative facing materials. Their report shows the huge potential for out-of-plane bowing when GFRC is attached to ceramic tile. When they tested GFRC over face mix concrete the results showed that, even when two shrinkable portland cement based materials are composite, the potential for bowing is at least half that for the composite ceramic tile panel.

In the previous example of the Olympic/ Four Seasons Hotel, the GFRC was not covered by other decorative materials except paint. This was probably a contributing factor in the successful behavior of the GFRC panels on that project. In contrast to the direct anchor method of attaching GFRC to the metal stud framing used at the Olympic Hotel, WJE recommends a sliding connection that allows the GFRC to move. (See Figure 5) This connection allows the panel to breathe while transferring gravity and wind loads to the building frame. This does not restrain the panel, or attempt to keep the panel from moving thermally or from moisture absorption. As a result, these GFRC panels will move, and joints must be designed and sized accordingly.

Early in this article it was shown how Fiber reinforcing can add strength to concrete by holding it together after its "first crack strength". Glass Fiber Reinforcing Concrete, however, has a tendency to "age", or loose ductility with time. Tests conducted by WJE show that GFRC can loose over half of its strength when subjected to aging tests, and when testing naturally ages samples and comparing with data when the sample was new. This is important when considering using GFRC in members that support other members.

Figure 6: GFRC is not to be used for anchoring other building systems, i.e., windows, storefronts, or even building signs.

Generally GFRC is not considered to be a material that can support something else. For example, when an exterior wall panel contains a window, the window must not be anchored to the GFRC, but to some other supporting member. It is not sufficient to merely imbed steel plate inside the GFRC to accommodate an attachment. Figure 6 shows an incorrect attachment of a window. As the GFRC ages, the forces acting on the connection can crack the GFRC flange. Figure 7 illustrates a connection to the steel tube framing. The only stress on the GFRC return is from the backer rod.

Figure 7: This detail shows additional structural support for anchoring a window.

Design and construction of GFRC has proven to be more complex than originally anticipated. When composite materials are used, panels can build stresses that cause panel failure. Further, the strength values used to design GFRC must be evaluated due to its propensity to age and become weaker. Many of the standards in GFRC construction such as Precast/Prestressed Concrete Institute (PCI) Recommended Practice for Glass Fiber Reinforced Concrete Panels (MNL 128) were written in the middle to late 1980's and are not up-to-date relative to the failures that have occurred in recent years. New building designers would be well advised to bring on board an expert like WJE who has data beyond that which is generally available through the trade organizations.

It has been proven through laboratory testing that the addition of steel, glass and nylon fibers improves the strength and durability of concrete. These fibers allow concrete to be used where added strength, impact resistance, and/or reduced weight are desirable. GFRC has been used to cover bare earth in tunnel construction, and to form lightweight building panels where precast would be too heavy. Nylon and other plastics are being used to give slabs on grade added strength, crack resistance and even moisture resistance. Fibers are also used to reinforce mortar and plaster. We can look to new uses as these products are tested and improved. As always, caution is advised when using any new ideas and technology. Care in using fiber reinforced concrete can result in exceptional architecture of lasting beauty and performance.

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