Fiber:

A Supplemeant to Reinforced Steel

When was the last time you thought about hairy concrete? If your thoughts were of disgust, you aren’t alone. Concrete reinforced with fibers, such as hair or straw, has been used successfully for thousands of years - since the Roman Empire, in fact. The "hairy" appearance the fibers created earned them a bad reputation. The Roman Empire is long gone, but many of its concrete structures remain. The fact that these structures have endured is a true testament that the Romans may have been on to something. Today, fibers give all the benefits to concrete that they gave the Romans — but with a clean shave.

In the precast industry, many precasters are looking to fiber-reinforced concrete (FRC) as a means to increase efficiency and product quality. FRC can provide many benefits to precasters, but they should understand exactly what the fibers can do for them and maintain realistic expectations. Precasters and end-users alike can rest assured that advances in fiber technology have nearly eliminated the hairy appearance of FRC of the past.

Why Does Concrete Need Reinforcement?

Since so many applications for reinforced concrete exist - ranging from simple bumper blocks to complex bridge sections and culverts - we should first consider that the basic properties of concrete are characterized by a fundamental lack of adequate tensile strength to make use of its potential structural capacity. Consequently, it is uneconomical to use plain concrete in sections that are subject to tension in whole or in part (as in sections subjected to bending). This is the basic reason reinforcement in concrete is required. In precast concrete, reinforcement typically consists of deformed reinforcing steel bars (rebars) or welded wire reinforcement (WWR). The primary function of both rebar and WWR is to provide additional tensile strength to the internal structure of the reinforced concrete section. Reinforcing steel is placed in specific areas in the concrete product to resist the anticipated tensile forces at these particular spots. This type of structural reinforcement is generally referred to as "primary" reinforcement. Reinforcing steel is also used to provide additional compressive capacity and confinement in concrete sections in compression, such as columns or walls.

Concrete also undergoes significant volumetric changes during the course of its lifetime, typically from expansion and contraction due to temperature and moisture changes. Also, concrete tends to shrink slightly both during and after curing. Plastic shrinkage occurs at the surface of fresh concrete from rapid drying. Although subtle, these volumetric changes cause internal stresses within the concrete matrix, which could cause cracking if not otherwise restrained by reinforcement. To reduce this type of cracking, "secondary" reinforcement is required.

There are misconceptions about whether precast-concrete products are considered structural or nonstructural. Some argue that many products are nonstructural since they do not ultimately become part of a larger, more complex structure such as a bridge. However, at the very least, all products - big or small - must support their own weight. The majority of precast-concrete products requires additional structural strength. Internal reinforcement allows proper functioning by providing a long life, low maintenance, and sufficient durability - regardless of whether the product is considered structural or nonstructural.

Desired Properties of Precast Concrete

For precast concrete in particular, the anticipated exposure conditions define which properties should be maximized. Desirable concrete properties for precasters generally include:

* Easy batching and mixing.
* Flowable, workable, and placeable concrete mix.
* Rapid setting time, hydration, and strength gain.
* Resistance to cracking from drying and plastic shrinkage.
* Resistance to abrasion (for hydraulic structures).
* Resistance to cracking due to thermal and/or moisture stresses.
* Ability to withstand handling stresses and impacts at an early age.
* Ability to adequately resist loads and stresses after installation.
* Low permeability and watertightness.

Most precast-concrete products are manufactured in a controlled environment, and extended curing times are usually not possible. So precasters are also looking for efficiency in their production materials. They are also looking for ways to reduce material and preparation time prior to casting.

The durability of concrete is also a hot topic these days. The word "durable" is defined as having resistance to wear or decay. In general, concrete fits that description. However, it is possible - and appropriate - to enhance the natural properties of concrete to increase durability. Since many precast products are buried, doesn’t it seem appropriate to provide a more durable product? Even if precast products are not buried, conditions such as thermal or freeze-thaw cycles also require enhanced durability. Precasters should maximize the durability of their products for both above and below grade.

Effects of Fiber Reinforcement

The primary use of fibers in concrete is to enhance the properties of concrete containing conventional reinforcement. The enhancements of concrete properties include:

* Resistance to crack propagation due to plastic and drying shrinkage.
* Resistance to thermal and moisture stresses.
* Increased ductility.
* Increased impact and abrasion resistance.
* Increased tensile, flexural, and fatigue strength.
* Decreased permeability.
* Decreased mix-water bleed rate.
* Decreased handling damage during transport of dry-cast products to curing areas.

All these enhancements depend on the type of fibers used and the concentration in the mix. Trial batching can optimize the best use of fibers. Some enhancements also can be achieved with modifications to the concrete mix design, curing, and other production operations rather than using fibers.

Types of Fiber Reinforcement

The six general types of fiber reinforcement materials are:

* Steel - including high-tensile strength and stainless steel.
* Glass - either ‘E’ or alkali-resistant.
* Synthetic - including polypropylene, polyethylene, polyester, acrylic, and kevlar.
* Carbon - either high-modulus or high-strength.
* Natural - including wood, sisal, coconut, bamboo, jute, akwara, and elephant grass.

Currently in the United States, steel, glass, and synthetic fibers are the most widely used fiber types. Blends of steel and synthetic fibers are also available. This article covers only the most commonly used fibers.

Steel Fiber Reinforcement

The majority of steel fibers used today has hooks or other deformations at the ends or are rough, crimped, or undulated along the length of the fibers. These designs are intended to improve the bond between the fiber and the surrounding concrete. Usually, either carbon or stainless steel is used. Fibers are typically one to two inches long.

The amount of fiber in concrete mixes typically ranges from 0.5 percent to 2.0 percent by volume, although smaller amounts have been used successfully in reduction of plastic- and drying-shrinkage cracking. According to the Portland Cement Association, steel fiber contents greater than 2.0 percent result in poor workability and fiber dispersion within the concrete mix.

With the addition of steel fibers, the concrete can be expected to have increased tensile, flexural, and fatigue strength. The exact amount of increased strength depends on many variables, especially fiber content. With fiber contents of 1.5 percent to 2.0 percent by volume, direct tensile strength will increase 30 to 40 percent, and flexural strength (first crack) will increase 50 to 150 percent.

So don’t expect to see concrete more than triple its original strength. Increased resistance to impact is probably one the greatest benefits to precasters of using steel fibers.

When choosing steel fibers, consider the durability of the fibers themselves and the aesthetics of the concrete surface. Like other embedded metal objects, steel fibers can rust if the surrounding concrete is cracked. Also, some surface-rust staining is possible, although corrosion-resistant steel fibers are available.

Glass Fiber Reinforcement

Glass fiber-reinforced concrete (GFRC) is used primarily for production of precast architectural cladding panels, building and site products, and many other applications. During the early stages of development in the 1940s by the Soviets, the glass fibers degraded quickly due to attack from the alkaline environment inside the concrete. Later, in the 1960s, alkaline-resistant fibers were developed. In the 1970s, the use of GFRC gained widespread acceptance in the United States.

The advantage of using GFRC for cladding is the capability of producing a relatively lightweight panel (on the order of 10 to 25 pounds per square foot, depending on surface treatments) and very thin sections (1/2-inch minimum). High early strength allows for reduced damage from handling. Also, GFRC products can be made in an almost endless array of shapes and sizes.

Synthetic Fiber Reinforcement

The most common types of synthetic fibers used in the precast industry are polypropylene and nylon fibers. Synthetic fibers consist of relatively short (1/8 inch to 2 inches), very thin lengths of material. Fiber contents of 0.1 percent to 2.0 percent by volume are typically used. These fibers interact by mechanically bonding to the concrete’s internal matrix and do not generally affect the curing process chemically. However, nylon absorbs water, while polypropylene does not. With fiber contents greater than 0.2 percent by volume, the water absorption of nylon fibers should be considered in the mix design.

Polypropylene fibers are by far the most common used in the precast industry and are available in two varieties: monofilament or fibrillated. Monofilament fibers are drawn though a circular die in a continuous process then cut to length. Fibrillated fibers are extruded though a rectangular die to create thin sheets. These thin sheets are then mechanically distressed, or fibrillated, to produce a two-dimensional fiber network. Fibrillated fibers utilize the mixing action to break the fibers apart prior to casting.

Synthetic fibers can best benefit concrete at early ages when it is most vulnerable. This is especially beneficial to precasters who need to strip the product from the forms and handle it within a day or so of casting. The fibers can decrease plastic- and drying-shrinkage cracking and increase impact resistance in young concrete products.

Mix Design and Testing Considerations

The concentration of fibers can affect the mix’s workability because of the relatively high surface area of fibers that must be coated with mortar (cement, fine aggregates, and water). An increase in the amount of fine aggregate to coarse aggregate ratio cement, and/or a high-range, water-reducing admixture (superplasticizer) will typically improve workability. Batch operators should resist the temptation to just add water to improve workability, because this causes many additional problems unrelated to fiber reinforcement. Fiber manufacturers have developed coarse-filament fibers in order to reduce the surface area of the fibers and, consequently, the demand for water.

In addition to the normal battery of fresh concrete tests, the inverted slump-cone test can be performed. This test measures the time it takes for the uncompacted concrete to flow through a slump cone with a vibrator inserted into the cone. Also, quality-control personnel should be aware that fibers will reduce slump from one to four inches.

It may be necessary to perform flexural toughness testing and/or residual strength testing when using a FRC mix. A testing laboratory can perform these tests easily.

Production Considerations

Batching of fibers has traditionally been performed manually by the mixer operator. Synthetic fibers are typically available in sealed paper bags that are added "wholesale" to the mix. These bags disintegrate during mixing. Bags of synthetic fibers come in incremental weights to accommodate various batch sizes and fiber contents. A new system for automated batching has been developed and is used in a few U.S. precast plants. This system is charged with bulk fibers and uses a special weigh hopper to batch very precise amounts of fibers. This is useful for mixing smaller-than-normal batches or for easily varying the mix’s fiber content.

Standard placing and vibration equipment typically can be used during production. It is likely that, in order to achieve adequate consolidation, increased vibration efforts will be required. Mixes with high fiber contents (especially steel fibers) generally require the use of external vibration or vibration tables. Special finishing techniques may be necessary to keep surface fibers properly embedded. Because of the additional production requirements associated with using FRC, especially steel FRC, additional employee training may be necessary.

Structural Considerations

Currently, the precast-concrete industry is under pressure to improve productivity and efficiency. The use of fiber reinforcement has been suggested as a means of reducing the amount of primary standard reinforcing steel required in a particular product. Precasters should not replace reinforcing steel with fiber reinforcement and expect the product to behave in the same manner. Standard steel reinforcement and fiber reinforcement are two completely different materials. Reinforcing steel, including rebar and WWR, are continuous reinforcing elements that are placed discreetly in the product to provide tensile resistance in specific areas, as mentioned earlier. This makes the placement of reinforcing steel critical. Because fiber reinforcement is discontinuous and is dispersed randomly throughout the concrete mix, placement of fibers is not as critical. This discontinuity and random orientation typically does not allow enough bond strength to develop in the concrete to fully utilize the fibers’ tensile strength. For optimal performance, however, both fiber and standard reinforcement should be used together, making use of the beneficial properties of both.

Currently, performance testing of final products is the best indicator of the structural capabilities of FRC. According to Mel Galinat of Synthetic Industries, no standard design methodology currently exists for FRC in the United States. An appropriate structural design approach would be completely different than that currently used for design of reinforced-concrete structures.

In Europe, some design methodologies already have been developed and are being studied by U.S. researchers. It’s only a matter of time before standard design and structural behavior prediction methods will be available. Until then, it is up to FRC advocates to convince regulators and end-users that the use of fiber reinforcement as a replacement for primary steel reinforcement is appropriate and acceptable. In addition, performance testing should be done that closely matches the loading conditions of a particular precast product after installation.

With appropriate design criteria and performance-testing requirements in place, the use of fiber reinforcement along with standard reinforcing steel will likely increase dramatically, and with adequate understanding of the potential benefits and realistic expectations of FRC, precasters can increase efficiency, quality, value, and permanence inherent in precast-concrete products.

(By Dean A. Frank, P.E., Dean Frank is Director of Technical Services at NPCA.)