Technical Bullentins & Memos

Fiber

Fiber reinforced concrete is concrete made of hydraulic cements containing fine or fine and course aggregates and discontinuous discrete fibers.  Continuous meshes, woven fabrics, and long rods are not considered to be discrete fiber type reinforcing elements in this memo. 

Fibers have been produced from steel, plastic, glass, and natural materials in various shapes and sizes.

A convenient numerical parameter describing a fiber is its aspect ratio, defined as the fiber length divided by and equivalent fiber diameter.  Typical aspect ratios range from about 30 to 150 for length dimensions of 0.25 to 3 inches.

For a given mix, the fiber content, fiber geometry, and the distribution of the fibers are important to the properties of the fiber reinforced concrete.  However, the variables that are important for strength, durability, and workability(water/cement ratio, air content, density,ect.) also have a significant influence on the properties.  The variables that influence the bond affect the strength properties of fiber reinforced concrete.

Two mechanisms have been proposed for predicting the “first crack strength” or the proportional limit of fiber reinforced concrete.  One mechanism relates the “first crack strength” to the spacing of the fiber reinforcement while the other mechanism relates the proportional limit to the volume, orientation, and aspect ratio of the fibers.  The fiber spacing mechanism is based on crack arrest action by the fibers and linear elastic fracture mechanics concept. *  The other mechanism is based on the laws of mixtures of composite materials. 

It is generally agreed that the ultimate strength is relatively insensitive to fiber spacing and depends primarily on the volume, aspect ratio, and the bond characteristics of fibers. 

A fiber mix will generally require somewhat more vibration to move the mix and consolidate it into forms.  Properly controlled internal vibration is acceptable, but external vibration of the forms and exposed surface is preferable to prevent fiber segregation.  The fibrous nature of the mix makes the use of shovels or hoes difficult.  Forks and rakes are preferred for handling low slump mixes.  Standard screeding methods are acceptable, such as wood screed boards, portable vibrator mounted on boards, or by the mechanical method of the slipform paver and sidewalk paver.

 Metal trowels, tube floats, and rotating power floats can be used to finish fibrous concrete.  A texture surface can be obtained by brooming with a stiff brush, but should be delayed  as long as possible to prevent pulling fibers to the surface.  Fibrous concrete should be cured and protected using the same methods and techniques as for conventional concrete.  

Slump of concrete is significantly reduced by fibers.  Reliance on slump tests often results in the use of excessive amounts of water in an attempt to improve workability.