Structure of Fiber Reinforced Concrete
Fiber reinforced concrete (FRC) is a concrete that composes of cement, water, aggregates and fibers. The basic concept behind the development of FRC is explained here>>. The study of structure of fiber reinforced concrete is performed by studying three main components:
- The structure of the bulk cementitious matrix
- The shape and the distribution of fibers
- The structure of fiber-matric interface
The properties and behavior of fiber reinforced cementitious materials are dependent on the structure of the composites. The behavior of FRC during several loading conditions are dependent on the internal structure of FRC.
Lets discuss the internal structure of FRC in detail.
1. Structure of FRC Matrix
The structure of bulk cementitious matrix of fiber reinforced concrete does not have much difference compared to ordinary cement or concrete mix. It can have two main components namely:
- Paste or Mortar - Cement or sand water mixture
- Concrete - Cement-sand-coarse aggregate and water mixture
1. Paste or Mortar- Cement or Sand Water Mixture
When we consider fiber reinforced cement mortar or paste mixture, the fibers added in it act as primary reinforcement. The amount of fibers in this mixture ranges from 5 to 15 percent of the total volume. Usually, such FRC mixtures are used to construct thin sheet components and requires special production methods for their manufacture.
2. Concrete - Cement-sand-coarse aggregate and water mixture
When it comes to fiber reinforced concrete mixtures, the volume of fiber content is less than 2% and they act as secondary reinforcement.
To know more on fibers as primary and secondary reinforcement in cement mortar or concrete read the article : Fiber Reinforced Concrete (FRC)
Fibers used here serves the objective to control cracks. The production of these concrete is conducted by conventional method. By controlling the rheology and microstructure of the concrete mix, advanced matrix formulations can developed and used to incorporate higher content of fibers using simple mixing technology.
These advanced formulation combines uses fillers and dispersants in the process like DSP, RPC etc. These process provides a dense microstructure to these composites with an improved rheology. This would facilitate uniform dispersion of 2 to 6 percent by volume of short fibers which in turn act as an effective reinforcement.
2. Shape & Distribution of Fibers
The fibers that are used in concrete can have different mechanical, physical and chemical properties. These features are considered while selecting a fiber as a reinforcement for cement or concrete matrices. The fiber reinforcing array can take various geometries. While we characterize the nature of these geometries, two main levels of geometric description have to be considered.
- The shape of the individual fibers
- The dispersion of these fibers in cement matrix
2.1. Shape of Individual Fibers
Mainly individual fibers are classified into two:
- Monofilaments
- Fiber Assemblies
Monofilament fibers are fibers that are separated from one another like steel fibers as shown in figure-1 below. These are used for cement reinforcement. These type of reinforcement can deform to form various shape and configurations in order to improve the fiber-matrix interactions through mechanical anchoring.
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| Fig.1. Steel Fibers - Monofilament Configuration (a) deformed (b) hooked |
The fiber assemblies are usually made of bundles of filaments with each fiber having diameter not greater than 10μm. The bundled fiber is generally a combination of man-made fibers. It maintains its bundled nature in the composite.
2.2. Dispersion of Fibers
The fiber-reinforcing arrays can be of two types:
- Continuous Reinforcement
- Discrete Short Fibers
Continuous reinforcement takes the form of long fibers. These are included in the matrix by using techniques like filament winding or by laying up of fiber mats. These type of fibers can be aligned in required orientation that is controlled by the production process.
Discrete short fibers have less lesser than 50 mm that are placed in concrete either by spraying or mixing. These disperse in matrix in a more uniform manner and they take random orientation.
3. Structure of Fiber-Matrix Interface
The cementitious composites are characterized by the interfacial transition zone (ITZ). The size and nature of the transition zone is dependent on the type of fiber and the production technology. It might change with time. This influence the bond and debonding of the fiber-matrix interface.
The matrix closer to the fiber is more porous than the bulk paste matrix. This is reflected in the microstructure of the composite with the advancement of hydration process.
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