Hybrid Fiber Reinforced Concrete – Advantages and Applications in Construction

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A hybrid fiber reinforced concrete is a composite of two or more fibers in concrete. Types, mix proportions,  advantages and applications of hybrid fiber concrete is discussed.

History of Hybrid Fiber Concrete (HFC)

The concept of using fibers as a reinforcement in the concrete mixture is not a new study. The use of fibers has been carried out from ancient times. There are different types of fiber reinforced concrete that are categorized based on the fiber that is employed. If steel fiber is used we get steel fiber reinforced concrete. Similarly, nylon reinforced concrete, glass fiber reinforced concrete, carbon fiber reinforced concrete etc. are some of the types. A composite can be stated as a hybrid when two or more type of fibers is used in a combined matrix to produce a composite that will reflect the benefit of each of the individual fiber used. This will finally provide a synergetic response to the whole structure. Such a composite of concrete is termed as the Hybrid Fiber Reinforced Concrete (HFC). The mechanical properties of concrete are enhanced appreciably using short lengthened fibers. This increases the modulus of elasticity of the concrete. This will reduce the chances of brittleness and hence small crack formation, as small cracks are the main factors behind propagation and larger cracks formation. Fiber debonding or chances of pull out are less as this cause requires large energy absorption. This is the reason that provides fracture resistance and toughness to HFC during dynamic as well as cyclic loads.

Fibers Used in Hybrid Fiber Reinforced Concrete

It is observed that the use of a combination of both metallic and non-metallic type of fibers help in improving the concrete properties extensively. This also results in the overall reduction of cost in concrete production. The fibers can be divided into two groups:

1. Fibers that have a moduli value lesser than the cement matrix. Examples are: Nylon, cellulose and polypropylene
2. Fibers that have a greater moduli value than cement. Examples are the glass, steel, asbestos fibers etc.

Type-1 fibers are said to increase the strain performance of the concrete, while the type 2 fiber has greater modulus than cement and provide greater strength performance for the concrete. The addition of more fibers into the concrete mixtures make it more homogeneous and isotropic in nature. This brings a conversion of concrete nature from brittle to the ductile condition. This enhances the ductility behavior of the concrete under critical loads.

Hybrid Fiber Reinforced Concrete- Mixture Composition

The hybrid fiber concrete employs different type of fibers whose outcome is based on the type of orientation selected for the fibers in the concrete. This is a key factor that controls the application of HFC. It is recommended in an ideal situation that one should know to control the orientation of the fibers so that the desired load carrying capacity is gained. One of the best methods to obtain desired load carrying capacity condition is to have an HFC mix that is flowable and is stable in nature. This mix must also gain constant rheological properties. This is because a mix that flows with its own weight can only promote the design as well the control the orientation of the fibers that are intended to use. If the mix is self compacting, it should be able to satisfy this demand. A self-compacting hybrid fiber reinforced concrete mix is a flowable mix. Such a matrix while prepared should be adjusted and optimized as the incorporation of fibers should not badly affect the matrix. So, selection of optimum cement quantity, optimum cement type and optimum aggregate type and quantity is carried out with respect to the fibers combination and quantity used. When analyzed at the microscopic level, it is essential that the cement matrix around each fiber used in the concrete must be dense enough that it must efficiently make use of the fiber when a pull-out is possible to occur during an opening or any kind of crack bridging. The pull-out test is carried out in single as well as short fibers to stimulate the bridging of cracks by the fibers. This test finally aims at bringing a cement matrix best suited for the fibers decided to use so that an efficient response to pull-out is obtained. The main parameters that govern these tests are:

1. The concrete composition: This is the composition from which the fibers pull out is intended to test
2. Presence of secondary fibers: These are the short fibers that are used
3. Inclination Angles: The angle at which the fibers are laid and the pull out is carried out

Tensile Behavior of Hybrid Fiber Concrete

The main aim regarding the tensile strength of hybrid fiber reinforced concrete is to attain maximum tensile strength at minimum use of fibers. Hence the optimum amount of fibers will be used in the concrete is based on the strength obtained from the concrete. For this, a series of tensile strength tests is carried out. Initially, to determine the combination of fibers to be employed, flexural tests are carried out. This is followed by Uniaxial tensile tests, which is performed only on the selected HFC. This is to finally evaluate the tensile properties. The test results provide the main basis for HPC utilization. The basic factors of fibers that affect the tensile properties of Hybrid Fiber concrete are:

1. Type of Fiber
2. Quantity of Fiber
3. Orientation of Fiber
4. Number of Each Fiber

Hence, after the evaluation of tensile properties, each property (mentioned above) will be further evaluated in detail for complete assurance. A detailed analysis and results on the fracture process and behavior of fibers in HFC can be done through the analytical modeling. Analytical modeling provides the contribution of each fiber type that is used in HFC. The single fiber pull-out test and the Uniaxial tensile test provides the results to model the Uniaxial tensile behavior. This model is followed by the analysis of fiber number and their respective orientation. Proper utilization of hybrid fiber concrete can be enabled by creating appropriate stress-strain relationships. The Uniaxial tensile tests will provide results for this.

Basic Design Principles of Hybrid Fiber Concrete

The basic principles behind the design of HFC are:

• Use of short and long steel fibers together in the same concrete mix
• To ensure homogeneous distribution of the fibers in the mix
• To ensure that each fiber employed in the mix is effectively used

Principle 1: Use of Long and Short Steel Fibers

This criterion is based on the fracture mechanism that is taken by the fibers in the concrete mix. This is the case of long or short steel fibers or hooked end steel fibers. Initially, the cracks are formed in the interfacial zone with the tension load. These micro cracks propagate with addition of more tension load. Hence the cracks will connect forming larger cracks and cause the failure of the concrete. The figure-1 below shows the effectiveness of short and long fibers in micro and macro cracks respectively. So, this brought an idea of combining both the highlights. An idea of using both short and long fibers are shown in figure-1(b).

Fig.1. (a) Bridging of micro cracks by short fibers (b) Bridging of macro cracks by long fibers and micro cracks by short fibers.

The short fibers are very thin and will be large in number when compared with long fibers. This helps them to bridge micro cracks more efficiently. This is shown in figure-1(a). The short fibers enable an enormous increase in the tensile strength if the factor of consideration is the formation of micro crack and crack bridging. But with load, these micro cracks grow to larger macro cracks. In this situation, the hooked-end fibers act more effectively in the bridging of the macro cracks. This will increase the ductility and the tensile strength. A post-peak response is hence provided using long fibers. As the load increases, the crack widens. This situation makes the short fibers to pull out more easily making it inactive. Hence, the combination of both the fibers proves good and an optimum solution. Hence, tensile and ductility property is increased.

Principle 2: Homogeneous Distribution of Fibers in the Mix

It is very essential to have homogeneity all over the structural element at the structural level. The spacing between the adjacent fibers must be equal throughout the mixture. Hence, this must not be too close nor too away. Too close arrangement will result in the segregation of the fibers. These requirements can be achieved best by employing self-compacting concrete. As such special concrete flows by its own weight, the fibers will itself arrange in proper and appropriate direction and orientation. This will avoid segregation and change in orientation are made due to compaction. Hence a proper and desirable homogeneous distribution of fibers is obtained.

Principle 3: Effective Use of Each Fiber Used

The tensile stress in concrete creates cracks. Every fiber present can resist these micro or macro cracks. This provides high resistance against the crack opening. This resistance can only be achieved if the tensile strength of each fiber used in the concrete is completely utilized. This will be called as the efficiency of the fibers. The efficiency of the fibers depends upon the cement matrix on which the fiber is lying. The factors that determine the fiber efficiency are the w/b ratio, the inclination of the angle of the fibers, the type and length of the fiber, a number of short fibers and the cement quantity.

Advantages of Hybrid Fiber Reinforced Concrete

1. Crack Bridging at two stages is carried out: As two types of fibers are used, one will treat the initial micro cracks. Further chances of macro cracks are treated by next type of fibers. This is not achieved by a single type of fiber. 2. Two or more types of system: One type provides strength and stiffness. The other type will gain flexibility and ductility. 3. It can use fiber with different durability. The strength and toughness are increased by using durable fiber.

Applications of Hybrid Fiber Reinforced Concrete

1. Hybrid Fiber Reinforcement in Concrete Pavements The figure below shows the use of HFC in the construction of concrete pavement for warehouses. The slab thickness is of 22cm. The fibers used are 30kg/meter cube of steel and 1kg/meter cube of polypropylene fibers. The addition of fibers didn’t affect the aesthetics appearance of the concrete pavement.

Fig.2. The final appearance of the concrete pavement (HFC) before and after swept

The maintenance report showed that no kind of cracks were reflected despite the heavy traffic loads and climatic conditions. 2. Rehabilitation of Bridge Deck Using Hybrid Fiber Reinforced Concrete The Rogotin Bridge 's deck had undergone a repair by using HFC. This proves that the hybrid fibers can be used in both young and hardened concrete, by preventing the cracks.

Fig.3. The concrete placing work carried out during the bridge deck work

Other applications of hybrid fiber reinforced concrete include: 3. Construction of machine foundation: The machine foundation is subjected to large dynamic loads. The Hybrid fiber concrete responds appreciably towards dynamic and cyclic loads. This hence is a better solution over conventional machine foundation. 4. Tunnel Linings: Tunnel lining is an essential construction that follows difficulty in durability. The use of HFC will prove good in tunnel construction. Read More:

Applications of Fiber Reinforced Concrete in Pavements

Steel Fiber Reinforced Concrete Mix Preparation and Uses