Composites of fibers and cement is called as fiber reinforced cement composites. Behavior of fiber reinforcement cement and high performance FRC composites are discussed.
- Introduction to Composites of Cement and its Performance Improvement
- Fiber Reinforced Cement (FRC) Composites
- High Performance Fiber Reinforced Cement Concrete (HPFRC)
- Deflection Hardening and Deflection Softening Applications
Introduction to Composites of Cement and its Performance Improvement
In recent years, fiber reinforced cement has attained greater momentum in the concrete construction industries. Understanding and controlling the matrix and the mechanism within these composites have resulted in the development of a mix with better fiber-matrix interface.
These developments facilitated by composite production process and the overall continual improvement in terms of their performance have rated fiber reinforced cement composites in a higher position.
This continuous improvement in performance was bought by any of the following means:
Introduction of new generation additives
The Superplasticizers and viscous agents are the new generation additives. These bring enormous strength to the matrix that is readily achieved. The loss of workability by their addition is negligible.
Use of active or inactive micro-fillers
Understanding the behavior of fly ash or silica which are referred as active and inactive micro fillers, when added in increasing amount into the mix will help in identifying the effect on strength, porosity, and durability.
Increasing wide variety fibers
The fiber influence in the concrete varies with the type, size and their respective properties, based on which the concrete mix can be designed. This has demanded the enormous use of fibers with concrete.
Use of Polymers
The polymer addition or any kind of impregnation increases the bonding strength of the concrete. The fiber and matrix bond is increased by this method, which will increase the efficiency of fiber reinforcement in design mix.
Self – Consolidation and Self – Compacting innovations
To bring a uniform mix when the quantity to manufacture is high can be attained by certain innovations.
The innovation of self-consolidation and self-compacting have increased the production of fiber reinforced cement concrete production and application. This innovation has the advantage of porosity reduction and mixing in large quantities.
Fiber Reinforced Cement (FRC) Composites
The composites with the components fibers and the matrix are termed as the fiber reinforced cement composites. The matrix is of cement, which itself is a composite formed of several materials. So, this is mentioned as matrix and the second main component as the fiber.
The fiber is discontinuous and its orientation is random in nature. The fibers are distributed within the volume of the whole composite. The fiber and the matrix together work to provide proper bonding and strength to the whole mix. These two components work together to form an effective composite.
The matrix will be either a paste or mortar or concrete with specified aggregates and additives. Any air voids contained within the matrix is considered as its part. The figure below shows the composite model of fiber and matrix with the help of a chart.
Fig.1: The Two component system – Fiber and Matrix; considered in a composite model
High Performance Fiber Reinforced Cement Concrete (HPFRC)
The HPFRC can be defined in terms of strain hardening behavior. One of the main parameters that can be employed to understand whether the fiber reinforced cement concrete qualifies as HPFRC, is the strain hardening behavior of the composite material which is obtained from the stress strain curve.
In the case of a high performance composite mixture, the strain hardening or pseudo hardening mechanism is said to happen immediately after the first crack. But is a case of a normal FRC composite mix sample, strain softening is said to happen just after the first crack.
Hence , High performance Fiber reinforced concrete composites is a type of FRC composites, which is distinguished by the strain hardening phenomenon in tension after the first crack. This later undergoes multiple cracking at higher strain levels.
Based on this hardening characteristics, fiber reinforced cement composites are of two kinds:
- Strain Hardening -HPFRCC
- Soft Hardening
The figure below shows the strain hardening nature (both strain hardening and softening)
Fig.2: Stress Strain Curve in tension till complete separation. The curve represents a conventional FRC composite undergoing soft hardening.
The strain hardening is a process accompanied by multiple cracking and this property is considered desirable. This gains large energy absorption capacity. The above definition mentioned for high performance fiber cement concrete composite is based on discussions carried out in five international symposia.
Fig.3: Stress-Strain Curve in tension till complete separation. The curve represents an HPFRCC type FRC composite undergoing strain hardening.
If the post cracking strength of the sample is and the cracking strength is represented as , then condition of achieving the strain hardening behavior can be expressed as
The post cracking strength and the cracking strength of a conventional FRC and HPFRCC are explained in the stress strain curve shown in the figure below. The load is direct tension load.
Fig.4: Typical Stress strain curve of Fiber Reinforced Cement (a) and HPFRCC (b) representing the soft and strain hardening behavior respectively. The post cracking strength and the cracking strength is represented in the cure.
From the above figure (b), the curve of HPFRCC starts with a steep initial increasing curve, till the first cracking. This forms the part I (Figure-3). Next, there is the occurrence of cracks in multiple amounts, which is the part II.
The stress and strain coordinates () represent the point of first cracking, figure-3. The maximum post cracking point is represented by the stress and strain coordinates, (). This point is the peak point where one of the cracks becomes critical. This will cause the crack localization resulting in the drop of resistance. This point will further develop and no other crack formation is found to occur.
After this part III is created, where the descending curve is observed. This is the effect of the opening of critical crack with the application of further load. At this situation, two possibilities can happen. Either the fibers can pull out or the fibers can fail or the combination of both can happen. At this, a small contribution towards cracking is provided by the cement matrix.
Based on whether soft or strain hardening is prevailing, there are chances for elongation of the composite to get translated as tensile strain, before the crack localization. Once the crack localization is carried out, the opening of the critical crack will control the behavior.
In the case of conventional FRC (Figure-2.), the starting of stress-strain curve is same as in HPFRCC. But the localization is occurring just below the first cracking. Here no strain hardening and formation of multiple cracks are found to occur.
Stress Formed at the First Cracking
The first cracking represents the visible crack that is formed first or the point where the deviation of the linearity in the curve is said to occur. There may be numerous cracks that are formed within the structure microscopically.
But this crack is called the “percolated crack“, a crack that creates a complete separation in the structural member (tensile member). The opening of the crack with the load application may be small or sometimes might be invisible for the naked eye to observe.
The percolation crack may not be formed perpendicular to the application of tensile load. It is formed by a combination of smaller or micro cracks. For the modeling purpose, the crack can be considered normal to the direction of load application. As mentioned before () represents the first cracking point.
Maximum Post Cracking Stress
The post cracking stress is the stress formed after the formation of percolation crack. The maximum value of post cracking stress is . The condition for strain hardening composites is . The condition is for soft hardening composites, as shown in figure-3 and figure-4. is the maximum post cracking strain.
Strain Hardening and Deflection Hardening of FRC Composites
The general classification of FRC composites as shown in figure-5, was suggested by a fourth international workshop on High Performance Reinforced Cement Composites (HPFRCC). This classification is based on the key response of the FRC sample, whether strain or soft hardening.
Fig.5: General Classification of Fiber Reinforced Cement Composites
The bending response of the structural member under the tensile action is explained through the figure-6. This shows the lead of the response either to deflection hardening or deflection softening. It has been observed that;
- Deflection hardening elements are formed by the strain hardening elements
- Either deflection hardening or deflection softening behavior can be shown by a soft hardening composite
- Compared to strain softening composite, the strain hardening composite gives a better performance.
Deflection Hardening and Deflection Softening Applications
Areas of structures, where bending prevails, the criteria for deflection hardening works better. The application of deflection softening mention lower areas- where it is used to avoid plastic shrinkage in concrete to a larger area for the construction of concrete pavements and slabs.
Fig.6: Volume of Fibers for Strain Hardening
The figure-6 above also represents the volume of a fraction of fibers that are essential to achieve the strain hardening or the deflection hardening as per the requirement.