The efficient passage of stress between and the fibers determines many of the features of FRC. The following variables affect the traits and functionality of the FRC matrix.
- Types of Fibers
- Aspect Ratio
- Fiber volume and spacing
- Orientation of fibers
- Mix and compaction factor
The combined effects of the fibers and matrix determine the characteristics of fiber composites. The characteristics of a composite depend on the fiber parameters, such as diameter, density, rupture modulus, chemical resistance, Poission’s ratio, elongation, etc. Steel fibers are superior to natural and synthetic fibers because of their high Young’s modulus. Natural fibers are not frequently employed in modern construction because of their low elasticity modulus and susceptibility to insect and fungus attack.
Aspect Ratio: The diameter and length of a fiber affect its strength. The aspect ratio is the proportion of a fiber’s length to its diameter. Compressive strength of SFRC is increased by increasing aspect ratio up to 60%, after which the rate of increase decreases. There is an ideal aspect ratio, which varies depending on the type of fibers. The strength of SFRC decreases if the ratio is below the optimal level. Fibers ball up if the value is higher than the ideal. The propensity of fibers to band together while combining with the matrix and aggregate is known as balling of fibers. Concrete loses some of its workability as a result of balling.
Fiber Spacing and Volume:
Fiber volume is stated as a proportion of concrete volume. It is discovered that the strength of FRC is a linear function of fiber volume. The amount of fibers can only be increased so much before hitting a ceiling. The amount of fibers known as the critical fiber volume are those that could still support the weight of the FRC even after the matrix has cracked. Following matrix cracking, the fiber volume fraction determines the fibers’ ability to support loads. Spacing reduces with increasing fiber capacity. (7) The number of fibers crossing a unit area of a particular plane section through material or the distance between the centroids of individual fibres may be used to compute the spacing.
A typical reinforcing bar is oriented in the desired direction while fibers are oriented randomly, which is one of the contrasts between conventional reinforcement and fiber reinforcement. The resistance to crack propagation and tensile strength are influenced by the distribution direction of the fibers. Parallel fibers are around 30% more tough than randomly dispersed ones, however perpendicular fibers have no resistance to crack propagation and no tensile strength. (18) It’s noteworthy to note that randomly oriented fibers have a tensile strength ratio of 0.85 compared to parallel fibers. It closely resembles the ratio for the fiber reinforced composite’s elastic modulus that was theoretically expected.
Workability and Compaction of Concrete
The workability is significantly reduced by the addition of steel fiber. The consolidation of fresh mix is negatively impacted by this condition. Concrete cannot be compacted by continuous external vibration. The fiber volume at which this condition is obtained is determined on the fiber’s length and diameter. The dispersion of the fibers is not uniform, which is another effect of poor workability. In general, increasing the water/cement ratio or adding some sort of water-reducing additive can improve the mix’s workability and compaction standard.
Coarse aggregate size
To prevent a noticeable decrease in the strength of the composite, the maximum size of the coarse aggregate should be limited to 10mm. Additionally, fibers serve as an aggregate. Despite having a straightforward geometry, they have a complicated impact on the characteristics of freshly cast concrete. The orientation and distribution of the fibers, and subsequently the characteristics of the composite, are controlled by the inter-particle friction between the fibers and between the fibers and aggregates. The mix can be greatly enhanced by admixtures that lessen friction and enhance cohesion.
Concrete with fiber reinforcement must be mixed under strict circumstances to prevent fiber balling, segregation, and generally problematic uniform mixing of the constituents. The challenges and tendency to ball up are made worse by an increase in the aspect ratio, volume percentage, and size and amount of coarse aggregate. It is challenging to blend materials with an aspect ratio of more than 100 and a steel fiber concentration of more than 2% by volume. It is crucial that the fibers are evenly distributed throughout the mixture, which can be accomplished by adding the fibers before the water. Fiber distribution will be improved when introduced through a wire mesh basket when mixing in a laboratory mixer. Other workable techniques must be used for field use.