FRC and SCC

Concrete Preparation

Concrete is a composite material made of cement, aggregates (fine and coarse), and water.

Mixing Sequence

1. Dry Mixing: Cement, aggregates, and powdered additives (e.g., fly ash) are mixed dry.

2. Wet Mixing: Water and liquid admixtures (like superplasticizers for SCC) are added.

3. Fiber Incorporation (for FRC): Discrete fibers are added gradually and mixed thoroughly to ensure uniform dispersion and random orientation, which is crucial for their performance.

Additives and Admixtures

• Additives (SCMs): Finely divided materials (e.g., Fly Ash, Silica Fume) that replace cement and modify long-term properties like durability and strength.

• Admixtures (Chemicals): Chemicals added in small quantities to alter properties.

  • Superplasticizers: Essential for SCC; dramatically increase workability (flow) while keeping the water-to-cement ratio low.

  • Viscosity-Modifying Admixtures (VMA): Used in SCC to prevent segregation (separation of materials).

Fiber Reinforcement vs. Steel Reinforcement

It is important to note that fiber reinforcement is not entirely the same as traditional steel reinforcement (rebar).

Feature	Fiber Reinforcement (FRC)	Traditional Steel Reinforcement (Rebar)
Primary Role	Crack Control, Toughness, Impact Resistance.	Carrying Primary Tensile/Bending Loads.
Placement	Randomly distributed throughout the entire concrete volume.	Placed strategically in the tension zones of the structure.
Effect on Cracking	Prevents or delays the formation of micro-cracks and bridges them once formed.	Used to carry load after the concrete has cracked; prevents catastrophic failure.

The primary role of fiber reinforcement is to prevent cracking and improve the concrete's post-cracking performance (toughness and residual strength).

Cracking in Normal Concrete

Two main types of cracking are controlled by FRC: plastic shrinkage (in fresh concrete) and drying shrinkage (in hardened concrete).

1. Plastic Shrinkage Cracking

How it occurs in Normal Concrete:

• This happens while the concrete is still plastic (fresh), typically within the first few hours of placement.

• When the rate of evaporation of surface water exceeds the rate at which bleed water can rise to the surface, the surface of the concrete begins to dry rapidly.

• The surface layer experiences capillary tension as the water leaves the pores, causing the paste to shrink.

• Since the fresh concrete below is still wet and resists this movement, tension develops on the surface, leading to short, shallow, and usually parallel plastic shrinkage cracks .

   

2. Drying Shrinkage Cracking

How it occurs in Normal Concrete:

• This happens in hardened concrete over weeks or months.

• As the concrete dries out and loses moisture to the surrounding environment, the cement paste shrinks.

• If this shrinkage is restrained (e.g., by the foundation, underlying ground, or other structural members), internal tensile stresses develop.

• When the tensile stress exceeds the low tensile strength of the hardened concrete, drying shrinkage cracks form.

How FRC Prevents Cracking

FRC mitigates both types of shrinkage cracking through the principle of crack bridging.

• Mechanism: The short, discrete fibers, distributed throughout the concrete matrix, act as internal reinforcement at the micro-level.

• Plastic Shrinkage Prevention: As the concrete surface attempts to shrink, the fibers resist the movement by effectively holding the paste together. They transfer the shrinkage stress across potential crack planes, which significantly reduces the width and prevents the formation of many plastic shrinkage cracks.

• Drying Shrinkage Prevention: In hardened concrete, if a micro-crack begins to form due to restrained drying shrinkage, the fibers span the crack faces. This bridging action restrains the crack opening, forces the stress to be transferred across the crack, and limits the crack width to very small, acceptable sizes, thereby preserving the concrete's durability.

Properties and Applications

1. Fiber Reinforced Concrete (FRC)

• Key Properties: Greatly enhanced Toughness (energy absorption capability), high Impact Resistance, and superior Crack Control.

• Applications: Industrial Floor Slabs (heavy-duty traffic), Airport Pavements, and Tunnel Segments (where post-cracking performance is critical).

2. Self-Compacting Concrete (SCC)

• Definition: Concrete that flows and consolidates under its own weight without vibration.

• Key Properties: Filling Ability (flows into formwork), Passing Ability (flows through rebar), and Segregation Resistance (remains homogeneous).

• Applications: Complex Structural Elements (heavily reinforced columns and walls) and Precast Architectural Shapes (requiring a flawless surface finish).