Special Concrete
Introduction and Context¶
Special concrete refers to modified concrete formulations where chemical and mineral admixtures are incorporated into normal concrete to enhance its strength, stability, and overall performance characteristics. These specialized formulations have been developed to address specific engineering challenges and environmental conditions that conventional concrete cannot adequately handle.
Applications and Advantages of Special Concrete¶
Special concrete is engineered for use in demanding conditions and applications:
- Extreme weather conditions: Suitable for both hot and cold climates
- Large structures: Provides enhanced performance for major construction projects
- Marine environments: Offers excellent protection against chloride attack, preventing corrosion of steel reinforcement
- Chemical exposure: Used in pipes and structures to resist chemical attack
Key Performance Characteristics¶
- High Young's modulus (stiffness)
- High flexural strength
- Low shrinkage capacity
- Reduced creep
- Excellent durability
- Low heat of hydration
- Low resistance to de-icing salts (requires consideration in cold climates)
Classification of Special Concrete¶
Based on specific properties and applications, special concrete can be classified into the following types:
- Lightweight concrete
- High strength concrete
- Fibre reinforced concrete
- Ferrocement
- Shotcrete
- Polymer concrete
- High performance concrete
- Vacuum concrete
- Geopolymer concrete
1. Lightweight Concrete (LWC)¶
Definition and Purpose¶
Lightweight concrete has a density ranging from 300 to 1850 kg/m³, compared to normal concrete which has a density of 2200 to 2600 kg/m³. The primary objective is to reduce the dead weight of structures while maintaining equivalent load-carrying capacity. This reduction in self-weight is particularly valuable in high-rise buildings, long-span structures, and applications where reducing foundation loads is beneficial.
Methods of Production¶
Lightweight concrete is achieved through three main approaches:
- Using lightweight aggregates to replace conventional dense aggregates
- Introducing air bubbles into the mortar matrix to create a cellular structure
- Omitting fine aggregate to create a porous structure with interconnected voids
Types of Lightweight Concrete¶
1.1 Lightweight Aggregate Concrete¶
This type utilizes aggregates with lower specific gravity than conventional aggregates. The aggregates can be categorized as:
Natural Lightweight Aggregates: - Pumice (volcanic glass) - Diatomite (fossilized algae) - Scoria (volcanic rock) - Volcanic cinder - Sawdust (organic) - Rice husk (organic)
Artificial Lightweight Aggregates: - Artificial cinder - Coke breeze - Foamed slag - Bloated clay (expanded clay) - Expanded shale and slate - Sintered fly ash - Expanded perlite
These lightweight aggregates significantly reduce the overall density of the concrete while maintaining structural integrity.
1.2 Aerated or Cellular Concrete (Foamed Concrete)¶
This concrete is produced by introducing air or gas into the mortar mix. When the mixture sets and hardens, a uniform cellular structure is formed throughout the matrix. This cellular structure consists of millions of tiny air bubbles that dramatically reduce the overall weight.
Gas Generation Method: - Aluminum powder or zinc is added to the mix - These metals react chemically to produce hydrogen gas - The gas creates bubbles that become trapped in the setting concrete - Results in a lightweight, insulating material
1.3 No-Fines Lightweight Concrete¶
In this variant, fine aggregate is completely omitted from the mix, using only coarse aggregate bound together by cement paste.
Characteristics: - Contains large interconnected voids between coarse aggregate particles - The voids reduce density and provide drainage capabilities - Particularly suitable for pavement construction and applications requiring permeability
[Space for diagram showing the three types of lightweight concrete structure]
2. High Strength Concrete (HSC)¶
Definition¶
High strength concrete is defined by its compressive strength at 28 days of water curing. When the concrete grade exceeds M35 (35 MPa compressive strength), it is classified as high strength concrete. Modern HSC can achieve compressive strengths exceeding 100 MPa.
Context and Need¶
Conventional concrete made with only cement, aggregate, and water has limitations in achieving very high strengths. High strength concrete is essential for:
- High-rise buildings (reducing column sizes)
- Long-span bridges
- Offshore structures
- Structures requiring high durability and low permeability
Production Methods¶
Producing HSC requires specialized techniques beyond conventional mixing:
(a) Seeding: Introducing nucleation sites to control crystal growth
(b) Re-vibration: Applying vibration at two stages to improve compaction
(c) High-speed slurry mixing: Intensive mixing to improve cement particle dispersion
(d) Use of admixtures: - Chemical admixtures (superplasticizers) for workability - Mineral admixtures (silica fume, fly ash) for strength and density
(e) Inhibition of cracks: Techniques to minimize microcracking during curing
(f) Sulphur impregnation: Filling pores with sulphur to increase strength
(g) Use of cementitious aggregates: High-quality aggregates that bond better with cement paste
3. Fibre-Reinforced Concrete (FRC)¶
Composition and Concept¶
Fibre-reinforced concrete consists of cement, aggregate, and uniformly dispersed fibres throughout the mix. Unlike conventional reinforcement (rebars) which is placed strategically, fibres are distributed randomly in three dimensions throughout the entire concrete matrix.
Fibre Characteristics¶
Fibres are characterized by their aspect ratio, which is the ratio of fibre length to its diameter. This ratio significantly influences the fibre's effectiveness in reinforcing the concrete.
Types of Fibres¶
Metallic Fibres: - Steel fibres (most common)
Synthetic Fibres: - Carbon fibres (high strength, expensive) - Glass fibres (corrosion-resistant) - Polypropylene fibres (economical)
Mineral Fibres: - Asbestos fibres (rarely used now due to health concerns)
Organic/Natural Fibres: - Sisal fibres - Jute fibres - Coir fibres - Other plant-based fibres
[Space for diagram showing fibre distribution in concrete matrix]
Advantages and Performance Benefits¶
1. Enhanced Tensile Strength: Fibres bridge microcracks, significantly improving the concrete's tensile capacity and ductility.
2. Reduced Defects: Fibres help minimize the formation of air voids and water channels (inherent characteristics of the gel structure), resulting in a denser matrix.
3. Improved Durability: Better resistance to cracking leads to enhanced long-term durability and reduced permeability.
4. Creep Resistance: Fibres (particularly carbon and graphite) provide excellent resistance to creep deformation under sustained loads.
5. Shrinkage Control: Differential shrinkage and associated cracking is minimized, reducing deformation in the hardened concrete.
6. Enhanced Dynamic Properties: Improved impact resistance and energy absorption, making it suitable for structures subject to dynamic or seismic loads.
4. Ferrocement¶
Definition and Composition¶
Ferrocement is a thin-walled reinforced concrete structure consisting of hydraulic cement mortar reinforced with closely spaced layers of continuous and relatively small wire mesh. This mesh, sometimes called "chicken mesh," can be metallic or made from other suitable materials.
Material Components¶
1. Cement Mortar Matrix: - Fine sand and cement - No coarse aggregate (unlike conventional concrete) - Provides the binding medium
2. Skeletal Steel Reinforcement: - Multiple layers of fine wire mesh - Closely spaced to provide three-dimensional reinforcement - Creates a composite material with unique properties
[Space for diagram showing ferrocement layered structure]
Advantages¶
- Versatility: Can be formed into complex curved shapes easily
- Cost-effective: Significant cost savings in materials and construction
- Flexible: Easy cutting, drilling, and joining
- Appropriate technology: Well-suited for developing countries with limited resources
- Fire resistance: Good performance in fire conditions
- Impermeability: Excellent waterproofing characteristics
- Low maintenance: Minimal maintenance requirements over service life
- Reduced labor: Lower skilled labor requirements
- No formwork: Eliminates need for conventional formwork, enabling rapid construction
- Waterproof: High resistance to water penetration
- Lightweight: Reduced dead weight compared to conventional concrete
Applications¶
- Boat and ship hulls
- Water tanks and reservoirs
- Roofing elements
- Shell structures
- Repair and rehabilitation works
5. Shotcrete (Sprayed Concrete)¶
Definition and Process¶
Shotcrete is concrete that is pneumatically projected at high velocity onto a surface. The term comes from "shot" (propelled) and "concrete." The high-velocity application results in excellent compaction and bonding with the substrate.
Applications¶
Repair and Restoration: - Repairing damaged concrete surfaces - Breeze degree applications (severely deteriorated concrete) - Fire and earthquake damage repair - Strengthening existing walls and structures
Marine Structures: - Repair and protection of marine structures - Application in tidal and splash zones
Underground Construction: - Rock support in underground excavations - Tunnel lining - Mine shaft stabilization
Specialty Construction: - Swimming pools (forming complex curved surfaces) - Tanks and reservoirs with well-compacted sub-base - Architectural features requiring complex geometries
[Space for diagram showing shotcrete application process]
6. Polymer Concrete¶
Definition and Process¶
Polymer concrete is produced through polymerization, which involves impregnating monomers into the pores of hardened concrete, then initiating polymerization through thermal treatment or chemical catalysts. This process fills the pore structure with polymer, creating a denser, stronger, and more durable material.
Types of Polymer Concrete¶
- Polymer Impregnated Concrete (PIC): Existing hardened concrete is impregnated with monomers
- Polymer Cement Concrete (PCC): Polymer is added to fresh concrete mix
- Polymer Concrete (PC): Aggregate bound entirely with polymer (no cement)
- Partially Impregnated and Surface Coated: Selective impregnation and surface treatment
Common Monomers Used¶
- Methyl methacrylate (MMA) - most common
- Styrene
- Acrylonitrile
- T-butyl styrene
- Various thermoplastic monomers
Applications¶
Industrial and Power: - Nuclear power plants (radiation resistance) - Chemical processing plants
Infrastructure: - Curbstones - Prefabricated structural elements - Precast deck slabs - Road works (overlays and repairs) - Marine structures (piers, docks)
Specialized Structures: - Pre-stressed concrete elements - Irrigation works (canals, gates) - Sewage works (resistant to chemical attack) - Waterproofing systems for buildings - Food processing facilities (hygenic surfaces)
7. Vacuum Concrete¶
Concept and Rationale¶
A fundamental challenge in concrete technology is balancing two competing requirements: - High water content: Needed for workability during placement - Low water content: Required for high strength and durability
Vacuum concrete resolves this dilemma through a specialized technique that allows placement with high workability while achieving the benefits of low water-cement ratio.
Process¶
- Concrete is mixed with higher water content than ultimately needed (for workability)
- After placement and initial consolidation, the concrete is subjected to vacuum pumping
- Excess water—not needed for the hydration process—is extracted through vacuum suction
- This removal occurs before initial setting, while concrete is still workable
- Results in concrete with high initial workability but ultimate properties of low water-cement ratio concrete
Benefits¶
- High quality concrete with improved strength
- Better durability due to reduced porosity
- Enhanced density and reduced permeability
- Faster strength gain
- Reduced bleeding and segregation
[Space for diagram showing vacuum concrete process]
8. Geopolymer Concrete¶
Definition and Environmental Context¶
Geopolymer concrete is an eco-friendly alternative to conventional Portland cement concrete. It represents a significant innovation in sustainable construction, as Portland cement production is responsible for approximately 8% of global CO₂ emissions.
In geopolymer concrete, Portland cement is replaced by geopolymer materials—primarily industrial by-products—that are activated by alkaline solutions to form the binding matrix.
Materials Used¶
Primary Binders (Geopolymer Precursors): - Fly ash: By-product from thermal power plants (coal combustion) - GGBS (Ground Granulated Blast Furnace Slag): By-product from steel manufacturing
Other Components: - Fine aggregate (sand) - same as conventional concrete - Coarse aggregate - same as conventional concrete - Alkaline activator solution: Typically sodium hydroxide and sodium silicate solutions that activate the geopolymer reaction
Advantages¶
- Significant reduction in carbon footprint
- Utilizes industrial waste materials
- Excellent durability and chemical resistance
- High strength potential
- Reduced environmental impact
- Lower energy consumption in production
Environmental Impact¶
By replacing Portland cement with industrial by-products, geopolymer concrete: - Reduces CO₂ emissions from cement production - Provides beneficial use for waste materials - Decreases landfill requirements for industrial by-products - Contributes to sustainable construction practices
Conclusion¶
Special concrete types have been developed to address specific engineering challenges and environmental conditions that conventional concrete cannot adequately meet. Each type offers unique properties and advantages:
- Lightweight concrete reduces structural dead loads
- High strength concrete enables taller and longer-span structures
- Fibre-reinforced concrete improves tensile strength and crack control
- Ferrocement provides thin, strong, and versatile construction
- Shotcrete enables repair and construction in difficult access areas
- Polymer concrete offers exceptional durability and chemical resistance
- Vacuum concrete optimizes workability and strength simultaneously
- Geopolymer concrete provides an environmentally sustainable alternative
Understanding these special concrete types allows engineers to select the most appropriate material for specific applications, optimizing performance, economy, and sustainability in construction projects.