Civil Engineering: Concrete Technology I Chapter 4

Water in Concrete

• Water is an essential ingredient in concrete, participating in the chemical reaction with cement.
• The quality of water is crucial, as impurities can interfere with cement setting, affect concrete strength, or cause surface staining and reinforcement corrosion.
• Suitable water for mixing and curing purposes should be considered, including tap water with a pH between 6 and 8 that doesn't taste saline or brackish.
• Sea water or water with high chloride content should not be used, especially when appearance is important or a plaster finish is applied, as it can cause corrosion of the reinforcement.

Chemical Admixtures

• Admixtures are materials added to concrete, excluding cement, water, and aggregates, to achieve specific properties.
• Major reasons for using chemical admixtures include:
  • Achieving certain properties effectively
  • Maintaining quality during mixing, transporting, placing, and curing
  • Overcoming emergencies during concreting operations
  • Economy
• Admixtures are added in quantities no larger than 5% by mass of cement.

Types of Admixtures

• Admixtures can be classified into various types based on their function in concrete, including:
  • Type A: Water-reducing
  • Type B: Retarding
  • Type C: Accelerating
  • Type D: Water-reducing and retarding
  • Type E: Water-reducing and accelerating
  • Type F: High-range water-reducing or superplasticizing
  • Type G: High-range water-reducing and retarding, or superplasticizing and retarding

Accelerating Admixtures

• Accelerating admixtures accelerate early strength development in concrete.
• They are useful in low-temperature placements, precast concrete, or urgent repair work.
• However, they can cause excessive heat of hydration and shrinkage cracking at high temperatures.

Retarding Admixtures

• Retarding admixtures delay the setting of cement paste.
• They are useful in hot weather concreting, preventing cold joints, and prolonging the time for transportation, placement, and compaction.
• Retarders include sugar, carbohydrate derivatives, soluble zinc salts, and soluble borates.

Water-Reducing Admixtures

• Water-reducing admixtures reduce the water content of the mix, usually by 5-10%, up to 15% in high-workability concretes.
• They allow for a reduction in the water-cement ratio while maintaining workability or improving it.
• Water-reducing admixtures can be used in pumped concrete or concrete placed by a tremie.

Superplasticizers (High-Range Water Reducers)

• Superplasticizers permit a reduction of water up to 30% without reducing workability.
• They are water-soluble organic polymers synthesized through a complex process, making them relatively expensive.
• Superplasticizers wrap themselves around cement particles, giving them a highly negative charge, resulting in deflocculation and dispersion of cement particles.
• They increase workability, allowing for flowing concrete with a high slump, which is useful for heavily reinforced sections, inaccessible areas, and rapid placement.

Other Types of Admixtures

• Air-entraining admixtures
• Pozzolanic or mineral admixtures
• Damp-proofing and waterproofing admixtures
• Gas-forming admixtures
• Air-detraining admixtures
• Alkali-aggregate expansion-inhibiting admixtures
• Workability admixtures
• Corrosion-inhibiting admixtures
• Bonding admixtures
• Fungicidal, germicidal, insecticidal admixtures
• Colouring admixtures
• Concrete curing compounds
• Polymer bonding agents
• Polymer-modified mortar for repair and maintenance
• Mould-releasing agents
• Protective and decorative coatings
• Installation aids
• Floor hardeners and dust-proofers
• Non-shrink high-strength grout