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

This quiz covers the fourth chapter of Concrete Technology I, focusing on water and admixtures in concrete. It explores the quality of water and its importance in concrete production.