JNTU B.Tech III CONCRETE TECHNOLOGY Previous Question Paper (October - 2020 - R13)

An admixture is a material other than water, aggregates, hydraulic cement, and fiber reinforcement, used as an ingredient of concrete or mortar and added to the batch immediately before or during its mixing. Admixtures are used to modify the properties of concrete in the fresh or hardened state to make it more suitable for specific work or to save costs.

The primary functions of admixtures include:

• Workability enhancement: Water-reducing admixtures (plasticizers) improve flowability without increasing the water-cement ratio.
• Setting time control: Accelerators speed up the initial set for cold weather or rapid construction, while retarders delay the set for hot weather or long-distance transport.
• Durability improvement: Air-entraining agents improve resistance to freeze-thaw cycles, while waterproofing agents reduce permeability.
• Strength modification: High-range water reducers (superplasticizers) allow for high-strength concrete by enabling very low water-cement ratios.
• Economic efficiency: Admixtures can reduce the required cement content or allow the use of marginal aggregates.

Heat of hydration is the exothermic chemical reaction that occurs when cement comes into contact with water. As the cement compounds, primarily tricalcium silicate (C3S) and tricalcium aluminate (C3A), react to form hydration products, they release a significant amount of thermal energy.

The impact of this heat on concrete quality is significant:

• Thermal Cracking: In massive concrete structures (like dams or thick foundations), the heat generated in the core cannot dissipate quickly. This creates a large temperature gradient between the hot interior and the cooler surface, leading to tensile stresses that cause surface cracking.
• Delayed Ettringite Formation (DEF): If internal temperatures exceed approximately 70°C, it can lead to expansive reactions within the hardened concrete, causing long-term structural damage and loss of durability.
• Strength Development: While higher temperatures initially accelerate the rate of hydration and early strength gain, they can sometimes result in lower ultimate strength due to a coarser pore structure.
• Mitigation: To manage these effects, engineers often use low-heat cements (such as Portland Pozzolana Cement or Low-Heat Portland Cement), replace a portion of cement with supplementary cementitious materials like fly ash or slag, use chilled water or ice in the mix, or implement cooling pipes within the concrete mass.