North Carolina Building Contractor License Practice Exam

What percentage of silica fume, used as a cementitious material for concrete exposed to de-icing chemicals, is required?

1. 1%

2. 2%

3. 3%

4. 4%

The correct answer is: 2%

The correct answer is 2%. Silica fume is recognized for its ability to enhance the durability and strength of concrete, especially in environments affected by de-icing chemicals. When utilized as a cementitious material, incorporating about 2% of silica fume by weight of cement can significantly improve the concrete's resistance to chemical attack, particularly from chlorides present in de-icing salts. At this percentage, silica fume helps to reduce permeability and enhances the binding properties, which helps protect the concrete from potential damage caused by freeze-thaw cycles and the aggressive actions of de-icing agents. This concentration strikes an effective balance, ensuring the beneficial effects of silica fume without negatively impacting the overall mix workability and strength development. Higher percentages might provide further benefits but can also lead to difficulties in mixing, excessive viscosity, and challenges in achieving the intended workability, making 2% an optimal choice for practical applications in most concrete mixes exposed to such harsh conditions.