Which statement about masonry components when heated is most accurate?

• A. Stones and concrete may lose small portions of their surface when heated.
• B. Concrete blocks may crack and lose their strength, causing structural collapse.
• C. Mortar between bricks, blocks, and stone will ignite quickly, causing structural collapse.
• D. Stones and concrete stand up well to fire and intense heat, but may crack or shatter when cooled with water.

Answer: (C)

Masonry materials are largely noncombustible, so they can tolerate high heat, but they are vulnerable to damage from heat and subsequent cooling. When heated, moisture inside bricks, blocks, and mortar turns to steam and causes expansion; the different expansion rates of stone/brick, concrete, and mortar can lead to cracking or spalling. If water is applied after heating, rapid cooling creates thermal shock that can crack or shatter the masonry. Among the statements, the one that best reflects this behavior is that stones and concrete stand up well to fire and intense heat, but may crack or shatter when cooled with water. Mortar is not flammable, so it does not ignite quickly, though heat can weaken the bond and degrade the masonry.