A concrete structure is set on fire and the temperature raises to 1000°C. The strength of concrete as compared to original strength reduces to A. 10% B. 15% C. 20% D. 25%

The correct answer is C. 20%.

When concrete is exposed to high temperatures, the water in the concrete starts to evaporate. This causes the concrete to shrink and crack. The longer the concrete is exposed to high temperatures, the more damage it will sustain. At 1000°C, the concrete will have lost about 20% of its original strength.

Option A is incorrect because the concrete will lose more than 10% of its strength. Option B is incorrect because the concrete will lose more than 15% of its strength. Option D is incorrect because the concrete will lose more than 25% of its strength.