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Difficulty: MediumCommon Building Materials

Reinforced concrete is a widely used composite building material that combines poured concrete with steel reinforcing bars (rebar). While the steel is primarily added to provide tensile strength, what other critical physical property must the steel and concrete share to prevent the structure from cracking or tearing itself apart during seasonal temperature changes?

  1. A
    They must heat up and cool down at exactly the same rate when exposed to the environment.
  2. They must have nearly identical coefficients of thermal expansion.Answer
  3. C
    They must both be highly ductile materials that stretch easily without fracturing to absorb thermal shock.
  4. D
    They must both act as effective thermal insulators to block heat from penetrating the structure.

Answer

The steel and concrete must have nearly identical coefficients of thermal expansion so they expand and contract together at the same proportional rate when the temperature changes.
For a composite material like reinforced concrete to remain structurally sound across differing environments, its component materials must expand and contract at the same proportional rate when the temperature changes. Because structural steel and concrete share nearly identical coefficients of thermal expansion, they change size together in harmony. This prevents the creation of internal stresses that would otherwise destroy the bond and crack the brittle concrete.

Step-by-Step Solution

1
Identify the effect of temperature changes on physical materials.
Almost all building materials expand when heated and contract when cooled (thermal expansion).
Temperature changes are an unavoidable environmental stressor on outdoor structures.
2
Analyze the mechanics of a composite material like reinforced concrete.
Steel is embedded tightly inside the concrete, creating a solid, bonded structural unit.
The materials must work together to distribute loads.
3
Evaluate what happens if the two bonded materials react differently to heat.
If steel expanded significantly more than concrete when heated, the internal swelling of the rebar would shatter the brittle concrete from the inside.
Mismatched expansion rates create massive internal shear and tensile stresses.
4
Conclude the necessary shared property.
They must expand and contract by the exact same amount per degree of temperature change, meaning they have the same coefficient of thermal expansion.
This compatibility ensures the composite material maintains structural integrity year-round.

Key Concept

Thermal expansion compatibility in composite building materials
Estimated Time:45s
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Topics

Common Building Materials

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