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Difficulty: MediumCausal Detail Extraction

During the construction of large civil structures such as dams and bridge piers, mass concrete pours generate significant internal heat due to the exothermic reaction of cement hydration. Because concrete is a poor thermal conductor, the interior core remains extremely hot while the outer surfaces cool rapidly in ambient air. Consequently, a steep temperature differential develops between the core and the exterior shell. This gradient causes the cooler outer concrete to contract against the expanding warm core, resulting in tensile stresses that exceed the material's tensile strength and produce severe surface cracking.

According to the passage, what directly causes surface cracking in mass concrete structures?

  1. A
    The exothermic breakdown of poor thermal conductors within the ambient air
  2. B
    The rapid expansion of the outer surface pulling away from a cooling interior core
  3. Tensile stresses produced when the cooler exterior contracts against the expanding hot coreAnswer
  4. D
    Moisture evaporation from the interior core before cement hydration takes place

Answer

Tensile stresses produced when the cooler exterior contracts against the expanding hot core
The passage explicitly explains the causal chain: the temperature differential causes the cooler outer concrete to contract against the expanding warm core, generating tensile stresses that exceed the material's strength and lead directly to surface cracking.

Step-by-Step Solution

1
Locate the target effect in the passage.
Identified 'produce severe surface cracking' in the final sentence of the text.
Finding the specific effect mentioned in the prompt anchors the search for the direct cause.
2
Identify the causal linking phrase and trace the preceding cause.
The text states that the cooler outer concrete contracts against the expanding warm core, 'resulting in tensile stresses that exceed the material's tensile strength'.
The phrase 'resulting in' directly connects the mechanical stress of differential contraction and expansion to the surface cracking.

Key Concept

Causal Detail Extraction in Technical Passages
Estimated Time:1m 0s
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Topics

Causal Detail Extraction

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