During a concrete pour, the ready-mix truck is delayed and concrete begins to set before placement is complete. What is the most appropriate action?
Correct Answer
C) Reject the concrete and order fresh material
Once concrete has begun to set, it should be rejected and not used. Adding water or admixtures after initial set compromises the concrete's strength and durability. Fresh concrete must be ordered to ensure structural integrity.
Why This Is the Correct Answer
Once concrete has begun its initial set, the chemical hydration process has started and cannot be reversed. Using concrete that has already begun setting will result in compromised structural integrity, reduced strength, and potential failure of the concrete element. The only safe and code-compliant option is to reject the concrete and order fresh material that meets proper workability and timing requirements.
Why the Other Options Are Wrong
Option A: Add water to the mix to restore workability
Adding water to concrete that has begun setting (tempering) is strictly prohibited by ACI standards and building codes. This practice significantly reduces the concrete's compressive strength, increases permeability, and compromises durability. Water addition after initial mixing violates the designed water-cement ratio.
Option B: Add a retarding admixture to slow the setting time
Adding retarding admixtures after concrete has already begun to set is ineffective and prohibited. Retarders must be added during initial mixing to be effective. Adding admixtures to concrete that has started setting will not restore workability and may create chemical incompatibilities that further weaken the concrete.
Option D: Vibrate the concrete more aggressively to restore plasticity
Aggressive vibration cannot restore plasticity to concrete that has begun setting. Excessive vibration of setting concrete can actually cause segregation, bleeding, and further damage to the concrete matrix. Vibration is only effective on fresh, workable concrete and cannot reverse the chemical setting process.
Memory Technique
Think 'SET = REJECT' - once concrete has SET, you must REJECT it. No water, no additives, no vibration can fix concrete that has started setting.
Reference Hint
ACI 318 Building Code Requirements for Structural Concrete, Chapter 26 - Quality Control; ACI 301 Specifications for Structural Concrete, Section 4.2 - Fresh Concrete Properties
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