Underwater concreting — select the correct set of requirements (Workability, cement content, aggregate proportions, and dewatering practice.)

Introduction:
Underwater concreting requires special provisions to control washout of cement paste and to maintain cohesion and workability. This question validates understanding of mix adjustments and construction practice when placing concrete below water level.

Given Data / Assumptions:

• Concrete is placed under water using tremie, pump, or bucket methods.
• Washout risk and segregation must be minimized.
• Workability must be higher than for dry placement.
• Dewatering during and immediately after placement is prohibited.

Concept / Approach:
To resist washout, cement content is typically increased and fines proportion improved. Slump is kept high (about 100–180 mm) for continuous flow. Coarse aggregate volume is commonly 1.5–2.0 times that of fine aggregate to maintain grading without harshness. Dewatering induces flow through fresh concrete, promoting segregation and should be avoided for at least 24 h.

Step-by-Step Solution:
1) Select a well-graded mix with higher paste content (≥10% more cement).2) Target slump 100–180 mm for tremie/pumped flow without segregation.3) Maintain coarse:fine aggregate volume between 1.5 and 2.0.4) Prohibit dewatering during placement and for about 24 h afterwards.

Verification / Alternative check:
Field guides for tremie concreting specify anti-washout measures (higher cement/fines, admixtures), and strict controls on water movement.

Why Other Options Are Wrong:
Options B–D contradict best practice on cement content, slump, aggregate proportion, or dewatering, and would risk segregation and washout. Option E cannot be correct because the sets conflict.

Common Pitfalls:
Using low slump; starting or resuming dewatering too early; allowing free fall through water; inadequate continuous flow in tremie pipe.

Final Answer:
At least 10% more cement than dry work; coarse aggregate 1.5–2 times fine aggregate; slump 100–180 mm; no dewatering during placement or for 24 h thereafter.