MCQs on Concreting in Adverse Conditions

Answer: C. High compressive strength

Bridges are primarily compression-dominant structures. Concrete's high compressive strength (20–80 MPa) makes it ideal for piers, abutments, arches, and deck soffits. Combined with reinforcing steel (to handle tension), reinforced concrete efficiently carries all bridge loads. Durability and excellent formwork flexibility are additional advantages.

Answer: B. Use of an air-entraining agent

Air-entraining agents introduce microscopic air bubbles that absorb ice expansion during freeze-thaw cycles — a protection needed only in cold weather. Options A (sprinkle cooled water) and C (cool aggregates) are hot-weather precautions to lower mix temperature. Option D is used in both conditions for different reasons.

Answer: B. (i), (ii), (iii), and (iv)

All four precautions are required for sub-zero concreting. Pre-heating raises the initial temperature; economical heating manages costs; anti-freezing admixtures lower the pore water freezing point; electrical heating maintains concrete temperature during curing. No single measure is sufficient at sub-zero temperatures.

Answer: B. Higher C₃S and lower C₂S

C₃S (Tricalcium Silicate) hydrates rapidly, generates high heat of hydration, and develops high early strength — all critical in cold weather to prevent freezing before the concrete gains strength. C₂S (Dicalcium Silicate) is slow-hydrating and contributes minimal heat and early strength. Rapid Hardening Portland Cement (high C₃S) is ideal for cold weather.

Answer: D. (i), (ii), and (iii)

All three effects are caused by cold weather: delayed setting (hydration slows below 10°C), early-age freezing (damage if water freezes before concrete gains ≥3.5 MPa), and freeze-thaw cycling (long-term surface scaling and microcracking). Each is a distinct and well-documented cold weather effect.

Answer: C. (i), (ii), (iii), and (iv)

All four are recognised methods for placing concrete underwater. The Tremie method (most common) was not listed. Each method addresses the key challenge: concrete must not fall freely through water. Pumping and tremie keep concrete submerged in itself; toggle bags and bagged concrete are placed by divers for smaller-scale applications.

Answer: A. Formation of ice lenses in capillary cavities

When freshly placed concrete freezes, water in capillary cavities turns to ice with ~9% volumetric expansion. These ice lenses exert enormous internal pressure on the weak nascent cement gel matrix, rupturing it before it gains any meaningful strength. The result is a permanently disrupted structure with severe strength loss.

Answer: D. All of these

All three specifications are correct for pumped concrete: pipeline diameter typically 100–200 mm (not exceeding 300 mm); slump 50–80 mm (medium workability for pumpability); w/c ratio 0.5–0.65 (fluid enough to pump; cohesive enough to resist segregation). All three must be satisfied simultaneously for effective pumping.

Answer: B. Decreases the strength of concrete

High temperatures (>38°C) reduce concrete strength. Rapid initial hydration produces coarser, less dense C-S-H gel microstructure with larger pores. Accelerated evaporation of mixing water creates more capillary voids. Plastic shrinkage cracks form at the surface. The net result is 10–20% or more reduction in 28-day and ultimate compressive strength versus concrete placed at 20–27°C.

Answer: A. Soundness

'Soundness' specifically describes an aggregate's ability to resist disintegration due to weathering (freeze-thaw, wet-dry cycles, temperature changes) and volume instability from chemical reactions with cement hydration products. It is tested by IS 2386 Part 5 using sodium or magnesium sulphate immersion cycles. Crushing strength, abrasion resistance, and impact value measure different mechanical properties.

Answer: C. Chemical reaction

Leaching is a chemical deterioration process where water dissolves calcium hydroxide [Ca(OH)₂] from the cement paste and carries it out through capillary pores. The dissolution reaction Ca(OH)₂ → Ca²⁺ + 2OH⁻ is a chemical reaction. The visible white deposit (efflorescence) forms when dissolved Ca²⁺ reacts with atmospheric CO₂ to form CaCO₃ at the surface.

Answer: A. Increases strength

(SSC JE Official Answer) Low non-freezing temperatures result in slower cement hydration, which produces a denser, more uniform C-S-H gel microstructure with fewer defects. This ultimately yields higher final compressive strength compared to concrete cured at higher temperatures. Note: while early-age strength (7-day) is significantly lower at low temperatures, the long-term ultimate strength is higher than the standard reference (20–27°C curing).