Freezing of freshly laid concrete seriously impairs structural integrity and causes strength loss primarily due to:

Detailed Explanation: Ice Lenses in Capillary Cavities Cause Freezing Damage

Why A (Formation of ice lenses in capillary cavities) is correct: When freshly placed concrete freezes, the mix water (including pore water in the nascent capillary pore system) turns to ice. Ice has approximately 9% greater volume than liquid water. This expansion within the confined capillary cavities generates enormous internal tensile pressure — far exceeding the tensile strength of the unset or early-age cement gel — resulting in disruption of the cement matrix, loss of structural integrity, and permanent strength loss.

Mechanism of Freeze Damage in Fresh Concrete

Stage	What Happens	Effect on Concrete
1. Placement (T >0°C)	Concrete is plastic; cement hydration has just begun; C-S-H gel network is embryonic and very weak	Concrete is vulnerable — gel has essentially zero tensile strength
2. Temperature drops below 0°C	Free water in capillary cavities and bleed water freezes; ice lenses form in capillary pores and at paste-aggregate interfaces	9% volume expansion inside confined pore spaces
3. Ice lens expansion	Expansion creates tensile stress in surrounding cement gel; gel cannot resist; micro-cracks form at multiple locations	Rupture of gel structure; separation of paste from aggregates
4. Thawing	Ice melts; water drains from newly created cracks; large void spaces remain	Permanent internal void network; severely reduced strength; increased permeability

Why Other Options Are Wrong

Option	Why Incorrect
A. Ice lenses in capillary cavities	Correct — the actual mechanism of freeze damage
B. High workability	High workability is not a consequence of freezing; it is a mix design property unrelated to freeze damage
C. Endothermic reaction	Hydration of cement is actually exothermic (heat-generating); freezing is endothermic but the damage is from ice expansion, not from the endothermic process itself
D. Air-entraining agents	Air-entraining agents PREVENT freeze damage; they don't cause it

Ice Lens Expansion: Volume of water = V Volume of ice = 1.09 × V (9% expansion) Pressure generated in confined pore: can exceed 200 MPa Tensile strength of fresh concrete gel: essentially 0 MPa Result: Tensile stress >> Tensile resistance → rupture guaranteed

• Freezing of fresh concrete damages it via ice lens formation in capillary cavities — 9% volumetric expansion creates irresistible internal tensile forces.
• Concrete must gain ≥3.5 MPa (ACI 306) before it can safely withstand one freeze-thaw cycle without damage.
• Air-entraining agents provide deliberate empty bubbles to absorb ice expansion, preventing ice-lens-induced cracking in hardened concrete.