In cold weather, the subgrade soil must be free of _____ before concrete placing begins.

In cold weather, the subgrade soil must be free of _____ before concrete placing begins.

• A. frost
• B. snow
• C. ice
• D. water

Answer: (A)

Frost in the subgrade is the critical issue. When water in the soil freezes, it forms ice lenses that grow and push the soil apart, causing frost heave. That movement under a fresh concrete slab creates differential bearing and can lead to cracks or uneven settlement as the concrete cures. Frozen soil also doesn’t compact well and provides an inconsistent base, compromising the slab’s strength and long-term performance. Snow or surface ice isn’t the key problem once frost is present in the subgrade; the dangerous factor is the formation of frost within the soil. Keeping the subgrade frost-free ensures a stable, uniform bearing layer and reduces the risk of cracking and settlement during curing.

Frost in the subgrade is the critical issue. When water in the soil freezes, it forms ice lenses that grow and push the soil apart, causing frost heave. That movement under a fresh concrete slab creates differential bearing and can lead to cracks or uneven settlement as the concrete cures. Frozen soil also doesn’t compact well and provides an inconsistent base, compromising the slab’s strength and long-term performance. Snow or surface ice isn’t the key problem once frost is present in the subgrade; the dangerous factor is the formation of frost within the soil. Keeping the subgrade frost-free ensures a stable, uniform bearing layer and reduces the risk of cracking and settlement during curing.