Which of the following is a common risk factor when placing a concrete slab directly on compacted earth?

Moisture penetration is indeed a common risk factor when placing a concrete slab directly on compacted earth. When a concrete slab is poured on soil without a proper vapor barrier, moisture from the ground can seep up through the slab. This can lead to a variety of issues, such as surface spalling, mold growth, and damage to flooring finishes over time due to excessive humidity. Proper moisture control is essential to ensure the longevity and durability of the structure, which is why a vapor barrier is often recommended to mitigate this risk.

While factors like insufficient strength, excessive heat retention, and high insulation value are important considerations in construction, they are less directly related to the placement of a concrete slab on compacted earth. Insufficient strength typically relates to the quality and mix of the concrete itself, rather than its placement. Excessive heat retention is more associated with the materials used in construction and their thermal properties, and high insulation value pertains to energy efficiency, which is unrelated to the immediate risk of moisture issues from ground contact.