What is the purpose of isolation joints in a concrete slab on grade?

Isolation joints in a concrete slab on grade serve the essential purpose of allowing movement due to expansion and contraction resulting from temperature changes, moisture variations, and differential settlement. These joints are typically placed between the slab and adjacent structural elements such as foundations, walls, or columns to accommodate these movements without causing cracking or structural damage to the concrete.

By having isolation joints, the design permits the concrete slab to shift independently from other structures while maintaining its integrity, thus enhancing its durability and performance over time. This characteristic is especially crucial in environments where temperature fluctuations are significant, or where soil conditions may lead to differential settling.

The other options do not accurately reflect the function of isolation joints. For instance, while it might seem intuitive to think they could be used to prevent movement, that would lead to issues such as cracking and structural failure. Creating decorative effects and minimizing water drainage are not related to the primary function of isolation joints in a concrete slab.