Final answer:
The minimum recommended thickness for a control joint in a 6-inch thick concrete slab is 1 1/2 inches, which is equivalent to one-fourth the thickness of the slab.
Step-by-step explanation:
The minimum thickness of a control joint in a 6-inch thick concrete slab on ground is typically 1/4 inches. Control joints are planned fractures that allow movements caused by temperature changes and drying shrinkage. The rule of thumb is that the depth of a control joint should be at least one-fourth the thickness of the slab. Therefore, for a 6-inch thick slab, a control joint with a minimum depth of 1.5 inches (1/4 of the slab thickness) is adequate. However, option 1) suggests a depth of 1/4 inches which is not enough for a 6-inch slab, while option 2) 1 inch and option 3) 1 1/2 inches are closer to the recommended depth. Option 4) 3 inches would exceed the necessary depth and is not typically needed for a control joint. Most professional guidelines would suggest option 3) 1 1/2 inches as the minimum depth for a control joint in this scenario.