Concrete pavement is often unreinforced, depending on the type of concrete used, the base it is placed on, and design considerations specific to the project. Flexible concrete actually performs better without reinforcing bars except at stress locations such as angled segments and sharp curves. Because the design mix allows the hardened, cured concrete to flex before it cracks/breaks, rebar would actually contribute to cracking if it flexes.
Where reinforcing is critical is in bridging, that is, where a section isn’t on solid base material. When placed on structural base where a sufficient CPR or LBR (California Bearing Ration or Limestone Bearing Ratio) is achieved, the base is a material that is compacted and stiff/hard enough to support the full weight of the concrete plus the designed load of traffic. In locations where soft material isn’t or cannot be mitigated by overexcavating and filling with stronger material for subgrade and base, reinforcing bars are needed.
One thing to note is that even though the slab may not need to be reinforced, joints are usually dowelled with steel bars so that slabs remain flush if settling does occur. These are often 3/4 inch or one inch steel bars that are slipped through the formwork so that half is in one slab, and half in the adjacent. One ends is sometimes greased to allow the dowel to slide when the slabs expand and contract.
Other methods are used in highway pavement, such as steel fiber reinforcement, polyester fiber reinforcement, and others, that are blended into the concrete when it is mixed.
Still other techniques are used to mitigate cracks caused by expansion/shrinkage in pavement slabs, like entraining air, using shrinkage reducing admixtures, controlling slump during placing, and curing with membranes to slow dehydration after the concrete is placed. This answer is applicable to the region where I work, in the southeast U.S., and probably different climates have different needs, particularly very cold climates, or places where stable ground conditions are difficult to achieve.
Its not essential. . . but it can be used to assist in crack control.
I have designed thousands of square m of airfield pavement (which takes far higher loads than roads) which has all been unreinforced.
