Surface Asphalt is different–why? Because its is the part that has to deal directly with tires and traffic and the elements: The top layer of an asphalt structure is  often called the “wearing course”. It is designed to have traffic directly on it. This layer is designed to receive load and to resist skidding, traffic abrasions and weather. This is why overlays are so important to get right! The aggregate size if different as compared to the other layers. and mineral filler is of a smaller size than the intermediate or base courses of asphalt and its to accommodate the conditions mentioned above.