My asphalt driveway is crumbling where it meets the cement threshold of my garage. The culprit is water dripping down over the years from the gutters. There is a small void on one side of the garage door frame I've tried to fill in with sand but this is just a quick fix. There is also a hollow sound when I tap around the threshold. Fortunately I only use the garage for storage so I'm not concerned about the weight of a car on this area. What is the best way to prevent any real damage?