What Causes Bleeding in HMA

Dear Friends!

I have a question regarding surface diffects (distress) which often occurs in HMA mixes. This is bleeding or flushing as others tend to name it.

In my view I tend to think this problem is caused by too much bitumen in the mix which fill all air voids. What amount is too much bitumen?

In design we tend to limit the amount of bitumen which retains air voids of 3-6% by volume of total mix

Too much voids have effects of making the layer too open which can allow water ingress and shorten the life.

Some literature suggest, bleeding is resulted from low air voids below 3% after secondary compaction.

To avoid the mix should be designed to have  4% to 8% air voids, care should be taken to ensure voids are not connected. This can be achieved by ensuring proper grading.

Robert Ngalu,

Tanga.

13/1/2012