Question

An engineer took asphalt cores on a roadway that has seen rutting and bleeding over the past three years. If the calculated air voids were 2.5% but the design air void content was 4%, explain what effect the calculated air voids had on the rutting and bleeding seen on the asphalt roadway.

Answer 1

Solution

Rutting is the formation of depressions or grooves on the road pavement due to wearing action of wheel movement. The calculated air voids of 2.5 % is a low value. At lower air voids it is observed that the amount of rutting which occurs in an asphalt pavement is inversely proportional to the air void content. At low air voids percentage the binder completely fills the void giving the mix some degree of fluidity and thus the pavement becomes less resitant to rutting. The general variation of rutting with air voids is depicted in the following graph.

As you can see below 3 % rutting rate increases rapidly.

Bleeding mostly occurs in hot weather conditions when asphalt binder fills all the voids and expands onto the pavement surface. As we have low calculated air void percentage there is not enough room for the asphalt to expand into and thus it bleeds out on to the pavement surface.