to prime or not to prime... 3

[cvg]

Just wondering what the current concensus is on applying a bituminous prime coat to the aggregate base course on roadway prior to placing asphalt concrete (assuming major urban arterial road)

 

[genomty]

Prime or not to prime? that's the question, Im not totally agree with THE QUESTION

In my humble opinion, if final cover (course layer PCC or HMAC) is going to be inmediately placed over the base, there is not a real reason to use the prime coat, I'll directly apply the tack coat and then pave, however, in the real world this situation usually does not happen, even in the states (I assume that those who refer to DOT's are US based.)

Perhaps the most common practice arround the globe, is to place the base and some days, some weeks or even several weeks later place the course layer. Meanwhile final layer is placed the base is used for multiple purposes, as socking deck, as parking lot, as course surface. In a hihgway dumptruks, ready mix concrete trucks, HMAC dump trucks and any other heavy piece of equipment you can said will road over the base. In comercial or industrial facilities is always the same history. So,I think that prime coat is intended not to protect the base against moiture entering or to provide and additional strenght or bond, my opinion is that prime coat it is intended to protect base against moisture lost and maintain surface aggregates bonded. Water entering should not be a problem since one of the multiple functions of the base is precisely provide an adequate sub-drainage in order to keep the pavement structure as dry as possible, if base accumulate water, there is not a problem related with traffic estimation, it is probably a problem with designing considerations, in highway construction, base usually can freelly drain the entering water through both sides of the roadway, however, in situations like parking lots or urban roadways this free drainage does not exist in that case when designing a higher value of lost support should be used and consequently a thicker pavement structure will result.

Guys nevertheless, this prime coat is not the panacea, it is just a temporary and quite limited protection and if base is under heavy loads or bad weather, perhaps the layer should be repaired before final layer will be placed, maintaining the layer in moisture and compaction will be necessary even if base remains unprotected for a few days with no traffic, so, i think that the question should be:

To place the base now or to rapair it later?