Grade of Asphalt cement for tropical country 2

[Leongoring]

We are moving away from pen. graded asp. cement to viscosity grade. Specs on contract now running ask for AC20, with min. pen described as 6o. We are recieving bitumen with pen. of 92 but having prescribed viscosity as per spec. Want to limit this to penetration of 80. So we now want viscosity of 1600 - 2400 poises and pen. of 60 - 80.(inferred from AASHTO M226.) Any suggestions here.

 

[VAD]

What was the reason for moving away from pen graded to viscosity graded asphalt cement?. Was the reason based on research of the behaviour of asphalt cements and performance of resulting mixes or was this just because someone decided to use a specification because it was coined from a North American jurisdiction.

It would be beneficial if you examine the reasons for this change as you will find that in your climate there is really no need to be sophisticated but to use what has worked well in relation to pen grade asphalt cements.

I presume that you were accustomed to use 60-70 pen asphalt. Hence you are wary about not getting similar penetrations but now faced with the fact that the viscosity satisfies the specifications and you are not certain what those numbers mean to your ultimate pavement performance.

However,it seems that you require to meet both the pen and vis requirements. If this is the case then the bitumen with pen 92 is not acceptable. Or is your specification based on viscosity only. In that case, you may have to institute changes, but may face contractual problems.

Too often specs are imported from another jurisdiction without anyone truly understanding the reasons why the specs were developed for that jurisdiction. This seems to be a case of specs copied by a Consultant carte blanche from standard specifications of some jurisdiction. Perhaps, the flair of sophistication of the "viscosity" tests was a ploy to demonstrate that the specifier has some advanced knowledge.

It is a pity that effort is not taken to understand first what has worked well over the years in your climate by those who wish to promote something different to those who may be less knowledgeable, but perhaps have a lot of local experience.

If the specs were concoted by a consultant let him decide what do.

Finally, use your local experience to discuss the pros and cons of pen grade versus viscosity graded asphalts in your climate. I would bet that the Consultant would be hard pressed to provide suitable and acceptable reasons.

 

[BigH]

- VAD!

Here in India (humid, wet, hot) we use 60/70 penetration grade. This, I think is too soft and 40/50 would be better but not made here. Typically, pen grades at 30/40, 40/50, 60/70 and 80/100 (although AASHTO has it as 85/100) - here there was problem on my last job that 85/100 had to be used but we could only get 80/100! Luckily all our pens were >85. To overcome this and other problems there is a move (not yet in all specs) to CRMB60 (crumb rubber modified) or PNB40 (polymer modified). You didn't say but are you using virgin asphalt or blown asphalt? Or is there other additives to modify the bitumen properties?

Whatever grade you use - you are in a dilemma to "meet the specs" but to achieve a good end product and the first doesn't always achieve the second if the specs are out of whack. Example, overcompaction of the mix (less than desirable air voids at initial compaction) - minimum bitumen contents that are too high, etc. These all can lead to trouble and - well, the contractor says, "I was in the centre of the specs - all the way!" Hard to fight that.

 

[Leongoring]

VAD: Correct: 1. Specs imported. 2. Consultant hard pressed. 3. Spec based on viscosity only, and ask for a min. pen. of 60. BigH: Do you mean <85?. Silicone additive asked for.
This shipment pen. is 96!. Getting worried, have no idea of performance at this pen. May have to resort to what we know - 60/70.

 

[VAD]

What is the purpose of using silicone additive. Some use it to assist in mix laydown. Generally, it assists in this case to mask any surface imperfections such as segregation. You should find out the real reason for use.

Be careful in accepting asphalt cement with such high pen values, rutting could develop with such mixes. Your Consultant should be taking the lead on this issue and providing you with the necessary comfort factor. If in doubt go back to what you know has worked well over the years.

Good Luck