Hot new asphalt mix sticks to its promise

Publisher’s version / Version de l'éditeur:

Heavy Construction News, 42, March 3, pp. 34-35, 1998-03-01

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Hot new asphalt mix sticks to its promise

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III

R E S E A R C H

Hot new asphalt mix sticks

HCH Stoff Report

Iigbt Custom pbureteiYes

".ooHu

",,-The extra asphalt cement gives the pavement the flexibil-ity to withstand the deformation that leads to cracking, even in Canadian winters.

For the contractor, however, there are special problems

ass0-ciated with the new

rnD<.

In fact, Bert Hen-driks says, "at first, I didn't think we were going to be able to produce the mix,"

Hendriks is general manager of Beaver Road Builders Ltd., the firm that did a number of paving jobs for the Regional Municipality of

o

Ita wa- Carle ton (RMOC) last summer using the new mix. Included were some urban intersections, some of the transitway stations, and a section of expressway,

The mix is so spe-cialized, Hendriks says, that plant modi-fications were needed. Even the aggregate posed special prob-lems.

Dr. Mohamed explains that the aggregate that goes into themixis critical. Most mixes contain aggregates crushed to a larger size (19 mm), Abon: SevereruItilg

aIlramitMly.

A

new design procedure (or

hot-mix asphalt, developed by Canadian scientists, promises to solve some of the special problems associated with urban roads and, at the same time, offer a measure of relief for hard-pressed municipal

tax-payers.

The problems are well-known: rut-ting and cracking, caused by heavy

ttaffic -especiallybus traffic - and the severity of Canadian winters.

In Ottawa., the problem was espe-cially bad along the transitway. an expressway reserved for buses. Offi-cials watched with concern while the ruts at the stations grew deeper and deeper. Finally. they turned to the National Research Council's Institute

for Research in Construction(IRe). '"1be way roads are designed now, including Ihe theory behind the design, is simply not working," says Dr. EJ Hussein H. Mohamed, manag-er of urban roads research for IRC's Urban Infrastructure Rehabilitation Program. "Rehabilitation measures (overlays), for instance, were designed to last for 15 years, but they rarely do. Most of them will show signs of pre-mature failure within three to eight years."

In the case of the transitway in Ottawa, those signs were showing up within a year, or a little more so Dr. Mohamed., and the material scientists who work with him, came up with a newmixdesign procedure and imple-mented ittodevelop a recipe for "crit-ically loaded" roadways, such as bus routes and areas around intersections.

The mix uses smaller aggregate in sufficient quantity to establish stone-to·slone contact, forming an "aggre-gate skeleton," instead of having the aggregate "noating in a river of asphalt"

That means itis the aggregate, and not the asphalt cement binder, that bears the load on the road. That, in tum, means more asphalt cement is accommodated without the likelihood of bleeding under the traffic loading.

ＢＮ｟ｾ __Ｎ｟ｾ .. __ ._ ...._ .T wafW'nl'nrrr

Pladng andtnlingnew asphalt lakes plate under high·lemperalllle silualian.

.i.

Camera

Ieme

(DpgimKale 10 densityaf stone-la·stone mix.

hopefully, if it continues to be used, then )'OU ha\'e the experi-ence, the know· how, and you might have a bit of a com-petitive edge."

Stephen Lee, pave-ment and materials engineer for RMOC

who worked with Dr. Mohamed on thc project, is also ver), enthusiastic..

Lee says lite customized pavement will cost more, but it will last three times aslong. "We think the objective

was met in tenns of delivering some-thing that will prolong the life cyde of our pavement. \Ve calculate that we willsee minimum life-cyde savings of

30%."

Hendriks adds, "Ithink the taxpay-ers need a break, and Ihis should help."

It's a sentiment echoed by Dr.

Mohamed.

"Our roads right now arcn't doing well, so we have to do beller." he says. "Taxpayers can't afford 10 pay

any more."

lie."

compaction, he says.

On top of everything else, the mix has 10 be

applied at 140 deg to 150 deg C, instead of Ihe conventional 110 deg to 120 deg C.

To achieve that high-er application temphigh-era- tempera-ture, Hendriks says, the mix had to be about

165des: to 170degC in the paver.

Dr. Mohamed stresses the impor-lance of this. He says lite only really criticaJ quality control mcasure in me field is achicving thc correct com-paction tcmperature.

"II's critical to the quality. It's diffi-cult to achieve, but it's very impor-tant," he says.

Hendricks sees the problems encountered as a series of learning curves - "one for the crushing, one in the asphalt plant, another wilh the application and rolling."

lbrough all that, is Hendriks com-plaining?

Not abit.

"It was interesting work, kind of exciting. You like to be at the cutling edge of new technology because, and the shape of the individual

gran-ules, while important, is not criticaL

But with the new mix, the granular

material must be smaller (13 rum), wilh individual granules much less

elongated than is permissible in

con-ventional mixes. Itmust alsobe

high-strength material - trap rock, or, in the

Ottawa area, dolomitic sandstone.

It look quite a bit of experimenta-tion with cone crushers to get the size and shape nceded, Hendriks says. $e\'enty per cent of the aggregate used

had to pass through a 6.7-mm sieve, withnone passing a 4.75-mm sieve.

In the asphalt plant, the mix required tbe addition of 8 to 10 per cent mineral filler (again, dolomitic sandstone), plus cellulose fibres and a

bit oflime. Also, a performance-grade

asphalt is used, which makes the mix CXlrcmcly sticky, Hendriks says.

"It sticks to pavers, to rollers, everything. I think we had to go out about three times and buy all the soap in Ottawa."

The stickiness means traffic has to be kept ofT the new paving for a while, and that required working at night and rolling patterns were different because it seems even one extra pass by the rollers causes the mix to lose