Europe has centuries-old bridges, requiring a more intimate knowledge of
stone masonry than normally required in this country. In the U.S., a large
proportion of bridges are a direct result of the "40-years young"
interstate highway system, so a more thorough understanding of steel and
concrete construction is essential. Ergonomic and operator safety standards
are often tougher in Europe; large projects with large crews are more often
the norm in the U.S.
This last point-the relative size of jobs and crews in each of these parts
of the globe-plays a large role in the relatively small role onboard compaction
or density meters on compactors play in the U.S. According to one industry
insider, "In Europe they lay less asphalt per day-if they lay 5,000
tons, that's a big day. Over here, a contractor that is not putting down
50,000 tons per day is going to have to start looking for another job. It's
just a different way of approaching things."
Compaction meters tend to pay off for jobs that are either time-sensitive-such
as runway-paving projects-or where the compactor operator is the primary
source of quality control.
"The Europeans have a flair for technological 'bells and whistles,'
" say Jim McEvoy, Ingersoll-Rand Co.'s marketing manager for North
America. "There seems to be a lot of focus on the operator over there,
ergonomics and concerns with operator comfort and productivity. They seem
to put a great deal of stock in what their operators can do. Here in the
U.S., we have a tendency to have the operator move the unit from point A
to point B, and then the crew members do the rest."
While many factors figure into the meter-use equation-cultural differences,
labor-pool sizes and costs, average job sizes-the sum of these variables
is different on each side of the Atlantic. Climb into the cab of an asphalt
or soil compactor in Europe, and odds are you'll find some type of compaction
meter. In the U.S., the odds are very much in favor of not detecting such
a unit.
As applied to onboard units, the term "compaction meter" is often
a misnomer. Most units do not "meter" compaction-they serve primarily
to give comparative values, as opposed to definitive figures. Currently,
only nuclear-density meters measure "compaction" in terms of actual
density. For most contractors, a true density reading is essential.
And obtaining "true"-i.e. accurate-density readings with an onboard
unit is something manufacturers appear to have yet to perfect. While many
individuals contacted by ROADS & BRIDGES had definite opinions about
the meters, few were willing to go on record with them. This is indicative
of the lack of confidence-warranted or otherwise-that U.S. industry members
have in the devices, as well as the relatively low profile these units have
in this country.
While most compactor companies contacted offer some version of a compaction
meter on either their soil or asphalt units, many were unable to supply
an information sheet or photo of the unit. Those who were able to provide
information usually sent product sheets describing the meter's use on the
company's European-marketed products. There just doesn't appear to be that
great a demand for such devices in the U.S.
HAMM Compactors Inc., Irving, Texas, has one of the longest histories of
onboard compaction meters, but they currently only appear on the company's
soil compactors. (Not surprisingly, HAMM's parent company is the European
HAMM Walzenfabrik GmbH.) "HAMM is looking at putting meters in the
asphalt compactors, but they are not there yet," says Udo H. Boersch,
the company's national sales and marketing manager. "The meters on
asphalt compactors are usually nuclear meters; the HAMM unit meters on roller
impact, so it is not the same thing."
The equipment manufacturer offers two models of compaction meters on its
soil compactors: The HCM, which displays information on an in-dash gauge;
and the CDS (Compaction Documentation System), which uses a liquid-crystal
display. Both units work on the same operating principle: A sensor in the
roller detects the vibratory motion of the roller drum and transfers this
signal to the meter's processor. Increasing reaction forces correspond with
growing compaction, which is displayed as a numerical value (on the HCM
unit) or graphically (on the CDS).
The key word in the preceding sentence is "correspond": Because
the meter does not measure true density, readings obtained are only relative
values. Ingersoll-Rand's DD-130-an asphalt compactor-comes with a similar
system, and even McEvoy is reluctant to call the measuring device a "meter."
"The impact meter on the DD-130 is actually a gauge," he explains.
"What it gives the operator is impact spacing relative to travel speed.
What we are doing is sensing the true ground speed of the unit. The rule
of thumb in the industry is 10 impacts per foot, so you can correlate the
VPMs with travel speed to get the correct impact spacing."
Thus impact meters don't measure true density; they merely give an operator
a sense of how compaction at point A compares to compaction at point B (or
point A before and after a certain number of passes).
That's not to say such meters serve no purpose: Especially on large jobs
where high productivity is a priority, the use of such meters by a skilled
operator can help reduce the time needed to obtain proper compaction. By
using the meter against a known value (such as one obtained with a handheld
nuclear-density device), a knowledgeable operator can use the comparative
readings an onboard meter delivers to optimize a compactor's performance.
For all the technical wizardry available to the construction industry, many
phases of the paving process are still guided by an individual's skilled
intuition. Onboard compaction meters are just another tool to help a crew
foreman or compactor operator make informed decisions.
As mentioned above, the nuclear-density meter is the industry standard for
measuring asphalt density in the field. Troxler Electronics Lab, Research
Triangle Park, N.C., manufacturers such handheld units, and it also offers
the Model 4545 for onboard use.
While the unit has been thoroughly tested by the company, Troxler is in
the process of conducting market and user evaluations of the unit. "Basically,
we are looking at the unit for a possible redesign and improvement,"
explains Bill Worrell, Troxler's product services director. "We currently
don't sell the gauge, we only lease it. This is the way our customers seem
to like working. Many times these gauges are used on big asphalt jobs-airports
and long-distance stretches of asphalt paving-and these jobs don't come
along all the time. So they really prefer to lease rather than own the gauge."
Worrell estimates the unit's selling price might approach the $15,000 mark,
so leasing is an attractive (i.e. low-cost) alternative.
Alan Reynolds, the company's central branch manager for sales, says the
4545 can be used on "just about any roller." The meter uses a
cesium-gamma ray source; Reynolds says the unit measures to a depth of almost
4 in. with a precision of plus or minus 1 PCF lb. Yet even with this method,
true relative density is difficult to obtain (absolute density is obtained
by measuring the specific gravity of core samples).
"The 4545 is a little different than our handheld units," Reynolds
concedes. "It measures over a larger area than the handheld units,
but it has been very well accepted by those who use it." While not
as specific about the density of a particular point on a mat, the broad
coverage does provide an operator with more compaction information than
traditional test methods provide.
Echoing Worrell's comment, Reynolds says the primary market for such a unit
are large contractors who do high-volume, time-sensitive work.
Reynolds cautions that courser mixes, such as open-graded courses or stone-matrix
mixes, will read lower than their actual value due to surface voids. But
again, a knowledgeable operator will know how to compensate for this variance
by making the necessary adjustment to the meter's offset or calibration
curve. Even with high-tech, high-priced units like nuclear-density meters,
the accumulated experience of the compactor operator is invaluable.
The high cost of high-tech meters is a factor that strongly affects sales.
An onboard impact meter will, on average, run about $10,000; an onboard
nuclear-density meter can cost thousands more. Most U.S. contractors can't
justify this type of expense-they'll never get an equitable return on their
investment. Only contractors with large budgets and large projects will
see the units as cost effective.
In Europe, however, labor costs are higher and operator control is more
of an issue, so the meters can pay dividends despite the high capital outlay.
As with many other international issues, cultural factors come into play
in the meter arena. Americans have traditionally been a "fly by the
seat of your pants" country, using that "old American know-how"
and Yankee ingenuity to get the job done. Ask 12 contractors how to determine
an optimum rolling pattern, and you're likely to get a dozen answers (all
of which may be correct). It's a point of pride for Americans to do things
in an idiosyncratic manner.
Europeans, with traditions that extend well back before the birth of our
nation, have a tendency to conform to accepted norms. In addition, they
put great stock in the ability and judgment of their compactor operators,
and they seem to feel an investment in an onboard compaction meter is money
well spent.
What matters is the final result. With the emphasis on quality in paving jobs and the introduction of warranties into paving contracts, America's contractors are definitely shooting for the highest quality they can obtain. European contractors appear to be aiming for the same goals, but have a different way of approaching them. In Europe, onboard meters appear to be a runaway hit; in the U.S., it seems they aren't quite ready for widespread use.
