Quality of Cold-Mix Asphalt Deserves Attention

Asphalt Article December 28, 2000
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Today, more municipalities and other users of cold-mix asphalt
patching materials are looking for suppliers who offer an added
measure of quality. However, defining what constitutes quality
in exact terms is not an easy task.

To manufacturers of
high-performance, cold-mix patching products, quality runs
deeper than surface appearances. Quality pertains to the
appearance and consistency of the patching product, how well it
performs, and the unique benefits it offers.

However,
quality also pertains to how business is conducted--the
commitments producers and suppliers make to their customers and
the responsibility taken to ensure that customers are getting
what they expect.

What customers expect is cost
effectiveness. This value is determined by the cost of the cold
patch, its expected life, and the manpower needed to make
repairs. A U.S. government study on pothole repair (SHRP-H-353)
conducted by the Strategic Highway Research Program recommends
that the best materials available should be used to reduce the
need for repatching.

According to the study, the cost of
repeatedly patching the same potholes, because of poor-quality
material, quickly offsets the savings from purchasing a less
expensive mix. The study further indicates that in most cases,
the poorer performance associated with inexpensive cold mixes
will result in greater overall costs for patching because of
increased costs for labor, equipment, traffic control and user
delay.

To a large extent, the quality of the material begins
and ends with the relationship between the cold-patch mix
supplier and the asphalt producer. With higher performance
mixes, the supplier should work closely with the asphalt
producer to ensure that the finished cold patch meets everyone's
expectations.

By the same token, a quality producer should
take responsibility for the success of the product in the end
user's application. Producers should strive to maximize quality
by understanding how the material performs, and by fine-tuning
their production process in order to achieve the best possible
results.

Factors affecting production

Providing
consistent, high-quality asphalt patching material is not a
routine task. A number of factors affect quality, including raw
material selection, how the materials are stored and handled,
and the type of production facility being used. In addition,
many environmental factors, such as extreme heat or cold, and
the presence of moisture or excess dust can dramatically change
the complexion of the asphalt mix and cause problems.

For
the most part, hot-mix asphalt is produced by coating crushed
aggregate with asphalt cement. Standard cold mixes are produced
by coating aggregate with an emulsion or with a cut-back
asphalt, and some mixes even have some type of modifier in them.
In the case of high-performance cold-patch materials, additives
specially formulated for each aggregate are incorporated to
allow the cold mix to be applied in lower temperatures, and to
achieve a "slow cure," which helps to promote long life for the
end user. This, along with individual mix designs and quality
control procedures, ensures a high quality patching material.

Given the above variables, the only way to achieve
consistent quality is for the cold mix blend supplier to monitor
the asphalt producer's production on-site to observe and to act
as a consultant during every production. To minimize production
problems and to maximize the effectiveness of high-performance
products, some manufacturers have developed a formalized
evaluation program that each batch undergoes in every step of
the process.

In the production of its UPM Blend, Unique
Paving Materials (UPM) conducts three major tests on asphalt
cement: viscosity, penetration, and specific gravity/pound per
gallon. The viscosity test ensures that the thickness and flow
of the mix is appropriate at a given range of temperatures. The
relative hardness of the asphalt is determined by the
penetration test. Another test, measuring the specific
gravity/pound per gallon determines the weight of the asphalt as
compared to the weight of water. Because of the extreme variety
of weather conditions around the U.S. these tests help to
determine which raw materials are suitable for making cold mix
in a given region of the country.

Raw material selection and
testing

On the surface, what could be easier than raw
material selection? For one thing, there are many different
types of aggregates in the U.S.--from the granite and basalt
typically available on the West Coast to the limestone and
traprock used on the East Coast. Sometimes there are major
differences in the rock from the same quarry. These differences
can result from diverse factors, such as the depth at which the
rock was excavated, the hardness or softness of the rock, and
water saturation of the rock over time.

The relative size
and surface area of each individual aggregate stone is important
because in the production of asphalt, the aggregate is coated
with a blend of asphalt and additives. Leading manufacturers use
standard testing methods, such as ASTM (American Society for
Testing and Materials), to determine the quality of the
aggregate that goes into their products.

Before production,
a battery of tests should be performed to ensure a quality
product. These tests would determine certain characteristics
that affect the finished product, including the viscosity of the
blend, and the appropriate additive package to make sure the
product performs to expected standards.

UPM recommends
several tests to conduct on the aggregate to determine how well
it will perform.

Surface area: A sieve analysis test would
measure the aggregate size and surface area of the rock to be
coated. In conjunction with this, a -200 Wash test determines
how much dust is present on the rock, which affects the true
size of the surface area. The surface area is measured in units
of square feet per pound and is used along with the absorption
and abrasion of the rock to determine starting Job Mix Formula.

Soundness: A five-cycle sodium test would determine how the
rock will stand up to road salt, in addition to simulating the
wetting and drying action of five freeze/thaw cycles.

Abrasion: The most rugged of these tests is the L.A. Abrasion
test that places 30 lb of aggregate into a steel drum with five
steel balls for 500 revolutions. This test determines how much
the surface area will increase due to break up during
production.

Formulation testing: A series of tests are
conducted to help allow for the differences in aggregate and to
compensate the amount of water and additives needed to achieve
production consistency.

One of these is a "stripping test"
that simulates a production of cold patch using various additive
packages to determine which is best suited for a particular
aggregate and to determine if any special precautions or methods
are needed during production.

The major cause of
stripping--when the asphalt does not adhere to the aggregate--is
the aggregate not being dry enough before the asphalt blend is
added.

Another cause of stripping is the presence of excess
dust and fines in the aggregate. Measures can be taken during
production, such as running the aggregate through a dryer with a
dust collector system and raising the temperature to help
eliminate the dust.

Production testing

The key to
achieving a good blend is to match the right asphalt mixture
with the right additives and tweak the process to work well with
the type of aggregate available at the plant.

During the
production of cold patch, several on-site tests should be
conducted throughout the mixing procedure to ensure the
consistency of the blend.

High-performance patching
materials can be affected by contamination in the stockpile area
after production. The remedy is as simple as it seems: storage
requirements should call for a clean, paved stockpile area.

To ensure quality control at each production site,
representatives should attend each production and to complete a
series of forms on each production of cold patch. These records
would help laboratories look for trends and to be able to
anticipate and solve problems when similar circumstances arise.

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