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KERCHER ENGINEERING, INC. “If a job is worth doing, it is worth doing right!” |
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Probably the most versatile of all the different types of surface treatments available, micro-surfacing can be a cost-effective maintenance solution to a wide variety of maintenance problems. Micro-surfacing is basically a “high-tech” slurry seal that starts with the same ingredients as a slurry seal, however, higher quality materials are used. Micro-surfacing was developed in the late 1960’s and early 1970’s by German scientists trying to fill narrow ruts on the Autobahn by using multiple layers of traditional slurry seals. Through experiments, the scientists found that using high quality materials, polymers and other special additives allowed the modified mixture to be applied much thicker than conventional slurry seals. Also, the new mixture had better performance characteristics. Today, micro-surfacing is used for a variety of reasons on a broad range of pavements such as low volume residential streets, high volume interstates, and runways for airports in many parts of the world including the United States, Europe and Australia.
Figure 1 – 5-Year Old Micro-Surfaced Pavement Wearing Very Well Applications Much like other surface treatments, one of micro-surfacing main uses is to seal the surface of aged pavements to slow down the oxidation process and to seal out water. As stated above, micro-surfacing was developed for rutting on the Autobahn. As such, micro-surfacing is an excellent rut filling material in many situations. Since micro-surfacing is very impermeable, in many cases, it will provide excellent performance as a surface coat placed on asphalt or chip sealed pavements that are experience severe levels of bleeding (or flushing). Additionally, micro-surfacing provides excellent skid resistance.
Table 1 – Potential Applications of Micro-Surfacing Materials The composition of micro-surfacing consists of aggregate, polymer-modified asphalt emulsion, water, mineral filler and other additives that are carefully mixed together. Polymer additives are used to increase the stiffness of the mix and improve the adhesion of the binder while reducing the temperature susceptibility. By reducing the temperature susceptibility of the binder, it can resist higher temperatures in the summer (before it softens) and resist lower temperatures in the winter (before it cracks). Mineral fillers such as portland cement are used to reduce aggregate segregation and to help control the rate at which the mixture breaks. The aggregate gradation is normally either ISSA
(International Slurry
Surfacing Association) type II or type III. While both types have
a maximum aggregate size of 3/16 inch, the type III gradation is
coarser
(more larger-sized materials). Type II is used for most
applications
while type III is typically used for filling depressions. The
application
rate of the aggregate is approximately 20 pounds per square yard of
aggregate
for a single layer. If a second layer is used, the application
rate
is 30-25 pounds per square yard. Curing Process Unlike other types of paving materials, micro-surfacing
cures through
an electro-chemical process which ejects the water from the slurry
mixture.
Since the micro-surfacing hardens through a curing process (similar to
concrete), it does not require compaction. This is a great
benefit
because compaction problems seem to plague many other road maintenance
techniques. Surface Preparation As with any maintenance technique, surface preparation is vital to the long-term performance of micro-surfacing. Cracks and joints wider than 3/16 of an inch should be sealed. Areas of deterioration such as alligator cracking or patches that are failing should be repaired. Inadequate preparation can result in poor long-term performance of micro-surfacing. In order to achieve proper adhesion, the surface must be
relatively
free of dust, dirt and loose material. A tack coat should be used
on asphalt concrete pavements if the surface is aged and raveled, and
on
all portland cement concrete surfaces. Application Techniques The modified slurry mixture is made and applied to the road surface using a mobile mixing unit that is specially designed for micro-surfacing (see Figures 2 and 3). The machine contains several compartments that store the aggregate, mineral filler, emulsion, water and additives. All the ingredients are carefully metered to assure that the proper quantities are mixed together. The materials are mixed together in a pug mill with dual shafts while the machine is traveling down the road. The mixed material then flows into a spreader box and is applied in a thin layer.
Figure 2 - Micro-Surfacing Machine
Figure 3 – Special Spreader Box For pavements with depressions, a scratch coat is applied to fill in the low spots. This is accomplished by using a steel strike-off box that is set to make contact with the high points on the road surface. The scratch coat should be allowed to properly cure before a second layer is applied. Areas with excessive depressions may require a hot mix asphalt scratch coat. In order to prevent premature breaking in hot weather, the
pavement
should be pre-wetted using a spray bar placed in front of the spreader
box. The water also serves as a lubricant that helps the
slurry-like
mixture to be spread more easily onto the road surface. When to Apply Micro-Surfacing When using micro-surfacing as a preventive maintenance
treatment,
timing is very important to maximize its cost-effectiveness. Applying
it
sooner is better than later. A good rule of thumb to remember is,
it is better to apply it one year too early than one year too
late.
As Figure ??? illustrates when the pavement is first starting to
show minimal amounts of deterioration, the pavement is still in
relatively
good condition and requires little or no preparation work.
Therefore,
the unit cost could range from $1.00 to $1.50 per sq. yd. depending
upon
the size of the project, and the long-term performance of the
micro-surfacing
should be excellent. However, a few years later, pavement is fair
condition and will require more preparation work. Therefore, the
unit cost could range from $1.50 to $2.50 per sq. yd. depending upon
the
size of the project, and the micro-surfacing will perform not be as
good.
Figure 4 - Timing of Application
Benefits When properly constructed and used in the right application, micro-surfacing is a cost-effective maintenance technique that can provide a variety of benefits including:
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