Upgrade Micro Surface and Slurry Sealing Operations
In part one of our two-part series on the best practices of micro surfacing and slurry seal applications, we share all of the basics and best preparation practices. Part two, forthcoming in the Fall 2020 issue of Preservation Pro, will dig into the best practices on the job site.
Experimentation with slurry sealing may have begun in the U.S. in the 1930s, but it wasn’t until the 1960s–with the advent of truck-mounted slurry seal pavers and the development and integration of quicker-setting cationic emulsions–that the treatment began to grow in popularity.
The surface treatment is designed to protect the pavement from oxidation and raveling, provide skid resistance, and stop moisture intrusion into the asphalt pavement, while minimizing disruptions to the traveling public with its quick construction times.
Its sister surface treatment, micro surfacing, didn’t reach the U.S. market until the 1980s, but has also grown in popularity since its introduction–both for its leveling capabilities and quick set capabilities.
“In the last one or two years, we’ve had customers who used to bid every micro surfacing project they could find having to pick and choose because there’s so much of this work out there now,” said Jimmy Kendrick, Director of Sales at Bergkamp, Inc., Salina, Kan. “The more educated people become on these applications, the more popular we’ve seen them become. They just need to see the benefits of it.”
What is Slurry Sealing?
What is Micro Surfacing?
Both slurry seal and micro surfacing are specifically-designed mixtures of asphalt emulsion, aggregate, water, mineral filler and additives, mixed in set proportions, and uniformly spread across a prepared surface.
Slurry seal is about one stone thick, just under ¼ of an inch or smaller, while micro surfacing is usually applied at multiple-stone thickness. Although slurry sealing can contain polymer-modified asphalt emulsion, micro surfacing always contains polymer-modified asphalt emulsion.
When to Micro Surface or Slurry Seal
Slurry seal and micro surfacing is ideal for pavements suffering from flushing, polished aggregate, raveling, oxidation and light to moderate cracking, said Dr. Andrew Braham of the University of Arkansas. Micro surfacing can also fill minor rutting and prevent cracks from growing wider, due to its use of polymer-modified emulsion. However, neither treatment will prevent cracks from returning, Braham said.
Despite their benefits, these treatments have no effect on the structural capacity of the existing pavement. As such, they should only be used to extend a roadway’s service life, not in the hopes of repairing structural damage.
Benefits of Micro Surfacing and Slurry Sealing
Both slurry sealing and micro surfacing are cost-effective treatments that will seal the pavement surface, fill small top-down cracking and restore surface friction.
They are suitable on a variety of asphalt pavement types experiencing various levels of traffic, from highways and collector streets to residential streets and culs-de-sac, according to the Pavement Preservation and Recycling Alliance (PPRA). Both treatments maintain drainage patterns and curb reveal.
Additionally, slurry sealed surfaces can be returned to traffic usually between one and four hours after application, and micro surfaced pavements, typically within one hour.
“Another benefit of micro surfacing is that it can be used alongside other applications,” Kendrick said. For example, it can be used alongside a cape seal, where a chip seal is covered with a slurry seal or micro surface.
Slurry sealing can extend a pavement’s service life between five and seven years, when applied correctly, and micro surfacing can extend service life between six and eight years.
Materials for Micro Surfacing and Slurry Sealing
There are five primary materials in micro surfacing and slurry seals: emulsion, aggregate, mineral filler, additives and water.
According to Pat Denney, Micro Surfacing Manager for Road Science, Tulsa, Oklahoma, slurry seal emulsions can be produced with a cationic quick-set emulsifier (CQS), a cationic slow-set emulsifier (CSS), or an anionic slow-set emulsifier (SS). CSS and SS emulsifiers depend largely on evaporation for curing, making them much slower to cure.
“By contrast, a micro surfacing CQS emulsifier will have a chemical break that begins to kick water out of the system,” Denney said. “Basically the system is stabilized during mixing, and then destabilized chemically to trigger a quicker curing process.”
Aggregates used for these treatments should be highly angular with minimal dust or clay, high resistance to abrasion and high soundness.
• Type I (#200 x 1/8 in. [3 mm]) offers maximum crack penetration and seals minor raveling and is used in low traffic areas.
• Type II (#200 x 1/4 in. [6 mm] stabilizes moderate to severe raveling, provides a smoother surface and quieter ride than Type III, and is used in moderate traffic conditions.
• Type III (#200 x 3/8 in. [9.5 mm]) corrects severe raveling, offers high skid resistance and structure for leveling, and is used in heavily trafficked areas.
These are the aggregate types, as outlined by the International Slurry Surfacing Association (ISSA). However, many state specs use their own aggregate types, which may differ from ISSA’s.
Check out AsphaltPro’s article “New AASHTO Pavement Preservation Standards, Explained” at www.theasphaltpro.com.
The mineral filler, additives and water are the same for slurry sealing and micro surfacing.
Mineral filler, such as portland cement, hydrated lime or limestone dust, can adjust breaking properties and improve mixture consistency. Additives, such as emulsifier and aluminum sulfate, can accelerate or decelerate the break time
Water should be free of contaminants or harmful salts.
“Water can have some effect on mixture characteristics,” Denney said. “The use of pond water in a micro surfacing application is not recommended due to increased risk of the presence of deleterious materials or other types of contamination that could negatively affect mixing and cure rates.”
Mix Designs for Micro Surfacing and Slurry Sealing
To ensure proper performance of a project, a mix design must be conducted to determine the compatibility of the aggregate, emulsified asphalt, water, mineral filler, and other additives.
Mix designs should be used to determine the compatibility and ratio of the above materials. They should be performed according to ISSA A105/ASTM D3910 for slurry sealing and ISSA A143/ASTM D6372 for micro surfacing.
For project success, the lab performing the mix design should have experience in mix designs for these treatments and should be aware of your project’s parameters. Since temperature, humidity and wind can affect cure time, these variables should also be considered.
Micro Surfacing and Slurry Sealing Equipment
These treatments can be applied either with a truck-mounted paver or a continuous paver. Truck-mounted pavers are typically used on projects with shorter pulls, while continuous pavers are used on longer pulls, such as highway jobs and can minimize construction joints.
Typically, jobs with truck-mounted pavers use two or more on the job, since they must load material at the stockpile. Mobile support units, or nurse trucks, deliver materials to the continuous pavers, which can load materials while continuing to apply the material.
Truck-mounted pavers require a smaller crew and can drive to the jobsite, while continuous pavers minimize construction joints. That’s why it’s important to have enough trucks on hand to maintain a steady supply.
Both types are equipped with a spreader box, which may or may not have augers to move material and may or may not have a drag on the back to texture the material.
The job may also require a distributor truck if tack is required, or a rubber tire roller if required.
Calibration and Test Strips
“You can have the best crew and the best site preparation in the world, but if the calibration isn’t right, you will have problems,” said Tim Harrawood of Vance Brothers, Inc., during a Transportation Research Board webinar. Calibration ensures compliance with the approved mix design, and the factors determined through the calibration process aid in application rate verification and overall material usage.
“It’s extremely important that application equipment is calibrated in the presence of an agency representative,” Harrawood said. “This could help eliminate potential issues caused by proportioning of the materials.”
Generally, Harrawood said, calibration will take place only once per project, though some agencies allow the transfer of calibrations. It can be performed on truck scales or platform scales, but all scales should be certified. He said a seasoned crew can perform the calibration in less than two hours. Each manufacturer has their own preferred calibration method, so consult the manufacturer’s manual.
Harrawood also recommends placing a test strip for evaluation and agency approval.
“Test strips assure adequate workmanship, aesthetics, and cure time of the mixture is achievable when applied with the personnel, equipment, and materials intended for use during execution of the project,” Harrawood said. He said the test strips should be performed in similar conditions as those expected during actual application. For example, test strips for a daytime project should be performed during the day.
For micro surfacing and slurry sealing jobs, the contractor will set up a staging area for the stockpiles of materials, equipment storage and maintenance.
“Your stockpile location is key,” said Zach Jensen, Product Support Manager at Bergkamp. “When the stockpile is so far away, it’s going to take longer for trucks to get back to the job site.”
The stockpile site should also be arranged for safe and optimal traffic flow.
The delivery site for the aggregate should be flat and free of vegetation. Denney recommends a gravel, cement, or asphalt surface.
“A lot of these jobs will be in remote areas where a concrete pad for your aggregate isn’t available,” Jensen said. “Use the first few loads to make a flat base for the rest of your aggregates to avoid digging into the underlying material.”
Regardless of the underlying material, loader operators should take care to avoid contaminating the aggregate with dirt, vegetation or other contaminants.
Verify that the onsite stockpiles of aggregate have not become segregated by wind or rain or contaminated with deleterious materials. Denney suggested conducting occasional checks for stockpile moisture content, as well.
Ensure the screening deck is free of contaminants and is in good working order. When screening the aggregate, Harrawood recommends screening it directly into the nurse truck. Any oversized stones could create drag marks in the mat behind the paver. “This is absolutely the best method to limit exposure to oversize aggregate,” he said. Some states with strict dust abatement rules, particularly western states, may not allow screening. “If that’s the case, work closely with the trucking firm to ensure the aggregate isn’t contaminated with oversize particles.”
It’s also important that the delivery transports, transfer tanks, and tanks on the project that will be used for the emulsion are also free of contamination, and that the screens on equipment are cleaned daily.
Additives may include both wet and dry additives. The storage area for these additives should be clean and dry, and additives should be covered by a tarp to protect them from moisture. It’s also important to ensure that the tanks, drum barrels or totes are contaminant-free.
There should be enough materials at the stockpile site and enough mobile support units to transport it to the job site to constantly supply the micro surface machine with material. This both maximizes production and minimizes transverse joints.
Stay tuned for the Spring 2020 issue of Preservation Pro to learn about the best practices of micro surface and slurry seal applications on the job site.
• FP2 Preservation Toolbox
• Federal Highway Administration (FHWA) and FP2’s slurry sealing and micro surfacing pocket guides
• Pavement Preservation and Recycling Alliance’s RoadResource.org Treatment Center
• Transportation Research Board’s micro surfacing and slurry sealing webinar
• AsphaltPro’s “How to Micro Surface” article
• Jimmy Kendrick and Zach Jensen of Bergkamp, Inc.