Surface preparation is the key for all successful HFST installations. Shot-blasting for concrete and high-pressure air washing for asphalt pavements are required prerequisites for HFST application.
Temperature is a very important condition to consider for HFST installation. Temperatures will be determined by the type of polymer resin binder being used as well as the method for broadcasting the aggregate.
- A good rule of thumb would be not to install HFST when the surface temperature of the pavement is below 50°F unless the resin binder system is formulated specifically for cooler temperatures.
- As temperatures rise, the resin binder system will reach initial gel faster. Automated methods of broadcasting are often capable of fully covering the resin binder before it begins to gel at these higher temperatures. As ambient temperatures begin to reach 95°F and above, most resin binder systems will begin to thicken and gel within minutes. It is important to determine that the aggregate can be fully broadcast at the specified coverage rate before the resin binder begins to thicken. If the aggregate is not broadcast to the specified coverage rate while the resin binder is still in its liquid state, the application is at risk of only partial embedment of the aggregate. This can lead to early loss of aggregate in the wheel path and failure of the HFST system to provide the required friction values. Many agencies require a test section to determine that the method of broadcasting aggregate will be sufficient to install HFST at the required temperatures. Always follow the guidelines of the State specifications and the manufacturer’s written instructions for the HFST binder.
Both surface and ambient temperatures are critical to a successful installation. If the aggregate can be broadcasted immediately, it can be installed at temperatures >100°F as long as the resin binder has enough time to penetrate the pavement surface prior to reaching initial gel. Hence, mechanical installation is always the preferred method.
It is generally not practical to remove small oil spots, but very large or heavily saturated oil spots may need to be removed by aggressive washing with approved detergents and rinsing with clean water. Proper drying time and/or techniques may be required. In some instances, heavily contaminated pavement should be removed and replaced.
For pavement surfaces, there should be no visible moisture present on the surface at the time of the binder application. Compressed air may be used to dry the surface. A plastic sheet left taped in place for a minimum of 2 hours, according to ASTM D 4263, may be used to identify significant moisture in the pavement during periods after rain, for example.
Utilities, drainage structures, curbs, joints, and any other structure within or adjacent to the treatment location should be protected against the application of the surface treatment materials. Existing pavement markings that are adjacent to the application surfaces to be preserved should be covered or care taken that the binder does not cover the markings.
High-tack adhesive tape can be used to outline the perimeter of the application area, cover pavement markings, or to protect joints etc. Any existing moving joints should be protected, or re-cut with the correct saw-cutting practices after the installation cures.
Pavement markings that are not covered or consist of material other than paint should be removed. The removal method may be grinding, water blasting, or other treatments; the surface should then be dried and swept clean prior to the polymer binder application. Pavement marking lines should be considered clean when the pavement has exposed aggregate showing through the existing marking. Care must be taken to not remove and/or gouge the pavement profile when removing pavement markings. Un-even surfaces should not be repaired with the HFST system. NOTE: HFST will not fully adhere to thermoplastic road markings.
Pavement cracks greater than 1/4 inch in width and depth should be sealed 30 days prior to HFST installation if rubberized asphalt or similar products are used. When sealing cracks, it is important to ensure the sealant is recessed slightly below the pavement surface and not overbanded onto the pavement. Joints and cracks may be pre-treated with the mixed polymer resin. Once the polymer in the pre-treated areas has gelled, the HFST binder and aggregate topping installation may proceed.
Repair all pavement defects such as spalls, pot holes, raveling, and rutting prior to placing HFST. Contact the binder resin manufacturer to review which materials will permit proper adhesion of the HFST system. Clean and fill all inadequately sealed joints, including shoulder areas. HFST may be applied over pavements exhibiting minor rutting or heaving; however, the product is not intended as a repair for these conditions and will not level pavements. Proper evaluation of pavement condition is important before considering HFST.
The polymer resin overlay should not be placed on Portland cement concrete that has been in place less than 28 days. Patching and cleaning operations should be inspected and approved prior to placing each layer of the overlay. After initial cleaning, any contamination of the deck or intermediate courses should be removed.
Concrete surfaces should be abrasively cleaned by shot blasting to remove oils, dirt, rubber, curing compounds, paint carbonation, laitance, weak surface mortar, and other potentially detrimental contaminants that may interfere with the bonding or curing of the overlay.
When using rapid strength concrete to shorten the 30-day waiting period prior to HFST installation, it is critical to qualify compatibility of the patching materials/rapid strength mortars with the polymer resin. Some patching materials may not provide a good bonding surface. No matter what material goes on top, installers should ALWAYS refer to the manufacturer’s installation instructions to determine if it qualifies.
Before placement of HFST on an asphalt surface, the entire surface should be cleaned by high-pressure air washing or approved washing methods to remove oils, dirt, rubber, paint, laitance, and other potentially detrimental contaminants that may interfere with the bonding or curing of the overlay. Acceptable cleaning is recognized as a surface with no oil spots, dirt, or debris.
Asphalt deposits larger than one inch in diameter and smaller areas spaced less than six inches apart should be removed.
For applications on new asphalt pavements, it is recommended to wait a minimum of 30 days after paving before installing HFST. Asphalt pavements that still appear to be “tacky” or look wet on the surface should be evaluated for possible extension to the waiting period prior to installing the HFST. On open-graded friction course asphalt surfaces, stone mastic asphalt, or pavement that has been treated with prior surface treatments, contact the resin binder manufacturer for guidance.
Several agencies have experimented with pressure washing or lightly shotblasting new asphalt surfaces to shorten the 30 day waiting period. This practice is still not recommended since the potential for trapping moisture or volatile oils can still be a risk and can contribute to the failure of the HFST installation.
Many HFST applications have been successfully installed on OGFC. However, OGFCs are relatively thin and vary in strength based on the aggregate used and the condition of the pavement. This has led to a few problematic installations since it is often difficult to detect existing pavement issues. Successful installations over OGFC have used a double layer treatment in order to seal the voids with the first layer and use the top course to maintain the proper binder depth, which is necessary for the aggregate embedment into the HFST riding surface.
If placing HFST over OGFC, the shoulder on the high side of the superelevation may need to be sealed to keep water from running through the OGFC and under the HFST, which can cause failure. It may also be necessary to extend the HFST further when the curve is on a grade in order to seal the OGFC to prevent water from running down the grade and under the HFST as well. Be aware that this adds to the quantity of HFST required.
Current methods of installing HFST can vary based on recent advancements and the automation of mixing and placement equipment. Although some agencies still allow hand mixing and placement for HFST, the level of experience with these methodologies can vary dramatically and should warrant stringent quality control procedures within a specification to ensure consistent and proper coverage rates. The selection of different methodologies depends on the size of the installation, site-specific conditions like storage and ready access, and the availability of acceptable detour routes or traffic restrictions.
Because the HFST system will have a greater coefficient of linear expansion than the pavement substrate, applying an even and consistent wet mil thickness for the resin binder is critical to the longevity of the system. Variable thickness within the HFST installation can increase the potential for cracking and spalling, especially when installed on pavements that show signs of degradation.
Consistent mil thickness of the HFST is also important for maintaining even contact with vehicles’ tires. Any high spots that are created because of un-even installation practices can create “point loading” once traffic travels over the HFST system. These high spots can potentially transfer very high dynamic impact to the underlying pavement and could potentially manifest as early age spalling and delamination of the HFST.
Prompt broadcasting of high-friction aggregates is important to ensure full embedment of the aggregate within the resin binder. As temperatures rise during summer months, resin binder systems tend to reach initial gel within minutes. When the binder gels, the viscosity begins to thicken which can inhibit the aggregates in reaching embedment properly. This could manifest in early loss of surface aggregates.
HFST may be placed within the width of the traveled lane only or from edge of pavement to edge of pavement. The treatment may also be applied to an area that is narrower than a lane to avoid removal of striping or pavement markings. This decision should be evaluated in the design phase of the project with respect the project budget, the behavior of vehicles, and the relevant crash data. Placement within the traveled lane only may result in lower required quantity of HFST, but may not provide enhanced friction on shoulders if traffic patterns or crash data indicate that vehicles travel partially within the shoulder area—behavior which is common on tight curves and ramps. It is generally not necessary (and is much more costly) to place HFST within 1 foot of obstructions to traffic like barriers, guardrail, or attenuators. When placing HFST on paved shoulders, it is also not usually necessary to follow a variable or degraded roadway edge exactly, but rather to establish a path that provides for a consistent width for friction enhancement.
HFST may be placed in conjunction with rumble strips for additional safety enhancement. If HFST is installed on a roadway with existing rumble strips, it is generally not placed over them but rather on a near-parallel offset a few inches from the rumble strip alignment. If the rumble strips are being added to the project, they may be placed first with the HFST installed in an offset alignment later, or they may be ground in after HFST placement by grinding through the HFST. Similarly, recessed striping, recessed markers, or other recesses may be achieved by grinding through the HFST with milling (cutting) style machinery. Diamond grinding or abrasive grinding machinery will not work as well for this application due to the hardness of the HFST aggregate.
HFST can be installed using any of three main application methods: manual, machine-aided, and fully automated. The preferred application method depends on the size of the project, the number of installations within the contract, and specific project needs. For example, manual installation may be used for small spot-treatments while a fully automated process might be used for larger systemic contracts consisting of numerous sites. Most transportation agencies contract out their HFST installations to avoid purchasing HFST-specific equipment.
- Manual – Manual HFST application is ideal for small spot locations of 200 sq. yd. or less where it may be difficult or not economical to bring in automated installation equipment. For a completely manual application, the resin binder components and any additives are manually mixed on site in buckets. The mixed resin binder is poured onto the prepared surface and spread to the proper mil thickness using squeegees. Next, the aggregate is broadcast on top of the binder. There is no preferred method to broadcast the aggregate, but the binder must be completely covered. While the manual application method is ideal for keeping costs low on small spot-treatments, this method has a few drawbacks. Working under live traffic conditions increases safety risks by prolonging workers’ exposure to traffic. Further, given the opportunity for human error and inconsistency, quality and uniformity in manually applied treatments are a concern. In addition, the prolonged presence of a work zone can potentially cause secondary crashes.
- Machine-Aided Manual – As its name implies, machine-aided manual application of HFST involves a combination of manual labor and machine-aided application. The exact combination largely depends on the type of application truck. Trucks are equipped with any combination of a mixing machine, binder spreader, and aggregate spreader. The contractor will typically use equipment that mixes the binder components at the correct ratio. Some binder materials are mixed with catalysts and accelerators at varied rates based on temperature to speed up cure rates in cooler weather. The machine often pumps the binder out a spigot located directly behind the truck as the truck slowly drives down the lane. Workers use squeegees to evenly spread the binder behind the truck. This installation method allows the roadway surface to be covered more quickly than manual application, but is still subject to human error and inconsistency in resin application. On larger projects, the reduced labor hours can offset the cost of additional equipment, resulting in a lower installation unit cost.
- Fully Automated – A fully automated HFST application typically involves a truck that is customized to mechanically mix the binder components in accordance with the current ambient or surface temperature. It is also capable of applying a consistent wet mil thickness of the binder and controlled rate of aggregate to the pavement surface without any manual squeegeeing or spreading. The truck contains the binder and aggregate in large bulk containers on its chassis. The application can be customized to the intended lane width of the treatment. The fully automated method minimizes lane closures due to quick installation time and reduces the number of workers on the roadway. The installation speed and the uniformity of application of binder and aggregate ensures a higher quality application and reduces overall project costs on large systemic installations.