This is not extremely common, but can be caused by an isolated spot on the pavement that was not cleaned well or had undetected solvent on the pavement. However, if small localized sections of HFST de-bond from the pavement, it can easily be repaired by milling back to a well-bonded area, cleaning the area well, and applying polymer resin and aggregate by hand to blend the patch in with the existing material.
More common is when a small section “pops out,” but the HFST is perfectly attached to a fragment of pavement. If this is a limited occurrence, this does not indicate a catastrophic failure of the HFST. Generally, this means the underlying pavement had a weak spot and lost its bond with a lower layer of pavement. The pavement should be repaired and HFST reapplied if the area is large enough to cause roadway issues. If asphalt is used to patch the pavement, HFST should not be placed for at least 30 days, and if concrete is used, then any curing compound must be removed and the surface cleaned before applying the HFST, since curing compounds act as bond breakers.
No reactions have been found with magnesium chloride deicers. Some concrete repair materials may contain magnesium phosphate and caution is advised. Always consult the HFST resin binder manufacturer prior to the installation of any repair or patching material to confirm compatibility and installation requirements.
Common contributing issues for installation problems appear to be due to a combination of the following improper practices:
- Improper mixing of the resin binder system, either manually or by automated equipment that did not have fail safes or monitoring devices. This can result in early loss of aggregates and premature wear in the wheelpath. Installations for which the written instructions of the HFST binders have not been followed are always subject to potential problems.
- Improper placement of the resin binder by installing either too thin or too thick a layer of resin material or by disturbing the resin after it has begun its gel phase.
- If the resin material is placed more thinly than indicated by the specification, it may not have sufficient depth to embed and retain the aggregate for long-term durability.
- If the resin material is placed more thickly than indicated by the specification, it may fully encapsulate the aggregate material, leading to a lower friction (glassy) resin surface and lower macrotexture to channel water away. If the material is too thick on steep or banked roadways, it may also exhibit creep or drift towards the lower side of the roadway.
- If the resin is disturbed after placement as it begins to build its strength in the gel phase, it may lead to a reduction in the strength properties of the final cured resin material.
• Application of aggregate after resin binder has begun to gel: Aggregate should be broadcast onto the resin binder immediately to ensure proper embedment. Resin binder tends to gel very fast at high temperatures, and gelled resin binder will resist aggregate embedment. This is a primary advantage of automated continuous application systems which broadcast the aggregate within seconds after placement of the binder resin.
• Improper embedment depth: Embedment depth should typically be 50 percent of the nominal aggregate size to ensure that the aggregate is held firmly in place by the resin binder while still providing adequate texture for frictional resistance and bulk water drainage.
• Application of HFST to immature pavements: The HFST should not be applied to asphalt that is less than 30 days old or concrete pavements less than 28 days old. For concrete pavement, all curing compounds must be removed and the surface properly shot-blasted. Proper shot-blasting will remove all concrete surface paste, dirt, dust, debris and other surface contaminants.
• Application of HFST to pavements in poor condition: Existing pavement may have experienced significant cracking prior to HFST placement, so that over time the asphalt or concrete beneath the HFST failed. In these instances, the resin binder and aggregate have remained completely bonded, but the underlying pavement failed, so it was a poor candidate for HFST.
• Environmental requirements were ignored: The pavement surface must be dry at time of installation, and the ambient temperature must be above the manufacturer’s minimum recommendation. For most polymers, a minimum temperature of 50 degrees F and rising is typical, while some recommend even higher minimum temperatures. Other polymer binders are available that can be installed at lower temperatures per the manufacturers direction. Please see general temperature requirement under INSTALLATION.
The answer to this question likely depends on: (1) the condition of the existing pavement, since HFST only lasts as long as the pavement it is placed on, and (2) the severity and type of crash the location is experiencing. For a deteriorating pavement with substandard friction characteristics and minimal pavement life, the problem can be dealt with using more conventional pavement treatments to restore standard friction characteristics, at least for the short term. However, HFST may still be a good idea if the location has challenging geometric features and high incidences of speeding, since this tends to escalate pavement polishing and the problem may return even with new pavement installation.
In North Carolina and in Colorado, two different problems combined to cause the underlying asphalt pavement to be delaminated: poorly draining pavement that trapped water underneath the HFST and cracking of the underlying asphalt pavement that reflected through the HFST. However, the HFST remained intact on the asphalt material.
Any pattern cracking that is visible on the pavement such as map cracking and/or alligator cracking likely makes the pavement a poor candidate for an HFST installation. Replace un-sound pavement prior to installing HFST to increase service life expectancy. Follow guidelines such as a 28-day hydration period for new concrete pavement and 30 days waiting period for new asphalt pavements prior to installing HFST.