Best Practices for High Friction Surface Treatment (HFST) Installations: Understanding Substrate Conditions
Services: High Friction Surface Treatment
Mark Archer, International Technical Operations Manager at WJ Surface Treatments, shares best practices for successful HFST installations and highlights what engineers should consider when assessing substrate conditions.
High Friction Surface Treatment (HFST) is a proven safety solution designed to reduce skidding and improve vehicle control, particularly in high-risk locations. While its adoption across the United States is still emerging, HFST has been widely implemented in countries like the UK, where it’s considered a standard part of road safety strategy. To shed light on its practical application, performance, and suitability for different environments, we spoke with Mark Archer, International Technical Operations Manager at WJ, to answer some of the most common questions raised by engineers and highway superintendents in the field.
Why is substrate condition important for HFST installations?
While a sound substrate is ideal for high friction surface treatment (HFST) installations, there are situations where budget constraints make full surface repairs unfeasible. In cases where improving friction is the priority, even if the existing surface is distressed, it is important for specifying organisations, clients, and engineers to understand that HFST can still be successfully applied. These treatments can significantly improve surface friction and road safety, although the durability may be reduced compared to applications on fully sound surfaces.
How does HFST compare to other pavement solutions?
HFST stands out from other pavement solutions like high friction asphalts and micro asphalts because it can achieve significantly higher levels of texture and skid resistance. This makes it particularly effective in locations such as curves and intersections where maximum friction is required to keep vehicles safe.
What risks are associated with overly smooth asphalt?
In extreme cases on asphalt surfaces, either due to a very fine, close-graded asphalt mix or the effects of traffic wear and polishing from repeated tyre contact, the surface can become overly smooth and dense. This results in little to no macro or micro texture, which can hinder the proper bonding of high friction surface treatment (HFST) resin. Without sufficient surface texture, there is limited physical adhesion, making it difficult for the resin to adhere effectively.
It’s worth considering pretreatment surface preparation such as captive shotblasting or contacting a resin supplier for advice if the asphalt surface is to be treated with HFST.
How can shot blasting help?
Shot blasting is a highly effective and cost-efficient method for preparing smooth asphalt surfaces. This process involves propelling abrasive material at high velocity onto the surface to remove contaminants and create a rougher texture. Shot blasting increases the surface profile, providing the necessary macro texture for the HFST resin to bond effectively. The level of shot blasting required depends on the smoothness of the asphalt. For very smooth surfaces, a medium to heavy shot blast might be necessary to achieve a minimum texture depth of 1.0 mm as measured by the sand patch test. Consider a test patch to determine the optimal level.
What happens if the surface is too open-textured?
If the substrate to be treated with HFST has an open textured surface, particularly more than 2mm as measured by the sand patch method, there is a risk of resin binder will drain or seep into the surface. This can result in insufficient resin remaining to properly hold the bauxite in place.
Can this issue be resolved with additional binder?
To address this, the surface may need to be resurfaced, or additional resin binder can be applied. Both options involve extra cost. If more resin is used than specified by either the state or the material manufacturer’s recommended spread rate, the increased usage must be agreed upon in advance and may require a formal change order.
What are the specific concerns with chip seal surfaces?
Similar to substrates with pronounced positive macrotexture, an open-textured surface, particularly one with a texture depth exceeding 2mm as measured by the sand patch method, there is a risk that the resin binder may drain or seep into the surface, leaving insufficient binder on top to hold the bauxite aggregate in place. Chip seal surfaces can also cause issues due to the irregular spacing and rough texture of the aggregate, as well as potential areas of free bitumen, both of which may interfere with proper adhesion.
To mitigate these risks, options include resurfacing the area or applying additional resin binder. However, before proceeding, it is advisable to arrange a site visit by either the resin manufacturer or the installation contractor. This allows for a proper assessment of the surface condition and agreement on the most effective pretreatment to ensure a successful and durable installation.
Summary of Best Practices
To ensure successful HFST installations:
- Assess the surface texture: Conduct a sand patch test to determine the existing macro texture. Identify if the surface is overly smooth or open-textured.
- Pretreatment is critical: For smooth surfaces, shot blasting is highly recommended to enhance texture. For open-textured surfaces, consider resurfacing or be prepared for increased resin usage.
- Consult with the supplier: Contact your resin supplier well in advance to discuss specific substrate conditions and obtain recommendations.
- Site visits are valuable: For challenging surfaces like chip seals, arrange for a site visit by the supplier or contractor to assess and agree on the best approach.
- Document changes: Any deviations from the recommended resin spread rate must be agreed upon and documented with a change order if necessary.
By adhering to these best practices, HFST installations can effectively enhance road safety and durability, even in situations with challenging substrate conditions.