Transport Infrastructure Ireland’s new high-friction surfacing specification
12 July 2016
The March 2015 revision of Transport Infrastructure Ireland (TII, formerly the National Roads Authority) ‘Specification for Roadworks Series 900’ is completely new for high-friction surfacing (HFS), commonly termed ‘anti-skid’.
This new ‘performance’ specification within document CC-SPW-00900-07 supersedes the old Clause 924, and allows more opportunity of choice for highway engineers. This is a very constructive development by TII for due commendation. However, it has also generated hidden caveats, disparity and ambiguities which are examined in this article.
Clause 7.3 of CC-SPW-00900-07 relates to HFS constituents and composition. Installation requirements are contained in Clause 10.2.4. Binder and aggregate requirements are detailed in tables 23a, 23b and 23c. Client-specific contract requirements are to be detailed in Contract Appendices 7/10 and 7/11. Other documents for cross reference are:
|New TII Document||Former NRA Document|
|CC-SPW-00900||Series 900 Specification|
|CC-GSW-00900||NG900 Guidance Notes|
|AM-PAV-06045||HD28 – Management of Skid Resistance|
|DN-PAV-03023||HD36 – Pavement Design & Maintenance|
|DN-PAV-03024||HD37 – Bituminous Mixtures, Surface Treatments, etc|
|DN-PAV-03074||HD300 – Design|
|DN-PAV-03075||HD301 – Approval of Specific Products (prTAITs)|
The previous NRA Clause 924 Specification was very specific in using only a cold applied ‘thermosetting’ (resin bonded) ISO 9001 or equivalent accredited system, of a resin binder in combination with a minimum 72 PSV (polished stone value) calcined bauxite aggregate. This system type was first investigated in the USA in the 1950s and has a long track record of proven performance.
Key specification changes are:
- DN-PAV-03074 defines a HFS Design Procedure;
- A contractor designed HFS system must have Factory Production Control (FPC), for purchaser assessment within a contractor-compiled Appendix 7-11 document;
- Hot-applied thermoplastic (pre-mixed) systems are permissible;
- Calcined bauxite or 70+ PSV natural aggregate are now permissible;
- One year completed prTAITs (provisional Type Approval Installation Trials), regarding site categories and traffic counts defined in TII document DN-PAV-03075_ Annex 2C: Table 2C.1, provide expected in-service performance to the purchaser. Performance of HFS systems is monitored over five years to enable a purchaser evaluate service life and whole-life cost of a specific contractor-proposed system;
- HFS systems as standard will have a five-year ‘design working life’;
- New field testing schedules include for surface macrotexture (sand patch test), visual and sensory assessment (perceptible properties check) and a bond to substrate (pull-off) test.
New HFS approval process
A prTAIT is completed by carrying out site assessment and performance tests on a HFS installation, after one year since completion to constraints of DN-PAV-03075_Table 2.1. DN-PAV-03075, which lays out the performance requirements and control procedures for the installation of HFS on national roads only. The writer’s non-exhaustive understanding of the HFA approval procedure is as follows:
- Purchaser identifies HFS requirement per DN-PAV-03023, and compiles an Appendix 7/10 and 7/11 for the defined site(s);
- Pending the purchaser agrees, the contractor submits work proposals detailing the design and installation of the HFS prTAIT. Compiled per FPC Appendix 7-11 documentation and schedule of prices. The prTAIT must have be 50-100m of an installation. The purchaser representatives commit to co-operate in the prTAIT process, after contractor consultation with TII regarding agreement on prTAIT assessment procedure. This prior agreed approach shall detail the use of competent personnel to undertake the assessment(s) and the methodology for conducting the assessment(s) e.g. laser-crack measurement system, high definition video, site walk, etc;
- Purchaser then assesses the contractor proposals for specification adherence and whole-life cost, and agrees to terms of the prTAIT;
- HFS Installed after purchaser instruction to proceed;
- After a year of trafficking, pending TII observation and an agreed performance-testing schedule performed within one month of completion, the HFS system will be approved for a specific prTAIT family and traffic count constraints per DN-PAV-03075/Annex 2C_Table 2C.1;
- The contractor shall continue to monitor the prTAIT annually for five years and declare the performance characteristics retained within their FPC Appendix 7-11 documentation, supported by data attained from TII annual surveys on the National Road network.
Approved HFS systems
The TII designated ‘prTAIT family’ for an approved specific HFS system will now be deemed as suitable for future use on similar defined national roads without the need for additional performance testing. Approved systems will only require two bond-to-substrate (pull-off) tests per installation.
To date, enquiries to accredited testing houses have intimated that a coring rig is required for the pull-off test, which is generally a two-day process. This is because steel dollies must be bonded to the surface by resin and will need to harden overnight before the dollies can be pulled off.
The authors favour the option of requesting a departure from the in situ pull-off tests. We suggest the use of HAPAS-approved (Highway Authorities Product Approval Scheme) standardised stone-mastic asphalt slabs to perform this test within an Irish National Accreditation Board laboratory on prepared site samples. This approach would reduce exposure to site risks and traffic-management costs. It would also allow a standardised textured slab for accurate result comparison between sites. After installation, performance is then monitored against ‘design working life’ by the purchaser.
From the writer’s perspective, there are uncertainties with regard to how the new HFS specification and approval system will be implemented, and how it will affect end performance:
1). Natural aggregates should warrant very careful evaluation regarding the five-year ‘design working life’. Calcined bauxite has undisputed proven history of retaining adequate micro and macrotexture for skid resistance. The undefined ‘declared’ aggregate abrasion value (AAV) limits for natural aggregate, has potential to expose highway engineers to erroneous whole-life cost for this HFS system. Surely a maximum AAV of 4 should be set for natural aggregates as per the calcined bauxite requirements? Care must be taken in use of high PSV natural aggregate, as research has proven PSV is gained at the expense of almost every other property such as strength and durability (Woodward, 1995).
PSV has historically been over-emphasised amongst highway engineers, to the detriment of other properties required for HFS. As noted in DN-PAV-03023_Annex A: ‘It is also recognised that the PSV test (IS EN 1097-8) has limitations and, whilst a good indicator, may not always accurately predict the in service skid resistance of aggregates for all stress and traffic combinations.’
The science of determining wet skidding resistance of a surfacing is complex, and no one factor can be attributed to the performance of a pavements wet skid resistance. Factors include: road geometry and hydraulic conductivity (drainage), traffic nature and counts, surface coarse nature of contact pressure between pavement and tyre (positive or negative texture), aggregate flakiness/AAV and PSV, season and climatic conditions at time of installation, etc. It seems extraordinary that the performance specification does not include a measurement of skid resistance, which is surely the fundamental purpose of the HFS installation.
2). What specification covers the non-national road network? The TII has AM-PAV-06045 as a ‘skid resistance policy’ on national roads. What is the ‘skid resistance policy’ for non-national and local roads? These low-speed roads are typically maintained by a local authority, where microtexture is more vital than macrotexture, compared to national roads where macrotexture is critical. DN-PAV-03023_4.23 notes: ‘The fine scale microtexture of the surface aggregate is the main contributor to skidding resistance and is the dominant factor in determining skidding resistance at lower speeds.’
3). How will TII standardise the prTAIT approval system for all stakeholders? Who in TII confirms compliance of a new HFS system to ensure consistency? Will a transparent web database be maintained by TII of approved systems? Testing in near wheel track in lane one (slow lane) is not defined, as is usual practice. Pull-off testing is destructive to the pavement. Is this wise, especially with one location in a wheel track? What will be the approved repair post-test?
Is a one-year prTAIT sufficient to guarantee performance? In comparison, the long-standing HAPAS HFS accreditation only approves after a two-year monitoring period. Minimum installer training or experience is not defined, whereas HAPAS installers hold HFS formal National Vocational Qualifications. The British Board of Agrément performed a study on the performance of HAPAS-accredited HFS. They reported in December 2015, after assessing 272 HFS locations, that 95% of sites were performing after five years. Failed sites were condemned due to poor substrate, were not assessed properly at prestart or failed due to poor workmanship. It has also been reported that visually acceptable HFS when tested underperformed, highlighting site walk-overs are not appropriate alone for prTAIT approval, as implied in DN-PAV-03075. It also seems strange that tried and tested HAPAS-approved HFS systems do not receive automatic TII approval.
4). What additional cost will new performance testing add to HFS? (Estimates are €500 per installation, with recovery required of €5,000 per prTAIT family performed.)
It will take at least five years before it can be determined whether the new HFS specification is betterment on the old Clause 924. In the interim there may be misperception within the Industry as specifiers, clients and contractors adapt to the new requirement.
Read the first article in our two-part series on road surfacing here.
John Cagney, ROI special products manager, Northstone Materials
Paul McBride, NI special products manager, Northstone Materials
(Northstone Materials, a CRH company)