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The BBA Assessment Report
Hoben Industrial Minerals has funded what is believed to be the first independent assessment of coloured high friction surfacing to measure colour change in addition to the traditional mechanical properties commonly quoted for such materials.
As with any other material used on the highway, the engineer looks to define the relevant properties of a coloured surface in a non-subjective manner for tenders and specifications - this has been exceptionally difficult with colour retention: its perception is so dependant on the observer and the fact the changes such materials undergo when laid cannot be observed in a short length of time, make trial sites inappropriate as a precursor for projects needing materials in the following months rather than following years.
By establishing a number of laboratory tests to rank the performance of coloured materials, potential equivalents of routinely specified existing tests to predict relative in-service performance for aggregates like the AAV (Aggregate Abrasion Value) and PSV (Polished Stone Value)- might be found to measure potential for in-service colour change. PSV is routinely used to rank materials resistance to polishing but with a complex traffic dependant relationship with in-service performance (as measured using the pendulum, SCRIM or Griptester). Perhaps the parameter for overall colour change may end up being a PSV for colour?
At the very least a benchmark level of performance for coloured materials can now be established where nothing exists at present. The Engineer must remember... the long-term performance of coloured materials cannot be predicted by their as-laid appearance!
Hoben Industrial Minerals sought the advice of TRL, the British Board of Agrèment (BBA) and the Paint Research Association when considering an appropriate test programme, the valuable advice they received led to the tests being carried out by the BBA as a Product Assessment (using ASTM, BS and TRL tests) with the tests chosen to closely reflect the processes acting upon SightGRIP as-laid.
The BBA Product Assessment compares the relative performance of SightGRIP, a non-SightGRIP red coated calcined bauxite (Product A) and a red coloured high-friction thermoplastic material (Product B). SightGRIP is manufactured by coating a graded calcined bauxite aggregate with a coloured epoxy resin binder. Fig 1 illustrates a typical SightGRIP application.
|Test||Property||SightGRIP Red Epoxy-Resin
Coated Calcined Bauxite
SRV @ 10k Passes
SRV @ 60k Passes
|TRL Freeze-Thaw + Scuff||E1
|TRL Diesel + Scuff||E1
|BS 500hrs Salt Spray||1.35|
The results showed that "the SightGRIP (red) has a greater resistance to colour loss after scuffing than the alternative materials tested". The Scuffing Test reproduced in the laboratory within a period of nine minutes the appearance of a well-trafficked surface - the action of the scuffing tyre exposing the aggregate by abrasion of the coating, the colour change through loss of coating varied dramatically as the results in Table 1 illustrate and the difference between the SightGRIP material and the non-SightGRIP red coated bauxite/red thermoplastic was obvious both by visual inspection and by measuring the colour change using the spectrophotometer
In the Wear Test, "(the) SightGRIP (red) aggregate used with a proprietary polyurethane binder had a better final (sand-patch) texture depth than the non-SightGRIP products (after the wear test)". Again a laboratory simulation reproduced the appearance of a well trafficked road surface in a relatively short time, all materials showed a similar degree of colour change in the Wear test, however the Non-SightGRIP red coated bauxite showed the greatest colour change of any of the materials tested (See Table 1).
The action of the UV-A and moisture produced colour change to a varying degree in all samples. Table 1 gives the results. The only red coloured high friction thermoplastic tested showed 30% more UV-A induced colour change than the cold applied Red SightGRIP over the same test duration.
"Exposure to a neutral salt spray solution had no significant effect on any SightGRIP colour evaluated except for a noticeable loss in gloss". It was thought unlikely that any significant colour change would result from the salt exposure, however the degree of colour change of the non-SightGRIP red coated bauxite with =2.11 was 50% more than the measured maximum value for all the other materials (maximum SightGRIP=1.38 for Green, minimum SightGRIP for Oxide Red).
The nature of the colour change seen in the Scuffing and Wear tests was one of erosion of the coatings at the point contact between the test tyre and the coarse aggregate particle - a similar degree of colour change was observed for all the materials in the Wear test but the final texture depths varied between the materials tested.
More variation in colour change was seen between the materials in the Scuffing Test but with little difference in terms of final texture depth.
Conditioning of Scuffing Test panels using diesel exposure or repeated freeze-thaw cycles failed to produce detrimental effects on performance of the Red SightGRIP both in terms of colour change and scuffing resistance.
Salt Spray tests showed the non-SightGRIP red coated bauxite to have the least resistance to the effects of salt of any of the materials tested. The red coloured high friction thermoplastic had 30% more Accelerated Weathering (QUV-A) induced colour change than the Red SightGRIP.
The differences in colour change due to the variation in the resistance of the coloured coatings to erosion observed during the Scuffing and Wear tests have been quantified in the colour changes (Delta E or ) measured by the spectrophotometer.
The reversible nature of the solidification of the red coloured high friction thermoplastic may have led to softening/melting during the high-stress short-duration scuffing test allowing rubber from the test tyre to be picked up by the sample contributing towards the highest colour changes measured in the Scuffing Tests.
The non-SightGRIP red coated bauxite visibly had a greater loss of coating, this resulted in nearly twice the measured colour change in the Scuffing Test seen for the SightGRIP samples. The red coloured high friction thermoplastic also showed more observed and measured colour change in the Scuffing Test but precisely how much of this was due to coating loss could not be established owing to the colour change caused by rubber pick up (see Table 1).
The strength and abrasion resistance of the shoulder-to-shoulder bauxite matrix of the cold applied high-friction surfaces potentially offers a level beyond which further erosion of the surface texture is extremely difficult, the mix of bauxite, other aggregates, and the thermoplastic in the red coloured high friction thermoplastic does not appear to possess this "base-level" which may account for the relatively high loss of texture seen for this material in the Wear Tests.
Colour change as a response to Salt and UV-A exposure resulted from a combination of loss of transparency (chalking) of the coating material entraining the pigments and changes in the pigments themselves - the red coloured high friction thermoplastic showed visual evidence of chalking in the QUV-A tests and this could be measured as a greater value compared to other non-chalked samples. The Non-SightGRIP red coated bauxite also showed a numerically low but still unexpectedly high value of in the Salt spray tests in comparison to the range of values obtained for the other materials tested.
Certain coated bauxite materials obtained from elsewhere in the past have shown an almost total loss of coating when exposed to salt solution, the routine use of a salt exposure test may eliminate the possibility of improving colour retention elsewhere by sacrificing the resistance of the coating to salt.
SightGRIP has also been shown to exceed the minimum levels of SRV (SRV >=65) required for classification as a high-friction aggregate. The BBA has confirmed " SightGRIP aggregate may be used as an alternative approved source of calcined bauxite by a HAPAS High Friction System Agrèment Certificate holder ".
The tests have shown that the relative performance of an aggregate such as SightGRIP using high quality pigments and applied using a commercially available binder can be measured in the laboratory against others which may be being considered as equivalent in terms of as-laid appearance (screed lines in hot applied materials not withstanding) offering the user a means of comparing as-laid costs with the long term colour performance of the materials under consideration.
The degree of variation seen in the performance of only three materials supports the continuing need for a specification for the minimum properties of coloured high-friction surfaces (potentially based around the tests carried out here) to ensure they are controlled in a manner commonplace with materials used elsewhere in the highways field.
As the BBA Product Assessment concluded: "At present there are no guidelines or requirements for the colour retention properties of coloured aggregates. In addition there is no correlation between the UV ageing period, under UK climatic conditions, for this type of product". However the results "...give a reasonable indication of which materials are the most resistant to colour change over any given period and conditions. In practice the loss of colour will depend on specific site conditions including traffic levels and the position of the site."
The values obtained by the BBA for SightGRIP represent the first laboratory determined numerical representation of the colour retention properties of a road proven product, the extension of this work to provide a national specification for the colour retention of all coloured high-friction aggregates/systems via HAPAS may well require a larger study incorporating site monitoring. The philosophy of basing a specification on measurements made for time-proven products is still a very valid one.
1: Transport Research Laboratory (TRL) Report 176 (15.10.96 Draft) Appendix G. Simulates the turning action of traffic and assesses the potential for de-bonding.
2: Transport Research Laboratory (TRL) Report 176 (15.10.96 Draft) Appendix H. Simulates long term wear caused by turning traffic.
3: Transport Research Laboratory (TRL) Report 176 (15.10.96 Draft) Appendix L. 16-17hrs @ -20+/- 2oc / 7-8 hrs @ room temperature cycles before Scuffing.
4: Transport Research Laboratory (TRL) Report 176 (15.10.96 Draft) Appendix M. Surface of sample soaked in diesel for 48 hours before Scuffing.
5: BS 3900: Part F12: 1985(1991) - Methods of tests for paints "Determination of Resistance to neutral salt spray": 500 hours exposure at 35oc.
6: ASTM G53-96 Standard Practice for "Operating Light and Water - Exposure Apparatus (Fluorescent UV-Condensation Type) for Exposure of non-metallic Materials": 2000 hours exposure of cycles of four hours UV-A light at 45oc followed by four hours condensation at 50oc.
7: = Delta E= Overall Colour Change CIELAB 1976 Method determined by spectrophotometer (CIE: Commission Internationale d I'Eclarage): the larger the the greater the colour change.
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