coatings

Article In Situ Study of Marking Durability Using Glass Microbeads and Antiskid Aggregates as Drop-On Materials

Andrés Coves-Campos 1, Luis Bañón 1 , José Andrés Coves-García 1,2 and Salvador Ivorra 1,* 1 Department of Civil Engineering, University of Alicante, Ctra. San Vicente del Raspeig s/n, 03690 San Vicente del Raspeig, Alicante, Spain; [email protected] (A.C.-C.); [email protected] (L.B.); [email protected] (J.A.C.-G.) 2 Servicio Territorial de Carreteras de Alicante, Conselleria de Vivienda, Obras Públicas y Vertebración del Territorio, Generalitat Valenciana, Avda. Aguilera 1, 03007 Alicante, Spain * Correspondence: [email protected]; Tel.: +34-965-903707

 Received: 20 September 2018; Accepted: 20 October 2018; Published: 21 October 2018 

Abstract: Road markings play an important role in road safety because they provide significant information to drivers about the road. For that reason, they must be replaced when they are not correctly perceived by road users. To analyse which are the main factors that affect road marking perception over time, a test section was designed in a two- rural , running actual traffic over 18 different types of markings fabricated with different combinations of drop-on materials. Chromatic coordinates, luminance, and retroreflectivity of each sample were measured during 18 months in order to study their evolution over time. The results obtained show different behaviours depending on the aggregates and application method used. An increment of the durability has been observed with the use of different layers and mixtures of glass microbeads with different sizes.

Keywords: road markings; glass microbeads; drop-on materials; retroreflectivity; skid resistance; road safety

1. Introduction Until now, four main engineering treatments have generally been carried out in order to improve road safety: (i) highway updating; (ii) road signing refurbishment; (iii) repainting of road markings; and (iv) pavement resurfacing. Updating the horizontal or vertical signage is the most affordable option and, in addition to this, road safety increases up to 35% [1]. Road markings provide continuous information to road users about roadway alignment and positioning [2]. Whether lines or pictograms are used, both should transmit an understandable message for any user. In rural highways, where the traffic average speed is high, it is especially important that this kind of signs is clear and understandable [3]. Road marking should meet certain standards of durability, resistance, and visibility [4]. The service threshold is established according to drivers’ safety needs and traffic requirements, and for that reason it is requested [5]: • Defining road marking’s essential features for that they achieve their function, from the users’ point of view. • Setting the minimum acceptable value for these features, according to traffic circumstances and the intended objectives. To ensure a good road marking quality, it is very important to make a proper selection of the materials to be used, for which we should consider economic, environmental, application, and use

Coatings 2018, 8, 371; doi:10.3390/coatings8100371 www.mdpi.com/journal/coatings Coatings 2018, 8, 371 2 of 17 or location criteria. Amongst all them, their specific characteristics and suitability for use must be principally considered; road marking must fulfil some requirements of durability, skid resistance, and visibility [6]. Daytime visibility is achieved through visual contrast between road marking and pavement [3,4]. Nighttime visibility is reached by using glass microbeads embedded in the surface, so the drivers can receive a proper amount of reflected light, providing them an adequate road visibility at night and increasing their safety [7]. The correct retroreflection value is attained through the combination of good quality microbeads and base material (white painting) in a correct dosage. Microbeads efficiency must be assessed for their different compositions and densities in the painting embedment [8]. Previous studies show that microbeads must be embedded from 1/3 to 2/3 of their diameter to optimise light retroreflection [7]. Furthermore, road markings must ensure safety for any road user that drives on them. For this reason, it is also necessary to provide them with antiskid properties (skid resistance) [9]. Road marking should be replaced when their colour, retroreflectivity, or skid resistance measurements drop below a certain value established by regulations [10]. The evolution of these properties (daytime and nighttime visibility, skid resistance, and durability) depends on the quality of the products used, their resistance against traffic conditions, application system, connection between the paint coating and the pavement, base material, traffic volume, etc. [9]. Another aspect that has a vital importance for the behaviour of drivers is how they perceive road safety because of the road markings. On one hand, depending on the situation, this perception makes the users feel more comfortable and protected, but on the other hand it is more likely that if they do not feel like this, they will react impulsively increasing the risk of having an accident on road. It is a delicate issue that must be studied and considered thoroughly by the responsible of every road marking project [11]. This paper is focused on obtaining the best composition for road markings by using different combinations of drop-on materials and postmix application systems: glass microbeads, transparent and non-transparent antiskid aggregates, mixed in different proportions with the same base material, and applied in one or two layers, in order to determine which key aspects should be considered in order to improve visibility distance and skid resistance of road markings, both crucial factors for road safety.

2. Experimental program

2.1. Test Section For this piece of research, an in situ test section was set on the CV-904 rural two-lane highway from Crevillente to Catral, located at the existing on 4 + 400 kilometric point (KP), in the municipality of Catral, province of Alicante, Spain (see Figure1). There is virtually no traffic diversion in this junction because this section is only connected with minor rural paths, so it guarantees a continuous traffic flow along the testing samples (transversal stripes). In addition, speed reduction of passing is negligible, preventing skid marks that could pollute the samples, altering the wearing process. Furthermore, placing the test section in a crossing makes easier to create a third virtual traffic lane to divert the traffic while making or measuring the samples. The tests were performed under actual traffic and weather conditions, which provide more accurate results than those obtained in laboratory simulations. Coatings 2018, 8, 371 3 of 17 Coatings 2018, 8, x FOR PEER REVIEW 3 of 17

(a)

(b)

FigureFigure 1. 1Test. Test section sectionlocation: location: (a)) in Spain; ( (bb)) at at CV CV-904-904 two two-lane-lane highway highway (Crevillente (Crevillente-Almorad-Almoradí). í ).

InIn the the technical technical datasheet datasheet ofof the traffic count count station station ID ID 904020, 904020, located located at atthe the CV CV-904-904 highway, highway, coveringcovering traffic traffic flow flow from from 1 1 + +300 300 KPKP toto 7 + 000 000 KP KP [12 [12],], we we can can observe observe that that traffic traffic volume volume is balanced is balanced betweenbetween the the two two , lanes, approximately approximately 50% in direction direction to to Catral Catral (ascending) (ascending) and and 50% 50% in indirection direction to to CrevillenteCrevillente (descending). (descending). TableTable1 shows 1 shows that that that that traffic traffic flow flow is regular is regular over over time, time, with with Annual Annual Average Average Daily Daily Traffic Traffic (AADT) (AADT) values around 4000 vehicles/day, and a decreasing percentage of trucks (PT). values around 4000 vehicles/day, and a decreasing percentage of trucks (PT).

Table 1. Annual Average Daily Traffic (AADT) and PT (2009–2013), traffic count station ID 904020. Table 1. Annual Average Daily Traffic (AADT) and PT (2009–2013), traffic count station ID 904020. AADT2009 PT,2009 AADT2010 PT,2010 AADT2011 PT,2011 AADT2012 PT,2012 AADT2013 PT,2013 AADT2009 4055PT,2009 –AADT 20104507 PT,20106.0% AADT42762011 P4.9%T,2011 4AADT406 20123.6%P T,20123734AADT 3.4%2013 PT,2013 4055 – 4507 6.0% 4276 4.9% 4406 3.6% 3734 3.4% We also notice (see Table 2) that the average traffic flow rates during working days are greater than on weekends, which means that it is a rural road with a predominant mobility function. Annual We also notice (see Table2) that the average traffic flow rates during working days are greater Average Daily Truck Traffic (AADTT) remains stable throughout the year, being less intense on than on weekends, which means that it is a rural road with a predominant mobility function. Annual weekends as well. Average Daily Truck Traffic (AADTT) remains stable throughout the year, being less intense on weekends as well. Table 2. Traffic flow rates in 2013, traffic count station ID 904020.

AverageTable Day 2. Traffic flowWorking rates in D 2013,ays traffic countSaturdays station ID 904020. Sundays Month AADT AADTT PT AADT AADTT PT AADT AADTT PT AADT AADTT PT May 3897 Average134 Day3.4% 4144 Working169 Days4.1% 3555 Saturdays64 1.8% 3007 Sundays29 1.0% Month November 7144AADT 280 AADTT 3.9%P T 7659AADT AADTT356 4.6%PT 6AADT566 AADTT124 P1.9%T AADT5146 AADTT58 PT1.1% May 3897 134 3.4% 4144 169 4.1% 3555 64 1.8% 3007 29 1.0% 2.2.November Materials and 7144 Application 280 Sys 3.9%tems 7659 356 4.6% 6566 124 1.9% 5146 58 1.1% We have analysed different compositions of road marking materials and application systems in 2.2.order Materials to study and their Application damage Systemsover time. We have analysed different compositions of road marking materials and application systems in order2.2.1. to Base study Material their damage over time. Base material used in all testing samples is white styrene acrylic painting. Technical 2.2.1.specifications Base Material are described in Table 3. Base material used in all testing samples is white styrene acrylic painting. Technical specifications Table 3. White styrene acrylic painting specifications. are described in Table3. Parameter. Standard Value and Unit

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Table 3. White styrene acrylic painting specifications.

Parameter. Standard Value and Unit Consistency UNE 48076 [13] 86 ± 10 KU Relative density ISO 2811-1 [14] 1.59 ± 0.02 g/cm3 Drying time UNE 135202 [15] 15 min Colour ISO 11664-1 [16](x,y) inside the polygon specified by regulations Solids content EN 12802 [17] β = 0.89 ± 0.02—LF7 class Coating powder UNE 135213 [18] 72% ± 2% Bleed resistance EN 1871 [19] Rc = 0.96 ± 0.01 Bleed stability EN 1871 β ≤ 0.05—BR2 class Consistency variation ≤ 5 KU. Without skins, Stability in full container UNE 48083 [20] neither clots nor solid traces ∆β ≤ 0.05—Type UV1. (x,y) inside the polygon, UV ageing EN 1871 without alterations Storage stability EN 1871 8 Alkali resistance EN 1871 Non-skin effect

2.2.2. Glass Microbeads We used four different types of glass microbeads, with different grading and surface treatment: (i) Echostar 5 (125–710 µm); (ii) Echostar 20 (1180–125 µm); (iii) Duolux 125 H1 (425–850 µm); and (iv) Ultralux (600–850 µm). Technical information provided by their manufacturers is shown from Tables4–8, according to EN 1423 [21] and EN 1424/A1 [22] European Standards.

Table 4. Echostar 5 glass microbeads specifications.

Attribute Specification Sphericity ≥80% manual method (microscope) (% good quality beads) (EN 1423 and Annex D from EN 1423) Sieve (microns) Min Max 1000 0 2 710 0 10 Grading 600 5 40 (according to ISO 2591-1 [23], sieve according 355 40 80 to series R 40/3 from ISO 565 [24]) 212 70 100 125 95 100 Bottom 100 100 Type A ≥ 1.5 Refractive index (EN 1423 and Annex A from EN 1423) Colour According to EN 1423 and EN 1424 Physicochemical resistance (H O, HCl, 2 According to EN 1423 CaCl2, Na2S) SBP (Aromatic -based ) Surface treatment SBP ECO (Non-aromatic solvent-based paints) Certifying agencies CE-marking LCPC/ESE BENOR COPRO AENOR (France) (Belgium) (Schönborn) (Spain) Coatings 2018, 8, 371 5 of 17

Table 5. Echostar 20 glass microbeads specifications.

Attribute Specification Sphericity ≥80% manual method (microscope) (% good quality beads) (EN 1423 and Annex D from EN 1423) Sieve (microns) Min Max 1400 0 2 1180 0 10 1000 5 20 Grading 850 10 30 (according to ISO 2591-1, sieve according to 600 20 60 series R 40/3 from ISO 565) 355 50 90 212 70 100 125 95 100 Bottom 100 100 Type A ≥ 1.5 Refractive index (EN 1423 and Annex A from EN 1423) Colour According to EN 1423 and EN 1424 Physicochemical resistance (H O, HCl, 2 According to EN 1423 CaCl2, Na2S) Surface treatment SBP (Aromatic solvent-based paints) Certifying agencies CE-marking LCPC/ESE BENOR COPRO AENOR (France) (Belgium) (Schönborn) (Spain)

Table 6. Duolux 125 H1 glass microbeads specifications.

Attribute Specification Sphericity (% good quality beads) 91% Sieve (microns) % Retained % Passed 1000 0–2 98–100 Grading (EN 1423) 850 0–10 90–100 600 60–75 25–40 425 95–100 0–5 Type A ≥ 1.5 Refractive index (EN 1423 and Annex A from EN 1423) Colour According to EN 1423 and EN 1424 Surface treatment Adhesion Coating Certification number 0913-CPD-2007/002

Table 7. Duolux 121 H1 glass microbeads specifications.

Attribute Specification Sphericity (% good quality beads) 91% Sieve (microns) % Retained % Passed 1400 0–2 98–100 1180 0–10 90–100 1000 5–15 85–95 820 10–30 70-90 Grading (EN 1423) 600 20–50 50–80 425 40–80 20–60 250 80–100 0–20 180 90–100 0–10 125 95–100 0–5 Refractive index ≥1.54 Colour According to EN 1423 and EN 1424 Surface treatment Adhesion Coating Certification number 0913-CPD-2007/002 Coatings 2018, 8, 371 6 of 17

Table 8. Ultralux glass microbeads specifications.

Attribute Specification Sphericity (% good quality beads) 91% Sieve (microns) % Retained % Passed 1000 0–2 98–100 Grading (EN 1423) 850 0–10 90–100 710 85–95 5–15 600 95–100 0–5 Refraction index ≥1.5 Colour According to EN 1423 and EN 1424 Surface treatment Adhesion Coating Certification number 0913-CPD-2007/002

2.2.3. Antiskid Aggregates We have used white sand (Macael 400) as non-transparent antiskid aggregate and sodium-calcium glass particles (Glass 600) as transparent antiskid aggregate. Macael 400 aggregate properties are described in Table9.

Table 9. Macael 400 white marble sand specifications.

Attribute Specification Water absorption 0.16% Apparent density 2.72 g/cm3 Knoop’s micro-hardness 140.4 kg/mm2 Physical and Mechanical Features Shock resistance 45 cm Wear resistance 0.36 mm Compression strength 803.9 kg/cm2 Bending strength 211.9 kg/cm2

CO2 43.55% CaO 55.19% MgO 0.02% FeO/Fe2O3 0.21% Al2O3 0.00% Na O 0.01% Chemical Analysis 2 SiO2 0.19% K2O 0.00% SO3 0.00% Insoluble waste 0.26% Total 99.43% Calcite (CaCO3) 98.55% Retained above Lower Limit Upper Limit 0.001 mm 0.0% 0.0% 0.01 mm 1.1% 3.3% 0.05 mm 15.0% 18.6% Sieve Analysis 0.2 mm 33.1% 40.5% (ISO 3310.1:2016) [25] 0.4 mm 25.9% 28.5% 0.5 mm 5.4% 7.2% 0.6 mm 4.9% 7.3% 0.7 mm 3.4% 6.2% 0.8 mm 0.0% 0.6% 1 mm 0.0% 0.0%

Transparent antiskid Glass 600 calcium and sodium granules have irregular shapes and sharp edges to be mixed with glass microbeads, in order to generate roughness. Technical specifications are shown in Table 10. Coatings 2018, 8, 371 7 of 17

Table 10. Glass 600 granules specifications.

Attribute Specification Colour Transparent Hardness 6/7 Mohs Physical and Apparent density 2.60 g/cm3 Mechanical Features Non-magnetic metals: <5 g/t Impurities Magnetic metals: 0 g/t (However it is acceptable one particle smaller than 0.05 g in each delivery) Infusible: <25 g/t

SiO2 70.00%–74.00% Na2O 12.00%–14.00% CaO 7.00%–11.00% Chemical Analysis MgO 3.00%–5.00% Al2O3 0.50%–2.00% FeO/Fe2O3 <0.3% K2O 0.20%–1.00% Sieve Size (mm) Quantity (%) Sieve Analysis (ISO 3310.1:2016) <0.3 10.0 Max >0.6 5.0 Max Coatings 2018, 8, x FOR PEER REVIEW 7 of 17

2.2.4. Application SystemsSystems TwoTwo differentdifferent typestypes ofof applicationapplication systemssystems werewere usedused inin thisthis study:study: • “Monolayer”“Monolayer” or single layer system (AS1): it is thethe traditionaltraditional systemsystem ofof application.application. First, the base materialmaterial isis sprayedsprayed andand immediately immediately after, after, mixture mixture of of drop-on drop-on material material is projectedis projected (it can(it can be justbe just glass glass microbeads microbeads or a mixture or a mixture of microbeads of microbeads with transparent with transparent or non-transparent or non-transparent antiskid aggregates).antiskid aggregates). • “Bilayer”“Bilayer” or or double double layer layer system system (AS2): (AS2): the first the stepfirst is step to create is to a create complete a complete AS1 layer, AS1 as explained layer, as before;explained once before; it is dry, once another it is dry, identical another layer identical is applied layer overis applied the previous over the one. previous one.

2.3. Test SectionSection DesignDesign andand SampleSample FabricationFabrication Following thethe indicationsindications ofof thethe Spanish Spanish regulations regulations on on road surface marking marking design, design included, included in thein the Ministerial Ministerial Order Order FOM/3053/2008 FOM/3053/200 [826 [],26 we], w sketchede sketched the the placement placement of theof the testing testing samples samples on theon pavementthe pavement with with chalk chalk and adhesive and adhesive tape. tape. As shown As shown in Figure in Figure2, stripes 2, werestripes painted were painted in pairs in (except pairs the(except first the one, first that one, is just that one is just stripe), one stripe), spaced spaced 15 m between 15 m between groups, groups, except except for the for single the stripe,single stripe, that is 20that m is apart. 20 m Theapart. last The stripe last isstripe 50 m is away 50 mfrom away the from intersection, the intersection, considered considered as a conflict as a conflict point. point.

Figure 2. Test section general layout.

There are nine stripes per traffic direction. In addition, every stripe is divided into two testing samples covering half of the lane, so that both are treaded by a tyre at the same time, totalling 18 testing samples with different combinations of road marking materials (see Table 11). The scheme was repeated in the opposite direction, obtaining a total of 36 testing samples, labelled as shown in Figure 3. As AADT is similar in both directions, it is possible to compare and contrast the results obtained from each testing sample.

Table 11. Test samples composition and application system.

Testing Glass Particle % Drop-on Material Applic. Group Sample Microbeads Size (µm) Microbeads Macael 400 Glass 600 System S1 Echostar 5 125–710 100 – – AS1 S2 Echostar 20 125–1180 100 – – AS1 G1 S3 Duolux 125 H1 425–850 100 – – AS1 S4 Duolux 121 H1 125–1180 100 – – AS1 S5 Ultralux 600–850 100 – – AS1 S6 Echostar 5 125–710 80 20 – AS1 S7 Echostar 20 125–1180 80 20 – AS1 G2 S8 Duolux 125 H1 425–850 80 20 – AS1 S9 Duolux 121 H1 125–1180 80 20 – AS1 S10 Ultralux 600–850 80 20 – AS1 S11 Echostar 5 125–710 80 – 20 AS1 G3 S12 Echostar 20 125–1180 80 – 20 AS1

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There are nine stripes per traffic direction. In addition, every stripe is divided into two testing samples covering half of the lane, so that both are treaded by a tyre at the same time, totalling 18 testing samples with different combinations of road marking materials (see Table 11). The scheme was repeated in the opposite direction, obtaining a total of 36 testing samples, labelled as shown in Figure3. As AADT is similar in both directions, it is possible to compare and contrast the results obtained from each testing sample.

Table 11. Test samples composition and application system.

Testing Particle Size % Drop-on Material Group Glass Applic. System Sample Microbeads (µm) Microbeads Macael 400 Glass 600 S1 Echostar 5 125–710 100 – – AS1 S2 Echostar 20 125–1180 100 – – AS1 G1 S3 Duolux 125 H1 425–850 100 – – AS1 S4 Duolux 121 H1 125–1180 100 – – AS1 S5 Ultralux 600–850 100 – – AS1 S6 Echostar 5 125–710 80 20 – AS1 S7 Echostar 20 125–1180 80 20 – AS1 G2 S8 Duolux 125 H1 425–850 80 20 – AS1 S9 Duolux 121 H1 125–1180 80 20 – AS1 Coatings 2018, 8, x FORS10 PEER REVIEW Ultralux 600–850 80 20 – AS1 8 of 17 S11 Echostar 5 125–710 80 – 20 AS1 S12 Echostar 20 125–1180 80 – 20 AS1 S13 Duolux 125 H1 425–850 80 – 20 AS1 G3 S13 Duolux 125 H1 425–850 80 – 20 AS1 S14S14 Duolux Duolux 121 H1 125 125–1180–1180 8080 –– 2020 AS1 AS1 S15S15 Ultralux Ultralux 600 600–850–850 8080 –– 2020 AS1 AS1 S16S16 Ultralux Ultralux 600 600–850–850 100100 –– –– AS2 AS2 G4G4 S17S17 Ultralux Ultralux 600 600–850–850 8080 2020 –– AS2 AS2 S18S18 Ultralux Ultralux 600 600–850–850 8080 –– 2020 AS2 AS2

Figure 3. Testing samples labelling: ( a) Crevillente direction; (b) Catral direction.

While fabricating eacheach ofof inin situsitu testingtesting samples,samples, wewe obtainedobtained specimensspecimens in in a a15 15 cm cm× × 30 cm metal plate placed onon thethe pavementpavement whilewhile paintingpainting to to examine examine them them under under a a laboratory laboratory microscope. microscope Figure. Figure4 shows4 shows the the sample sample fabrication fabrication process. process.

a b

c d

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S13 Duolux 125 H1 425–850 80 – 20 AS1 S14 Duolux 121 H1 125–1180 80 – 20 AS1 S15 Ultralux 600–850 80 – 20 AS1 S16 Ultralux 600–850 100 – – AS2 G4 S17 Ultralux 600–850 80 20 – AS2 S18 Ultralux 600–850 80 – 20 AS2

Figure 3. Testing samples labelling: (a) Crevillente direction; (b) Catral direction.

While fabricating each of in situ testing samples, we obtained specimens in a 15 cm × 30 cm metal Coatings 2018, 8, 371 9 of 17 plate placed on the pavement while painting to examine them under a laboratory microscope. Figure 4 shows the sample fabrication process.

a b

c d

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e f

g h

Figure 4.FigureSample 4. Sample fabrication fabrication process: process: (a) traffic (a) traffic diversion; diversion; (b) sample (b) sample sketching sketching with with chalk chalk and and adhesive adhesive tape; (c) spraying of base material; (d) application of drop-on material; (e) in situ test sample tape; (c) spraying of base material; (d) application of drop-on material; (e) in situ test sample finished; finished; (f) laboratory specimen fabrication; (g) transversal stripe completed; (h) finished test section, (f) laboratory specimen fabrication; (g) transversal stripe completed; (h) finished test section, ready to ready to be opened to traffic. be opened to traffic. 2.4. Tests Performed 2.4. Tests Performed We have tested the fundamental parameters of the in situ testing samples: (i) luminance factor Weβ haveand colour tested (chromatic the fundamental coordinates X parameters,Y) for daytime of visibility, the insitu using testing a Spectro samples:-Guide 45/0 (i) gloss luminance factor βspectrophotomand colour (chromaticeter (BYK GmbH, coordinates Wesel,X ,Y) for daytime); (ii) retrorefle visibility,ctive using luminance a Spectro-Guide coefficient 45/0 gloss spectrophotometer(retroreflectivity) in dry (BYK (RL) GmbH, and wet Wesel, conditions Germany); (RW) for nighttime (ii) retroreflective visibility, using luminance a ZRM 6013 coefficient retroreflectometer (Zehntner GmbH, Sissach, Switzerland); and (iii) skid resistance coefficient (SRT) (retroreflectivity) in dry (R ) and wet conditions (R ) for nighttime visibility, using a ZRM 6013 using the TRRL penduluLm (Controls SpA, Milan, Italy)W. All the tests were performed inside the wheel retroreflectometerpath area. As (Zehntnerstandards require, GmbH, each Sissach, factor was Switzerland); measured a minimum and (iii) of skid three resistance times in every coefficient testing (SRT) using thesample TRRL per pendulum direction, (Controls and SpA,subsequently Milan, the Italy). average All thevalue tests is calculated. were performed Therefore, inside for each the wheel path area.kind As of standardsmixture tested require, (S1–S18), each at factorleast six was values measured for each aparameter minimum and of period three of times time inassessed every testing samplewere per drivingdetermined direction,. and subsequently the average value is calculated. Therefore, for each For the durability study, we have measured those parameters at different times: just produced, after 1, 6, 12, and 18 months, for a quantitative and qualitative analysis supported by data charts, pictures of in situ testing samples and laboratory specimens using a Leica EZ4D microscope (Leica Microsystems GmbH, Wetzlar, Germany). The weather conditions during data collection are shown in Figure 5.

Coatings 2018, 8, 371 10 of 17 kind of mixture tested (S1–S18), at least six values for each parameter and period of time assessed were determined. For the durability study, we have measured those parameters at different times: just produced, after 1, 6, 12, and 18 months, for a quantitative and qualitative analysis supported by data charts, pictures of in situ testing samples and laboratory specimens using a Leica EZ4D microscope (Leica Microsystems GmbH, Wetzlar, Germany). The weather conditions during data collection are shown in

Figure5. Coatings 2018, 8, x FOR PEER REVIEW 10 of 17 Coatings 2018, 8, x FOR PEER REVIEW 10 of 17

Figure 5. Surface temperature and relative humidity (RH) measured in the test section during Figure 5. Surface temperature and relative humidity (RH) measured in the test section during data collection. data collection. 3. Results and Discussion 3. Results andFigure Discussion 5. Surface temperature and relative humidity (RH) measured in the test section during data collection. 3.1. Daytime Visibility 3. Results and Discussion 3.1. Daytime VisibilityFigure 6 shows the average results of the chromatic coordinates (X,Y) measured on the 18 in situ 3.1.testing Daytime samples Visibility over time. The results obtained are all inside the colour polygon defined by its Figure6vertices shows for thethe white average permanent results colour, of hence the chromatic this parameter coordinates is not a limiting ( Xcondition,Y) measured according on the 18 in situ testing samplesto theFigure aspects 6 overshows studied time. the in average this The paper. results of obtainedthe chromatic are coordinates all inside (X the,Y) measured colour polygonon the 18 in defined situ by its vertices fortesting the white samples permanent over time. Thecolour, results hence obtained this are parameter all inside is the not colour a limiting polygoncondition defined by according its to vertices for the white0.38 permanent colour, hence this parameter is not a limiting condition according the aspectsto studied the aspects in studied this paper. in this paper. 0.37 0.38

0.36 0.37

0.35 0.36

0.34Y 0.35 0.33

0.34Y 0.32

0.33 0.31

0.32 0.30 0.28 0.29 0.30 0.31 0.32 0.33 0.34 0.35 0.36 0.31 X Acceptance Criteria: Inside the Polygon Chromatic Coordinates X,Y of the 18 testing samples over time 0.30 Figure 6. Chromatic0.28 coordinates0.29 (X0.30,Y) of the0.31 18 testing0.32 samples0.33 during0.34 the0.35 18-month0.36 testing period.

X Acceptance Criteria: Inside the Polygon Chromatic Coordinates X,Y of the 18 testing samples over time Figure 6. ChromaticFigure 6. Chromatic coordinates coordinates (X,Y (X), ofY) of the the 18 18 testingtesting samples samples during during the 18 the-month 18-month testing period testing. period.

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TheThe luminance luminance factor factor evolution evolution over over ti ti timememe is is shown shown in in the the Figure Figure 7.7 7.. An AnAn analysis analysisanalysis of ofof the thethe collected collectedcollected datadata shows shows that that between between the the sample sample groups groups with with AS1 AS1 AS1 application application application system system (monolayer), (monolayer), the the best best performanceperformance is is obtained obtained obtained by by by the the the group group group G1 G1 samples, G1 samples, samples, that that only that only use only glass use use microbeads glass glass microbeads microbeads as drop-on as as drop material,drop-on-on material,followedmaterial, follo follo by groupwedwed by by G3, group group whose G3, G3, samples whose whose samples containsamples contain glass contain microbeads glass glass microbeads microbeads (80%) and (80%) (80%) glass and and 600 glass glass (20%), 600 600 and(20%) (20%) the, , andlastand the onethe last last is G2,one one that is is G2, G2, has that that a mixture has has a a mixture ofmixture glass of beadsof glass glass and beads beads non-transparent and and non non-transparent-transparent antiskid antiskid aggregatesantiskid aggregates aggregates as drop-on as as dropmaterials.drop-on-on materials. materials. We should We We underline should should underline thatunderline group that that G4 group (bilayer)group G4 G4 results (bilayer) (bilayer) follow results results a similar f ollowfollow trend, a a similar similar but getting trend, trend, better but but gettingoutcomesgetting better better than outcomes outcomes the equivalent than than the the AS1 equivalent equivalent group with AS1 AS1 the group group same with with kind the the of same drop-onsame kind kind aggregates. of of drop drop-on-on aggregates. aggregates.

0 0Months Months 1 1Month Month 0.750.75 6 6Months Months 1212 Months Months 0.700.70

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0.300.30

0.250.25 S1S1 S2S2 S3S3 S4S4 S5S5 S6S6 S7S7 S8S8 S9S9 S10S10 S11S11 S12S12 S13S13 S14S14 S15S15 S16S16 S17S17 S18S18 G1G1 G2G2 G3G3 G4G4 Sample Sample FigureFigure 7 7..7 Evolution. Evolution of of the the average average luminance luminance factor factor β β in in the the 18 18 testing testing samples samples samples.. .

From the qualitative analysis of the in situ pictures taken from every testing sample over time, we FromFrom the the qualitative qualitative analysis analysis of of the the in in situ situ pictu picturesres taken taken from from every every testing testing sample sample over over time, time, notice that larger glass microbeads become detached first, leaving craters which are filled by dust, dirt, wewe notice notice that that larger larger glass glass microbeads microbeads become become detached detached first, first, leaving leaving craters craters which which are are filled filled by by dust, dust, and rubber from tyres, making the road markings darker and losing its luminance factor, as observed dirtdirt, ,and and rubber rubber from from tyres, tyres, making making the the road road markings markings darker darker and and losing losing its its luminance luminance factor, factor, as as in Figure8a. However, we realize that glass microbeads with an adhesive coating (as in sample S4) are observedobserved in in Figure Figure 8a 8a. .However, However, we we realize realize that that glass glass microbeads microbeads with with an an adhesive adhesive coating coating (as (as in in detached with more difficulty, even if they are in bigger sizes. Therefore, a lesser number of craters are samplesample S4) S4) are are detached detached with with more more difficulty, difficulty, even even if if they they are are in in bigger bigger sizes. sizes. Therefore, Therefore, a a lesser lesser created, and the luminance stays better over time, as seen in Figure8b. numbernumber of of craters craters are are created, created, and and the the luminance luminance s taysstays better better over over time, time, as as seen seen in in Figure Figure 8b 8b. .

aa bb

FigureFigure 8. 8. In In situ situ surface surface detail detail after after 1 1month month of of service: service: ( a( a) )S2 S2 sample; sample; ( b (b) )S4 S4 sample. sample.

Coatings 2018, 8, 371 12 of 17

CoatingsCoatings 2018 2018, 8, ,8 x, xFOR FOR PEER PEER REVIEW REVIEW 1212 of of 17 17 Regarding G2 and G3, apart from the craters caused by the detached microbeads, the antiskid RegardingRegarding G2 G2 and and G3, G3, apart apart from from the the craters craters caused caused by by the the detached detached microbeads, microbeads, the the antiskid antiskid aggregates retain dust and rubber particles, which also contributes to reduce luminance (Figure9). aggregatesaggregates retain retain dust dust and and rubber rubber particles, particles, which which also also contributes contributes to to reduce reduce luminance luminance (Figure (Figure 9). 9).

aa bb

FigureFigure 9. 9. InIn In situ situ situ surface surface surface detail detail after after 12 12 months months of of service: service: ( (a ()a S8) S8 S8 sample; sample; ( b ()b S13 ) S13 sample. sample.

FromFrom the the global global analysis analysis of of the the luminance luminance factor factor in in all all the the 18 18 18 testing testing testing samples, samples, samples, we we we notice notice that that they they fulfilfulfilfulfil the the minimum minimum acceptance acceptance standard standard standard of of of β β = =0.30 0.300.30 establishedestablished established by byby the thethe Spanish SpanishSpanish regulati regulationsregulationsons [ [27,2827[27,28,28]],,] , beingbeing that that that some somesome testing testingtesting samples samples slightly slightly under under under this this this value value value in in inat at 18 at 18 18months months months age, age, age, as as in asin S12 inS12 S12and and andS17, S17, S17,but but couldbutcould could be be still still be acceptable. still acceptable. acceptable. That That i Thats i swhy why is we whywe consider consider we consider that that the the that luminance luminance the luminance factor factor β factorβ is is not not βthe theis main not main the limiting limiting main factorlimitingfactor for for factor durability. durability. for durability.

33.2..2.3.2. Nighttime Nighttime V Visibility Visibilityisibility

Figure 10 shows the evolution of the retroreflectivity in dry conditions,L R . Samples from FigureFigure 10 10 shows shows the the evolution evolution of of the the retroreflectivity retroreflectivity in in dry dry conditions, conditions, R RL. Samples. SamplesL from from groups groups groups G2 and G3 reached lowerL R values than those from G1, which only has glass microbeads as G2G2 and and G3 G3 reached reached lower lower R RL values values Lthan than those those from from G1, G1, which which only only has has glass glass microbeads microbeads as as drop drop-on-on material.drop-onmaterial. material.

0 Months0 Months 1 Month1 Month 6 Months6 Months 1212 Months Months 1818 Months Months 0-300-30 days days ≥ 300≥ 300 (mcd/m2/lx) (mcd/m2/lx) 3030-180-180 days days ≥ 200≥ 200 (mcd/m2/lx) (mcd/m2/lx) 180180-365-365 days days ≥ 150≥ 150 (mcd/m2/lx) (mcd/m2/lx) 365365-730-730 days days ≥ 100≥ 100 (mcd/m2/lx) (mcd/m2/lx) 700700

600600 500

/lx) 500

/lx)

2 2 400400

(mcd/m 300 (mcd/m

L 300

L

R R 200200

100100

0 0 S1S1 S2S2 S3S3 S4S4 S5S5 S6S6 S7S7 S8S8 S9S9 S10S10S11S11S12S12S13S13S14S14S15S15S16S16S17S17S18S18 G1G1 G2G2 G3G3 G4G4 SampleSample

Figure 10. Evolution of RL on test samples during the first 18 months of service. L FigureFigure 10. 10. Evolution Evolution of of R RL on on test test samples samples during during the the first first 18 18 months months of of service service. .

FromFrom the the qualitative qualitative analysis analysis of of the the pictures pictures taken taken in in laboratory, laboratory, we we realized realized that that in in G2 G2 t hethe non non- - transparenttransparent antiskid antiskid particles particles were were taking taking up up spaces spaces that that could could be be covered covered with with glass glass microbeads. microbeads.

Coatings 2018, 8, 371 13 of 17

From the qualitative analysis of the pictures taken in laboratory, we realized that in G2 the Coatings 2018, 8, x FOR PEER REVIEW 13 of 17 non-transparent antiskid particles were taking up spaces that could be covered with glass microbeads. Consequently,Consequently, these these opaque opaque particles particles do do not not let let the the light light go through go through them. them. Moreover, Moreover, they cast they shadows cast Coatings 2018, 8, x FOR PEER REVIEW 13 of 17 shadowsto the glass to the microbeads glass microbeads placed just placed behind just them behind (Figure them 11 ).(Figure That is 11). the That reason is whythe reason this sample why groupthis has a considerable loss of retroreflectivity compared to G1. sampleConsequently, group has a considerable these opaque loss particles of re troreflectivity do not let the lightcompared go through to G1. them. Moreover, they cast shadows to the glass microbeads placed just behind them (Figure 11). That is the reason why this sample group has a considerable loss of retroreflectivitya b compared to G1.

a b

FigureFigure 11. 11. MicroscopeMicroscope detail detail of of S9 S9 sample: sample: (a ()a ×)16× 16magnifications; magnifications; (b) ( b×)35× magnifications35 magnifications.. Figure 11. Microscope detail of S9 sample: (a) ×16 magnifications; (b) ×35 magnifications. SomethingSomething similar similar happens happens with with G3 G3 samples. samples. As As they they have have mixed mixed glass glass beads beads plus plus transparent transparent Something similar happens with G3 samples. As they have mixed glass beads plus transparent antiskidantiskid aggregates, aggregates, the the last last ones ones let let part part of of the the light light go through themthem andand bebe retroreflectedretroreflected (Figure (Figure 12 ). antiskid aggregates, the last ones let part of the light go through them and be retroreflected (Figure That is why G3 samples obtain better results than in G2, but not as good as in G1. 12). That12). is Thatwhy is G3 why samples G3 samples obtain obtain better better results results than than in in G2 G2, but, but not not asas good asas in in G1. G1.

a a b b

FigureFigure 12. Microscope 12. Microscope detail detail of of S15 S15 sample: sample: ( a(a))× ×1616 magnifi magnifications;cations; (b) ( b×35) × magnifications.35 magnifications. Figure 12. Microscope detail of S15 sample: (a) ×16 magnifications; (b) ×35 magnifications. FollowingFollowing the acceptance the acceptance criteria crite [ria27 , [2827,28], we], we find find that that from from the the eighteenth eighteenth month, month, some some G2 G2 testing testing samples have a RL value below Spanish standard requirements (100 mcd/m2 2/lx). For G3, samplesFollowing have the a R L acceptancevalue below crite Spanishria [27,28 standard], we find requirements that from the(100 eighteenth mcd/m /lx). month, For someG3, testing G2 testing samples S13 and S14 are those whose perform best along the different tests, achieving the best testingsamples samples S13 and have S14 a are RL those value whose below perform Spanish best standard along the requirements different tests, (100 achieving mcd/m2 the/lx). best Forresults G3, results during service life. during service life. testing samplesFrom S13 both and qualitative S14 are thoseand quantitative whose perform analysis, best we along can theconclude different that tests,initial achieving retroreflectivity the best results Fromresultsduring bothimprove service qualitative whenlife. microbeads and quantitative used have analysis,a greater size. we canOn the conclude other hand, that the initial smaller retroreflectivity the glass resultsFrombeads improve both are, qualitativethe when more microbeads they and embed quantitative usedin the havebase analysis, material, a greater we getting size. can Onlessconclude the retroreflectivity other that hand, initial at the first. retroreflectivity smaller However, the glass resultsbeads asimprove are, they the detach when more with theymicrobeads more embed difficulty, used in the haveretroreflectivity base a material,greater size.is getting preserved On the less betterother retroreflectivity along hand, time, the smallerextending at first. the However,road glass beadsas they are,marking detach the more service with they more life. embed The difficulty, samples in the retroreflectivity fabricatedbase material, with getting medium is preserved less-sized retroreflectivity betterglass microbeads along time, at (425first. extending– 850However, µm) road asmarking they obtaineddetach service with good life. more results The difficulty, initially samples and retroreflectivity fabricated over time. with Another is medium-sizedpreserved important better aspect glass along to microbeads consider time, extending is that(425–850 glass road µm) markingobtainedmicrobeads service good life. resultswith The an initially adhesive samples andcoating fabricated over are time. detached with Another medium from the important-sized paint glasswith aspect more microbeads to difficulty, consider (425 extending is–850 that µm) glass road marking performance. obtainedmicrobeads good with results an adhesive initially coating and over are time. detached Another from importantthe paint with aspect more to difficulty,consider extending is that glass road In the G4 testing samples group, a retroreflection increase is observed after twelve months of microbeadsmarking performance. with an adhesive coating are detached from the paint with more difficulty, extending service. This behaviour could be explained because the upper paint layer is progressively worn out In the G4 testing samples group, a retroreflection increase is observed after twelve months of road markingby traffic, performance. revealing the microbeads embedded in the lower layer (Figure 13), improving service.In retroreflectivity.the This G4 behaviourtesting Thissamples could process group, be explainedstarts a from retroreflection becausethe sixth themonth increase upper of service paint is observed layer and extends is progressivelyafter th twelvee service months worn life of out of by service.bilayer This be roadhaviour markings, could reducing be explained retroreflectivity because decay the upper over time. paint layer is progressively worn out by traffic, revealing the microbeads embedded in the lower layer (Figure 13), improving retroreflectivity. This process starts from the sixth month of service and extends the service life of bilayer road markings, reducing retroreflectivity decay over time.

Coatings 2018, 8, 371 14 of 17 traffic, revealing the microbeads embedded in the lower layer (Figure 13), improving retroreflectivity. This process starts from the sixth month of service and extends the service life of bilayer road markings, reducing retroreflectivity decay over time. CoatingsCoatings 2018 2018, 8,, x8 ,FOR x FOR PEER PEER REVIEW REVIEW 14 of14 17 of 17

FigureFigure 13. S18 13. sampleS18 sample in situ in situ detail detail after after 12 12 months months of of service. service. Emergence Emergence ofof glass microbeads from from the Figure 13. S18 sample in situ detail after 12 months of service. Emergence of glass microbeads from bottomthe layer bottom is layer observed. is observed. the bottom layer is observed. FigureFigure 14 shows 14 shows retroreflectivity retroreflectivity evolution evolution over over time time in in wet wet conditions.conditions. From From this this figure, figure, it itcan can be be Figure confirmed 14 shows that fromretroreflectivity six months evolution old, none over of the time G1, in G2 wet, and conditions. G3 testing From samples this figure, reach theit can confirmed that from six months old, none of the G1, G2, and G3 testing samples reach the minimum be minimum confirmed value that required from six by months Spanish old, regulatio none ns of [ 27,28 the G1,]. We G2 also, and notice G3 that testing samples samples with reachbigger the value required by Spanish regulations [27,28]. We also notice that samples with bigger microbeads minimummicrobeads value get requiredbetter RW byresults Spanish than thoseregulatio withns smaller [27,28 ones,]. We because also notice they thatprotrude samples over withthe water bigger get better R results than those with smaller ones, because they protrude over the water surface. microbeadssurface.W Furthermore, get better R asW results bigger microbeadsthan those withare detached smaller ones,easier, because RW drops they quickly protrude over time,over theas seen water Furthermore,surface.on Figure Furthermore, 14. as bigger as microbeads bigger microbeads are detached are detached easier, easier,RW drops RW drops quickly quickly over over time, time, as as seen seen on Figureon Figure 14. 14. 0 Months 1 Month 6 Months 12 Months 0 18Months Months 0-130 Month days ≥ 50 (mcd/m2/lx) 6 30Months-180 days ≥ 35 (mcd/m2/lx) 18012- 730Months days ≥ 25 (mcd/m2/lx) 150 18 Months 0-30 days ≥ 50 (mcd/m2/lx) 30-180 days ≥ 35 (mcd/m2/lx) 180-730 days ≥ 25 (mcd/m2/lx) 150135 120 135 105

120 /lx)

2 90 105

75 /lx)

2 90

(mcd/m 60

W 75 R 45

(mcd/m 60

30 W

R 45 15 30 0 15 S1 S2 S3 S4 S5 S6 S7 S8 S9 S10 S11 S12 S13 S14 S15 S16 S17 S18 0 G1 G2 G3 G4 S1 S2 S3 S4 S5 S6 S7 S8 S9SampleS10 S11 S12 S13 S14 S15 S16 S17 S18

G1 G2 G3 G4 Figure 14. Evolution of RW on test samples during the first 18 months of service. Sample

As happened with RL values, RW increase from the sixth to the twelfth month is observed in G4 FigureFigure 14. 14.Evolution Evolution of ofR RWW onon testtest samples during during the the first first 18 18 months months of of service service.. samples, due to the same reason explained before. This effect makes that these three samples achieve the minimum RW values established by the Spanish regulations [27,28]. AsAs happened happened with withR LRLvalues, values,R RWW increaseincrease from from the the sixth sixth to to the the twelfth twelfth month month is isobserved observed in inG4 G4 samples,samples, due due to to the the same same reason reason explained explained before.before. This effect effect makes makes that that these these three three samples samples achieve achieve 3.3. Skid Resistance thethe minimum minimumR WRWvalues values established established byby thethe Spanish regulatio regulationsns [ [27,2827,28].]. Figure 15 shows skid resistance results of the 18 testing samples over time. Results were the 3.3.opposite Skid Resistance of those obtained for β, RL, and RW. So, the best group for SRT value is G2, followed by G3 Figure 15 shows skid resistance results of the 18 testing samples over time. Results were the opposite of those obtained for β, RL, and RW. So, the best group for SRT value is G2, followed by G3

Coatings 2018, 8, 371 15 of 17

3.3. Skid Resistance

CoatingsFigure 2018 15, 8shows, x FOR PEER skid REVIEW resistance results of the 18 testing samples over time. Results were15 of the17 opposite of those obtained for β, RL, and RW. So, the best group for SRT value is G2, followed by G3and and finally finally G1. Therefore, G1. Therefore, non-transparent non-transparent aggregates aggregates have better have roughness better results roughness than resultstransparent than transparentones. ones.

0 Months 1 Month 6 Months 12 Months 18 Months Acceptance criteria > 45 65

60

55

SRT 50

45

40 S1 S2 S3 S4 S5 S6 S7 S8 S9 S10 S11 S12 S13 S14 S15 S16 S17 S18 G1 G2 G3 G4 Sample

FigureFigure 15. SkidSkid resistance resistance coefficient coefficient (SRT (SRT)) evolution evolution on test on samples test samples during duringthe first the18 months first 18 of months service. of service. For G4, testing samples follow same the trend that those testing samples that have the same dropFor- G4,on materials testing samples but are applied follow using same AS1 the system. trend that Sample those S17 testing shows samplesthe best results that have in this the grou samep, drop-onfollowed materials by S18 but and are S16. applied using AS1 system. Sample S17 shows the best results in this group, followed by S18 and S16. 4. Conclusions 4. Conclusions All sample groups reach the luminance factor acceptance requirements over time. Thus, luminanceAll sample is groups not the reach most the determining luminance factor factor acceptance in order torequirements choose the over optimum time. Thus, combination luminance of is notmaterials the most for determining road marking factor selection in order amongst to choose all the the samples optimum studied. combination The craters of materials left on the for base road markingmaterial selection by the amongst detached all drop the- sampleson aggregates studied. are Thefilled craters with dust left onand the rubber base materialparticles, by darkening the detached the drop-onroad aggregatesmarkings and are making filled with them dust lose and luminance rubberparticles, and therefore darkening daytime the visibility road markings over time. and making them loseReg luminancearding the and chromatic therefore coordinates, daytime visibility all the testing over time. samples analysed are inside the polygon thatRegarding defines the permanent chromatic white coordinates, colour during all the testingthe 18-month samples testing analysed period. are inside As happens the polygon with the that definesluminance permanent factor, white colour colour is not during determinant the 18-month to find the testing optimum period. drop As-on happens mixture. with the luminance factor, colourRetroreflectivity is not determinant of road marking to find the samples optimum made drop-on of just microglass mixture. beads as drop-on material is greaterRetroreflectivity than those ofwhich road have marking a mixture samples of microbeads made of just and microglass transparent beads antiskid as drop-on aggregates, material and is greaterthese than ones those obtain which better have results a mixture than those of microbeads made with andopaqu transparente antiskid antiskidaggregates. aggregates, and these ones obtainMacael better 400 results particles than take those the space made that with could opaque have antiskid been covered aggregates. by glass microbeads and block theMacael incident 400 light, particles which take reduces the space RL values that could in G2 have samples. been coveredGlass 600 by particles glass microbeads also fill that and space, block reducing RL value; however, as they are transparent, they let part of the light go through them, the incident light, which reduces RL values in G2 samples. Glass 600 particles also fill that space, obtaining slightly better RL values in the G3 sample group. reducing R value; however, as they are transparent, they let part of the light go through them, As Lthe size and quality (less impurities and defective beads) of microbeads increase, better initial obtaining slightly better RL values in the G3 sample group. RL values are obtained. On the other hand, largest microbeads are detached from the painting easier As the size and quality (less impurities and defective beads) of microbeads increase, better initial and faster than the smaller ones, because the first ones are poorly embedded in the base material, RL valueslosing areretroreflection obtained. Oncapacity the other over hand,time. Moreover, largest microbeads microcraters are left detached by missing from beads the are painting posteriorly easier andfilled faster by than dirt, the dust smaller, and ones, rubber because from thetyres, first causing ones are also poorly extra embeddedluminance inloss. the Microbeads base material,’ surface losing retroreflectioncoating gives capacity them extra over time. adhesion Moreover, to remain microcraters embedded left in by the missing base material, beads are getting posteriorly really good filled by dirt,results, dust, even and in rubber the largest from diameters. tyres, causing The alsobest extraRW values luminance are obtained loss. Microbeads’ with the largest surface microbead coating givessizes, them because extra adhesiontheir upper to part remain remains embedded above the in thewater base and material, continues getting providing really some good retroreflection results, even capacity.

Coatings 2018, 8, 371 16 of 17

in the largest diameters. The best RW values are obtained with the largest microbead sizes, because their upper part remains above the water and continues providing some retroreflection capacity. Bilayer road markings (G4 samples) results follow the trend of their equivalent monolayer sample. It should be pointed out that what we achieve with this application system is to decrease the wearing of road markings from the first month and to keep constant retroreflectivity values from the sixth month to the end of the testing period, even recovering this feature from the sixth to the twelfth month of service because of the emergence of the bottom layer after the wearing of the upper layer. The relation between skid resistance is opposite to daytime and nighttime visibility: road markings having better SRT results (G2 sample group) obtain lower values of luminance and retroreflectivity. That is because rougher mixtures capture a greater amount of dirt and rubber particles, darkening the markings surface.

Author Contributions: Conceptualization, A.C.-C., J.A.C.-G. and S.I.; Methodology, A.C.-C and S.I.; Validation, L.B. and S.I.; Formal Analysis, A.C.-C. and L.B.; Investigation, A.C.-C. and J.A.C.-G.; Resources, J.A.C.-G.; Writing-Original Draft Preparation, A.C.-C. and L.B.; Writing-Review & Editing, L.B. and S.I.; Visualization, L.B.; Supervision, S.I. and L.B.; Project Administration, S.I. Funding: This research received no external funding. Acknowledgments: The authors gratefully acknowledge the Conselleria de Vivienda, Obras Públicas y Vertebración del Territorio from the Valencian Government for their collaboration to develop this study. They also want to acknowledge the Ministerio de Economía y Competitividad from the Spanish Government for funding the PhD grant for Andrés Coves-Campos. Conflicts of Interest: The authors declare no conflict of interest.

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