Section 4: Pavement Marking Material Descriptions
Anchor: #i1020819Introduction
A majority of the pavement markings placed on TxDOT roadways over the past five years fall into one of three categories: thermoplastic, water-based paint, and preformed tape. However, other materials exist that have shown positive performance either in Texas or elsewhere, which warrants their discussion in this handbook.
Each material contains three primary components: binder (glue), surface glass beads (reflectors), and pigment (color). For thermoplastic markings, glass beads are also intermixed into the material and often become exposed as the binder material is worn down by traffic. The various pavement marking materials are often categorized by the type of binder material used.
Table 2-4 shows materials and their uses described in this section.
|
Material |
Brief Usage Note |
Special Approval Required* |
|---|---|---|
|
thermoplastic |
See DMS-8220 |
- |
|
water-based paint |
See DMS-8200 |
- |
|
preformed tapes |
See DMS-8240 |
- |
|
epoxy |
experimental use in Texas |
Yes |
|
polyurea |
experimental use in Texas |
Yes |
|
modified urethane |
experimental use in other states |
Yes |
|
methyl methacrylate (MMA) |
extensive use in other states |
Yes |
|
profiled thermoplastic |
experimental use in Texas |
- |
|
contrast markings |
experimental use in Texas |
- |
|
heated-in-place thermoplastic |
transverse lines, words, and symbols only |
- |
|
ceramic buttons |
previously used extensively in Texas |
- |
|
* Materials designated here as experimental require special approval from TRF or CST-MAT for use. |
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This section includes in-depth descriptions of commercially available materials and typical uses. Information on pavement marking material selection based on characteristics of a given roadway can be found in Section 7 of this chapter. Although every attempt was made to include descriptions of all commercially available materials, new materials and formulations frequently become available. Therefore, materials that are not described herein may be used with approval from TRF or CST-MAT on an experimental basis until substantial performance data is obtained. Use of experimental materials may be considered for problem areas where other typical materials may not have provided the desired performance.
Anchor: #i1020845Summary of Material Use
Each of the materials, with the exception of heated-in-place thermoplastic, may be used as either a transverse (shortline) or longitudinal (longline) application, although performance in either case may vary. Further details on the performance of these materials are provided later in this chapter. Ceramic buttons are currently not an approved marking material due to the fact that they are not a retroreflective material and therefore provide poor nighttime visibility. Table 2-5 provides a summary comparison of the characteristics of each material described in this section.
|
- |
Use Based on Pavement Surface |
- |
- |
- |
||
|---|---|---|---|---|---|---|
|
Material |
Conc. |
Asp. |
Seal. |
Lane Closure Required |
TxDOT Specifications |
See Table # |
|
Thermoplastic |
L |
Y |
Y |
No |
Yes |
2-6, 2-7, 2-8 |
|
Water-Based Paint |
Y* |
Y* |
Y* |
No |
Yes |
2-9, 2-10 |
|
Preformed Tape |
Y |
Y |
N |
Yes |
Yes |
2-11 |
|
Epoxy |
Y |
Y |
L |
Yes |
Yes** |
2-12, 2-13 |
|
Polyurea |
Y |
Y |
L |
Yes |
Yes** |
2-14 |
|
Modified Urethane |
L |
L |
L |
Yes |
Yes** |
2-15 |
|
Methyl Methacrylate |
L |
L |
L |
Yes |
No |
2-17 |
|
Profiled Thermoplastic |
Y |
Y |
N |
No |
Yes |
- |
|
Contrast Markings |
Y |
Y |
L |
No |
No |
- |
|
Heated-in-Place Thermoplastic (not for use in longlines) |
Y |
Y |
Y |
Yes |
Yes |
- |
|
Ceramic Buttons |
L |
N |
N |
Yes |
No |
2-15 |
|
Y = Suitable for use. N = Not recommended. L = Limited use. * = Refer to Table 2-10 for traffic volume conditions. ** = Refer to Multipolymer Specification SS 1513. |
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Anchor: #i1020854
Supplemental Material Performance Information
The National Transportation Product Evaluation Program (NTPEP) performs performance evaluations on a number of different pavement marking materials on various pavement surface types and under various traffic volume levels and environmental conditions. Reports documenting these evaluations are available through NTPEP or the American Association of State Highway and Transportation Officials (AASHTO). TxDOT also maintains similar field test decks used for performance comparison of various marking materials.
Anchor: #i1020864Thermoplastic Markings
Thermoplastics have been used as a pavement marking material in the United States since the late 1950s and have been the most common pavement marking material used on roadways in Texas for years. Thermoplastic is so named because the mixture of plasticizer and resins that serves to hold all of the other ingredients together exists as a solid at room temperature, but becomes liquid when heated. The popularity of thermoplastic markings can be attributed to several factors including:
- readiness for immediate use
- high durability
- good retroreflectivity
- relatively low cost.
When properly formulated for a given roadway surface and correctly applied, thermoplastic pavement markings have been known to last from 5 to 8 years depending on traffic volumes, but research has shown that usual service lives range from 2 to 3 years depending on traffic volumes.
Thermoplastic materials are very sensitive to the variables governing application, warranting strict quality control during application. The following are the key variables that influence the durability and retroreflectivity performance of thermoplastic markings:
- material composition
- application procedure
- roadway surface
- traffic
- environment.
If applied properly, thermoplastic materials provide durability and retroreflective performance that far surpass that of standard traffic paints. However, because thermoplastic materials are very sensitive to the variables involved with application, they may not be the most suitable material for certain situations. The following segments describe the types, application, performance, and suggested uses of thermoplastic materials.
Anchor: #i1020934Thermoplastic Marking Material Characteristics
Thermoplastic materials are classified by TxDOT as a Type I pavement marking material, with material specifications falling under DMS-8220 and tested using Test Method Tex-863-B. Thermoplastic materials consist of four general components: binder, pigment, glass beads, and filler material (usually calcium carbonate, sand, or both). Thermoplastic materials are classified into two basic categories based on the type of binder: hydrocarbon and alkyd. Hydrocarbon thermoplastic is made from petroleum-derived resins, while alkyd thermoplastics are made from wood-derived resins. Alkyd thermoplastics are currently the standard thermoplastic allowed for use by TxDOT as a pavement marking material under DMS 8220. Hydrocarbon thermoplastics are currently only allowed for use by TxDOT through a special provision to DMS 8220. A side-by-side comparison of the two types of thermoplastic materials is shown in Table 2-6.
|
- |
Type of Thermoplastic |
|
|---|---|---|
|
Characteristic |
Hydrocarbon |
Alkyd |
|
Binder Source |
Petroleum |
Wood |
|
Application Temperature |
Approximately 420° |
Approximately 420° |
|
Oil Soluble? |
Yes |
No |
|
Heat Stability |
More |
Less |
|
Sensitivity to Changes in Application Properties |
Better Suited |
Not As Well Suited |
|
Durability |
Less |
More |
|
Expected Life Under Normal Conditions |
Up to 5 years |
Up to 5 years |
The ability for thermoplastic materials to bond to the roadway surface is based on the thermal properties of the thermoplastic binder and the roadway surface along with the porosity of the surface. Thermoplastic is well suited for use on asphalt surfaces because the thermoplastic develops a thermal bond with the asphalt via heat fusion. When applied to hydraulic cement concrete surfaces, bond formation occurs by the liquid thermoplastic seeping into the pores of the concrete and forming a mechanical lock to the concrete surface. Primers are recommended prior to thermoplastic application on all hydraulic cement concrete surfaces and asphalt surfaces that are more than two years old, heavily oxidized, or have exposed aggregates.
Anchor: #i1020951Thermoplastic Marking Application Methods
Most of the thermoplastic longlines placed on TxDOT roadways are applied by spraying the hot thermoplastic onto the surface. Therefore, sprayed applications are the major focus of the information in this segment. However, it is worth noting that there are other methods of applying thermoplastic markings to the roadway surface, such as gravity extrusion and ribbon application.
Gravity Extrusion. Gravity extrusion was the application method of choice for many years, and is well suited for application of thicker markings. Extrusion occurs by pouring hot thermoplastic into a trough or shoe, which has a gate set to produce a desired thickness. However, extrusion is a relatively slow process (3 mph).
Ribbon Application. Ribbon application uses a pressurized gun to apply thermoplastic in the same manner as the gravity extrusion method.
Hot-Sprayed. Hot-sprayed thermoplastic is the most common application method used for striping on TxDOT roadways and provides many advantages over other application types. The main difference between sprayed thermoplastic and the other application methods is that the hot thermoplastic is combined with pressurized air in spray applications. The primary advantage to sprayed application is that striping can occur at higher speeds (2–8 mph) and markings are ready for traffic in one minute. Sprayed applications often produce a better surface bond than extrusion and ribbon applications. However, sprayed thermoplastic markings are not as well suited for applying markings thicker than 100 mils when compared to extrusion and ribbon applications. Figure 2-10 shows a typical sprayed thermoplastic application.
Figure 2-10. Typical sprayed thermoplastic operation.
Anchor: #i1020987Thermoplastic Marking — Surface Preparation
The temperature of the thermoplastic material is a very important factor in the performance of the thermoplastic-roadway surface bond. Suitable application temperatures range from 400–450°F, with 420°F as the recommended temperature for most applications. For proper bonding, the following conditions must exist:
- The pavement surface must be free of dirt, dust, and other contaminants.
- The pavement surface must be free of poorly adhered existing markings, glass beads, and curing compound.
- The pavement surface must be free of moisture.
- Pavement and air temperatures must be at least 50°F and 55°F, respectively, to ensure proper rate of cooling.
It must be emphasized that a clean and dry pavement surface is critical for thermoplastic materials to achieve proper bonding with the pavement surface. If primers are used, they must be given proper cure time (until tacky) prior to applying the marking material. Special pavement heating devices are now available that are effective for removing pavement surface moisture and elevating the pavement surface temperature, allowing for better thermoplastic bonding.
Table 2-7 describes problems and solutions associated with the application of thermoplastic markings.
|
Trips |
Tips |
|---|---|
|
Debonding |
|
|
Bubbles in line |
|
|
Flowing Line (i.e., no defined edge), line with excessive rounded edges |
|
|
Cracks in line |
|
|
Rough line surface or crumbly edges |
|
|
Smooth, shiny, glossy line |
|
|
Smooth line with slight dimples |
|
|
Cratered line |
|
|
Greenish yellow appearance |
|
|
Splattering |
|
|
Dingy or dull white color |
|
|
Lumps in line |
|
Anchor: #i1021026
Thermoplastic Marking — Effect of Pavement Surface on Performance
Because of the thermal bonding characteristics between thermoplastic and asphalt, nearly all thermoplastic materials are well suited for application on new HMAC surfaces. This includes both hydrocarbon and alkyd thermoplastics. As HMAC surfaces wear and become more brittle through traffic exposure and oxidization, the use of a primer is suggested when applying thermoplastic directly to the pavement surface. Suitable minimum thickness of thermoplastic on new HMAC surfaces is 90 mils. Suitable restripe thickness on HMAC surfaces is 60 mils.
Not all thermoplastic materials have been shown to provide suitable durability on concrete surfaces. In other words, thermoplastics that are suitable on asphalt surfaces may not be suitable for concrete surfaces. Because the thermal bond is not available between thermoplastic materials and concrete surfaces, mechanical bonding is relied upon. For mechanical bonding to occur, the concrete surface must be porous enough to allow the liquid thermoplastic to seep into the pores and create a tight mechanical bond after cooling. This is often not the case with new concrete pavements, suggesting the use of primers. A major thermoplastic bonding failure on concrete is shown in Figure 2-11.
Figure 2-11. Major thermoplastic failure on concrete.
Certain thermoplastic products have been identified that provide suitable bonding with concrete either with or without a primer. Suitable minimum thickness of thermoplastic on new HCC surfaces is 90 mils. Suitable restripe thickness on HCC surfaces is 60 mils.
When thermoplastic markings are applied to coarse surfaces such as surface treatments, thickness plays a major role in the durability and retroreflective performance over time. As with all asphalt surfaces, suitable thermal bonding is achieved. However, much of the thermoplastic material seeps into the voids between the aggregates, leaving very little material on the top of the aggregates. This lack of material at the top of the aggregate leads to accelerated wear of the thermoplastic and premature bead loss. Research has shown that thermoplastic thicknesses of 100 mils or greater provide better performance than thinner applications on surface treatments. In most cases, a restripe thickness of 60 mils is suitable on surface treatments, although a greater thickness may sometimes be necessary.
Anchor: #i1021061Thermoplastic Marking — Effect of Traffic on Performance
Durable pavement markings, such as thermoplastic, are desirable on high-traffic-volume roadways because they require fewer restripe cycles, reducing the amount of worker exposure and delay to traffic. However, on very low-volume roads, paint may provide comparable performance to durable materials at a much lower cost.
Thermoplastic materials provide suitable performance for a broad range of traffic volumes. Because thermoplastic dries very quickly, it is well suited for higher traffic volume roadways. However, thermoplastics may not be the most durable marking in areas where very high-traffic volumes or heavy weaving result in increased tire wear. Other two-component materials or permanent tapes may provide better durability under extreme traffic conditions.
Anchor: #i1021076Thermoplastic Marking — Environmental Effects on Performance
One drawback to thermoplastic materials is that they are sensitive to environmental changes — especially the alkyd materials used extensively throughout Texas. Many sources state that surface moisture and air temperature are the two most important factors that influence the long-term durability of thermoplastic markings. Thermoplastics are especially susceptible to moisture-associated bonding failures. Therefore, thermoplastic materials may not be the best material for pavement markings in locations that are very humid or susceptible to dew formation during times that would affect striping operations, such as striping at night. Thermoplastics should also not be used unless the pavement temperature is 50°F and the air temperature is 55°F. Thermoplastics perform better than standard traffic paints in areas where snowplowing is common but still suffer some loss in durability depending on the number of snowplow passes.
Anchor: #i1021086Thermoplastic Marking Use Summary
Table 2-8 summarizes the recommended uses of thermoplastic pavement markings.
|
- |
Asphalt |
Concrete |
Surface treatments |
||||||
|---|---|---|---|---|---|---|---|---|---|
|
- |
AADT <1,000 |
AADT 1,000 –10,000 |
AADT >10,000 |
AADT <10,000 |
AADT 10,000 –50,000 |
AADT >50,000 |
AADT <1,000 |
AADT 1,000 –10,000 |
AADT >10,000 |
|
Use2 |
Y |
Y |
Y |
L |
L |
N |
Y |
Y |
Y |
|
Material suggestions |
TxDOT standard |
specific concrete formulation |
- |
TxDOT standard |
|||||
|
Typical minimum thickness (new) |
90 mils |
90 mils |
90 mils |
- |
100 mils |
||||
|
Typical minimum thickness (restripe) |
60 mils |
60 mils |
60 mils |
- |
60 mils |
||||
|
Surface prep. |
Clean & dry. |
Clean, dry, & primer-sealer (refer to Item 678 or manufacturer recommendations). |
- |
Clean, dry, & remove loose stones. |
|||||
|
Expected Service Life |
up to 4 years |
up to 4 years |
up to 3 years |
up to 4 years |
up to 4 years |
- |
up to 4 years |
up to 4 years |
up to 3 years |
|
Approx. bid price for new surface in 2002 (per lf) |
$0.20 |
$0.35 |
- |
$0.20 |
|||||
|
Estimated cost per year of service life (per lf) |
$0.05 |
$0.05 |
$0.07 |
$0.07 |
$0.09 |
- |
$0.05 |
$0.05 |
$0.07 |
|
Footnotes: 1. TxDOT Specification Thermoplastic unless noted otherwise. 2. Y = suitable for use; N = not recommended; L = limited use. |
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Anchor: #i1021095
Water-Based Paint
Traffic paints are the oldest and most widely used pavement marking materials in existence. Paint is a common pavement marking material used by TxDOT, although its use has recently declined as thermoplastic has become more popular. Paint still remains the most inexpensive of all pavement marking materials, although its cost has increased slightly as new formulations have been introduced. Paint is almost exclusively used for longline applications, often in maintenance projects, and is currently the only longline material applied by state forces.
Anchor: #i1021105Water-Based Paint — Material Characteristics
Paints are classified by TxDOT as Type II pavement markings, with material specifications falling under DMS-8200, WPT-12, and YPT-12 for white and yellow, respectively. The current DMS-8200 specification specifically calls for Rohm-Haas Fast Track HD-21A or Dow DT-400 acrylic emulsion resins. Check with TxDOT materials laboratory for approved formulations, as they are subject to change.
Conventional traffic paint consists of three interwoven elements: pigment, binder, and glass beads. Each element is important because different paints react differently to different pavement surfaces, traffic wear, and environmental wear. Binder materials (or resins) are usually latex or acrylic materials that provide the adhesive and cohesive properties of the material.
A myriad of traffic paint formulations exists, including solvent-based and water-based. Increased environmental awareness in the late 1980s and early 1990s led to a decline in use of volatile organic compounds (VOCs) in the United States. This awareness also led to a decline in the use of solvent-based paints and an increase in the use of water-based paints, which contain far fewer VOCs. TxDOT has not allowed solvent-based paint for many years, relying exclusively on water-based formulations. In addition, most state DOTs have disallowed solvent-based paints.
Water-based paints are environmentally friendly, are much easier to handle than solvent-based paints, and greatly decrease the safety hazards to workers. Water-based paints also become track-free much quicker than solvent-based paints. Humidity has very little effect on the track-free times of water-based paints because these paints begin to set as a result of the drop in pH due to evaporation of the ammonia.
Anchor: #i1021133Water-Based Paint Application
Most of the water-based paint placed for longline applications is applied by spraying the paint onto the surface using a striping truck. Paint adheres to the pavement surface through mechanical bonding within the pores of the pavement surface. Although paints are less sensitive to changes in temperature, moisture, or other environmental characteristics than thermoplastic materials, surface preparation is still important. To achieve proper bonding, the following conditions must exist: <