Quality problems and cause analysis for thermoplastic road marking on concrete

Part-One, Major problems that may cause

After concrete pavement marking construction, there will be a variety of quality issues. These problems not only affect the effect of the marking, but also affect the life of the markings, therefore these problems should be given high priority. Many factors may cause marking quality problems, not only including marking paint itself quality reasons ( including the quality of raw materials and coating formulations ), but also there are reasons of pavement condition, construction steps and process, as well as environmental, construction weather and construction machinery and workers and so on. After our investigation and detection analysis of cement pavement markings, quality problems could be classified as the following five categories: bubble (micro holes), cracking off, reflective effect, color and appearance shape.

1. Bubble problems

Bubble problems include bubbles and pores. Bubble is formed in the case of the thermal expansion material vaporization from the top coating under the high temperature of thermoplastic marking material, such as the instantaneous vaporization of the water and the bottom some of the low-boiling components of primer coating and so on. Some subsequently break through the coating to form pores. Such problems not only affect the appearance of the marking, but also reduce the actual combined area of the coating with the road, and seriously affect paint adhesion and service life, but also affect the reflective effects.

There are following main reasons produce bubbles:

a. Paint quality: If material contains a lot of impurities inside, such as inferior quality petroleum resin contains air or other impurities, it will inevitably lead to the generation of bubbles.

b. Not waiting primer dry and construction directly. Primer contains some the low-boiling components, which is very easy to vaporized to form bubbles when suddenly contact high temperature.

c. Particles on the ground. Construction ground surface is not clean up, too much particles object on the ground will lead to increase in air gap, and it will cause blowholes.

d. Air moisture in road surface.  Humid air, road surface humidity is too large, the vaporization of water in case of high temperature of road marking paint forms bubbles.

e. Construction in winter. Winter construction is more likely to produce bubbles, lower surface temperature in winter, road surface humidity is too large, the bottom primer is not easy to dry, and larger fog is also a factor.

f. Road surface itself reasons. The deeper pavement sculptured, sculptured trench more easily hidden moisture, primer is also difficult to dry, more easily produce bubbles

2. Cracking off class

Cracking off means quality problems of surface cracking, even block off. Such problems will affect the appearance of the markings, incomplete marking off serious loses marking. The reason is mainly the following five.

a. Paint reasons. Resin performance is not good, or brittle formulation or formulation does not match with the construction season, all of them are likely to lead paint itself is more brittle and cohesive force with the road, and thus prone to fall off naturally or by the wheel in driving the process rolling or vibration shedding.

b. Primer reasons. To improve the adhesive force between the road surface and the coating, road surface need to be coated primer. The viscosity of primer is the key factor which affect marking falling off or not. Applying excessive, or too little or lose the viscous because of too long time, all of them will reduce the adhesive force between the road surface and coating.

c. The road surface cleanliness reasons. Road is not clean up or the road surface treatment is not good before the construction, it will inevitably lead coating fall off easily.

d. Low temperature reasons. Winter construction , coating contract when cooled , resulting in stress, the stress concentration is easy to crack , and the construction temperature is too low , the viscosity rapidly thickens the paint after contact with the road , which affects the adhesion of the coating and pavement fastness , thus affecting paint adhesive force between the road surface. Therefore, winter construction in general should not be lower than 5 ℃.

e. Road surface quality problem, such as off quality concrete

3. Reflective Effect

The reflective effect of traffic lines is achieved by pre-mixed glass beads and drop-on glass beads. When the light is directed to traffic lines, it refracts at the pre-mixed glass beads and reflexes to the direction of light source , thereby improving the legibility of pavement traffic lines. The reflective effect is determined by inverse reflectance values measured by inverse reflectometry. National standard: white≥150 mcd•lx-1•m-2, yellow≥100 mcd•lx-1•m-2. There are four factors affecting the reflective effect of traffic lines.

• a. The quality of glass beads. Glass beads play a central role in the reflective effect, and retroreflectivity is the feature of colorless and transparent spherical glass beads, so glass beads for road marking shall have high refractivity, high roundness, fewer impurities, large particle size, uniform particles, not excessive oval, acute-angle, opaque glass beads and glass powder.
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• b. The quality of paint. The quality of paint includes the quality of raw material and formula, such as resin quality, content of glass beads, content of titanium dioxide in white paint and so on, which may affect the reflective effect of traffic lines. Construction temperature. It proves that the reflective effect is the best when half of glass beads are embedded into the paint; if the temperature is too high, much glass beads sink, if too low, few glass beads sink and fall off easily. Meanwhile, it is not better to spray too much drop-on glass beads; if too much, they are not so adhesive to pavement and fall off easily, and also affect the reflective effect due to overlapping each other. Therefore, glass beads shall be sprayed as evenly as possible.
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• c. The reduction of reflective values. It proves that immediate reflective effect is quite good, but reduces very fast. As Chart 1 shows, when newly marked lines are tested within 10 min, the surface temperature doesn’t cool down, paint performance is not stable, so reflective value is relatively low as 10~20; after 6 h, reflective value reaches the top, but it begins to reduce quickly because of wind and sun, running tyres, surface contamination,etc; 3 months later, the inverse reflectance value reduces from about 150 to 80; half a year later, drop-on glass beads almost fall off completely and only pre-mixed glass beads can be slightly reflective. Reflectivity lasts not long, which is a universal problem and more effective ways are not found yet.

4. Color

For national and provincial traffic lines, center lines are yellow while edge lines are white, and center lines can be changed to orange yellow, orange red or others. If yellow center lines are not so satisfactory, one way is to consider adjusting formula, the other is to note the heating temperature to prevent yellow paint from changing color caused by over heating, because yellow paint has poor temperature resistance. White lines changing to yellow is a greater problem and is  not allowed because the legibility is seriously affected. Generally speaking, there are three main reasons:

a. Unqualified resin. Our thermoplastic paint is made of petroleum resin which has four catagories: C5(Aliphatic class), C9 (Aromatic hydrocarbons), DCPD (Cycloaliphatic diene) and pure monomer. Resin works as binder of paint, and C5 resin is the most suitable for thermoplastic road marking. If the resin is not pure (e.g. C9 or other impurities are mixed), the quality can not be guaranteed.

b. Low content of titanium dioxide. Titanium dioxide is used as a white pigment of road marking paint and has the strongest hiding power among all the white pigments. If the content is too low, white color may not be guaranteed.

c. Color change caused by high heating temperature or being heated too long. Resin material tends to worsen when heating temperature reaches about 200 degrees. If heating temperature is too high or heating too long, thermal degradation may occur which will reduce paint performance and cause paint to change color.

5. Shape problem.

Shape problem includes overall linetypes, partial deformation, surface streaks, etc. Overall linetypes mainly have relationship with skills of manipulators and leveling degrees of pavements. If the manipulators are not adept at operating road marking machine or road surfaces are rugged with some plates being cataclastic or malposed, the marking lines will surely look distorted. Partial deformation means some partial marking lines bulge to the side, and it is mainly caused by uneven pavements. When some pavements suddenly sink, There will be gap between the diamond side baffle of road marking machine and pavement, thus road making paint will flow from the gap, and external bulges will take shape. Surface longitudinal streaks are caused by the drag marks of hard granules. This may be caking residuals of road marking paints that have been burnt partially and excessively or some foreign matters mixed into the paints, such as unclean pavements, screes blew from passing cars, etc, which are probable causes. Transverse streaks are mainly due to the vibration of paint bucket tanks, too fast speeds or uneven pavements. Now cement pavements are required to be skid-resisting and their cutting grains are comparatively deep generally, so the making lines are required not to be too thin. Generally speaking, they should be slightly thicker than what is required by the national standard, otherwise it will be very easy to have transverse stripes or leakage grooves take shape.