Coating systems based on polymers

Coatings produce a continuous protective layer on the surface of concrete with a certain specified thickness. They are applied in order to rule out the ingress of harmful substances, to increase the mechanical resistance of concrete, and to bridge cracks—moving and non-moving. Typical thicknesses of coatings are between 0.1 and 5.0  mm, which might be enlarged due to special boundary conditions.

Depending on the intended use, coatings can be made, e.g., out of epoxy resins, polyure-thanes, acrylates, polymer dispersions, or cement- polymer compounds. They might also contain aggregates, which are usually quartz sand with a grain size below 1 mm.

Coatings usually consists of more than one layer in order to meet the requirements given by EN 1504-5. Each layer features different functions, which are as follows:

• Impregnation (see above)

• Levelling layer

• Main protective layer

• Wear- out layer

• Top coat

Figure 5.18 Water drops on top of a concrete surface treated with hydrophobic agent.

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This list of single layers is not defined in EN 1504-2. Exemplarily in the following para-graphs typical coating systems based on the German guideline for repair and rehabilitation of concrete (RL- SIB 2001) are described. The experience with surface protection systems has led to the knowledge that most of the requirements can only be achieved with more than one layer. So the following paragraphs briefly describe the function of each possible layer of a surface protection system.

Especially when the concrete surface is uneven and rough, the so- called levelling layer is applied. This layer has the following tasks:

• Levelling of the concrete surface

• Closing of pores, voids, and small defects of the concrete surface

This levelling layer is required because otherwise the next layers that represent the protec-tive layers cannot be applied in a consistent thickness. The levelling layer is usually made out of polymer- modified cement mortar with a maximum grain size of 0.5  mm, and in exceptional cases 1 mm. The maximum thickness of the entire layer is about 3 mm. After applying the polymer- modified mortar on the surface, the surface is levelled so that peaks are not covered and the valleys of the surface are filled with mortar.

The main protective layer has one or more than one of the following features:

• Impermeable to water or harmful substances

• Impermeable to CO₂

• Crack bridging

• Resistivity to mechanical impact (abrasion or stresses caused by temperature changes)

• Resistivity to chemical attack

Analogous to the crack injection materials, surface protection systems can be classified into one of the following group:

Figure 5.19 Application of a surface protection system by hand.

• Sealing surface protection system

• Crack bridging surface protection system for dynamic crack openings

• Wear- out layers

Depending on the main feature, the materials that are used for the specific layers are selected. Generally for surface protections systems that should only seal a surface, stiff poly-mer resins, such as epoxy resins, are used to realise the layer. The thickness of such layers depends on the exposure of the layer. On vertical surfaces the layer thickness is approximately 0.3 mm; horizontal surfaces exposed to traffic require thicknesses around 5 mm. Due to the material properties, these layers not only seal the surface, but also work as a wear- out layer.

In order to ensure a sufficient roughness, the layer is gritted with quartz sand or corundum, which has a higher resistance against abrasion than quartz sand (see Figure 5.20).

Figure 5.20 Application of sand to the freshly applied surface protection system.

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If the surface protection system has to be crack bridging, flexible polymers such as poly-urethanes are used. Additional reinforcement layers made out of glass fibre textiles can be added to increase the resistivity of the crack bridging layer. The thickness of a main protec-tive layer is usually between 1 and 4 mm. If these surface protection systems are exposed to traffic, the wear- out layer is also made out of polyurethane, but the material is not as flexible as the crack bridging layer. Also, these layers are highly filled with sand in order to ensure a sufficient durability.

In order to prevent the wear- out of the sand, a top coat has to be applied to the surface protection system. A top coat is usually made out of the same polymer as the protective layer and can be filled with pigments in order to meet optical requirements.

The layer thickness of a surface protection system represents one of the main material properties, which has to be specified by the manufacturer based on the requirements defined by EN  1504-2. The specified layer thickness is considered to be a minimum value that should not be fallen below to fulfil all necessary requirements selected by the designer.

The minimum layer thickness is usually specified as the thickness of the layer after dry-ing and not durdry-ing application. Due to the chemical hardendry-ing processes of the different polymers, the layer during the application is usually significantly thicker than the resulting layer after hardening. Thus, the technical data sheet of the manufacturer includes advice on how to calculate the resulting layer thickness based on the consumption during application.

Figures 5.21 to 5.27 show different types of surface protection systems according to the German guideline RL- SIB (RL- SIB 2001). The pictures show the real system as well as the schematic drawing of the entire system. The first system (OS-1) is a hydrophobic treatment, and the last one (OS-11) is a crack bridging surface protection system for parking decks. It can be seen that the increase of requirements leads to a significant increase of complexity and thickness of the entire surface protection system.