To attain satisfactory results with a coating, it is not enough with a technically perfect product. Surface preparations, surface temperatures, moisture conditions and concrete strength are factors that are of very great importance. All contaminations on a surface have negative effects on adhesion. The surface that is to be coated must be strong enough to hold a coating in place. Furthermore, it can be disastrous to apply a diffusion-tight coating on a concrete substrate in which moisture wanders towards the surface. The temperature must naturally be high enough so that hardening takes place within a reasonable time.
Substrate’s surface strength
Concrete and similar cement-based products often constitute the substrate for coatings of epoxy plastic. The strength of these primarily depends on the composition, but the application and the conditions during the application can have a major effect on the upper layer’s tensile and compression strength. One must keep in mind that concrete and epoxy plastic have different properties in regards to, for example, coefficients of thermal expansion and modulus of elasticity. When heating or cooling a floor, the epoxy coating moves significantly more than the concrete. Stress occurs in the boundary layer. If the substrate does not have sufficiently high surface tensile strength, the stress can cause the coating to detach. Practical tests have shown that a concrete or cement-based substrate needs a surface tensile strength of at least 1.5 MPa (about 15 kp/cm²) to be able to retain a coating. A normal floor concrete holds about 3 MPa.
Cement skin, also called laitance layer, has a tensile strength down to 0.1 MPa, and must always be removed prior to coating. This is best accomplished with steel-ball blasting or grinding.
One can even use diluted hydrochloric acid (10–20%) to remove the cement skin. The method is quick but entails certain disadvantages; machinery and iron objects in the area rust quickly in air containing hydrochloric acid. Furthermore, the salts that form during the hydrochloric acid’s influence on the concrete must be removed; this is done with high-pressure water rinsing and brushing. One must check that the concrete surface after rinsing has a neutral or weakly alkali reaction; test with pH paper. If there are cracks in the concrete, the hydrochloric acid can force its way down and cause rust formation on the reinforcement. Another disadvantage is that one thoroughly wets the surface and must therefore wait with coating until the surface is dry.
The tensile strength can only be determined through onsite measurement. The method used is called the pull-off method, and involves measuring the force that is expended to pull off glued dollies.
A suitable instrument is an Elcometer 106 with the 0–3.5 MPa scale.
The procedure is as follows:
So that the dolly is not glued askewed in relation to the instrument’s pulling angle, a NM Centring unit shall be used. The centring unit consists of a steel ring and a rectangular plexiglass sheet with a track.
Place the steel ring on the cleaned concrete surface. Place a dolly in the track on the plexiglass sheet. Place the sheet on the ring so that the sides align with the ring’s periphery.
The dolly shall be possible to move back and forth in the track without touching the concrete surface.
Lift the sheet with the dolly. Blow the concrete free from dust. Apply glue to the dolly. Make sure that the dolly is pressed as far as possible into the track. Place the sheet on the ring so that the edges align with the ring’s periphery. Push the dolly so that excess glue is pressed out. The dolly now rests on the plexiglass sheet.
When the glue hardens, the sheet is removed. Take away the glue excess that is beyond the text fixture with a hole saw. Slide the Elcometer’s grab unit onto the dolly and adjust the ring so that the outer part of the Elcomenter’s pads align with the ring’s periphery. Place the scale’s release indicator at zero.
Hold the instrument in place with one hand and softly turn the knob. Read the value when the release indicator stops.
A test should always encompass at least five dolly’s.
Note that other instruments and other measuring methods can result in deviating values.
Substrate’s degree of cleanliness
All contaminants on a surface negatively affect adhesion. Commonly occurring contaminants are dust, oil, grease and chemicals. Dust on the surface, for example, constitutes a block for the epoxy plastic in makings its way down into the concrete pores where it can anchor itself. It must be removed and this is done through powerful vacuum; sweeping the surface with a broom is insufficient.
Grindings from concrete grinding can be difficult to remove by vacuum. Water rinsing can be necessary to clean the surface.
If the concrete is contaminated with machine oil, grease or similar substance, it can be removed with an emulsifying agent. Thinner and ligroin, however, may not be used because these agents only dissolve the oil and spread it over a larger surface or deeper down into the concrete. After emulsification, the surface shall be rinsed with water.
If oil has forced its way deep into the concrete, it can be necessary to mill the surface layer. Animal and vegetable oils can be removed by washing with 10% caustic soda; the oil is saponified and becomes water-soluble.
Metal surfaces that shall be coated must be free from oil, grease, dust, rust, oxide scaling, etc. The best way to clean metals is by sandblasting. If the surface is oily, it can be necessary to wash with solvent, such as thinner or toluole prior to sand-blasting. A freshly blasted surface is very quickly attacked by humidity and oxygen; the coating shall therefore be performed as soon as possible after sandblasting is completed. If sandblasting cannot be performed, one must grind the surfaces with a grinding disk; steel brushing should be avoided.
Stainless steel requires special primer.
Concrete surface’s degree of moisture
After construction, concrete contains a surplus of water. The amount of the surplus depends on the concrete quality. It can generally be said that it takes a very long time for surplus water, which is also referred to as construction water, to dry out of a concrete construction. The concrete is considered dry when its relative humidity is equal to the surrounding air’s relative humidity. The drying time can in some cases be one year.
In wet concrete, the water rises capillary through the concrete’s pore system, up towards the surface.
This process does not cease when one applies a diffusion barrier. Because it takes a relatively long time for the epoxy plastic to harden, the water will concentrate in the concrete-epoxy boundary layer. Theoretically, it is enough with a water film of 5 ångstrom (5·10-7mm or 0.5 millionths of a millimetre) thick for adhesion to fail. This applies to all diffusion-tight materials that are intended to adhere to a substrate. Concrete that is dry to the moisture balance can be coated with a diffusion-tight coating without problem.
Concrete that has not dried to the moisture balance can only be coated with a material that permits water vapour to pass during the hardening process. Such material is called diffusion-open. The amount of water vapour that can pass during a specific time is dependent on the layer thickness.
By combining a diffusion-open epoxy plastic with a diffusion-tight, it is possible to apply tight coatings to wet substrates.
The concrete’s degree of moisture can be measured with a hygrometer, but it is not always necessary to know the exact measurement value. A covered hygrometer on the concrete substrate provides a quick indication of if water is wandering towards the surface and evaporating. This check is made by placing a bit of polyethene foil on the floor. The hygrometer is placed above the plastic foil and remains there for about 15 minutes to adjust itself to the floor temperature. The hygrometer is now read and the value noted.
The hygrometer is thereafter placed under the plastic foil and the edge sealed with, for example, masking tape. After an hour, the hygrometer is read and the value compared to the first. If the concrete’s moisture is in balance with the air’s, the second value is the same as the first.
Before coating with epoxy plastic, the surface temperature must be checked. Temperatures between +13 and +25°C seldom lead to problems.
For temperatures under +8°C, some form of heating should be applied; under 4°C hardening is so slow that one should avoid treating with epoxy. Aqueous emulsified epoxy generally requires at least +10°C at the substrate in combination with good ventilation.
A factor to include in calculations is that the air in the concrete pores expands as the temperature rises, for example, in sunshine or under substantial heating. This can cause blistering.
It is advantageous to coat the concrete when the temperature is decreasing.
If there is risk that a coating or similar covering with epoxy plastic will be substantially cooled during the hardening process, as is the case with night frost, heat shall be applied. It is namely so that a reaction that stops can be difficult to restart.
Wood and wood fibreboard
Epoxy plastic has very good adhesion to wood, but one must take into consideration that wood is a living material and moves considerably in comparison to other materials. While wood swells when subjected to moisture, epoxy plastic does not, which gives rise to powerful stresses. On wood fibreboard, however, coatings can be applied with good results.
Necessary layer thickness
A floor covering’s layer thickness is primarily determined by two factors:
- Grain maximum, i.e., the largest filler.
- Load size in relation to the substrate.
Normally, the general rule applies that states that the minimum layer thickness shall be three times the grain maximum.
Most epoxy coatings have very high compression strengths in comparison to concrete. Floor concrete usually has a compression strength of 25–35 MPa, while an epoxy coating has a compression strength of 70–100 MPa. By increasing the layer thickness, the load can be distributed over a larger area of concrete. The distribution area’s size is shown in the figure below.
Example of use
A concrete floor of K30 concrete (compression strength 30 MPa) shall be used for truck traffic with a calculated load of 70 MPa. What is the least recommended layer thickness for a suitable epoxy coating?
To attain the distribution area, 70 is divided by 30, which is equal to 2.3. The distribution area should therefore be 2.3 times larger to withstand the heavier load. Find 2.3 on the vertical axis, go out to the curve, go straight down to the horizontal axis and read 2.6 mm.
A 3 mm epoxy coating is sufficient under the condition that the coating has a compression strength that exceeds 70 MPa.
All epoxy products are two- or multiple-component products. As described previously, an epoxy resin must be mixed with a hardener to become a plastic. The reciprocal amounts of resin and hardener are carefully determined. They vary of course from case to case, depending on the type of resin and hardener, but in each individual case, the specified mixing ratios may not be changed.
What is supposed to happen is a chemical reaction between the oxygen in the epoxy resin and the hydrogen in the amines in the hardener. This requires that the hardener be mixed very carefully with the epoxy resin so that all hardener molecules come in contact with the epoxy molecules. They shall hook onto one another to form the plastic’s large molecule.
Schematically, one can draw it like this:
Epoxy molecules’ bonding to a hardener molecule
The circles with two hooks are epoxy molecules and the circle with four rings is a hardener molecule. If not all of the hooks fasten in the rings, there will be no complete epoxy plastic molecule.
The glue, coating or whatever it is, would be weakened, soft patches would arise, and strength and chemical resistance would decline.
If the relation between the epoxy resin and hardener (mixing ratio) is faulty, it does not help with perfect mixing. The excess of one or the other component would always be non-reacted.
This would naturally degrade the epoxy plastic’s physical properties. The importance of using kit-packaged epoxy and hardener cannot be over emphasised. If the components are weighed at the factory with a functioning quality system, the risk for faulty blend proportions as good as non-existent.
Should a kit package be split, despite all, it shall be done with the help of a scale with sufficient accuracy. The mixing ratio, or mixing proportions, indicated in weight units shall be printed on the package. Splitting packages using volume measurements can cause very serious faults.
Small amounts, that is to say up to about 1 kg, can be mixed by hand with a mixing paddle that must be rectangular.
To try to mix the epoxy with a round rod, screwdriver or a piece of reinforcement bar is doomed to failure. Large amounts are impossible to satisfactorily mix by hand, and thus require electrical or pneumatically powered mechanical mixers, for example a low-speed drill with splash-guard and suitable mixing wings (see figure). Epoxy concrete and other masses with low binding agent content can be mixed in a concrete mixer or paddle mixer. Epoxy resin and hardener shall always be carefully mixed before filler is added.
Before mixing the epoxy plastic, it can be necessary to temper the components. This can be done with hot air or in a water bath.
Open flame may not be used.
It is best to store the products in heated indoor areas. At work-sites, there are often various types of epoxy and hardener. Always sort them by type to avoid mistakes. At larger work-sites, set up a mixing station where one person is responsible for mixing; this is the most important part of the work process.
For all painting and coating work, it is important to ensure that moisture does not condense on the substrate while work is under-way. Water in the boundary layer can prevent adhesion between the epoxy and substrate. Condensed water is called dew in everyday language.
Water vapour is soluble in air up to a certain level that depends on the air’s temperature. The warmer the air, the more water vapour it can dissolve.
When the air is saturated with water vapour, one has reached the saturation vapour content.
At + 20°C, for example, the air’s saturation vapour content is 17.3 g / m³. This can also be expressed as the air’s relative humidity (RH) being 100%.
Air that at + 20°C contains 10.4 grams of water per cubic metre thus has a relative humidity of (10.4 / 17.3) x 100 = 60.1%.
When air that contains a certain amount of moisture is cooled, it will eventually reach the saturation vapour content. This temperature is called the dew point.
By measuring the air’s relative humidity using a hygrometer and with the aid of a dew point table, one can establish the dew point for the current air temperature.
In the table, one can see, for example, that the dew point at an air temperature of +20°C and a relative humidity of 60% is +12°C, i.e. if the temperature falls from +20°C to +12°C, dew forms. To ensure that dew does not form on the surface to be coated, it should have a temperature of at least 3°C above the dew point, or +15°C in the above example.
Another phenomenon that can influence adhesion between hardened epoxy and a new layer of epoxy is the epoxy hardener’s capacity to form insoluble amine carbonates and amine carbamates.
This occurs during the hardening process upon the amines’ reaction with the air’s content of carbon dioxide (CO2) and water. Different hardener types have different susceptibilities to carbonation. Pure amines have, for example, greater susceptibility than amine adducts. Low hardening temperature also benefits carbonate formation.
To ensure good adhesion, it is necessary to remove any carbonate film. It can be difficult to determine if an epoxy layer is coated with a carbonate film or not; it is namely not visible to the naked eye.
Sanding or rather flatting is often sufficient. It can often suffice with priming with aqueous emulsified epoxy since the amine carbonate is somewhat water-soluble.
Dew point table
|Dew point in °C at relative humidity of|
Relationship between relative humidity, air temperature and dew point.