Find out how to prepare self levelling compound application to get perfectly even and smooth surface.
ambient temp. +5°C to +25°C
residual moisture below 2% (cement screed)
a trowel on a stick
moisture measuring tool
a spiky roller
Check the ambient and surface temperature
Temperature is one of the factor that has influence on SLC drying and hardening speed. The higher temperature the faster drying proces, the lower temperature the slower drying process. Check temperature and compare with TDS requirements range:
• General purpose SLC – usually +5°C to +25°C for substrate and air temperature.
• Advanced types SLC – above +15°C for substrate and 18°C for air temperature.
• Special cases SLC – e.g. hot climate – up to +45°C for air temperature.
Recommended relative air humidity content is below 75%. The higher humidity content the slower SLC drying proces. Optimal conditions for SLC application and usually taken as reference conditions are: air temperature +23°C and air humidity 50%.
Determine the type and absorbency level of the substrate. Determine substrate condition with a hammer test and scratch test. Inspect the substrate for cracks and joints. In many cases they are caused by lack of joints which lead to uncontrolled stresses caused by thermal or mechanical movements. Substrate ready for SLC must be free of stresses. It also applies to minimum maturing time for mineral substrates e.g. 28 days for cement screeds.
Measurement of residual moisture
Check the residual moisture with relevant for mineral screeds, CM method. The expected moisture value are:
- for cementitious screeds: max. CM 2% for unheated and 1.8% for heated screeds.
- for gypsum screeds: max. CM 0.5% for unheated and 0.3% for heated screeds.
If the values above are exceeded, some dehumidifiers could be used to speed up drying process. Alternatively, use special epoxy primers (Ceresit R 755) if maximum 6% residual moisture is allowed.
Grinding, hoovering and priming
Remove any substances which may impair adhesion. The best solution is to grind and vacuum the surface. Prime the substrate with appropriate product.
Primers need to be completely dry before SLC application, otherwise, the compound will dissolve primer and connection will be weaker. On non-absorbent substrate primer’s layer should be done as thin as possible but as much as homogenous coverage. In case of anticipated heavy mechanical stresses best option is epoxy primer sprinkled with sand to expand bonding surface.
Movement joints are to be done in whole cross section of screed (floating or release layer). They are allowing movement of screed caused by thermal or mechanical loads.Recommended field for cementitious screed with movement joint is max. 6 m x 6 m and as square in shape as possible. The ratio of the length to the width of the field should not exceed 1: 1.5. If there are movement joint in existing substrate they must be copied in SLC.
How to calculate consumption of compound
Measure the surface: divide it into squares and add them all (m²). Measure average thickness needed (mm). Check material consumption rate for 1 mm thickness per 1 m² (kg/m²). Multiply values: surface x thickness x consumption = weight needed (kg). For bags amount, divide by 25* (kg/bag) = bags amount. For pallet amount divide by 40* (bags/pallet) = pallets amount.
* typical amount is 25 kg per 1 bag of SLC and 40 bags per pallet.
Tips for SLC application
Before mixing prepare levelling markers. They can be used as special indicators of SLC level and be removed just after application. When pouring SLC on substrate, spread it for required thickness immediately, smooth with a trowel and release air bubbles with a spiky roller. Provide continuous application.
Start application from the furthest point from the entrance and move towards the exit. Spiky shoes will allow you to walk on still liquid consistency material without destroying it.
How to mark a required level all over the surface?
Make 1 cm wide square markers and set them up over the entire area to be levelled. They will indicate when the expected level will be reached. You can make these markers of a piece of old tile or slab.