Preparing Concrete, Screeds & Self Leveller for Microcement

Whilst microcement is very strong, smooth & flexible, it relies entirely on the surface (aka substrate) that it’s going onto to be completely smooth and free from movement.

The surface must be flushed out and the chance of movement eliminated before microcement is applied.

Golden Rules:

• Surfaces should be completely smooth – as if you were going to paint onto them.
• Surfaces should be well consolidated, so that any chance of movement is eliminated.

Additional Important Rules:

• Never Microcement directly onto natural timber or across cold joints/expansion joints.
• Surfaces should be free from oil stains, dirt, dust and excessive laintance.
• Surfaces must be fully cured with a moisture content of no more than 5%.
• Microcement can only be applied to surfaces with a compressive strength of at least 16N/mm2.
• There must be no risk of water getting to the microcement during the install process.
• Surfaces must be damp-proofed so that no water can get to the Microcement from underneath at anytime.
• Allow 3-4mm as the finished depth.

 

Before microcement is applied to concrete, screeds or self leveller, the following actions must have taken place:

Cured to less than 5% Moisture Content
If there is still moisture in the substrate, the microcement could blister or de-laminate as moisture will be trapped under the surface. Floors that aren’t fully cured are also more likely to crack as they do so whilst curing. The moisture content can be tested with a moisture meter or by checking with the manufacture of the screed. The drying time will depend on the depth of floor & the type of product used. Be aware that Anhydrite screeds can take a lot longer to cure than sand and cement.

Special Prep for Anhydrite Screeds
Remove laintance by grinding back the surface until there is no more loose dust. Vacuum dust away, apply Epoxy, Mesh & Sand Blind. It’s not essential that you include mesh with the Epoxy, but it is advisable since that is the strongest option.

Put Underfloor Heating through a Full Cycle
Once cured, take underfloor heating up to max temperature & back down to off @ small intervals – as per screed manufacturers recommendations, no more than 5°C per day. The initial thermal expansion & contraction may force out cracks which must be repaired.

Repair Cracks under 3mm Wide
Use a 5” grinder to open up the crack (take care not to cut into any underfloor heating), hoover out any dust then mix Epoxy with Epoxy Hardener (Ratio 5:2) and pour into the crack. Leave to cure fully, this will take for at least 12 hours. It may need topping up depending on how deep the crack is.

Repair Cracks over 3mm Wide
Use a 5” grinder to open up the crack and grind approx 100mm “stitches” horizontally across the length of the crack around 40mm apart (take care not to cut into any underfloor heating), hoover out any dust then insert steel helical stitching bars into the stitches. Mix Epoxy with Epoxy Hardener (Ratio 5:2) and pour into the crack. Leave to cure fully this will take for at least 12 hours. It may need topping up depending on how deep the crack is.

Epoxy, Mesh & Sand Blind
Where crack repairs have taken place, the best way to reduce the chance of cracks transferring through to the surface is to apply Epoxy, Mesh & Sand Blind over the entire floor. Many professional installers choose to use it on all floors as it ensures the floor dries at a uniform rate, helps to prevents stains in the substrate ghosting through to the microcement and strengthens the base. Use a trowel to apply an even coat of Epoxy over the sub-floor and mesh. You’ll need to ensure you have completely filled any voids in the sub-floor and also completely covered the mesh.

Whilst the Epoxy is still wet, spread the quartz sand across the surface. You can scatter it by hand, starting at the entrance to the room and walking on the sand into the room. Or by walking on the Epoxy with spiked shoes. The following day, sand any high spots and hoover up all the loose sand.

Epoxy, mesh & sand are contained within the Floor Strengthening Kit or by selecting High Performance Primer as part of your kit when you buy it from relentlessmicrocement.com.

Priming the Surface
Whether you have applied Epoxy & Sand or you are going straight onto the screed, you will need to prime the surface with Primer 100. If you are going directly onto concrete, screeds & self leveller, apply 1 x Primer 100 diluted with 50% water, leave to dry and then apply a neat coat of Primer 100 with Fibreglass Mesh. For Epoxy & Sand you can apply a neat coat directly onto the sand. The Primer is applied with a roller and dries in about 1 hour. You must apply the 1st base coat within 12 hours.

Never Microcement over Cold Joints/Expansion Joints
Microcement doesn’t need expansion joints itself but where they are in the sub floor, they must be carried through to the surface. Use 4mm dural expansion joint beads – these need to be screwed into the floor and epoxied over so that there is no chance of them moving.