The compressive strength of conventional underfloor heating backfill mortar (typically C15–C20) generally fails to meet the minimum substrate strength requirement of MicroRock (≥25 MPa). The backfill layer must be strengthened and strictly inspected to qualify as a proper base.

The MicroRock system demands high substrate firmness and stability. Below are the specific requirements and standards for the structure of “underfloor heating backfill layer + MicroRock topcoat”.

 

I. Requirements for Underfloor Heating Backfill Mortar

 

The backfill layer serves both as protection for underfloor heating and as the load-bearing substrate for MicroRock, and its performance must fully comply with specifications.

Requirement Item	Specific Standard	Basis & Explanation
Compressive Strength	≥25 MPa (28-day curing age)	General minimum requirement for MicroRock substrate; ordinary C15 (15 MPa) backfill mortar is unqualified. C25 or higher fine aggregate concrete is mandatory.
Bond Strength (Pull-off)	≥1.5 MPa	Ensures firm bonding between topcoat and substrate to prevent delamination.
Density & Flatness	1. No hollowing, sanding, cracking or looseness on surface 2. Flatness ≤3mm / 2m	Substrate defects will reflect directly on the finished surface, affecting appearance and service life.
Moisture Content	≤4% (measured before construction)	MicroRock is moisture-sensitive; excessive moisture easily causes blistering and delamination.
Curing & Drying	Minimum 28 days curing after backfill placement; turn on underfloor heating for forced drying before MicroRock application to fully evaporate internal moisture.	Critical special requirement for underfloor heating scenarios.

 

II. Key Substrate Treatment Procedures Above Backfill Layer

 

After the backfill layer meets all standards, the following treatments must be completed before MicroRock installation:

Recommended grid spacing: 0.8m × 0.8m

Groove dimension: depth and width equal to twice the designed topcoat thickness (e.g. 10mm×10mm groove for 5mm topcoat)

Dense grid grooves resist temperature stress caused by heating and cooling cycles of underfloor heating.

 

1. Strength & Defect Inspection

Test substrate strength with a rebound hammer or pull-off tester; chisel out hollow and loose areas, then level with high-strength repair mortar.

 

2. Shot Blasting (Sand Blasting) Treatment

The backfill surface must be shot-blasted to expose solid aggregate and enhance mechanical bonding force.

 

3. Cutting Anchor Grooves

Recommended grid spacing: 0.8m × 0.8m;

Groove size: depth and width twice the planned topcoat thickness (e.g. 10mm×10mm groove for 5mm overlay);

Dense grid layout relieves thermal stress from underfloor heating temperature cycles.

 

4. Edge Treatment

Cut parallel boundary grooves 8–10cm away from walls, column bases and other perimeters.

 

5. Thorough Cleaning & Dust Removal

Remove all dust and particles with an industrial vacuum cleaner.

 

6. Application of Special Primer

On a dry and clean substrate, apply the matched polyurethane primer of the MicroRock system to seal capillary pores and improve interlayer adhesion.

 

III. Comprehensive Recommendations

 

1. Clarify Requirements at Design Stage

Specify ≥25 MPa compressive strength for underfloor heating backfill in early communication. Adopt ready-mixed C25 or higher fine aggregate concrete to avoid unstable on-site mixing ratios.

 

2. Strict Acceptance of Backfill Layer

On-site inspection of strength, moisture content and flatness before polyurethane flooring construction; do not proceed if any index fails.

 

3. Prepare Special Construction Plan

Regard shot blasting, grooving, cleaning and priming as standard procedures for MicroRock underfloor heating projects, and require constructors to implement strictly.

 

Summary

 

The MicroRock system can withstand temperature differences of underfloor heating, yet overall performance depends entirely on substrate quality. Conventional backfill mortar must be upgraded to high-strength backfill, and strict substrate treatment processes must be followed to ensure long-term stability of MicroRock flooring with no cracking or delamination.