The compressive strength of conventional underfloor heating backfill mortar (normally C15–C20) is generally insufficient to meet the minimum substrate strength requirement of MicroRock (≥25 MPa). The backfill layer must be strengthened and strictly inspected before it can serve as a qualified base substrate.

The MicroRock system imposes high requirements on the firmness and stability of the substrate. Below are the specific specifications and standards for the structure of “underfloor heating backfill layer + MicroRock wearing course”.

 

I. Requirements for Underfloor Heating Backfill Mortar

 

The backfill layer acts both as protective covering for underfloor heating and as the load‑bearing substrate for MicroRock, and must fully comply with performance standards.

Item	Specification	Remarks
Compressive Strength	≥25 MPa (28‑day cured)	Minimum general requirement for MicroRock substrate; standard C15 (15 MPa) backfill mortar is unacceptable. C25 or higher fine aggregate concrete shall be adopted.
Bond Strength (Pull‑off)	≥1.5 MPa	Ensures reliable bonding between surface layer and substrate to prevent delamination.
Density & Surface Flatness	1. No hollowing, surface dusting, cracking or loose areas 2. Flatness tolerance ≤3mm per 2m	Substrate defects will reflect directly on the finished floor, affecting appearance and service life.
Moisture Content	≤4% (measured prior to application)	MicroRock is sensitive to moisture; excessive moisture may cause blistering and delamination.
Curing & Drying	Minimum 28 days curing after backfill placement; switch on underfloor heating for forced drying before MicroRock installation to fully evaporate internal moisture.	Essential special requirement for underfloor heating installations.

 

II. Key Substrate Treatment Procedures above Backfill Layer

 

Once the backfill layer meets all specifications, the following treatment steps must be completed before laying MicroRock:

Recommended grid spacing: 0.8m × 0.8m

Groove size: depth and width equal to twice the designed wearing‑course thickness
(Example: 10mm×10mm groove for a 5mm surface layer)

The dense grid structure relieves thermal stress generated by heating and cooling cycles of underfloor heating.

 

1. Strength & Defect Inspection

Test substrate strength using a rebound hammer or pull‑off tester; remove hollow and loose sections, then level the surface with high‑strength repair mortar.

 

2. Shot Blasting Treatment

The backfill surface must be shot blasted to expose coarse aggregate and create a strong mechanical bonding profile.

 

3. Cutting Anchor Grooves

Recommended grid spacing: 0.8m × 0.8m;

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

The dense grid layout effectively counteracts thermal expansion stress from underfloor heating temperature cycling.

 

4. Edge Treatment

Cut continuous perimeter grooves 8–10cm away from walls, column bases and other boundaries.

 

5. Thorough Cleaning & Dust Removal

Remove all dust and debris completely using an industrial vacuum cleaner.

 

6. Application of Special Primer

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

 

III. General Recommendations

 

1. Define Requirements at Design Stage

Clearly specify a minimum compressive strength of ≥25 MPa for the underfloor heating backfill during early design communication. Ready‑mixed C25 or higher fine aggregate concrete is recommended to avoid inconsistent on‑site mixing ratios.

 

2. Strict Acceptance of Backfill Layer

On‑site inspection of strength, moisture content and flatness is mandatory before polyurethane flooring application. Construction shall not proceed if any index fails to meet standards.

 

3. Formulate Special Construction Specification

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

 

Summary

 

The MicroRock system can fully withstand temperature cycling of underfloor heating; however, overall performance relies entirely on proper substrate preparation. Conventional backfill mortar must be upgraded to a high‑strength structural layer, and full substrate treatment procedures strictly followed, to ensure long‑term stable performance of MicroRock flooring with no cracking or delamination.