Repair mortar cracking and debonding are among the most common quality complaints. Many blame poor substrate preparation or inadequate curing. But one key variable is often overlooked: whether RDP dosage and glass transition temperature (Tg) are properly matched.

RDP is the core component that provides flexibility and bond in repair mortar. Dosage determines “how much polymer” is present. Tg determines “how flexible the polymer is.” When the two are mismatched, cracking and debonding become inevitable.

RDP’s Dual Role in Repair Mortar

RDP (redispersible polymer powder) forms a continuous polymer film within the mortar, weaving between cement hydrates and aggregates. This film serves two functions: absorbing shrinkage stress and temperature movement to prevent micro-crack growth, and anchoring to the old substrate surface for strong interfacial bonding.

Both functions relate directly to RDP dosage and Tg. Insufficient dosage leaves the polymer film discontinuous, with nowhere for stress to release. Too high a Tg makes the film rigid and poor at deforming. Too low a Tg makes it too soft, weakening cohesive strength.

Experiment Design: Two-Factor Comparison

Fix the base repair mortar formula and set the following variables:

RDP dosage: 3%, 5%, 8% (by binder weight)
RDP Tg: -15°C, -5°C, +5°C

Each group tests three key indicators: tensile bond strength (original and after water immersion), elongation at break, and shrinkage cracking index (crack count and width under restrained conditions).

What the Results Reveal

Dosage effect on flexibility
When dosage increases from 3% to 5%, elongation at break improves significantly and the shrinkage cracking index drops noticeably. The polymer film becomes more continuous, and stress absorption improves. Beyond 5%, however, the elongation gain slows. Further addition brings diminishing returns on flexibility while cost rises sharply.

Tg effect on flexibility
When Tg drops from +5°C to -5°C, elongation at break improves clearly. The film becomes more flexible and crack resistance increases. But at Tg -15°C, although elongation is highest, bond strength after water immersion drops. The film is too soft, and cohesive strength and water resistance suffer.

Interaction between dosage and Tg
High dosage with low Tg gives the best flexibility but the highest cost. Low dosage with high Tg is cheapest but carries high cracking risk. The optimal combination is usually medium dosage (5%) with medium-to-low Tg (-5°C to -10°C), balancing flexibility, bond strength, and cost.

Selection Guide by Application

Standard repair (room temperature, non-structural)
RDP dosage 3%–5%, Tg -5°C to 0°C. Meets general crack resistance and bonding needs at moderate cost.

High-movement substrates or large temperature swings
RDP dosage 5%–8%, Tg -10°C to -15°C. Higher polymer content and a more flexible film are needed to accommodate substrate movement and thermal cycling.

Wet or humid environments
RDP dosage 5%–8%, Tg -5°C to 0°C. Avoid the water resistance loss from too-low Tg while ensuring adequate bond strength.

Structural repair or high-load applications
RDP dosage 8%+, Tg 0°C to +5°C. Higher cohesive strength and early hardness are needed, with flexibility appropriately compromised.

Validation Points

After adjustment, measure these indicators to confirm performance: tensile bond strength (original, after water immersion, after heat aging), elongation at break, and restrained shrinkage cracking tests. All three must meet targets together — never rely on a single data point

Conclusion

Cracking and debonding in repair mortar come down to an imbalance between flexibility and bond. RDP dosage determines the continuity of the polymer film. Tg determines the film’s softness or hardness. Only when the two are properly matched can the repair layer both hold firm and resist cracking.