Why is my screed lifting?
Introduction
Delamination, commonly known as "drummy" screed, is a condition where a screed fails to adhere properly to the substrate, resulting in potential issues such as cracking and movement. While the screed itself may not be at fault, the failure lies in the adhesion between the screed and the concrete slab. Rectifying a delaminated screed poses challenges, and various options are available, but prevention is always the best approach. This article delves into the causes of delamination and offers insights into how to avoid and address this problem effectively.
Root Causes of Delamination
1. Failure to Prepare the Substrate Properly
In most cases, delamination occurs due to improper substrate preparation. Several scenarios fall under this category:
a) Smooth Slab: A shiny, helicopter-finished slab with a closed surface matrix prevents the adhesive from bonding effectively. Grinding or scabbling the slab creates an open surface for better adhesion.
b) Contaminated Slab: Dust, dirt, oil, or residues of old adhesive, tile or carpet glues can hinder the adhesive's ability to form a strong bond. Thoroughly clean and remove any such contaminants from the slab.
c) Insufficient Mechanical Key: Scabbling or grinding the slab is recommended to ensure a strong mechanical key that enhances adhesive bonding.
2. Slab Contamination or Problems
a) Contaminants in the Slab: Some slabs, both old and new, may contain curing agents or chemicals that impede proper adhesion. Identifying potential contaminants beforehand can help avoid issues or use specialized adhesives to work with contaminated slabs.
b) Contaminants on the Slab: Hydrophobic agents, sealers, or other substances on the slab surface can interfere with adhesive bonding. Properly scabbling back the slab can remove surface contamination.
c) Rising Moisture: Ground floor slabs with a high water table or inadequate sealing can suffer from rising moisture, degrading the adhesive bond and causing delamination.
d) Vibration: Areas with high vibration, such as train stations or airports, can gradually degrade the adhesive bond over time.
e) Suspended Slabs with Movement: Suspended slabs with substantial expansion and contraction can compromise adhesive bonds due to their constant movement.
3. Adhesive Failures
a) Incorrect Mixing: Failure to follow adhesive mixing instructions can result in a sub-optimal adhesive mixture, affecting bonding strength.
b) Dilution: Adding excess water to the adhesive can dilute the slurry and weaken the bond.
c) Using Extra Ingredients: Deviating from recommended adhesive specifications by adding non-standard products may lead to sub-optimal bonding.
d) Adhesive "Filming Over": Allowing the adhesive to sit exposed on the slab before installing the screed can result in a thin film forming on the surface, hindering integration.
e) Applying Adhesive on Hot or Dry Slab: Fast-drying slabs can prevent proper bonding if the adhesive cures before forming a strong bond.
f) Applying Adhesive on Slab with Standing Water: Adhesive should not be applied on slabs with puddles of water, as it may dilute the adhesive or prevent it from soaking into the screed.
g) Using Incompatible Adhesive: Always use the recommended adhesive for the screed to avoid delamination.
h) Slow Drying Adhesive with Fast Drying Screed: Mismatched curing rates between the adhesive and screed can lead to inadequate bonding.
i) Insufficient Adhesive: Using less adhesive than recommended can compromise the bonding strength.
j) Trying to Bond to Unsuitable Substrates: Some substrates may require specialized adhesives, while others may be impossible to bond.
k) Cement Slurry Instead of Polymer-based Adhesive: Traditional cement and water slurry adhesives lack the necessary flexibility to maintain a strong bond.
l) Incorrect Application: Not following adhesive installation instructions, such as using the wrong trowel or application method, can result in sub-optimal bonding.
4. Installation Failures
a) Inadequate Screed Compaction: Proper screed compaction is essential for adhesive penetration and forming a strong bond.
b) Lack of Expansion Joints: Failure to install expansion joints in the screed can lead to uncontrolled cracking and stress on the adhesive, contributing to delamination.
c) Excessive Screed Thickness: Bonded screeds should not exceed the recommended thickness to avoid excessive curling during drying, which can lead to delamination.
d) High Doses of Polymer Admixes: High polymer admix doses can cause screeds to dry faster, increasing the curling effect and the risk of delamination.
e) Fast Drying of Screed: Screeds drying too quickly can curl and lift before the adhesive forms a strong bond.
f) Combining Factors: Delamination is often the result of multiple factors, and their combined effect may lead to failure.
Conclusion
Preventing delamination in bonded screeds is crucial for ensuring a successful installation. Proper substrate preparation, selecting the right adhesive, following installation instructions, and addressing potential slab contamination or issues are vital steps. By paying attention to these factors, installers can minimize the risk of delamination, leading to successful projects with long-lasting results.
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