Most anhydrite screed problems come back to three causes: moisture left in the screed, laitance left on the surface, and a screed laid too thin or on a badly prepared base. Get those three right and most of the cracking, debonding, hollow spots and covering failures described below never happen. The rest of this article explains each failure, but this is the short version: dry it properly, test it properly, sand off the laitance, and lay it to the thickness the standard requires.
Anhydrite screeds have become increasingly popular in recent years as an alternative to traditional sand and cement screeds. Made from calcium sulfate, anhydrite screeds offer several advantages, such as faster drying times, higher strength, and excellent compatibility with underfloor heating systems. However, as with any building material, anhydrite screeds can also run into problems if not installed correctly.
Common Issues
Some of the most common issues that can arise with anhydrite screeds include:
Cracking and Debonding
If the substrate or base is not properly prepared or if contraction joints are not properly installed, cracking and debonding can occur as the screed dries and contracts. Cracks allow moisture underneath the screed which can cause damage over time.
Efflorescence
This white, powdery deposit on the surface of the screed is caused when excess moisture reacts with salts in the screed. While not harmful, efflorescence indicates moisture issues.
Hollow Spots
Low spots or hollow areas can form in the screed due to inaccurate screed installation or issues with the substrate. These make the floor feel uneven underfoot.
Incompatible Materials
Using the wrong adhesive or screed additive can cause the screed to fail prematurely. Always check compatibility.
Why Do These Issues Happen?
There are several reasons why anhydrite screed problems can occur:
Improper Mixing and Installation
If the screed is not mixed correctly or proper installation procedures are not followed, issues like cracking, hollow spots, strength problems, and more can develop.
Moisture Imbalances
Too much water during installation or improper drying can cause debonding, efflorescence, and affect strength. However, too rapid drying also causes issues. Maintaining proper moisture levels is essential.
Base Preparation Problems
Contaminants, unevenness, loose materials, and other issues with the substrate can transfer to the screed causing bonding problems, uneven surfaces, and cracks.
Adding Unapproved Materials
While plasticizers, additives, and fibers can enhance screeds, adding the wrong materials can severely impact strength and durability.
How to Prevent Anhydrite Screed Problems
Many anhydrite screed problems can be prevented by:
Choosing a Reputable Installer
Select an installer with extensive anhydrite screed experience and training. Check references for examples of their work.
Following Manufacturers Guidelines
Strictly adhere to the manufacturer’s recommendations for mixing ratios, water content, drying times, and preparation methods.
Suitable Substrate
The substrate under the screed should be properly analyzed, prepared, cleaned, leveled, and primed prior to screed installation.
Installation Best Practices
Allow proper drying times between layers, fully compact all screed layers correctly, and properly cut contraction joints for the screed to perform optimally.
Use Compatible Products
Always verify adhesives, sealers, additives, and underfloor heating systems are approved for use with the specific screed.
Fixing Existing Anhydrite Screed Problems
If you already have anhydrite issues, here are some potential fixes:
Efflorescence
This can often be removed with cleaning. Improve ventilation and drying to address excessive moisture.
Cracks and Hollow Spots
Grind down high edges of cracks. Fill and seal cracks with approved filler materials. Fill hollow spots.
Debonded Areas
Remove loose screed sections. Re-prime and re-screed problem areas.
Uneven Surfaces
Level with self-leveling compound compatible with anhydrite.
Be aware that some issues may require complete replacement of the problematic screed sections. Preventing problems is more cost effective than repairing them.
Hiring Professionals for Repairs
With significant or widespread issues, it is best to hire a professional installer to assess and repair the screed correctly. They have the proper tools, materials, and expertise to remedy problems and prevent future failure. Check their experience working specifically with anhydrite screeds.
Be wary of contractors suggesting sand cement overlays as this can cause more problems long-term due to incompatibility with anhydrite screeds. Make sure to get a binding warranty for any repair work should issues reoccur.
Avoiding Anhydrite Screed Problems in New Installations
For new installations, preemptively avoiding issues is vital. Here are key tips:
Hire the Right Installer
Select an installer specially trained on modern anhydrite screed installation with numerous years of experience. Check references from past anhydrite jobs.
Use Quality Materials
Choose screed materials from reputable manufacturers that adhere to all relevant standards. Verify compatibility for any additives or fibers used.
Careful Substrate Prep
Make sure the substrate under the screed meets specifications for flatness and proper preparation for bonding.
Follow All Instructions
Strictly follow manufacturer instructions for ratios, drying times between layers, curing procedures, and contraction joint cutting.
Control Conditions
Carefully control temperature, humidity levels, and ventilation during and after installation for proper curing.
The numbers that prevent most failures
Calcium sulphate screed to BS 8204-7 is laid at a minimum of 25 mm bonded to a prepared slab, 30 mm unbonded over a separating layer, and 40 mm floating over insulation (35 mm floating in domestic work), with at least 25 mm cover over underfloor heating pipes. Those are minimums at the thinnest point, so specify above them where the slab is uneven. Plan drying at about 1 day per millimetre up to 40 mm, then 2 days per millimetre beyond that, so a 40 mm screed plans at about 40 days before testing. Do not cover an impermeable finish until a carbide test reads below 0.5 percent moisture. Separately, remove laitance by light sanding once the screed is dry enough to sand, and vacuum the dust fully, because adhesives and primers bond to laitance, not to the screed, and a covering laid over laitance can lift with the skin.
Diagnose it by symptom
Use this table to match what you can see or hear to the most likely cause and the first check to make. It narrows the fault down; it does not replace a moisture test or a professional assessment of a widespread failure.
| Symptom | Most likely cause | First check |
|---|---|---|
| White powdery deposit on the surface | Efflorescence from excess moisture | Improve ventilation and drying, then moisture test before covering |
| Hollow sound when tapped, or a loose area | Debonding or a void under the screed | Tap survey the area and check base preparation and joint layout |
| Cracks across the floor | Missing joints, screed too thin, or dried too fast | Check thickness at the crack and whether joints were cut as specified |
| Floor covering lifting or bubbling | Laitance left on the surface, or moisture still in the screed | Check whether the surface was sanded, then carbide test |
Conclusion
When installed correctly, anhydrite screeds offer exceptional performance and durability. However, they also come with a unique set of potential issues inherent to calcium sulfate screeds. Following manufacturer guidelines closely, hiring experienced professionals, taking climate control seriously, and carefully preparing the substrate are keys to avoiding problems like cracking, hollow spots, efflorescence and more. But if issues do pop up, calling in a specialist to professionally remedy the problem is advisable to prevent further expensive damage. Overall, some extra care taken during installation and curing goes a long way to harnessing all the advantages anhydrite screeds provide while avoiding headaches later on.
FAQs
What is the main cause of cracking in anhydrite screeds?
The main causes of cracking are inadequate contraction joints, substrate movement, excessive water in the mix, and uncontrolled shrinkage and expansion from temperature changes.
Does efflorescence cause structural problems in anhydrite screeds?
No, while unsightly, efflorescence itself does not cause structural or strength issues. It mainly indicates excessive subsurface moisture that should be addressed to prevent potential long-term problems.
What strength does an anhydrite screed reach after 28 days?
A typical anhydrite screed will reach a compressive strength of 20-30 N/mm2 after 28 days which is as strong or stronger than traditional sand cement screeds.
Can you apply a sand cement screed over an existing anhydrite screed?
No, a sand cement overlay should never be placed over an existing anhydrite screed as it can cause bonding issues leading to debonding and hollowness over time.
What thickness should an anhydrite wearing screed be?
Anhydrite wearing screeds require a minimum thickness of 35 mm over the substrate to provide adequate strength and durability as a finished floor. Thinnersections may require an additional compatible floor covering.