ᐅ The screed installer used different additives than agreed upon.

Hello everyone,

I have a question for the experts here, especially those with experience in screed work.

Originally, a cement screed with Retanol Xthinn was proposed to achieve high strength, fast readiness for covering, and low shrinkage despite limited build-up height.

However, on the day of installation, a different combination was used without prior consultation, namely:

• ML7 Q7 additive for cement screed,
• steel fibers,
• Floorwitt Cement CEM I N.42.5.

I was told this combination would provide results comparable to Retanol Xthinn.

My question to you:
Can this mixture be considered a qualitatively equivalent substitute for Retanol Xthinn? Especially regarding strength, drying and readiness times for covering, shrinkage behavior, and dimensional stability in cases of limited build-up heights?

I would appreciate professional assessments or experiences – also any advice on what to pay attention to afterwards or which values I should possibly have tested.

Thank you in advance!

Sometimes I wonder if there are absolutely no problems on other construction sites when such an effort is made for this one. 😀
I wouldn’t worry about it at all, but I’m not a screed expert.

@"Bulle2k1"
The topic you brought up is complex for a non-expert, so here is an explanation from my side:
First of all, and also from a legal perspective, what was contractually planned or agreed upon at the time regarding the screed strength is crucial!
You did not mention this, and you probably could not have known the need to specify it when you started your inquiry.
Furthermore, according to DIN 18560 Part 2 (screed work), different installation thicknesses are specified for residential construction, depending on the strength class and whether it is a heated screed or not.
You mentioned a "limited build-up height."
What exactly is meant by this?
In general, a standard screed installation thickness of at least 45mm (1.8 inches) is required for residential construction with a bending tensile strength (BTS) class of F4.
With a CEM I N.42.5 cement, a bending tensile strength of F5 will likely be achieved, meaning the screed mortar can be installed 5mm (0.2 inches) thinner.
The product "Retanol Xthinn" is a screed additive for faster early strength development and higher final strength (CT-C40-F6).
In my experience, such products MAY work, but do not necessarily have to.
Choosing a cement with higher compressive strength is always the safer way to achieve a more durable screed (compressive strength is unrelated to bending tensile strength), compared to relying on screed additives.
If nothing was contractually agreed on at the start, you should accept the approach of using a stronger cement.
You asked about
* Strength
* Drying or readiness for covering times
* Shrinkage behavior (dimensional stability is essentially the same as shrinkage behavior in terms of impact)
I have already addressed strength.
The drying time for cement screeds is not affected by the (here higher) strength class.
Regarding shrinkage behavior, it is the case that with increasing inherent strength (see: cement CEM 42.5), shrinkage behavior and thus the risk of cracking also increase.
But be careful: no need to panic now!
The screed installer was certainly aware of this; otherwise, they would not have reinforced the screed mortar with fibers.
Steel fibers have been "out" for around 15 years; today polypropylene fibers are used.
However, the fiber additive, whether steel or plastic, minimizes the risk and width of cracks during drying.
If occasional cracks appear with widths of 0.1mm to 0.2mm (0.004 to 0.008 inches), that is normal and not a defect.
--------------------------
Summary:
Higher mortar strengths are certainly and primarily achieved by using a cement with higher strength.
This has nothing to do with drying behavior.
Slightly higher shrinkage and the resulting risk of cracking during drying is largely neutralized by fiber additives used in the screed production (assuming the appropriate amount is applied).

I hope I was able to reassure you here?!
Regards to all, KlaRa

You have received a great and professional answer already.

Bulle2k1 schrieb:

Can this mixture really be considered a qualitatively equivalent replacement for Retanol Xthinn?

However, I wonder why you don’t trust the experts managing your construction.
I miss questions about mortar, masonry, and formwork, whether the material used is actually suitable 😉