When a screed that is intended to be bonded the substrate doesn't adhere as intended (or delaminates) this is often called "drummy" screed due to the sound the screed makes when tapped, which is how it the problem is normally diagnosed.
A delaminated screed is not a failure of the screed itself but a failure of the adhesion between the screed and the concrete slab. However, once a screed delaminates this can increase the risk of the screed cracking or moving in the future.
Once a screed either fully or partially delaminates the options are generally:
• remove the screed and replace it (having solving the root cause of the delamination before installing the replacement screed)
• injecting additional bonding agents between the screed and the slab try and re-bond them - this is expensive and difficult and often doesn't work
• considering whether the project can accept a drummy screed
These can be difficult choices and none are ideal. Its always better to avoid a drummy screed in the first instance. So what causes screeds to delaminate?
Failure to properly prepare the substrate
In almost all cases, the root cause is a failure to properly prepare the substrate. There are lots of scenarios within this including:
1. Slab too smooth
A helicopter finished slab is too shiny and the surface matrix is not open enough to allow the adhesive to fully absorb into the slab creating a good bond. The slab should have been ground back or scabbled.
2. Slab surface dusty, dirty or oily
The slab should be dry, clean and free of dust or contaminants. Any of these will prevent the adhesive from forming a good bond with the slab.
3. Old adhesive or screed on the slab
Any traces of old adhesive, tile or carpet glues or old screed should be fully removed. They will interfere with the adhesive.
4. Not getting a strong mechanical key into the slab
If in doubt, all slabs should be either scabbled or ground back. The courser the slab the better the adhesive will bind into the slab.
Slab contamination or problems
1. Contaminants in the slab
Sometimes slabs both old and new have contaminants in them such as curing agents from the concrete or chemicals that have over time impregnated the slab. This can be very difficult to determine both prior to installation or afterwards in the case of a delamination. A laboratory might be able to determine this but it is better avoided if possible.
If it is suspected that there could be contamination in the slab the approach would be to either scabble back the slab to achieve a maximum coarseness and then combine this with a specialist adhesive that can work with contaminated slabs or install an unbonded screed isolated from the slab using plastic.
2. Contaminants on the slab
Similarly, sometimes slabs (even new slabs) can be contaminated with products such as hydrophobic agents or sealers used to protect walls from graffiti, paint used in spraying walls or any of a number of possible agents. If in doubt, scabble back the slab to remove any possible surface contamination.
3. Rising moisture in the slab
Ground floor slabs whether there is a high water table or where the slab has not been sealed can suffer from rising moisture which can degrade the adhesive bond and cause the screed to delaminate.
4. Vibration affecting the slab
Areas of high vibration (train stations, airports etc) can degrade the bond of the screed over time.
5. Installing over suspended slabs with substantial movement or expansion and contraction
Suspended slabs can have high degrees of movement and can expand and contract more rapidly due to heating and cooling. This movement can cause the adhesive bond to degrade.
Adhesive failures
1. Not mixing the adhesive correctly
Adhesives are mixed on site by the installers in buckets normally. Failure to follow the mixing instructions will result in the adhesive not being mixed to the correct ratios and can result in sub-optimal adhesive.
2. Diluting the adhesive with water
Some adhesives allow the addition of potable water and others do not. Water will dilute the adhesive slurry and too much water can "split" the slurry. Too much water in a slurry will reduce the bonding strength of the slurry.
3. Adding extra ingredients to the adhesive
Sometimes installers have their "secrete recipe" for their preferred adhesive slurry which sometimes involves the addition of non-standard products such as adding a tile glue to an already polymer/latex adhesive. Going off-specification for any adhesive system should always be carefully considered as it may result in a sub-optimal bonding.
4. Letting the adhesive "film over" before applying the screed
Most adhesives need to be applied wet-on-wet meaning that the damp screed should be applied onto the fresh adhesive. If the adhesive is allowed to sit on the slab exposed to the conditions (even only a few minutes of sun or wind) the adhesive can have a thin film form on the surface which results in the screed not integrating with the adhesive. The adhesive needs to soak into the underside of the screed to form a good bond, just in the same way that the adhesive needs to soak into and bond to the slab.
5. Installing the adhesive on a hot or dry slab
If the slab is hot or dry it can very quickly dehydrate and cure the adhesive before it has a chance to form a good bond with the slab. This can cause the adhesive to completely fail to bond to the underlying slab and "come off clean".
6. Installing the adhesive of a slab with standing water
Any water from rain or pre-wetting the slab should be fully absorbed prior to applying adhesive in most cases. If adhesive is added to a slab with puddles of water, it will result in the adhesive being diluted (which reduces its strength) or the adhesive may not absorb into the screed because of all the water. Either way, standing water on a slab is never a good idea.
7. Using the wrong adhesive for the screed
If the screed has a recommended adhesive, it is always a good idea to use it and no go off specification. Whether there are ingredients in the screed which are incompatible with non-specified adhesives or differences in strengths or drying time, using the wrong adhesive can result in delamination.
8. Using a slow dying adhesive with a fast drying screed
Screeds and adhesive cure at different rates. If the screed dries before the adhesive forms its bond the natural curling effect of the screed during the drying process can result in the screed not bonding to the slab. This can particularly be an issue if screeding in hot conditions or if the screed is a fast drying screed or has high levels or polymer admixes added which can decrease the drying time. The adhesive should be drying and forming its bond between the screed and the slab at an appropriate rate having regard to how fast the screed will dry.
9. Not using enough adhesive
Skimping on the amount of adhesive can be a costly, cost-saving exercise. Always use the recommended amount of adhesive. A thin adhesive can of course result in a sub-optimal bond.
10. Trying to bond to polyurethane, plastic or foam
Some substrates cannot be bonded to or require specialist adhesives. Always check that your adhesive is appropriate for the substrate.
11. Using a cement slurry instead of a quality latex or polymer based adhesive
Traditional cement powder and water slurry adhesives are notorious for not forming a strong bond and for not having the ability to flex with the slab or screed to remain bonded despite movement in the slab. We always recommend the use of a good quality latex or polymer based adhesive.
12. Not applying the adhesive in the correct manner
Some adhesives have specific installation instructions such as using a notched trowel, or a broom application or requiring the slab to be wetter prior to installation. Always follow the instructions, they are there for a reason.
Installation failures
1. Not fully compacting the screed
A well compacted screed is essential for lots of reasons but it will also help impregnate the adhesive into the screed such that a good quality bond is formed between the screed and the adhesive.
2. Not installing expansion joints
Expansion joints or control joints minimise uncontrolled cracking of concrete and screeds. Any expansion joints in the screed should mirror those in the slab and should be installed in accordance with Australian Standards, usually every 16 - 20sqm. If expansion joints are not installed, the screed can crack or move in unexpected ways which places stress of the adhesive and can contribute to delamination.
3. Installing a bonded screed too thickly
Bonded screeds have a maximum recommended thickness of normally 40mm. This is because thick screeds have a stronger "curling" effect when drying which can contribute to screeds delaminating. The risk of this can be reduced through using strong adhesives and expansion joints but thick screeds will always have a higher risk of delaminating.
4. Using high doses of polymer admixes or glues in screeds
Polymers and other admixes typically cause the screeds to dry much faster and increase the curling effect during the drying process. High admix doses should be approached with caution consideration should be given to the adhesive used in such situations. This is a common cause of delamination as the polymer admix causes the screed to dry faster than the adhesive and preventing a good quality bond being formed in the first instance.
5. Allowing the screed to dry too quickly
In hot conditions or with high polymer doses, if the screed dries too fast it can curl and lift prior to the adhesive forming a strong bond. In some cases it is necessary to protect the screed from the elements or rehydrate it so that it doesn't cure too quickly before the adhesive has formed its bond
Combination of factors
In cases of adhesive failure we often see a combination of factors. If it had just been one factor, the screed probably would not have delaminated, but if multiple things go wrong (even just in a small way) the effect can result in delamination.
For example, a typical scenario could be a "custom" adhesive mix that the installer has been using for years but is non-standard, applied a tiny bit too thin to a screed that is a bit too thick on a hot day when the screed dried too fast on a slab that probably should have been scabbled back first; or
A new slab that has had some over-spray of a hydrophobic wall sealer/anti-graffiti product that has lightly misted the slab and the slab being a bit too smooth which should probably have been either scabbled or ground back: or
A hot day on an exposed rooftop so the installer soaks the slab with water but doesn't let it fully absorb which means the adhesive is diluted in the puddles when its installed and then the screed dries fast because it wasn't protected from the heat and the thin diluted adhesive isn't strong enough to form a bond; or
A suspended carpark slab where the mix is an unusually high polymer admix dose and the slab is a bit too smooth and has a high degree of movement and no expansion joints being installed resulting in the adhesive not dying fast enough to form a strong bond.
Conclusion
There is nothing worse for us to supply a great screed and then see it fail to form a good bond to the slab and then requiring it to be replaced. It is costly for the installers and always results in finger pointing. Often there are multiple factors at play and it can be hard to diagnose one specific cause after the fact.
Our advice is always to pay particular attention to the slab, if in doubt grind or scabble it back so that the adhesive has the best chance of forming a strong bond and match the adhesive to the screed and follow good installation practices. If the installers do all these things, they give themselves the best chance of avoiding adhesive problems.