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What are the consequences of a drummy screed?

The dreaded words..."the screed has gone drummy".

What does this mean and what are the consequences?

The term drummy screed refers to the sound that tapping on screed makes when it has failed to bond to the concrete substrate. It makes a "tock tock" sound rather than a higher "tick tick" sound. Other terms used are that the screed has delaminated from the substrate or that the screed has a hollow sound.

A common misconception is that the delamination of the screed has something to with the screed. It actually has nothing to do with the screed and is entirely to do with a failure of the slurry to bond to the slab. It is a failure at the interface between the slurry and the slab.

For a discussion on whether or not screed will bond to a smooth concrete substrate, read our blog post here.

So what are the consequences of a drummy screed?

So a drummy screed basically means that you have an unplanned, unbonded screed which may be fully (across the entire floor) or partially (in patches) unbonded from the concrete slab substrate.

An unbonded screed in itself is not necessarily a problem, in fact we regularly install screeds that are unbonded by design and they have a number of advantages in certain circumstances. However, unbonded screeds need to be thicker than bonded screeds as bonded screeds obtain their strength in part from their bond to the slab and can therefore be thinner and unbonded screeds do not gain strength from their slab bond and therefore need to be thicker. A common problem at this point is that the drummy screed may be thinner than the recommended minimum thickness for an unbonded screed.

Also, if the screed is has patches that are bonded and patches that are unbonded, which typically indicates a low bond strength generally across the floor but just that in some areas the screed has stuck and others it has come away (and the areas that have stuck may in fact go drummy in the future due to the general low bond strength), when you walk across the floor it will sound different from one area to another, which is not the effect anyone wants.

Other than the sound differences between a bonded and drummy screed, there is an increased risk of cracking in the screed, especially if there are some sections that are bonded and others that are unbonded. As the slab moves or expands and contracts, the bonded sections will move with the slab and the unbonded sections will not. This can result in differential movement in the screed and cracking.

Also, as the screed goes drummy and the bond "lets go" it is common for the screed to crack as the bond releases and the screed "pops up". This is especially the case if the screed is thin.

Overall, if the screed is thin and goes drummy then depending on what product is installed on top and how much strength that contributes to the screed (eg heavy concrete pavers v vinyl or heavy tiles v thin tiles) the life of the screed can be reduced and the screed and/or top product may crack.

A drummy screed is best avoided and whilst some projects can accept a drummy screed, others will decide to remove it, rectify the issue that caused the drummy screed in the first place. You can read or blog on what causes a drummy or delaminates screed here.


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