Dealing with Construction Moisture

Why Construction Moisture Can Severely Delay a Development Programme.
Multi-Million Pound RC Framed Residential Development

We dealt with two issues this week relating to problems caused by construction moisture. A plaster beetle infestation and an investigation into the reasons why anhydrite floor screeds were not drying out within a multi-million pound residential development block in London. More on the plaster beetle infestation in a separate blog to follow quite soon.

Our client, was understandably concerned that pumped anhydrite floor screeds, which on some floors had been pumped in 6 weeks earlier, had failed to dry. The installation process for this particular screed required that the laitance be sanded from the surface of the floor once dry, this usually occurs at 3-7 days after installation, however any attempt at sanding the floor immediately resulted in a clogged rotary sanding pad.

Laitance occurs on the surface during settlement/compaction. During this compaction process, bleed water migrates to the screed surface. This brings with it fine particulates within the screed. Laitance is subsequently formed as a result of the evaporation of the bleed water, and once hardened can impede the drying process .

The advantages of laying a pumped anhydrite screed, as opposed to laying concrete is that you can generate quite significant savings on labour and time but only if optimum environmental conditions for drying the screed are present.

Other benefits of using  pumped calcium sulfate screeds are that they are self compacting, have very little shrinkage and rarely require movement joints. They are particularly suited to underfloor heating systems because they dissipate heat far better than a concrete floor slab would.

Wall channel between anhydrite screeds to each room.

Pumped anhydrite screeds do not provide a wearing surface so are overlaid with a suitable wearing course; this may be a compatible smoothing compound, carpets, floor tiles or any other finished flooring. Many floor screed failures that we investigate concern buildings with underfloor heating systems and a latex wearing screed. There can be a number of complex issues involved relating to material compatibility and poor commissioning of the UFH system but this particular issue was far simpler to identify. The developer had long since overrun the usual 3-7 day drying and six weeks later some floors were still soft on the surface and therefore unsuitable for sanding to remove the surface laitance. In anhydrite screeds, the binder reacts with the water in order to produce gypsum crystals. Around 80% of the anhydrite is converted to gypsum and this reaction uses a large proportion of the mixing water but the remaining moisture is lost by evaporation.

The block, containing around 56 flats over 4 floors, had almost all windows and doors installed but no heating or MVHR system, which would be installed too late in the programme to be of any help. What we consistently found is incredibly high internal relative humidity of around 87% and therefore very little difference between ambient temperature (15 degrees centigrade)  and dew point temperature (13.6 degrees centigrade).

Floor temps below dew point temperature
Scanning for relative moisture content across slab.

Floor temperatures were consistently recorded as being below dew point temperature, therefore proving that condensation damp was constantly rewetting the anhydrite floor screeds. To rule out any other potential moisture sources we like to scan the whole floor for relative readings, which shows up any unusually high peaks in moisture that may suggest another source of moisture, but we found consistent levels of moisture across all floor slabs on each storey. We tested the floor slabs for total moisture content (TMC) over all levels using calcium carbide and found readings ranging from 0.2% TMC at first floor level to 2% TMC on other floors. You would ideally look for a TMC of 0.3%, but no more than 0.5%.

Interestingly, we were able to prove that the simple action of opening windows made very little difference to this problem by recording internal relative humidity two hours after opening windows in the block. This did nothing to reduce relative humidity and of course there was no difference between external and external temperatures so even ambient temperature remained the same.

There may sometimes be a case for installing dehumidifiers and maybe even supplementary heating but in this particular case we do not think that was the right strategy, since no internal doors or partitions had been installed and doors were often wedged open for trade activities. You need a sealed environment to stand any chance of success with a dehumidifier otherwise you are attempting to dry the world. In this particular case we recommended both positive pressure and negative pressure ventilation. We calculated the cubic volume of the block and recommended fan that would give us 12 air changes in the building over a 24 hour period. Fans would set up to force air in at one end of the building and extract air from the opposite end of the building. As a general principal, we do not believe that you should expect these high levels of construction moisture to take take care of themselves and a supplementary airflow is often needed if you are to keep your development programme on target. We make precise recommendations based on construction progress, material testing and environmental conditions found on site.

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