Very many thanks to all those who responded to my query about concrete drying out times.   I summarise the responses received so far below as I’m sure the helpful hints will benefit others planning new strongrooms.   If you’d like to add to the debate or have any further tips, please let me know and I’ll issue an updated summary in the next couple of weeks. 

 

Drying out - general:

·         In good drying conditions, rule of thumb = 1mm per day.  But also depends on ventilation, weather, water-cement ratio, concrete structure, thickness, finishes etc.  

·         Build strongrooms first to allow maximum time for drying out.

·         Put roof on as soon as you can (otherwise rain will make situation worse).

·         Air tightness of building required de-humidifiers to draw out moisture.

·         Avoid putting floor finish on when RH of concrete is too high.

·         Don’t lay final floor covering or install racking until concrete floor has properly dried out.

·         If heavily insulated for stable/passive environment, can take longer for escape of moisture.  Improving air flow (fans and open doors) can help.

·         Be proactive initially (e.g. fans) in order to reduce timescales.  Some areas won’t dry out naturally.

·         Actively manage drying out from start – ventilation and mechanical dehumidification

·         Specify dehumidifier and active ventilation during drying out period.  Very difficult to achieve stable environment without this.

·         Humidity levels will be very high just after construction.  Drying out for first 60 days is generally faster; the rate slows down after this.

·         Important not to use gas heaters (as these emit moisture).

·         Addition of boxes of paper increases thermal inertia and helps stabilisation.

·         BUT if concrete floors dry out too quickly, they crack.   If so, you’ll need painted substrate on top otherwise it is a nice breeding ground for bugs etc. 

·         Strongroom walls – need to be fully dried out before plastered and painted.  Some offices leave the bare concrete.  Moisture cannot be extracted once completely sealed. 

Legal:   Strongly recommend putting clause in contract for build so that you don’t take handover of building until strong room conditions have been tested for at least 3/6 months and are satisfactory.  

Measurements.    Regular measurements of air, but also walls and floor.  Good idea to have archive boxes (with duplicate material or disposable material) to test how these are reacting too in different parts of the building.   Moisture levels of walls and floors took longer than air to reach appropriate levels. 

Take note of dates when pour concrete, build walls, what weather conditions are to accompany your data logger measurements etc. 

Moisture meters/sonar meters for walls/floors.  Look for patterns/comparative results over time rather than specific figures

Timescales:   between 6 and 18 months

·         With air conditioning, RH and temperature were both spot on within 6 months of building being finished. 

·         First six months – considerable variation – just keep measuring.

·         Passive stores take longer.   Recommend allowing it to go through full set of seasons. 

·         18 months x 1

·         12 months x 2

·         6 months x 4.

Harwell and Polygon:

·         Recommended by several folk, particularly when things go wrong, e.g. post construction mould outbreaks, e.g. where floors or shelving have been laid prematurely before concrete has dried out properly.

·         Industrial dehumidifiers available for hire.  No doubt there are other sources too.

·         Moisture measurement – specialists in drying out buildings after floods so have instruments and knowledge for measuring and certifying appropriate moisture content  (e.g. drying certificates for insurance purposes after buildings have flooded).

 

MANY THANKS,

 

Heather

 

Heather Forbes

Head of Archives Service

Gloucestershire Archives, Clarence Row, Alvin Street, Gloucester GL1 3DW

Tel:  01452 425299; email [log in to unmask]

 

 

 

 

 

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