Normal warehouse floors are designed to carry the loads that will be imposed on them, including forklifts, pallet racking systems or bulk product. These concrete floors are reinforced both to carry load and to stop unwanted cracking both during the curing process and thereafter for the life of the store. Storing new products should be done with care. I once visited a good friend whose office was on the third floor of an old CMT (cut, make and trim) building. All of a sudden his office door wouldn’t close. It didn’t take much investigation to reveal the cause. 12 pallets of sugar weighed rather more than fabric and were forcing the concrete floor downwards.
All concrete floors will crack. However this process can be managed by way of fibre, reinforcing, anti-crack mesh, expansion/contraction joints and saw cut joints. If rails are installed in the concrete floor either for cranes or mobile racking, then increased reinforcing is likely both to assist in carrying and spreading the increased rail point loads and to prevent the concrete floor from cracking around the rails. This can happen as embedded rails are crack inducers.
The actual design of normal concrete warehouse floors depends on the supporting ground beneath. If it is of a variable nature or not weight supporting, and the imposed loads will be relatively large, then piling may be needed.
Freezer store concrete floors or wearing slabs must incorporate all the requirements of normal warehouse floors with a few extras for freezing. If the temperature of the subfloor approaches freezing, usually plus 4 degrees C is taken as the danger point, then any water moisture in the subfloor will freeze. When water becomes ice, it expands in volume and has to be accommodated. This normally leads to the concrete floor rising and cracking as the rise is variable over the floor area. Ice will lift just about any weight which is particularly important when pallet racking is present.
As well as underfloor insulation, all freezer floors should have added protection from frost heave. In South Africa the current standard is the electric heater mat which is normally laid in three circuits in case there is a circuit malfunction. The mat only uses energy if the subfloor temperatures reaches 4 degrees or less. Some larger freezer stores are now using a warm piped glycol/water mixture which absorbs the waste heat given off by the condensers. These run continuously. The Australians favor underfloor 200mm diameter pipes through which warm air should be blown via a fan system. The pipes are laid at a slight angle to prevent moisture from lying in them as this might freeze, gradually closing the pipe.
I was in one freezer where frost heave had raised portions of the floor by over 800mms. The store had a good underfloor air ventilation system but the openings had been blocked with checker plate to stop rats from entering.
Mobile racking is especially affected by “frost heave” as the drive systems are designed to only operate on reasonably level floors. Freezer floors also experience a secondary contraction when cooled to below zero temperatures and it’s not unusual to see a 10 to 15mm gap between the concrete floor and the insulated wall panels. It is not a good idea to ‘cool’ a new freezer store quickly as it’s more likely to crack – especially around the door, a high traffic area.
In freezer stores there can be either contraction or construction joints. I have seen freezer stores up to 1600m2 without contraction joints where the reinforcing is designed to make the floor shrink inwards. In this situation one would expect to have about 120 to 130 kgs of reinforcing per m3 of poured concrete. Contraction joints should be designed so that both sides of the joint can move, but that the same level is maintained. These should be armored when in high use areas like gangways. We try and design mobile layouts s that any contraction joints are under fixed racks. In such instances where a double fixed rack spans a joint, the one side should only be bolted down once the floor has stabilized at its sub- zero temperature. Although tempting in a mobile store, construction joints should not be placed at rails as this encourages cracking and voids.
Cold store floors are the most important part of a freezer store and should be seen not just as a wearing surface but also as a foundation for the racking system. It is therefore worth taking time to get the floor right as subsequent repairs on a large scale at subzero temperatures are extremely difficult. For those who do have the occasional floor problem, Barpro stocks Spalpro 2000 and Febset 45.
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