Conventional methods of construction are not adequate to produce floor surfaces that are flat or level enough to support the operation of the sophisticated lift trucks in these wire guided, defined traffic, narrow aisle, high rack environments. Specialized methods of construction incorporating strip pour construction techniques are required (pictured above). High performance, low cost, safe operating environments are key benefits of Superflat floor installations.
Superflat floors are constructed to far tighter tolerances than conventional floor slabs using a specialized tolerance system call the “Fmin” (F minimum) tolerance system. The Fmin system is based upon defining the maximum variation in a floor surface for a particular lift truck. Optimizing the floor surface simply optimizes the performance of the lift truck operation. The operation of these specialized lift trucks on conventional surfaces can increase maintenance costs and create picking problems (including rack impact).
The control of highly accurate transverse and longitudinal tolerances is achieved through pouring the concrete in narrow strips using specialized formwork, materials and methods of construction. As exhibited below, these floor profile graphs indicate the differences over a 25′ length between a good quality random traffic floor surface (FF30/FL20) and a Super Flat floor (approx. Fmin80).
The FF30/FL20 floor (on the left) is a moderately flat random traffic floor tolerance. The FF117/FL110 floor profile (on the right) exhibits the typical flatness/levelness of a Super Flat floor. The special “Fmin” system of tolerance definition and measurement requires that any defects in excess of the maximum allowable variation be corrected by grinding, unlike the FF/FL tolerance system.
While the tolerance definition for each particular environment is somewhat complex, the table below provides a rough approximation of the Fmin tolerance requirements for any floor based upon the maximum rate of change per foot of travel in the longitudinal direction for a lift truck. Note that data should be obtained from your lift truck manufacturer.
|Maximum Rate of Change Per Foot of Travel||0.040″||0.050″||0.060″||0.070″||0.080″|
|Approximate Fmin Tolerance||Fmin100||Fmin80||Fmin65||Fmin55||Fmin50|
Super Flat Essentials
We strongly recommend the following for building owners and their representatives when specifying and designing Superflat floors:
- Strip pouring is mandatory. Construction methods for narrow-aisle high-rack automatic guided vehicle (AGV) environments necessitate the use of strip pour techniques because of the difficulty in achieving these very demanding longitudinal and transverse tolerances.
- There are NO transverse sawcuts in superflat floors. Tolerance losses caused by drying shrinkage curling at transverse contraction joints can easily render a superflat floor unusable. Transverse cracking is therefore to be expected (which remarkably stays flat). The designer should reinforce against these longitudinal shrinkage stresses as well as for applied loading. Longitudinal drying shrinkage stresses can create wide cracks if left unrestrained and it is desirable to restrict crack widths to avoid damage to equipment, to minimize the cost of crack repairs and to reduce any future maintenance costs.
- It is recommended that adjacent strip placements be smooth steel doweled to prevent differential vertical movement across construction joints and preserve future change of use.
- Fmin tolerances apply to defined traffic floors. “FF/FL” F-Number tolerances are for random traffic surfaces and not for defined traffic environments. FF/FL simply does not address the longitudinal and transverse equipment tolerances.
- Floor tolerances must be measured on a daily basis using highly sensitive “Profilograph” equipment. If measurements are not taken daily, then defects in construction can be compounded unnecessarily. If the floor isn’t measured, then the floor surface quality is unknown and cannot be “certified”.
- Ensure that embedded continuous steel is located so as to not interfere with the guide wire electrical signals (Note: carefully review proposed existing surfaces). Steel fibres have proven to not interfere with guide wire signals. Ensure that any floor coatings are also reviewed for their ability to emit a static charge as well.
- Generally, moderate and higher rates of dry-shake hardeners may be incompatible with the achievement of Superflat tolerances. Common surface solutions rely upon a reduced wear concrete mix (maximum 0.50 w/c) and a liquid densifier (each environment requires careful consideration).