The floor is one of the most important parts of the building. In an industrial facility it’s the workspace on which all operations take place. Not only is it part of the structure, it is also an integrated part of the materials handling and workflow system. When the floor system becomes damaged it affects the efficiency of the facility and the bottom line profitability of the operation. Managing stress in slab on ground is key to ensuring minimum cracking, curling and warping and joint stability to ensure the lowest cost slab over the lifecycle of the facility.
The most vulnerable place on an Industrial Floor Slab is at the Joints. Spalling is the critical failure mode at the joint in a slab that will carry lift truck traffic of varying kinds. Spalling damage is the primary cause of reductions in the operating efficiency of industrial facilities contributing to downtime as well as high maintenance and rectification costs over the lifecycle of the floor.
Spalling is typically a result of curling stress. Differential drying shrinkage between the top and bottom of the slab causes the free edges of the slab panel to lift and lose contact with the base resulting in a cantilever effect at the perimeter of the slab panel. This curling or warping condition allows vertical deflection at the edge of the slab when loaded by the lift truck traffic. Joints break down when the lift truck moves towards the joint, deflecting the edge of the loaded slab down and then bumping into the edge of the adjacent panel causing spalling damage to the edges of the panels as the lift trucks move back and forth over the joint.
Load transfer in slab on ground can be described as the transfer of concentrated loads across a joint to an adjacent abutting floor slab. When a wheel load is applied to a slab, the load deflects the free edge of the slab end downwards. When an appropriate load transfer system is used at the joint both the loaded and the unloaded slabs deflect equally, spreading the load across both slab panels while keeping the top edges of the panels level to limit spalling damage.
Load Transfer Efficiency is essentially the measure of the magnitude of reduction in stresses and deflections by a joint utilizing appropriate load transfer controls in comparison to a joint where free edges are allowed to deflect.
Joint Stability is the measure of vertical deflection or movement at the joint created when a load passes over the joint from one adjacent slab panel to the next. Joint Stability measurements of 0.25mm for hard wheeled materials handling equipment and 0.51mm for soft or rubber wheeled materials handling equipment are deemed to provide good service life according to ACI Standards.
A properly designed dowel system limits the allowable deflection and transfers the load from one slab panel to the next across the joint to minimise the difference in vertical height between the loaded slab and adjacent slab to significantly limit damage and extend the working lifecycle of the floor.
The F-Number System is the American Concrete Institute (ACI117) and Canadian Standards Association (CSA A23.1) standard for the specification and measurement of concrete floor flatness and levelness. There are two F-Numbers: FF for flatness and FL for levelness. ‘Flatness’ relates to the bumpiness of the floor, while ‘levelness’ describes the local tilt or pitch of the slab. The higher the F-Number, the better the characteristic of the floor. F-Numbers are linear, so an FF 20 is twice as flat as an FF 10, but only half as flat as an FF 40. Industrial slab-on-ground floors are often specified with an FF number and an FL number.
Danley’s PD3® Dowel Cradles and Danley® Dowels conform to the requirements of the following industry design guidance documents:
-TR66: External In-situ Concrete Paving
-TR34: Concrete Industrial Ground Floors, A Guide To Design And Construction
-ACI 330.2R-17 Guide for the Design and Construction of Concrete Site Paving for Industrial and Trucking Facilities
-ACI 360R-10 Guide to Design of Slabs-on-Ground
The most efficient method of reducing stress in a newly poured slab is with an early-entry Soff-Cut made as soon as the concrete will support the saw and operator. This is generally within 2-3 hours of last pass of power trowel but needs to be assessed by experienced operators on the day of the pour to take into account the ambient temperature conditions and other influences. The purpose of the Soff-Cut is to relieve stress in the slab panel at predetermined locations to create planned cracks and limit unplanned cracking due to drying shrinkage and stress. Saw cut depth should be approx. 1/4 of the slab. It is critical that all joints are activated to relieve the stress and distribute the shrinkage evenly across the slab panel. This will maximise the reduction of stress and minimise the shrinkage and the joint gap at the free edge of the slab.
Plate Dowels have a wider cross sectional area than Round or Square dowels creating a wider bearing surface to more evenly distribute load, therefore reducing point loading and stress to lower the risk of concrete cracking or bursting. The PD3™ Dowel Cradle tapered shape allows for the lateral movement of the dowel without the need for plastic sleeves to create cavities. The absence of a plastic sleeve allows direct contact between the steel plate and the concrete therefore providing minimal chance of deflection, greatly reducing the risk of damage to the joint edges.
All Danley dowels (PD3™ and Diamond™ Plate Dowels, Round & Square) are made from Grade 300 mild steel.
The Dowel Cradle wire basket construction ensures the accurate placement and alignment of dowels at the correct centres and height within the slab. This significantly reduces onsite labour costs associated with processing formwork and reduces the risk of dowel placement and alignment error.
The patented PD3® design features a tapered and sleeveless plate dowel that provides world’s best performance in limiting joint deflection to provide superior joint stability. The PD3® Dowel provides the highest deflection control tolerance in line with ACI Standards recommendations to limit joint spalling, facilitate load transfer and provide the lowest risk of restraint to ensure the best serviceability outcome for the slab design. As drying shrinkage occurs the tapered plate forms a cavity that facilitates lateral movement to eliminate the risk of restraint without the need for a sleeve system. The elimination of the sleeve system ensures that there is direct contact between the concrete and plate dowel with no sleeve interference to provide the highest level of deflection control and therefore superior joint stability and serviceability outcomes. Use of the PD3® system extends the lifecycle of the floor to provide the highest level of floor efficiency for tenants and the highest return on investment for the asset owner.
Please call our customer service department and they will be happy to offer some suggested alternatives for consideration or direct you to our appropriate representative to call on you to discuss the needs of the project.