Eliminate rocking floor slabs

If you’re responsible for the maintenance of a concrete floor slab that supports materials handling equipment (MHE), there is a good chance you’ll encounter a problem with a rocking floor slab. Alan Yuill, Director of CG Flooring Systems looks at the remedial solutions.

A rocking floor slab will become apparent at a loose floor joint, where the concrete slabs on either side of the joint move up and down independently of one another. This is generally noticeable when a dynamic load, such as a moving forklift truck passes over the slab and exposes the joint arris (edge) to impact damage from the MHE. A clue to this could be the thudding sound heard as the truck drives over the joint. The resultant spalling and chipping of the joint arris can cause problems for forklift drivers and operational efficiency.

What is the cause?

There are a few possible reasons that could be the cause of a loose floor joint, such as curling of the slab or excessive drying shrinkage but the main reason is a lack of effective load transfer across the joint. If left untreated causes damage to the joint arris, structural failure of the floor and increased maintenance of the forklift trucks that traffic the floor.

A well-designed floor joint has the ability to transfer a vertical load. When the MHE drives over it, the concrete floor will move as one. In order to achieve this, floor designers will specify load transfer mechanisms typically dowel bars or steel reinforcement, both of which have a good record of success. Designers can also rely on aggregate interlock for load transfer, although this is less dependable.

In contrast, a loose or rocking joint has little or no effective load-transfer capability. This can happen if designers or contractors do not install a load transfer mechanism or if the specified device is inadequate.

Repair options

It is essential that the vertical movement at the loose joint is arrested. You can measure the amount of vertical deflective movement in the slab using a dipstick, an instrument used to measure floor flatness and levelness, and a forklift truck, preferably loaded. Simply set the dipstick across the joint, drive the forklift over the joint passing close to the measuring device and note the readings on the dipstick just before and after the truck rolls across the joint. A reading of zero indicates perfect load transfer; a reading above zero will mean stabilising the floor joints should be considered.

Once it has been determined which floor joints need to be stabilised there are five repair options available:

• total slab replacement

• sub-slab grout injection

• retrofit dowel bars

• full depth joint replacement

• joint stabiliser.

Total slab replacement is rarely done, unless the floor slab is seriously defective in addition to the floor joints and the full depth joint replacement leaves two joints in place of one. While both these options can provide a long-term solution, they are expensive and extremely disruptive to an on-going warehouse operation. Sub-slab grout injection doesn’t address the root problem of poor load transfer, therefore fails to provide a long-term fix.

A retrofit dowel is a solution where saw cuts are made in the slab across the joint, generally at 300mm centres, cutting approximately half way through the slab thickness. The dowel bars are placed in the slots and locked in place with a strong mortar. Getting the details right is essential and can be difficult, but when done well this method provides very effective load transfer.

The CoGri Joint Stabiliser is a mechanical device that provides similar results to those from dowel bars. It was developed for warehouse floors and is an aluminium cylinder, 76mm in diameter, split lengthwise, that is installed vertically into a core hole. An internal mechanism pushes the two sides apart with around 115MPa of force, locking the stabiliser into the concrete slabs and effectively bridging the joint. Internal springs enable the stabiliser to maintain the connection with the slabs as the joint opens and closes. Spaced at approximately 1m apart, the joint stabilisers can reduce vertical differential movement to less than a fraction of a millimetre. The joint stabiliser is less disruptive, requiring less downtime than any other joint stabilisation repair method and is usually more cost-effective.

The Solution

The issue of a rocking floor slab is specific to each organisation. There are a number of repair solutions available on the current market which work to varying degrees of success. The Joint Stabiliser, however, is a proven device tailored to each repair, which allows clients to maintain production with minimum disruption at a reasonable cost. Warehouse users should contact industrial floor specialist for advice on the most appropriate solution to suit their needs.

CG Flooring Systems Ltd

www.cg-flooring.com