Dealing with wide-opening joints

There are occasions when floor slab usage requirements force designers to have joints which they know will open very wide. A wide joint in a floor slab is clearly a disadvantage because it will lead to damage to the trucks bearings (especially with small hard wheels as on motorized pallet trucks), damage to the items being carried and of course damage to the floor arris edges.

Damage to floor arris edges can be obviated with the use of a suitable Permaban armouring system in the design. Damage to existing slab arris edges can also be dealt with by using the Permaban Arris Repair System. However, the problems created by wide-opening joints remain a costly nuisance and impede efficient operation of the activities in the building.

Permaban  have experienced unavoidable wide-opening joints around the edges of post-tensioned floor slabs, in doorways to freezer store floor slabs and some large bay joint less floor slabs.

When the temperature of a concrete slab is reduced to -30 degrees  Centigrade it shrinks far more than, but additional to, the normal drying shrinkage. This causes the saw cut joints and the free-contraction joints to open much wider than normal. By producing joint sealants which can be installed at these very low temperatures, Permaban facilitate for the joint to be sealed after all the drying and thermal shrinkage have had their full effect on the joint opening.

There is a limit to how wide a joint can be treated only with joint sealant when the joint istrafficked  - Permaban would suggest 8 mm is a maximum limit for saw cut shrinkage control joints. Above this, and always for free-contraction joints in heavy duty situations, Permaban  would always recommend steel armouring of the arris edges by use of AlphaJoint as recommended in BS 8204 Part 2.

For situations where the joints are expected to open 20 to 40 mm then a sliding armoured joint with replaceable top-plates is required.

Image  1 shows a conventional approach to this, which Permaban provided for Morrisons 65,000 sqm distribution centre in Kettering in 2005 installed by Stanford Industrial Concrete Flooring (see "Concrete" Sept 2005 page 6).. The main floor area was post-tensioned resulting in a slab without joints but with a larger opening at the perimeter and to the slab adjacent to the dock levellersslab (See image 2).

The joint used at Kettering comprises 15mm thick x 60 x 60 mm angles supporting the 10mm thick x 100mm wide removable plate with 2 10mm thick side strips robust enough to protect the concrete arris. Installation and micrometer-accurate levelling was achieved by the use of Permaban AlphaFix Installers with the post-tensioning cables passing under the joint. No reinforcement or load transfer across the joint was required in this 300 mm deep slab. Before completion the screws down one side of the removable plate were taken out to allow free movement.

After twelve months the joint had opened some 40 mm. Permaban attended to make measurements for replacement plates and then attended site while the 140 mm wide stainless steel replacement plates were fitted. The overall width of the joint was 200mm necessitating great care when installing to ensure the joint was level horizontally across the joint.

By these means the 40 mm movement was accommodated whilst fully protecting the joint arris edges. Traffic passes over the joint without bumping and with no damage to the goods, trucks nor floor.

Particularly in food related premises, the design of the joint should prevent the gap under the joint allowing the ingressof vermin.

Since 2005, in discussions with various clients throughout Europe, we have seen the need for further development from the conventional approach. Also the other types of joints available for these situations such as the "S" configuration or "interlocking finger-type" joints are very wide to start with and unsuitable from hygiene and long term performanceconsiderations'  This  led to the development by Permaban of a new range of armoured surface-bridging joints with much better tolerance than can be achieved with conventional steel angles and overcomes the limitations of replacement plate type joints, "S" type and "finger type" joints with an entirely new approach of filling the shrinkage opening with an steel infill strip. The width of the new AlphaSlide joint is only 70mm, this reducing the potential for the joint being installed out of level horizontally, but still permits 40mm joint opening.Also Plate Dowels  are incorporated to provide load transfer and allow movement in 2 planes. Image 3 shows the AlphaSlide as cast with no joint opening.

When the joint opens as a result of the concrete shrinking the AlphaSlide  joint opens leaving a gap (See image 4).  This gap is then filled with a steel infill strip of the appropriate width  eliminating any gap in the joint. The 10mm thick steel infill strips are produced in various widths between 10 mm and 40 mm, are pre-drilled and counter-bored and  installed using socket head-cap screws into the captive nuts located on the support plate (See image 5).  If the joint opens further the infill strips can be simply replaced to accommodate any increase in the width  of the gap  thus ensuring  a smooth ride across the joint with no gap for dirt and detritus to enter and feed any rodents.  

Medipav recently completed a 9800 sqm post-tensioned slab in Perugia County, Italy. The Front Cover picture shows construction of this slab after the concrete has been placed and levelled with Somero SXP-D laser screed. The AlphaSlide joint shown is precision-installed with AlphaFix installers (See image 5).

The development of AlphaSlide compliments the existing range of robust metal leave-in-place joints which are available to suit all types of floor uses.

Some free contraction joints in concrete floor slabs can be predicted to open wider than should be acceptable to the building occupier. In these cases the designer is now able to specify a pre-fabricated joint detail to satisfy client needs for smooth-running, durability and hygiene.

1. [References]:BRITISH STANDARDS INSTITUTION, BS 8204, Part 2
2. Concrete Magazine, September 2005 Issue, Page 6: Joint armouring and load transfer