The construction of the foundation has to be a fairly exact science. If everything is not calculated correctly, many issues can arise. Experts have devised many construction tricks and techniques that are used to avoid potential pitfalls during the building process. One of these involves the usage of plastic slip sheets. Using a slip sheet as reinforcement has been successfully used for many years in construction.
What is a Plastic Slip Sheet?
A slip sheet is a piece of plastic, typically polyethylene. It can be made in varying thickness, but it is often used in a 10ml form. Like the name suggests, the slip sheet is used to reduce friction and prevent movement or slippage. Using plastic slip sheets as reinforcement in foundation building helps to prevent cracking.
The Post-Tension Process
Slab-on-grade foundations are one of the most commonly used types of foundations in the United States. These foundations are quick and inexpensive to build (which is why they are so popular); however, they can be prone to cracking during the curing stage. Concrete can shrink as it dries; this can cause cosmetic damage to future floor finishes like tile, vinyl, wood, and carpeting. The type of soil - called expansive soil - is what ultimately causes the shrinkage and cracks. Expansive soil shrinks and swells as the water content changes.
To avoid the cracks, a process using plastic slip sheets is generally used. This process is called post-tensioning. Post-tensioning compresses the concrete, which helps it to resist shrinking and cracking that can be caused by the difficult soil conditions. In order to post-tension a slab foundation, steel strands are run through the concrete in a squared grid pattern. Each strand of steel is covered in plastic sheathing that protects the steel from corrosion and allows it to move during the stressing operation. The strands have an anchorage on each end. Each strand, along with its sheathing and anchorage, is called a tendon.
During the post-tensioning process, the tendons are usually stressed along one end; however, sometimes they are stressed along both ends. When they are stressed, the steel stretches -- but it is kept in place by the anchorage. This works to compress the concrete, which, in turn, works to keep the concrete from cracking.
How are Plastic Slip Sheets used with Post-Tensioning?
The plastic slip sheets are installed directly below the slab, or between the slab and the bearing surface. They work to allow the tendons to move the slab smoothly across the ground. Without the slip sheets, the tendons would not have a flat, even surface across which to move. The tendons must be able to move easily across the ground in order to correctly stress the steel, as this is the only way in which the concrete will be compacted in order to avoid shrinkage and cracks.
If builders were to neglect to install plastic slip sheets - or if they were to install them incorrectly - the slab would resist moving, which would prevent the post-tensioning process from being successful.
Getting goods from A to B is hard enough. Finding the right way to carry those goods is another headache. Once, we loaded and unloaded goods item by item - those were the days when labour was cheap. Then the Second World War came. This mother of many inventions brought us the wooden pallet. This, combined with a fork lift truck, enabled goods to be moved quickly and with less labour.
The wooden pallet was a great idea. So good was the idea that it is still going strong to this day. However, things have changed. Pallet pooling can be an expensive business. Rental and purchase costs have risen to reflect the increase in timber costs and the extra burden of fumigation and quality control.
Escalating fuel costs have taken their toll on the inevitable dedicated trips that are made to repatriate empty pallets when imbalances occur in the supply chain. These fuel costs - and driver shortages - have also forced an examination of the space and weight taken up by the ubiquitous pallet.
The pallet is a bit like the air that we breathe. It is all around us yet we don't see it. It is easy to forget the extra cost that the pallet adds to the value chain. Perhaps now the time has come when, in some eyes, the pallet has outstayed its welcome. Rather than being the carrier, is the pallet itself being carried by the value chain? Rather than wait for another world war, a ready solution has been found and is tried and tested. That is the Slip Sheet.
Enter the Slip Sheet
Slip sheets have been around for some time, especially in the
U.S. where the free market has always had the knack of taking the path of least resistance. European countries tend to prefer centrally planned systems and standardization. Slip sheets lack firmly agreed international standards and this may be one of the reasons why they have not been popular in Europe. The greatest reason is ignorance. Many simply have no understanding of slip sheets and how they work - so they don't bother.
So perhaps it's time to right this wrong here and now.
How Slip Sheets Work
1. Slip sheets are durable sheets of material that are designed to carry a uniform load of goods with plan dimensions (or "footprint") of roughly 1 metre square/40 inches square (typically they may be designed to carry a footprint of 40 inches x 48 inches - 1m x 1.2m). The slip sheet is made to these dimensions plus a pull tab - an extension to the sheet - of up to 6 inches/15cm. There may be just one pull tab sticking out from under the load on one side. There could, however be pull tabs on 2, 3 or all 4 sides. Slip sheets can be made of
fibreboard or plastic.
2. Goods are loaded directly onto the slip sheet within the creases that border any pull tabs. A push-pull attachment on a forklift truck is used to drag the load onto the truck by gripping (one of) the pull tab(s). The push-pull attachment has very flat ground-level forks that are designed to go under the
slip sheet whilst it is under load
3. To unload, the forklift truck pushes the load, complete with slip sheet off of itself and withdraws
Pros and Cons
Slip sheets are not the panacea to all distribution needs. For one thing, all parties in any supply chain will need to invest in the push-pull attachments, which are not cheap. The staff training that will be required, especially for forklift truck operators, can also be expensive. Perhaps the greatest factor, though, is the uniformity of the load itself. This exposes the slip sheet's greatest advantage - and greatest disadvantage. Slip sheets may not be able to cope when boxes are awkwardly shaped or where there is a mixed load of different shapes. Conversely, if a company is shipping out large quantities of the same sized box, then slip sheets are ideal. As such, slip sheets lend themselves to operations further upstream of the supply chain. As loads are broken down further downstream, then pallets may be more appropriate. The good thing is that loads of slip sheets can be easily transferred to pallets. A good example of the advantages of using slip sheets is where parts for goods are manufactured and shipped through a long distance sea route. The space and weight saving can result in a significant financial boost for the value chain.
Co-operative Value Chains
Another great factor in all this is uniformity of slip sheet systems. All parties in the supply chain need to be using the same specification of slip sheet and ensure that all staff are fully trained to ensure delivered loads are configured correctly for the receiving party. This requires value chains that are highly co-operative, or, to be realistic, it requires a value chain that has one dominant party who will simply dictate their requirements. The U.S. has pioneered slip sheets and has used them for many years. However, the dominance of the supermarket sector in some European countries may be forcing the change there also. The ability of large supermarkets to influence their supply chains means that they are in a position to take a strategic view of the benefits that slip sheets can bring. They are in a position to arrange for both senders and receivers to be ready to handle products on slip sheets. Things are changing. Perhaps the pallet industry is about to get a jolt.
Watch this space. There are tremendous savings to be made and significant environmental benefits to be gained. Full conversion to slip sheets for any supply chain is not going to happen, but a partial conversion for particular product lines is not only desirable, but inevitable - even in Europe.