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.
Acrylic is an extremely versatile material which is used in a wide range of environments. Acrylic sheeting is available in a huge range of variations, including; clear, coloured, mirrored, frosted and even with a brushed aluminium effect.
Extruded acrylic and cast acrylic are the two main types of sheet acrylic you will come across. Although they may initially look similar, the two have quite different properties and one is also much more expensive than the other.
It is important to be aware of the differences between the two so you can pick the type most suitable for your needs.
Extruded and cast acrylic sheets differ mainly in their thermal and chemical properties which are caused by the manufacturing process. There are also several other differences as outlined below which you may wish to consider when choosing acrylic sheet for your home DIY project.
Key differences you should be aware of when working with acrylic sheets:
Due to the way the sheets are made cast acrylic tend to have slight variations in the thickness of the sheet whereas extruded sheets are much more uniform, with next to no variation. This makes extruded acrylic ideal for vacuum forming or situations where more detailed precise shapes are needed.
Cast acrylic has an isotropic response to temperature meaning there is a maximum shrinkage of 2% in all directions. It may be important to note that acrylic expands and contracts at a much greater rate than glass. Extruded acrylic, on the other hand, shows differences in shrinkage depending on the thickness and direction of extrusion.
Benefits of Extruded sheets
Benefits of Cast acrylic sheets
Both types of acrylic benefit from being durable and long lasting, showing excellent resistance to natural aging. Most manufactures will guarantee against noticeable yellowing for approximately 10 years.
Often it may not be made obvious which type of acrylic you are buying, especially online. Cast acrylic can be as much as double the price of extruded acrylic so if nothing is mentioned and it seems cheap, the chances are it is extruded acrylic.