Not all concrete structures are created equal. Also, not all “stressed” concrete is created equal. This article takes a detailed look into the difference between pre-stressed and post-tensioned concrete…
The difference between pre-stressed and post-tensioned concrete is the time at which the stressing bars or cables are tensioned in relation to the construction sequence of the concrete element being built. Pre-stressed concrete cables or bars are stressed prior to pouring and curing of the concrete, post-tensioned concrete is stressed following pouring and partial curing of the concrete.
Principles of Pre-Stressed and Post-Tensioned Concrete
For a detailed look at exactly how post-tensioned concrete slabs and beams work, take a look at THIS article. For a quick crash course, read on below…
Both pre-stressed and post-tensioned concrete operate under the same principle. With both systems, compressive stresses are introduced to the concrete element which reduces the tension stresses that may be caused due to bending. This plays to concretes inherent strength (good performance under compression) and reduces exposure to its inherent weakness (poor performance under tension).
The name “post-tensioned concrete” is a little misleading. The tensioning really occurs to the stressing bars or cables within the concrete, not the concrete itself. On the contrary, the resultant tensioning of the cables or bars produces a compressive reaction within the concrete element itself. We will look into this a little further now…
What is Post-Tensioned Concrete
Post-tensioned concrete systems are mostly used in settings where the concrete element is constructed in-situ (on-site). Prefabricated concrete members generally adopt pre-stressing systems.
Post-tensioned concrete systems generally comprise the following elements:
- A “live” end where the cable or bar is stressed
- A “dead” end where the cable or bar is embedded into partially cured concrete.
- Post-tensioning cable or bar (depending on the type of system adopted)
- A grout duct which houses the cable or bar and prevents contact of the cable or bar with the poured concrete of the member.
The construction sequencing of post-tensioned concrete may take the form of the following steps:
- Formwork is set-up and propped
- Bottom layer of conventional reinforcing is placed on the workwork
- Post-tensioning ducts are placed above the bottom layer of reinforcement, then post-tensioning cables are fed into the ducts ensuring that the cables protrude from the ducts at both ends.
- The top layer of reinforcing bar is laid above the post-tensioning ducts/cables
- The concrete is poured and its surface smoothed. Initial curing is undertaken (for more information on concrete curing and how curing may cause slabs to crack, take a look at THIS article).
- 24 hours after the concrete has been poured, around 25% stress load is applied to the post-tensioning cables or bars using a stressing jack. This is to control the effects of cracking caused by shrinkage of concrete during the curing process. At around 5 days after the concrete has been poured, the remainder of the stress load is applied to the post-tensioning cables or bars using the stressing jack.
- After the cables are stressed, the ducts are grouted providing a bond between the post-tensioning cables or bars with the concrete element which has been constructed. The propping is removed, and the concrete beam/slab is loaded as per its design loading.
What is Pre-Stressed Concrete
Pre-stressed concrete is more widely used in a prefabrication or factory setting. This is because rigid formwork elements are required to restrain the stressing bars or cables during the construction sequence.
Pre-stressed concrete systems generally comprise the following elements:
- Pre-cast casting bed with end restraints which support the dead and live ends of the stressing bar or cable
- A “live” end where the cable or bar is stressed
- A “dead” end where the cable or bar
- Stressing cable or bar (depending on the system being used)
As you can see, the elements of a pre-stressing system are very similar to the post-tensioning system. The main difference is the sequence of the construction. Let’s take a look at the construction sequence of a pre-stressed concrete element now…
- Bottom reinforcement is placed in the pre-cast bed
- Stressing bars or cables are placed within the pre-cast bed
- Tension is introduced to the bars or cables using a stressing jack
- Top reinforcement is installed above the stressing bars/cables
- The concrete is cured and finished
- The stressing bars/cables are released from the pre-cast bed which results in a “transfer” of the stress from the cable/bar to the concrete member.
- The concrete member is generally then craned or lifted into its final position and loaded as per its design loading.
When the stressing bar is tensioned, it undergoes elongation (that is to say that it becomes longer). After the concrete has been poured and cured, when the stressing bar is released, it tends to return back to its original non-elongated state.
However, the presence of the concrete around the bar and its bond with the stressing bar itself prevents this shortening from occurring and is the mechanism by which the tension stress within the bar is “transferred” to the concrete element, resulting in a compression force within the section.
A product which is used widely in Australia and utilises the pre-stressing system is the Hollowcore Plank. These are prefabricated and pre-stressed sections which can be placed side-by-side to construct a floor slab. The planks are then structurally tied together through an in-situ topping slab which is poured after the planks are installed.
What are the Advantages of Post-Tensioning over Pre-Stressing?
Both systems of post-tensioning and pre-stressing offer advantages and disadvantages over one another.
Post-tensioning systems do not require specialised restraint as part of the formwork system as it uses the concrete member itself to restrain the cables during the stressing procedure.
The nature of post-tensioning also allows cables to be profiled (draped or curved) which allows for flexibility in design. This allows the design engineer to place the tensioning cable at the most efficient and appropriate location within the member which may vary along the members length. Pre-stressing however is generally restricted to a straight bar application, this means that the stressing bar is located in a fixed position within the member for the full length of the member.
Because pre-stressing generally occurs in a factory setting, this can allow for higher grade concrete mixes to be adopted. This can result in larger spanning members and higher performance.
Pre-stressing also allows the flexibility for prefabricated members to be constructed off-site and lifted into place. Depending on the site constraints and the structure in question, sometimes this construction technique is desired for safety and site access reasons.
The following table provides a summarised comparison between the two systems…
|Pre-Stressed Concrete||Post-Tensioned Concrete|
|Stressing of bar or cable occurs before concrete is poured||Stressing of bar or cable occurs after concrete is poured|
|Generally constructed off-site in a factory||Can either be constructed off-site or in-situ on-site|
|Stressing bar or cable runs flat within concrete section for the length of the member||Stressing bar or cable can be profiled (draped) to allow varying height along the length of the member.|
|Can cater for high strength concrete applications||Less flexibility when it comes to high-strength concrete for cast in-situ applications.|
|Due to construction constraints generally, single spans can be constructed||Can cater for multiple span and continuous large floor plate arrangements|
|Friction losses of stressing bar or cable are lower||Friction losses of stressing bar or cable are generally higher|
|Level of quality control is higher due to being constructed in a controlled environment||Level of quality control is generally lower due to applications of in-situ construction and latent site conditions.|