A post tensioned (PT) concrete slab uses the compressive force of post tensioning cables to reduce the tension within the slabs cross-section allowing it to span further and have a thinner profile compared with a conventionally reinforced concrete slab.
How does a Post Tensioned Concrete Slab Work?
When a beam or slab bends downwards, the top portion of the slab goes into compression, and the bottom portion goes into tension. This is usually taught in Engineering 101 taken during the first year of University.
We also know that concrete is far stronger in compression than it is in tension. This is the reason for providing reinforcing bars within conventionally reinforced concrete slabs. The reinforcing bars are strategically located in the tension zones of the concrete to increase its tension capacity.
The introduction of post tensioning within the slab can result in the full cross section of the slab remaining in compression when it is bending. Taking the simple example from the previous image, we can cut a cross section through a portion of slab to see the stress distribution across the section with and without post-tensioning.
This can be demonstrated with a “toilet role test”. If you take several toilet paper rolls (its a COVID thing) and line them up with no connection to one another, the collection of toilet roles has zero bending capacity. This is because the system is missing a key element to allow bending to occur, tensile capacity.
However what if we apply a compressive force at either end of the series of toilet rolls and lift them, something interesting happens. The compressive force (applied by two hands for example) resembles that of the compressive forced applied to a post tensioned concrete slab by the post tensioning cables. The system can now bend because we have applied enough compressive force to the toilet rolls with a magnitude larger than the tension within the toilet rolls generated by bending. Tension capacity has not been added to the system but rather the need for tension capacity has been removed. This example, believe it or not, demonstrates how a post tensioned concrete slab works.
Components of a Post Tensioned Concrete Slab
In simple terms, the components which make up a post tensioned slab include:
- Concrete: Puts the concrete into PT Concrete Slab
- Post Tensioning: The assembly of the post tensioning cables ducts and embedment systems. This system provides the compressive force to the slab.
- Conventional Reinforcing Bar: Reinforcing bar (or rebar) is also used in conjunction with post tensioning cables within PT slabs.
Post tensioning cables have different parts which act together to make them work. Lets explore some terminology you may hear regarding post tensioning cables.
A post tensioning duct is a cold formed steel hollow tube which houses the PT Cables. The PT Duct is cast into the concrete slab during construction. The primary role of the PT Duct is to allow the PT Cables within it to freely move within the slab. The ducts may come in a flat or round profile (flat mostly being used in PT concrete slabs and round used in beams and bridge applications)
A post tensioning cables (or strands) are steel tension cables constructed from wound steel wires. Often several PT cables are located within the one PT Duct.
The live end protrudes from the concrete slab allowing the ends of the PT cables to be exposed. The live end is where the stressing jack is used to tension the PT cables. Within the live end mechanism is the flute, stressing block, lock wedges and cap.
The dead end of the PT cable is located at the opposite end to the live end. The dead end is where the PT cables exit the PT duct and are embedded within the poured concrete slab. The embedment into the concrete slab allows the cables to be stressed from the live and without being pulled out. The end of each PT cable is usually crimped to form an “onion” which allows the concrete to bond around each individual wound wire.
At locations where the live end of the PT cable is located away from the slab edge, a stressing pan needs to be provided. This is to allow the jack access to the cables for stressing. The pan is formed by a triangular plastic wedge which is embedder into the surface of the slab. After the cables are stressed and grouted, the stressing pan zone is also grouted to form a flat finish to the slab.
Edges stressing is where the cables are stressed on the vertical face of the slab/beams edge.
The stressing jack is the mechanism used to provide the tensioning load into the post tensioning cables. The jack is placed over the cables and uses a hydraulic slider to tension them
Anti Burst Reinforcement
Anti burst reinforcement (or spiral reinforcement) is reinforcement provided at the dead and live ends to allow effective transfer of the stress within the cables to the slab mass. High stresses are concentrated at both the dead and live end of the cables, this may cause local compressive failure of the slab. Introduction of the anti burst reinforcement prevents this from occurring.
How is a Post Tensioned Concrete Slab Constructed
The construction sequencing for a post tensioned concrete slab differs from a conventionally reinforced slab. Below is a step-by-step sequence of the construction methodology of a PT slab (similarities with a conventional RC slab highlighted in Green):
- Formwork is set-up and propped.
- Bottom layer of conventional reinforcing bar is laid on the formwork
- Post tensioning ducts are laid above the bottom layer of reinforcement
- The PT cables are fed into the ducts ensuring that the cables protrude from the ducts at both ends.
- Anti burst reinforcement is slipped over the dead and live ends of the cables
- The dead end onion is crimped onto the dead end of the cables.
- The live end Anchor is installed and the stressing pans (if required)
- The duct is taped at either end and the grout tube as well as breather tube is fed into each end of the duct.
- The top layer of reinforcing bar is laid above the PT ducts/cables.
- The concrete is poured and its surface smoothed
- Initial curing is undertaken (for more on curing and why slabs crack see the article at THIS link)
- 24 Hours after the concrete has been poured around 25% stress load is applied to the cables using the stressing jack. This is to control shrinkage cracking.
- At around 5 days after the concrete is poured the remainder of the stress load is applied to the cables using the jack. The total jacking force for each standard 12.7mm (0.5 inch) cable is 156kN (35,000 pounds).
- After the cables are stressed, the ducts are grouted along with the live end stressing pan or edge stress pocket.
What is the Benefit of a Post Tension Slab?
PT Slabs have several benefits compared to reinforced slabs. Due to the concrete being utilised to its strengths (compression), this allows longer spans and thinner slabs.
The self weight of a concrete slab in a high-rise building can be close to half of the design load on the structure. Therefore reducing the slab thickness can allow reduction in column sizes as well as depth and volume of foundations.
Developers of tall buildings usually prefer post tensioned slabs because the depth reduction either allows overall reduction of a buildings height which saves cost, or introduction of additional floors for the same height envelope. For example, a thickness reduction of 50mm (1.9inch) on a slab across 50 levels of a high-rise tower would allow another full floor of sellable floor area to be added to a building.
Keeping the building mass lower also reduces the seismic load during an earthquake. This is especially valuable in seismically sensitive regions.
The construction sequence of a post-tensioned concert slab may appear more laborious from the previous section. The reality is that while there are more steps involved in constructing a PT slab, the labour and therefore time involved in completing a full slab is less. This is because there are much fewer reinforcing bars to place in the top and bottom layer of the slab. Laying rebar can be one of the most time consuming and labour intensive activities when constructing a concrete slab.
How to Identify a Post Tension Slab
A post tensioned slab can be identified by the grouting of the live stressing end of the cables. Once the cables are tensioned and grouted, the live ends where the jack applies the stressing is also grouted. The grouting is usually a different mix to the concrete slab itself so usually has a different colour. This leaves a distinct tell tail sign of a post tensioned concrete slab. If the live ends are pan stressed, the grouting pockets are found on the top of the slab, if the cable are edge stressed, they are visible on the exposed slab edge.
Can I Drill into a Post Tension Slab?
This depends on the type of post-tensioning used. In Australia the most common form of post tensioning used in concrete slabs is bonded post tensioning. Bonded post tensioning simply means that after stressing of the cables has occurred, the PT ducts are fully grouted, this allows for a progressive transfer of the force from the cables to the slab. For this type of post tensioning, it is generally OK to drill or cut into the slab, provided necessary investigation works have been undertaken and advice from a qualified structural engineer is given (see below for more on this).
In most parts of Canada and the US however unbonded post tensioning is much more common (meaning the PT ducts are not grouted). This makes it much more dangerous when cutting or drilling into a PT slab. Cutting of the cables can result in quite an unsafe situation and negative effects on the slab.
Lets take a look at the bonded post tensioning case in Australia and what may happen if a PT cable is damaged or cut…
There may be up to around 5 cables within a post tensioning duct. Each cable has a higher yield stress than a conventional rebar. In Australia, the comparison between the two is as follows:
|Yield Stress of Reinforcing Bar
|Yield Stress of Post Tension Cable
|500 MPa (72 kip/square inch)
|1500 MPa (217 kip/square inch)
This means that cutting one of the cable is equivalent to cutting 3 conventional reinforcing bars. On top of this, because up to 5 cables may be bunched together within the one duct, all 5 cables may be compromised at once. In summary, this means that cutting one PT duct is equivalent to cutting over 15 conventional rebars.
Because of this, it is always best practice to scan the slab first using Ground Penetrating radar (GPR). This allows you to accurately locate the PT ducts and reinforcing bars before drilling or cutting the slab. Specialty contractors can carry this work out such as this contractor and this contractor who are both located in Melbourne.
Guidance and advice from a qualified Structural Engineer should be sought before drilling or cutting into any structure.
Under no circumstances shall a post tensioned concrete slab be drilled or cut before all the PT ducts are fully grouted.
What are the Design Considerations of a Post Tensioned Concrete Slab
To design a post tensioned slab effectively the full benefits of the compressive force needs to be exploited. This means that an adequate quantity of post tensioned needs to be provided within the slab to reduce the conventional rebar requirement as much as possible.
However too much post tensioning can have the reverse effect and require additional rebar, or worse, result in the compressive failure of the concrete. Therefore a “Goldilocks” approach needs to be taken. Below is a guide which can be used to provide stress targets for different post tensioned slab types.
|Less than 1.4MPa (0.2 kip/square inch)
|1.4 to 3.5MPa (0.2 to 0.5 kip/square inch)
|Good Target Range
|Greater than 3.5MPa (0.5 kip/square inch)
|High, may be used for transfer beams, take care to ensure limited slab restraint
The layout of the post tensioning also need to consider the installation and jacking sequencing. The live end locations need to be positioned to allow for the stressing jack to be placed on the PT cables. A clear gap of around 1000mm is usually adequate from live end to wall/obstruction.
The compressive force applied by the PT cables adds to the drying shrinkage effects of the slab while it is curing. The elastic shortening of the slab is additional to the shortening which the slab undergoes due to the concrete drying effects. For this reason, additional care needs to be taken to reduce the restraints upon the slab during construction. The slab should be free to shorten and shrink by introducing elements such as temporary movement joints and pour strips. Refer to THIS link for more explanation on why slabs crack and the effects of drying shrinkage on restrained slabs.