Post-tensioning is a method of reinforcing concrete in which steel tendons are cast into the element inside protective ducts, then tensioned with a hydraulic jack after the concrete has hardened and gained a specified strength. Tensioning the tendons puts the concrete into permanent compression, which offsets the tensile stresses the element will see in service.
It does not increase the compressive strength of the concrete itself. A post-tensioned slab and a conventionally reinforced slab cast from the same mix reach the same compressive strength. What post-tensioning changes is how much load the element can carry before it cracks, because the concrete has to be pulled out of compression first. That is why post-tensioned members span further, run thinner, and crack less than the equivalent conventionally reinforced member.
The strength that matters on site is the strength at the moment of stressing. Tendons cannot be tensioned until the concrete has reached the compressive strength stated in the project specification, because the anchorage zones carry very high local stresses. Stress too early and you risk crushing at the anchorages and cracking around them. Wait longer than you need to and the whole floor cycle slips.
Understanding Compressive Strength vs. Tensile Strength
Ensuring your concrete is durable and stable isn’t just about achieving an acceptable level of compressive strength, which is the concrete’s ability to withstand loads that crush or squeeze it. You also need to consider its tensile strength, and the concrete’s ability to withstand loads that will likely elongate or stretch it. Since concrete naturally has a relatively high compressive strength and low tensile strength, contractors will often apply reinforcement techniques to increase the latter.
Concrete structures such as office buildings, parking lots, bridges, and sports stadiums require high tensile strength. Without them, the concrete will struggle under the weight it has to carry due to a lack of flexibility. This leads to cracking and weakening of the structure. For example, if a parking lot’s beams are subjected to heavy loads of cars without any reinforcement, it’s likely that the bottoms of the beams will expand. Even slight elongation in the concrete can cause cracking.
To prevent this from happening, steel reinforcing bars, or rebar, are embedded into the concrete to increase the concrete’s flexibility. Unfortunately, rebar only provides passive reinforcement. This means it will only bear loads or force once the concrete has already cracked.
Post-tensioned concrete comes into play as active reinforcement, unlike steel reinforcing bars.
What Is Post-Tensioning and How Is it Implemented in a Concrete Structure?
Post-tensioning was first patented by P.H. Jackson in the 1800s in San Francisco. It was refined to its contemporary form in the 1920s by Eugene Freyssinet in France. Unbonded post-tensioning was introduced to North America in the 1950s and has been widely used in buildings ever since.
The post-tensioning process works as follows:
First, you must install steel tendons into the formwork and position them before the concrete is placed. Steel tendons are prestressing steel cables inside plastic protective ducts or sleeves, each one with anchors on each end to transmit the forces into the structure. Because of the sleeves, the tendons are not placed in direct contact with the concrete.
Then, once the concrete has gained the required level of strength but before the service loads are applied, you’ll pull the cables tight (aka tensioning) by using a hydraulic jack and anchored against the outer edges of the concrete. Think of them as behaving like rubber bands. Keep in mind that the required strength before stressing the tendons is set by your project specification, not a fixed percentage, so confirm the target with your engineer of record before you begin.
What’s the benefit of tensioning or pulling the tendon-filled sleeves after the concrete is placed? Doing so allows them to be readily formed into the desired shape, such as incorporating vertical and/or horizontal curvature. Once the tendons are tensioned, it creates a force that counteracts the weight that is subsequently applied to the hardened concrete structure. This prevents cracking and increases its service life.
Steel tendons used for post-tensioning typically have a tensile strength of 270,000 pounds per square inch (psi), are about 1/2 inch in diameter, and are stressed to a force of 33,000 pounds. In contrast, a typical piece of rebar will yield about 60,000 psi.
Monitor your concrete strength during post-tensioning with maturity!
Confirming Concrete Strength Before Post-Tensioning
What has to be confirmed before tendons are stressed
| Item | Why it matters | How it is confirmed |
|---|---|---|
| Concrete compressive strength at the anchorage zone | Anchorages concentrate very high local stress into a small area of concrete. This is the governing check | The strength value stated in the project specification, demonstrated by the method the specification accepts |
| Which element the strength applies to | A cylinder cured in a tank is not the slab. Post-tensioning is a decision about the element in front of you | Field-cured cylinders, or an in-place method such as maturity monitoring to ASTM C1074 |
| Age and curing history of the pour | Cold placement, a cold night, or an unheated deck all slow strength gain regardless of what the calendar says | Continuous temperature record for that element |
| Stressing sequence and jacking force | Stressing out of sequence changes the load path through a partially cured element | Post-tensioning supplier’s approved stressing drawings and calculations |
| Formwork and shoring status | Shoring frequently has to stay in place through stressing | Approved shoring and reshoring plan |
| Sign-off authority | Stressing is not a field call | Engineer of record or their delegate, per the specification |
Everything above depends on one number: the in-place compressive strength of the element you are about to stress. The strength required before stressing is set by the engineer of record and stated in the project specification, and it is normally expressed either as a minimum value in psi or MPa or as a percentage of the specified compressive strength. It is not a rule of thumb, and it varies with the anchorage system and the element.
The complication is that a cylinder does not tell you about the slab. Standard-cured cylinders show what the mix can do under ideal curing. Field-cured cylinders get closer, but they still sit beside the element rather than inside it, and on a cold deck the difference is real.
SmartRock® sensors address that directly. They are fixed to the rebar before the pour and log the concrete’s internal temperature continuously from inside the element. Under the maturity method (ASTM C1074), that temperature history is converted into a maturity index and then into an estimated in-place compressive strength, using a strength-maturity curve calibrated in advance for the specific mix. The estimate reflects the curing conditions the element actually experienced.
Because the data is continuous, the team can see the element approaching its stressing strength rather than discovering after the fact that it got there overnight. Results are viewed in the SmartRock app on site and in SmartRock® Web, the cloud dashboard, where thresholds can be set so the crew is notified when an element reaches the value the specification requires. The sensor is non-destructive and stays in the concrete, so there is nothing to retrieve and nothing to repair.
Maturity data supports the decision. It does not make it. The strength target, the acceptable method of demonstrating it, and the sign-off all come from the project specification.
Post-tensioned concrete strength FAQ
What strength does concrete need before post-tensioning?
The required strength is set by the engineer of record and written into the project specification, usually as a minimum compressive strength in psi or MPa or as a percentage of the specified compressive strength. It depends on the anchorage system and the element, so there is no universal figure. Confirm the value in your specification, then demonstrate the element has reached it using a method the specification accepts.
Does post-tensioning make concrete stronger?
Not in the sense of compressive strength. The concrete’s compressive strength is a property of the mix and is unchanged by post-tensioning. What post-tensioning adds is a permanent compressive force that has to be overcome before the element goes into tension, which raises the load the member can carry before it cracks and allows longer spans and thinner sections.
What is the difference between pre-tensioning and post-tensioning?
In pre-tensioning, the strands are tensioned before the concrete is placed, and the force transfers to the concrete once it has cured and the strands are released. That is almost always done in a precast plant. In post-tensioning, the tendons sit in ducts, the concrete is placed and cures first, and the tendons are tensioned afterwards, on site, and anchored at the ends.
What happens if tendons are stressed too early?
The anchorage zones carry very high local stress, so stressing before the concrete has reached the required strength risks crushing the concrete at the anchorages and cracking around them. That damage is expensive to remediate and sits at exactly the point where the prestressing force enters the structure.
Can maturity data be used to authorise stressing?
On many projects, yes, provided the specification accepts the maturity method (ASTM C1074) as a means of demonstrating in-place strength and the calibration curve for the mix has been established beforehand. It is an accepted in-place method, not an automatic substitute for whatever the contract requires. Confirm acceptance before you plan the cycle around it.
**Editor’s Note: This post was originally published on August 12, 2020 and has been updated for accuracy and comprehensiveness in October 2026.
