Post-tension concrete is a stronger, more durable form of concrete that resists cracking without the need for heavy reinforcements. The post tensioning process yields a product that is just as lightweight as untreated concrete, but with a higher degree of tensile strength. The result is a versatile building material ideal for foundations, parking structures, and similar applications — something every construction crew wants.
If you’re considering replacing an aging concrete structure or building a new one, post-tensioned slabs might provide the most durable and cost-effective material for your needs. This guide will walk you through everything you need to know about the material, including what it is and why it’s being used more frequently in concrete construction.
What is Post-Tensioned Concrete?
Concrete is versatile, but it has several significant weaknesses — such as low tensile strength. It holds up well to compression, meaning it is unlikely to fail under heavy loads, but is also prone to cracking when stretched. A few things to keep in mind:
- Tensile stresses come from normal service loads and natural thermal expansion, both of which apply stretching forces to a concrete structure.
- Left in its natural state, concrete easily cracks under the strain.
- Steel, on the other hand, is known for its excellent tensile strength.
Post-tensioned concrete is concrete that has gone through a prestressing process — known as “post-tensioning” — to improve its tensile strength. The process leverages these mechanical properties to reinforce concrete with lightweight, high-strength steel strands, eliminating the need for heavy rebar while still improving durability.
How to Post-Tension Concrete to Help Withstand Tensile Stresses
The post-tensioning process puts concrete into a permanent compressive state, which helps to counteract stretching forces that might be encountered later in the concrete’s working life. The post-tensioning process involves the following steps:
- Run tendons through preformed ducts in the concrete slabs.
A post-tensioning tendon is composed of one or more long, coated steel strands encased in a protective duct or sheathing. Each is made from high-strength steel that can withstand more than 200,000 psi (compared to approximately 60,000 psi for the average piece of rebar).
- Secure the reinforcing steel to the exterior of the concrete members.
Each tendon, whether it contains a single strand or multiple, must be clamped securely to the slab edge.
- Stretch the cables.
Engineers apply a predetermined stretching force to each tendon, pulling the strands with a hydraulic stressing jack until an optimum tensioning force is reached. Proper tensioning force is typically around 33,000 pounds of pressure, although more or less tension may be required depending on the thickness of the slab. As the prestressing steel stretches, the concrete compresses.
- Anchor each tendon in place.
At the ideal tensioning force, each tendon and strand is permanently anchored in place. This ensures the steel remains in tension while the concrete remains in compression, effectively neutralizing future tensile stresses.
Post-tensioning works because concrete that is being compressed can better resist tensile forces. Conversely, steel that is being stretched can better resist compressive forces. Compressing concrete while stretching steel puts each material into its strongest state, preventing slabs from cracking when they encounter future stretching forces.
Post-tensioned concrete offers a number of key benefits:
- Lighter materials than reinforced or rebar concrete
- Stronger-than-ordinary concrete
- Reduced necessary depths for concrete structures
- Versatile enough to be used with both long spans and short, sturdy foundations
- Increased cost efficiency during and after construction
- Can be performed at the job site to increase efficiency
These benefits have made post-tensioning an extremely popular method of prestressing structural concrete.
What is a Post-Tensioned Concrete Slab?
A post-tensioned concrete slab is a block of structural concrete that has undergone the post-tensioning process. These slabs contain prestressing steel tendons anchored at the ideal length to keep each strand under tension.
Post-tensioned concrete slabs enable lighter, stronger concrete construction projects of all kinds. Some of the primary applications include:
- Parking structures
- High-rise apartments and condos
- Commercial buildings
- Roofing structures
- Slab-on-ground foundations
In general, if a project can be completed with standard concrete slabs, it can likely benefit from the reinforcement of post-tensioned concrete slabs.
Prestressed versus Post-Tensioned Concrete
While all post-tensioned concrete is prestressed, not all prestressed concrete is post-tensioned. The difference is that some prestressed concrete goes through pre-tensioning rather than post-tensioning. Both techniques are prestressing methods that use steel tendons to counteract tensile strain, but they differ in when the tensioning step occurs in construction.
This process involves setting tendons through ducts after the concrete has been placed. The tendons go through ducts in the concrete, and the tendons are then stretched to create tensile stress.
This process involves setting cables, wires, or tendons before the concrete has even been poured. As in post-tensioning, the tendons are set at a predetermined length to optimize stretch, but are laid out in the mold in advance of pouring.
As a result, different types of prestressed concrete gain strength in different ways. In post-tensioning, tendons are inserted at their normal length, then stretched and anchored in the hardened concrete slab. The compression comes from the concrete resisting the tension of the sheathed, anchored steel cables.
In pre-tensioning, the steel begins at its point of maximum tensile strain and the anchors are released once the concrete has hardened over it. The tendons “want to” return to their normal length, but they can’t because they’re encased in concrete. This tension is what exerts compressive force on the concrete without ducts and anchors.
Contact the Experts for Help with Your Questions
The team at Limitless Paving & Concrete is made up of experts in concrete construction and reinforcement, including rebar and prestressing. We understand concrete’s benefits and limitations, and we have extensive experience in developing custom construction solutions for clients across industries. If you have a project coming up, our experts are on hand to answer your questions. Contact Limitless Paving & Concrete today.