Prestressed Cables are used in prestressed concrete, where concrete is cast into pre-assembled forms with rebar and steel cable reinforcement. These cables are tensioned to approximately three-quarters of their ultimate strength before concrete curing.
In this article we will cover:
- What are prestressed concrete cables?
- Prestressing cables – how it’s done
- Prestressed cable specification
What are prestressed concrete cables?
Prestressed or pre-tensioned cables are the core component to prestressed concrete used in construction. The process simulates the forces to which the material will be exposed under normal circumstances, which increases its tensile (pulling) strength. Rather than pressurizing the concrete itself; however, cables within the concrete material are prestressed. These cables or tendons can be made of a variety of materials, like steel, carbon fiber polymers, or other synthetic fibers. They are then woven or braided into cables of different thicknesses depending on the need. The pre-tensioning process means the resulting concrete has a much higher tensile strength than concrete with only reinforcing bars (rebar) and is virtually indestructible when compared to concrete poured without either rebars or prestressed cables.
Prestressing cables – how it’s done
Prestressing cables is a precise feat of engineering. The cables are set into channels within a concrete mold and anchored firmly, before they are stressed in a carefully planned and calibrated process. The tensions can be stretched to up to 75% of their total load capacity. After the tensioning process is complete, the concrete is poured and allowed to harden and cure. Once the curing process is complete, the end anchors holding the cables are released. When the cable anchors are released, Newton’s Law of Static Motion ensures that the pressure of the cable tension is transferred to the concrete itself, allowing the material to counteract the tensile stresses encountered after installation.
As long as the bond between the prestressed cables and the concrete itself is complete, the prestressed concrete is capable of withstanding forces much higher than plain poured concrete. Although the cables can be produced in any number of thicknesses before being stressed; thinner cables are generally used, because this increases the surface area of contact between the cured concrete and the cables, which allows more tension to transfer into the concrete.
The production process of prestressed concrete has been developed over the years to the point where concrete can be poured in long forms with the cables embedded within. This allows for long, continuous pieces of concrete to be created and then moved to the construction site, which minimizes the number of single pieces needed for a project. By minimizing the individual pieces of concrete needed to complete a build, fault lines are decreased, which increases the strength of the overall project. Additionally, the strength of the prestressed cables allows for less supporting structures, which facilitates open-space design concepts and allows bridges to cross wide expanses without support.
When considered a construction project where you want to maximize the tensile strength of a concrete surface or structure, it is important to work with a certified, experienced, and trusted contractor, who can advise about the choice of prestressed cables, particularly in relation to post-stressed cables, which are tensioned after the concrete as been poured, hardened, cured, and bonded. Post-stressed cables have different uses than prestressed cables, and your contractor will know which is appropriate for the project.
Prestressed cable specification
The United States, the United Kingdom, and Australia have regulatory standard requirements for the specifications of prestressed cables. In the United States, builders must adhere to standards like Rule 318 from the American Concrete Institute: Building Code Requirements for Reinforced Concrete. The specifications differ based on the application, and include the following parameters: material, standards, diameter, length, tensile strength, yield strength, elongation, relaxation, protective coatings, anchorage systems, overall stress, and types of ducts, and overall testing and certification process.
Materials – A prestressed concrete steel strand is most often composed of 2, 3, 7, or 19 strands of high-strength steel wire twisted together; although occasionally the strands are made of carbon fiber or synthetic polymers.
Diameter & Length – The individual tendons twisted together range in size from 9.0 mm to 29 mm. If bars are necessary instead of tendons, the width ranges from 10 mm to 50 mm. The length of prestressed cables varies entirely based on project requirements.
Tensile Strength – Tensile strength is calculated according to the maximum force that can be applied to a cross section of material. Tensile strength specifications for prestressed cables range from 1,860 Megapascals (MPa) to 2,230 MPa.
Yield Strength – Yield strength corresponds to the upper limit of force that can be applied to a material before it loses elasticity. The yield strength of prestressed cables is around 90% of the ultimate tensile strength of the material.
Elongation & Relaxation – Stress relaxation is the gradual tension loss in a cable, which results in a longer length. Reference standards for the highest thickness strand (28.6 mm) are 0.2% to 3.5% elongation of the original length and 2.5% maximum allowable relaxation of the original tensile strength.
Protective Coatings – To prevent corrosion, steel strands are coated with epoxy and all internal voids between the twisted strands are also filled with epoxy.
Anchorage Systems – The anchorage location for the cables can be a potential failure zone, so the prestressed cable anchorage zones must be designed for a minimum strength of 3,500 to 4,000 pounds per square inch (psi).
