Prestressed Concrete Properties

[Karlyn Hemmerling]

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Prestressed concrete is a structural material that uses steel to create predetermined engineering stresses within concrete members to counteract the stresses that will occur when they are subject to loading. This combines the high strength compressive properties of concrete with the high tensile strength of steel.

In ordinary reinforced concrete, stresses are carried by the steel reinforcement, whereas prestressed concrete supports the load by induced stresses throughout the entire structural element. This makes it more resistant to shock and vibration than ordinary concrete, and able to form long, thin structures with much smaller sectional areas.

Prestressed concrete was patented by San Franciscan engineer P.H Jackson in 1886, although it did not emerge as an accepted building material until 50 years later when a shortage of steel, coupled with technological advancements, made prestressed concrete the building material of choice during European post-war reconstruction.

Prestressed concrete is commonly used for floor beams, piles and railways sleepers, as well as structures such as bridges, water tanks, roofs and runways. Generally, prestressed concrete is not necessary for columns and walls, however, it can be used economically for tall columns and high retaining walls with high bending stresses.

As a general rule, traditional reinforced concrete is the most economic method for a span of up to 6 m. Prestressed concrete is more economical when spans are over 9 m. Between 6 and 9 m, the two options must be considered according to the particular requirements as to which is the most suitable option.

Wire is made by cold-drawing a high carbon steel rod through a series of reducing dies. The wire diameter typically ranges from 3-7 mm and may be round, crimped or indented to give it better bond strength. Another form of tendon is strand which consists of a straight core wire around which is wound in helixes around further wires to give formats such as 7 wire (6 over 1) and 19 wire (9 over 9 over 1). Similar to wire tendons, strand can be used individually or in groups to form cables.

Pre-tensioning prestressed concrete involves the stressing of wires or cables by anchoring them at the end of a metal form, which may be up to 120 m in length. Hydraulic jacks stress the wire as required, often adding 10% to accommodate creep and other pre-stress losses that may be incurred. Side moulds are then fixed and the concrete placed around the tensioned wires. The concrete hardens and shrinks, gripping the steel along its length, transferring the tension from the jacks to exert a compressive force in the concrete.

Once the concrete has reached the desired strength, the tensioned wires are released from the jacks. A typical concrete strength of 28 N/mm2 can be achieved by 24-hour steam curing, as well as using additives.

Post-tensioning prestressed concrete follows the reverse method to pre-tensioning, that is, the concrete member is cast and the prestressing occurs after the concrete is hardened. This method is often used where stressing is to be carried out on site after casting an insitu component or where a series of precast concrete units are to be joined together to form the required member.

The wires, cables or bars may be positioned in the unit before concreting, but bonding to the concrete is prevented by using a flexible duct or rubber sheath which is deflated and removed when the concrete has hardened.

Stressing is carried out after the concrete has been cured by means of hydraulic jacks operating from one or both ends of the member. Due to the high local stresses at the anchorage positions it is common for a helical (spiral) reinforcement to be included in the design. When the required stress has been reached, the wire or cables are anchored to maintain the prestress. The ends of the unit are sealed with cement mortar to prevent corrosion due to any entrapped moisture and to assist in stress distribution.

Anchorages used in post-tensioning depend on whether the tendons are to be stressed individually or as a group. Most systems use a form of split cone wedges or jaws which act against a form of bearing or pressure plate.

There are many different post-tensioning systems. For example, the Freyssinet system enables the stressing strands to be tensioned simultaneously using centre hole tensioning jacks, anchored by tapered jaws. This is suitable for pre-stressing elements up to 50 m in length.

The Macalloy system on the other hand, involves applying stress to the concrete by means of a solid bar, usually with a diameter of 25-75 mm. The bar is anchored at each end by a special nut which bears against an end plate to distribute the load.

Precast concrete uses raw materials mixed and poured into different shapes (slabs, panels) using a mold. The concrete then cures in a highly controlled environment for optimal quality. Builders can order the cured concrete slabs or panels, choosing from a multitude of shapes and surface finishes, and get the precast concrete slabs delivered to the doorstep.

Since the concrete that builders order is cast and cured in a precast plant beforehand, it is also called prefabricated concrete or pre-made concrete. Precast concrete manufacturing offers greater control than the traditional method of pouring concrete at the job site and waiting for weeks so it can cure in uncontrollable environmental conditions.

Precast concrete is considerably stronger than site-poured concrete. The main reason for its higher strength lies in the superior quality of materials, some of which cannot be locally sourced, and the curing process in a highly controlled environment. Additionally, for structural components, the strength of a precast concrete structure can be increased further by adding prestressed steel strands. This concrete is then known as prestressed concrete.

The compressive strength of concrete is the load it can withstand before failure. It is the best way to know the strength of concrete structures. The compressive strength of precast concrete with steel strands is about 7,000 psi (48 MPa). This concrete can withstand loads of more than 100,000 pounds, which is an astonishingly high value. Therefore, it is ideal for any construction, including high-rise buildings. Even without reinforcing strands, conventional precast has a minimum concrete compressive strength of around 4,500 psi (31 MPa).

Keep in mind that to obtain these high compressive strength values, it is important to use quality precast concrete components. Premier Precast is the industry leader in high-strength precast concrete production, testing the products and materials before delivering them to you.

Yes, precast concrete is much stronger than other types of concrete, such as traditional pouring concrete on site. Precast concrete construction is also more durable and saves more construction time than using formwork and cast-in-place concrete onsite. Experts regard precast concrete as the most durable option, and it uses more sustainable materials (MK Hurd).

However, for precast concrete, the curing takes place indoors in a carefully controlled environment. Factors such as humidity and temperature are optimal, providing ideal curing conditions for the concrete, and enabling it to achieve its maximum potential strength.

Professionals working with cast-in-place concrete often forget about permeability, which is a huge factor in determining the durability of any construction material, whether brick, stone, or concrete. Permeability measures how much water can pass through the pores of a material. Concrete with high permeability would allow water to seep inside and lead to the corrosion of the steel reinforcement within it. Therefore, concrete must have low permeability.

Precast concrete manufacturing employs the latest technology and techniques to ensure the proper mixing of concrete, resulting in extremely low permeability. There is no water seepage via the concrete pores, so corrosion of the reinforcement strands or bars is not a concern. Additionally, its low permeability and the mixing technologies used in precast concrete minimize voids, resulting in denser concrete with increased strength.

Precast concrete contains several chemical additives, so the panels are resistant to weather, acids, alkalis, ASR reactions, seawater corrosion, sulfate attacks, and many other harmful elements. Adding these aggregates is impossible for cast-in-place concrete due to sourcing difficulties of some chemicals or cost restraints. However, precast concrete utilizes economies of scale, so it is possible to use all the best materials without significantly increasing the cost of a project.

The use of concrete and EPS foam in manufacturing precast wall panels results in excellent thermal insulation properties. They resist hot and cold temperatures and do not need secondary outer insulation. Other additives within concrete products improve properties such as sound insulation and fire resistance.

Its long lifespan is another one of the precast concrete advantages. Precast products are designed to last for at least 100 years once installed. Additionally, they do not require any maintenance or repairs during their lifetime. In contrast, with a conventional concrete mix, the end product is expected to last a few decades, after which it needs significant repairs.

As the numbers dictate, precast concrete has significantly more durability and strength than concrete poured at the construction site. These factors are why many builders use precast concrete for most building requirements.

Premier Precast of Delray Beach, Florida, specializes in precast concrete solutions. It is one of the leading precast manufacturers in the US that can provide customized precast concrete panels in any shape, size, and surface finish you require.

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