Pre-stressed concrete beams are one of the modern materials in the construction industry, featuring unique characteristics such as the ability to be used in longer spans compared to other concrete ceilings and increasing the speed of execution operations.

This feature, along with other advantages that we will become more familiar with later, has led to these prefabricated components gradually gaining more applications and becoming a replacement for various common beams in construction, including block beams.

What is a pre-stressed beam?

Concrete is a material that is very resistant to compressive stresses but very weak in tension. If a concrete member with a compressive strength of 35 megapascals is subjected to axial tensile stress, when the average stress in the concrete reaches about 2 to 3 megapascals, the concrete cracks. In addition to the low resistance, the behavior of concrete in tension is unpredictable, and its failure is also sudden.

To eliminate this weakness in concrete, steel reinforcements are used to strengthen the concrete member, which ultimately results in a reinforced concrete section (concrete with steel reinforcement).

The use of reinforced concrete has been accepted and utilized for years in the construction of various building and industrial structures. Although this composite material has many advantages, it also has disadvantages such as increasing the weight of the structure and not utilizing the full load-bearing capacity of the concrete.

For this reason, and after extensive research conducted in the last century, a new type of concrete called pre-stressed concrete was invented and manufactured. Pre-stressed concrete is a type of reinforced concrete in which the steel reinforcement is stretched using mechanisms, and this stretching creates pressure in the concrete, allowing for greater utilization of its capacity. A pre-stressed concrete beam is a member made of pre-stressed concrete along with high-strength steel. In other words, a pre-stressed beam or "Schpenit" beam is one that is constructed using high-strength steel wires (1400 to 1900 newtons per square millimeter) and concrete with a minimum compressive strength of 35 megapascals in an inverted T-shaped cross-section.

Steps to manufacture pre-stressed beams

Now that we are familiar with the primary characteristics of pre-stressed concrete beams, it is necessary to see how these materials are made and what steps must be taken in the construction and execution of pre-stressed beams. We will then examine the advantages, disadvantages, and limitations of using them as one of the common types of beams in the construction industry. It is important to note that manufacturing pre-stressed beams requires technology and a production line with specific characteristics, along with engineers and skilled individuals in this field who possess special capabilities. In the previous section of the article, we became acquainted with the specifications that pre-stressed beams must have, such as the strength of the concrete and the required pre-stressing in the pre-stressing cables.

What types of pre-stressed beams are there?

These rings are designed for ease of transportation, and the hook-beam is installed in the main beam using hooks during installation and does not require a stabilizer.

Introduction to hook pre-stressed beams

Appearance of pre-stressed beams

The surface of pre-stressed beams is not flat, and this beam has a T-shaped cross-section. If you want to know how many rebars are used in this beam, just take a look at the beginning or end of this beam.

What sizes do pre-stressed beams come in?

After high strength and durability, this beam has another important feature, which is the length of the beam. This beam is manufactured in sizes from 1 to 12 meters, and its weight and volume are less than those of older beams. Ultimately, the appropriate strength and light weight of this beam have made buildings that use this beam more resistant to hazards such as earthquakes.

The weight of a pre-stressed beam should be greater than or equal to 28 kilograms per meter in standard condition. It should be noted that the reason for the high weight of pre-stressed beams compared to other beams is that the pre-stressed beam is entirely made of concrete in its cross-section.

Sometimes, there is a misconception that pre-stressed beams make the ceiling heavy; however, this view is completely incorrect, and on the contrary, pre-stressed beams actually lighten the ceiling. Calculations have shown that if a pre-stressed beam is used in a ceiling, approximately 7 to 9 kilograms of weight is reduced per square meter.

Execution of pre-stressed beams

One-way slab ceilings are formed from a series of secondary beams or prefabricated pre-stressed beams placed close together, with the space between them filled with clay, concrete, or polystyrene blocks up to 50 to 70 centimeters wide, and covered with a concrete slab with a thickness of 50 to 100 millimeters. In normal conditions, this slab does not require additional steel other than thermal and shrinkage reinforcements.

In Figure number 1, a ceiling with prefabricated pre-stressed beams is displayed. Dead and live loads are transferred through the slab to the beams and then from the beams to the main beams and finally to the columns and foundation. Additionally, instead of using lightweight permanent blocks, plastic or composite molds can be used, which can lead to significant savings in execution. These molds are placed between the beams and are poured with concrete as before; the mentioned molds are removed after the concrete pouring is completed.

What are the methods of producing pre-stressed beams?

The production method for pre-stressed beams is carried out using an extruder system and molds. We will explain these two methods below:

Extruder system method: This method is the only correct and principled way to produce pre-stressed beams and can be performed using European machines. In this method, concrete without slump is placed on a vibrating line and is completely compacted. Then it is processed using steam and heat. The use of Chinese machines in this method is not recommended as it reduces quality and production time. Additionally, manufacturers using Chinese machines will face problems obtaining standard certification in Iran.

Mold method: The use of molds for producing pre-stressed beams has been obsolete for some time due to numerous problems that arise in the produced pre-stressed beams. Some of these problems include cracking and hole formation in the pre-stressed concrete beam and a significant reduction in strength due to the use of fluid concrete. Furthermore, in this method, it is not possible to use concrete without slump due to the presence of molds. In addition to these issues, obtaining a standard license in Iran is also not possible in this method.

Quality of beams is an important point for buyers

Pre-stressed beams are mass-produced in large factories, and if this product has a smooth and polished surface, it is certainly of poor quality; because this type of beam contains voids that, due to the invalid production method, will damage the main structure in the future.

In factories, these beams are tested after production to obtain an exit permit, and the only factor for receiving an exit permit is the manufacture of this product according to international standards. Therefore, if you are using these beams for construction, be assured that the final quality of your building is equivalent to global standards.

Know the advantages of pre-stressed beams

The points we have mentioned so far regarding pre-stressed beams are part of their beneficial characteristics. Next, we intend to take a closer look at the advantages of using these ready-made beams. These advantages include:

1– Use of high-strength concrete (usually 40 to 50 megapascals) and high-strength rebars with a tensile strength of 1725 megapascals

2– Approved by domestic regulations (Chapter Nine of the National Building Regulations, Publication 250 Appendix ABA and Publication 543 and Publication 82 of the Management and Planning Organization of the country) and foreign regulations (American Concrete Institute ACI 318-19 and PCI and EN 15037-1:2008, etc.)

3– Approved by the Iranian Standard and Industrial Research Organization and has a standard certificate (ten-digit code 9081454998).

4– Reduction in the use of scaffolding and reduction of shoring under the ceiling by up to 75 percent.

Continuous quality control "considering the sensitivity of the production process" at a fully industrial production level using fully automated machines, eliminating manual concrete pouring and welding from the production process of beams. Quality control is performed on all components including wires, sand, and concrete, etc.

6– Given that the entire cross-section of the beam is under pressure from the stretched wires, the section is fully tested before loading.

7– Reduction in the amount of concrete used in the ceiling due to additional concrete around the beam of about two cubic meters for every 120 square meters of ceiling

8– No need to execute upper rebars (which must be executed in ordinary beams) and only the execution of shrinkage and thermal rebars and negative moment rebars

9– Resistant to cracking due to the initial compressive stress present and consequently impermeability to moisture, leading to longer durability and lifespan of the structure

10– Use of dry concrete with a minimum water-to-cement ratio (without slump) with a special mix design in the production of this type of beam

12– Possibility of execution up to a span of 12 meters and elimination of intermediate columns in the building

How do pre-stressed beams perform in buildings?

As mentioned at the beginning of this page, pre-stressed beams are a very suitable replacement for concrete-booted truss beams and are responsible for transferring the loads applied to the ceiling to the main beams. Interestingly, in reinforced concrete structures, beams are designed and executed as a two-ended beam structure. To create this beam, a minimum of three centimeters of concrete cover is essential.

Concrete-booted truss beams have a thickness of about 4 centimeters. However, a three-centimeter cover is not suitable for this beam, and this is just one of the problems associated with using this type of beam. Nevertheless, with the continuous advancement of technology and the increasing awareness of designers and engineers, the use of new products has become a priority. Therefore, pre-stressed beams today have more applications than other beams.

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