Index
What is Stressing?
Stressing is the process of inducing known persistent stress in a concrete structure or element prior to the application of the full or live load. Tensioned highly tensile strands, cables, or rods are mechanically attached to the prestressed portion to induce these stresses.
The following are two basic classification techniques for the Prestressing System:
Pre - Tensioning Prestressing simply implies that the tensile steel is strained (drawn or tensioned) prior to the service loads being applied to the concrete.
Post - Tensioning The concrete is poured first, and then the tension is added; nonetheless, it is still strained before the load is added, although it is still prestressed.
Equipment used in Stressing
Post-tensioning / pre-stressed concrete works involve the use of stressing equipment. Cemtech Infra Solution uses high-quality stressing equipment and accessories, such as the Stressing Jack, Compression Fitting Machine, and others.
Stressing Jack
Stressing Jack (also termed as Multi Strand Jack) is a short, compact, and very effective tool for simultaneously stressing all of the strands of a tendon. It is essential for speed and precision that all strands are stressed at the same time. It has a lot of versatility when it comes to matching different types and configurations of anchors.
Pump
A pump is a machine that is used to move fluids from one place to another. It's a hydraulic device that transports fluids through low to high pressure and lifts them from low to high levels. The pump moves fluid by transforming the mechanical energy of the fluid into pressure energy (hydraulic energy).
Grouting pump
A highly compact electrically operated cement grout pump, model J-600, is used to pour the cement inside the cable duct. This is a revolving positive displacement pump with dual cylinders. This is a more advanced version of the Grout Pump. It is used to grout vertical cables in cylindrical buildings such as concrete silos, chimneys, nuclear reactor containment walls, and so forth.
Hand operated grout pump is used for special applications in shorter cables and compact constructions. When the quantity of structural Grouting Mortar pumped is minimal and the cable hollow is tiny, this pump is employed. Because the pump is hand-operated and therefore does not require electricity, it is mostly used in remote areas facing electricity scarcity.
Materials used in Stressing
HT strand Low Relaxation Pre-Stressed Concrete Strands (LRPC strands or HT Strands) are used in pre-stressing concrete for various building projects. They are employed in concrete slabs that go on the rooftops of commercial and residential buildings, as well as bridges, tunnels, and aqueducts.
Wedges Wedges are used in pre-tensioned and post-tensioned concrete manufacturing. Wedges are heat-treated, tapered pieces of high tensile strength steel with embedded serrations that pierce the pre-stressing material during prestressing force transmission.
Barrels A barrel is a cylinder-shaped component that is installed over a single pre-stressing strand and distributes the strand stress to the anchor through tapered wedges situated within its interior conical groove.
Hydraulic Oil It is a non-compressible oil used in hydraulic machinery and equipment to transfer power. It's also a fluid located inside a piece of machinery that's utilized to convey power and is usually powered by a pump.
Process of Prestressing & Post-stressing
The process of intentionally creating permanent stress in a construction structure or system, to increase its durability is called prestressing.
You can study the diagram below for a more clear understanding.
Source: image
Need for pre-stressing:
Commonly in reinforced concrete consisting of steel and concrete, the compressive stresses are carried by the concrete, whereas the tensile stress is solely borne by the steel.
In this case, the concrete serves only as a binding medium and does not resist external stress.
In comparison, the prestressed concrete works on the principle that compressive strength is produced in the concrete structure by solid steel tendons before the loads are applied. This helps in increasing the beam's strength, thus balancing the tensile stress induced in the structure.
A proper combination of high-strength concrete and high tensile steel is important to get the most out of prestressed concrete.
The cube strength of concrete used in pre-stressing work should be 35 N/mm2 for post-tensioned structures and 45 N/mm2 for pre-tensioned structures.
Pre-stressing is most commonly used for mass manufacture of short span constructions pre-stressing, such as railway bridges, electricity polls, fences, pre-tensioned slabs, pre-stressed concrete bridges, etc.
Method of Prestressing:
There are two methods to induce compressive stresses in prestressed concrete:
Pre-Tensioning
Pre-Tensioning is a process of pre-stressing steels before they are concreted.
In pre-tensioning, a number of similar structures are positioned between the two stiff abutments or reaction bolsters.
Hydraulic jacks stretch the steel tendons or wires to the appropriate tension, generally adding 10% to account for pull and other pre-stress effects.
After that, they are sealed at two abutments after reaching the required elongation and then the concrete is poured into the blocks with stressed steel in place.
The cement hardens and shrinks, holding the steel across its length and transmitting the jacks' strain to the concrete, thus compressing it.
The prestressed wires are removed from the jacks after the concrete has achieved suitable strength.
Post-Tensioning
Source: image
Post-tensioning is a process where prestressing steel is stressed after the concrete has reached its initial strength.
Two of the structure's extreme ends are termed reaction faces, against which the force is applied. The formwork is then filled with ducts and reinforcement, and the concrete is poured.
After the concrete has reached the ultimate strength, a certain number of Prestressing Steel is introduced into both ducts for stressing purposes.
After the cement has been hardened, a spiral reinforcement is commonly used in the design given the high tension at the anchoring sites.
The wire or strands are fastened to retain the prestress after the required stress is obtained.
The unit's ends are cemented with the concrete mix to prevent corrosion from retained moisture and to help distribute tension.
Advantages of Pre-stressing:
To increase the holding strength of the structure - The tensile stresses in the concrete are not applied until after the final shape is achieved. Pre-stressing reduces concrete's tensile instability. These components are durable and remain free of fractures and can withstand the impacts of stress, jolt, and load reversal more effectively than R.C.C. structures.
To reduce cracking propagation in the structure - The concrete acts as a binder, stiffening the structure.
To reduce freezing in cold regions - To prevent frost heaving, the steel is typically uncoated.
To reduce settlement and cracking - The steel wires tense the concrete within the blocks.
It allows precast component manufacturers to greatly extend the design variation available.
The disadvantage of Pre-stressing:
Prestressing is an expensive procedure when compared to other methods of strengthening concrete. Predominantly high tensile steel is used in the process, which results in the expensive unit prices of high strength materials. Moreover, the usage of prestressing machinery, labour and shipping costs and its installation incur additional upfront costs.
Process of Grouting
The process of cement grouting, in civil engineering, means injecting pumpable materials into a soil or rock formation to modify its physical qualities. During civil engineering projects, it is one of the ways for regulating groundwater.
Grouting is appropriate when soil permeability would place a high reliance on pumping or where subsurface conditions would make drilling wells economically inefficient.
Grout can also be used to build pile foundations, ground anchors, under-reaming, underpinning, roads, dams, and other structures. Grouting is often done by drilling pipes or boring pores into the ground and then pumping a high-pressure grout solution via inserted tubes.
Investigation of ground conditions and computation of a boring pattern determine the amount of structural grouting necessary for a certain region. This takes into account the hole's size, spacing, and depth.
Composition of correct grout mix
Fill a drum halfway with water, then add the grout powder and stir thoroughly. To minimise air infiltration, mix grouting powder mechanically with water in an optimum ratio using an electric mixer at low speed (max 500 RPM). The mixing ratio can be modified to get the required consistency and flow characteristics.
Major Stressing Equipment Manufacturers in India
JK Prestressing
Industrial equipment, particularly the ones used in the construction industry, is designed with durability and load-carrying capability as the primary considerations. The threshold of workmanship is elevated even higher when it comes to large industrial machines and equipment that must withstand extreme loads without jeopardising the structural system integrity they support.
J K Prestressing Co., which was founded in 1954, is known in the industry for its constant innovation and products that live up to their quality and durability claims. They are a prominent manufacturer, supplier, wholesaler, distributor, and exporter of huge industrial tools and materials such as Pot Bearings, Machinery Lathes, Expansion Joints, Elastomeric Bearings, and more. They have a large and well-functioning infrastructure unit in Haryana, India, that assists us in producing a great assortment of items that meet worldwide standards.
Dynamic Prestress
Prestressing technology is becoming a rapidly increasing sector in the construction business, especially for constructions, bridges, metro, viaducts, nuclear power plants, and other structures. Dynamic Prestress, an ISO-certified prestressing expert, offers a state-of-the-art facility for the development, manufacture, and testing of relevant products and equipment required in the prestressing business.
For the previous two decades, this company has been delivering custom prestressing solutions to the construction sector. They have engineers and supervisors that are technically trained and have extensive expertise in the prestressing sector. They've contributed supplies and technical skills to a variety of international projects in a variety of nations.
Gamzen India
Gamzen is a civil construction machine manufacturer and exporter with extensive expertise in making construction machinery. They have Reversible Drum Mixers, Mobile Batching Plants, Pilling Tools, Piling Winch Machines, and other equipment.
Gamzen is a company that explores new possibilities in mechanical engineering for a variety of construction industry areas. They assure the finest quality products for their consumers through quality management at every manufacturing stage. All of their products are widely supplied across domestic and international markets.
Centpro Engineering
Centpro Engineering Pvt. Ltd., situated in Pune, India, provides a diverse range of construction products and services. They have a wealth of knowledge and expertise in designing process equipment to relevant standards and norms.
They are also ISO certified and follow the highest quality standards. They are a one-stop shop for all of the process industry's needs, such as high-quality materials and equipment, on-time delivery of all orders, and so on.
Jay Pee Engineers
JayPee Engineers is a high-quality pre and post-tensioning wedges and barrels manufacturer situated in India. It is an ISO-certified company that sells quality products in both domestic and international markets.
Barrels and Wedge Strands for all sorts of constructions have become a speciality of the company. Prestressing anchorage coupling systems, post-tensioned anchors, prestressed and post-tensioning grip systems, open grip barrels and wedges, and other products are among their specialities.
Summary
Prestress concrete is a type of construction concrete. During manufacture, it is "prestressed" (tightly compacted) in a way that reinforces it against tensile pressures that will occur in service.
When compared to ordinary reinforced concrete, prestressed concrete allows for greater spans, lower structural thickness, and material savings in a variety of construction and civil projects. Tall structures, household slabs, foundation structures, bridge and dam constructions, silos and tanks, industry pavements, and nuclear containment buildings are examples of typical uses of prestressed concrete.
Stressing is a highly technical and specialised process in the construction sector, and requires heavy machinery. Cemtech Infra Solution Pvt. Ltd. specialise in pre & post-tensioning of PSC girders & structure construction.
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