Why use Sheaths for Prestressed Concrete

High tensile strength metal cabling being drawn through a sheathing of corrugated steel, has become a common observation on modern-day construction sites. Using this sheathing process in the production of prestressed concrete has immense integral value. It assists this revolutionary product to be used in a variety of applications where normal reinforced concrete products struggle to compete. For example, the construction of high-rise buildings, roofs, floors, dams, and bridges with longer, unsupported spans are among its numerous uses.

All building projects are required to conform to local, national, and sometimes international structural quality regulations. The processes of producing the finished product of prestressed concrete are not exempt from these. Therefore, the elements of the product’s composition have to equal or better these stringent regulatory standards.

When civil engineers, building contractors, and other participants in the South African construction sector select to utilise corrugated steel sheathing, the national regulatory stipulations are very clear. The corrugated steel sheathing needs to comply with regulations pertaining to its integrity, tensile strength, and other technical requirements.


How does Sheathing Assist in the Production of Prestressed Concrete?

Before discussing the various benefits of using corrugated steel sheathing in prestressed concrete, let’s first analyse an example of how sheathing assists in the production of this unique product.

Inherently, concrete is resistant to compressive stresses, but its resistance to tension is much lower. To assist in providing concrete with tension strength (or tensile capacity), the concrete has to be reinforced through compression. To accomplish this procedure, the concrete is tensioned, either during manufacture or during construction.

The principle behind tensioning is such that compressive stresses are introduced intentionally. High-tensile strength steel cables or rods, called tendons, are used to induce compression in the concrete before weight loads are applied to them. This will allow an equal but opposite force that will balance out applied stresses imposed on the product throughout its lifetime.

There are essentially two methods of performing the tensioning process, however, tendons are more popularly encased in, or pulled through a corrugated steel sheathing. This example is referred to as post-tensioning.

In post-tensioned concrete, the tendons are tensioned after the concrete has been poured. In one sought-after method of prestressing, the sheathing can be pre-set within the concrete pour (before casting). In this case, the concrete is cast around the sheathing which has been included in the pouring mould.

The sheathing forms a duct within the concrete bed, and tendons are placed in or drawn through the often-used corrugated steel casing. Once the concrete casting has cured sufficiently, the tendons are tensioned with hydraulic machinery and anchored into place. In this example, which represents bonded-type tendons, the cavity left inside the sheathing, is grouted with a sufficient amount of compressed mortar slurry, to ensure a fixed bond.


Corrugation is Key with Armco

Armco Superlites’ corrugated steel sheathing doesn’t look good only for aesthetic appeal. Each crest and trough of the spiralled corrugation wave is specifically designed to meet South Africa’s strict building codes. The Armco sheathing waves are carefully calculated to maximize the complete filling of the cavities between the tendons and the sheathing walls when the high-pressure mortar slurry is applied. This is crucial in ensuring a tight bond between the inner wall of the corrugated steel and the tensioned tendons.

Armco’s corrugated steel design allows more than sufficient bonding to secure the tendons to the sheathing wall. It also ensures the subsequent bonding from the outer wall of the corrugated steel to the structural concrete cast. Not only does it more than compensate for the transfer of force required for the compression process to take place, but the completed product also guarantees corrosion protection for the encased tendons.


Slurry Tight with Armco Superlite

The ability to withstand the transfer of force for tensioning to take place is only one of the many durability tests that Armco’s corrugated steel casings have to undertake through the prestressing process. During the pouring of the concrete cast, Armco’s sheathing stands up to any examination and surpasses all of the South African and African building regulation codes.

Correctly positioned low in the moulding, it is rigid enough to support the weight of the concrete casting. The tough, corrugated steel, also excels at forming the cavity required for the tensioning tendons, with minimal to no intrusions from the concrete into the prestressing duct. Additionally, its solid corrugation design prevents damage to the tubing as the tendons are passed through them.

Inversely, leakage from the applied high-pressure mortar slurry mixture into the cavity, for the bonding of the tendons to the inner wall, is equally minuscule. The grouting process is completed effortlessly, without water and other mixing liquids being forced out of the sheathing and into the concrete casting. Thus, preventing clogging or the segregation of the intrusion slurry.


Armco Armour, The Corrugated Steel Stalwart

Armco’s corrugated steel sheathing resists leakage and remains indentation and buckling-free, withstanding the harshest construction site conditions. From manufacture to prestressing, sheathing has to endure rigorous mechanical stresses. Because of this, Armco Superlite products are manufactured from lightweight, low-cost, 0.4mm, uncoated mild steel.

With its characteristic spirally locked seam, rendering the tubing virtually waterproof, the final product is tough and resistant to harsh treatment, providing outstanding quality with favourable characteristics. For example, precise sizing uniformity, shape consistency, exceptional strength, and rigidity, whilst maintaining flexibility.

The sheathing is manufactured in diameters ranging from 40 to 120mm and is available in standard lengths of 12m, or shorter upon request. Additionally, the corrugated steel tubing is provided with a strong, robust screw coupling to enable on-site assembly.


Malleable is Meaningful

Our corrugated steel sheathing is tough and durable enough to sustain difficult site conditions whilst forming the prestressing ducts. It is also rigid enough to support itself between supporting points without sagging. However, Armco Superlite takes pride in the knowledge that the sheathing remains malleable enough to remain sufficiently flexible to accept the course of the tendons being drawn through it.

The resultant ability of our corrugated steel to sustain rigidity, whilst remaining flexible, remains one of its most valuable assets. Tendons inserted post-casting without internal stiffening, are when Armco’s sheathings’ longitudinal rigidity becomes invaluable. Equally, the corrugated steel’s flexibility is as valuable when factory-manufactured tendons, supplied to the construction site on spools in looping rings, need to be ducted.

Using Armco’s corrugated steel in the manufacture of our sheathing due to its ability to sustain rigidity and malleability, has these and many other benefits. The ability to be stretched and concertinaed during the positioning and installation of the product is advantageous. However, to have the ability to retain its transversal tensile strength throughout the prestressing process is another of its indispensable attributes.


Armco’s Benefits for the Budget

Any construction project aims to complete a lasting example of engineered excellence on time and within budget. From the very first spade-full of turf, however, there are many obstacles in the way, conspiring against these ultimate outcomes. Thankfully, with the advancements in modern engineering, there are inversely equal, but opposite solutions to these challenges to balance out the budget.

Armco’s corrugated steel has all the attributes to become your sheathing product of choice when prestressed concrete is a prerequisite. It is tough enough to endure any construction site, saving thousands in damaged or destroyed sheathing lengths, and has all the additional advantages already mentioned.

Setting the fact that these structural benefits will be delivering perfectly required results aside, our corrugated steel sheathing is manufactured with lightweight materials and is extremely cost-effective. Additionally, it saves days in labour time wages because of how quick it is to install.

It decreases concrete costs significantly because of the shallower pour, therefore having a direct impact on delivery transport costs. It is easily stored on-site and Armco’s renowned reputation for service delivery excellence will give you a consistent supply, on demand. As tough as our products are, however, we can still supply them protected in bubble wrap for your storage convenience.


The Right Brand for the Right Product

Armco Superlite has had its foundations firmly founded in the construction industry since the 1930s. We have an accomplished history of accolades and successes. This history and our outstanding, target-driven service delivery have recognised us to be an internationally renowned, leading brand name within this industry.

We are listed in accordance with the BSI ISO 9001: 2015 quality scheme which ensures the quality of all our products and services provided by Armco Superlite. Our corrugated steel sheathing products are fully compliant with all the rules and regulations relating to the African construction industry. Both our Isando and Randfontein plants have been licensed under the National Environment: Air Quality Act (Act No. 39 of 2004) which is constantly overseen and strictly managed.

If you are planning a build where an integral part of the construction includes prestressed concrete, consider consulting with Armco Superlite. Allow one of our carefully selected civil engineers to professionally and accurately advise you of our offerings. We cater to all sectors of trade and industry and include both private and public companies on our proud listing of previous projects. We have a nationwide and inter-Africa footprint that has left a trail of resounding success. Contact us for more information.


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