Aug. 06, 2024
Hardware
Growth in the global construction industry is expected to explode over the next decade, with output volume increasing 85 percent from to , according to a study by consultancy Global Construction Perspectives and Oxford Economics. The report, titled Global Construction , forecasts that China, the United States and India will lead the way with 57 percent of that increase, and most spending will go into infrastructure.
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More construction should translate into higher demand for construction lubricants, from the engine and transmission oils that go into mobile equipment to hydraulic fluids used in cranes and greases used in machinery and other processes.
Kolkata, India-based lubricant and grease maker Balmer Lawrie has been focusing part of its research and development efforts on greases used for concrete reinforcement, specifically for use with steel cables used for post-tensioning systems. In recent years, use of a pre-stress technique has increased because it can actively prevent cracks in the finished concrete, said N. Parameswaran, chief manager of R&D at the company.
With the conventional method of reinforcing concrete with steel rebar, the concrete slab or beam starts to bend when heavily loaded, and the concrete structure gradually starts to crack, Parameswaran explained at the National Lubricating Grease Institute India Chapters annual meeting in February. While the steel holds the cracks together, it cannot prevent them.
Pre-stressing the reinforced concrete uses steel strands, typically referred to as tendons, to hold the concrete together and prevent cracks from forming even under heavy loads. Pre-stressing can be done by applying tension to the steel tendons either before the concrete is poured or after it has hardened.
In pre-tensioning, tension is applied to the tendons using a jack against end abutments to which the tendon is secured. The concrete is cast, then the ends of the tendon are cut free from the abutments. In post-tensioning, the concrete is cast and the jack is placed against the concrete beam itself. Tension is applied to the tendon, which is then held in place by anchors at the ends of the beam.
The popularity of the post-tensioning method has risen recently due to its inherent advantages in the construction process, Parameswaran said at the meeting in Guwahati, India. In the U.S., Europe and also in China, the post-tensioning process is widely used in the construction industry, and in the last 10 years, the volume of construction by post-tensioning process has tripled, he said, adding that the method is also gaining ground in India.
Advantages of post-tensioning include the ability to use less concrete, significant reduction in building weight, higher load-bearing capacity with thinner beams and slabs, longer structure life, better corrosion resistance and flexible construction options, he noted.
Post-tensioning can be used in all facets of construction, such as office and apartment buildings, parking structures, bridges, and rock and soil anchors.
Basic Requirements
Post-tensioning tendons are manufactured from seven strands of steel wire. These strands are coated with corrosion-inhibiting grease during tendon formation and encased in a high-density polyethylene protective sheath.
In addition to preventing corrosion of the steel tendons once they are inside the concrete, the post-tensioning grease must also prevent corrosion at the anchorage; reduce the friction between the strands as well as between the tendon and the polymer sheath; resist hardening or softening the plastic sheath; and otherwise be compatible with the plastic material.
Other important properties include minimal oil separation, resistance to water emulsification, and a passing score on long-duration salt-water corrosion testing and salt-water soak testing, Parameswaran said.
Performance requirements for post-tensioning grease are governed by the United States-based Post-Tensioning Institute and Switzerland-based International Federation for Structural Concrete (FIB).
Parameswaran said the selection of thickener, base oil and additives are critical considerations for meeting PTI and FIB specifications. Lithium or lithium-calcium thickeners can be used to meet the required dropping point of 150 degrees Celsius or higher (ASTM D566).
The right combination of base oils is required for compatibility with the polymer sheath material, causing less than a 15 percent change in hardness, 10 percent change in volume and 30 percent change in tensile strength (ASTM D, PTI only). Base oil also affects oxidation stability, which the FIB requires to be less than 0.06 megapascal pressure change at 100 C in ASTM D942 after 100 and 1,000 hours.
A superior type of antioxidant is required to meet the severe oxidation resistance requirements, Parameswaran noted. Sturdy corrosion inhibitors allow the grease to meet PTI and FIB limits in several different corrosion tests, including ASTM B117 (Standard Practice for Operating Salt Spray (Fog) Apparatus).
Polymer Compatibility Study
Balmer Lawrie, which is one of Indias largest grease suppliers, formulated lithium grease with API Group I, Group II and naphthenic base oils to evaluate their compatibility with the HDPE polymer sheath used in post-tensioning systems.
The government-run company developed greases with four different combinations of base oils: Grease A (Group I only), Grease B (Group II only), Grease C (naphthenic only), and one with 25 percent each Group I and Group II and 50 percent naphthenic base oil-Grease D.
Results from ASTM D (Standard Test Method for Elastomer Compatibility of Lubricating Greases and Fluids) showed that the greases with Group I and II oils increased shore hardness and reduced the volume of the polymer, while the naphthenic grease softened the polymer and marginally increased the volume. The grease with a mixture of all three base oils produced the least change in hardness and volume.
Parameswaran noted the polymer compatibility values of all four greases were within the acceptable limits, but the grease based on a mixture of all three base oils showed the best results. The company selected Grease D for corrosion resistance and emulsion test using selected additives.
Corrosion & Emulsion Test
The researchers developed post-tensioning grease based on Grease D with two different corrosion protection additives. Three greases with different amounts of corrosion inhibitors were tested: 3.5 percent CP Additive I (Grease E), 3 percent CP Additive II (Grease F) and 4 percent CP Additive II (Grease G). All three formulations contained 1 percent of the same antioxidant additive.
The company evaluated all three greases for corrosion protection with the ASTM B117 method, using salt-water spray and distilled-water spray, and with the Emcor rust test, an FIB requirement. The greases were also assessed using salt fog and a soak (emulsion) test required by the PTI specification.
The experiment showed that Grease E, based on CP Additive I, met FIB requirements but failed to meet corrosion and emulsion test specifications required by PTI. Grease F and Grease G, which included different amounts of CP Additive II, passed the requirements for both sets of specifications.
Grease G far exceeded the number of hours to failure as required for salt fog test and emulsion test as per PTI requirement, Parameswaran concluded.
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The fully formulated Grease F and Grease G were further tested for all PTI and FIB parameters, such as oil separation, dropping point, oxidation stability and polymer compatibility. Parameswaran said the results showed these two greases had the best properties for the post-tensioning process.
Parameswaran believes that use of the post-tensioning method in Indias construction and infrastructure segment will double or triple over the next few years. Suitable greases will ensure long-term protection of steel tendons used in such applications, he noted.
It&#;s no secret why builders use post-tensioning systems in their construction; these systems are critical to strengthening concrete used in modern large-scale building projects. The same argument applies to post-tension slabs&#;a method used when pouring the slab foundation in commercial or residential construction. This article further explores post-tension slabs and some of the advantages and challenges of their usage in construction projects.
The concrete industry started experimenting with strategies and techniques to strengthen and reinforce concrete in the s. The prestressing concept, now a standard in residential and commercial building construction, involves using a steel cable or tendon to squeeze and compress concrete before the concrete endures a structural load. As the construction industry continued refining its techniques to standardize prestressing, most builders eventually began using post-tensioning techniques for reinforced concrete slab foundations. The Federal Housing Administration officially endorsed the use of post-tensioned slabs in , and the building method is now embraced and co-opted as the preferred method for builders.
Post-tensioned slabs contain a tendon, or steel cable, that runs crisscrossed throughout the center of the slab after the concrete has already hardened. As the poured concrete dries, the post-tensioned cables stretch and tighten, which applies a significant force to the concrete slab. This technique strengthens and compresses the concrete, which helps reduce cracking and structural instability caused by unstable soil conditions in marshes, swamps, and lake areas. The process helps residential homes and commercial buildings achieve greater load-bearing strength. Post-tension slabs are more durable and stable than traditional slabs that lack the reinforcement of steel cables.
Concrete slabs have inherent structural weaknesses; they are especially susceptible to expansion and contraction due to seasonal fluctuations and changing temperatures. Anyone that has experienced winter knows that thawing temperatures usually lead to potholes, broken water mains, and cracks in the pavement. Concrete slabs are no exception. The swelling and contraction of soils based on their contact with water is another element that adversely affects the integrity of concrete slabs. Poor and unstable soils found in Southwestern states and Mexico lead to issues with soil expansion, making it difficult for builders to lay concrete slabs.
Builders depend on post-tension slabs to reduce the risks of cracking, contraction, and soil expansion and strengthen the foundation&#;s structural integrity.
Post-tension slabs are stronger than traditional concrete slabs; however, that does not necessarily mean a post-tension slab is without challenges. Read on to learn more about builders&#; challenges when using post-tensioned slabs.
Post-tensioned slabs are an excellent foundation for a new structure, but builders must plan accordingly for proper drainage. Gutters, downspouts, roof pitches, and balconies redirect rainfall. Builders need to be aware of the location of drainage as it can seep into the foundation and lead to cracks.
Laying the foundation of a post-tension slab requires professional workers, quality materials, and expert knowledge to understand the job&#;s complexity. In the building and construction industry, it&#;s essential to research the service provider and contractors you partner with before signing a contract.
Since post-tension slabs require the addition of steel tendons to strengthen and reinforce the concrete, the tendons must precisely align as shown on the blueprints. Improper tendon locations can cause uplifting, as the tendon applies more force than the weight of the concrete. As a result, the concrete might be physically lifting the slab.
Aside from producing crack-free tennis courts, smooth parking garages, and a secure foundation for someone&#;s new home, post-tension slabs offer builders an array of advantages compared to traditional concrete slabs. Here are a few of the major benefits.
Post-tensioned slabs require less concrete than traditional ones, saving builders money. Post-tensioned slabs are an investment that helps homeowners reduce the risks of concrete contraction and cracks, which inevitably costs money to repair. That&#;s why post-tensioned slabs are an initial investment worth it in the long run.
A lighter yet stronger slab of concrete allows builders to design their structure with less material and overall surface space, providing more space to build supporting pillars, walls, columns, and beams. Post-tensioned slabs are also thinner, giving builders more creative freedom when designing floors.
No matter how much planning goes into a construction project, things happen, and the idea for builders is to minimize risks. Post-tensioned slabs may help prevent cracks from forming, but concrete is not immune to the elements. The good news is that if cracks form on post-tensioned slabs, they are held together and do not spread as quickly as those on traditional foundation slabs. Therefore, post-tensioned slabs are a more durable solution.
Designers use post-tensioning as a way to reinforce concrete by prestressing it. In prestressed members, compressive stresses are introduced into the concrete to reduce tensile stresses resulting from applied loads including the self weight of the member (dead load). Prestressing steel, such as strands, bars or wires, is used to impart compressive stresses to the concrete. Pre-tensioning is a method of prestressing in which the tendons are tensioned before concrete is placed and the prestressing force is primarily transferred to the concrete through bond. Post-tensioning is a method of prestressing in which the tendons are tensioned after the concrete has hardened and the prestressing force is primarily transferred to the concrete through the end anchorages.
Designers use post-tensioning as a way to reinforce concrete by prestressing it. In prestressed members, compressive stresses are introduced into the concrete to reduce tensile stresses resulting from applied loads including the self weight of the member (dead load). Prestressing steel, such as strands, bars or wires, is used to impart compressive stresses to the concrete. Pre-tensioning is a method of prestressing in which the tendons are tensioned before concrete is placed and the prestressing force is primarily transferred to the concrete through bond. Post-tensioning is a method of prestressing in which the tendons are tensioned after the concrete has hardened and the prestressing force is primarily transferred to the concrete through the end anchorages.
Unlike pre-tensioning, which can only be done at a precast manufacturing facility, post-tensioning is performed on the jobsite in cast-in-place applications. The concrete component is cast with steel reinforcing strands installed in a way that protects them from bonding with the concrete. This practice gives designers the flexibility to further optimize material use by creating thinner concrete members.
Unlike pre-tensioning, which can only be done at a precast manufacturing facility, post-tensioning is performed on the jobsite in cast-in-place applications. The concrete component is cast with steel reinforcing strands installed in a way that protects them from bonding with the concrete. This practice gives designers the flexibility to further optimize material use by creating thinner concrete members.
The materials used to post-tension concrete members are ultra-high-strength steel strands and bars. Horizontal applications (like beams, slabs, bridges, and foundations) typically employ strands. Walls, columns, and other vertical applications usually utilize bars. Steel strands used for post-tensioning typically have a tensile strength of 270,000 pounds per square inch (psi), are about 1/2 inch in diameter, and are stressed to a force of 33,000 pounds.
While concrete is strong in compression, it is weak in tension. Steel is strong under forces of tension, so combining the two elements results in the creation of very strong concrete components. Post-tensioning can help create innovative concrete components that are thinner, longer, and stronger than ever before.
While concrete is strong in compression, it is weak in tension. Steel is strong under forces of tension, so combining the two elements results in the creation of very strong concrete components. Post-tensioning can help create innovative concrete components that are thinner, longer, and stronger than ever before.
Many of today&#;s &#;high-performance&#; concrete structures, including many landmark bridges and buildings, employ some type of prestressing. Parking garages, high-rise residential towers, and many other kinds of structures also employ post-tensioning techniques.
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