Post Tension Slab Problems and Advantages - Post Tensioning

Post Tension Slab Problems & Advantages

There are two reasons why builders used post tensions slabs during the construction of a home or building.

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A post tension slab is reinforced with stranded steel cables that are tensioned (tightened) after the concrete hardens. The cables are slid inside a plastic sheathing, like a wrapping. This prevents the cables from touching the concrete. When the concrete has sufficiently cured, the plastic sheathing is stretched. Stretching the post-tensioned cables applies significant force to the concrete system, lifting the slab into a compressed state, which reduces shrinkage cracks and cracks caused by difficult soil conditions.

Problems With Post Tensions Slabs

One of the biggest problems with a post tension slab is cracks. The cables laying in the concrete are generally not tightened until at least 7-10 days after the concrete is poured. Since the cables are not stretched or elongated, they cannot provide any crack control in those first few days.

Another reason for cracks is when a vehicle or another large object is placed upon the structure, and its concrete slab undergoes tension. To solve this problem, post tensioned steel tendons are placed when the concrete is poured and tensioned after with conventional reinforced bars.

A second issue with post tension slabs is that the footings anchor the foundation in place. The tension required to bind the additional concrete in the footings plus the dirt between the footings can’t be met.

Defects such as slab cracks can be costly to repair after construction is finished. Picture ripping up flooring or carpet to repair a small crack you may never notice. Trying to fix cracks can be time-consuming and expensive. Depending on the extent of damage, a family may have to leave their home while the work is being done. Talk about prices adding up.

Benefits of Post-Tensioning

Architects, Structural Engineers, General Contractors, Real Estate Developers, and Owners incorporate post-tensioning systems on their projects in order to achieve economy, efficiency, quicker construction and lower lifetime cost of the structure. Why? Because Post-Tensioning enhances concrete strength under both compressive and tensile stresses.

Post-tensioning tendons are installed in a parabolic profile prior to placing the concrete and later stressed to a specified concrete strength. This process introduces both compressive forces in the concrete and stresses that counterbalance service loads. The benefits are substantial.

Post-tensioning frequently solves design and construction challenges that other construction methods simply cannot. Some key advantages include:

MATERIAL SAVINGS

Thinner concrete member sizes; reduction in concrete is approximately 20%

Rebar in floor elements is reduced by 60% to 75%

Decreased dead load reduces rebar and concrete in columns and foundations

Reduction in building height decreases the cost of building cladding, vertical mechanical/service elements, and rebar and concrete in shear walls

QUICKER CONSTRUCTION

Potential pour cycle of 3-4 days

Reduced re-shoring requirements

Coordination with embeds and MEP openings

INCREASED PERFORMANCE

Improved seismic behavior

Reduced deflection and vibration

Improved crack control and waterproofing properties—especially beneficial for parking garages and balconies

Longer spans and fewer columns give greater flexibility in floor layouts in office/residential buildings and better lighting in parking garages which enhances personal safety.

REDUCED LIFETIME COSTS

Lower overall maintenance and lifecycle costs of the structure

Reduced building height also results in energy savings, especially for office buildings

Slab-on-grade vs. post-tension foundation design

Another advantage of post-tension foundation design is construction efficiency. The use of post-tensioning can expedite the construction process as it requires fewer concrete materials and simplifies the installation of reinforcing tendons. This can lead to faster project completion times and potential cost savings.

Post-tension foundation design does come with its own set of challenges that need to be considered by the contractor and end user.

Cost: Post-tension foundations typically require specialized design, materials, and installation techniques, which can lead to higher upfront costs compared to traditional foundations. However, the long-term benefits may outweigh the initial investment.

Expertise Required: Designing and constructing post-tension foundations require specialized engineering expertise and experienced contractors. Hiring professionals with knowledge in post-tensioning is crucial to ensure proper installation and performance.

Maintenance Challenges: While post-tension foundations are generally durable, they may require periodic inspections to monitor the integrity of the tendons and address any potential issues. Maintenance and repairs, if needed, can be more complex and costly compared to conventional slab foundations.

Limited DIY Options: Post-tensioning is a specialized technique that is typically not suitable for do-it-yourself (DIY) projects. Professional expertise and equipment are necessary to ensure the proper installation and tensioning of the tendons.

Risk of Failure: Although rare, if post-tensioned tendons are not properly installed or maintained, there is a risk of failure. This could lead to structural issues, including cracks or even collapses. However, with proper design, installation, and ongoing maintenance, these risks can be minimized.

An additional limitation of post-tension slab design is that it can’t always be used. If there is not enough space for the equipment or a grade beam does not have a defined exterior face to place the dead and live sides of the wire for the tensioning operating, those instances will require conventional rebar in a slab-on-grade design.

Post-Tensioning Institute > Education > PT Applications > Slab-on-Ground

Post-tensioned slabs-on-ground provide a cost-efficient, high-performance solution for problems associated with ground-supported residential foundations on shrink-swell soils. The compressive stresses resist the anticipated tension stresses induced by the soil movements, enhancing the performance over a non-prestressed foundation. Cost benefits are achieved by reductions in quantities of concrete, steel and excavations, which in turn reduce labor costs.

In less expansive soils, a uniform thickness foundation is utilized. Typical thickness ranges from 7.5- to 12-inches and any increases in material quantities are compensated by reductions in labor and equipment costs. With the elimination of stiffening ribs, a post-tensioned foundation can be constructed rapidly eliminating labor and equipment to dig the ribs and dispose of excavations. This is a substantial benefit in sandy soils where trenches require shoring. Post-tensioned foundations are also used in areas with stable soils to reduce cracking, reduce or eliminate control joints, increase flexural capacity and improve constructability. Reducing the control joints also improves the serviceability and eliminates durability problems.

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Advantages of using post-tensioning for your next slab-on-ground project:

Stronger/more efficient: Less concrete and steel are needed for the same structural capacity and the slab stiffness is increased so that the slab is better able to resist bending caused by differential soil movements.

Minimizes and Controls Cracking: Post-tensioning will reduce cracking and keep any cracks that might form tight, preventing entry of insects and reducing possible water penetration, which can damage flooring and cause mold problems.

Controls deflections: The strength and added stiffness of a post-tensioned foundation reduces the amount the slab will bend under load.

Faster Installation: With fewer pieces to handle and less concrete to place, a post-tensioned slab can often be installed more quickly than a comparable rebar- or wire mesh-reinforced slab.

More Reliable: An engineered solution, post-tensioning is designed to exacting standards and code requirements, has an excellent performance record and offers increased reliability.

Economical: Cost benefits are achieved by reductions in quantities of concrete, steel and excavation, which in turn reduce labor costs. Beams are smaller and slab thickness is less, therefore savings in excavation and site preparation are possible.

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Growth in the global construction industry is expected to explode over the next decade, with output volume increasing 85 percent from 2015 to 2030, according to a study by consultancy Global Construction Perspectives and Oxford Economics. The report, titled Global Construction 2030, 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.

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 D4289, 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 D4289 (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.

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.

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