This giant checker board is actually the guts of an unbonded post tension slab. Post tension slabs are a form of prestressed slabs which means they are compressed prior to use so that the slabs are able to bear weight or service loads beyond their own natural weight. This hybridisation helps achieve the formation of a much thinner slab with a longer span devoid of any columnfree spaces. The posttensioning method is the best way to create stronger, more reliable concrete slabs. Posttensioned foundations are particularly susceptible to this problem because, until the stressing is applied to the posttensioned cables, there is no reinforcement in these slabs. In addition to the tendons, the posttension slabs pt1 and pt2 had a rebar of 10 mm diameter spaced at 250 mm from center to. Post tension concrete dubaiuae, cairoegypt and riyadh. Posttensioning compresses concrete, enabling it to resist both shrinkage cracks and cracks caused by difficult soil conditions. Bonded post tension slab cracking due to restraint.
Experimental investigation on the effect of impact loading. Homeowners are getting crushed by flawed posttension cable foundations the switch from steel rebar to posttension cable by houston area home builders has delivered unmistakable results posttension cable foundations are failing earlier and. Posttension basics how posttensioned slabs are built. Home builders can avoid cracks in a posttensioned concrete slabongrade foundation by preventing certain errors made during installation. Post tensioning also results in lesser reinforceme. A post tension slab is a concrete slab or foundation that contains high tension cables running through the concrete to increase strength and reduce cracking typically in areas with poor soil conditions. It is a concrete slab that has steel cables running through it that have been put under lots of stress to reduce cracking of. Balanced load in posttension concrete my post tension elevator speech is pretty well rehearsed at this point. I am building a new home in longview, texas, and they poured our post tension slab 7 days ago. Pt slabs have lesser thickness than rcc slabs as their deflection can be controlled by post tensioning. A post tension slab is a concrete slab or foundation that contains high tension cables running through the concrete to increase strength and reduce cracking. The only concern is to always remember not to cut or drill into post tensioned concrete slabs, since once a tendon has been cut, it is very difficult to repair.
Posttensioned concrete means that the concrete is poured and then the tension is appliedbut it is still stressed before the loads are applied so it is still prestressed. My question is simply what is the problem with post tension slabs that have a great deal of restraint cracking due to this not being taken into consideration of the design. Hair line crackings in garage floor post tension slab. More information on posttensioning, including example case studies, can be found at the posttensioning association pta website at. Shrinkage crack considerations for post tensioned foundations june 1, 2015 shrinkage cracks in concrete slab onground foundations are a significant public relations problem to home builders. This is usually one to two weeks after the concrete is. This is accomplished by prestressing cables which run throughout the slab and by compressing the concrete. This area is typically shored and reformed after the primary forming has been removed. When it comes to posttension concrete, leave it to the professionals. These tendons were posttensioned by stress in the wires with a hydraulic jack after casting the concrete slabs to reach a stress equal to 1440 mpa. Many post tensioned slabs will be stamped to alert the owner and any renovation contractors that the slab is post tensioned. This stronger, thinner slab also saves money on concrete. The use and proper construction of slip details is therefore critical for the performance and aesthetics of post tensioned concrete.
After decades of building and inspecting in the posttensioning pt. Many people dont understand how they work and the benefits and challenges. There are a wide variety of reasons for utilizing post tension slabs versus conventional slabs. For this, the concrete slab is tensioned with a process called prestressing and the slab of concrete which comes out after this process is known. The slab was decorative concrete it had been stained a dark brown, scored and polished. Lots of cracks in new post tension slab inspection news. Similarly, a slab will crack if too much weight is placed in one spot, or if the soil settles unevenly, bending the slab. For example, a concrete wall can support tons of weight from above, but will crack easily if pushed sideways forcing it to bend. The only concern is to always remember not to cut or drill into posttensioned concrete slabs, since once a tendon has been cut, it is very difficult to repair. Until the cables are stretchedelongated, they cannot provide any crack. By the third day, i noticed a crack in the back patio, and by the fourth, there were sprawling cracks throughout the entire foundation. An introduction to specifying posttensioned slabs, an article extracted from concrete quarterly, spring 2020. Posttensioned concrete slabs have been used since the 1930s, and have become commonplace in the last 30 years.
In addition to the already mentioned general features of posttensioned construction, the following advantages of posttensioned slabs over reinforced concrete slabs may be listed. This concrete has cable conduits cast into it and that cable is stretched, or tensioned, hydraulically after the concrete has hardened. Consequently, although posttension slabs will generally hold together, they have a higher propensity for cracking than do slabs built with plenty of rebar. All concrete will shrink during the curing process which lasts up to a year after the time concrete is placed in the forms. Homeowners and builders are frequently very concerned to see the cracking in a brand new slab foundation. And we keep a long and growing list of the mistakes weve encountered along the way. Posttensioning is used in this manner in millions of square feet of warehouse floors, sport courts, housing and specialized paving applications. Post tensioned foundation information for the homeowner. Post tension slab advantages and disadvantages of post. The typicalconventional pour strip which is a leaveout of 8 to 4 feet with the post tension concrete slabs stopping at each side of the leaveout and reinforcing bars running from the initial pour into the leaveout and bars from the second pour running to the face of the initial pour.
My firm was recently hired to look at cracks that had developed in a posttensioned floor slabongrade. But since a post tensioned slab does not have excess rebar to minimize and distribute cracking, restraint cracks will be large and very noticeable. In this article, ill be giving homeowners some necessary information about the characteristics, advantages, disadvantages, and common questionsconcerns that i. The aci 302 provision setting a reasonable level of cracking first appeared in the 1996 revision of aci 302.
Posttensioning can be used for repairs, modification and strengthening of reinforced concrete and posttensioned structures. A posttensioned foundation is a concrete slab foundation that is reinforced with. A posttension slab is a slab of concrete that has been prestressed using a specific method to increase the strength of the concrete. This is a typical concern for those purchasing new construction who are not familiar with post tension cable reinfored foundation construction. Help bad cracks in post tension slab pics inspection news. Posttension cables are steel wires inserted into plastic sheathing laid in place while pouring the concrete slab. This tension makes the concrete slab and foundation much stronger than concrete without reinforcement and helps reduce cracking. Post tension slab is a combination of conventional slab reinforcement and additional protruding highstrength steel tendons, which are consequently subjected to tension after the concrete has set. The use and proper construction of slip details is therefore critical for the performance and aesthetics of posttensioned concrete.
Lots of cracks in new post tension slab inspectionnews. More economical structures resulting from the use of prestressing steels with a very high tensile strength instead of. This was present throughout the home, no floor covering anywhere. Pt slabs on ground can be placed and stamped just like with any other concrete slab. Post tension slab although concrete is a strong material for construction, it is not resistant to breakdown especially where longer concrete slabs are required like in a bridge or a length of a beam. Several methods exist for prestressing concrete, with posttensioning being a very common one. Ppl provide services in the construction field by increasing the loading capacity of concrete elements like beams, slabs, bridges, silos, slabs on ground etc. Most often the cracks extend across the entire foundation and occur prior to tensioning of the tendon cables. Q recently we completed construction of a hotel parking garage. Cracks in concrete floors or slabs occur in poured concrete slabs may be found both in basement and in slab on grade or patio home construction and have a variety of causes and cures that we discuss here as we explain how to repair cracked concrete floors and as we describe slab on grade construction or patio home construction cracks and as. Post tension slab eliminating cracks and joints in concrete. Posttensioning methods for reinforcing concrete the. Sections are tensioncontrolled when the tensile strain in the extreme tension.
Many people dont understand how they work and the benefits. Top 10 biggest mistakes made in posttensioned concrete design. Posttension concrete slabs are used to create a monolithic single pour slab that is stronger than a traditional slab without. Control of cracks posttensioned slabs control cracks much better than other reinforced slabs, due to the squeezing pressure tension of the cables compressing the cracks. Post tension slabs are used to create a monolithic slab that is stronger than the typical slab and foundation that is poured in stages. How to create a concrete slab foundation using the post. Post tensioned slabs are a great tool to help reduce joints and control cracks. Posttensioned concrete slabs are typically expected to crack less than other slabs because posttensioning produces a. To create a slab using this method, plastic sleevecovered steel cables are laid in a grid running in both directions. Preventing cracks on posttensioned concrete slabs pro. As a result, even though post tensioning is meant to help prevent a slab from cracking, the slab can still crack if the tensioning process is done incorrectly. Homeowners are getting crushed by flawed posttension. Posttensioning elevated concrete beams and slabs can result in thinner concrete sections andor longer spans between supports.
The article gives guidance to practicing engineers and contractors on techniques. Im seeing some deep cracks in a posttension slab residential build 7 years old. Shrinkage crack considerations for posttensioned foundations. Over tensioning, bad prepration design layingcuring of concrete, bad structural design, bad load estimation and temperature, right. I had a call this morning from a prospective client who was concerned about cracks in the concrete foundation of the home they were building. Build your dream home, find out more at canyoncovehomes. A concrete slab is susceptible to soil expansion and contraction, which can lead to cracking. Any post tension slab inspectors out there i am building a new home in longview, texas, and they poured our post tension slab 7 days ago. Each strand is housed inside plastic sheathing that protects the steel from corrosion and allows it to move during the stressing operation. Some of the cracks are more than 30 feet long, and i can feel a slight difference in the rise on each side you can even. Posttensioning is a form of prestressing concrete to provide reinforcement and overcome concretes weaknesses.
Now assuming all these are taken care of, no cracks should be there. Usually a sign in the garage or stamp in the slab will indicate if your home has a post tension slab. Time savings post tensioned can be installed much faster that other reinforced foundation systems. Preventing cracks on posttensioned concrete slabs pro builder. Posttensioned prestressed concrete as a building material, concrete is strong in compression, but weak in tension. Posttensioned foundation a typical posttensioned foundation with the forms set and ready for concrete. The best way to ensure this doesnt happen is to educate your construction teams on the best practices for designing and constructing post tensioned slabs. Pt cables are used to compress the concrete and reduce cracking, whereas rebar is dormant until the concrete cracks.
Slab was poured about 9 days ago and they have not tightened the tension cables yet. But since a posttensioned slab does not have excess rebar to minimize and distribute cracking, restraint cracks will be large and very noticeable. Structural engineers design concrete slabs with posttension cables to obtain greater span lengths between supporting columns and to allow thinner slabs. Shrinkage cracks in concrete slabonground foundations are a significant. Post tension slabs answers to questions buyers ask. Most precast, prestressed concrete is actually pretensionedthe steel is pulled before the concrete is poured. Post tension slab working principle, components and. Cracks, cracks, cracks what is a reasonable level of cracking. A typical residential foundation takes 34 hours to install. This is done by exploiting the force of high tensile steel to make concrete in continuous compression. Flexural cracks develop when the tensile strength of concrete is exceeded under load. Whats the solution of cracks in the post tensioning slab. Top 10 biggest mistakes made in posttensioned concrete. Steel strands are run through the concrete in a grid pattern.
I had a call this morning from a prospective client who was concerned about cracks in the concrete foundation of the home they. Posttensioned concrete design limit is the tensile strain in the reinforcement at the balanced strain condition, which is taken as the yield strain of the reinforcement, f e aci 10. Rebar slabs may 03 2016 0 comments concrete, nishkian dean 0 reinforced concrete slabs have been used in buildings from the middle of the 19 th century. Types of pour strip options pros and cons tech sales. The proposed research will examine, using numerical analysis, the impact of rock fall on prestressed concrete slabs with equivalent moment capacity reinforced concrete slabs.
Why post tension slabs are used in phoenix, gilbert, and. This method is a better way to construct a slab foundation to be more resistant to cracking and deflection. Concrete can support a lot of weight in compression, but is weak in tension. Posttension concrete is a method of reinforcing concrete. Steel reinforcement bars are therefore required in the slab. After decades of building and inspecting in the post tensioning pt concrete construction industry, weve seen the best and the worst of pt construction. Before a posttension slab is poured, highstrength steel strands or cables, called tendons, are laid in a tight grid. Post tensioned slabs are used when spans are large and normal rcc slabs become uneconomical. Reinforcement helps extend the life of concrete, overcoming cracking that is inherent in the material and preventing its failure. Over tensioning, bad prepration design laying curing of concrete, bad structural design, bad load estimation. The lobby level of the garage has a 4inch topping on a 10inch posttensioned slab with. Rowing correct, it isnt that clear, let me try again.
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