This hybridisation helps achieve the formation of a much thinner slab with a longer span devoid of any columnfree spaces. Posttensioned structural systems eliminate closely spaced joints and help ensure watertightness by placing the floor in biaxial compression, thereby controlling and counteracting shrinkage and flexural cracks. Many post tensioned slabs will be stamped to alert the owner and any renovation contractors that the slab is post tensioned. Cracks, cracks, cracks what is a reasonable level of cracking. The posttensioning method is the best way to create stronger, more reliable concrete slabs. Aaron erickson structural engineer talks about his work on repair post tension structures. Post tensioned concrete is used to strengthen structures such as bridges, parking structures, buildings and tanks. We recommend using pt for temperature and shrinkage reinforcement. In this article, ill be giving homeowners some necessary information about the characteristics, advantages, disadvantages, and common questionsconcerns that i receive regarding posttensioned foundations. Reduction of shrinkage and cracking in slabs which results in the use of fewer or even no joints. Post tensioned floors are relatively crackfree, resulting in minimal maintenance issues. Posttensioned floors are relatively crackfree, resulting in minimal maintenance issues. Nov 25, 2008 post tensioned concrete is a term heard more and more in the construction industry today. Many posttensioned slabs will be stamped to alert the owner and any renovation contractors that the slab is post tensioned.
Defects such as slab cracks can be costly to repair after construction. In a posttensioned beam, the tendons are stressed and each end is anchored to the concrete section after the concrete has been cast and has attained sufficient strength to. Pretensioning and posttensioning in prestressed concrete. Post tension repair post tension pt tendon systems are primarily used for large slab construction projects. How to create a concrete slab foundation using the post. 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. The use and proper construction of slip details is therefore critical for the performance and aesthetics of post tensioned concrete. This method is not applicable if the crack is active, the cracks are large in number, or. Oct 24, 2012 my firm was recently hired to look at cracks that had developed in a post tensioned floor slab ongrade. There are a number of post tension slabs advantages, including. Photo 3 close view of a balcony slab edge with cracks and deteriorated coating. We pride ourselves by providing the highest quality custom finishes available today. The companies that make prestressed concrete tanks are crom, dyk, natgun, and preload. But, not all cracks are created equal and you need a trained professional to look at the slab.
Post tension slab advantages and disadvantages of post. Since the early 1950s, post tension tendons have been used in the design of concrete structures. Once the slab cures, qualified personnel apply tension and grout the cables. Post tension slab eliminating cracks and joints in concrete. My firm was recently hired to look at cracks that had developed in a posttensioned floor slabongrade.
All concrete will shrink during the curing process which lasts up to a. When the cables are tensioned, the concrete is placed in compression. Lots of cracks in new post tension slab inspection news. Before a posttension slab is poured, highstrength steel strands or cables, called tendons, are laid in a tight grid. Cracks that form in plastic concrete can be categorized as either plastic shrinkage cracking or plastic settlement cracking. Often the culprit is not the concrete or post tensioning work. Structure magazine posttensioned concrete construction. A posttension slab is a slab of concrete that has been prestressed using a specific method to increase the strength of the concrete. Asphalt irregularities, including cracks, settling and uncorrected deviations in the planarity of the asphalt base or wear course, can telegraph or. 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.
Removing the interior floor finishes to make repairs to the slab is both. Neglecting to design general anchorage zones or delegating this responsibility. The foundation in question was a posttension cable reinforced foundation as opposed to a foundation which is reinforced with steel rebar see the typical photograph in this article. Contractors sometimes need to drill or cut into a posttensioned slab for repair or renovation work. This means the foundation is reinforced by cables laid in a grid arrangement and tensioned after the concrete cures sufficiently with the intent to create compression in the foundation concrete to resist cracking. Post tensioned slab construction uses a combination of common rebar and highstrength tendon reinforcing, which is stressed after the concrete is poured. Posttension cables are steel wires inserted into plastic sheathing laid in place while pouring the concrete slab.
Preventing cracks on posttensioned concrete slabs pro. 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. Shortening crack mitigation crack evaluation tn451010616. 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. 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. Most often the cracks extend across the entire foundation and occur prior to tensioning of the tendon cables. Due to the biaxial precompression in posttensioned floor systems, concrete can sustain longer spans between adjacent, parallel tendons without diminishing the service. Techniques for repairs of posttensioning tendons are basedupon time of discovery of the problem, type of tendonsinvolved, and structure type. Shrinkage crack considerations for posttensioned foundations june 1, 2015 shrinkage cracks in concrete slabonground foundations are a significant public relations problem to home builders. Jimenez t he use of unbonded posttensioned pt reinforcement in concrete structures has been popular for many decades. The post tensioning method is the best way to create stronger, more reliable concrete slabs. Concrete water tanks are often posttensioned to reduce crack width and leakage. As part of a proactive effort, the owner selected the investigatedesignbuild idb team led by structural and structural technologies to provide assessment, engineering and solution implementation for the parking structure. In the early stages of development of posttensioned concrete in europe, posttensioning without bond was also used to some extent for example in 193637 in a.
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 as post tension slab. Whether it be conventional slab on grade, heavily reinforced structural decks, or waffle slabs rjg can service your concrete repair needs. Advantages of using posttensioning for your next repair, rehabilitation and. If you have many cracks in the posttensioned slab or have reason to believe that cracks will continue to develop in it, you may want to consider some means of sealing the top surface of the topping to keep water from penetrating it and ultimately the posttensioned slab as well. 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.
The robinson center parking garage, located in downtown little rock, was showing signs of deterioration within the slabs and beams. Jan 20, 2016 rebar reinforcement in this case is technically allowed by code but in our experience, a properly designed post tensioned concrete slab will minimize cracking due to shrinkage. The only cracking i noticed were a few beams have 1 to 3 vertical hairline cracks a couple of feet away from the beam column intersection. All concrete will shrink during the curing process which lasts up to a year after the time concrete is placed in the forms. These types result from the bleeding and segregation process that occurs when fresh concrete is placed. Masonry walls can be post tensioned this is usually done with a solid steel bar fastened to the foundation and stressed with a nut at the walls top. Over tensioning, bad prepration design layingcuring of concrete, bad structural design, bad load estimation and temperature, right. As a building material, concrete is strong in compression, but weak in tension. Preventing cracks on posttensioned concrete slabs pro builder. This method of reinforcing concrete enables a designer to take advantage of the considerable benefits provided by prestressed concrete while retaining the flexibility afforded by the castinplace method of building concrete structures.
Over tensioning, bad prepration design layingcuring of concrete, bad structural design, bad load. By the third day, i noticed a crack in the back patio, and by the fourth, there were sprawling cracks throughout the entire foundation. The use of post tension tendons allows the engineer to design for larger slab construction spans typically used in parking structures or high rise. Techniques for repairs of post tensioning tendons are basedupon time of discovery of the problem, type of tendonsinvolved, and structure type. Lots of cracks in new post tension slab inspectionnews. If there arent many cracks in the posttensioned slab and you have good reason to believe that there wont be many more, you might successfully seal them from.
In posttensioned floor slab construction, the placement of. If cracks do form they are minimal and are held together. Although aalami and barth were conservative in estimating the lineal ft of restraining cracks requiring repair, it seems difficult to believe that there are 4 times more lineal feet of cracking in a post tensioned slab than in a jointed slab onground. Posttensioning institute education pt applications. Post tension slabs superior foundations services jackson. By using posttensioned tendons to compress the concrete.
At granite, we perform slab foundation repairs as well as pier and beam foundation repairs a concrete slab foundation problem needs prompt evaluation and. Several methods exist for prestressing concrete, with posttensioning being a very common one. Although aalami and barth were conservative in estimating the lineal ft of restraining cracks requiring repair, it seems difficult to believe that there are 4 times more lineal feet of cracking in a posttensioned slab than in a jointed slabonground. Contractors sometimes need to drill or cut into a post tensioned slab for repair or renovation work. Depending on the extent of the damage, you may also have to put up the homeowners in a hotel during the repair. Beverly hills surgeon reveals at home fix no creams needed. Techniques for repairs of post tensioning tendons are based upon time of discovery of the problem, type of ten dons involved, and structure type. At first glance it appears the concrete was too wet. I had a call this morning from a prospective client who was concerned about cracks in the concrete foundation of the home they. Sep 24, 20 aaron erickson structural engineer talks about his work on repair post tension structures. Post tension pt tendon systems are primarily used for large slab construction projects. Intermittent concrete keys were specified to transfer slab shear forces from one side of the construction joint to the other. Post tensioning repair robinson parking garage structural. Epoxy injection for concrete crack repair in foundations, basements, beams, columns, slabs, walls and other concrete structures injection of epoxies under pressure may be used to bond the cracks having greater than or equal to 0.
Such cracks usually appear one to six hours after concrete placement. Introduction to the icripti joint document pti dc80. The first post tensioned slabs were erected in the usa in 1955, already using. Vector engineers and technicians have specialized knowledge and training in the deterioration and repair of posttensioned concrete. 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. Jun 04, 2014 this leaves the middle of a post tensioned slab as an unreinforced plain concrete slab, just like an improperly built wire mesh or steel bar reinforced slab with the steel lying on top of the plastic moisture barrier and not embedded in the thickness of the slab. B9472, is staffed with an experienced team of skilled professionals who undertake all aspects of structural concrete rehabilitation, restoration, repair and reinforcement for a variety of commercial and industrial buildings, parking structures and slabs. Typical slip details use felt, building paper, or plastic to. Posttensioned foundation a typical posttensioned foundation with the forms set and ready for concrete. In slabs on ground, the friction between the slab and the ground acts to resist this shrinkage and results in what are commonly referred to as shrinkage cracks. 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. Post tension slab working principle, components and.
First, slab and beam cracking is a common concern and an indication that attention and repairs are needed. Shrinkage crack considerations for posttensioned foundations. To create a slab using this method, plastic sleevecovered steel cables are laid in a grid running in both directions. 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. Because all shaw homes slabs are posttension concrete, cracks are. This method is a better way to construct a slab foundation to be more resistant to cracking and deflection. Vector engineers and technicians have specialized knowledge and training in the deterioration and repair of post tensioned concrete. Post tensioned structural systems eliminate closely spaced joints and help ensure watertightness by placing the floor in biaxial compression, thereby controlling and counteracting shrinkage and flexural cracks. Reinforcement helps extend the life of concrete, overcoming cracking that is inherent in the material and preventing its failure. 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. Now assuming all these are taken care of, no cracks should be there. Conventional rebar slab construction uses common bar reinforcing throughout the slab distributed in two layers.
Once the tendons are tensioned pulled or stressed, they are grouted in the slab. This leaves the middle of a posttensioned slab as an unreinforced plain concrete slab, just like an improperly built wire mesh or steel bar reinforced slab with the steel lying on top of the plastic moisture barrier and not embedded in the thickness of the slab. Post tensioned foundation information for the homeowner. Mar 09, 2020 depending on the extent of the damage, you may also have to put up the homeowners in a hotel during the repair. Having all the facts before you begin repairs will help you choose the right company and avoid unexpected cost and delays. Whats the solution of cracks in the post tensioning slab. In post tensioned floor slab construction, the placement of post tensioning tendons and associated nonprestressed reinforcement does not strictly follow the practice of nonprestressed slabs. The crack is very small in width, you would be hard pressed to fit a hair into the crack. Most, but not all, cracks extended from edge to edge. Posttensioned slab foundations houston slab foundations. For instance, sps crack injection repair is most effective in restoring concrete. Posttensioned concrete is used to strengthen structures such as bridges, parking structures, buildings and tanks.
In this article, ill be giving homeowners some necessary information about the characteristics, advantages, disadvantages, and common questionsconcerns that i. Discovering the necessity to repair a tendon prior to concrete placement is easier than. But since a post tensioned slab does not have excess rebar to minimize and distribute cracking, restraint cracks will be large and very noticeable. A posttensioned slabonground foundation is simply a groundsupported concrete slab foundation that is reinforced with flexible cables that are tensioned after the concrete hardens. Concrete water tanks are often post tensioned to reduce crack width and leakage.
Five underlying features of post tensioned construction allow greater flexibility in reinforcement layout. Post tensioned slabs are a great tool to help reduce joints and control cracks. Shrinkage cracks in concrete slabonground foundations are a significant. Often the culprit is not the concrete or posttensioning work. This stronger, thinner slab also saves money on concrete. Top 10 biggest mistakes made in posttensioned concrete.
Post tension cables are steel wires inserted into plastic sheathing laid in place while pouring the concrete slab. In general, its more expensive to build a post tensioned slab ongrade foundation than a traditional one. Slab foundation repair cost what you can expect to pay. I recently made a site visit for a cast in place post tensioned parking garage that we designed. Slab thicknesses as a function of span lengths recommended limis slendernesses 1. This article describes the types of cracks that occur in poured concrete slabs or floors and explains the risks associated with each, thus assisting in deciding what types of repair may be needed.
This was present throughout the home, no floor covering anywhere. The cables are not normally tensioned until at least 7days and sometimes not until 30days after concrete placement. Since the early 1950s, post tension tendons have been used in. Posttensioning institute education pt applications repair. This can lead to a dilemma when the need arises to repair the slab edges due to corrosion of the steel bursting bars, particularly if the. Bridge designers have used pt both for castinplace concrete and for precast segmental construction.
The slab was decorative concrete it had been stained a dark brown, scored and polished. Posttensioned concrete slabs have been used since the 1930s, and have become commonplace in the last 30 years. Also, because aci 302 says a reasonable level of cracking is from 0% to 3%, it implies that. Posttension basics how posttensioned slabs are built. In general, its more expensive to build a posttensioned slabongrade foundation than a traditional one. Issues in posttensioned construction concrete construction. 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. 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. Techniques for repairs of posttensioning tendons are based upon time of discovery of the problem, type of ten dons involved, and structure type. Usually a sign in the garage or stamp in the slab will indicate if your home has a post tension slab. Slab foundation repair cost can be expensive and may even cause collateral damage with extra costs as well. Tce has extensive experience in evaluation and repair of post tension structures. Many people dont understand how they work and the benefits.
Homeowners are getting crushed by flawed posttension. Sealing cracks in posttensioned deck concrete construction. Discovering the necessi ty to repair a tendon prior to concrete placement is easier. The alternative to pretensioning is posttensioning. 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. Such a plain concrete slab only provides a hard surface, but has no significant. Posttension basics how posttensioned slabs are built the. Cracking in post tensioned beams structural engineering.
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