This was present throughout the home, no floor covering anywhere. When errors are made onsite during the posttensioning process, the ability of the reinforcement to prevent cracking is decreased. Have you ever cut a posttensioned slab forum archinect. Most residential foundations in the united states are constructed with rebar or wire mesh as their reinforcement to minimize shrinkage cracks and to resist the stresses created by the. Most often the cracks extend across the entire foundation and occur prior to tensioning of the tendon cables. Unlike conventional inactive rebar, which only helps keep the slab intact after cracking, posttension tendons continually contribute to the structural integrity of home. Before a post tension slab is poured, highstrength steel strands or cables, called tendons, are laid in a tight grid. The stranded cables are slid inside of plasticsheathing, which prevents the stranded steel tendons from touching the concrete.
As of today, march 3 no one has come back out to stress the tendons in our slab. Five underlying features of post tensioned construction allow greater flexibility in reinforcement layout. 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. This is usually one to two weeks after the concrete is placed. The best way to ensure this doesnt happen is to educate your construction teams on the best practices for.
Rebar reinforcement in this case is technically allowed by code but in our experience, a properly designed posttensioned concrete slab will minimize cracking due to shrinkage. Twoway slab construction with unbonded and bonded post tensioning tendons. Cracking in posttensioned concrete box girders generally results, in a broad sense, for the following reasons. Preventing cracks on posttensioned concrete slabs pro builder.
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. Preventing cracks on posttensioned concrete slabs pro. We recommend using pt for temperature and shrinkage reinforcement. Pt cables are used to compress the concrete and reduce cracking, whereas rebar is dormant until the concrete cracks. A concrete slab is susceptible to soil expansion and contraction, which can lead to cracking. Lots of cracks in new post tension slab when they pulled the tension on the cables, this will close some of the cracks. Hair line crackings in garage floor post tension slab. The lobby level of the garage has a 4inch topping on a 10inch posttensioned slab with. We had one customer come back with the ve the slab to post tension and the cost savings was offset by the redesign fee. Known cases add significant amounts of rebar to attempt to better control the cracking. Post tensioned foundation information for the homeowner. Over tensioning, bad prepration design laying curing of concrete, bad structural design, bad load estimation. Consequently, although posttension slabs will generally hold together, they have a higher propensity for cracking than do slabs built with plenty of rebar.
Posttension parking garage repair structural technologies. With their excellent performance, relative ease of construction. I am building a new home in longview, texas, and they poured our post tension slab 7 days ago. Span lengths up to 30 feet are common for twoway pt slab systems. 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. Mar 09, 2020 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.
These categories of cracks are caused by entirely different phenomena and have very different characteristics. Usually a sign in the garage or stamp in the slab will indicate if your home has a post tension slab. Cracks caused by the restraint to axial slab shortening rts cracks. 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. Shrinkage cracks in concrete slabonground foundations are a significant. A post tensioned foundation is a concrete slab foundation that is reinforced with stranded steel cables that are tensioned after the concrete hardens. Five underlying features of posttensioned construction allow greater flexibility in reinforcement layout. Before a posttension slab is poured, highstrength steel strands or cables, called tendons, are laid in a tight grid. The compressive stresses resist the anticipated tension stresses induced by the soil movements, enhancing the performance over a nonprestressed foundation. Read more about post tension slabs and why you should not cut or core one if a garage has a post tension slab there is usually either a sign stamped in the concrete floor saying it is a post tension slab or a plastic sign on the garage wall stating its a post tension slab.
All concrete will shrink during the curing process which lasts up to a year after the time concrete is placed in the forms. Twoway slab construction with unbonded and bonded posttensioning tendons. Cracking in groundsupported post tensioned slabs on. However, as the slab shortens due to shrinkage, creep, elastic shortening, and cooling, it is restrained by supporting elements. Lots of cracks in new post tension slab inspectionnews. The advantages of pt as noted in the opening page are the lack of cracking or at least very narrow cracks and the ability to span farther. Bonded post tension slab cracking due to restraint. When moment is drawn on the tension side, the diagram. We had a tenant fit out project in a cheap, junky strip mall where the very unskilled gc accidentally cut through multiple posttension strands in the foundation slab. Posttensioned foundations use highstrength steel to actively reinforce residential foundations to minimize cracking and increase performance. Types of pour strip options pros and cons tech sales. Whats the solution of cracks in the post tensioning slab. The original installer of the posttension strands came out and repaired it under the supervision of the original engineer.
It reduces or eliminates shrinkage crackingtherefore no joints, or fewer joints, are needed. Posttension concrete slabs are used to create a monolithic single pour slab that is stronger than a traditional slab without. Post tension slab services post tension concrete, cable. Ccl posttensioned slab systems can be integrated into any type of slab, including flat, ribbed or waffle slabs, which may include drop caps, drop panels or band beams.
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. This tension makes the concrete slab and foundation much stronger than concrete without reinforcement and helps reduce cracking. As a result, even though posttensioning is meant to help prevent a slab from cracking, the slab can still crack if the tensioning process is done incorrectly. Posttension basics how posttensioned slabs are built. Post tension slab design software structural engineering. In posttensioned floor slab construction, the placement of posttensioning tendons and associated nonprestressed reinforcement does not strictly follow the practice of nonprestressed slabs. Post tension services of texas ptst is a full service post tensioning company based in dallas. Posttension services of texas ptst is a full service posttensioning company based in dallas. The cause of cracking in posttensioned concrete box. The cause of cracking in posttensioned concrete box girder. Also, because aci 302 says a reasonable level of cracking is from 0% to 3%, it implies that no cracking, 0%, is a possible, reasonable level. Any reinforcement extending from one slab into the other will act as a tension tie, restrain the relative movement of the two slabs, and most likely cause cracking. 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. Homeowners and builders are frequently very concerned to see the cracking in a brand new slab foundation.
The post tension slab can be made only by skilful professionals. A posttension slab is a slab of concrete that has been prestressed using a specific method to increase the strength of the concrete. Lots of cracks in new post tension slab inspection news. Over tensioning, bad prepration design layingcuring of concrete, bad structural design, bad load estimation and temperature, right. Neither aci nor ec2 requires a strength check at the initiation of cracking p998.
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 during bending. Sealing cracks in posttensioned deck concrete construction. Many people dont understand how they work and the benefits. 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. Post tensioned slabs are a great tool to help reduce joints and control cracks. On the premise that those who ignore history will be condemned to relive it, this paper attempts to document those types of problems that lead. Structural reliability ccl post tensioned slabs show reduced cracking. Crack control in posttensioned oneway slab systems authors. It reduces or eliminates shrinkage cracking therefore no joints, or fewer joints, are needed. But since a posttensioned slab does not have excess rebar to minimize and distribute cracking, restraint cracks will. Neither aci nor eq requires a stmngth check at the initiation of cracking 1998. Throughout the garage, on every slab, posttensioned tendons needed replacement and deteriorating concrete required repair. Post tension slab eliminating cracks and joints in concrete.
It allows slabs and other structural members to be thinner. It can withstand strong mother nature storms, cushioned systems can be applied on the slab and works great both indoor and out. Because the garage needed to remain in operation, the owners major concerns included maintaining safety, minimizing disruption and keeping as much of the garage available for use during the extensive repairs. It has the following benefits as compared to unbonded post tensioning. You will need to get the foundation plans and have an engineer look at them. Most, but not all, cracks extended from edge to edge. This allow for more flexible tenant spaces and better sightlines. Post tension slabs answers to questions buyers ask. In order to ensure that the posttension slab used will have the best chance of avoiding. Types of pour strip options pros and cons home post tension. There are several post tension slab problems which can be avoided. I had a call this morning from a prospective client who was concerned about cracks in the concrete foundation of the home they. Now i understand that there is some cracking that can happen due to shrinkage and the fact that the pt cables are unbonded to concrete.
Im told that the cables can be partially stretched to help offset the cracking in. Avoiding posttension errors which result in cracking. This hybridisation helps achieve the formation of a much thinner slab with a longer span devoid of any columnfree spaces. Several methods exist for prestressing concrete, with posttensioning being a very common one. Two broad categories of cracking can occur in groundsupported posttensioned slabs. By the third day, i noticed a crack in the back patio, and by the fourth, there were sprawling cracks throughout the entire foundation. Shrinkage crack considerations for posttensioned foundations. Avoiding post tension errors which result in cracking. Posttensioned concrete courts provide many advantages, the design of the system will greatly decrease the chances for cracking, settling and ponding. Bonded posttensioned concrete is the descriptive term for a method of applying compression after pouring concrete and the curing. Slab foundation repair terms structured foundation. What is the difference between normal slab and post. Decorative post tensioned concrete since pt is simply reinforcement, there really arent any specific decorative applications related to post tensioning.
Q recently we completed construction of a hotel parking garage. A psc is applied with a compressive stress on the cross section in advance before the application of loading on. Today i noticed, what i the homeowner consider, severe cracking in the slab. On the other hand post tension slabs allow for the soil compaction due to decay and are strong enough to avoid the associated cracking. Using rebar for shrinkage and temperature reinforcement in parking garage slabs. Posttensioned concrete slabs are typically expected to crack less than other slabs because posttensioning produces a compressive stress that must be exceeded before enough tensile stress develops to cause cracking. Posttensioning is a method of reinforcing strengthening concrete or other materials with highstrength steel strands rebars, typically referred to as tendons. Apr 05, 2020 a post tension slab is a slab of concrete that has been prestressed using a specific method to increase the strength of the concrete. Post tension slab working principle, components and. A normal slab is basically a rcc reinforce cement concrete slab where as a post tentioned slab is the psc prestressed concret slab.
Current structural concrete design standards require that posttensioned slab systems be provided. 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. I steer customers towards conventional every opportunity im asked. Post tension slab design software sethguthrie civilenvironmental 18 aug 16 19. Looks like precast on a poured concrete wall to me, not slab on grade and no real indication that the slab is post tensioned, if it was. A posttensioned foundation is a concrete slab foundation that is reinforced with. Sep 18, 2006 the general philosophy of post tensioning is to apply a large enough force to the concrete such that any tensile stresses are reduced to acceptable levels.
Placing the concrete in a compressed state will minimize the potential for cracking and improve the integrity of the slab. 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. Posttensioning will reduce cracking and keep any cracks that might. The cause of cracking in post tensioned concrete box girder bridges and retrofit procedures walter podolny, jr. It is a concrete slab that has steel cables running through it that have been put under lots of stress to reduce cracking of the concrete and increases the strength of the slab. Cracks, cracks, cracks what is a reasonable level of cracking. On the other hand posttension slabs allow for the soil compaction due to decay and are strong enough to avoid the associated cracking. We specialize in new construction post tension slab projects and post tension repair work for both commercial and residential applications in dallas, fort worth, arlington, keller, plano, mansfield and throughout the metroplex, texas, oklahoma, colorado, louisiana and new mexico. The original installer of the post tension strands came out and repaired it under the supervision of the original engineer.
Bridge division office of engineering federal highway administration washington, d. Since pt is simply reinforcement, there really arent any specific decorative applications related to post tensioning. Pdf evaluation of cracks in post tension slabs researchgate. It lets us design longer spans in elevated members, like floors or beams. Top 10 biggest mistakes made in posttensioned concrete.
Several methods exist for prestressing concrete, with post tensioning being a very common one. Post tensioning in concrete slab benefits and advantages. Post tension slab stressing timing general inspection. Posttension basics how posttensioned slabs are built the. Issues in posttensioned construction concrete construction. Slab onground post tensioned slabsonground provide a costefficient, highperformance solution for problems associated with groundsupported residential foundations on shrinkswell soils. Post tension slabs superior foundations services jackson.
Post tensioned slabs typically have less cracking and less permeability when detailed properly. Posttensioned slabs typically have less cracking and less permeability when detailed properly. We had a tenant fit out project in a cheap, junky strip mall where the very unskilled gc accidentally cut through multiple post tension strands in the foundation slab. Cracks caused by applied loads flexural or shear cracks. Posttensioned foundations are particularly susceptible to this problem because, until the stressing is applied to the. 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. Bonded post tension slab cracking due to restraint rapt structural 19 mar 14 22. We had one customer come back with the ve the slab to post. Posttensioning, which is a form of prestressing, has several advantages over standard reinforcing steel rebars. Posttensioning is a form of prestressing concrete to provide reinforcement and overcome concretes weaknesses. About the only way to know if you have enough cables is what jim has said. 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.
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. In order to ensure that the post tension slab used will have the best chance of avoiding. Mar 03, 2014 we recently had a 5,696 square foot post tension slab poured here in north texas on 2142014. All the post tension compress forces are intended to go into the slab. When the foundationconcrete has sufficiently hardenedcured, the. Home builders can avoid cracks in a posttensioned concrete slabongrade foundation by preventing certain errors made during installation. The slab was decorative concrete it had been stained a dark brown, scored and polished. Post tensioning, which is a form of prestressing, has several advantages over standard reinforcing steel rebars. 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. Very good video showing step by step post tension slab. Post tension slabs are excellent ways to construct stronger structures at an affordable price. Structure magazine cracking moment and safety of post. It is very questionable of how much of the post tension compression forces actually go into the concrete slab and how much is in the restraint elements i.
Now assuming all these are taken care of, no cracks should be there. My firm was recently hired to look at cracks that had developed in a posttensioned floor slabongrade. All rebar and post tensioning must be lap spliced inside the width of the pour strip. It allows us to build slabs on expansive or soft soils. Advantages of using posttensioning for your next slabonground project. In addition to having more movement, a posttensioned slab will have substantially less rebar than a conventional system, which is one of the main economic benefits. But since a posttensioned slab does not have excess rebar to minimize and distribute cracking, restraint cracks will be large and very noticeable. A posttensioned foundation is a concrete slab foundation that is reinforced with stranded steel cables that are tensioned after the concrete hardens.
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