The vast majority of post tensioning tendons used in these slabs have been unbonded. It has been used for decades, even centuries, all around the world. A pt concrete podium slab provides better crack control due to the precompression applied to the structure. Post tension slab design software sethguthrie civilenvironmental 18 aug 16 19. Mar 09, 2020 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. 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. You can prevent cracks in ceramic tile that is laid over concrete in any number of ways. Tulsa home builders 918 6885660 can a post tension slab. A normal slab is basically a rcc reinforce cement concrete slab where as a post tentioned slab is the psc prestressed concret slab. 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. Good quality concrete should reach 2000 psi in approximately 4 days under normal curing conditions, and certainly within 7 days. 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.
About the only way to know if you have enough cables is what jim has said. The tendon geometry can easily be adjusted to work around the existing utility pipes and ducts and apply the. 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. Avoiding post tension errors which result in cracking. Gina, do you have a third party engineer or inspector. Signs of foundation problems concrete slab foundations. If a garage has a post tension slab there is usually either a sign stamped in the. Strengthening structures with posttensioning tendons can be a costeffective solution, because new, external highstrength steel tendons can be easily installed in the structure so that strengthening forces are applied where needed. The dsld engineer said the cracks are normal in post tension slabs but i don. Avoiding posttension errors which result in cracking. Nov 25, 2008 posttensioning now is used extensively in bridges, elevated slabs parking structures and residential or commercial buildings, residential foundations, walls, and columns. 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.
Several methods exist for prestressing concrete, with post tensioning being a very common one. Post tensioned slabs can be designed to be crack free and therefore waterproof slabs are possible. Aug 28, 2015 there are many reasons why concrete slab foundations crack. 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. Post tensioning pt delivers roughly four times the tensile strength compared to conventional reinforcement and significantly reduces or eliminates concrete cracking, thus enabling thinner slab construction reducing the environmental impacts, saving material and labor costs, and shortening construction schedules. Im told that the cables can be partially stretched to help offset the cracking in. Post tensioned tendons are used in bridge construction, building construction, box girder roadways, tennis courts, dams and other large concrete structures. 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. Home builders can avoid cracks in a posttensioned concrete slabongrade foundation by preventing certain errors made during installation. While these materials will not insure a crack free slab, they will hold the concrete together such that small harmless hairline cracks occur instead of cracks in excess of 18 inch or more. 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. Post tensioned foundation information for the homeowner. Post tension slabs have wound cables running through them instead of rebar. Values for the three parameters listed previously must be established before the required amount of post tensioning can be determined.
Issues in posttensioned construction concrete construction. Post tension slabs versus floating slabs probest pest. Posttension basics how posttensioned slabs are built the. In the united states alone, it is estimated that there are 2.
Quite a few property owners in the dallas, fort worth, tx area have discovered that poor construction, soil conditions, excessive moisture, plumbing leaks and improper grading are common causes of foundation cracks in concrete slabs. Post tensioned slabs should be stressed when the concrete strength designated by the engineers plan is reached. As concrete shrinks during the drying process it would not crack if it were supported by a perfectly smooth frictionless surface, but in reality this is not the case. Cracking in groundsupported post tensioned slabs on. But since a posttensioned slab does not have excess rebar to minimize and distribute cracking, restraint cracks will be large and very noticeable. Experimental and numerical assessment of bonded and unbonded posttensioned concrete members. But it is still critical to check and correct for longterm deflection at the design stage. Typically shrinkage cracks occur prior to post tensioning and erecting the walls, so these would likely have been noticed when the floor was stained. If you notice a post tension cable sticking out of the concrete wedge, you need to call in a post tension cable specialist to fix the concrete and ensure proper cable tension. Unless your going to be setting anchors over 58 bolt size, not od of the anchor you have nothing to worry about.
Top 10 biggest mistakes made in posttensioned concrete design. Call us today to schedule a free foundation evaluation. We dont sale a house without a post tension slab unless they require pier and beam. Especially in hot weather, shrinkage cracks can occur as early as a few hours after the slab has been poured and finished. The cracks are perpendicular to the uniform tendons direction. Cracks caused by the restraint to axial slab shortening rts cracks. Post tensioning can be used for repairs, modification and. By applying post tension, we increase the overall compression force in the cross sect. Preventing cracks on posttensioned concrete slabs pro. Post tension slabs answers to questions buyers ask. One thing most posttension slabs are not reinforced for is vertical movement. Cracking due to shrinkage after tensioning is not common and if they do occur they would be very small. The default load combinations for post tensioning are defined in the following sections.
Oct 24, 2012 typically shrinkage cracks occur prior to post tensioning and erecting the walls, so these would likely have been noticed when the floor was stained. Now that you know the benefits of building with a post tensioned concrete. Achievement of this objective depends upon careful design, detailing and construction. Post tensioned slabs are a great tool to help reduce joints and control cracks. I have tried looking for it in my states roc rules but they have nothing for what is considered to be a structural crack in a post tension concrete slab. As concrete shrinks during the drying process it would not crack if it were supported by a. Post tensioned tendons are used in bridge construction, building. In order to ensure that the posttension slab used will have the best chance of avoiding cracking, as well as ensuring the longevity of post tension slabs, there are several possible errors to consider. Sep 19, 2019 concrete can support a lot of weight in compression, but is weak in tension.
Pdf evaluation of cracks in post tension slabs researchgate. During this time cracks often form, and once the crack has occurred, there little you can do to eliminate it. In this article, ill be giving homeowners some necessary information about the characteristics, advantages, disadvantages, and common questionsconcerns that i receive regarding posttensioned foundations. A posttensioned foundation is a concrete slab foundation that is reinforced with. You must register before you can view photos and images. Shrinkage crack considerations for posttensioned foundations. Jul 22, 2019 post tensioned slabs are a great tool to help reduce joints and control cracks. What is the difference between normal slab and post. The best way to ensure this doesnt happen is to educate your construction teams on the best practices for designing and constructing posttensioned slabs. Ensure proper vibration to ensure consistency of concrete and no honeycomb voids between aggregate where no. Whats the solution of cracks in the post tensioning slab. Found at almost every maybe 50% location of at the uniform tendons anchorage end. Post tensioning of foundations, transfer beams and plates, post tensioned masonry and the combination of precast elements with castinplace concrete by means of post tensioning offer other interesting opportunities. However, out of the 800 there were 42 call backs for shrinkage crack investigation reports.
There are several post tension slab problems which can be avoided. There are not any special considerations or requirements for the formwork beyond what is used in nonposttensioned construction, and the deck forms can be cycled as soon as the tendons are stressed, resulting in fast construction cycle times. The reason that post tension or even pre tension concrete does not show shrinkage cracks as readily is that the tension placed through the tendons is enough to overcome the friction caused by shrinkage. Whitton concrete along with whitton plumbing can help you get this seeming devastating problem back to a minor inconvenience. The curing time before cables can be tensioned may vary, but most builders dont tension for 10 days or more. Homeowners and builders are frequently very concerned to see the cracking in a brand new slab foundation. If each panel contained one crack that was fullpanel lengthabout 15 ft the total crack length for the 100,000 ft2 floor would be 210 lineal ft. The practice of mlaw has been to apply stress when the concrete reaches 2000 psi. A psc is applied with a compressive stress on the cross section in advance before the application of loading on. Especially in areas with expanding or shifting soils, concrete slab foundations can heave, crack, split, move, even have trees growing up through them. Posttension basics how posttensioned slabs are built.
Mar 03, 2014 hey chris, what you are describing seems normal for a post tension slab. Post tension slab working principle, components and. This is usually one to two weeks after the concrete is placed. If you are still horribly paranoid, you can always have the slab xrayed. Everything you need to know about building with concrete. Without post tension tendons large slabs of concrete would crack and collapse under load. You may not know it but if you own a modern home you probably have a post tension slab. 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. Many people dont understand how they work and the benefits and challenges. Their deflection can be controlled by varying the amount of posttensioning to balance any portion of applied loads immediately after stressing. What is the maximum allowable crack size dimension in a post tension concrete slab before it is considered a structural crack, in arizona, usa. Nov 18, 2018 without post tension tendons large slabs of concrete would crack and collapse under load.
Do you say you dont know anything, or can you give them a short. Jul 11, 2017 the post tensioning allows the slab to be thinner. Each set of plans that i have seen with a post tension slab has been issuesd or stamped by an engineer due to the greater design requirements cable placement, tension strength, etc. Why is a posttensioning slab more economical than a cast in. Afterwards, once the concrete has gained strength but before the service loads are applied, the cables are pulled tight, or tensioned, and anchored against the outer edges of the concrete. Why concrete cracks concrete crack solutions ardex ireland.
Cracks, cracks, cracks what is a reasonable level of cracking. The most common type of cracking found in posttensioned slabs on ground is caused by restrainttoshortening rts. 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. Over tensioning, bad prepration design laying curing of concrete, bad structural design, bad load estimation. They give a conservative estimate of the total length of anticipated cracks requiring repair as 0. Pt slabs on ground can be placed and stamped just like with any other concrete slab. Many people dont understand how they work and the benefits. The choice of concrete mix and curing methods along with quality workmanship also play a key role. Post tensioned concrete slabs have been used since the 1930s, and have become commonplace in the last 30 years. Lots of cracks in new post tension slab when they pulled the tension on the cables, this will close some of the cracks.
Help bad cracks in post tension slab pics inspection news. Preventing cracks on posttensioned concrete slabs pro builder. Pt beams and slabs have an inherent advantage over nonpt members when it comes to deflection. Pdf experimental and numerical assessment of bonded and. Post tension slab eliminating cracks and joints in concrete. Hairline cracks x 8 long consistently happen at uniform tendons dead end location at bottom of slab. 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. Many posttensioned slabs will be stamped to alert the owner and any renovation contractors that the slab is post tensioned.
You will need to get the foundation plans and have an engineer look at them. This location is typically congested with tendons near. Post tension tendon inspection services how to inspect a. Sealing cracks in posttensioned deck concrete construction. When the stress becomes too great for the now hardened concrete, the slab will crack in order to relieve tension. A post tension slab is a slab of concrete that has been prestressed using a specific method to increase the strength of the concrete. In part a a posttensioned member on very flexible supports shortens under the precompression, forcing the supports to follow the members moment. Lots of cracks in new post tension slab inspection news. The idea is simple, concrete is good in compression and bad in tension. 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. Similarly, a slab will crack if too much weight is placed in one spot, or if the soil settles unevenly, bending the slab. One thing most post tension slabs are not reinforced for is vertical movement. Small cracks do not normally create a significant concern.
Bonded post tension slab cracking due to restraint. Micro cracks are caused by bad curing, especially intermittent water curing, as is done in india. Post tensioning in buildings is not limited to floor slabs. Jim rogers bending and flexing creates high tensile forces that can cause the concrete floor slab to crack. Developers, architects, engineers, contractors, educators and. However, in my opinion, the main culprit is micro cracks and voids, which causes reduction in the surface to take up tension.
We can also repair your cracked, ponding or heaving porch or garage floors. Twoway post tensioned slabs have been an extremely popular floorframing system in american building construction. Shrinkage cracks in concrete slabonground foundations are a significant public relations problem to home builders. Even longer spans can be achieved by using beam and slab construction, such as in a parking structure where typical posttensioned beams can span 60 to 65 feet. 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. Usually a sign in the garage or stamp in the slab will indicate if your home has a post tension slab. For post tensioned concrete design, the user can specify the prestressing load pt by providing the tendon profile. Structural engineers design concrete slabs with post tension cables to obtain greater span lengths between supporting columns and to allow thinner slabs. This hybridisation helps achieve the formation of a much thinner slab with a longer span devoid of any columnfree spaces.
Stretching or elongating the posttensioned cables applies significant force to the concrete system, lifting the slab into a compressed state, which reduces the negative effects from tensile forces cracking. I am assuming that all other factors are controlled. Post tension slab eliminating cracks and joints in. Aug 11, 2012 post tension slabs versus floating slabs. As this settling takes place the slab will often develop a few cracks due to this.
Two broad categories of cracking can occur in groundsupported posttensioned slabs. For example, a concrete wall can support tons of weight from above, but will crack easily if pushed sideways forcing it to bend. These categories of cracks are caused by entirely different phenomena and have very different characteristics. Before a post tension slab is poured, highstrength steel strands or cables, called tendons, are laid in a tight grid. Cracks caused by applied loads flexural or shear cracks. When curling in a concrete slab application cannot be tolerated alternate options include the use of shrinkage reducing admixtures, shrinkagecompensating concrete, post tensioned slab construction or vacuum dewatering. Post tension slab design software structural engineering. If a slab does not have post tension cables in it, this would be a huge concern. To provide any crack control benefit, the slabs on each side of the pour strip have to be completely separate when the tendons are stressed.
Over tensioning, bad prepration design layingcuring of concrete, bad structural design, bad load estimation. Shortening crack mitigation crack evaluation tn451010616. Lots of cracks in new post tension slab inspectionnews. Builders can partially tension tendons during the curing process to reduce this occurrence but it rare in production construction. Posttensioned foundation a typical posttensioned foundation with the forms set and ready for concrete. The aci 302 provision setting a reasonable level of cracking first appeared in the 1996 revision of aci 302. Posttensioning elevated concrete beams and slabs can result in thinner concrete sections andor longer spans between supports. Many times inexperienced contractors may drill or shoot column collars into concrete on top of the slab. Post tension slabs superior foundations services jackson. Posttensioned slabs show reduced cracking, improved durability and lower maintenance costs. This impediment to its free movement creates stress that can literally pull the slab apart.
Buyers have questions about post tension slabs and if theyre stronger or better than a. Signs of foundation problems in concrete slab foundations require prompt attention. From time to time even the post tension slab could crack or allow entry point for termites to get through, the floating slab has concrete meeting concrete and therefore has a crack where termites can make entry into the home. 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.
Structure magazine posttensioned concrete construction. Nevertheless, it may be pointed out here that post tensioning can also be of economic interest in the following components of a multistorey building. Whats involved with creating a post tensioned concrete podium. 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. In order to ensure that the post tension slab used will have the best chance of avoiding cracking, as well as ensuring the longevity of post tension slabs, there are several possible errors to consider. Five underlying features of post tensioned construction allow greater flexibility in reinforcement layout. Aug 27, 2015 tulsa home builders 918 6885660 can a post tension slab have cracks.
Posttensioning is used in this manner in millions of square feet of warehouse floors, sport courts, housing and specialized paving applications. The cable is going to be in the centre of the slab. Tulsa home builders 918 6885660 can a post tension. The first thing to do is add steel and fibers to the concrete slab.
Rts cracking occurs when the normal tendency of a posttensioned slab to shorten due to the effects of elastic shortening, shrinkage, and axial creep, are restrained5. By the third day, i noticed a crack in the back patio, and by the fourth, there were sprawling cracks throughout the entire foundation. Rowing correct, it isnt that clear, let me try again. Top 10 biggest mistakes made in posttensioned concrete. Post tension slab stressing timing general inspection. This type of cracking can be managed easily with control joints basically make it crack where you want it. But since a posttensioned slab does not have excess rebar to minimize and distribute cracking, restraint cracks will.
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