The purpose of this document is to explore implementation methods of posttensioning using the turnofthe nut procedure, to apply posttensioning on inverted tbeam specimens. How home builders can avoid cracks in a posttensioned slabongrade foundation by preventing certain errors made during the installation process. I would not be alarmed at cracks of this magnitude but would monitor and make sure that any excessive shrinkage that might endanger the post tension cable system should be sealed. The contract work to be performed under this section consists of furnishing all required labor, materials, equipment, implements, parts and supplies necessary for, or appurtenant to, the. Usually a sign in the garage or stamp in the slab will indicate if your home has a post tension slab. Friends in this video lecture i show you the method of the crack repairing. Hair line crackings in garage floor post tension slab. Over tensioning, bad prepration design layingcuring of concrete, bad structural design, bad load estimation and temperature, right. Additionally, since posttension slabs are placed without expansion joints, there has to be at least enough compression to mitigate shrinkage cracks. In this article, ill be giving homeowners some necessary information about the characteristics, advantages, disadvantages, and common questionsconcerns that i. These categories of cracks are caused by entirely different phenomena and have very different characteristics. Post tensioned slabs are a great tool to help reduce joints and control cracks. 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. In posttensioned floor slab construction, the placement of posttensioning tendons and associated nonprestressed.
This was present throughout the home, no floor covering anywhere. Post tensioning or normal concrete floors will crack as you described two hours after it was poured. Signs of foundation problems concrete slab foundations. Post tension slabs must be marked or stamped with an identifier placard, as they cannot be drilled or cut into by a lay. If you notice a post tension cable sticking out of the. Post tensioned foundation information for the homeowner. When tensioning the strands these allow the slab to withstand a greater load both punctual and distributed, compared to a slab of the same thickness. Bonded post tension slab cracking due to restraint. Signs of foundation problems in concrete slab foundations require prompt attention. Types of pour strip options pros and cons tech sales. By using posttensioned tendons to compress the concrete. After decades of building and inspecting in the post tensioning pt concrete construction industry, weve seen the best and the worst of pt construction.
Buyers have a number of options when it comes to permits. A normal slab is basically a rcc reinforce cement concrete slab where as a post tentioned slab is the psc prestressed concret slab. Bonded post tension slab cracking due to restraint aaronpteng structural op. I had a call this morning from a prospective client who was concerned about cracks in the concrete foundation of the home they. Shrinkage cracks appear as a displacement or separation in the surface of the concrete and. The concrete is poured, allowed to dry, and the tensioners removed.
Two broad categories of cracking can occur in groundsupported posttensioned slabs. Shrinkage cracks in concrete slab onground foundations are a significant public relations problem to home builders. The slab was decorative concrete it had been stained a dark brown, scored and polished. Posttensioned foundation a typical posttensioned foundation with the forms set and ready for concrete. Shrinkage generates tension within the concrete until at some point concrete will separate yielding a shrinkage crack. Post tension slabs have steel cables that run through them, similar to rebar but are further apart. Posttensioning is commonly used in large commercial and industrial projects such as garages and towers. Whats the solution of cracks in the post tensioning slab. After about seven days these cables are stretched tightly with. For example, a concrete wall can support tons of weight from above, but will crack easily if pushed sideways forcing it to bend. Concrete can support a lot of weight in compression, but is weak in tension. The lobby level of the garage has a 4inch topping on a 10inch posttensioned slab with. Things you should know about cracks in slab foundation.
Flexural cracks develop when the tensile strength of concrete is exceeded under load. Lots of cracks in new post tension slab inspectionnews. Steel reinforcement bars are therefore required in the slab. It reduces or eliminates shrinkage crackingtherefore no joints, or fewer joints, are needed. My firm was recently hired to look at cracks that had developed in a post tensioned floor slab ongrade. Top 10 biggest mistakes made in posttensioned concrete design. Cracks caused by applied loads flexural or shear cracks.
Foundation crack in garage common, nonstructural crack in a garage floor. You can expect shrinkage and cracks in the slab foundation and these are very common. There are several post tension slab problems which can be avoided. But since a posttensioned slab does not have excess rebar to minimize and distribute cracking, restraint cracks will be large and very noticeable. This hybridisation helps achieve the formation of a much thinner slab with a longer span devoid of any columnfree spaces. The cracks are perpendicular to the uniform tendons direction. 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.
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. Posttensioning will reduce cracking and keep any cracks that might. Over tensioning, bad prepration design laying curing of concrete, bad structural design, bad load estimation. They usually do not compromise the structural integrity of the home. Several methods exist for prestressing concrete, with posttensioning being a very common one.
Also, because aci 302 says a reasonable level of cracking is from 0% to 3%, it implies that no. Help bad cracks in post tension slab pics inspectionnews. Over tensioning, bad prepration design layingcuring of concrete, bad structural design, bad load estimation. In concrete we mainly face the cracks so cracks can me repaired by using this method. Posttensioned foundations are particularly susceptible to this problem because, until the stressing is applied to the. 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.
Post tension slab eliminating cracks and joints in concrete. Similarly, a slab will crack if too much weight is placed in one spot, or if the soil settles unevenly, bending the slab. In addition to having more movement, a post tensioned slab will have substantially less rebar than a conventional system, which is one of the main economic benefits. 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. Even before saw cut control joints are attempted at times. Before a posttension slab is poured, highstrength steel strands or cables, called tendons, are laid in a tight grid. 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. The dsld engineer said the cracks are normal in post tension slabs but i don. The use and proper construction of slip details is therefore critical for the performance and aesthetics of posttensioned concrete. Many people dont understand how they work and the benefits. I am building a new home in longview, texas, and they poured our post tension slab 7 days ago. Avoid the errors that cause cracks in posttensioned concrete slab foundations.
Avoiding posttension errors which result in cracking. 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. But since a post tensioned slab does not have excess rebar to minimize and distribute cracking, restraint cracks will be large and very noticeable. 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. 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. When cracks like this are evident, there are usually cracks in sheetrock and masonry or brick. Hairline cracks x 8 long consistently happen at uniform tendons dead end location at bottom of slab. Then, each cable is torqued up to 30,000 pounds, and held into place. Cracking in groundsupported post tensioned slabs on. By michael dickens march 9, 2020 slabongrade foundations are wellsuited to southern u. Should i get a foundation engineer to come out to look at it.
Call us today to schedule a free foundation evaluation. What is a post tension slab does your home have one. Most often the cracks extend across the entire foundation and occur prior to tensioning of the tendon cables. Advantages of using posttensioning for your next slabonground project. Preventing cracks on posttensioned concrete slabs pro. The cause of cracking in posttensioned concrete box girder bridges and retrofit procedures walter podolny, jr. Top 10 biggest mistakes made in posttensioned concrete. Cracks caused by the restraint to axial slab shortening rts cracks. By the third day, i noticed a crack in the back patio, and by the fourth, there were sprawling cracks throughout the entire foundation. Sections are tensioncontrolled when the tensile strain in the extreme tension. We had one customer come back with the ve the slab to post tension and the cost savings was offset by the redesign fee.
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. 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. Posttensioning, which is a form of prestressing, has several advantages over standard reinforcing steel rebars. Lots of cracks in new post tension slab inspection news. The cause of cracking in posttensioned concrete box. A posttension slab is a slab of concrete that has been prestressed using a specific method to increase the strength of the concrete. 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. Cracks that are developed due to the restraint by supports to the free shortening of the slab require special attention and treatment in rebar detailing. 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.
Shrinkage crack considerations for post tensioned foundations june 1, 2015. Are post tension concrete slabs better than slabs with rebar or no rereinforcement. Vertical endregion posttensioning is a developing method for preventing websplitting cracks. If there are cracks, they should be hairline cracks only. The conventional pour strip provides both moment and shear continuity. Homeowners are getting crushed by flawed posttension. What is the difference between normal slab and post. Found at almost every maybe 50% location of at the uniform tendons anchorage end. 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. All concrete will shrink during the curing process which lasts up to a year after the time concrete is placed in the forms. Posttension basics how posttensioned slabs are built. The effect is a springloaded slab that is resistant to any movement and most cracks. Post tension slab working principle, components and. 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.
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. Similarly with temperature changes, expansion and contraction can happen. Does eliminate slab cracks caused by slab volume changes due to post tension stressing and concrete curing. In short, posttension concrete construction gives you more strength per dollar than conventionally reinforced concrete and when properly designed and constructed, less cracks. The current version of the aci 318 states that in no circumstances may the precompression in a slab drop below 125 psi. This is a typical concern for those purchasing new construction who are not familiar with post tension cable reinfored foundation construction methods and soils conditions in our area. Preventing cracks on posttensioned concrete slabs pro builder. A psc is applied with a compressive stress on the cross section in advance before the application of loading on. It allows slabs and other structural members to be thinner. The aim of the post is not to identify the types of cracks or anything, it is more about we all have seen poorly detailed structures failing to take into account restraining effects. 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.
Due to humidity, things like wood floors, trim, wood framing can shrink and get acclimated to lower inside humidity. I also like to add a top layer of wire mesh above the tendons to eliminate shrinkage cracks, which also adds cost to. A post tension slab will usually have a stamp located by the garage entry either just inside the garage door our just outside the garage door. Does not provide for continuity of diaphragm chord steel, required for system lateral resistance. 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. Bridge division office of engineering federal highway administration washington, d. These natural cracks are known as shrinkage cracks.