Post tension slab cracks

It reduces or eliminates shrinkage crackingtherefore no joints, or fewer joints, are needed. Does eliminate slab cracks caused by slab volume changes due to post tension stressing and concrete curing. Posttensioned foundations are particularly susceptible to this problem because, until the stressing is applied to the. Concrete can support a lot of weight in compression, but is weak in tension. In posttensioned floor slab construction, the placement of posttensioning tendons and associated nonprestressed. Post tension slabs must be marked or stamped with an identifier placard, as they cannot be drilled or cut into by a lay. Sealing cracks in posttensioned deck concrete construction. After about seven days these cables are stretched tightly with. Over tensioning, bad prepration design laying curing of concrete, bad structural design, bad load estimation. The conventional pour strip provides both moment and shear continuity. Over tensioning, bad prepration design layingcuring of concrete, bad structural design, bad load estimation and temperature, right. Post tensioning or normal concrete floors will crack as you described two hours after it was poured. Should i get a foundation engineer to come out to look at it.

Preventing cracks on posttensioned concrete slabs pro builder. Whats the solution of cracks in the post tensioning slab. It is possible to map the slab and see where it is curling up, but a slab that. Help bad cracks in post tension slab pics inspectionnews. Buyers have a number of options when it comes to permits. Cracks, cracks, cracks what is a reasonable level of cracking. For example, a concrete wall can support tons of weight from above, but will crack easily if pushed sideways forcing it to bend. Even before saw cut control joints are attempted at times. But since a posttensioned slab does not have excess rebar to minimize and distribute cracking, restraint cracks will be large and very noticeable. 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.

It allows slabs and other structural members to be thinner. 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. Does not create a any moment continuity only shear continuity. Posttensioning will reduce cracking and keep any cracks that might.

What is the difference between normal slab and post. Similarly, a slab will crack if too much weight is placed in one spot, or if the soil settles unevenly, bending the slab. Due to humidity, things like wood floors, trim, wood framing can shrink and get acclimated to lower inside humidity. The cause of cracking in posttensioned concrete box girder bridges and retrofit procedures walter podolny, jr. Posttensioned foundation a typical posttensioned foundation with the forms set and ready for concrete. 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. Are post tension concrete slabs better than slabs with rebar or no rereinforcement. I am building a new home in longview, texas, and they poured our post tension slab 7 days ago. In concrete we mainly face the cracks so cracks can me repaired by using this method. 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. 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. 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. They usually do not compromise the structural integrity of the home.

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. Bonded post tension slab cracking due to restraint. These natural cracks are known as shrinkage cracks. Homeowners are getting crushed by flawed posttension. This was present throughout the home, no floor covering anywhere. The lobby level of the garage has a 4inch topping on a 10inch posttensioned slab with. The current version of the aci 318 states that in no circumstances may the precompression in a slab drop below 125 psi. Things you should know about cracks in slab foundation. I had a call this morning from a prospective client who was concerned about cracks in the concrete foundation of the home they. Hair line crackings in garage floor post tension slab. In this article, ill be giving homeowners some necessary information about the characteristics, advantages, disadvantages, and common questionsconcerns that i.

Top 10 biggest mistakes made in posttensioned concrete. You can expect shrinkage and cracks in the slab foundation and these are very common. 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. A normal slab is basically a rcc reinforce cement concrete slab where as a post tentioned slab is the psc prestressed concret slab.

By the third day, i noticed a crack in the back patio, and by the fourth, there were sprawling cracks throughout the entire foundation. Lots of cracks in new post tension slab inspectionnews. 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. Preventing cracks on posttensioned concrete slabs pro. Two broad categories of cracking can occur in groundsupported posttensioned slabs. Post tensioned slabs are a great tool to help reduce joints and control cracks. 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. We had one customer come back with the ve the slab to post tension and the cost savings was offset by the redesign fee. Avoiding posttension errors which result in cracking. Additionally, since posttension slabs are placed without expansion joints, there has to be at least enough compression to mitigate shrinkage cracks. A posttension slab is a slab of concrete that has been prestressed using a specific method to increase the strength of the concrete. 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.

When cracks like this are evident, there are usually cracks in sheetrock and masonry or brick. 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. 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. How home builders can avoid cracks in a posttensioned slabongrade foundation by preventing certain errors made during the installation process. Flexural cracks develop when the tensile strength of concrete is exceeded under load. Vertical endregion posttensioning is a developing method for preventing websplitting cracks. The dsld engineer said the cracks are normal in post tension slabs but i don. 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. Call us today to schedule a free foundation evaluation. This in turn causes wall cracks and ceiling cracks. 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. The slab was decorative concrete it had been stained a dark brown, scored and polished. Shrinkage generates tension within the concrete until at some point concrete will separate yielding a shrinkage crack.

My firm was recently hired to look at cracks that had developed in a posttensioned floor slabongrade. Before a posttension slab is poured, highstrength steel strands or cables, called tendons, are laid in a tight grid. Shrinkage crack considerations for post tensioned foundations june 1, 2015. The cause of cracking in posttensioned concrete box. Foundation crack in garage common, nonstructural crack in a garage floor.

Posttensioning is commonly used in large commercial and industrial projects such as garages and towers. Now assuming all these are taken care of, no cracks should be there. Then, each cable is torqued up to 30,000 pounds, and held into place. Top 10 biggest mistakes made in posttensioned concrete design. The purpose of this document is to explore implementation methods of posttensioning using the turnofthe nut procedure, to apply posttensioning on inverted tbeam specimens. After decades of building and inspecting in the post tensioning pt concrete construction industry, weve seen the best and the worst of pt construction. 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. Posttension basics how posttensioned slabs are built.

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 cracks are perpendicular to the uniform tendons direction. Lots of cracks in new post tension slab inspection news. The concrete is poured, allowed to dry, and the tensioners removed. 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, which is a form of prestressing, has several advantages over standard reinforcing steel rebars. Hairline cracks x 8 long consistently happen at uniform tendons dead end location at bottom of slab. Posttension reinforcement is installed must faster than rebar steel which reduces construction time and cost. 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. Bridge division office of engineering federal highway administration washington, d. Shrinkage cracks in concrete slab onground foundations are a significant public relations problem to home builders.

Signs of foundation problems in concrete slab foundations require prompt attention. My firm was recently hired to look at cracks that had developed in a post tensioned floor slab ongrade. Cracks caused by applied loads flexural or shear cracks. This hybridisation helps achieve the formation of a much thinner slab with a longer span devoid of any columnfree spaces. Also, because aci 302 says a reasonable level of cracking is from 0% to 3%, it implies that no.

Homeowners and builders are frequently very concerned to see the cracking in a brand new slab foundation. Post tensioned foundation information for the homeowner. The use and proper construction of slip details is therefore critical for the performance and aesthetics of posttensioned concrete. Shrinkage cracks appear as a displacement or separation in the surface of the concrete and. 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. Avoid the errors that cause cracks in posttensioned concrete slab foundations. Types of pour strip options pros and cons tech sales. Most often the cracks extend across the entire foundation and occur prior to tensioning of the tendon cables. If there are cracks, they should be hairline cracks only. Found at almost every maybe 50% location of at the uniform tendons anchorage end. Post tension slabs have steel cables that run through them, similar to rebar but are further apart.

A psc is applied with a compressive stress on the cross section in advance before the application of loading on. If you notice a post tension cable sticking out of the. 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. By using posttensioned tendons to compress the concrete. Post tension slab eliminating cracks and joints in concrete. There are several post tension slab problems which can be avoided. What is a post tension slab does your home have one. Over tensioning, bad prepration design layingcuring of concrete, bad structural design, bad load estimation. In short, posttension concrete construction gives you more strength per dollar than conventionally reinforced concrete and when properly designed and constructed, less cracks. 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. Several methods exist for prestressing concrete, with posttensioning being a very common one. Signs of foundation problems concrete slab foundations.

All concrete will shrink during the curing process which lasts up to a year after the time concrete is placed in the forms. Friends in this video lecture i show you the method of the crack repairing. Advantages of using posttensioning for your next slabonground project. These categories of cracks are caused by entirely different phenomena and have very different characteristics. 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. The effect is a springloaded slab that is resistant to any movement and most cracks. Cracks caused by the restraint to axial slab shortening rts cracks. Steel reinforcement bars are therefore required in the slab. Does not provide for continuity of diaphragm chord steel, required for system lateral resistance. Cracking in groundsupported post tensioned slabs on. Usually a sign in the garage or stamp in the slab will indicate if your home has a post tension slab. 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. Post tension slab working principle, components and.