Cracking in groundsupported posttensioned slabs on expansive soils by ken bondy1 1 introduction posttensioned slabs are widely used as foundations for light residential construction in areas of expansive soils. Posttensioning 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. Posttensioned parking garages can be either standalone structures or one or more floors in an office or residential building. Reinforcement helps extend the life of concrete, overcoming cracking that is inherent in the material and preventing its failure.
Additionally, the concrete of the slab is in heavy compression. The bond condition of tendons may influence the flexural and shear performance of. On the other hand post tension slabs allow for the soil compaction due to decay and are strong enough to avoid the associated cracking. How to create a concrete slab using the posttensioning method. Shrinkage crack considerations for posttensioned foundations june 1, 2015 shrinkage cracks in concrete slabonground foundations are a significant public relations problem to home builders. Whats the solution of cracks in the post tensioning slab. All concrete will shrink during the curing process which lasts up to a. 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. Shrinkage cracks in concrete slabonground foundations are a significant.
Professionals helpline 0800 80 80 85 is available free to those. Reinforced concrete slabs have been used in buildings from the middle of the 19 th century. A posttensioned foundation is a concrete slab foundation that is reinforced with stranded steel cables that are tensioned after. Nov 25, 2008 post tensioned concrete is a term heard more and more in the construction industry today. Founded in 1904 and headquartered in farmington hills, michigan, usa, the american concrete institute is a leading authority and resource worldwide for the development, dissemination, and adoption of its consensusbased standards, technical resources, educational programs, and proven expertise for individuals and organizations involved in concrete. In bonded post tensioning, the prestressing force in cracks is transferred to the concrete by bond stresses on either side of the crack. Post tension slab advantages and disadvantages of post. Architectural benefits post tensioned slab has an advantage over others as it makes a very efficient base for floor design with thin slabs and columnless spaces in larger spans. The first edition of the posttensioning manual was published in 1972, and as of the time of this writing 2018, they are up to their 6th edition. Based on extensive observations and a survey of the performance of unbonded post tensioned buildings, paper presents a categorization of the common cracks in beams and slabs, and the principal causes of crack formation due to restraining effects of supporting structures are discussed in detail. Professionals helpline 0800 80 80 85 is available free to those who work. Posttensioned concrete construction is a little different than standard reinforced concrete construction. The posttensioning institute designed posttensioned foundations. Post tension concrete dubaiuae, cairoegypt and riyadh.
Prestressed concrete is a form of concrete used in construction. On the other hand posttension slabs allow for the soil compaction due to decay and are strong enough to avoid the associated cracking. Private bag 50 908, porirua 5240, new zealand phone. But since a posttensioned slab does not have excess rebar to minimize and distribute cracking, restraint cracks will be large and very noticeable. Usually a sign in the garage or stamp in the slab will indicate if your home has a post tension slab. The tension in the tendons results in an equivalent compression in the concrete, which causes the member to shorten fig. All concrete will shrink during the curing process which lasts up to a year after the time concrete is placed in the forms. The post tensioning method is the best way to create stronger, more reliable concrete slabs. It reduces or eliminates shrinkage crackingtherefore no joints, or fewer joints, are needed. This stronger, thinner slab also saves money on concrete. Mar 09, 2020 with concrete, weather is an important factor, as wet or dry conditions can significantly change the timing of when you stress the tendons.
Shrinkage crack considerations for posttensioned foundations. Itll come out at the anchor where they tensioned it since that anchor relies on the tension to clamp down directional. Understanding the causes of some of these problems should help contractors build better post tensioned structures. Avoiding posttension errors which result in cracking. Top 10 biggest mistakes made in posttensioned concrete. 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. Posttensioned pt slabsonground also experience thermal and drying shrinkage behavior, yet these slabs do not normally contain contraction joints.
Steel reinforcement bars are therefore required in the slab. So lets consider another crack prediction sourcethis time for post tensioned slabs. Cracks, cracks, cracks what is a reasonable level of cracking. A concrete slab is susceptible to soil expansion and contraction, which can lead to cracking.
Shrinkage crack considerations for post tensioned foundations june 1, 2015 shrinkage cracks in concrete slabonground foundations are a significant public relations problem to home builders. There are several post tension slab problems which can be avoided. Post tensioning, which is a form of prestressing, has several advantages over standard reinforcing steel rebars. It reduces or eliminates shrinkage cracking therefore no joints, or fewer joints, are needed. This method is a better way to construct a slab foundation to be more resistant to cracking and deflection.
Lots of cracks in new post tension slab inspectionnews. Since pt is simply reinforcement, there really arent any specific decorative applications related to post tensioning. Post tension slab eliminating cracks and joints in concrete. Post tensioned foundations are also used in areas with stable soils to reduce cracking, reduce or eliminate control joints, increase flexural capacity and improve constructability. To create a slab using this method, plastic sleevecovered steel cables are laid in a grid running in both directions.
In part a a posttensioned member on very flexible supports shortens under the precompression, forcing the supports to follow the members moment. In post tensioned construction, the tendons are stressed and anchored after the concrete member they are embedded in has developed sufficient strength fig. Posttensioned concrete courts are reinforced by sheathed and greased steel cables that are placed within the slab. It is substantially prestressed during production, in a manner that strengthens it against tensile forces which will exist when in service 35 this compression is produced by the tensioning of highstrength tendons located within or adjacent to the concrete and is done to improve the performance of the concrete in service. After the concrete has reached sufficient strength, the cables are tightened, placing the slab under permanent compression. Very good video showing step by step post tension slab youtube. Shortening crack mitigation crack evaluation tn451010616. Avoiding post tension errors which result in cracking. Posttension concrete slabs are used to create a monolithic single pour slab that is stronger than a traditional slab without. 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. 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. Since there are a number of tendons and wires within the post tensioned concrete, it can result in corrosion. The foundation in question was a post tension cable reinforced foundation as opposed to a foundation which is reinforced with steel rebar see the typical photograph in this article.
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. Structural reliability ccl post tensioned slabs show reduced cracking. Types of pour strip options pros and cons tech sales. Post tensioning tendons, which are prestressing steel cables inside plastic ducts or sleeves, are positioned in the forms before the concrete is placed. Many people dont understand how they work and the benefits and challenges. Post tensioned concrete is a term heard more and more in the construction industry today. 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 cast in place method of building concrete structures. But since a post tensioned slab does not have excess rebar to minimize and distribute cracking, restraint cracks will be large and very noticeable. Understanding the causes of some of these problems should help contractors build better posttensioned structures. The lobby level of the garage has a 4inch topping on a 10inch posttensioned slab with.
Post tension slab working principle, components and construction subhajit basak. Steel tensionedbefore concrete is placed typically precast 2. Jul 17, 2017 conventional concrete slabs are limited in length and thickness. The importance of monitoring temperature during posttensioning.
Post tensioned concrete construction is a little different than standard reinforced concrete construction. Advantages of using posttensioning for your next slabonground project. 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. Journal articles on cracking in concrete delamination cracking behavior of curved post tensioned concrete structures. It allows us to build slabs on expansive or soft soils. Post tension slabs answers to questions buyers ask. The extent of the restraint cracking in a posttensioned member depends on a number of factors, including the stiffness of the supports. In order to ensure that the post tension slab used will have the best chance of avoiding. Pt slabs have lesser thickness than rcc slabs as their deflection can be controlled by post tensioning. Why polishing suspended concrete slabs is more likely to. Post tensioned foundation information for the homeowner. Advantages and disadvantages of each of these slab designs are provided. Structural engineers design concrete slabs with post tension cables to obtain greater span lengths between supporting columns and to allow thinner slabs.
Flexural cracks develop when the tensile strength of concrete is exceeded under load. The posttensioning method is the best way to create stronger, more reliable concrete slabs. Posttensioning will reduce cracking and keep any cracks that might. Post tensioned pt slabs onground also experience thermal and drying shrinkage behavior, yet these slabs do not normally contain contraction joints.
Once the slab cures, qualified personnel apply tension and grout the cables. There is the expectation that post tensioning will prevent cracking and for pt warehouse slabs the reason for using post tensioning is to eliminate contraction joints in order to save on. There are several construction issues with post tensioned concrete that are found consistently among engineers and contractors. Posttensioning is used for many reasons, including economy and building efficiency as well as the reduction of deflection and cracking it provides. In this article, ill be giving homeowners some necessary information about the characteristics, advantages, disadvantages, and common questionsconcerns that i receive regarding posttensioned foundations. Mar 04, 2016 flatness and levelness values for formed and shored post tensioned or reinforced concrete suspended slabs, however, could be as high as those for slabs onground f f 50f l 35. Home builders can avoid cracks in a posttensioned concrete slabongrade foundation by preventing certain errors made during installation. Reducing the control joints also improves the serviceability and eliminates durability problems.
Posttension basics how posttensioned slabs are built. Design criteria and technology for posttensioned slabs have been developed to a level comparable to that available for conventionally reinforced slabs. Grisolia, with almost a decade of experience in the home building industry, has assisted housing giants affected by quality issues like cracking in posttensioned slabs in southwest markets. Pdf experimental and numerical assessment of bonded and. Longer spans can be achieved due to prestress, which can also be used to counteract deflections. Such slabs generally have much less cracking than conventionally reinforced slabs. Post tensioned slabs show reduced cracking, improved durability and lower maintenance costs. Structural reliability ccl posttensioned slabs show reduced cracking. Posttensioned slabs use highstrength tensioned steel strands to compress the slabs, keeping the majority of the concrete in compression. Steel tensioned after concrete is placed and hardened typicallycast. Very good video showing step by step post tension slab. They require a specific level of thickness to maintain their strength usually at least 4 inches for a slabongrade foundation. Have you ever cut a posttensioned slab forum archinect. The demand for post tensioned slabs, throughout the world, continues to increase because of.
Post tensioned concrete slabs are typically expected to crack less than other slabs because post tensioning produces a compressive stress that must be exceeded before enough tensile stress develops to cause cracking. 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. 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. Post tensioning also results in lesser reinforceme. Current structural concrete design standards require that posttensioned slab systems be provided with mild steel or posttensioning tendons to. 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. Posttensioning tendons, which are prestressing steel cables inside plastic ducts or sleeves, are positioned in the forms before the concrete is placed. Architects like concrete slabs because the designs can allow for longer spans and thinner slabs. It allows slabs and other structural members to be thinner.
Branz corporation, concrete slabs and conrol joints, build, augsept 2005, branz, moonshine road, judgeford, porirua city 5381, new zealand post. Issues in posttensioned construction concrete construction. We recommend using pt for temperature and shrinkage reinforcement. 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. Structure magazine posttensioned concrete construction. Long concrete slabs elevated above the ground are limited in length due to the danger of cracking. With concrete, weather is an important factor, as wet or dry conditions can significantly change the timing of when you stress the tendons. A posttensioned pt concrete member can be reinforced with either bonded or unbonded prestressing tendons. Posttension basics how posttensioned slabs are built the. Top 10 biggest mistakes made in posttensioned concrete design.
Post tensioned slabs use highstrength tensioned steel strands to compress the slabs, keeping the majority of the concrete in compression. 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. Basically you have a steel rod, tensioned to around 30,000lbs, in a greased tube. Based on extensive observations and a survey of the performance of unbonded posttensioned buildings, paper presents a categorization of the common cracks in beams and slabs, and the principal causes of crack formation due to restraining effects. Cracking in groundsupported post tensioned slabs on. There are several basic advantages for a post tension slab. This tension makes the concrete slab and foundation much stronger than concrete without reinforcement and helps reduce cracking. However, in my opinion, the main culprit is micro cracks and voids, which causes reduction in the surface to take up tension. 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.
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. Preventing cracks on posttensioned concrete slabs pro. Additional reinforcing bars have been used throughout the entire slab to better distribute the cracking due to concrete slab volume change. As a building material, concrete is strong in compression, but weak in tension. The use and proper construction of slip details is therefore critical for the performance and aesthetics of post tensioned concrete. I had a call this morning from a prospective client who was concerned about cracks in the concrete foundation of the home they. There are several construction issues with posttensioned concrete that are found consistently among engineers and contractors. A posttensioned foundation is a concrete slab foundation that is reinforced with. Posttensioning, which is a form of prestressing, has several advantages over standard reinforcing steel rebars. Post tensioned slabs are used when spans are large and normal rcc slabs become uneconomical. Control joints in residential post tension slabs trakjoint. There is the expectation that posttensioning will prevent cracking and for pt warehouse slabs the reason for using posttensioning is to eliminate contraction joints in order to save on. Using rebar for shrinkage and temperature reinforcement in parking garage slabs.
After decades of building and inspecting in the posttensioning pt. Pt slabs offer the thinnest slab type, as concrete is worked to its strengths, mostly being kept in compression. By the third day, i noticed a crack in the back patio, and by the fourth, there were sprawling cracks throughout the entire foundation. Many people dont understand how they work and the benefits. Post tensioned slabs are a great tool to help reduce joints and control cracks.
Posttensioned foundation a typical posttensioned foundation with the forms set and ready for concrete. Pt concrete courts are relatively unaffected by temperatures. Nov 27, 2018 many post tensioned slabs are often stamped to alert workers, as well as owners, in case any renovations take place in the future. In areas where there are expansive clays or soils with low bearing capacity, posttensioned slabsonground and mat foundations reduce problems with. Flatness and levelness values for formed and shored posttensioned or reinforced concrete suspended slabs, however, could be as high as those for slabsonground f f 50f l 35. Preventing cracks on posttensioned concrete slabs pro builder. In detail, the advantages of the use of posttensioned slab can be summarized as follows. Q recently we completed construction of a hotel parking garage. As the posttensioned slabs have lesser thickness, the quantity of concrete and reinforcement used is reduced upto 20% 30% when compared to conventional concrete slabs. 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. So lets consider another crack prediction sourcethis time for posttensioned slabs. Posttensioned slabs show reduced cracking, improved durability and lower maintenance costs. Post tension slab working principle, components and. The importance of monitoring temperature during post.
Post tensioned concrete is a variant of prestressed concrete where the tendons are tensioned after the surrounding concrete structure has been cast 25 the tendons are not placed in direct contact with the concrete, but are encapsulated within a protective sleeve or duct which is either cast into the concrete structure or placed adjacent to it. Posttensioning is a form of prestressing concrete to provide reinforcement and overcome concretes weaknesses. Posttensioned concrete slabs have been used since the 1930s, and have become commonplace in the last 30 years. Architectural benefits posttensioned slab has an advantage over others as it makes a very efficient base for floor design with thin slabs and columnless spaces in larger spans.
Jan 24, 2015 post tension slab 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. Since there are a number of tendons and wires within the posttensioned concrete, it can result in corrosion. Sealing cracks in posttensioned deck concrete construction. Post tensioning is used for many reasons, including economy and building efficiency as well as the reduction of deflection and cracking it provides.
A typical no pour strip with a single construction joint only and keeping the post tensioning continuous between restraints i. 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. Im told that the cables can be partially stretched to help offset the cracking in. In order to ensure that the posttension slab used will have the best chance of avoiding. Posttension cables are steel wires inserted into plastic sheathing laid in place while pouring the concrete slab. Post tension slab 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. Many posttensioned slabs are often stamped to alert workers, as well as owners, in case any renovations take place in the future. As we all know, concrete tends to crack over time it helps spread the load weight of the home over the entire slab area verses just on the footing area. Post tension cables are steel wires inserted into plastic sheathing laid in place while pouring the concrete slab. Post tension slabs superior foundations services jackson. Jul 22, 2019 post tensioned slabs are a great tool to help reduce joints and control cracks.
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