Pt cables are used to compress the concrete and reduce cracking, whereas rebar is dormant until the concrete cracks. Mar 27, 2018 concrete is the most important construction material on earth and foundation of our modern society. When a heavy live load is brought upon a structure, its concrete slab undergoes tension, which leads to the formation of cracks and ultimately deformation occurs. Post tensioning steel cables or rods are added to exterior of concrete beams or the underside of prestressed slabs to increase the structural capacity of these members. Cracking in hardened concrete can result from any one of many causes. Precast posttensioned segmental construction is defined as a method of construction for bridges, buildings, tanks, silos, cooling towers, tunnels and other structures in which primary load carrying members are composed of precast concrete segments post tensioned together. Shortening crack mitigation crack evaluation tn451010616. When an owner sees a crack in his slab or wall, especially if the concrete is relatively new, he automatically assumes theres something wrong. Posttensioning actively loads the structure one of the main differences between a posttensioned beam or slab and a typical reinforced concrete element is that the strands drape or profile inside the concretetheir vertical location changes along the length of the member. 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 working principle, components and construction. Shrinkage crack at slab penetration to combat random shrinkage cracks, control joints often mistakenly referred. Shrinkage crack considerations for posttensioned foundations. Use of shrinkagecompensating concrete in post tension buildings john wayne airport parking structures a2 and b2, santa ana, ca over 70,000 linear feet of cracks were measured and repaired in the 480,000 square feet of floor area in both structures. Post tensioning concrete increases the many benefits associated with a concrete framed building. Shrinkage cracks in concrete slabonground foundations are a significant. Dec 25, 2014 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. The cause of cracking in post tensioned concrete box girder bridges and retrofit procedures walter podolny, jr. This tension makes the concrete slab and foundation much stronger than concrete without reinforcement and helps reduce cracking. 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 a post tensioned beam, the tendons are stressed and each end is anchored to the concrete section after the concrete has been cast and has attained sufficient strength to. Significant overbalancing of the weight of the concrete with post tensioning can actually lift and crack the slab. Lots of cracks in new post tension slab inspectionnews.
What is the difference between normal slab and post. Pretensioning and posttensioning in prestressed concrete. Its a shame, because those cracks wreck havoc on vehicles and make for really bumpy rides. Sep 02, 2019 pre tensioned and post tensioned concrete is not well understood. In a post tensioned beam, the tendons are stressed and each end is anchored to the concrete section after the concrete has been cast and has attained sufficient strength to safely withstand the prestressing force as shown in fig. Concrete, cement explain the principles of prestressing. By the third day, i noticed a crack in the back patio, and by the fourth, there were sprawling cracks throughout the entire foundation.
Post tension slab advantages and disadvantages of post. The concrete cannot shrink smaller than the object it is poured around, and this causes enough stress to crack the concrete see figure 2. Bridge division office of engineering federal highway administration washington, d. At first glance it seems rudimentary, but there is a tremendous amount of complexity involved in. Posttensioning tendons, which are prestressing steel cables inside plastic ducts or sleeves, are positioned in the forms before the concrete is placed. The disadvantages of pouring concrete over existing concrete. Post tensioned concrete courts provide many advantages, the design of the system will greatly decrease the chances for cracking, settling and ponding. They require a specific level of thickness to maintain their strength usually at least 4 inches for a slabongrade foundation. The purpose of this publication is to widen the understanding of post tensioned floor construction and show the considerable benefits which include minimum storey heights, rapid construction, economy, maximum design flexibility, minimum number of columns, optimum clear spans, jointfree, crack free. Cracking in groundsupported post tensioned slabs on. Compressive stresses are introduced in prestressed concrete either by pre tensioning or post tensioning the steel reinforcement. They illustrate there is no better value than castinplace concrete parking structuresnow and for the future.
The alternative to pre tensioning is post tensioning. Aug 17, 2018 munson specializes in post tensioned concrete and other types of play courts, and has won national and state awards for its concrete tennis court installations. Reinforcement helps extend the life of concrete, overcoming cracking that is inherent in the material and preventing its failure. The alternative to pretensioning is post tensioning. A psc is applied with compressive stress on the crosssection in advance before the application of loading on the member so that the stresses after loading are fully or partially balanced by the prestressedstresses developed due to the tensioning of steel.
Pretensioning and posttensioning in prestressed concrete design. 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. 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. 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. Epoxy injection high strength epoxy is injected into cracks in concrete to weld back concrete to monolithic properties.
Post tensioning is a technique for reinforcing concrete. Eurocode recognises that cracking is normal in concrete subjected to bending, shear, torsion, and restraint from movement clause 7. Moisture infiltration can occur from numerous sources including initial construction, ground water, roof leakage, and poor quality grout pockets. Four reasons why concrete cracks bright hub engineering. The strain corresponding to ultimate stress is usually around 0. These projects cover the spectrum of castinplace concrete garages, both in size and application. Posttensioned concrete is a unique structural system that is widely used to construct many types of structures such as bridges, parking structures, buildings and tanks. Post tensioned concrete is a term heard more and more in the construction industry today. Posttensioning methods for reinforcing concrete the. Curling is the distortion of a slab into a curved shape by upward or downward bending of the edges.
Concrete that has cracked and concrete that is going to crack. The best that a contractor can do is to try to control the cracking. All concrete will shrink during the curing process which lasts up to a year after the time concrete is placed in the forms. Whats the solution of cracks in the post tensioning slab. 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. When a garage floor is first poured, the concrete begins to harden contractors say cure and when this process takes place the concrete shrinks a little. 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. Over tensioning, bad prepration design layingcuring of concrete, bad structural design, bad load estimation and temperature, right. 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. 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. Flexural and tensile crack widths can be expected to increase with time for members subjected to either sustained or repetitive loading. Afterwards, once the concrete has gained strength but before the service loads are applied, the cables are pulled tight, or tensioned. The cement is a powder substance and when it is combined with water, it hardens into concrete.
Since contractors and engineers know that most concrete is going to crack, they like to control where it is going to crack. Post tension nightmare post tension foundation problems. 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. Cracking in post tensioned concrete box girders generally results, in a broad sense, for the following reasons. The tension in the tendons results in an equivalent compression in the concrete, which causes the member to shorten fig. 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. Cracking is assumed to occur when the restrained strain exceeds the tensile strain capacity of the concrete. In addition, recommendations are made for crack mitigation and maintenance of buildings that are post tensioned with unbonded tendons.
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. Shrinkagecompensating concrete in post tension buildings part 2 of 2 this is the second of a twopart article with case studies of four projects that show the effective use of shrinkagecompensating concrete to mitigate restrainttoshortening rts cracking in post tensioned concrete buildings. Post tensioned slabs are a great tool to help reduce joints and control cracks. 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. Flexural cracking due to sustained post tensioning forces is believed to be more severe than cracking due to transient loading. Post tensioned concrete uses a sheathed cable called tendons which are placed before the concrete is poured.
This means that for cracking to occur, some part or the whole of the concrete section. In post tensioned construction, the tendons are stressed and anchored after the concrete member they are embedded in has developed sufficient strength fig. Similarly, a slab will crack if too much weight is placed in one spot, or if the soil settles unevenly, bending the slab. Ensure proper vibration to ensure consistency of concrete and no honeycomb voids between aggregate where no. Preventing cracks on posttensioned concrete slabs pro. One alternative to removing old concrete is to pour a new layer over the existing slab.
Apr 24, 2019 the cracking moment is the moment corresponding tensile bending stress at which concrete will start to crack. Surface cracks are also caused by surface dressing with a mortar having too rich in cement. Jul 17, 2017 conventional concrete slabs are limited in length and thickness. Principles of prestressed concrete in concrete, a large prestressing force is applied by tendons embedded into it to develop high bearing stresses at. Cracking in groundsupported posttensioned slabs on expansive soils by ken bondy1. Top 10 biggest mistakes made in posttensioned concrete design.
Bonded post tension slab cracking due to restraint rapt structural 19 mar 14 22. May 14, 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 tensioning actively loads the structure one of the main differences between a post tensioned beam or slab and a typical reinforced concrete element is that the strands drape or profile inside the concrete their. The cause of cracking in posttensioned concrete box girder.
Compressive stresses are introduced in prestressed concrete either by pretensioning or post tensioning the steel reinforcement. 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. Concrete can support a lot of weight in compression, but is weak in tension. 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. The advantages of pt as noted in the opening page are the lack of cracking or at least very. The best way to ensure this doesnt happen is to educate your construction teams on the best practices for designing and constructing post tensioned. 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.
A normal slab is basically a rcc reinforce cement concrete slab whereas a post tensioned slab is the psc prestressed concrete slab. Two broad categories of cracking can occur in groundsupported posttensioned slabs. Consequently, although posttension slabs will generally hold together, they have a higher propensity for cracking than do slabs built with plenty of rebar. Types of cracks in fresh and hardened concrete causes and. Significant overbalancing of the weight of the concrete with posttensioning can actually lift and crack the slab. Hair line crackings in garage floor post tension slab. Many concrete contractors will say that there are two types of concrete.
Post tension cable polished concrete, epoxy floor, concrete repair. For more information about post tension foundation repair contact james at 6024182970 or you can always go to for more information. 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. Bonded post tension slab cracking due to restraint. I had a call this morning from a prospective client who was concerned about cracks in the concrete foundation of the home they. The advantages of castinplace post tensioned concrete parking structures discussed above are shown by the case histories that follow. Information about how posttensioned concrete is constructed. We have decided to do two or three slabs in post tension and we will keep track of. New homes cracks that occur in the first few months or years. Comparing pre tensioned and post tensioned concrete.
Used to resist cracking caused by curing shrinkage or thermal expansioncontraction in slabs and walls. This video describes the benefits and challenges of both methods and describes how they are built. These causes include a drying shrinkage, b thermal stresses, c chemical reactions, d weathering, which involves heating and cooling and is linked to thermal stresses, e the corrosion of steel reinforcing, f poor construction practices. Throughout the garage, on every slab, post tensioned tendons needed replacement and deteriorating concrete required repair. It can withstand strong mother nature storms, cushioned systems can be applied on the slab and works great both indoor and out. The occurrence is primarily due to differences in moisture andor temperature between the top and bottom surfaces of a concrete slab.
Many people dont understand how they work and the benefits. All concrete will shrink during the curing process which lasts up to a year after the time concrete. Post tensioned concrete repair post tensioned concrete corrosion the principal cause of corrosion of post tension tendons is the presence of moisture and air voids around the strands. Removing an existing concrete slab is a lengthy, laborintensive process. Cracks in concrete are extremely common but often misunderstood. Munson specializes in post tensioned concrete and other types of play courts, and has won national and state awards for its concrete tennis court installations. Miyagi construction concrete repair,post tension cable repair.
One method of avoiding such hair cracks is to remove the surface skin of the concrete by brushing it with a stiff brush soon after setting. For this reason it needs help in resisting tensile stresses caused by bending forces from applied loads which can result in cracking and ultimately failure. Some say it doesnt matter, and concrete will crack either way. The biggest drawback of asphalt courts is not that they crack, for even a post tensioned slab may crack from shrinkage or even develop minor structural cracking, but that the cracks. Conventional concrete slabs are limited in length and thickness. 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.
But since a post tensioned slab does not have excess rebar to minimize and distribute cracking, restraint cracks will be large and very noticeable. Posttensioned concrete cable repair contractors concrete. Why concrete cracks concrete crack solutions ardex ireland. Tensioning building structures march 12, 2020 2020 educode las vegas. Post tensioning tendons, which are prestressing steel cables inside plastic ducts or sleeves, are positioned in the forms before the concrete is placed.
For example, a concrete wall can support tons of weight from above, but will crack easily if pushed sideways forcing it to bend. The ultimate stress is reached when a large crack network is formed within the concrete, consisting of the coalesced microcracks and the cracks in the cement paste matrix. Flexural cracks due to eccentric post tensioning forces will remain open throughout the life of the wall, and may create problems related to water penetration, freezethaw or corrosion. Too much water, insufficient curing, or from over trowelling. Now assuming all these are taken care of, no cracks should be there. Preventing cracks on posttensioned concrete slabs pro builder. 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 design. Two of these projects were built more than 40 years ago, one has been in service for 12 years, and one is new, completed just 19 months before this writing.
Prestressed concrete is a highly versatile construction material as a result of it being an almost ideal combination of its two main constituents. 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. On the basis of limited data, the following expression has been suggested to estimate the maximum crack width in direct tension aci 224r. Before a post tension slab is poured, highstrength steel strands or cables, called tendons, are laid in a tight grid. Cracks, cracks, cracks what is a reasonable level of cracking. Several methods exist for prestressing concrete, with post tensioning being a very common one. Shrinkagecompensating concrete in posttensioned buildings. Long concrete slabs elevated above the ground are limited in length due to the danger of cracking. However, only the use of special chemical expanding cement will eliminate it. Concrete, although strong in compression, is weak in tension. Sealing cracks in posttensioned deck concrete construction. Issues in posttensioned construction concrete construction.