The most common complaintconcern that i hear is in regards to its propensity to develop long unsightly curing cracks, which will be forever visible in the garage and rear patios. Post tension slab eliminating cracks and joints in. Construction of post tensioned slabs on grade is very similar to using reinforcing steel, except for the tensioning step. Usually a sign in the garage or stamp in the slab will indicate if your home has a post tension slab. New house concrete slab foundation cracks home improvement. If there are cracks, they should be hairline cracks only. 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 post tensioned slab than in a jointed slabonground. Sep 18, 2006 post tensioned foundations use highstrength steel to actively reinforce residential foundations to minimize cracking and increase performance. Typically shrinkage cracks occur prior to post tensioning and erecting the walls, so these would likely have been noticed when the floor was stained. Avoiding post tension errors result in the best chance of a crackfree post tension foundation.
Most concrete slab foundations were and are built with steel rebar or post tension cables for strength. 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 feel the uneveness with your shoe if you rub it across the cracks. For residential construction, tendons at 48 inches on center are common. Issues in posttensioned construction concrete construction. Sep 28, 2017 diagonal foundation cracks are caused by differential settling of a foundation, which is where one side of a homes foundation settles lower than the rest of the foundation. The tendon profile, which is typically in a concave shape.
Things you should know about cracks in slab foundation. Post tension foundations are what tract home and apartment builders use so therefore they are lower quality. Posttensioned foundations use highstrength steel to actively reinforce residential foundations to minimize cracking and increase performance. Cracks, cracks, cracks what is a reasonable level of cracking.
Flexural cracks develop when the tensile strength of concrete is exceeded under load. Mar 09, 2020 depending on the extent of the damage, you may also have to put up the homeowners in a hotel during the repair. 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. Foundation cracks happen as a result of many factors that include upheaval and settlement. Detailed information about why these problems happened and what repairs are needed. Homeowners and builders are frequently very concerned to see the cracking in a brand new slab foundation. In continuous or indeterminate post tensioned structures the moments due to the prestressing.
Should it make any difference to a buyer what kind of slabonground foundation a house has. Concrete is very weak in tension but brick is very strong in compression, so the concrete loses and the corner cracks. Posttension basics how posttensioned slabs are built. Whats the solution of cracks in the post tensioning slab. Yes, this is a post tension slab, in the last picture the cables can be seen sticking out, not sure if this means it has been tensioned or not. By using post tensioned tendons to compress the concrete, the formation of visible shrinkage cracks can be greatly reduced or even eliminated. Non uniform thickness of the soil underneath the foundation results in. Most residential foundations in the united states are constructed with rebar or wire mesh as their reinforcement to minimize shrinkage cracks and to resist the stresses created by the.
Cracking due to shrinkage after tensioning is not common and if they do occur they would be very small. The pti design procedure is used to design a slabon grade foundation that will respond elastically to forces the foundation is exposed to including those that are the consequence of soil pressure and soil deformation. Since pt is simply reinforcement, there really arent any specific decorative applications related to post tensioning. Before a post tension slab is poured, highstrength steel strands or cables, called tendons, are laid in a tight grid. If a slab does not have post tension cables in it, this would be a huge concern. Posttensioned slab on ground foundations concrete construction. Over tensioning, bad prepration design layingcuring of concrete, bad structural design, bad load estimation and temperature, right. One thing most posttension slabs are not reinforced for is vertical movement. 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. 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. One thing most post tension slabs are not reinforced for is vertical movement. This is usually one to two weeks after the concrete is placed. Posttension basics how posttensioned slabs are built the. Similarly with temperature changes, expansion and contraction can happen.
Information about how posttensioned concrete is constructed. Many home foundations have cracks and theyre generally not serious, that just looking at a foundation crack or cracks will not really tell you whether a home has serious structural issues. 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. 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 during bending. Most often the cracks extend across the entire foundation and occur prior to tensioning of the tendon cables. But since a posttensioned slab does not have excess rebar to minimize and distribute cracking, restraint cracks will be large and very noticeable.
There is no hard and fast rule for when foundation repairs are needed, individual opinions can vary widely always get an opinion from an engineer rather than a foundation company that has an interest in selling you repairs. Post tension foundations cost about the same, and in some instances, more than rebar due to the extra steps needed after foundation has been poured. Post tension slabs superior foundations services jackson. Post tension slab working principle, components and. In simple span beams the primary post tensioning pt moments induced by the prestressing force are directly proportional to the eccentricity of the tendons with respect to the neutral axis of the member i. 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. The cracks that develop from corner popping of a slab are a result of shear stress at the cold joint between the brick mortar joint on the concrete surface of the slab foundation. Post tensioning is used in this manner in millions of square feet of warehouse floors, sport courts, housing and specialized paving applications. Jun 22, 2017 homeowners are getting crushed by flawed post tension cable foundations the switch from steel rebar to post tension cable by houston area home builders has delivered unmistakable results post tension cable foundations are failing earlier and in far greater numbers than steel rebar foundations. The foundation in question was a posttension cable reinforced foundation as opposed to a foundation which is reinforced with steel rebar see the typical. Lots of cracks in new post tension slab inspectionnews. Many people dont understand how they work and the benefits and challenges. Those post tension cables can break one day and go flying out and might kill someone. 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.
The dsld engineer said the cracks are normal in post tension slabs but i don. Home builders can avoid cracks in a posttensioned concrete slabongrade foundation by preventing certain errors made during installation. Several methods exist for prestressing concrete, with post tensioning being a very common one. Cables are arranged as indicated by the engineer and chaired to run through the center of the slab. Aug 18, 2019 foundation inspections, stem walls, post tension, monolithic foundations, cracks, heave crack, rebar oxidation, structural crack stitching phoenix arizona. They usually do not compromise the structural integrity of the home. 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. There are also a few disadvantages associated with a post tensioned foundation. Jul 06, 2018 this causes a tension on the foundation just like described above. Hydronic radiant floor heating in post tension slabongrade. Post tensioned slabs are a great tool to help reduce joints and control cracks. Help bad cracks in post tension slab pics inspectionnews. Preventing cracks on posttensioned concrete slabs pro builder. The ray post tension repair difference from our initial consultation to foundation cable repair completion, we are with you at every milestone.
Preventing cracks on posttensioned concrete slabs pro. The builder used a post tension foundation with footings. Slabs built with rebar and cables have a degree of flexibility that allow bending or flexing over a period of time. This change was made because posttension slabs are cheaper which. I suppose this is not unusual, based on other posts ive read. Concrete is exceptionally strong in compression, but generally weak when subjected to tension forces or forces that pull it apart. This is usually one to two weeks after the concrete is. I have a post tension foundation with cracks in it. Posttensioning is the method of achieving prestressing after the concrete has hardened, thus taking advantage of concretes inherent compressive strength. Aug 28, 2015 other things that can cause cracks in concrete slab foundations are a lack of proper reinforcement and poor quality concrete mix. 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. Aug 25, 2018 post tension foundations were developed to combat the ill effects of expansive soils, that are well documented throughout this region in chandler arizona. Posttensioning institutes design of posttensioned slabsonground, pti design procedure 3rd edition.
This type of uneven tension then causes diagonal cracking. Also, because aci 302 says a reasonable level of cracking is from 0% to 3%, it implies that no. Jan 20, 2017 hydronic radiant floor heating in post tension slabongrade foundations home articles hydronic radiant floor heating in post tension slabongrade foundations the structural department recently designed and observed the construction of a post tension slabongrade foundation that incorporated a hydronic radiant floor system. Steel strands are run through the concrete in a grid pattern. 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. This hybridisation helps achieve the formation of a much thinner slab with a longer span devoid of any columnfree spaces. Signs of foundation problems concrete slab foundations. As mentioned in the article above, a common disadvantage of a posttensioned foundation is the common occurrence of cracking.
This is also why many diagonal cracks have the crack wider at one end. The wise owner or buyer must really look at the big picture to determine the seriousness of foundation cracks. In general, its more expensive to build a post tensioned slabongrade foundation than a traditional one. What are conventionally reinforced slabonground foundations. Roots may cause cracks, however, foundations rarely crack from the roots pushing up, but from them drawing the moisture out of the soil under the foundation due to transpiration. Lots of cracks in new post tension slab inspection news. What begins as a hairline crack can become a bigger complication. Many people dont understand how they work and the benefits. A guide to foundation cracks decker home inspection services. The type of reinforcement used when preparing the foundation rebar or post tension cables, as well as the spacing of the rebar or cables, can affect the strength of the foundation. Jul 22, 2019 post tensioned slabs are a great tool to help reduce joints and control cracks.
Post tension v conventional cph structural engineering, inc. But this tension is not in a straight horizontal vector, but horizontal and vertical. Posttension slab alert magnolia home inspection services. You can expect shrinkage and cracks in the slab foundation and these are very common. Shrinkage crack considerations for posttensioned foundations. 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. Post tensioned foundation information for the homeowner. For the most part, post tension is just that, tightened after the concrete foundation has had time to cure out to full strength. Post tension foundation repair expert chandler arizona.
The foundation cracks, but cracks at an angle rather than straight upanddown. Post tensioned foundations are particularly susceptible to this problem because, until the stressing is applied to the post tensioned cables, there is no reinforcement in these slabs. Jul 28, 2007 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. Should i get a foundation engineer to come out to look at it. Signs of post tension foundation problems concrete repairman llc. We guarantee your satisfaction every step of the way. Thus, the cables and slab are able to handle over 33,000 pounds of tension, making the slab stronger and less likely to crack. The advantages of pt as noted in the opening page are the lack of cracking or at least very narrow cracks and the ability. 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. There are also a few disadvantages associated with a posttensioned foundation. Because all shaw homes slabs are posttension concrete, cracks are. Post tension slab eliminating cracks and joints in concrete. 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.
Foundation repair phoenix arizona concrete repairman. Nov 12, 2009 the post tensioning will typically keep it closed up tight each of those tendons has about 30,000 lbs of tension on them. Homeowners are getting crushed by flawed posttension cable. Due to humidity, things like wood floors, trim, wood framing can shrink and get acclimated to lower inside humidity.
Posttensioning compresses concrete, enabling it to resist both shrinkage cracks and cracks caused by difficult soil conditions. By the third day, i noticed a crack in the back patio, and by the fourth, there were sprawling cracks throughout the entire foundation. Steel reinforcement bars are therefore required in the slab or beam to provide adequate tensile resistance. What are the significant differences between conventionally reinforced and post tensioned slabonground foundations.
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