The posttension construction using cable under tension holds the slab intact in. Two broad categories of cracking can occur in groundsupported posttensioned slabs. 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. Mohammed zaid post tension slabs vs flat slabs post.
Can the house still be settling or are there other issues. There are some contractors who will say that it is normal for the concrete to have a few cracks even if you have just poured it the day before. 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. Id like to install a tile floor over the slab, but im worried that the tile will crack. Things you should know about cracks in slab foundation. The rest of it i noticed very few small hairline cracks and the sides and edges look good. What begins as a hairline crack can become a bigger complication. Further, the risk of cracking varies between cured, uncured and reinforced concrete. The cracks are often caused by the hardening or curing process. Tom meehan, a tile installer and owner of cape cod tileworks in harwich, massachusetts, replies.
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 some of us who experienced cracking in the concrete that we have just poured in some parts of our house. The lobby level of the garage has a 4inch topping on a 10inch posttensioned slab with. The cause of cracking in posttensioned concrete box girder bridges and retrofit procedures walter podolny, jr. Im not sure if these are just surface shrinkage cracks or they go right through the slab. Especially in hot weather, shrinkage cracks can occur as early as a few hours after the slab. Hairline cracks at the surface should be repaired if they are widening with time, are a hazard, collect dirt or are unsightly shrinkage cracks occur while the concrete is curing and can be reduced or prevented with proper joint placement. Help bad cracks in post tension slab pics inspection news. These cracks result as a structural failure where elevation changes have caused one side of the crack to be lower than the other. If a garage has a post tension slab there is usually either a sign stamped in the. A cracked slab also may be the result of shrinkage in the concrete or building materials, foundation settling or slab foundation movement caused by frost heaves andor expansive soil. The danger posed by a crack in slab foundation depends on its direction, width, and depth. To adhere the port to the concrete, apply a small amount of epoxy around the bottom of the port base.
Shrinkage crack considerations for post tensioned foundations june 1, 2015 shrinkage cracks in concrete slab onground foundations are a significant public relations problem to home builders. Cracking in groundsupported post tensioned slabs on. The cracks are perpendicular to the uniform tendons direction. Telling meaningful cracks from inconsequential cracks is important. Hairline cracks x 8 long consistently happen at uniform tendons dead end location at bottom of slab. If a hairline crack in slab foundation is deep, it can lead to wider, more severe cracking inside the slab over time. The cracks are all hairline i cant fit my fingernail in them, although the major one has grown wider in the last year but still not able to fit a penny in it or anything. But since a posttensioned slab does not have excess rebar to minimize and distribute cracking, restraint cracks will. Tulsa home builders 918 6885660 can a post tension slab have. Once the slab cures, qualified personnel apply tension and grout the cables. Hair line transverse crack in concrete garage floor in.
Cracking, though unsightly, depends on the use that the floor will have. 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. And one tile had a crack completely across it larger than a hairline. Below are some specific types of cracks that are found in slab foundations. Shrinkage crack considerations for posttensioned foundations june 1, 2015 shrinkage cracks in concrete slabonground foundations are a significant public relations problem to home builders. Shrinkage cracks are common and can occur as early as a few hours after the slab has been poured and finished. Post tension construction uses cables to reinforce the concrete and resist the movement caused by the expansive soil. I am wondering if this is something i should be concerned about it. Frost heaves or expansive soil damage can cause substantial damage to basement, crawl space, or garage floor slabs in some conditions. Post tension slabs have steel cables that run through them, similar to rebar but are further apart. If its to be finished and used as a living area, then cracks shouldnt be of a size that they will reflect.
Shrinkage crack considerations for posttensioned foundations. These categories of cracks are caused by entirely different phenomena and have very different characteristics. In addition to having more movement, a posttensioned slab will have substantially less rebar than a conventional system, which is one of the main economic benefits. As the name implies, these cracks are very small about 0. Contractors sometimes need to drill or cut into a posttensioned slab for repair or renovation work.
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. Hairline cracks are just a reality of concrete and a natural occurrence. By using posttensioned tendons to compress the concrete. Concrete cracks due to inclusions of shale and iron sulfide mineral pyrrhotite cracking. If your home is exhibiting symptoms of foundation problems like cracks in walls, sticking and swinging doors, or uneven floors it may need foundation repair. Concrete can support a lot of weight in compression, but is weak in tension. But since a posttensioned slab does not have excess rebar to minimize and distribute cracking, restraint cracks will be large and very noticeable. As a general engineers guideline, cracks that are hairline up to 18 inch in width are considered negligible to slight, 316 916 inch are moderate, 916 1 inch are severe and over 1 inch are very severe forensic geotechnical and foundation engineering, r. Tulsa home builders 918 6885660 can a post tension. Most shrinkage cracks are the inconsequential variety usually. By the third day, i noticed a crack in the back patio, and by the fourth, there were sprawling cracks throughout the entire foundation. The florida home i just purchased has many cracks in the concrete slab, some of them 10 ft. If a garage has a post tension slab there is usually either a sign stamped in the concrete floor saying it is a post tension slab or a plastic sign on the garage wall stating its a post tension slab. Twoway posttensioned design amsysco post tensioning.
So cracks, if they occur, should be hairline only basement floors. Went into contract with a 2story house in austin this week. A slab crack that continues up into the homes wall is a cause for concern and should be examined by a qualified repair specialist. Especially in hot weather, shrinkage cracks can occur as early as a few hours after the slab has been poured and finished. Often, plastic shrinkage cracks are only a hairline in width and are barely. Found at almost every maybe 50% location of at the uniform tendons anchorage end. As a rule of thumb, injection ports should be placed 8 apart along the length of the crack. It was built in dec of 1993 by lennar and we bought it in 1998 from the original owners. As the slab loses moisture while curing it gets a bit smaller. Cracks, cracks, cracks what is a reasonable level of cracking. You could replace the slab 5 times and get cracks every time.
All concrete will shrink during the curing process which lasts up to a year after the time concrete is placed in the forms. 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. Cracks in slab floors can also result from tree roots. Do not allow epoxy to block the port or the crack under it, this is. I just moved 2 weeks ago and i saw some hairline crackings in the garage post tension slab concrete. Heres an overview of crack types and what to do about them. Post tension slab eliminating cracks and joints in concrete. Just look at all the cracks in concrete bridges that literally have tons of rebar to hold them together. Shrinkage cracks in concrete slabonground foundations are a significant. When a homes slab is poured, it quickly begins to dry as moisture evaporates from the concrete. Usually a sign in the garage or stamp in the slab will indicate if your home has a post tension slab. They are usually reinforced with rebar or posttensioned tendons. Hairline cracks can be left alone, however a crack that begins. It is not uncommon for cracks to appear in the concrete as it dries.
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. Inspection showed hairline cracks in parts of about 8 tiles 18 inch builder grade floor tiles. The cracks were mostly in random spots, no two tiles in a row with 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. Iv tried to insert a pin into the crack but it will not go in. If there are cracks, they should be hairline cracks only. Such cracks in newly formed slabs are called shrinkage cracks. The cracks often reflect the location of the reinforcement of the slab from rebar, wire mesh, ibeams or footers. Q recently we completed construction of a hotel parking garage. Concrete has relatively high compressive strength, but significantly lower in tensile strength, and as such is usually reinforced with materials that are strong in. Post tensioned foundation information for the homeowner. Whats the solution of cracks in the post tensioning slab.
Also, because aci 302 says a reasonable level of cracking is from 0% to 3%, it implies that no. Very thin hairline cracks in random or irregular hexagonal patterns that are very. Im getting ready to install a 2 post car lift in my garage. Hairline crack in concrete causes, repair and prevention. Burns professor of civil engineering the university of texas at austin austin, texas. Hello all i am very concerned about the foundation of my home in houston, tx. Ive read that post tension is made to resist movement but not cracking ie it will still crack but it will withstand and hold the foundation to prevent sinking in. Should i get a foundation engineer to come out to look at it. The house is built on a concrete slab post tension. When are foundation cracks normal signs of foundation. If large trees are growing too near your home, removal of some roots, or possibly even the trees, may be warranted. Because all shaw homes slabs are posttension concrete, cracks are nothing.
Yes, this is a posttension slab, in the last picture the cables can be seen sticking out, not sure if this. Until the cables are stretchedelongated, they cannot provide any crack. It so happends one of the post will have to be anchored over some hairline cracks. Posttension cables are steel wires inserted into plastic sheathing laid in place while pouring the concrete slab.
Hi all, i just bought a new townhouse and when i moved in i did not have inspector visit assuming that new house will have less trouble boy i was in for a rude awakening. 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. The cracks in this case ranged in width measured across the crack from hairline less than 116 to. Why concrete cracks concrete crack solutions ardex ireland. Sealing cracks in posttensioned deck concrete construction. When the stress becomes too great for the now hardened concrete, the slab will crack in order to relieve tension.
Another quality issue that requires a careful study on what rectification method to apply is the concrete hairline cracks. Preventing cracks on posttensioned concrete slabs pro builder. Lots of cracks in new post tension slab inspectionnews. The crackings are randoms so they are not one of those controlled points. When we first looked at the house, we noticed a few cracks in the sheetrock around a column and one on a bumpout over a window in the kitchen. Place the port at one end of the crack and repeat until the entire crack is ported. We were told they were just from bad tape and float in the sheetrock. The cause of cracking in posttensioned concrete box. Hair line crackings in garage floor post tension slab. A combination of these values will determine the slab stresses at the time of stressing and at service loads. Hairline cracks in post tensioned slab rapt structural 31 jan 05 00. Foundation cracks happen as a result of many factors that include upheaval and settlement. Posttensioning for crackfree superflat floors concrete construction.
Cracking in post tensioned beams structural engineering. For example, a concrete wall can support tons of weight from above, but will crack easily if pushed sideways forcing it to bend. Issues in posttensioned construction concrete construction. 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. As the concrete shrinks, the slab could crack in order to relieve tension. The dsld engineer said the cracks are normal in post tension slabs but i don. Settlement cracks in a slab indicate inadequate site preparation, such as failure to compact fill on which a slab was poured. 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. Bridge division office of engineering federal highway administration washington, d. Home builders can avoid cracks in a posttensioned concrete slabongrade foundation by preventing certain errors made during installation. Cracks caused by the restraint to axial slab shortening rts cracks.
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