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Cracks
13
Cracks
Concrete floors often crack. A crack occurs w h e n the tensile stress on the concrete exceeds the concrete's tensile strength. Many cracks are harmless, but some cause serious problems. Designers and builders cannot guarantee crack-free floors. Some cracks can be prevented, however, and m a n y cracks n o t p r e v e n t e d can be controlled so they do not make trouble for the floor user. Because it costs m o n e y to prevent and control cracks, designers need to decide how far to go in dealing with each type of crack. Not every crack causes trouble, and the elimination of cracks is not always worth the cost. This chapter discusses the following types of cracks, with suggestions on how to prevent or control each type: 9 plastic-shrinkage cracks; -~ 9 plastic-settlement cracks;_] i n p l a s t i c
concrete
9 crazing; -~ 9 drying-shrinkage cracks; [ 9 thermal-contraction cracks;[ i n h a r d e n e d 9 structural cracks.
concrete
_]
Plastic-shrinkage cracks These occur w h e n newly placed, still-plastic concrete undergoes severe drying. The cracks are widest at the top and seldom penetrate the full depth or extend the full width of the slab (Figure 13.1).
208
Design and Construction of Concrete Floors
Figure 13.1 Plastic-shrinkagecracks usually stop short of the slab edge
Plastic-shrinkage cracking is most c o m m o n in dry climates but can occur in generally h u m i d areas like the UK, especially in high winds. These cracks can look alarming but seldom do real harm. Some guides recommend beating the floor surface to fill the cracks in, but I have never seen anyone do that. The more usual remedy, not always effective, is to fill the cracks w i t h mortar when the floor is floated. Here are ways to reduce the risk of plastic-shrinkage cracks: 9 Lay the floor indoors, where it is protected from w i n d and sunshine. 9 When laying floors outdoors, use curtains or tents shield the concrete from wind. 9 Avoid placing concrete on dry, w i n d y days. 9 Use misters u p w i n d of the fresh concrete to raise humidity. 9 Spray the fresh concrete w i t h a monomolecular film to slow evaporation.
Plastic-settlement cracks These occur w h e n plastic concrete settles relative to fixed objects such as reinforcing bars. The crack appears directly over the fixed object. In bad cases, cracks appear over every bar, resulting in a near-perfect grid of cracks. Plastic-settlement cracks are m o s t c o m m o n where reinforcing bars are thick and close to the surface. Thick slabs crack more t h a n t h i n ones, because t h e y settle more. Here are some ways to reduce the risk of plastic-settlement cracks: 9 Increase the c o v e r - the distance from the floor surface to the reinforcing steel. Make sure this does n o t c o m p r o m i s e the structural design. 9 Use smaller reinforcing bars. By using more bars of smaller diameter, you can m a i n t a i n the same a m o u n t of steel. 9 Take steps to reduce plastic settlement, such as increasing the size of the coarse aggregate and e l i m i n a t i n g gaps in the aggregate gradation.
Cracks
209
Crazing This is a network of closely spaced, shallow cracks at the floor surface. Other names for it include map cracking (because it resembles the pattern of roads on a map) and alligator or crocodile cracking (because it vaguely resembles the fissures in the skins of those reptiles). In American practice, crazing describes a network with less t h a n 50 m m (2 in) between cracks, while m a p cracks are separated by greater distances. Crazing occurs w h e n the floor surface shrinks relative to t h e concrete below it. This often h a p p e n s very soon after the concrete has set. C o n t r i b u t i n g factors are a high evaporation rate and a smooth, b u r n i s h e d finish. Poor curing or a delay in curing is sometimes blamed, but crazing sometimes occurs even where the floorlayers take great pains to cure the concrete well. Crazing is usually harmless. The narrower the craze cracks, the less likely they are to cause trouble (Figure 13.2). Narrower cracks are often associated with a closer crack spacing. This means a floor with a lot of craze cracks (that is, with a close crack spacing) may actually fare better t h a n one with a few. Wider craze cracks collect dirt and, in the worst cases, deteriorate u n d e r industrial traffic.
Figure 13.2 Craze cracks don't look pretty, but rarely cause real harm
210
Design and Construction of Concrete Floors
Ironically, some of the best, most long-wearing industrial floors have craze cracks. The reason for that is those floors are power-trowelled to the smoothest possible finish, which greatly increases the likelihood of crazing. Here are some ways to lower (but not eliminate) the risk: 9 Do not float or trowel the floor surface while bleed water is present. 9 Give the floor a b r o o m or float finish (not a good choice, t h o u g h , for industrial floors that must resist wear). 9 Lay the floor indoors, where it is protected from w i n d and sunshine. 9 W h e n laying floors outdoors, use curtains or tents shield the concrete from wind. Avoid placing concrete on dry, w i n d y days. 9 Use misters u p w i n d of the fresh concrete to raise humidity. Start curing as soon as possible. Most crazing does not need repair. Where it does, options include grinding and the application of a resin seal.
Drying-shrinkage cracks These occur because concrete shrinks as it dries. If a floor slab is restrained from m o v i n g freely, the shrinkage produces tensile stresses w i t h i n the slab. If the tensile stress exceeds the concrete's tensile strength, w h i c h is not great, the slab cracks. Unlike all the cracks described above, drying-shrinkage cracks usually extend over the full width and d e p t h of the slab. They tend to start near points of restraint (at building columns, for example) and at planes of weakness in the slab (at cold joints, for example). They sometimes follow the lines of reinforcing bars, but by no m e a n s always. It is hard to tell drying-shrinkage cracks from thermal-contraction cracks, described below. Both types are caused by restrained v o l u m e changes, and are identical in their effect on the floor. Some measures t h a t control drying-shrinkage cracks work equally well against t h e r m a l - c o n t r a c t i o n cracks. There are two general ways to deal with drying-shrinkage cracks: prevention and control. If prevention is the goal, the choices include: 9 9 9 9
shrinkage-compensating concrete; pre-tensioning with steel t e n d o n s (mainly used in precast units); post-tensioning with steel tendons; steel fibres at very high dosages - 40-60 k g / m 3 (70-100 lb/yd3).
Cracks
211
Bear in m i n d that n o n e of these preventive measures is foolproof. Most floor designers do not even try to prevent drying-shrinkage cracks. They try instead to control t h e m with reinforcement, joints or both. To control a crack is to m a n a g e its behavior in such a way t h a t it causes little or no harm. Suspended floors, if designed according to one of the structural codes, normally contain e n o u g h reinforcement to keep cracks narrow and trouble-free. G r o u n d - s u p p o r t e d floors often lack the q u a n t i t y of reinforcement needed to keep cracks tight and rely instead on joints to control where the cracks will occur. For more on this subject, see Chapter 16. T h o u g h reinforcement and jointing are our primary crack-control tools, we have m a n y other ways to reduce problems from drying-shrinkage cracks. Some m e t h o d s work by reducing concrete shrinkage, while others reduce restraint on the concrete. Here are some ways to reduce shrinkage: 9 Use bigger aggregate. 9 Avoid aggregates that show high drying shrinkage. 9 Use less water in the concrete mix (but beware the effect on workability and finishability). 9 Use less c e m e n t (but beware the effect on strength and finishability). And here are some ways to reduce restraint: 9 Isolate the floor from columns, walls and other fixed elements. 9 Use a polyethylene slipsheet directly u n d e r the slab. 9 Grade the sub-base with care.
Thermal-contraction cracks These occur because concrete contracts as it cools. They look the same and have the same effect as drying-shrinkage cracks. Thermal-contraction cracks can occur whenever the temperature falls, but are most c o m m o n in the early days w h e n the concrete is weak. Some evidence suggests that thermal contraction causes m a n y of the early cracks often attributed to drying shrinkage. (Kass and Campbell-Allen, 1973) Most of the m e t h o d s that prevent and control (albeit imperfectly) drying-shrinkage cracks work equally well against t h e r m a l - c o n t r a c t i o n cracks. The exceptions are the mix-design changes that aim to reduce drying shrinkage. Adjusting the concrete mix will not substantially reduce thermal contraction.
212
Design and Construction of Concrete Floors
We can prevent some t h e r m a l - c o n t r a c t i o n cracks by limiting temperature changes. This is hard to do after the floor is in use; no one heats or cools a building just to prevent floor cracks. We can, however, limit temperature swings during the floor's early life, w h e n the concrete is weak and highly vulnerable to cracks. Here are some measures t h a t have proven effective: 9 Lay the floor indoors, after the building envelope is complete. 9 Place concrete at night. 9 Insulate the newly slab with blankets or straw (but n o t p o l y e t h y l e n e sheets, w h i c h provide negligible insulation).
Structural cracks
These occur w h e n a floor bends u n d e r load, creating tensile stresses t h a t exceed the concrete's tensile strength. A structural crack m a y or m a y n o t denote a failure, d e p e n d i n g on the floor design.
Structural cracks in suspended floors These are normal. Being made of reinforced concrete, most suspended floors are designed to crack u n d e r load. Cracks are expected in areas of tension, and reinforcing steel is put there to resist that tension. A structural crack in a reinforced, suspended floor is n o t h i n g to worry about unless it grows wider t h a n expected. An unusually wide crack in a suspended floor may be the s y m p t o m of a very serious problem, however. The floor m a y be u n d e r d e s i g n e d or overloaded. Sometimes the cause lies with bad construction, such as missing or misplaced reinforcing steel. How wide m u s t a crack be before the alarm is raised? The a n s w e r varies with the design m e t h o d . Some designers try to limit crack w i d t h to a b o u t 0 . 3 r a m (O.01in), b u t others allow m u c h greater widths. American floors designed by the c u r r e n t s t r e n g t h - d e s i g n m e t h o d are allowed wider cracks t h a n older floors designed by the w o r k i n g stress m e t h o d . In a pre- or p o s t - t e n s i o n e d floor, any crack wider t h a n a hairline m a y be cause for concern. In any case, it is the floor design's job to predict crack w i d t h s a n d let o t h e r parties k n o w w h a t to expect (Figure 13.3).
Cracks
213
Figure 13.3 Measuring crack width with a 50x microscope
Structural cracks in ground-supported floors Unlike suspended floors, most ground-supported floors lack structural reinforcement and are designed not to crack under load. W h e n a structural crack appears in a typical ground-supported floor, something has gone wrong. G r o u n d - s u p p o r t e d slabs crack structurally for m a n y reasons, including: 9 9 9 9
loads that exceed design values; inadequate design; construction errors that result in a too-thin slab; subgrade failure.
214
Design and Construction of Concrete Floors
Overloading during construction causes m a n y cracks. Designers t e n d to focus on the user's loading, but some construction plant weighs far more t h a n a n y t h i n g the user m a y have in mind. Tilt-wall construction requires a heavy crane to lift the wall panels, and it is often c o n v e n i e n t to put that crane on the floor slab. Where this is allowed, structural cracks often appear a r o u n d the crane footpads. Even light construction loads can cause cracks if applied before the concrete has gained m u c h strength. The way to prevent structural cracks during construction is to specify and enforce m a x i m u m loads, and to forbid any substantial loading till the concrete is strong e n o u g h to take it. The safest course is to allow n o t h ing but foot traffic till the concrete has reached its design strength, but construction schedule d o n ' t always allow this luxury. T h o u g h structural cracks in a g r o u n d - s u p p o r t e d floor are evidence of failure, they are not always serious. An isolated crack seldom does real harm, especially if its cause was a t e m p o r a r y overload that is unlikely to recur. On the other hand, if cracks c o n t i n u e to form u n d e r n o r m a l loads, the floor will break up into every smaller pieces and eventually become unusable. Fortunately, this is rare.
Crack repair The time to t h i n k about crack repair is before the floor cracks. To prevent (well, reduce) arguments, every floor specification should include a section on crack repair. It should state the conditions u n d e r w h i c h a crack needs repair, and it should spell out the allowable m e t h o d or m e t h o d s of repair. M a n y cracks - perhaps most - do not harm. Cracks rarely need repair if h i d d e n u n d e r floorcoverings or exposed only to foot traffic. Even in an exposed industrial floor, a crack generally need not be repaired unless it meets one or more of these conditions: 9 Its w i d t h exceeds 1 r a m (1/32in) and it is exposed to hard-wheeled traffic. 9 It spalls u n d e r traffic, regardless of width. 9 It shows differential m o v e m e n t u n d e r traffic. Note that all three condition are related to traffic. Even in a busy warehouse or factory, cracks outside the main traffic areas can usually be left alone. Some users have special requirements, however. In a food or pharmaceutical plant, and in some electronics plants, concern for cleanliness may force the sealing of every visible crack (usually excluding craze cracks). Crack sealing may also be called for in floors exposed to corrosive chemicals.
Cracks
215
Some owners and designers insist on crack repairs solely for the sake of appearance, but that can backfire. The repair often ends up more obvious t h a n the original crack. Once you identify a crack as n e e d i n g repair, you m u s t decide h o w to do the job. Here are the four main methods: 9 9 9 9
Fill the crack with a sealant. Fill the crack with a semi-rigid joint filler. Inject glue into the crack. Install dowels across the crack.
The choice of m e t h o d depends n o t only on the nature of the crack, but also on h o w the floor is used.
Sealants These are described in Chapter 15. The choices include h o t - p o u r e d materials and cold-poured elastorners, which come in a bewildering variety. Sealants make a good repair if the only r e q u i r e m e n t is that the crack be sealed. They do n o t protect a crack's edges from spalling u n d e r traffic, nor can they reduce differential m o v e m e n t . Sealants require a m i n i m u m crack width, w h i c h varies according to the product. Most sealants need at least 3 rnm (1/8 in). If the crack is not wide e n o u g h to admit the sealant, widen it with a concrete saw, a crack router or an angle grinder fitted with a mason's tuck-pointing blade.
Semi-rigid joint fillers Based on either epoxy or polyurea, these are usually the best materials for cracks that have spalled, or are likely to spall, u n d e r hard-wheeled traffic. They even have some ability to stabilize cracks that show differential m o v e m e n t (Figure 13.4). Because semi-rigids are less elastic t h a n sealants, they will fail if cracks widen m u c h after repair. To minimize the risk, wait as long as possible before filling cracks, to give the concrete time to shrink. And if the floor is subject to seasonal temperature swings, apply semi-rigid fillers during the coldest period, w h e n thermal contraction is greatest. Semi-rigids should penetrate at least 25 m m (1 in) into the slab, and may need to go deeper where the goal is to reduce differential m o v e m e n t .
216
Design and Construction of Concrete Floors
Figure 13.4 This crack was patched with semi-rigid epoxy. Not every crack patch looks this good, however
Unless the crack is unusually neat and clean, chase it with a concrete saw, a crack router or an angle grinder fitted with a mason's tuck-pointing blade. Make a groove at least 25 m m (1 in) deep and wide e n o u g h to take out all spalled edges (see Figure 13.5). For the neatest repair, overfill the crack and sand or grind it flush after the filler has hardened. This works better with epoxy t h a n with polyurea.
Glues
Some builders inject strong glue into cracks. The glue is usually epoxy (not semi-rigid) or methylmethacrylate. Gluing is attractive because it is the only m e t h o d that restores the floor's flexural stiffness to its uncracked condition. Another benefit is that is works in very narrow cracks; some products will go into a crack only 0.05 m m (0.002 in) wide. Glue's strength is also its weakness. It locks up the crack, eliminating its ability to relieve future stress. If the conditions that caused the crack
Cracks
217
Figure 13.5 When repairing a crack with semi-rigid joint filler, always cut back to a clean, square edge
continue to exist, the floor is likely to crack again. Gluing is usually safe for structural cracks caused by temporary overloading, but not for dryingshrinkage and thermal-contraction cracks.
Dowelling Some of the worst crack problems occur when cracks exposed to heavy industrial traffic lose their ability to transfer load. The two sides of the crack then move up and down relative to each other every time a vehicle passes over them. Once this starts, crack edges can break down quickly and severely. If the differential m o v e m e n t is less t h a n 0.5 m m (0.030in), filling the crack with semi-rigid epoxy or polyurea may restore the lost load transfer. Failing that, the remedy is to install dowels, as described at the end of Chapter 14.
Cracks
Concrete floors often crack. A crack occurs w h e n the tensile stress on the concrete exceeds the concrete's tensile strength. Many cracks are harmless, but some cause serious problems. Designers and builders cannot guarantee crack-free floors. Some cracks can be prevented, however, and m a n y cracks n o t p r e v e n t e d can be controlled so they do not make trouble for the floor user. Because it costs m o n e y to prevent and control cracks, designers need to decide how far to go in dealing with each type of crack. Not every crack causes trouble, and the elimination of cracks is not always worth the cost. This chapter discusses the following types of cracks, with suggestions on how to prevent or control each type: 9 plastic-shrinkage cracks; -~ 9 plastic-settlement cracks;_] i n p l a s t i c
concrete
9 crazing; -~ 9 drying-shrinkage cracks; [ 9 thermal-contraction cracks;[ i n h a r d e n e d 9 structural cracks.
concrete
_]
Plastic-shrinkage cracks These occur w h e n newly placed, still-plastic concrete undergoes severe drying. The cracks are widest at the top and seldom penetrate the full depth or extend the full width of the slab (Figure 13.1).
208
Design and Construction of Concrete Floors
Figure 13.1 Plastic-shrinkagecracks usually stop short of the slab edge
Plastic-shrinkage cracking is most c o m m o n in dry climates but can occur in generally h u m i d areas like the UK, especially in high winds. These cracks can look alarming but seldom do real harm. Some guides recommend beating the floor surface to fill the cracks in, but I have never seen anyone do that. The more usual remedy, not always effective, is to fill the cracks w i t h mortar when the floor is floated. Here are ways to reduce the risk of plastic-shrinkage cracks: 9 Lay the floor indoors, where it is protected from w i n d and sunshine. 9 When laying floors outdoors, use curtains or tents shield the concrete from wind. 9 Avoid placing concrete on dry, w i n d y days. 9 Use misters u p w i n d of the fresh concrete to raise humidity. 9 Spray the fresh concrete w i t h a monomolecular film to slow evaporation.
Plastic-settlement cracks These occur w h e n plastic concrete settles relative to fixed objects such as reinforcing bars. The crack appears directly over the fixed object. In bad cases, cracks appear over every bar, resulting in a near-perfect grid of cracks. Plastic-settlement cracks are m o s t c o m m o n where reinforcing bars are thick and close to the surface. Thick slabs crack more t h a n t h i n ones, because t h e y settle more. Here are some ways to reduce the risk of plastic-settlement cracks: 9 Increase the c o v e r - the distance from the floor surface to the reinforcing steel. Make sure this does n o t c o m p r o m i s e the structural design. 9 Use smaller reinforcing bars. By using more bars of smaller diameter, you can m a i n t a i n the same a m o u n t of steel. 9 Take steps to reduce plastic settlement, such as increasing the size of the coarse aggregate and e l i m i n a t i n g gaps in the aggregate gradation.
Cracks
209
Crazing This is a network of closely spaced, shallow cracks at the floor surface. Other names for it include map cracking (because it resembles the pattern of roads on a map) and alligator or crocodile cracking (because it vaguely resembles the fissures in the skins of those reptiles). In American practice, crazing describes a network with less t h a n 50 m m (2 in) between cracks, while m a p cracks are separated by greater distances. Crazing occurs w h e n the floor surface shrinks relative to t h e concrete below it. This often h a p p e n s very soon after the concrete has set. C o n t r i b u t i n g factors are a high evaporation rate and a smooth, b u r n i s h e d finish. Poor curing or a delay in curing is sometimes blamed, but crazing sometimes occurs even where the floorlayers take great pains to cure the concrete well. Crazing is usually harmless. The narrower the craze cracks, the less likely they are to cause trouble (Figure 13.2). Narrower cracks are often associated with a closer crack spacing. This means a floor with a lot of craze cracks (that is, with a close crack spacing) may actually fare better t h a n one with a few. Wider craze cracks collect dirt and, in the worst cases, deteriorate u n d e r industrial traffic.
Figure 13.2 Craze cracks don't look pretty, but rarely cause real harm
210
Design and Construction of Concrete Floors
Ironically, some of the best, most long-wearing industrial floors have craze cracks. The reason for that is those floors are power-trowelled to the smoothest possible finish, which greatly increases the likelihood of crazing. Here are some ways to lower (but not eliminate) the risk: 9 Do not float or trowel the floor surface while bleed water is present. 9 Give the floor a b r o o m or float finish (not a good choice, t h o u g h , for industrial floors that must resist wear). 9 Lay the floor indoors, where it is protected from w i n d and sunshine. 9 W h e n laying floors outdoors, use curtains or tents shield the concrete from wind. Avoid placing concrete on dry, w i n d y days. 9 Use misters u p w i n d of the fresh concrete to raise humidity. Start curing as soon as possible. Most crazing does not need repair. Where it does, options include grinding and the application of a resin seal.
Drying-shrinkage cracks These occur because concrete shrinks as it dries. If a floor slab is restrained from m o v i n g freely, the shrinkage produces tensile stresses w i t h i n the slab. If the tensile stress exceeds the concrete's tensile strength, w h i c h is not great, the slab cracks. Unlike all the cracks described above, drying-shrinkage cracks usually extend over the full width and d e p t h of the slab. They tend to start near points of restraint (at building columns, for example) and at planes of weakness in the slab (at cold joints, for example). They sometimes follow the lines of reinforcing bars, but by no m e a n s always. It is hard to tell drying-shrinkage cracks from thermal-contraction cracks, described below. Both types are caused by restrained v o l u m e changes, and are identical in their effect on the floor. Some measures t h a t control drying-shrinkage cracks work equally well against t h e r m a l - c o n t r a c t i o n cracks. There are two general ways to deal with drying-shrinkage cracks: prevention and control. If prevention is the goal, the choices include: 9 9 9 9
shrinkage-compensating concrete; pre-tensioning with steel t e n d o n s (mainly used in precast units); post-tensioning with steel tendons; steel fibres at very high dosages - 40-60 k g / m 3 (70-100 lb/yd3).
Cracks
211
Bear in m i n d that n o n e of these preventive measures is foolproof. Most floor designers do not even try to prevent drying-shrinkage cracks. They try instead to control t h e m with reinforcement, joints or both. To control a crack is to m a n a g e its behavior in such a way t h a t it causes little or no harm. Suspended floors, if designed according to one of the structural codes, normally contain e n o u g h reinforcement to keep cracks narrow and trouble-free. G r o u n d - s u p p o r t e d floors often lack the q u a n t i t y of reinforcement needed to keep cracks tight and rely instead on joints to control where the cracks will occur. For more on this subject, see Chapter 16. T h o u g h reinforcement and jointing are our primary crack-control tools, we have m a n y other ways to reduce problems from drying-shrinkage cracks. Some m e t h o d s work by reducing concrete shrinkage, while others reduce restraint on the concrete. Here are some ways to reduce shrinkage: 9 Use bigger aggregate. 9 Avoid aggregates that show high drying shrinkage. 9 Use less water in the concrete mix (but beware the effect on workability and finishability). 9 Use less c e m e n t (but beware the effect on strength and finishability). And here are some ways to reduce restraint: 9 Isolate the floor from columns, walls and other fixed elements. 9 Use a polyethylene slipsheet directly u n d e r the slab. 9 Grade the sub-base with care.
Thermal-contraction cracks These occur because concrete contracts as it cools. They look the same and have the same effect as drying-shrinkage cracks. Thermal-contraction cracks can occur whenever the temperature falls, but are most c o m m o n in the early days w h e n the concrete is weak. Some evidence suggests that thermal contraction causes m a n y of the early cracks often attributed to drying shrinkage. (Kass and Campbell-Allen, 1973) Most of the m e t h o d s that prevent and control (albeit imperfectly) drying-shrinkage cracks work equally well against t h e r m a l - c o n t r a c t i o n cracks. The exceptions are the mix-design changes that aim to reduce drying shrinkage. Adjusting the concrete mix will not substantially reduce thermal contraction.
212
Design and Construction of Concrete Floors
We can prevent some t h e r m a l - c o n t r a c t i o n cracks by limiting temperature changes. This is hard to do after the floor is in use; no one heats or cools a building just to prevent floor cracks. We can, however, limit temperature swings during the floor's early life, w h e n the concrete is weak and highly vulnerable to cracks. Here are some measures t h a t have proven effective: 9 Lay the floor indoors, after the building envelope is complete. 9 Place concrete at night. 9 Insulate the newly slab with blankets or straw (but n o t p o l y e t h y l e n e sheets, w h i c h provide negligible insulation).
Structural cracks
These occur w h e n a floor bends u n d e r load, creating tensile stresses t h a t exceed the concrete's tensile strength. A structural crack m a y or m a y n o t denote a failure, d e p e n d i n g on the floor design.
Structural cracks in suspended floors These are normal. Being made of reinforced concrete, most suspended floors are designed to crack u n d e r load. Cracks are expected in areas of tension, and reinforcing steel is put there to resist that tension. A structural crack in a reinforced, suspended floor is n o t h i n g to worry about unless it grows wider t h a n expected. An unusually wide crack in a suspended floor may be the s y m p t o m of a very serious problem, however. The floor m a y be u n d e r d e s i g n e d or overloaded. Sometimes the cause lies with bad construction, such as missing or misplaced reinforcing steel. How wide m u s t a crack be before the alarm is raised? The a n s w e r varies with the design m e t h o d . Some designers try to limit crack w i d t h to a b o u t 0 . 3 r a m (O.01in), b u t others allow m u c h greater widths. American floors designed by the c u r r e n t s t r e n g t h - d e s i g n m e t h o d are allowed wider cracks t h a n older floors designed by the w o r k i n g stress m e t h o d . In a pre- or p o s t - t e n s i o n e d floor, any crack wider t h a n a hairline m a y be cause for concern. In any case, it is the floor design's job to predict crack w i d t h s a n d let o t h e r parties k n o w w h a t to expect (Figure 13.3).
Cracks
213
Figure 13.3 Measuring crack width with a 50x microscope
Structural cracks in ground-supported floors Unlike suspended floors, most ground-supported floors lack structural reinforcement and are designed not to crack under load. W h e n a structural crack appears in a typical ground-supported floor, something has gone wrong. G r o u n d - s u p p o r t e d slabs crack structurally for m a n y reasons, including: 9 9 9 9
loads that exceed design values; inadequate design; construction errors that result in a too-thin slab; subgrade failure.
214
Design and Construction of Concrete Floors
Overloading during construction causes m a n y cracks. Designers t e n d to focus on the user's loading, but some construction plant weighs far more t h a n a n y t h i n g the user m a y have in mind. Tilt-wall construction requires a heavy crane to lift the wall panels, and it is often c o n v e n i e n t to put that crane on the floor slab. Where this is allowed, structural cracks often appear a r o u n d the crane footpads. Even light construction loads can cause cracks if applied before the concrete has gained m u c h strength. The way to prevent structural cracks during construction is to specify and enforce m a x i m u m loads, and to forbid any substantial loading till the concrete is strong e n o u g h to take it. The safest course is to allow n o t h ing but foot traffic till the concrete has reached its design strength, but construction schedule d o n ' t always allow this luxury. T h o u g h structural cracks in a g r o u n d - s u p p o r t e d floor are evidence of failure, they are not always serious. An isolated crack seldom does real harm, especially if its cause was a t e m p o r a r y overload that is unlikely to recur. On the other hand, if cracks c o n t i n u e to form u n d e r n o r m a l loads, the floor will break up into every smaller pieces and eventually become unusable. Fortunately, this is rare.
Crack repair The time to t h i n k about crack repair is before the floor cracks. To prevent (well, reduce) arguments, every floor specification should include a section on crack repair. It should state the conditions u n d e r w h i c h a crack needs repair, and it should spell out the allowable m e t h o d or m e t h o d s of repair. M a n y cracks - perhaps most - do not harm. Cracks rarely need repair if h i d d e n u n d e r floorcoverings or exposed only to foot traffic. Even in an exposed industrial floor, a crack generally need not be repaired unless it meets one or more of these conditions: 9 Its w i d t h exceeds 1 r a m (1/32in) and it is exposed to hard-wheeled traffic. 9 It spalls u n d e r traffic, regardless of width. 9 It shows differential m o v e m e n t u n d e r traffic. Note that all three condition are related to traffic. Even in a busy warehouse or factory, cracks outside the main traffic areas can usually be left alone. Some users have special requirements, however. In a food or pharmaceutical plant, and in some electronics plants, concern for cleanliness may force the sealing of every visible crack (usually excluding craze cracks). Crack sealing may also be called for in floors exposed to corrosive chemicals.
Cracks
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Some owners and designers insist on crack repairs solely for the sake of appearance, but that can backfire. The repair often ends up more obvious t h a n the original crack. Once you identify a crack as n e e d i n g repair, you m u s t decide h o w to do the job. Here are the four main methods: 9 9 9 9
Fill the crack with a sealant. Fill the crack with a semi-rigid joint filler. Inject glue into the crack. Install dowels across the crack.
The choice of m e t h o d depends n o t only on the nature of the crack, but also on h o w the floor is used.
Sealants These are described in Chapter 15. The choices include h o t - p o u r e d materials and cold-poured elastorners, which come in a bewildering variety. Sealants make a good repair if the only r e q u i r e m e n t is that the crack be sealed. They do n o t protect a crack's edges from spalling u n d e r traffic, nor can they reduce differential m o v e m e n t . Sealants require a m i n i m u m crack width, w h i c h varies according to the product. Most sealants need at least 3 rnm (1/8 in). If the crack is not wide e n o u g h to admit the sealant, widen it with a concrete saw, a crack router or an angle grinder fitted with a mason's tuck-pointing blade.
Semi-rigid joint fillers Based on either epoxy or polyurea, these are usually the best materials for cracks that have spalled, or are likely to spall, u n d e r hard-wheeled traffic. They even have some ability to stabilize cracks that show differential m o v e m e n t (Figure 13.4). Because semi-rigids are less elastic t h a n sealants, they will fail if cracks widen m u c h after repair. To minimize the risk, wait as long as possible before filling cracks, to give the concrete time to shrink. And if the floor is subject to seasonal temperature swings, apply semi-rigid fillers during the coldest period, w h e n thermal contraction is greatest. Semi-rigids should penetrate at least 25 m m (1 in) into the slab, and may need to go deeper where the goal is to reduce differential m o v e m e n t .
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Design and Construction of Concrete Floors
Figure 13.4 This crack was patched with semi-rigid epoxy. Not every crack patch looks this good, however
Unless the crack is unusually neat and clean, chase it with a concrete saw, a crack router or an angle grinder fitted with a mason's tuck-pointing blade. Make a groove at least 25 m m (1 in) deep and wide e n o u g h to take out all spalled edges (see Figure 13.5). For the neatest repair, overfill the crack and sand or grind it flush after the filler has hardened. This works better with epoxy t h a n with polyurea.
Glues
Some builders inject strong glue into cracks. The glue is usually epoxy (not semi-rigid) or methylmethacrylate. Gluing is attractive because it is the only m e t h o d that restores the floor's flexural stiffness to its uncracked condition. Another benefit is that is works in very narrow cracks; some products will go into a crack only 0.05 m m (0.002 in) wide. Glue's strength is also its weakness. It locks up the crack, eliminating its ability to relieve future stress. If the conditions that caused the crack
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Figure 13.5 When repairing a crack with semi-rigid joint filler, always cut back to a clean, square edge
continue to exist, the floor is likely to crack again. Gluing is usually safe for structural cracks caused by temporary overloading, but not for dryingshrinkage and thermal-contraction cracks.
Dowelling Some of the worst crack problems occur when cracks exposed to heavy industrial traffic lose their ability to transfer load. The two sides of the crack then move up and down relative to each other every time a vehicle passes over them. Once this starts, crack edges can break down quickly and severely. If the differential m o v e m e n t is less t h a n 0.5 m m (0.030in), filling the crack with semi-rigid epoxy or polyurea may restore the lost load transfer. Failing that, the remedy is to install dowels, as described at the end of Chapter 14.