Cracks in Concrete Floors

Concrete floors often crack. A crack occurs when 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 many cracks not prevented can be controlled so they do not make trouble for the floor user. Because it costs money 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 exclusive article on engineersdaily discusses the following types of cracks, with suggestions on how to prevent or control each type:

Cracks in Plastic Concrete:

1. plastic-shrinkage cracks;
2. plastic-settlement cracks;

Cracks in Hardened Concrete:

1. crazing;
2. drying-shrinkage cracks;
3. thermal-contraction cracks;
4. structural cracks

Plastic-Shrinkage Cracks

Figure 1 Plastic-shrinkage cracks usually stop short of the slab edge

These occur when 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 1). Plastic-shrinkage cracking is most common in dry climates but can occur in generally humid areas 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 with mortar when the floor is floated. Here are ways to reduce the risk of plastic-shrinkage cracks:

1. Lay the floor indoors, where it is protected from wind and sunshine.
2. When laying floors outdoors, use curtains or tents to shield the concrete from wind.
3. Avoid placing concrete on dry, windy days.
4. Use misters upwind of the fresh concrete to raise humidity.
5. Spray the fresh concrete with a monomolecular film to slow evaporation.

Plastic-settlement Cracks

Figure 2 Plastic-settlement cracks

These occur when plastic concrete settles relative to fixed objects such as reinforcing bars ( Figure 2). 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 most common where reinforcing bars are thick and close to the surface. Thick slabs crack more than thin ones, because they settle more. Here are some ways to reduce the risk of plastic-settlement cracks:

1. Increase the cover – the distance from the floor surface to the reinforcing steel. Make sure this does not compromise the structural design.
2. Use smaller reinforcing bars. By using more bars of smaller diameter, you can maintain the same amount of steel.
3. Take steps to reduce plastic settlement, such as increasing the size of the coarse aggregate and eliminating gaps in the aggregate gradation.

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 than 50 mm (2 in) between cracks, while map cracks are separated by greater distances. Crazing occurs when the floor surface shrinks relative to the concrete below it. This often happens very soon after the concrete has set. Contributing factors are a high evaporation rate and a smooth, burnished 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.

Figure 3 Craze cracks don’t look pretty, but rarely cause real harm

Crazing is usually harmless. The narrower the craze cracks, the less likely they are to cause trouble (Figure 3). 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 than one with a few. Wider craze cracks collect dirt and, in the worst cases, deteriorate under industrial traffic.

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:

1. Do not float or trowel the floor surface while bleed water is present.
2. Give the floor a broom or float finish (not a good choice, though, for industrial floors that must resist wear).
3. Lay the floor indoors, where it is protected from wind and sunshine.
4. When laying floors outdoors, use curtains or tents shield the concrete from wind.
5. Avoid placing concrete on dry, windy days.
6. Use misters upwind of the fresh concrete to raise humidity.
7. 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 moving freely, the shrinkage produces tensile stresses within the slab. If the tensile stress exceeds the concrete’s tensile strength, which is not great, the slab cracks.

Unlike all the cracks described above, drying-shrinkage cracks usually extend over the full width and depth 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 means always. It is hard to tell drying-shrinkage cracks from thermal-contraction cracks, described below. Both types are caused by restrained volume changes, and are identical in their effect on the floor. Some measures that control drying-shrinkage cracks work equally well against thermal-contraction cracks.

There are two general ways to deal with drying-shrinkage cracks: prevention and control. If prevention is the goal, the choices include:

1. shrinkage-compensating concrete;
2. pre-tensioning with steel tendons (mainly used in precast units);
3. post-tensioning with steel tendons;
4. steel fibres at very high dosages – 40-60 kg/m³ (70–100 lb/yd³

Bear in mind that none of these preventive measures is foolproof. Most floor designers do not even try to prevent drying-shrinkage cracks. They try instead to control them with reinforcement, joints or both. To control a crack is to manage its behavior in such a way that it causes little or no harm. Suspended floors, if designed according to one of the structural codes, normally contain enough reinforcement to keep cracks narrow and trouble-free. Ground-supported floors often lack the quantity of reinforcement needed to keep cracks tight and rely instead on joints to control where the cracks will occur.

Though reinforcement and jointing are our primary crack-control tools, we have many other ways to reduce problems from drying-shrinkage cracks. Some methods work by reducing concrete shrinkage, while others reduce restraint on the concrete.

Here are some ways to reduce shrinkage:

1. Use bigger aggregate.
2. Avoid aggregates that show high drying shrinkage.
3. Use less water in the concrete mix (but beware the effect on workability and finishability).
4. Use less cement (but beware the effect on strength and finishability). And here are some ways to reduce restraint:
5. Isolate the floor from columns, walls and other fixed elements.
6. Use a polyethylene slipsheet directly under the slab.
7. 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 common in the early days when the concrete is weak. Some evidence suggests that thermal contraction causes many of the early cracks often attributed to drying shrinkage. Most of the methods that prevent and control (albeit imperfectly) drying-shrinkage cracks work equally well against thermal-contraction cracks. The exceptions are the mix-design changes that aim to reduce drying shrinkage. Adjusting the concrete mix will not substantially reduce thermal contraction.

We can prevent some thermal-contraction 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, when the concrete is weak and highly vulnerable to cracks. Here are some measures that have proven effective:

1. Lay the floor indoors, after the building envelope is complete.
2. Place concrete at night.
3. Insulate the newly slab with blankets or straw (but not polyethylene sheets, which provide negligible insulation).

Structural Cracks

These occur when a floor bends under load, creating tensile stresses that exceed the concrete’s tensile strength. A structural crack may or may not denote a failure, depending on the floor design.

Structural cracks in suspended floors 

Figure 4 Measuring crack width with a 5ox microscope

These are normal. Being made of reinforced concrete, most suspended floors are designed to crack under 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 nothing to worry about unless it grows wider than expected.

An unusually wide crack in a suspended floor may be the symptom of a very serious problem, however. The floor may be underdesigned or overloaded. Sometimes the cause lies with bad construction, such as missing or misplaced reinforcing steel. How wide must a crack be before the alarm is raised? The answer varies with the design method. Some designers try to limit crack width to about 0.3 mm (0.01 in), but others allow much greater widths. American floors designed by the current strength-design method are allowed wider cracks than older floors designed by the working stress method. In a pre- or post-tensioned floor, any crack wider than a hairline may be cause for concern. In any case, it is the floor design’s job to predict crack widths and let other parties know what to expect (Figure 4).

Structural Cracks in Ground-Supported Floors

Unlike suspended floors, most ground-supported floors lack structural reinforcement and are designed not to crack under load. When a structural crack appears in a typical ground-supported floor, something has gone wrong. Ground-supported slabs crack structurally for many reasons, including:

1. loads that exceed design values;
2. inadequate design;
3. construction errors that result in a too-thin slab;
4. subgrade failure.

Overloading during construction causes many cracks. Designers tend to focus on the user’s loading, but some construction plant weighs far more than anything the user may have in mind. Tilt-wall construction requires a heavy crane to lift the wall panels, and it is often convenient to put that crane on the floor slab. Where this is allowed, structural cracks often appear around the crane footpads. Even light construction loads can cause cracks if applied before the concrete has gained much strength. The way to prevent structural cracks during construction is to specify and enforce maximum loads, and to forbid any substantial loading till the concrete is strong enough to take it. The safest course is to allow nothing but foot traffic till the concrete has reached its design strength, but construction schedule don’t always allow this luxury.

Though structural cracks in a ground-supported floor are evidence of failure, they are not always serious. An isolated crack seldom does real harm, especially if its cause was a temporary overload that is unlikely to recur. On the other hand, if cracks continue to form under normal loads, the floor will break up into every smaller pieces and eventually become unusable. Fortunately, this is rare.