Reinforced concrete is concrete in which reinforcement bars ("rebars") or fibers have been incorporated to strengthen a material that would otherwise be brittle. In industrialised countries, nearly all concrete used in construction is reinforced concrete.
Much of the focus on reinforcing concrete is placed on floor systems. Designing and implementing the most efficient floor system is key to creating optimal building structures. Small changes in the design of a floor system can have significant impact on material costs, construction schedule, ultimate strength, operating costs, occupancy levels and end use of a building.
If a material with high strength in tension, such as steel, is placed in concrete, then the composite material, reinforced concrete, resists compression but also bending, and other direct tensile actions. A reinforced concrete section where the concrete resists the compression and steel resists the tension can be made into almost any shape and size for the construction industry.
The relative cross-sectional area of steel required for typical reinforced concrete is usually quite small and varies from 1% for most beams and slabs to 6% for some columns. Reinforcing bars are normally round in cross-section and vary in diameter (see rebar for more information). Reinforced concrete structures sometimes have provisions such as ventilated hollow cores to control their moisture & humidity.
Penetrating sealants typically must be applied some time after curing. Sealants include paint, plastic foams, films and aluminum foil, felts or fabric mats sealed with tar, and layers of bentonite clay, sometimes used to seal roadbeds.
Ultimate failure leading to collapse can be caused by crushing of the concrete matrix, when stresses exceed its strength; by yielding of the rebar; or by bond failure between the concrete and the rebar.
The water in the pores of the cement is normally alkaline. This alkaline environment is one in which the steel is passive and does not corrode. According to the pourbaix diagram for iron, the metal is passive when pH is above 9.5. The carbon dioxide from the air reacts with the alkali in the cement and makes the pore water more acidic, thus lowering the pH. Carbon dioxide will start to carbonate the cement in the concrete from the moment the object is made. This carbonatation process (in Britain, called carbonation) will start at the surface, then slowly move deeper and deeper into the concrete. If the object is cracked, the carbon dioxide of the air will be better able to penetrate into the concrete. When designing a concrete structure, it is normal to state the concrete cover for the rebar (the depth within the object that the rebar will be). The minimum concrete cover is normally regulated by design or building codes. If the reinforcement is too close to the surface, early failure due to corrosion may occur.
One method of testing a structure for carbonatation is to drill a fresh hole in the surface and then treat the surface with phenolphthalein. This will turn [pink] when in contact with alkaline cement, making it possible to see the depth of carbonatation. An existing hole is no good because the exposed surface will already be carbonated
Chlorides, including sodium chloride, promote the corrosion of steel rebar. For this reason, in mixing concrete only water suitible for drinking, cement and aggregates with a low chloride content should be used, and the use of salt for deicing concrete pavements is avoided where possible.
However, in the United States, it is common to use calcium chloride as an admixture to promote rapid set-up at the cost of lower ultimate strength and higher corrosion rates.
This is found when the cement is too alkaline, due to a reaction of the silica in the aggregates with the alkali. The silica (SiO2) reacts with the alkali to form a silicate in the Alkali silica reaction (ASR), this causes localised swelling which causes cracking. The conditions for alkali silica reaction are: (1) aggregate containing an alkali reactive constituent, (2) sufficiently high alkalinity, and (3) sufficient moisture, above 75%RH within the concrete. This phenomenon has been popularly referred to as "concrete cancer".
Concrete reinforced with fibers (which are usually steel, glass or "plastic" fibers) is less expensive than hand-tied rebar, while still increasing the tensile strength many times. Shape, dimension and length of fiber is important. A thin and short fiber, for example short hair-shaped glass fiber, will only be effective the first hours after pouring the concrete (reduces cracking while the concrete is stiffening) but will not increase the concrete tensile strength. A normal size fibre for European shotcrete (1 mm diameter, 45 mm length—steel or "plastic") will increase the concrete tensile strength.
Steel is the strongest commonly-available fiber, and come in different lengths (30 to 80 mm in Europe) and shapes (end-hooks). Steel fibres can only be used on surfaces that can tolerate or avoid corrosion and rust stains. In some cases, a steel-fiber surface is faced with other materials.
Glass fiber is inexpensive and corrosion-proof, but not as ductile as steel. Recently, spun basalt fiber, long available in Eastern Europe, has become available in the U.S. and Western Europe. Basalt fibre is stronger and less expensive than glass, but historically, has not resisted the alkaline environment of portland cement well enough to be used as direct reinforcement. New materials use plastic binders to isolate the basalt fiber from the cement.
The premium fibers are graphite reinforced plastic fibers, which are nearly as strong as steel, lighter-weight and corrosion-proof. Some experimeters have had promising early results with carbon nanotubes, but the material is still far too expensive for any building.
In some instances, the lifetime of the concrete structure is more important than its strength. Since corrosion is the main cause of failure of reinforced concrete, a corrosion-proof reinforcement can extend a structure's life substantially.
For these purposes some structures have been constructed using fiber-reinforced plastic rebar, grids or fibers. The "plastic" reinforcement can be as strong as steel. Because it resists corrosion, it does not need a protective concrete cover of 30 to 50 mm or more as steel reinforcement does. This means that FRP-reinforced structures can be lighter, have longer lifetime and for some applications be price-competitive to steel-reinforced concrete.
The main barrier to use of FRP reinforcement is the fact that it is neither ductile nor fire resistant. Structures employing FRP rebars may therefore exhibit a less ductile structural response, and decreased fire resistance.
However, the addition of short monofilament polypropylene fibres to the concrete during mixing may have the beneficial effect of reducing spalling during a fire. In a severe fire, such as the Channel Tunnel fire of 1996, conventionally reinforced concrete can suffer severe spalling leading to failure. This is in part due to the pore water remaining within the concrete boiling explosively; the steam pressure then causes the spalling. The action of fibres within the concrete is due to their ability to melt, forming pathways out through the concrete, allowing the steam pressure to dissipate.