Types of fiber used in concrete

There are many types of fibers that can be used in concrete including steel fibers, glass fibers, synthetic fibers and natural fibers. The characteristics of the fiber reinforced concrete depend on the fiber material, amount, geometry, distribution, orientation, and densities of the fibers used. Fibers are added to fresh concrete during the batching and mixing process to allow them to be equally distributed throughout the concrete.

Basics Of Fiber Reinforced Concrete

What Is Fiber Reinforced Concrete

Fiber reinforced concrete is a type of concrete that includes fibrous substance that increases its structural strength and cohesion. Fiber reinforced concrete has small distinct that are homogeneously dispersed and oriented haphazardly. used are s, synthetic , glass , and natural . The characteristics of fiber reinforced concrete are changed by the alteration of quantities of concretes, fiber substances, geometric configuration, dispersal, direction and concentration.

Why Fiber Reinforced Concrete Is Used

Portland cement concrete is believed to be a comparatively brittle substance. When un-reinforced concrete is exposed to tensile stresses, it is likely to fracture and fail. Since the beginning of the nineteenth century, studies were conducted to reinforce concrete by using steel. After the reinforcement of concrete by steel, it becomes a composite group in which the steel endures the tensile stresses. When concrete is reinforced by using fiber in the mixture, it further increases the tensile strength of the composite system. Research has revealed that the strength of concrete may be improved tremendously by the addition of fiber reinforcing. Since the stretching ability under load of reinforcing fiber is greater than concrete, initially the composite system will function as un-reinforced concrete. However, with additional loading the fiber reinforcing will be activated, to hold the concrete mix together.

Characteristics Of Fiber Reinforced Concrete

The characteristics of concrete depend upon the kind of fiber utilized, volume proportion of the fiber, and the ratio of length and diameter of the fiber. These conditions will improve the mechanical properties, including toughness, ductility, tensile strength, shear resistance and loading limit of the fiber reinforced concrete. Materials used for fiber reinforcement include steel, glass, polyester, rayon, cotton, and polythene. Most commonly used materials are steel and glass that are acid resisting. Natural being vulnerable to alkali attack are not much popular. Similarly, plastic have recently been introduced in the field of reinforcement and are still in the development phase. It is considered that the contribution of plastic to increase the static strength of concrete is limited. Nylons have the characteristics of a plastic material, and presently have a limited application in the slab technology. It is generally believed that nylon possess strength that is greater than the welded wire fabric in such slabs.

Fiber Reinforced Concrete

The concrete reinforcement technology is not new and fibers have been used for reinforcement since ancient times. Popular fibers are steel and glass fibers, while plastic and nylons have a limited use. Quantities, concentration, and dispersal influence the properties of fiber reinforced concrete.

SPECIAL TYPES OF CONCRETE

PRESTRESSED CONCRETEA prestressed concrete unit is one in whichengineered stresses have been placed before it hasbeen subjected to a load. When PRETENSION-ING is used, the reinforcement (high-tensile-strength steel strands) is stretched through theform between the two end abutments or anchors.A predetermined amount of stress is applied tothe steel strands. The concrete is then poured,encasing the reinforcement. As the concrete sets,it bonds to the pretensioned steel. When it hasreached a specified strength, the tension on thereinforcement is released. This prestresses theconcrete, putting it under compression, thuscreating a built-in tensile strength.

POST-TENSIONING involves a precastmember that contains normal reinforcing inaddition to a number of channels through which the prestressing cables or rods maybe passed. Thechannels are usually formed by suspending inflated tubes through the form and casting theconcrete around them. When the concrete has set,the tubes are deflated and removed. Once theconcrete has reached a specified strength, distressing steel strands or TENDONS are pulledinto the channels and secured at one end. They are then stressed from the opposite end with aportable hydraulic jack and anchored by one of several automatic gripping devices.Post-tensioning may be done where the member is poured or at the jobsite. Each membermay be tensioned, or two or more members maybe tensioned together after erection. In general,post-tensioning is used if the unit is over 45 ft longer over 7 tons in weight. However, some typesof pretensioned roof slabs will be considerably longer and heavier than this.When a beam is prestressed, either by pre-tensioning or post-tensioning, the tensioned steelproduces a high compression in the lower part of the beam. This compression creates an upwardbow or camber in the beam (fig. 7-19). When a load is placed on the beam, the camber is forcedout, creating a level beam with no deflection.Those members that are relatively small or that can be readily precast are normally pretensioned.These include precast roof slabs, T-slabs, floorslabs, and roof joists.

SPECIAL TYPES OF Concrete Special types of concrete are essentially those with unique physical properties or those produced with unusual techniques and/or reproduction processes. Many special types of concrete are made with portland cement as a binding medium;some use binders other than Portland cement.Lightweight ConcreteConventional concrete weighs approximately150 lb per cubic foot. Lightweight concrete weighs20 to 130 lb per cubic foot, depending on its intended use. Lightweight concrete can be madeby using either gas-generating chemicals.

Roller compacted concrete

• Roller compacted concrete, sometimes called rollcrete, is a low-cement-content stiff concrete placed using techniques borrowed from earthmoving and paving work.

• The concrete is placed on the surface to be covered, and is compacted in place using large heavy rollers typically used in earthwork.

• The concrete mix achieves a high density and cures over time into a strong monolithic block.

• Roller compacted concrete is typically used for concrete pavement. Roller compacted concrete dams can also be built, as the low cement content causes less heat to be generated while curing than typical for conventionally placed massive concrete pours

Pervious concrete

• Pervious concrete contains a network of holes or voids, to allow air or water to move through the concrete. This allows water to drain naturally through it, and can both remove the normal surface water drainage infrastructure, and allow replenishment of groundwater when conventional concrete does not.

• It is formed by leaving out some or the entire fine aggregate (fines), the remaining large aggregate then is bound by a relatively small amount of Portland cement.

• When set, typically between 15% and 25% of the concrete volumes are voids, allowing water to drain.

• The majority of pervious concrete pavements function well with little or no maintenance. Maintenance of pervious concrete pavement consists primarily of prevention of clogging of the void structure.

• In preparing the site prior to construction, drainage of surrounding landscaping should be designed to prevent flow of materials onto pavement surfaces. Soil, rock, leaves, and other debris may infiltrate the voids and hinder the flow of water, decreasing the utility of the pervious concrete pavement.

Shotcrete

• Shotcrete concrete uses compressed air to shoot concrete onto (or into) a frame or structure.

• Shotcrete is mortar or (usually) concrete conveyed through a hose and pneumatically projected at through a shortcrete nozzle with high velocity onto a surface. Shotcrete undergoes placement and compaction at the same time due to the force with which it is projected from the nozzle.

• It can be impacted onto any type or shape of surface, including vertical or overhead areas.

• Shotcrete is frequently used against vertical soil or rock surfaces, as it eliminates the need for formwork.

• It is sometimes used for rock support, especially in tunneling.

• Shotcrete is also used for applications where seepage is an issue to limit the amount of water entering a construction site due to a high water table or other subterranean sources.

• This type of concrete is often used as a quick fix for weathering for loose soil types in construction zones.