Reinforced Concrete Frame Structure

Reinforced concrete is one of the most widely used modern building materials. Concrete is ‘artificial stone’ obtained by mixing cement, sand, and aggregates with water. Fresh concrete can be molded into almost any shape, which is an inherent advantage over other materials. Concrete become very popular after the invention of Portland cement in 19th century. However, its limited tension resistance prevented its wide use in building construction. To overcome this weakness, steel bard are embedded in concrete to form a composite material called reinforced concrete. Developments in the modern reinforced concrete design and construction practice were pioneered by European engineers in the late 19th century. At the present time, reinforced concrete is extensively used in a wide variety of engineering applications.

The worldwide use of reinforced concrete construction stems from the wide availability of reinforcing steel as well as the concrete ingredients. Unlike steel, concrete production does not require expensive manufacturing mills. Concrete construction, does, however, require a certain level of technology, expertise, and workmanship, particularly in the field during construction. In some cases, single-family houses or simple low-rise residential buildings are constructed without any engineering assistance.

The extensive use of reinforced concrete construction, especially in developing countries, is due to its relatively low cost compared to other materials such as steel. The cost of construction changes with the region and strongly depends on the local practice. As an example, a unit area of a typical residential building made with reinforced concrete costs approximately $100/m² in India, $250/m² in Turkey, and $500/m² in Italy.

With the rapid growth of urban population in both the developing and the industrialized countries, reinforced concrete has become a material of choice for residential construction. Unfortunately, in many cases there is not the necessary level of expertise in design and construction. Design applications ranges from single-family buildings in countries like Colombia to high rises in China. Frequently, reinforced concrete construction is used in regions of high seismic risk.

Introduction

Steel reinforced concrete is a specific type that has had strong steel rebar or fibers added to it while wet, creating a very strong type of concrete that is able to withstand almost anything when it has dried. Because the result of using steel reinforced are so good for the strength of the building, most modern building today use steel reinforced concrete in the construction process. By adding thin steel bars to concrete can increase the strength of the concrete, making it better to use in variety of application. Today, many of the buildings located nations use reinforced concrete to make the buildings stronger and better able to in industrialized withstand the ravages of time and the weather. Reinforcing the concrete that will be used on the buildings add tensile strength to the concrete, making it much stronger and more flexible that regular concrete, which helps prevent cracking and breakage. Steel reinforced concrete can be used in a number of building applications, including floors, beams, supports, walls, and frames.

Steel reinforced concrete is a concrete in which steel reinforcement bars, plates or fibers have been incorporated to build up a material that would otherwise be fragile. 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 action. 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.

Reinforcing Steel

Before placing reinforcing steel in forms, all form oiling should be completed. As mentioned earlier, oil or other coating should not contact the reinforcing steel in the formwork. Oil on reinforcing bars reduces the bond between the bars and the concrete. Use a piece of burlap to clean the bars of rust, scales, grease, mud or other foreign matter. A light film of rust or mild film is not objectionable. Rebars must be tied together for the bars tore main in a desired arrangement during pouring. Tying is also a means of keeping laps or splices in place. Laps allow bond stress to transfer the load from one bar, first into the concrete and then into the second bar.

Advantages Behaviour

Materials

Concrete is a mixture of cement, stone aggregate, and small amount of water.

Cement hydrates from microscopic opaque crystal lattices encapsulating and locking the aggregate into a rigid structure.

Typical concrete mixes have low tensile strength.

Steel, is placed in concrete, then it will not only resists compression but also bending, and other direct tensile actions.

Steel also made the bonding of the aggregate in a concrete better.

Physical characteristics of steel reinforced concrete:

The coefficient of thermal expansion of concrete is similar to that of steel, eliminating internal stresses due to differences in thermal expansion or contraction.

When the cement paste within the concrete hardens this conforms to the surface details of the steel, permitting any stress to be transmitted efficiently between the different materials.

The alkaline chemical environment provided by calcium carbonate causes a passivating film to form on the surface of the steel, making it much more resistant to corrosion than it would be in neutral or acidic conditions.

Common Failure Modes of Steel Reinforced Concrete

Conventional steel reinforced concrete can failed due to inadequate strength, leading to mechanical failure, or due to a reduction in its durability. Corrosion and freeze may damage poorly designed or constructed reinforced concrete. When rebar corrodes, the oxidation products expand and tends to flake, cracking the concrete and unbonding the rebar from the concrete.

Typical mechanisms leading to durability problems are as below:

Mechanical failure

Steel reinforced concrete may be considered to have failed when significant cracks occur. Cracking of the concrete section cannot be prevented. However, the size and location of the cracks can be limited and controlled by reinforcement, placement of control joints, the curing methodology and the mix design of the concrete. Cracking defects can allow moisture to penetrate and corrode the reinforcement. This is a serviceability failure in limit state design. Cracking is normally the result of an inadequate quantity of rebar, or rebar spaced at too great a distance. The concrete then cracks either under excess loadings, or due to internal effects such as early thermal shrinkage when it cures. 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.

Carbonation

Carbonation or neutralisation, is a chemical reaction between carbon dioxide in the air and calcium hydroxide and hydrated calcium silicate in the concrete. The water in the pores of Portland Cement Concrete is normally alkaline with a pH in the range of 12.5 to 13.5. This highly alkaline environment is one in which the embedded steel is passivated and is protected from corrosion. The carbon dioxide in the air reacts with the alkaline 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 carbonation process will start at surface, then slowly move deeper and deeper into the concrete. If the object is cracked, the carbon dioxide in the air will be better able to penetrate into the concrete. Carbonated concrete only becomes a durability problem when there is also sufficient moisture and oxygen to cause electro-potential corrosion of the reinforcing steel.

Chlorides

Chlorides, including sodium chloride, can promote the corrosion of embedded steel rebar if present in sufficient concentration. So, only use fresh raw water or portable water for mixing concrete. It was once common for calcium chloride to be use as an admixture to promote rapid set-up of the concrete. It was also mistakenly believed that it would prevent freezing.

Alkali Silica Reaction

This is a reaction of amorphous silica sometimes present in the aggregates with alkali, for example from the cement pore solution. The silica reacts with the alkali to form a silicate in the Alkali silica reaction, this causes localize swelling which causes cracking. The conditions are: aggregate containing an alkaline reactive constituent, sufficiently availability of alkali ions and sufficient moisture. This phenomenon referred as concrete cancer. This reaction occurs independently of the presence of rebar.

Conversion of High Alumina cement

Resistant to weak acids and especially sulfates, this cement cures quickly and reaches very high durability and strength. However, it can lose strength with heat or time, especially when not properly cured.

Sulfates

Sulfates in the soil or in groundwater, in sufficient concentration, can react with the Portland cement in concrete causing the formation of expansive products which can lead to early failure of the structure.

Corrosion and Passivation of steel reinforcement

Exposed steel will corrode in moist atmospheres due to differences in the electrical potential on the steel surface forming anodic and cathodic sites.

Concrete as an environment

The environment provided by good quality concrete to steel reinforcement is one of high alkalinity due to the presence of the hydroxides of sodium, potassium and calcium produced during the hydration reactions. The bulk of surrounding concrete acts as a physical barrier to many of the steel’s aggressors. In such an environment steel is passive and any small breaks in its protective oxide film are soon repaired. However, the alkalinity of its surroundings are reduced, such as by neutralization are able to reach the steel then severe corrosion of the reinforcement can occur. This in turn can result in to staining of the concrete by rust and spalling of the cover due to the increase in volume associated with the conversion of iron to iron oxide.

Factors affecting corrosion rates of steel in concrete

The permeability of the concrete is important in determining the extent to which aggressive external substances can attack the steel. A thick concrete cover of low permeability is more likely to prevent chloride ions from an external source from reaching the steel and causing depassivation.

Alternatives for the reinforcing phase

Where an adequate depth of cover is difficult to achieve due to design considerations or where aggressive environments are expected such as in marine structures or bridge decks, additional protection may be required for the embedded steel. This may take many and varied forms and commercial interest in this field is strong. The steel reinforcement itself may be made more able to maintain its passivity by providing it with a protective coating. In extreme circumstances, solid stainless steel may be used, although the perceived additional cost restricts its use in all but the most specialized applications.

The ideal situation

There can be little doubt that the most effective way of protecting steel which is embedded in concrete is to provide it with an adequate depth of cover by high strength, low permeability concrete free from depassivating ions such as chlorides. However, in the real world, concrete is laid by the tone in all weathers and environments, exposed to industrial atmospheres, de-icing salts and seawater.

The real situation

Contaminated materials and poor workmanship are hard to avoid completely but by understanding the often complex chemical and electrochemical conditions that can exists it should be possible to develop ways of producing structures which will last long into the next century.

Deterioration mechanisms

The majority of reinforced concrete around the world performs adequately and gives few problems. A minority of structures have deteriorated due to either the action of aggressive components from the external environment or incompatibility of the mix constituents. Problems can arise as a result of incomplete or inaccurate site investigation, poor design, badly specified concrete, poor workmanship and a range of other factors.

Stages of deterioration

The mechanisms of deterioration are primarily chemico-physical in nature and occur in three discrete stages which are initiation, propagation, and deterioration.

Modes of deterioration

Deterioration may occur due to a number of mechanisms on which a large body of literature already exists. These include:

Corrosion of reinforcement due to chloride ions, carbonation and change in the rebar reinforcement.

Sulphate attack of concrete

Salt recrystallisation

Soft water or acid attack of concrete

Alkali aggregate reaction

Thermal incompatibility of concrete components

Shrinkage

Frost damage

Depth of cover

Inadequate cover is invariably associated with areas of high corrosion risk due to both carbonation and chloride ingress. By surveying the surface of a structure with an electromagnetic covermeter and producing a cover contour plot, the high-risk areas can be easily identified. A cover survey of newly completed structures would rapidly identify likely problem areas and permit additional protective measures to be taken.

Cracked cover

It should be remembered that reinforced concrete is intrinsically a cracked material because the steel stops the structure failing in tension but the brittle concrete cracks to the depth of the reinforcement. Only those cracks above a critical width which intersect the steel are liable to assist the corrosion processes.

Cost Incurred

After a period of unprecedented growth in prices during 2004, early date for 2005 indicates that the constructional steel market faces greater stability in the year ahead. Despite the price increases, demand for steel in the UK market remained at a very high level in 2004. One of the principal concerns for steel users was the availability of material, but the year ended with more steel in the supply chain than there had been at the beginning.

Structural steel frame costs

The leading benchmark cost unit for structural steelwork is its unit cost per tonne which includes the steel and the following elements:

Connection design, detail drawing, fabrication, testing, treatment and delivery, offloading, erection

These are calculated against the overall estimated tonnage for the building to generate an overall frame cost. Unit costs per tonne can vary enormously as there are a combination of factors that influence the overall cost. Care should be taken in considering each project’s characteristics in arriving at a tonnage rate. This can be calculated either on the number of beams and column in a building or a weight per m².

The relative costs of each element will vary depending on the nature of the project. The tonnage rate could be divided as follows:

Materials 30%

Engineering 5%

Fabrication 35%

Priming 8%

Delivery 2%

Erection 20%

The costs assume that the structural steelwork contractor will provide their own crane for all the projects with the exception of office buildings, for which the main contractor provides a tower crane. The early involvement of structural steelwork fabricators is the most effective way to value engineer cost savings into steelwork frame. For example, using more substantial and therefore more expensive steel columns in a design could remove the need for stiffeners. The steel may cost more but it is cheaper overall than paying for labour to fabricate and weld stiffeners to the column. If this value is adopted early enough in the project across the whole frame design, significant cost savings can be achieved.

The cost of a frame system alone should not dictate the choice of frame for a project. Rather it should be just one of a number of issues that should be considered when making the choice of frame material. The recent rises in reinforcement and steel prices have increased frame costs but the difference between steel and concrete frame costs remains insignificant. A 50% increase in European steel prices during 2004 has left many in the construction industry reviewing design solutions that have a heavy reliance on steel. The impact of the steel price rises and found that the whole project costs for concrete framed buildings are marginally less than for steel framed buildings.

Foundation costs

The foundations typically represent approximately 3% of whole project initial cost. For the heaviest reinforced concrete solutions, the foundations will be more expensive, but this represents only a small cost and can be offset by using post-tensioned slabs, which are typically 15% lighter.

Cladding costs

The thinner the overall structural and services zone, the less the cladding costs. Given that cladding can represent up to 25% of the construction cost it is worth minimizing the cladding area. The minimum floor-to-floor height is almost always achieved with a concrete flat slab and separate services zone.

Partitions

Sealing and fire stopping at partitions heads is simplest with flat soffits. Significant savings of up to 10% of the partitions package can be made compared to the equivalent dry lining package abutting a profiled soffit with downstands. This can represent up to 4% of the frame cost.

Services co-ordination/ Installation/ Adoptability

The soffit of a concrete flat slab provides a zone for services distribution free of any downstand beams. This reduces coordination effort for the design team and therefore the risk of errors. It permits flexibility in design and allows coordination effort to be focused elsewhere. Services installation is simplest below a flat soffit. This permits maximum off site fabrication of services, higher quality of work and quicker installation. These advantages should be reflected in cost and value calculations. Indeed, M&E contractors quote an additional cost of horizontal services distribution below a profited slab of up to 15%. Flat soffits also allowed greater future adaptability.

Fire protection

For concrete structures fire protection is generally not needed as the material has inherent fire resistance of up to four hours. This remove the time, cost and separate trade required to attend the site for fire protection.

Vibration

The inherent mass of concrete means that concrete floors generally meet vibration criteria at no extra cost and without any extra stiffening. For more stringent criteria, the additional cost to meet vibration criteria is small compared to other structural material.

Exposed soffit

A concrete structure has a high thermal mass. By exposing the soffits this can be utilized through fabric energy storage to reduce initial plant costs and ongoing operational costs. Furthermore, the cost of suspended ceilings can be reduced or eliminated.

Conclusion

As a conclusion, the majority of reinforced concrete structures show excellent durability and perform well over their design life. Adverse environments or poor construction practice can lead to corrosion of the reinforcing steel in concrete. The major mechanisms for corrosion are atmospheric carbon dioxide ingress and chloride attack from cast-in or diffused chlorides. The corrosion and deterioration mechanisms are essentially the same for both carbonation and chloride attack. Proper choice of materials, adequate cover to reinforcement, good quality concrete and attention to the environment during construction will enhance the durability of reinforced concrete structures. For cost incurred, concrete’s range of inherent benefits including fabric energy storage, fire resistance and sound installation means that concrete buildings tend to have lower operating costs and lower maintenance requirements.

For structure subjected to aggressive environments, combinations of moisture, temperature and chlorides may result in the corrosion of reinforcing and prestressing steel, leading to the deterioration of concrete and loss of serviceability. One preferred solution which has assumed the status of cutting-edge research in many industrialized countries, is the use of fiber reinforced polymer rebars in concrete. Fiber concrete is also becoming an increasingly popular construction material due to its improved mechanical properties over non-reinforced concrete and its ability to enhance the mechanical performance of conventionally reinforced concrete.

DEFINITION OF FIBRE REINFORCED POLYMER

Fibre-reinforced polymer (FRP), also known as fibre-reinforced plastic) are composite materials made of a polymer matrix reinforced with fibres. FRPs are commonly used in the aerospace, automotive, marine, and construction industries. FRPs are typically organized in a laminate structure, such that each lamina (or flat layer) contains an arrangement of unidirectional fibres or woven fibre fabrics embedded within a thin layer of light polymer matrix material. The fibres, typically composed of carbon or glass, provide the strength and stiffness. The matrix, commonly made of polyester, Epoxy or Nylon, binds and protects the fibers from damage, and transfers the stresses between fibers.

TYPES OF MATERIAL USED

Polymer

There are two main types of polymer used for resins: thermosets and thermoplastics. The thermosetting polymers used in the construction industry are the polyesters and the epoxides. There are many thermoplastic resins used in composite manufacture: polyolefins, polyamides, vinylic polymers, polyacetals, polysulphones, polycarbonates, polyphenylenes and polyimides.

Fibre

A wide range of amorphous and crystalline materials can be used as the fibre. In the construction industry the most common fibre used is glass fibre (there are 4 types of glass fibre: E-glass, AR-glass, A-glass and high strength glass). Carbon fibre, of which there are 3 types (Type I, II, III) can be used separately or in conjunction with the glass fibre as a hybrid to increase the stiffness of a structural member or the area within a structure, so that the stiffness exceeds the value possible using only glass fibre. Aramid fibres can be used instead of glass fibres to give increased stiffness to the composite. Today each of these fibers is used widely in industry for any applications that require plastics with specific strength or elastic qualities. Glass fibers are the most common across all industries, although carbon fiber and carbon fiber aramid composites are widely found in aerospace, automotive and sporting good applications.

Additives

For structural applications it is mandatory to achieve some degree of flame retardant. Fire retardants are usually incorporated in the resin itself or as an applied gel-coat. Fillers and pigments are also used in resins for a variety of purposes, the former principally to improve mechanical properties and the latter for appearance and protective action.

APPLICATIONS OF FRP IN CONSTRUCTION

There are three broad divisions into which applications of FRP in civil engineering can be classified: applications for new construction, repair and rehabilitation applications, and architectural applications.

NEW CONSTRUCTION

FRPs have been used widely by civil engineers in the design of new construction. Structures such as bridges and columns built completely out of FRP composites have demonstrated exceptional durability, and effective resistance to effects of environmental exposure. Pre-stressing tendons, reinforcing bars, grid reinforcement, and dowels are all examples of the many diverse applications of FRP in new structures.

REPAIR AND REHABILITATION

One of the most common uses for FRP involves the repair and rehabilitation of damaged or deteriorating structures. Several companies across the world are beginning to wrap damaged bridge piers to prevent collapse and steel-reinforced columns to improve the structural integrity and to prevent buckling of the reinforcement.

ARCHITECTURAL

Architects have also discovered the many applications for which FRP can be used. These include structures such as siding/cladding, roofing, flooring and partitions.

DESIGN CONSIDERATION

The strength properties of FRPs collectively make up one of the primary reasons for which civil engineers select them in the design of structures. A material’s strength is governed by its ability to sustain a load without excessive deformation or failure. When an FRP specimen is tested in axial tension, the applied force per unit cross-sectional area (stress) is proportional to the ratio of change in a specimen’s length to its original length (strain). When the applied load is removed, FRP returns to its original shape or length. In other words, FRP responds linear-elastically to axial stress.

FRP allows the alignment the glass fibers of thermoplastics to suite specific design programs. Specifying the orientation of reinforcing fibers can increase the strength and resistance to deformation of the polymer. Glass reinforced polymers are strongest and most resistive to deforming forces when the polymers fibers are parallel to the force being exerted, and are weakest when the fibers are perpendicular.

Thus this ability is can be an advantage or a limitation depending on the context of use. Weak spots of perpendicular fibers can be used for natural hinges and connections, but can also lead to material failure when production processes fail to properly orient the fibers parallel to expected forces. When forces are exerted perpendicular to the orientation of fibers, the strength and elasticity of the polymer is less than the matrix alone. In cast resin components made of glass reinforced polymers such as UP and EP, the orientation of fibers can be oriented in two-dimensional and three-dimensional weaves. This means that when forces are possibly perpendicular to one orientation, they are parallel to another orientation; this eliminates the potential for weak spots in the polymer.

COST

With the rising cost of nickel, FRP has become a very competitive material of construction. It is very competitive with acid brick or rubber-lined carbon steel and much less expensive than alloy-clad carbon steel. It is generally more expensive than resin-coated carbon steel but has a longer service life in most applications. Because FRP does not require insulation, FRP ductwork is actually less expensive than resin-coated carbon steel.

ADVANTAGES OF FRP

Composites offer the designer a combination of properties not available in traditional materials. It is possible to introduce the fibres in the polymer matrix at highly stressed regions in a certain position, direction and volume in order to obtain the maximum efficiency from the reinforcement, and then, within the same member to reduce the reinforcement to a minimal amount at regions of low stress value.

FRP products are a cost effective alternative to steel in many of the harshest industrial environments. The advantages of FRP products over other materials include:

Corrosion Resistant

Fibre Reinforced Polymer materials are designed to operate in aggressive environments. Little or no coating or treating required.

Low maintenance requirements

Designed and engineered to last, composite structural materials are virtually maintenance free.

Impact resistant

Inherent flexibility allows products to resist impact and failure.

Non-conductive and Non metallic

FRP constructions provide additional safety by stopping sparks and potential electrical hazards.

Fire Retardant

FRP has a low flame spread index when tested under ASTM E-84 and meets self extinguishing requirements of ASTM D-635.

High strength-to-weight ratio

The strong, but light weight alternative where heavy lifting or access is an issue.

Reduced installation time and cost

FRP products are easier and lighter to install. Normal ‘hand tools’ are used to make adjustments. Therefore FRP offers greater efficiency in construction compared with the more conventional materials.

DISADVANTAGES OF FRP

Structural failure can occur in FRP materials when tensile forces stretch the matrix more than the fibers, causing the material to shear at the interface between matrix and fibers, tensile forces near the end of the fibers exceed the tolerances of the matrix, separating the fibers from the matrix and tensile forces can also exceed the tolerances of the fibers causing the fibers themselves to fracture leading to material failure.

A serious matter relating to the use of FRPs in civil applications is the lack of design codes and specifications. For nearly a decade now, researchers from Canada, Europe, and Japan have been collaborating their efforts in hope of developing such documents to provide guidance for engineers designing FRP structures.

FRP plastics are liable to a number of the issues and concerns surrounding plastic waste disposal and recycling. Plastics pose a particular challenge in recycling processes because they are derived from polymers and monomers that often cannot be separated and returned to their virgin states, for this reason not all plastics can be recycled for re-use, in fact some estimates claim only 20% to 30% of plastics can be material recycled at all.

In addition, fibers themselves are difficult to remove from the matrix and preserve for re-use means FRP amplify these challenges. FRP are inherently difficult to separate into base a material that is into fiber and matrix, and the matrix into separate usable plastic, polymers, and monomers. These are all concerns for environmentally informed design today, but it must be noted that plastics often offer savings in energy and economic savings in comparison to other materials, also with the advent of new more environmentally friendly matrices such as bioplastics and UV-degradable plastics, FRP will similarly gain environmental sensitivity.

DIFFERENCES BETWEEN CONVENTIONAL STEEL REINFORCED CONCRETE AND FIBRE-REINFORCED POLYMER (FRP) CONCRETE

No.

Conventional Steel Reinforced Concrete

Fibre-Reinforced Polymer (FRP) Concrete

Definition:

Steel reinforced concrete is a specific type that has had strong steel rebar added to it while wet, creating a very strong type of concrete that is able to withstand almost anything when it has dried.

Definition:

FRP concrete is composite materials made of a polymer matrix reinforced with fibres and typically organized in a laminate structure, such that each lamina (or flat layer) contains an arrangement of unidirectional fibres or woven fibre fabrics embedded within a thin layer of light polymer matrix material.

Corrosion of steel reinforcement:

Exposed steel will corrode in moist atmospheres due to differences in the electrical potential on the steel surface forming anodic and cathodic sites.

Corrosion Resistant:

Fibre Reinforced Polymer materials are designed to operate in aggressive environments. Little or no coating or treating required.

Conductive and metallic:

Steel reinforced concrete constructions provide additional safety by stopping sparks and potential electrical hazards.

Non-conductive and Non metallic:

FRP constructions provide additional safety by stopping sparks and potential electrical hazards.

Increase installation time and cost:

Steel reinforced concrete products are difficult and heavy to install. Bar bending and bar cutting machine are used to make adjustments.

Reduced installation time and cost:

FRP products are easier and lighter to install. Normal ‘hand tools’ are used to make adjustments.

Have many design codes and specifications of steel reinforced concrete in civil applications which needed as guidance for engineers designing steel reinforced concrete structures.

The lack of design codes and specifications of FRP in civil applications which needed as guidance for engineers designing FRP structures.

The high costs of repair and maintenance of structures damaged by corrosion and heavy use.

Fibre Reinforced Polymer (FRP) is a relatively new class of composite material manufactured from fibers and resins and has proven efficient and economical for the development and repair of new and deteriorating structures in civil engineering.

Cost : Expensive (e.g. The material costs for the steel

reinforced bridge were $391,649.53)1

Cost : Cheap (e.g. The total material costs for the FRP reinforced

bridge were $632,718)1

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