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Reinforcement Steel

• satyendra
• May 10, 2013
• 4 Comments
• corrosion resistance, CTD bars, Ductility, Micro alloying, Rebars, reinforced concrete, reinforcement steel, TMT rebars, Yield strength,

Reinforcement Steel

Steel is the time proven match for reinforced concrete structures. Reinforced concrete structure is designed on the principle that steel and concrete act together to withstand induced forces. Reinforcement steel is embedded in the concrete in such a way that the two materials resist the applied forces together. The compressive strength of concrete and the tensile strength of steel form a strong bond to resist these stresses over a long span. Plain concrete is not suitable for most construction projects since it cannot easily withstand the stresses created by vibrations, wind, or other forces. Reinforcement steel is to be strong in tension and, at the same time, be ductile enough to be shaped or bent in cold condition. Steel reinforcement is also essential contributor towards crack control of concrete structures.

Reinforcement of concrete with steel is being done for more than 100 years. It is the time proven match for reinforcing concrete structures. At the very beginning of technology simple rounds and squares were used. Sometimes later rails and profiles were also used. The production of specific reinforcement steel started sometimes in 1930s when it was realized that beside the tensile strength, bond between steel and concrete plays an important role in terms of load transfer.

For a long time, steel bars in as rolled condition were being used for reinforcement purpose. Yield strengths of 350 MPa to 400 MPa were achieved in carbon-manganese steels by keeping C to around 0.35 % and Mn in the range of 0.9 % to 1.45 %. But due to the ingot casting, there was segregation of manganese which was causing brittleness of the steel at the place of segregation. At this stage micro alloying of steel was introduced to achieve the yield strength of the steel by micro alloying it with vanadium, niobium, or titanium. Since the carbon content also got reduced, these rods were weldable. But the advantage was got negated due to higher cost of these steels. Sometime during 1940s ribs were introduced in the reinforcement steels. Both straight long ribs and cross ribs were introduced to provide reinforcement steel better bonding with the concrete. These ribs were produced during rolling itself. This has resulted in saving of lot of fabrication work on the reinforcement steel.

Next important step in the development of reinforcement steel was cold twisting of the steel after rolling it in the mill. The cold twisting gives to the longitudinal rib a spiral shape. The reinforcement bars ( also known as rebars in short) produced by this method was popularly known as CTD (cold twisted deformed) or Torsteel bars. In these rebars, the strength of the steel is obtained through cold working and not by the adjustment in the chemistry of the steel. The process of cold working of reinforcement steel is a mechanical process which involves stretching and twisting of the rebars rolled from mild steel, beyond the yield plateau, and subsequently releasing the load. For many years, these rebars were dominating the use of reinforcement steel. Because of reasonable carbon in this reinforcement steel, these rebars were weldable. During late seventies, th e heat treatment process for manufacturing reinforcement steel was developed by Centre de Rechaerche Metallurgiques (CRM) Belgium and Thermex from HSE Germany.

Reinforced concrete is normally designed on the principle that steel and concrete act together in resisting force. Concrete is strong in compression but weak in tension. The tensile strength is normally rated around 10 % of the compression strength. For this reason, concrete works well for columns and posts which are compression members in a structure. But, when it is used for tension members, such as beams, girders, foundation walls, or floors, concrete is required to be reinforced to attain the necessary tension strength. Steel is the best material for reinforcing concrete since the properties of expansion for both steel and concrete are considered being approximately the same, i.e., under normal conditions, they expand and contract at an almost equal rate. However, at very high temperatures, steel expands more rapidly than concrete and the two materials separate.

Another reason because of which, steel works well as reinforcement material for concrete is since it bonds well with concrete. This bond strength is proportional to the contact surface of the steel to the concrete. In other words, the higher the surface of steel is exposed to the adherence of concrete, the stronger is the bond. A ribbed or deformed reinforcement bar adheres better than a plain, round, or square bar since it has a greater bearing surface. In fact, when plain bars of the same diameter are used instead of ribbed or deformed bars, around 40 % more bars are to be used. The rougher the surface of the steel, the better it adheres to concrete.

Besides the properties of thermal expansion for both steel and concrete being approximately the same. This along with excellent bendability property makes steel the best material for reinforcement in the concrete structures. Another reason that steel works effectively as reinforcement material is that it bonds well with concrete. When passive reinforcement (steel bars) is employed, the structure is known as reinforced concrete structure. In pre-stressed concrete structure, the reinforcement (steel wire) is stressed prior to subjecting the structure to loading, which can be viewed as active reinforcement. Passive steel reinforcement bars, also known as rebars, are necessarily to be strong in tension and at the same time, be ductile enough to be shaped or bent.

Rebar is an important component of reinforced concrete. It is normally formed from ridged carbon steel. The ridges give frictional adhesion to the concrete. It is used because concrete though very strong in compression is virtually without strength in tension. To compensate for this, rebar is cast into it to carry the tensile loads on a structure. While any material with sufficient tensile strength can conceivably be used to reinforce the concrete, steel is used since it has similar coefficients of thermal expansion. This means that a concrete structural member reinforced with steel experiences minimal stress as a result of differential expansions of the two interconnected materials due to temperature changes.

Reinforcement steel is used as conventional or pre-stressed reinforcing, depending on the design situation. In conventional reinforcing, the stresses fluctuate with loads on the structure. This does not place any special requirements on the steel. On the other hand, in pre-stressed reinforcement, the steel is under continuous tension. Any stress relaxation reduces the effectiveness of the reinforcement. Hence, special steels are required.

Reinforcement steel (Fig 1) can be used in the form of bars or rods which are either plain, ribbed or deformed, in the form of expanded metal, cold drawn wire, deformed welded rod or wire fabric, or sheet metal. Each type is useful for different purposes, and engineers design the concrete structures with those purposes in mind. Reinforcement steel is produced in three forms namely (i) plain bars, (ii) ribbed or deformed bars, and (iii) plain or deformed welded rod or wire fabrics.

Fig 1 Reinforcement steel

According to the surface pattern of the rebar, it can be classified into plain rebar and ribbed or deformed rebar. Plain rebar is typically a rod round in cross section without repeating patterns of ridges and depressions on its surface. These bars are frequently used in situations where the rebar sections need to slide, such as the highway pavements, which are easy to subject to the weather induced expansion and cracking. They are used in concrete for special purposes, such as dowels at expansion joints, where bars are to slide in a metal or paper sleeve, for contraction joints in roads and runways, and for column spirals. These are the least used of the rod type of reinforcement since these rods offer only smooth, even surfaces for bonding with the concrete.

Ribbed or deformed bars differ from the plain bars in that they have either indentation in them or ridges on them, or both, in a regular pattern. The ribs and depressions on its surface can increase the bond strength with concrete and prevent slippage. The patterns can be customized according to construction requirements. The twisted bar, for example, is made by twisting a plain, square bar in cold condition. The spiral ridges, along the surface of the deformed bar, increase its bond strength with concrete. Other forms used are the round and square corrugated bars. These bars are formed with projections around the surface which extend into the surrounding concrete and prevent slippage. Another type is formed with longitudinal fins projecting from the surface to prevent twisting.

Expanded metal or wire mesh is also used for reinforcing concrete. Expanded metal is made by partly shearing a sheet of steel. The sheet steel has been sheared in parallel lines and then pulled out or expanded to form a diamond shape between each parallel cut. Another type is square, rather than diamond shaped. Expanded metal is customarily used during plastering operations and light reinforced concrete construction, such as sidewalks.

Welded wire fabric is fabricated from a series of rods or wires arranged at right angles to each other and electrically welded at all intersections. Welded wire fabric has different uses in the reinforced concrete construction. In building construction, it is most frequently used for floor slabs on well-compacted ground. Heavier fabric, supplied mainly in flat sheets, is frequently used in walls and for the primary reinforcement in structural floor slabs. Additional examples of its use include road and runway pavements, box culverts, and small canal linings.

Mild steel rebars are the most common, as its price is relatively low while it provides material properties which are acceptable for many applications. Mild steel contains 0.15 % to 0.30 % carbon and hence it is neither brittle nor ductile. The mechanical properties of steel rebars affect the load bearing capacity of a particular structure. The properties most commonly used for rebars as a basis for specification and design are the specified minimum yield strength and the specified minimum ultimate strength, both obtained from tensile tests on the samples of the steel rebars.

Good strength, bond with concrete, thermal expansion characteristics (similar to concrete) and bendability are prime attributes which make steel rebars most effective reinforcement material for engineering of reinforced concrete structures. Besides strength, the durability of the structure depends upon rebar quality. Durability is the ability of the structure to maintain safety and serviceability criteria during its design life. Durability is dependent on the condition of concrete and reinforcement. Corrosion of rebars is one of the main factors which can impair durability. Corrosion can be either due to chloride intrusion or due to the effect of carbonation. Chemical composition of reinforcement plays an important role in this respect.

Two characteristics of rebars namely bendability and weldability, are important for the concrete construction. Bendability is essential from giving requisite shape to the rebar to suit the demand of the structures. Sometimes, welding of high diameter rebars is resorted to reduce congestion. Weldability of the rebars is also an important factor for fixing embedded parts in the concrete before pouring.

Enhancement of strength by cold working process or by changing chemical composition (for example increase in carbon content) has conflicting effect on the ductility and weldability. Hence, balancing of conflicting requirements is needed in fixing the characteristics of the rebar to strike an optimum balance between strength, ductility, durability and cost.

The bond between rebar and concrete depends upon many factors, such as shape, geometry of ribs. Steel rebars are normally round in cross section. To restrict longitudinal movement of the bars relative to the surrounding concrete, lugs or protrusions called deformations or ribs are rolled on to the bar surface. For appropriate bond strength, the deformations of ribs of rebar are to satisfy certain specifications.

Typical stress strain curve of monotonically loaded (tension) mild steel reinforcement bar is shown in Fig 2(i). The curve shows an initial elastic portion, a yield plateau (that is, a yield point beyond which the strain increases with little or no increase in stress), a strain hardening range in which stress again increases with strain, and finally a range in which the stress drops off until fracture occurs. The slope of the linear elastic portion of the curve represents the modulus of elasticity of steel. The stress at the yield point, referred as the yield strength, is a very important property of steel rebar since the rebars are normally characterized by its yield strength. Stress-strain curves of the steel in compression and tension are considered to be the same. In case of mild steel, yielding sometimes is accompanied by an abrupt decrease in stress, and the stress-strain diagram has two stress (yield) levels, which are marked as A and B in Fig 2(i). Points A and B are referred as upper and lower yield strengths respectively. The position of the upper yield point depends on the speed of testing, the shape of the section, and the form of the sample. The lower yield strength is normally considered as the true characteristic of the material and simply referred as yield strength.

In cold working (stretching and twisting) process, the mild steel bar is subjected to repeated loading. The steel follows a similar linear elastic path, as that of original mild steel till it reaches the point where unloading started, which becomes the new yield point as shown in Fig 2(ii). The cold working of steel can cause the shortening of the yield plateau or even eliminating it completely. Desired increase in yield strength is achieved by appropriate selection of unloading point. This is why high strength rebars normally do not show definite yield strength as that in case of mild steel.

Fig 2 (iii) shows a typical stress strain curve of cold twisted deformed (CTD) rebar. Cold working process is simple, reliable as well as cost effective, but reduces elongation of rebar compared to mild steel. CTD rebars does not show specific yield point and 0.2 % proof stress is taken as yield strength.  Stress-strain curve of TMT rebars as shown in Fig 2 (iv) is similar to that for mild steel rebars. But in case of TMT bars, there are no distinct yield plateau and two yield points. The actual yield strength of the rebar is normally somewhat higher than that considered in design. The specified yield strength normally refers to a guaranteed minimum value of the yield strength (lower yield strength in case of mild steel rebar).

Fig 2 Stress strain cures for rebars

Fatigue strength of reinforcement depends on its yield strength and rebars having higher fatigue strength have better capability of withstanding dynamic loads. Bond strength signifies its ability for holding concrete around it. It depends on the reinforcing properties of the bars, such as yield strength, adhesion with concrete matrix, indentation (configuration of deformed shape).

Ductility refers to ability of dissipating energy and large deformation. Ductility of rebar is expressed as the ratio of ultimate deformation at collapse to deformation at yielding. During the earlier period of reinforced concrete construction, requirement of ductility was considered synonymous with bendability. However, ductility of reinforcement has been found to have far reaching effects on the safety and durability of the structure. The physical property of rebar, which is responsible for ductility, is its elongation. Under the repetitive loading when the load is released before failure, the sample recovers along a stress-strain path that is parallel to the original curve with perhaps a small hysteresis and / or strain-hardening effect (Fig 2 (ii)). The virgin curve is then closely followed, as if unloading has not occurred. Hence, the monotonic stress-strain curve gives a good idealization for the envelope curve of rebar under repeated loading of the same sign.

Requirement of ductility is more important where the structure is subjected to cyclic loading (e.g. earth-quake load) or impact. Due to Bauschinger effect, that is, strain softening which takes place under reversed loading, the stress-strain curve becomes non-linear at a stress much lower than the initial yield strength. This behaviour of steel bars is strongly influenced by previous strain history with the time and temperature also has an effect. The unloading path follows the initial elastic slope. The behaviour with respect to ductility of rebar, against monotonic, repetitive and cyclic loading can be characterized by means of ultimate strain at fracture or total elongation. The requirement of minimum strain at fracture or minimum elongation is specified in standards. Such specification is essential for the safety of the structure and in order to ensure that the steel is ductile enough to undergo large deformations before fracture. It can be noted that CTD bar has lesser elongation before fracture than the mild steels.

Resistance of rebars against corrosion depends upon its chemical composition. Corrosion of rebars in reinforced concrete structure is a complex phenomenon. Corrosion of steel occurs due to a number of initiating causes which expose the rebars to moisture and oxygen either by carbonation or chloride intrusion. During the process of cement hydration, a thin protective alkaline passive film is formed around rebars. Corrosion process is initiated when this protective film is broken. Though good quality concrete is a pre-requisite for the corrosion resistance of the reinforced concrete structure, the quality of rebars has also a significant influence on it.

No carbon steel rebar can be termed as corrosion resistant steel. One type can have lower corrosion potential than the other. Experience shows that mild steel rebars are more corrosion resistant than CTD bars. Possible reasons for higher corrosion resistance of mild steel rebars compared to that of CTD and also of TMT rebar are described below.

In the production of mild steel rebars, a thin blue film (oxide) is formed around the bars during cooling operation, and this film acts as a barrier. This barrier retards the initiation of corrosion. In the case of TMT rebars the existence of this film depends on the process used for the production. In case of CTD bars the thin film is lost during the twisting process. Further, during the cold twisting process a part of residual strain is withheld in the periphery of the CTD bars. This locked-in strain initiates the corrosion process faster. Also, the level of induced stresses in CTD bars and TMT rebars are much higher than those in mild steel rebars which again enhances the potential of initiating corrosion.

Reinforcing bars are hot-rolled from a variety of steels in several different strength grades. An assortment of strengths for reinforcement bars is available. The high-strength of the rebars are achieved through three process stages namely (i) production of billets, (ii) rolling of billets into rebars, and (iii) process to impart further strength. Production of TMT rebars involves only the first two process stages. All the stages have significant influence on the properties of the rebars. In general, both the quality of basic materials used in rolling the rebars and its production process are important. Quality of metal scrap has utmost impact on the performance of rebars when re-rollable scraps are used for rebar production.

The so called mild steel rebars are rolled from general carbon steel billet without adopting any special measures or imparting further strength. The rebars are also being produced from the re-rollable scrap materials such as scrap rails, automobile scrap, defense scrap, defectives from steel plants, and scrap generated from ship breaking or discarded structures. In such cases, the composition of scrap steel is fixed based on the purpose of original usage from which the scrap is generated. In such cases, the composition is not always suitable for the production of rebars to meet the required characteristics.

It is necessary to refine the liquid scrap to control the contents of carbon, sulphur, phosphorus etc. to the desired levels. Though lower carbon content reduces the strength of steel, higher value makes steel brittle and unweldable. Higher sulphur and phosphorus content makes the steel brittle, even though higher phosphorus content can have beneficial effect like increasing strength and corrosion resistance. All these conflicting aspects indicate that certain level of refinement of the composition of steel is necessary.

Enhancement of rebar strength is normally achieved by three processes namely (i) cold working, (ii) thermo mechanical treatment (TMT), and (iii) micro alloying. The first process can be viewed as post rolling process while the second one is a part of rolling process, and the third one is associated with the billet production process. Proper equipment, manpower and overall good quality of raw material are necessary for achieving appropriate quality of the rebars. The process parameters are required to be established. For example, in case of the CTD bars, the tensile strength can be controlled by pitch of the twist. For the TMT process, proper control during cooling of the rebars is essential to ensure the quality of the finished products. Good quality of raw material (billet) and skilled manpower are of course the prerequisites for producing TMT rebars of desired quality. Corrosion resistance of TMT rebars is better than that of the CTD bars. TMT rebars are more ductile and have better capability to withstand dynamic loading as their elongation is expected to be better at higher strengths.

In micro alloying process, strengthening micro alloys like niobium, vanadium, boron, or titanium are added during the production of billet. When individual ingredient or combination does not exceed 0.3 %, the strength of rebars is increased. Other properties depend on other ingredients as usual. This is an expensive process and normally not used.

Performance by reinforced concrete structures largely depends on the quality of rebars. The danger is due to defective and / or substandard rebars. Defective or substandard rebars are produced due to several reasons such as lack of quality control in the basic material used in the billet production process, rolling process, and post rolling process. Defective rebars are those which can be detected by visual inspection. However, on many occasions, visual inspection fails to identify sub-standard rebars, which are normally identified by testing. Mechanical tests to determine strength and stress-strain curve are useful tools for this purpose. Sub-standard rebars are more dangerous than the defective ones, as they cannot be detected visually by the users, especially in smaller projects.

Variability of properties of rebars has very significant influence on the safety of structure. The variability can be minimized if the desired level of quality control in each phase of production is strictly adhered to. The coefficient of variation of the yield strength is to be within 5 %.

There exists a high risk in using rebars re-rolled from scrap materials which do not adhere to the quality requirement of basic material in line with the requirements as per standard. Again, it is not always be possible for small users of the rebars to have quality control measures before procurement. Branding system is useful in this respect. The branding system consists of marking on the rebars such information as (i) manufacturer’s identity mark, (ii) bar size, (iii) type of steel etc. However, the branding system is not a fool proof system. But, this is an effective system for a reasonable level of control, under the action of market dynamics, in selecting requisite quality of steel depending on their usage, especially for small users.

The mechanical properties of rebars, whose minimum values are normally given in most of the standards are (i) are yield strength (0.2 percent proof stress in case of CTD bars), (ii) ultimate strength (or maximum tensile strength), and elongation. In earthquake-resistant design where ductile behaviour of structure is required (for example, design against the earthquake forces), it is undesirable to have actual yield strength much higher than its minimum specified value that is considered in design. This is because higher is the actual yield strength of rebars, higher is the ultimate moment capacity of a reinforced concrete section. Again, increase in flexural strength of a member enhances the shear demand on the member under seismic loads. The increase in shear demand with the increase in yield strength for the reinforced concrete beam section can lead to higher risk of brittle shear failure of the member rather than a ductile flexure failure, which is against the spirit of safe a seismic design criterion of reinforced concrete section.

Standards normally do not specify any limitation on the statistical parameters of the reinforcement properties such as yield strength. However, the coefficient of variation of yield strength can increase due to variability in the production process. The safety in design is adversely affected with the increase in coefficient of variation of yield strength.

Versatility of the reinforcement steel

Good strength, bond with concrete, thermal expansion characteristics (similar to concrete) and bendability are main advantages of reinforcement steel as a material for reinforcement concrete. Reinforcement steel is the most versatile steel which meets the optimal reinforcement requirement for concrete. It include (i) load transfer is performed from concrete to reinforcement and vice versa over a limited development length, (ii) reinforcement steel has a high modulus of elasticity in order to gain a high degree of stiffness for the construction as a whole, (iii) the interaction between reinforcement steel and concrete is free of disadvantages in terms of chemical and physical disadvantages, (iv) the reinforcement steel has the appropriate delivery forms, shape and length which fit with various construction activities, (v) reinforcement steel provides spacious interfaces in a great variety and in a large range to match the calculated value of the reinforcement section, (vi) the reinforcement steel is flexible and easily bendable to match with the shape of the construction also the steel has a high ductility property, (vii) the reinforcement steel is capable of jointing by overlap, welding and forming mechanical connections, (viii) the reinforcement steel resists without significant deterioration of damage the rough conditions during transport, storage, bundling and placing on construction site (minor damage if it happens does not reduces significantly the performance characteristics), (ix) reinforcement steel provides to the construction a sufficient fatigue resistance because concrete is not resistant to dynamic loading, (x) reinforcement steel resist shear forces as well as tension and compression forces, (xi) reinforcement steel offers its performance characteristics in a sufficient long range of temperatures and also its behavior is predictable in extreme low and extreme high temperatures, and (xii) the quality level of reinforcement steel absorbs minor imperfections which normally happens during the construction activities.

Prevention of reinforcement steel from corrosion

Reinforcement steel is a durable material which provides several years of maintenance free life to the concrete structures. However, it is seen that in hostile environments even good quality concrete not necessarily protects reinforcement steel from corrosion for the long design life (typically 75 years to 120 years) presently specified for many reinforced concrete structures. To reduce and prevent the corrosion of reinforcement steel bars in concrete several methods are employed.  Some are related to the making of concrete while the others are related to the quality, composition and coating of steel used in the making of reinforcement of bars. The choice is normally made based on the cost. Coatings employed on the reinforcement bars are (i) hot dip galvanizing (ii) fusion bonded epoxy coating and iii) stainless steel cladding. Reinforcement bars of stainless steels are also being used.

Designers have choices for specifying the appropriate reinforcement steel material for concrete structures. The most commonly used reinforcement steel material is uncoated carbon steel, which has very little or no corrosion protection other than the concrete mix design, surrounding environment, and depth of cover. Fusion bonded epoxy coated and galvanized reinforcement steel bars are also specified, but each has its own inherent drawbacks. Fusion bonded epoxy coating provides good corrosion resistance but is damaged easily during bending and installation. Chips or cracks in the coating can contribute to corrosion. Protection from ultra violet light is required, if the coated rebar is stored outside for more than two months. In case of galvanized reinforcement steel field bending results in a breach of the coating at bend locations. The 30-year performance of galvanized reinforcement is marginally better than that of conventional uncoated carbon reinforcement steels. The use of stainless steel in the construction started in 1960s and since then its uses is steadily increasing despite its higher cost.