Lecture 11. Reinforcing steel and composite materials · PDF fileReinforcing steel and composite materials Prepared by: Fahim Al-Neshawy, D.Sc. (Tech.) Aalto University School of Engineering

CIV-E1010 Building Materials Technology (5 cr) (1/20)

Lecture 11. Reinforcing steel and composite materials

Prepared by:Fahim Al-Neshawy, D.Sc. (Tech.)Aalto University School of EngineeringDepartment of Civil EngineeringA: P.O.Box 12100, FIN-00076 Aalto, Finland


Table of ContentsLecture 11. Reinforcing steel and composite materials ............................................................................ 1

11.1 What is reinforced concrete? ...................................................................................................... 3

11.2 Reinforcing steel .......................................................................................................................... 3

11.2.1 Conventional reinforcing steel ................................................................................................ 3

11.2.2 Steel for prestressed concrete ................................................................................................ 8

11.3 Composite materials (Structural Composites) ......................................................................... 12

11.3.1 Classification of composite materials ................................................................................... 13

11.3.2 Composites in civil engineering applications: ...................................................................... 15

11.3.3 Properties of composite materials ........................................................................................ 16

11.3.4 Advantages and disadvantages of composite materials ..................................................... 20


11.1 What is concrete reinforcing?

Reinforced concrete, or RCC, is concrete that contains embedded steel bars, plates, or fibers thatstrengthen the material. The capability to carry loads by these materials is magnified, and because ofthis RCC is used extensively in all construction. In fact, it has become the most commonly utilizedconstruction material.

Reinforced materials are embedded in the concrete in such a way that the two materials resist theapplied forces together. The compressive strength of concrete and the tensile strength of steel form astrong bond to resist these stresses over a long span. Plain concrete is not suitable for mostconstruction projects because it cannot easily withstand the stresses created by vibrations, wind, orother forces.

Rebar (reinforcing bar) is an important component of reinforced concrete.

· Rebar is usually formed from ridged carbon steel; the ridges give frictional adhesion to theconcrete.

· Rebar is used because although concrete is very strong in compression it is virtually withoutstrength in tension. To compensate for this, rebar is cast into it to carry the tensile loads on astructure.

· Whilst any material with sufficient tensile strength could conceivably be used to reinforceconcrete, steel is used in concrete as they have similar coefficients of thermal expansion. Thismeans that a concrete structural member reinforced with steel will experience minimal stressas a result of differential expansions of the two interconnected materials due to temperaturechanges.

11.2 Reinforcing steel

Reinforcing steel could be used as conventional or pre-stressed reinforcing, depending on the designsituation.

· In conventional reinforcing, the stresses fluctuate with loads on the structure. This does notplace any special requirements on the steel.

· On the other hand, in pre-stressed reinforcement, the steel is under continuous tension. Anystress relaxation will reduce the effectiveness of the reinforcement. Hence, special steels arerequired.

11.2.1 Conventional reinforcing steel

Reinforcing steel (rebar) is manufactured in three forms: (i) plain bars, (ii) deformed bars, and (iii)plain or deformed wire fabrics.


Figure 1. Plain and deformed reinforcing steel bars.

According to the surface pattern of the rebar, it can be classified into plain rebar and deformed rebar asfollow:

Plain rebar: It is typically a round rod without repeating patterns of ridges and depressions on itssurface. They are often used in situations where the rebar sections need to slide, such as the highwaypavements, which are easy to subject to the weather induced expansion and cracking.

Deformed rebar: The majority of rebar are deformed. The ribs and depressions on its surface canincrease the bond strength with concrete and prevent slippage. The patterns can be customizedaccording to construction requirements. [1]

Some of the common rebar types as shown in Table 1.

Table 1. Common reinforcing steel bars used in concrete structures [1].

Rebar name Description

Black rebar

It is a traditional rebar without anti-rustcoating, which means it has the lowestprice compared to rebars with zinccoating or epoxy coating. According itsprice advantage, it is widely used in mostbuildings nowadays.

Epoxy-coated rebar

Different from black rebar, it has an anti-rust and corrosion-resistant coating, bywhich it can be used in humid and moistenvironment, such as marine structures.

[1] http://www.reinforcing-bar.com/


Galvanized rebar

It is a normal black rebar with a layer ofzinc coating, which can prevent the rebarfrom rusting and corrosion. Owing to theproperty the zinc coating, it can be usedin bridges and thruways.

Stainless steel rebar

This kind of rebar features its long-lifecycle and long term corrosion-assistance.Although it is the most expansive rebar,but it has superior cost effectiveness.

11.2.1.1 Manufacturing process for reinforcing steel [2]

Figure 2 illustrates the most common reinforcing steel process routes. The different process stagescan be split into:

· Steelmaking: There are two common steel-making processes used for reinforcing steels.(i)These are Basic Oxygen Steelmaking (BOS) and, (ii) perhaps the most common, Electric ArcFurnace (EAF) steel making.

· Ladle refining: Ladle refining of liquid metal is a proven technology to produce high qualitysteel. A Ladle Refining Furnace is used to raise the temperature and adjust the chemicalcomposition of molten metal.

· Continuous casting: Traditionally, after melting and refining, steel is cast into ingot moulds inorder to solidify. These moulds were then stripped, and the solidified steel was transferred toa mill for rolling in at least two stages; first to billet, then to the finished product.

[2] Guide to reinforcing steel: Part 2-Manufacturing process routes for reinforcing steels. Online at:http://www.ukcares.com/information/guides-to-reinforcing-steel


Figure 2. Reinforcing steel manufacturing process. [3]

· Hot rolling: Whichever casting process is used, the as-cast product always contains defectssuch as internal cracks, porosity and segregation, which are a result of the solidificationprocess. All reinforcing steels therefore go through a hot rolling operation in order toconsolidate the product, as well as change its shape. The reduction of cross-sectional areafrom the ingoing billet to the finished bar must be sufficient to weld up any internal defects,and improve the homogeneity in the product. In the hot rolling process, the cast billet isreheated to a temperature of 1100- 1200°C, and then rolled through a rolling mill to reduce itscross-section.

· Cold processing: In addition to hot process, there are reinforcing steels in which the propertiesare achieved by cold processing. The two methods commonly used are (i) cold rolling and (ii)cold drawing. The feedstock material for both processes is a hot rolled, round section rod. Incold rolling, typically used to manufacture bars in coil of diameters 12mm and below, the rodis deformed by passing it through a series of rolls. The material is forced into the gap betweenthe rolls, and so is compressed.

· Decoiling: All coil products have to be de-coiled before they can be used. Sometimes this isdone as part of the processing of cut and cut and bent shapes on an automatic link-bendingmachine. De-coiling processes are generally of two types; (i) “roller” and (ii) “spinner”. In theroller type, which is the more common, the coil is passed between two sets of rolls in a‘serpentine’ fashion. The product undergoes reverse bending stresses, and the rolls areadjusted so that the final product is straight, mostly followed by automatic bending to thedesired shape. In spinner straightening, which typically is used to produce straight lengths, thecoil passes through a set of rotating dies. The offset of the dies is adjusted along the length ofthe straightener to produce a straight product at the exit.

[3] Guide to reinforcing steel: Part 2-Manufacturing process routes for reinforcing steels. Online at:http://www.ukcares.com/information/guides-to-reinforcing-steel


· Fabrication, cutting bending of reinforcing steel: The fabrication of reinforcing steels, intoshapes suitable for fixing into the concrete formwork, is normally performed 'off-site'. Verylittle reinforcement is cut and bent on-site nowadays. The accuracy of cutting and bendingoperations is vital to ensure proper fit on site, and to maintain required lap lengths, anchoragelengths and cover.

· Manufacture of welded fabric: Welded wire steel fabric is manufactured with automaticwelding machines, where wires are welded together in square or rectangular grids. The wiresare welded by electric resistance in an automated state of the art machine which controlswelding parameters precisely.

11.2.1.3 Classification of reinforcing steel bars [4], [5]

Nowadays in Finland hot-rolled and cold-worked reinforcement are in use. The class of reinforcementshows the yield strength of steel and other special properties.

Figure 3. Example of concrete reinforcement classes.

Table 2. Classes of reinforcement steel that are mostly used in Finland [5].

Reinforcementclass

Description Yield strength fyk

[MPa]

A500HW Weldable hot-rolled ribbed steel 500

A700HW Weldable hot-rolled ribbed steel 700

B500K Cold-worked ribbed steel 500

B700K Cold-worked ribbed steel 700

B600KX Cold-worked stainless ribbed steel 600

[4] Alexey Pronozin, (2012). Comparison of Russian, Finnish and European norms for reinforced concrete structures.Saimaa University of Applied Sciences, Lappeenranta.

[5] SFS 1200, (1999) Betonirakenteiden yleiset teräkset. Lajit, nimikkeet ja merkinnät tuotteissa. MetalliteollisuudenStandardisointiyhdistys ry.


S235JRG2 Smooth round bar used as a lifting loops 235

S355J0 Smooth round bar used as a lifting loops 335

Table 3. Diameters of reinforcement bars used in Finland

Diameter[mm]

Cross-section area[mm²]

Weight for one running meter,[kg]

6 28.3 0.22

8 50.3 0.39

10 78.5 0.62

12 113.1 0.89

16 201.1 1.58

20 314.2 2.47

25 490.9 3.85

32 804.2 6.31

11.2.2 Steel for prestressed concrete [6]

Prestressed concrete is a method for overcoming the concrete's natural weakness in tension. It can beused to produce beams, floors or bridges with a longer span than is practical with ordinary reinforcedconcrete. Prestressing can be accomplished in three ways: pre-tensioned concrete, and bonded orunbounded post-tensioned concrete.

Prestressed concrete requires special wires, strands, cables, and bars. Steel for prestressed concretereinforcement must have high strength and low relaxation properties. High-carbon steels and high-strength alloy steels are used for this purpose.

11.2.2.1 Prestressing steel wires (7)

A prestressing steel wire is a single unit made of steel. Thenominal diameters of the wires are 2.5, 3.0, 4.0, 5.0, 7.0and 8.0 mm. The different types of wires are as follows.

1) Plain wire: No indentations on the surface.2) Indented wire: There are circular or elliptical

indentations on the surface.Figure 4. Wire prestressing steel.

[6] A. K. Sengupta and D. Menon, Pre-Stressed Concrete Structures – Course handouts. Online at:http://nptel.ac.in/courses/105106117/7 European Standard. Draft Document - prEN 10138-2:2000 - Prestressing steels - Part 2: Wire


According to the European Standard, the prestressing steel wire are names as shown in Figure 5.

Figure 5. Example of the prestressing steel wire naming.

Figure 6. Steel wire indentation and specified indentation dimensions.

11.2.2.2 Prestressing steel Strands (8)

A few wires are spun (kiertyä) together in a helical form toform a prestressing steel strand. The different types ofstrands are:

1) 3 - Wire strand: Three wires are spun together toform the strand.

2) Indented 3 – wire strand3) 7 - Wire strand: In this type of strand, six wires are

spun around a central wire. The central wire is largerthan the other wires.

4) Indented 7 – wire strand5) 7 - Wire compacted strand, class (G)

8 European Standard. Draft Document - prEN 10138-3:2000 - Prestressing steels - Part 3: Strand


Figure 7. Strand prestressing steel.

According to the European Standard, the prestressing steel wire are names as shown in Figure 8.

Figure 8. Example of the prestressing steel strand naming.

Figure 9. Steel strand indentation and specified indentation dimensions.


11.2.2.3 Steel tendons

A group of strands or wires are placed together to form a prestressing tendon. The tendons are usedin post-tensioned members. Figure 10 shows the cross section of a typical tendon. The strands areplaced in a duct which may be filled with grout after the post-tensioning operation is completed

Figure 10. Cross section and anchor head unit of typical steel tendons.

11.2.2.4 Prestressing steel bars (9)

Prestressing bars are hot-rolled, tempered from therolling heat, stretched and annealed, with a circularcross section. Bars are made of prestressing steel Y1050 H according to prEN 10138-4.

· Threadbars:Threadbars feature continuous hot-rolledribs, thus providing a right-hand thread alongthe entire length.

Figure 11. Prestressed concrete steel bars.

· Plain barsBoth ends of a plain bar, which is cut to suit individual project requirements, are provided withspecial cold-rolled threads.

The diameter of a bar is much larger than that of a wire. Bars are available in the following sizes: 10,12, 16, 20, 22, 25, 28 and 32 mm. According to the European Standard, the prestressing steel wire arenames as shown in Figure 12.

9 European Standard. Draft Document - prEN 10138-4:2000 - Prestressing steels - Part 4: Bars


Figure 12. Example of the prestressing steel bar naming.

11.3 Composite materials (Structural Composites) (10)

Composites are defined as materials which consist of not less than two different componentmaterials, neither of which are well suited for construction purposes on their own, but which incombination result in a very strong and rigid material, as shown in Figure 13.

Figure 13. Comparison of tensile properties of fiber, matrix, and composite.

Common names used in industry:

• RP: Reinforced Plastics• FRP: Fiber-reinforced plastics (Polymer) – the most popular one• GFRP / CFRP: Glass FRP/Carbon FRP

10 Michael S. Mamlouk and John P. Zaniewski, (2011). Materials for Civil and Construction Engineers, Chapter 11 -COMPOSITES


11.3.1 Classification of composite materials

As shown in Figure 14, Composite materials can be classified as:

1) Microscopic composites: include fibers or particles in sizes up to a few hundred microns2) Macroscopic composites: could have constituents of much larger size, such as aggregate

particles and rebars in concrete.

Figure 14. A classification scheme for composite materials.

Figure 15. Composites classified by function

11.3.1.1 Microscopic composites:

Many microscopic composite materials consist of two constituent phases:

1) a continuous phase, or matrix2) the dispersed phase or reinforcing phase, which is surrounded by the matrix. In most cases,

the dispersed phase is harder and stiffer than the matrix.


Microscopic composites fall into two basic classes based on the shape of the dispersed phase:

a) Fiber-reinforced:· include fibers dispersed in a matrix such as metal or polymer.· Fibers have a very high strength-to-diameter ratio, with near crystal-sized diameters.· Fibers are manufactured from many materials, such as glass, carbon and graphite,

polymer, boron, ceramic, and silicon carbide.· Because of their low cost and high strength, glass fibers are the most common of all

reinforcing fibers for polymer matrix composites.b) Particle-reinforced:

· Particle-reinforced composites consist of particles dispersed in a matrix phase.· The strengthening mechanism of particle-reinforced composites varies with the size of

the reinforcing particles.· When the size of the particles is about 0.01 micron to 0.1 micron, the matrix bears

most of the applied load, whereas the small dispersed particles hinder or impede themotion of dislocations.

· when the particles are larger than 1 micron, particles act as fillers to improve theproperties of the matrix phase and/or to replace some of its volume.

The matrix used in most microscopic composites is polymer (plastic) or metal. The matrix binds thedispersed materials (particles or fibers) together, transfers loads to them, and protects them againstenvironmental attack and damage due to handling. Polymers have the advantages of low cost, easyprocessability, good chemical resistance, and low specific gravity.

Figure 16 shows composites with continuously aligned fibers, random fibers, and random particles.The mechanism of strengthening varies for different classes and for different sizes and orientations ofthe dispersed shape.

Figure 16. Schematic of microscopic composites:(a) aligned fibers, (b) random fibers, and (c) randomparticles.


Figure 17. Pultrusion scheme used in fabricating structural shape fiber-reinforced composites.

Fabrication of microscopic composites includes the merging of the matrix and dispersed material intoa product with minimum air voids. Several methods have been used to fabricate the composites. Theselection of the fabrication process typically is based on:

i. the chemical nature of the matrix and of the dispersed phases,ii. the shape and strength requirements, andiii. the temperature required to form, melt, or cure the matrix.

Figure 17 illustrates fabrication of structural shape fiber-reinforced composites by using thepultrusion process. Pultrusion is an automated process for manufacturing fiber-reinforced compositematerials into continuous, constant-cross-section profiles.

11.3.1.2 Macroscopic composites:

Macroscopic composites are used in many engineering applications. Because macroscopic compositesare relatively large, how the load is carried and how the properties of the composite components areimproved vary from one composite to another. Common macroscopic composites used by civil andconstruction engineers include:

· plain Portland cement concrete,· steel-reinforced concrete,· asphalt concrete, and· engineered wood such as glued–laminated timber, and structural strand board.

11.3.2 Composites in civil engineering applications: (11)

· Fiber-Reinforced Plastic (FRP) shapes: panels, rods, tubes, beams, columns, cellular panels(highway bridge decks), etc.:

o Cables and Tendons as tension elements (pre- and post-tensioning of structures)

11 Pizhong Qiao, Composite Materials in Civil Infrastructure (Structural Composites). Lecture notes. Online at:http://pas.ce.wsu.edu/CE537-2/ce537-ch01.pdf


o Beams, girders and cellular panels to support large loads (vehicular and pedestrianbridges)

o Trusses in a wide variety of structures (bridges, transmission towers, and industrialplants)

o Columns, posts and pilings to carry vertical loads (bridge columns, marine pilings, andutility poles)

· Laminates and wraps to strengthen structures:o Fabrics for external reinforcement (wrapping) of concrete, wood, and even steel

(strengthening, rehabilitation, and retrofit (impact: retrofit-hardening))o Laminates (or plates) bonded to beams on the tension side (reinforcement and

strengthening and repair)o Filament winding of concrete and wood cores (railroad crossties and utility poles)

· Composite rebars and grids to reinforce concrete in bridge decks and highway barriers· Composite cables and tendons to prestress/post-tension concrete structures (bridges and

building)· Composites can also be used to strengthen and wrap columns and bridge supports that are

partially damaged by earthquakes and other environmental factors· Fiber-reinforced concrete is another composite material that has been used by civil engineers

in various structural applications. Different types of fibers, such as separate fibers, chopped-strands, or rovings, can be used to reinforce the concrete.

· Entrained air in concrete can also be considered as a component in a microscopic compositematerial. Entrained air increases the durability of concrete since it releases internal stressescaused by the freezing of water within the concrete.

Figure 18. Examples of using composites in civil engineering applications.

11.3.3 Properties of composite materials

The properties of composite materials are affected by:

1) the component properties,2) volume fractions of components,3) type and orientation of the dispersed phase, and


4) the bond between the dispersed phase and the matrix.

The properties of the composite can be viewed as the weighted average of the properties of thecomponents. Equations can be derived to estimate the composite properties under certain idealizedmaterial properties, loading patterns, and geometrical conditions. Assumptions that can be used tosimplify the analysis include the following:

· Each component has linear, elastic, and isotropic properties.· A perfect bond exists between the dispersed and matrix phases without slipping.· The composite geometry is idealized and the loading pattern is parallel or perpendicular to

reinforcing fibers.

11.3.3.1 Loading parallel to fibers

When load is applied to an aligned fiber-reinforced composite parallel to the fibers, as seen in Figure19 (a), both matrix and fiber phases will deform equally. Thus, the strains of both phases will be thesame (known as an isostrain condition) and are given by:

Where e = total strainec = composite strainem = matrix strainef = fiber strain

Also, the force applied to the composite Fc is the sum of the force carried by the matrix Fm and theforce carried by the fibers Ff:

Where Fi = force carried by component i (c = composite, m = matrix and f = fiber)si = stress of component iAi = area of component iEi = modulus of elasticity of component ivi = the volume fraction of each component and wm + vf = 1

e = e = e = e (1)

= + (2) = + (3)

= + (4)

= + (5)

= + (6), = + (7)


X = a property such as Poisson’s ratio, thermal conductivity, electrical conductivity, or diffusivity

The share of the load carried by the fibers can be determined as follows:

Figure 19. Patterns of loading continuously aligned fiber-reinforced composites: (a) loading parallel tofibers and (b) loading perpendicular to fibers.

11.3.3.2 Loading perpendicular to fibers

When load is applied to an aligned fiber-reinforced composite perpendicular to the fibers [Figure 19(b)], both matrix and fiber phases will be subjected to the same stress (isostress condition).

Where si = stress of component i (c = composite, m = matrix and f = fiber)

The elongation of the composite in the direction of the applied stress is the sum of the elongations ofthe matrix and fibers:

Dividing Equation 10 by the composite length Lc in the stress direction gives

Assuming that the fibers are uniform in thickness, the cumulative length of each component in thedirection of the stress is proportional to its volume fraction. Thus Lm = vmLc and Lf = vfLc

=ss

= = (8)

s = s = s = s (9)

∆ = ∆ + ∆ (10)

∆=∆

+∆

(11)


Where X = a property such as Poisson’s ratio, thermal conductivity, electrical conductivity, or diffusivity

The moduli in Equations 7 and 14 can be plotted as functions of the volume fraction of the fiber, asshown in Figure 20. Clearly, the fibers are more effective in raising the modulus of the compositewhen loading parallel to fibers thanwhen loading perpendicular to fibers.

Figure 20. Modulus of elasticity of the composite versus fiber volume fraction.

11.3.3.3 Randomly oriented fiber composites

Unlike continuously aligned fiber composites, the mechanical properties of randomly oriented fibercomposites are isotropic. The modulus of elasticity of randomly oriented fiber composites fallsbetween the moduli of loading parallel to fibers and perpendicular to fibers.

To estimate the modulus of elasticity of randomly oriented fiber composites, Equation 6 can berewritten as:

∆=

∆+

∆, e =

∆ e = e + e (12)

e ℎs

s=

s+

s

1= +

= +

(13)

Generally,

= +(14)

= + (15)


where K is a fiber efficiency parameter. For fibers randomly and uniformly distributed within threedimensions in space, K has a value of 0.2.

11.3.4 Advantages and disadvantages of composite materials (12)

Advantages of composite materials:

· High strength-to-weight ratio (specific strength)· High stiffness-to-weight ratio (specific stiffness)· Noncorrosive, nonmagnetic, nonconductive· High energy absorption properties: acoustic and seismic responses· High fatigue-life· Ability to incorporate sensors in the material to monitor and/or correct its performanceà

Smart composites· Ability to tailor the material (both fiber architecture and shape) for specific applications, and

to design the material with other inherent properties (UV light, flammability, smoke toxicity)· Ease of fabrication of large complex structural shapes or modulesà Modular construction

Disadvantages of composites

· Cost of raw materials and fabrication· Possible weakness of transverse properties· Weak matrix and low toughness· Environmental degradation of matrix· Difficulty in attaching· Difficulty with analysis

12 Pizhong Qiao, Composite Materials in Civil Infrastructure (Structural Composites). Lecture notes. Online at:http://pas.ce.wsu.edu/CE537-2/ce537-ch01.pdf

Lecture 11. Reinforcing steel and composite materials · PDF fileReinforcing steel and composite materials Prepared by: Fahim Al-Neshawy, D.Sc. (Tech.) Aalto University School of Engineering

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