Concrete is strong in compression, with crushing strengths typically in the range 20–40 MPa, and up
to 100 MPa for high-strength concretes. However, the tensile strength of concrete is usually only 10% of the compressive strength. Steel is the universally accepted reinforcing material as it is strong in tension, forms a good bond and has a similar coefficient of ther- mal expansion to concrete. The location of the steel within reinforced concrete is critical, as shown inFig.
3.14, to ensure that the tensile and shear forces are transferred to the steel. The longitudinal bars carry the tensile forces while the links or stirrups combat the shear forces and also locate the steel during the casting of the concrete. Links are therefore more con- centrated around locations of high shear, although inclined bars may also be used to resist the shear forces. Fewer or thinner steel bars may be incorpo- rated into reinforced concrete to take a proportion of the compressive loads in order to minimise the beam dimensions.
Steel reinforcement for concrete is manufactured, largely from recycled scrap, into round, ribbed, in- dented or ribbed and twisted bars (Fig. 3.15). Mild steel is frequently used for the plain bars to form bent links. Hot-rolled, high-yield steel is used for ribbed and indented bars. British Standard BS 4482: 2005 refers to 250 MPa yield strength steel for plain bars and to the higher grade 500 MPa steel for plain, ribbed and indented reinforcement of diameters between 2.5 and 12 mm. British Standard BS 4449: 2005 specifies high-yield steel (grade 500 MPa) with three levels of ductility A, B and C (highest) for ribbed bars from 6 to 50 mm diameters. Welded steel mesh reinforcement to BS 4483: 2005 is used for slabs, roads and within sprayed concrete.
Austenitic stainless steels may be used for con- crete reinforcement where failure due to corrosion is a potential risk. Grade 1.4301 (18% chromium, 10%
nickel) stainless steel is used for most applications, but the higher grade 1.4436 (17% chromium, 12% nickel, 2.5% molybdenum) is used in more corrosive envi- ronments. Where long-term performance is required in highly corrosive environments, the duplex grades of stainless steel may be used. The initial cost of stainless steel reinforcement is approximately eight times that of standard steel reinforcement, but in situations where maintenance costs could be high, for example due to chloride attack from sea water or road salts, the overall life cycle costs may be reduced by its use. Additionally, stainless steels have higher strengths than the standard carbon steels. Suitable stainless steels for the reinforce- ment of concrete are specified in BS 6744: 2001 + A2:
2009.
L I M E , C E M E N T A N D C O N C R E T E 8 5
Fig. 3.14 Reinforced concrete
Bond between steel and concrete
For reinforced concrete to act efficiently as a composite material the bond between the concrete and steel must
Fig. 3.15 Types of reinforcement for concrete and standard forms
be secure. This ensures that any tensile forces within the concrete are transferred to the steel reinforcement.
The shape and surface condition of the steel and the quality of the concrete all affect the bond strength.
To obtain the most efficient mechanical bond with concrete, the surface of the steel should be free of flaky rust, loose scale and grease, but the thin layer of rust, typically produced by short-term storage on site, should not be removed before use. The use of hooked ends in round bars reduces the risk of the steel being pulled out under load, but high bond strength is achieved with ribbed or indented bars which ensure a good bond along the full length of the steel. Steelrebars are usually either supplied in stock lengths, or cut and bent ready for making up into cages. Sometimes the reinforcement may be supplied as prefabricated cages, which may be welded rather than fixed with iron wire as on site. Steel reinforcement, although weldable, is rarely welded on site. Rebar joints can easily be made with proprietary fixings, such as steel sleeves fastened by shear bolts. Spacers are used to ensure the correct separation between reinforcement and formwork.
Good-quality dense concrete gives the strongest bond to the steel. Concrete should be well compacted
around the reinforcement; thus, the maximum aggre- gate size must not bridge the minimum reinforcement spacing.
Corrosion of steel within reinforced concrete
Steel is protected from corrosion provided that it has adequate cover of a good-quality, well-compacted and cured concrete. The strongly alkaline environment of the hydrated cement renders the steel passive. How- ever, insufficient cover caused by the incorrect fixing of the steel reinforcement or the formwork can allow the steel to corrode. Rust expansion causes surface spalling;
then exposure of the steel allows corrosion, followed by rust staining of the concrete surface (Fig. 3.16).
Calcium chloride accelerators should not normally be used in reinforced concrete as the residual chlorides cause accelerated corrosion of the steel reinforcement.
Additional protection from corrosion can be achieved by the use of galvanised, epoxy-coated or stainless steel reinforcement. The protective alkalinity of the concrete is reduced at the surface by car- bonation. The depth of carbonation depends on the
Fig. 3.16 Corrosion of steel reinforcement
permeability of the concrete, moisture content and any surface cracking. The nominal cover for concrete reinforcement is therefore calculated from the antici- pated degree of exposure (Table 3.16) and the concrete strength class as inTable 3.17. The recommended cover specified relates to all reinforcement, including any Table 3.16 Concrete exposure classes to Eurocode 2 (BS EN 1992-1-1: 2004)
Exposure classes Typical environmental conditions
No risk of reinforcement corrosion or attack on concrete
X0 Concrete with no reinforcement. Dry concrete Dry building interiors Corrosion induced by carbonation
XC1 Dry or permanently wet Interior of buildings and concrete under water
XC2 Wet and rarely dry Foundations
XC3 Moderate humidity Sheltered external concrete and high-humidity interiors
XC4 Cyclic wet and dry Concrete in occasional contact with water
Corrosion induced by chlorides
XD1 Humid environment Components exposed to airborne spray
XD2 Wet and rarely dry Swimming pools and contact to industrial waters
XD3 Cyclic wet and dry Exposed external concrete surfaces
Corrosion induced by seawater
XS1 Exposure to sea air Coastal structures
XS2 Submerged under sea water Submerged marine structures
XS3 Tidal and sea spray zone Parts of marine structures
Freeze/thaw deterioration
XF1 Moderate saturation Vertical surfaces exposed to rain and freezing
XF2 Moderate saturation with de-icing agent Vertical surfaces exposed to rain, freezing and de-icing
XF3 High saturation Horizontal surfaces exposed to rain and freezing
XF4 High saturation with de-icing agent Surfaces exposed to rain, freezing and de-icing or marine spray Chemical attack
XA1 Slightly aggressive agencies Soil and groundwater
XA2 Moderately aggressive agencies Soil and groundwater
XA3 Highly aggressive agencies Soil and groundwater
L I M E , C E M E N T A N D C O N C R E T E 8 7 Table 3.17 Minimum cover required to ensure durability of steel reinforcement in structural concrete for exposure classes to Eurocode 2 (BS EN 1992-1-1: 2004)
Exposure class X0 XC1 XC2/XC3 XC4 XD1/XS1 XD2/XS2 XD3/XS3
Recommended cover (mm) 10 15 25 30 35 40 45
Minimum cover (mm) 10 10 10 15 20 25 30
Strength class ≥C30/37 ≥C30/37 ≥C35/45 ≥C40/50 ≥C40/50 ≥C40/50 ≥C45/55
Notes:
The recommended cover relates to standard production with a design working life of 50 years.
Increased cover is required for a design working life of 100 years.
The minimum cover relates to very specific conditions combining high quality control for positioning of the reinforcement and the concrete production, additionally the use of 4% (minimum) air entrainment.
The Standard BS 8500-1: 2006 gives a more detailed set of recommendations relating strength classes to nominal cover including the option to add additional cover (c) for workmanship deviation.
Table 3.18 Indicative strength classes for durability of concrete to Eurocode 2 (BS EN 1992-1-1: 2004)
Corrosion risk XC1 XC2 XC3 and XC4 XD1 and XD2 XD3 XS1 XS2 and XS3
Indicative strength class C20/25 C25/30 C30/37 C30/37 C35/45 C30/37 C35/45
Damage to concrete X0 XF1 XF2 XF3 XA1 XA2 XA3
Indicative strength class C12/15 C30/37 C25/30 C30/37 C30/37 C30/37 C35/45
Notes:
The Standard BS 8500-1: 2006 details a more comprehensive relationship between minimum strength class and exposure class for frost resistance in relation to different maximum aggregate sizes and minimum cement contents.
wire ties and secondary reinforcement. Some reduc- tion in carbonation rate can be achieved by protective coatings to the concrete surface. It should be noted that the choice of an adequately durable concrete for the protection of the concrete itself against attack and for the prevention of reinforcement corrosion may result in a higher compressive strength concrete being required than is necessary for the structural design (Table 3.18).
Where the depth of concrete cover over reinforce- ment is in doubt it can be measured with acovermeter.
If reinforcement is corroding, cathodic protection by application of a continuous direct current to the steel reinforcement may prevent further deterioration and lead to realkalisation of the carbonated concrete.
Fibre-composite reinforced concrete
In most situations steel is used for reinforcing or prestressing concrete. However, for structures in highly aggressive environments high-modulus continuous fibres embedded in resin offer an alternative. The fibres, either glass, carbon or aramid, are encased in a thermosetting resin and drawn through a die by pultrusion to produce the required cross-section.
The extruded material is then overwound with fur-
ther fibres to improve its bond with concrete. The fibre-composite rods are used as reinforcement or as prestressing tendons within standard concrete con- struction.
Bendy concrete
Fibre-reinforced concrete of an appropriate mix may be continuously extruded into various sections to pro- duce sheets, cylinders or tubes. The product is more flexible and has a higher impact strength than ordi- nary concrete.Bendyconcrete may be drilled, cut and nailed without damage. It is lighter than ordinary con- crete and with its good fire resistance may be used as an alternative to other wall boards.
Fibre-reinforced aerated concrete
Polypropylene fibre-reinforced aerated concrete is used for making lightweight blocks, floor, wall and roofing panels, offering a combination of strength and insulation properties. The material, like standard aer- ated concrete, can be cut and worked with standard hand tools. Where additional strength is required, steel fibre-reinforced aerated concrete may be used for cast in situ or factory-produced units. The fibre-reinforced material has a greater resilience than standard aerated concrete. Roofing membranes and battens for tiling
Table 3.19 Typical cover to concrete reinforcement for fire resistance to Eurocode 2 (BS EN 1992-1-2: 2004)
Fire resistance (minutes) Typical cover to reinforcement (mm)
Beams Width (mm) Simply supported Continuous beams
R 30 80 25 15
R 60 120 40 25
R 90 150 55 35
R 120 200 65 45
R 180 240 80 60
R 240 280 90 75
Columns Minimum dimensions (mm) One face exposed
R 30 155 25
R 60 155 25
R 90 155 25
R 120 175 35
R 180 230 55
R 240 295 70
Walls Minimum dimensions (mm) One face exposed
REI 30 100 10
REI 60 110 10
REI 90 120 20
REI 120 150 25
REI 180 180 40
REI 240 230 55
Slabs Slab thickness (mm) One-way slabs Two-way slabs
REI 30 60 10 10
REI 60 80 20 10–15
REI 90 100 30 15–20
REI 120 120 40 20–25
REI 180 150 55 30–40
REI 240 175 65 40–50
Notes:
Fire resistance class:
R, load-bearing criterion; E, integrity criterion and I, insulation criterion in standard fire exposure.
All reinforcement cover requirements are also dependent on the dimensions and geometry of the concrete components and the degree of fire exposure (BS EN 1992-1-2: 2004).
Where low cover thicknesses are required for fire protection, a higher depth of cover may be required for corrosion protection (BS EN 1992-1-1: 2004).
can be directly nailed to roofing panels, whilst floor panels accept all the standard floor finishes.
Fire resistance of reinforced concrete
Concrete manufactured without organic materials is Class A1 with respect to reaction to fire. If more than 1% of organic materials are incorporated into the mix, then the material will require testing to the standard (BS EN 13501-1: 2007).
The depth of concrete cover over the steel reinforce- ment, to ensure various periods of fire resistance, is
listed inTable 3.19. Where cover exceeds 40 mm, addi- tional reinforcement will be required to prevent surface spalling of the concrete. The cover should prevent the temperature of the steel reinforcement from exceeding 550◦C (or 450◦C for prestressing steel).
PRESTRESSED CONCRETE
Concrete has a high compressive strength but is weak in tension. Prestressing with steel wires or tendons ensures that the concrete component of the composite
L I M E , C E M E N T A N D C O N C R E T E 8 9
Fig. 3.17 Prestressed concrete
material always remains in compression when subjected to flexing up to the maximum working load.
The tensile forces within the steel tendons act on the concrete putting it into compression, such that only under excessive loads would the concrete go into ten- sion and crack. Two distinct systems are employed; in pre-tensioning, the tendons are tensioned before the concrete is cured, and in post-tensioning, the tendons are tensioned after the concrete is hardened (Fig. 3.17).
Pre-tensioning
Large numbers of precast concrete units, includ- ing flooring systems, are manufactured by the pre-
tensioning process. Tendons are fed through a series of beam moulds and the appropriate tension is applied.
The concrete is placed, vibrated and cured. The ten- dons are cut at the ends of the beams, putting the concrete into compression. As with precast reinforced concrete it is vital that prestressed beams are installed the correct way up according to the anticipated loads.
Post-tensioning
In the post-tensioning system the tendons are located in the formwork within sheaths or ducts. The concrete is placed, and when sufficiently strong, the tendons are stressed against the concrete and locked off with special anchor grips incorporated into the ends of the concrete. Usually reinforcement is incorporated into post-tensioned concrete, especially near the anchor- ages, which are subject to very high localised forces.
In the bonded system, after tensioning the free space within the ducts is grouted up, which then limits the reliance on the anchorage fixing; however, in the unbonded system the tendons remain free to move independently of the concrete. Tendon ducts are typ- ically manufactured from galvanised steel strip or high-density polythene.
Post-tensioning has the advantage over pre- tensioning that the tendons can be curved to follow the most efficient prestress lines. In turn this enables long spans of minimum thickness to be constructed. During demolition or structural alteration work, unbonded post-tensioned structures should be de-tensioned, although experience has shown that if demolished under tension, structures do not fail explosively. In alteration work, remaining severed tendons may sub- sequently require re-tensioning and re-anchoring to recover the structural performance. However, the use of post-tensioning does not preclude subsequent struc- tural modifications.