2.1 REINFORCED CONCRETE STRUCTURES
Reinforced concrete is a composite material of steel bars embedded in a hardened concrete
matrix; concrete, assisted by the steel, carries the compressive forces, while steel resists
tensile forces. Concrete itself is a composite material. The dry mix consists of cement and
coarse and fine aggregates. Water is added and this reacts with the cement which hardens and binds the aggregates into the concrete matrix; the concrete matrix sticks or bonds onto the reinforcing bars.
The properties of the constituents used in making concrete, mix design and the principal
properties of concrete are discussed briefly. Knowledge of the properties and an
understanding of the behaviour of concrete is an important factor in the design process. The
types and characteristics of reinforcing steels are noted.
Deterioration of and failures in concrete structures are now of widespread concern. This is
reflected in the increased prominence given in the concrete code BS 8110 to the durability of
concrete structures. The types of failure that occur in concrete structures are listed and
described. Finally the provisions regarding the durability of concrete structures noted in the
code and the requirements for cover to prevent corrosion of the reinforcement and provide fire resistance are set out.
2.2 CONCRETE MATERIALS
2.2.1 Cement
Ordinary Portland cement (OPC) is the commonest type in use. The raw materials from which it is made are lime, silica, alumina and iron oxide. These constituents are crushed and blended in the correct proportions and burnt in a rotary kiln. The clinker is cooled, mixed with gypsum and ground to a fine powder to give cement. The main chemical compounds in cement are calcium silicates and aluminates.
When water is added to cement and the constituents are mixed to form cement paste,
chemical reactions occur and the mix becomes stiffer with time and sets. The addition of
gypsum mentioned above retards and controls the setting time. This ensures that the concrete does not set too quickly before it can be placed in its final position or too slowly so as to hold up construction.
Two stages in the setting process are defined in Page 10BS EN 197-1:2000: Cement. Composition, specifications and conformity criteria for common cementsBS EN 197-2:2000: Cement. Conformity evaluationThese are an initial setting time which must be a minimum of 45 min and a final set which must take place in 10 h.
Cement must be sound, i.e. it must not contain excessive quantities of certain substances
such as lime, magnesia, calcium sulphate etc. that may expand on hydrating or react with
other substances in the aggregate and cause the concrete to disintegrate. Tests are specified for soundness and strength of cement mortar cubes.
Many other types of cement are available some of which are:
1. Rapid hardening Portland cement: the clinker is more finely ground than for ordinary
Portland cement. This is used in structures where it is necessary for the concrete to gain
strength rapidly. Typical example is where the formwork needs to be removed early for reuse.
2. Low heat Portland cement: this has a low rate of heat development during hydration of
the cement. This is used in situations such as thick concrete sections where it is necessary to
keep the rate of heat generation due to hydration low as otherwise it could lead to serious
cracking.
3. Sulphate-resisting Portland cement: this is often used for foundation concrete when the
soil contains sulphates which can attack OPC concrete.
A very useful reference isAdam M Neville, Properties of Concrete, Prentice-Hall, 4th Edition, 1996.
2.2.2 Aggregates
The bulk of concrete is aggregate in the form of sand and gravel which is bound together by cement. Aggregate is classed into the following two sizes;
1. coarse aggregate: gravel or crushed rock 5 mm or larger in size
2. fine aggregate: sand less than 5 mm in size
Natural aggregates are classified according to the rock type, e.g. basalt, granite, flint.
Aggregates should be chemically inert, clean, hard and durable. Organic impurities can affect
the hydration of cement and the bond between the cement and the aggregate. Some aggregates containing silica may react with alkali in the cement causing the some of the larger aggregates to expand which may lead to the concrete disintegrating. This is the alkali-silica reaction. Presence of chlorides in aggregates, e.g. salt in marine sands, will cause corrosion of the steel reinforcement. Excessive amounts of sulphate will also cause concrete to disintegrate.
To obtain a dense strong concrete with minimum use of cement, the cement paste should
fill the voids in the fine aggregate while the fine aggregate and cement paste fills the voids in
the coarse aggregate. Coarse and fine aggregates are graded by sieve analysis in which the
percentage by weight passing a set of standard sieve Page 11sizes is determined. Grading limits for each size of coarse and fine aggregate are set out in BS EN 12620:2002: Aggregates for ConcreteThe grading affects the workability; a lower water-to-cement ratio can be used if the grading of the aggregate is good and therefore strength is also increased. Good grading saves cement content. It helps prevent segregation during placing and ensures a good finish.2.2.3 Concrete Mix DesignConcrete mix design consists in selecting and proportioning the constituents to give the required strength, workability and durability. Mixes are defined in BS 8500–1:2002: Concrete. Methods of Specifying and guidance for the specifierBS 8500–2:2002: Specifications for constituent materials and concreteThe five types are
1. Designated concretes: This is used where concrete is intended for use such as plain and
reinforced foundations, floors, paving, and other given in Table A.6 or A.7 of the code.
2. Designed concretes: This is the most flexible type of specification. The environment to
which the concrete is exposed, the intended working life of the structure, the limiting values
of composition are all taken account of in selecting the requirements of the concrete mix.
3. Prescribed concretes: This is used where the specifier prescribes the exact composition
and constituents of the concrete. No requirements regarding concrete strength can be
prescribed. This has very limited applicability.
4. Standardised prescribed concretes: This is used where concrete is site batched or
obtained from a ready mixed concrete producer with no third party accreditation.
5. Proprietary concretes: Used where concrete achieves a performance using defined test
methods, outside the normal requirements for concrete.
The water-to-cement ratio is the single most important factor affecting concrete strength.
For full hydration cement absorbs 0.23 of its weight of water in normal conditions. This
amount of water gives a very dry mix and extra water is added to give the required
workability. The actual water-to-cement ratio used generally ranges from 0.45 to 0.6. The
aggregate-to-cement ratio also affects workability through its influence on the water-tocement ratio, as noted above. The mix is designed for the ‘target mean strength’ which is the characteristic strength required for design plus a specified number of times the standard
deviation of the mean strength.
Several methods of mix design are used. The main factors involved are discussed briefly
for mix design according toTeychenne, R.E. Franklin and Entroy, H.C., 1988, Design of Normal Concrete Mixes.
(HMSO, London).
1. Curves giving compressive strength versus water-to-cement ratio for various types of cement and ages of hardening are available. The water-to-cement ratio is selected to give the required strength.
2. Minimum cement contents and maximum free water-to-cement ratios are specified in
BS8110: Part 1, Table 3.3, to meet durability requirements. The maximum cement content is also limited to avoid cracking due mainly to shrinkage.
3. In Design of Normal Concrete Mixes, the selection of the aggregate-to-cement ratio
depends on the grading curve for the aggregate.
Trial mixes based on the above considerations are made and used to determine the final
proportions for designed mixes.2.2.4 AdmixturesAdvice on admixtures is given in
BS EN 934–2:1998 Admixtures for concrete, mortar and grout.The code defines admixtures as ‘Materials added during the mixing process of in a quantity not more than 5% by mass of the cement content of the concrete, to modify the properties of the mix in the fresh and/or hardened state’.
Admixtures covered by British Standards are as follows:
1. set accelerators or set retarders
2. water-reducing/plasticizing admixtures which give an increase in workability with a
lower water-to-cement ratio
3. air-entraining admixtures, which increase resistance to damage from freezing and
thawing
4. high range water reducing agents/super plasticizers, which are more efficient than (2)
above.
5. hardening accelerators which increases the early strength of concrete.
The general requirements of admixtures are given in Table 1 of the code. The effect of new
admixtures should be verified by trial mixes. A useful publication on admixtures isHewlett, P.C (Editor). 1988, Cement Admixtures: Uses and Applications, (Longman Scientific and Technical).
2.3 CONCRETE PROPERTIES
The main properties of concrete are discussed below.
2.3.1 Compressive StrengthThe compressive strength is the most important property of concrete. The characteristic strength that is the concrete grade is measured by the 28 day cube strength. Standard cubes of 150 or 100 mm for aggregate not exceeding 25 mm in size are crushed to determine the strength. The test procedure is given in Page 13BS EN 12390:2:2000: Testing Hardened Concrete: Making and curing specimens for strength testsBS EN 12390:3:2000: Testing Hardened Concrete: Compressive strength of test specimens
2.3.2 Tensile StrengthThe tensile strength of concrete is about a tenth of the compressive strength. It is determined by loading a concrete cylinder across a diameter as shown in Fig.2.1 (a). The test procedure is given in BS EN 12390:6:2000: Testing Hardened Concrete: Tensile splitting strength of test specimens
2.3.3 Modulus of ElasticityThe short-term stress-strain curve for concrete in compression is shown in Fig.2.1 (b). The slope of the initial straight portion is the initial tangent modulus. At any point P the slope of the curve is the tangent modulus and the slope of the line joining P to the origin is the secant modulus. The value of the secant modulus depends on the stress and rate of application of the load.
BS 1881–121:1983 Testing concrete. Methods for determination of Static modulus of elasticity in compression.specifies both values to standardize determination of the secant or static modulus of elasticity.
The dynamic modulus is determined by subjecting a beam specimen to longitudinal vibration. The value obtained is unaffected by creep and is approximately equal to the initial
tangent modulus shown in Fig.2.1 (b). The secant modulus can be calculated from the dynamic modulus.BS 8110: Part 1 gives the following expression for the short-term modulus of elasticity inFig.2.1, the short-term design stress-strain curve for concrete. where fcu=cube strength and γm=material safety factor taken as 1.5. A further expression for the static modulus of elasticity is given in Part 2, section 7.2. (The idealized short-term stressstrain curve is shown in Fig.2.1.)
2.3.4 CreepCreep in concrete is the gradual increase in strain with time in a member subjected to prolonged stress. The creep strain is much larger than the elastic strain on loading. If the specimen is unloaded there is an immediate elastic recovery and a slower recovery in the strain due to creep. Both amounts of recovery are much less than the original strains under load.
Page 14
The main factors affecting creep strain are the concrete mix and strength, the type of
aggregate, curing, ambient relative humidity and the magnitude and duration of sustained
loading.BS 8110: Part 2, section 7.3, specifies that the creep strain εcc is calculated from the creep coefficient Φ by the equation where Et is the modulus of elasticity of the concrete at the age of loading. The creep coefficient Φ depends on the effective section thickness, the age of loading and the relative ambient humidity. Values of Φ can be taken from BS 8110: Part 2, Fig.7.1. Suitable values of relative humidity to use for indoor and outdoor exposure in the UK are indicated in the figure.
The creep coefficient is used in deflection calculations.Fig.2.1 (a) Cylinder tensile test; (b) stress-strain curve for concrete.
Page 15
2.3.5 ShrinkageShrinkage or drying shrinkage is the contraction that occurs in concrete when it dries and hardens. Drying shrinkage is irreversible but alternate wetting and drying causes expansion and contraction of concrete.
The aggregate type and content are the most important factors influencing shrinkage. The
larger the size of the aggregate is, the lower is the shrinkage and the higher is the aggregate
content; the lower the workability and water-to-cement ratio are, the lower is the shrinkage.
Aggregates that change volume on wetting and drying, such as sandstone or basalt, give
concrete with a large shrinkage strain, while non-shrinking aggregates such as granite or
gravel give lower shrinkages. A decrease in the ambient relative humidity also increases
shrinkage.
Drying shrinkage is discussed in BS8110: Part 2, section 7.4. The drying shrinkage strain
for normal-weight concrete may be obtained from Fig.7.2 in the code for various values of
effective section thickness and ambient relative humidity. Suitable values of humidity to use
for indoor and outdoor exposure in the UK are indicated in the figure. Values of shrinkage
strain are used in deflection calculations.
2.4 TESTS ON WET CONCRETE
2.4.1 WorkabilityThe workability of a concrete mix gives a measure of the ease with which fresh concrete can be placed and compacted. The concrete should flow readily into the form and go around and cover the reinforcement, the mix should retain its consistency and the aggregates should not segregate. A mix with high workability is needed where sections are thin and/or reinforcement is complicated and congested.
The main factor affecting workability is the water content of the mix. Plasticizing admixtures will increase workability. The size of aggregate, its grading and shape, the ratio of coarse to fine aggregate and the aggregate-to-cement ratio also affect workability to some degree.
2.4.2 Measurement of Workability(a) Slump testThe fresh concrete is tamped into a standard cone which is lifted off after filling and the slump is measured. The slump is 25–50 mm for low workability, 50–100 mm for medium workability and 100–175 mm for high workability. Normal reinforced concrete requires fresh concrete of medium workability. The slump test is the usual workability test specified. The following British standard covers slump test.
Page 16BS EN 12350–2: Testing fresh concrete-Part 2: Slump Test(b) Compacting factor testThe degree of compaction achieved by a standard amount of work is measured. The apparatus consists of two conical hoppers placed over one another and over a cylinder. The upper hopper is filled with fresh concrete which is then dropped into the second hopper and into the cylinder which is struck off flush. The compacting factor is the ratio of the weight of concrete in the cylinder to the weight of an equal volume of fully compacted concrete. The compacting factor for concrete of medium workability is about 0.9. The following British standard covers slump test.
BS EN 12350–4: Testing fresh concrete-Part 4: Degree of compatibility(c) Other testsOther tests are specified for stiff mixes and super plasticized mixes. Reference should be made to specialist books on concrete.2.5 TESTS ON HARDENED CONCRETE2.5.1 Normal TestsThe main destructive tests on hardened concrete are as follows.(a) Cube test: Refer to section 2.3.1 above.(b) Tensile splitting test: Refer to section 2.3.2 above.(c) Flexure test: A plain concrete specimen is tested to failure in bending. The theoretical maximum tensile stress at the bottom face at failure is calculated. This is termed the modulus of rupture. It is about 1.5 times the tensile stress determined by the splitting test. The following British standard covers testing of flexural strength.
BS EN 12390:5:2000: Testing Hardened Concrete: Flexural strength of test specimens(d) Test cores: Cylindrical cores are cut from the finished structure with a rotary cutting tool. The core is soaked, capped and tested in compression to give a measure of the concrete
strength in the actual structure. The ratio of core height to diameter and the location where the core is taken affect the strength. The strength is lowest at the top surface and increases with depth through the element. A ratio of core height-to-diameter of 2 gives a standard cylinder test. The following British standard covers testing of cores.
BS EN 12504–1: Testing concrete in structures-Part 1 Cored specimens-Taking examining
and testing in compression.2.5.2 Non-Destructive TestsThe main non-destructive tests for strength on hardened concrete are as follows.
Page 17(a) Rebound hardness testThe Schmidt hammer is used in the rebound hardness test in which a metal hammer held against the concrete is struck by another spring-driven metal mass and rebounds. The amount of rebound is recorded on a scale and this gives an indication of the concrete strength. The larger the rebound number is the higher is the concrete strength. The following British standard covers testing by Rebound hammer.
BS EN 12504-2: Testing concrete in structures-Part 2: Non-destructive testingDetermination of rebound number.(b) Ultrasonic pulse velocity testIn the ultrasonic pulse velocity test the velocity of ultrasonic pulses that pass through a concrete section from a transmitter to a receiver is measured. The pulse velocity is correlated against strength. The higher the velocity is the stronger is the concrete.(c) Other non-destructive testsEquipment has been developed to measure
1. crack widths and depths
2. water permeability and the surface dampness of concrete
3. depth of cover and the location of reinforcing bars
4. the electrochemical potential of reinforcing bars and hence the presence of corrosion
A useful reference on testing of concrete in structures isBungey, J.H. and Millard, S.G., 1996, Testing Concrete in Structures (Blackie Academic and Professional), 3rd Edition.2.5.3 Chemical TestsA complete range of chemical tests is available to measure
1. depth of carbonation
2. the cement content of the original mix
3. the content of salts such as chlorides and sulphates that may react and cause the concrete
to disintegrate or cause corrosion of the reinforcement. The reader should consult specialist literature
2.6 REINFORCEMENT
Reinforcing bars are produced in two grades: hot rolled mild steel bars have yield strength fyof 250 N/mm2; hot rolled or cold worked high yield steel bars have yield strength fy of 460
N/mm2. Steel fabric is made from cold drawn steel wires welded to form a mesh. It has a yield strength fy of 460 N/mm2.
Page 18The stress-strain curves for reinforcing bars are shown in Fig.2.2. Hot rolled bars have a definite yield point. A defined proof stress is recorded for the cold worked bars. The value of Young’s modulus E is 200 kN/mm2. The idealized design stress-strain curve for all
reinforcing bars is shown in BS8110: Part 1 (see Fig.2.2). The behaviour in tension and
compression is taken to be the same.
Mild steel bars are produced as smooth round bars. High yield bars are produced as
deformed bars in two types defined in the code to increase bond stress: Type 1 Square twisted cold worked bars. This type is obsolete. Type 2 Hot rolled bars with transverse ribsFig.2.2 Stress-strain curves for reinforcing bars.
2.7 FAILURES IN CONCRETE STRUCTURES
2.7.1 Factors Affecting FailureFailures in concrete structures can be due to any of the following factors:
1. incorrect selection of materials
2. errors in design calculations and detailing
3. poor construction methods and inadequate quality control and supervision
4. chemical attack
5. external physical and/or mechanical factors including alterations made to the structure
The above items are discussed in more detail below.2.7.1.1 Incorrect Selection of MaterialsThe concrete mix required should be selected to meet the environmental or soil conditions where the concrete is to be placed. The minimum grade that should be
Page 19used for reinforced concrete is grade 30. Higher grades should be used for some foundations and for structures near the sea or in an aggressive industrial environment. If sulphates are present in the soil or ground water, sulphate-resisting Portland cement should be used. Where freezing and thawing occurs air entrainment should be adopted. Further aspects of materials selection are discussed below.2.7.1.2 Errors in Design Calculations and DetailingAn independent check should be made of all design calculations to ensure that the section sizes, slab thickness etc. and reinforcement sizes and spacing specified are adequate to carry the worst combination of design loads. The check should include overall stability, robustness and serviceability and foundation design.
Incorrect detailing is one of the commonest causes of failure and cracking in concrete
structures. First the overall arrangement of the structure should be correct, efficient and robust.Movement joints should be provided where required to reduce or eliminate cracking. The overall detail should be such as to shed water.
Internal or element detailing must comply with the code requirements. The provisions
specify the cover to reinforcement, minimum thicknesses for fire resistance, maximum and
minimum steel areas, bar spacing limits and reinforcement to control cracking, lap lengths,
anchorage of bars etc.2.7.1.3 Poor Construction MethodsThe main items that come under the heading of poor construction methods resulting from bad workmanship and inadequate quality control and supervision are as follows. BS 8110, clause 6.2 gives guidance on many of the aspects discussed below.(a) Incorrect placement of steelIncorrect placement of steel can result in insufficient cover, leading to corrosion of the reinforcement. If the bars are placed grossly out of position or in the wrong position, collapse can occur when the element is fully loaded.(b) Inadequate cover to reinforcementInadequate cover to reinforcement permits ingress of moisture, gases and other substances and leads to corrosion of the reinforcement and cracking and spalling of the concrete.(c) Incorrectly made construction jointsThe main faults in construction joints are lack of preparation and poor compaction. The old concrete should be washed and a layer of rich concrete laid before pouring is continued. Poor
joints allow ingress of moisture and staining of the concrete face.
Page 20(d) Grout leakageGrout leakage occurs where formwork joints do not fit together properly. The result is a porous area of concrete that has little or no cement and fine aggregate. All formwork joints should be properly sealed.(e) Poor compactionIf concrete is not properly compacted by ramming or vibration, the result is a portion of porous honeycomb concrete. This part must be hacked out and recast. Complete compaction
is essential to give a dense, impermeable concrete.(f) SegregationSegregation occurs when the mix ingredients become separated. It is the result of
1. dropping the mix through too great a height in placing. Chutes or pipes should be used in
such cases.
2. using a harsh mix with high coarse aggregate content
3. large aggregate sinking due to over-vibration or use of too much plasticizer Segregation
results in uneven concrete texture, or porous concrete in some cases.(g) Poor curingA poor curing procedure can result in loss of water through evaporation. This can cause a
reduction in strength if there is not sufficient water for complete hydration of the cement.
Loss of water can cause shrinkage cracking. During curing the concrete should be kept damp
and covered. See BS 8110, clause 6.2.3 on curing.(h) Too high a water contentExcess water increases workability but decreases the strength and increases the porosity and permeability of the hardened concrete, which can lead to corrosion of the reinforcement. The correct water-to-cement ratio for the mix should be strictly enforced.2.7.1.4 Chemical AttackThe main causes of chemical attack on concrete and reinforcement can be classified under the
following headings.(a) ChloridesHigh concentrations of chloride ions cause corrosion of reinforcement and the products of corrosion can disrupt the concrete. Chlorides can be introduced into the concrete either during or after construction as follows.(i) Before construction Chlorides can be admitted in admixtures containing calcium chloride, through using mixing water contaminated with salt water or improperly washed marine aggregates.(ii) After construction Chlorides in salt or sea water, in airborne sea spray and from deicing salts can attack permeable concrete causing corrosion of reinforcement.
Page 21(b) SulphatesSulphates are present in most cements and some aggregates. Sulphates may also be present in soils, groundwater and sea water, industrial wastes and acid rain. The products of sulphate attack on concrete occupy a larger space than the original material and this causes the concrete to disintegrate and permits corrosion of steel to begin. Sulphate-resisting Portland cement should be used where sulphates are present in the soil, water or atmosphere and come into contact with the concrete. Super sulphated cement, made from blast furnace slag, can also be used. This cement can resist the highest concentrations of sulphates.(c) CarbonationCarbonation is the process by which carbon dioxide from the atmosphere slowly transforms calcium hydroxide into calcium carbonate in concrete. The concrete itself is not harmed and increases in strength, but the reinforcement can be seriously affected by corrosion as a result of this process.
Normally the high pH value of the concrete prevents corrosion of the reinforcing bars by
keeping them in a highly alkaline environment due to the release of calcium hydroxide by the
cement during its hydration. Carbonated concrete has a pH value of 8.3 while the passivation
of steel starts at a pH value of 9.5. The depth of Carbonation in good dense concrete is about 3mm at an early stage and may increase to 6–10 mm after 30–40 years. Poor concrete may
have a depth of carbonation of 50 mm after say 6–8 years. The rate of carbonation depends ontime, cover, concrete density, cement content, water-to-cement ratio and the presence of
cracks.(d) Alkali-silica reactionA chemical reaction can take place between alkali in cement and certain forms of silica in aggregate. The reaction produces a gel which absorbs water and expands in volume, resulting in cracking and disintegration of the concrete. The reaction only occurs when the following are present together:
1. a high moisture level in the concrete
2. cement with a high alkali content or some other source of alkali
3. aggregate containing an alkali-reactive constituent
The following precautions should be taken if uncertainty exists:
1. Reduce the saturation of the concrete;
2. Use low alkali Portland cement and limit the alkali content of the mix to a low level;
3. Use replacement cementitious materials such as blast furnace slag or pulverized fuel ash.
Most normal aggregates behave satisfactorily.(e) AcidsPortland cement is not acid resistant and acid attack may remove part of the set cement. Acids are formed by the dissolution in water of carbon dioxide or sulphur dioxide from the atmosphere. Acids can also come from industrial wastes. Good dense concrete with adequate cover is required and sulphate-resistant cements should be usedif necessary.2.7.1.5 External Physical and/or Mechanical FactorsThe main external factors causing concrete structures to fail are as follows.Fig.2.3 (a) Partial contraction joint; (b) expansion joint; (c) sliding joints; (d) hinge joints.
(a) Restraint against movementRestraint against movement causes cracking. Movement in concrete is due to elastic deformation and creep under constant load, shrinkage on drying and setting, temperature changes, changes in moisture content and the settlement of foundations. The design should include sufficient movement joints to prevent serious cracking. Cracking may only detract from the appearance rather than be of structural significance but cracks permit ingress of moisture and lead to corrosion of the steel. Various proprietary substances are available to seal cracks.
Movement joints are discussed in BS 8110: Part 2, section 8. The code states that the joints
should be clearly indicated for both members and structure as a whole. The joints are to
permit relative movement to occur without impairing structural integrity. Types of movement joints defined in the code are as follows.
1. The contraction joint may be a complete or partial joint with reinforcement running
through the joint. There is no initial gap and only contraction of the concrete is permitted.
2. The expansion joint is made with a complete discontinuity and gap between the
concrete portions. Both expansion and contraction can occur. The joint must be filled with a
sealer.
3. There is complete discontinuity in a sliding joint and the design is such as to permit
movement in the plane of the joint.
4. The hinged joint is specially designed to permit relative rotation of members meeting at
the joint. The Freyssinet hinge has no reinforcement passing through the joint.
5. The settlement joint permits adjacent members to settle or displace vertically as a result
of foundation or other movements relative to each other. Entire parts of the building can be
separated to permit relative settlement, in which case the joint must run through the full height of the structure.
Diagrams of some movement joints are shown in Fig.2.3. The location of movement joints
is a matter of experience. Joints should be placed where cracks would probably develop, e.g.
at abrupt changes of section, corners or locations where restraints from adjoining elements
occur.(b) AbrasionAbrasion can be due to mechanical wear, wave action etc. Abrasion reduces cover to reinforcement. Dense concrete with hard wearing aggregate and extra cover allowing for wear are required.(c) Wetting and dryingWetting and drying leaches lime out of concrete and makes it more porous, which increases the risk of corrosion to the reinforcement. Wetting and drying also causes movement of the concrete which can cause cracking if restraint exists. Detail should be such as to shed water and the concrete may also be protected by impermeable membranes.(d) Freezing and thawingConcrete nearly always contains water which expands on freezing. The freezing-thawing cycle causes loss of strength, spalling and disintegration of the concrete. Resistance to damage is improved by using an air-entraining agent.(e) OverloadingExtreme overloading will cause cracking and eventual collapse. Factors of safety in the original design allow for possible overloads but vigilance is always required to ensure that the structure is never grossly overloaded. A change in function of the building or room can lead to overloading, e.g. if a class room is changed to a library the imposed load can be greatly increased.(f) Structural alterationsIf major structural alterations are made to a building, the members affected and the overall
integrity of the building should be rechecked. Common alterations are the removal of walls or
columns to give a large clear space or provide additional doors or openings. Steel beams are
inserted to carry loads from above. In such cases the bearing of the new beam on the original
structure should be checked and if walls are removed the overall stability may be affected.(g) SettlementDifferential settlement of foundations can cause cracking and failure in extreme cases. The foundation design must be adequate to carry the building loads without excessive settlement.Where a building with a large plan area is located on ground where subsidence may occur, the building should be constructed in sections on independent rafts with complete settlement joints between adjacent parts.
Many other factors can cause settlement and ground movement problems. Some problems
are shrinkage of clays from ground dewatering or drying out in droughts, tree roots causing
disruption, ground movement from nearby excavations, etc.(h) Fire resistanceConcrete is a porous substance bound together by water-containing crystals. The binding material can decompose if heated to too high a temperature, with consequent loss of strength.The loss of moisture causes shrinkage and the temperature rise causes the aggregates to expand, leading to cracking and spalling of the concrete. High temperature also causes reinforcement to lose strength. At 550°C the yield stress of steel has dropped to about its normal working stress and failure occurs under service loads.
Concrete, however, is a material with very good fire resistance and protects the reinforcing
steel. Fire resistance is a function of member thickness and cover. The code requirements
regarding fire protection are set out below in section 2.9.2.
2.8 DURABILITY OF CONCRETE STRUCTURES
2.8.1 Code References to DurabilityFrequent references are made to durability in BS8110: Part 1, section 2. The clauses referred to are as follows.(a) Clause 2.1.3The quality of material must be adequate for safety, serviceability and durability.(b) Clause 2.2.1The structure must not deteriorate unduly under the action of the environment over its design
life. i.e. it must be durable.(c) Clause 2.2.4This states that ‘integration of all aspects of design, materials and construction is required to produce a durable structure’. The main provisions in the clause are the following:
1. Environmental conditions should be defined at the design stage;
2. The design should be such as to ensure that surfaces are freely draining;
3. Cover must be adequate;
4. Concrete must be of relevant quality. Constituents that may cause durability problems
should be avoided;
5. Particular types of concrete should be specified to meet special requirements;
6. Good workmanship, particularly in curing, is essential. Guidance on concrete
construction such as placing and compaction, curing, etc. are set out in section 6.2 of the code.
2.9 CONCRETE COVER
2.9.1 Nominal Cover against CorrosionThe code states in section 3.3.1 that the actual cover should never be less than the nominal cover minus 5 mm. The nominal cover should protect steel against corrosion and fire. The cover to a main bar should not be less than the bar size or in the case of pairs or bundles the size of a single bar of the same cross-sectional area.The cover depends on the exposure conditions given in Table 3.2 in the code.
These are as follows.Mild: concrete is protected against weatherModerate:concrete is sheltered from severe rain concrete under non-aggressive water concrete in non-aggressive soilSevere: concrete exposed to severe rain, alternate wetting and drying or occasional freezing or severe condensationVery severe: concrete occasionally exposed to sea water, de-icing salts or corrosive fumesMost severe: concrete frequently exposed to sea water, de-icing salts or corrosive fumesAbrasive: concrete exposed to abrasive action Limiting values for nominal cover are given in Table 3.3 of the code and Table 2.1. Note that the water-to-cement ratio and minimum cement content are specified. Good workmanship is required to ensure that the steel is properly placed and that the specified cover is obtained.
matrix; concrete, assisted by the steel, carries the compressive forces, while steel resists
tensile forces. Concrete itself is a composite material. The dry mix consists of cement and
coarse and fine aggregates. Water is added and this reacts with the cement which hardens and binds the aggregates into the concrete matrix; the concrete matrix sticks or bonds onto the reinforcing bars.
The properties of the constituents used in making concrete, mix design and the principal
properties of concrete are discussed briefly. Knowledge of the properties and an
understanding of the behaviour of concrete is an important factor in the design process. The
types and characteristics of reinforcing steels are noted.
Deterioration of and failures in concrete structures are now of widespread concern. This is
reflected in the increased prominence given in the concrete code BS 8110 to the durability of
concrete structures. The types of failure that occur in concrete structures are listed and
described. Finally the provisions regarding the durability of concrete structures noted in the
code and the requirements for cover to prevent corrosion of the reinforcement and provide fire resistance are set out.
2.2 CONCRETE MATERIALS
2.2.1 Cement
Ordinary Portland cement (OPC) is the commonest type in use. The raw materials from which it is made are lime, silica, alumina and iron oxide. These constituents are crushed and blended in the correct proportions and burnt in a rotary kiln. The clinker is cooled, mixed with gypsum and ground to a fine powder to give cement. The main chemical compounds in cement are calcium silicates and aluminates.
When water is added to cement and the constituents are mixed to form cement paste,
chemical reactions occur and the mix becomes stiffer with time and sets. The addition of
gypsum mentioned above retards and controls the setting time. This ensures that the concrete does not set too quickly before it can be placed in its final position or too slowly so as to hold up construction.
Two stages in the setting process are defined in Page 10BS EN 197-1:2000: Cement. Composition, specifications and conformity criteria for common cementsBS EN 197-2:2000: Cement. Conformity evaluationThese are an initial setting time which must be a minimum of 45 min and a final set which must take place in 10 h.
Cement must be sound, i.e. it must not contain excessive quantities of certain substances
such as lime, magnesia, calcium sulphate etc. that may expand on hydrating or react with
other substances in the aggregate and cause the concrete to disintegrate. Tests are specified for soundness and strength of cement mortar cubes.
Many other types of cement are available some of which are:
1. Rapid hardening Portland cement: the clinker is more finely ground than for ordinary
Portland cement. This is used in structures where it is necessary for the concrete to gain
strength rapidly. Typical example is where the formwork needs to be removed early for reuse.
2. Low heat Portland cement: this has a low rate of heat development during hydration of
the cement. This is used in situations such as thick concrete sections where it is necessary to
keep the rate of heat generation due to hydration low as otherwise it could lead to serious
cracking.
3. Sulphate-resisting Portland cement: this is often used for foundation concrete when the
soil contains sulphates which can attack OPC concrete.
A very useful reference isAdam M Neville, Properties of Concrete, Prentice-Hall, 4th Edition, 1996.
2.2.2 Aggregates
The bulk of concrete is aggregate in the form of sand and gravel which is bound together by cement. Aggregate is classed into the following two sizes;
1. coarse aggregate: gravel or crushed rock 5 mm or larger in size
2. fine aggregate: sand less than 5 mm in size
Natural aggregates are classified according to the rock type, e.g. basalt, granite, flint.
Aggregates should be chemically inert, clean, hard and durable. Organic impurities can affect
the hydration of cement and the bond between the cement and the aggregate. Some aggregates containing silica may react with alkali in the cement causing the some of the larger aggregates to expand which may lead to the concrete disintegrating. This is the alkali-silica reaction. Presence of chlorides in aggregates, e.g. salt in marine sands, will cause corrosion of the steel reinforcement. Excessive amounts of sulphate will also cause concrete to disintegrate.
To obtain a dense strong concrete with minimum use of cement, the cement paste should
fill the voids in the fine aggregate while the fine aggregate and cement paste fills the voids in
the coarse aggregate. Coarse and fine aggregates are graded by sieve analysis in which the
percentage by weight passing a set of standard sieve Page 11sizes is determined. Grading limits for each size of coarse and fine aggregate are set out in BS EN 12620:2002: Aggregates for ConcreteThe grading affects the workability; a lower water-to-cement ratio can be used if the grading of the aggregate is good and therefore strength is also increased. Good grading saves cement content. It helps prevent segregation during placing and ensures a good finish.2.2.3 Concrete Mix DesignConcrete mix design consists in selecting and proportioning the constituents to give the required strength, workability and durability. Mixes are defined in BS 8500–1:2002: Concrete. Methods of Specifying and guidance for the specifierBS 8500–2:2002: Specifications for constituent materials and concreteThe five types are
1. Designated concretes: This is used where concrete is intended for use such as plain and
reinforced foundations, floors, paving, and other given in Table A.6 or A.7 of the code.
2. Designed concretes: This is the most flexible type of specification. The environment to
which the concrete is exposed, the intended working life of the structure, the limiting values
of composition are all taken account of in selecting the requirements of the concrete mix.
3. Prescribed concretes: This is used where the specifier prescribes the exact composition
and constituents of the concrete. No requirements regarding concrete strength can be
prescribed. This has very limited applicability.
4. Standardised prescribed concretes: This is used where concrete is site batched or
obtained from a ready mixed concrete producer with no third party accreditation.
5. Proprietary concretes: Used where concrete achieves a performance using defined test
methods, outside the normal requirements for concrete.
The water-to-cement ratio is the single most important factor affecting concrete strength.
For full hydration cement absorbs 0.23 of its weight of water in normal conditions. This
amount of water gives a very dry mix and extra water is added to give the required
workability. The actual water-to-cement ratio used generally ranges from 0.45 to 0.6. The
aggregate-to-cement ratio also affects workability through its influence on the water-tocement ratio, as noted above. The mix is designed for the ‘target mean strength’ which is the characteristic strength required for design plus a specified number of times the standard
deviation of the mean strength.
Several methods of mix design are used. The main factors involved are discussed briefly
for mix design according toTeychenne, R.E. Franklin and Entroy, H.C., 1988, Design of Normal Concrete Mixes.
(HMSO, London).
1. Curves giving compressive strength versus water-to-cement ratio for various types of cement and ages of hardening are available. The water-to-cement ratio is selected to give the required strength.
2. Minimum cement contents and maximum free water-to-cement ratios are specified in
BS8110: Part 1, Table 3.3, to meet durability requirements. The maximum cement content is also limited to avoid cracking due mainly to shrinkage.
3. In Design of Normal Concrete Mixes, the selection of the aggregate-to-cement ratio
depends on the grading curve for the aggregate.
Trial mixes based on the above considerations are made and used to determine the final
proportions for designed mixes.2.2.4 AdmixturesAdvice on admixtures is given in
BS EN 934–2:1998 Admixtures for concrete, mortar and grout.The code defines admixtures as ‘Materials added during the mixing process of in a quantity not more than 5% by mass of the cement content of the concrete, to modify the properties of the mix in the fresh and/or hardened state’.
Admixtures covered by British Standards are as follows:
1. set accelerators or set retarders
2. water-reducing/plasticizing admixtures which give an increase in workability with a
lower water-to-cement ratio
3. air-entraining admixtures, which increase resistance to damage from freezing and
thawing
4. high range water reducing agents/super plasticizers, which are more efficient than (2)
above.
5. hardening accelerators which increases the early strength of concrete.
The general requirements of admixtures are given in Table 1 of the code. The effect of new
admixtures should be verified by trial mixes. A useful publication on admixtures isHewlett, P.C (Editor). 1988, Cement Admixtures: Uses and Applications, (Longman Scientific and Technical).
2.3 CONCRETE PROPERTIES
The main properties of concrete are discussed below.
2.3.1 Compressive StrengthThe compressive strength is the most important property of concrete. The characteristic strength that is the concrete grade is measured by the 28 day cube strength. Standard cubes of 150 or 100 mm for aggregate not exceeding 25 mm in size are crushed to determine the strength. The test procedure is given in Page 13BS EN 12390:2:2000: Testing Hardened Concrete: Making and curing specimens for strength testsBS EN 12390:3:2000: Testing Hardened Concrete: Compressive strength of test specimens
2.3.2 Tensile StrengthThe tensile strength of concrete is about a tenth of the compressive strength. It is determined by loading a concrete cylinder across a diameter as shown in Fig.2.1 (a). The test procedure is given in BS EN 12390:6:2000: Testing Hardened Concrete: Tensile splitting strength of test specimens
2.3.3 Modulus of ElasticityThe short-term stress-strain curve for concrete in compression is shown in Fig.2.1 (b). The slope of the initial straight portion is the initial tangent modulus. At any point P the slope of the curve is the tangent modulus and the slope of the line joining P to the origin is the secant modulus. The value of the secant modulus depends on the stress and rate of application of the load.
BS 1881–121:1983 Testing concrete. Methods for determination of Static modulus of elasticity in compression.specifies both values to standardize determination of the secant or static modulus of elasticity.
The dynamic modulus is determined by subjecting a beam specimen to longitudinal vibration. The value obtained is unaffected by creep and is approximately equal to the initial
tangent modulus shown in Fig.2.1 (b). The secant modulus can be calculated from the dynamic modulus.BS 8110: Part 1 gives the following expression for the short-term modulus of elasticity inFig.2.1, the short-term design stress-strain curve for concrete. where fcu=cube strength and γm=material safety factor taken as 1.5. A further expression for the static modulus of elasticity is given in Part 2, section 7.2. (The idealized short-term stressstrain curve is shown in Fig.2.1.)
2.3.4 CreepCreep in concrete is the gradual increase in strain with time in a member subjected to prolonged stress. The creep strain is much larger than the elastic strain on loading. If the specimen is unloaded there is an immediate elastic recovery and a slower recovery in the strain due to creep. Both amounts of recovery are much less than the original strains under load.
Page 14
The main factors affecting creep strain are the concrete mix and strength, the type of
aggregate, curing, ambient relative humidity and the magnitude and duration of sustained
loading.BS 8110: Part 2, section 7.3, specifies that the creep strain εcc is calculated from the creep coefficient Φ by the equation where Et is the modulus of elasticity of the concrete at the age of loading. The creep coefficient Φ depends on the effective section thickness, the age of loading and the relative ambient humidity. Values of Φ can be taken from BS 8110: Part 2, Fig.7.1. Suitable values of relative humidity to use for indoor and outdoor exposure in the UK are indicated in the figure.
The creep coefficient is used in deflection calculations.Fig.2.1 (a) Cylinder tensile test; (b) stress-strain curve for concrete.
Page 15
2.3.5 ShrinkageShrinkage or drying shrinkage is the contraction that occurs in concrete when it dries and hardens. Drying shrinkage is irreversible but alternate wetting and drying causes expansion and contraction of concrete.
The aggregate type and content are the most important factors influencing shrinkage. The
larger the size of the aggregate is, the lower is the shrinkage and the higher is the aggregate
content; the lower the workability and water-to-cement ratio are, the lower is the shrinkage.
Aggregates that change volume on wetting and drying, such as sandstone or basalt, give
concrete with a large shrinkage strain, while non-shrinking aggregates such as granite or
gravel give lower shrinkages. A decrease in the ambient relative humidity also increases
shrinkage.
Drying shrinkage is discussed in BS8110: Part 2, section 7.4. The drying shrinkage strain
for normal-weight concrete may be obtained from Fig.7.2 in the code for various values of
effective section thickness and ambient relative humidity. Suitable values of humidity to use
for indoor and outdoor exposure in the UK are indicated in the figure. Values of shrinkage
strain are used in deflection calculations.
2.4 TESTS ON WET CONCRETE
2.4.1 WorkabilityThe workability of a concrete mix gives a measure of the ease with which fresh concrete can be placed and compacted. The concrete should flow readily into the form and go around and cover the reinforcement, the mix should retain its consistency and the aggregates should not segregate. A mix with high workability is needed where sections are thin and/or reinforcement is complicated and congested.
The main factor affecting workability is the water content of the mix. Plasticizing admixtures will increase workability. The size of aggregate, its grading and shape, the ratio of coarse to fine aggregate and the aggregate-to-cement ratio also affect workability to some degree.
2.4.2 Measurement of Workability(a) Slump testThe fresh concrete is tamped into a standard cone which is lifted off after filling and the slump is measured. The slump is 25–50 mm for low workability, 50–100 mm for medium workability and 100–175 mm for high workability. Normal reinforced concrete requires fresh concrete of medium workability. The slump test is the usual workability test specified. The following British standard covers slump test.
Page 16BS EN 12350–2: Testing fresh concrete-Part 2: Slump Test(b) Compacting factor testThe degree of compaction achieved by a standard amount of work is measured. The apparatus consists of two conical hoppers placed over one another and over a cylinder. The upper hopper is filled with fresh concrete which is then dropped into the second hopper and into the cylinder which is struck off flush. The compacting factor is the ratio of the weight of concrete in the cylinder to the weight of an equal volume of fully compacted concrete. The compacting factor for concrete of medium workability is about 0.9. The following British standard covers slump test.
BS EN 12350–4: Testing fresh concrete-Part 4: Degree of compatibility(c) Other testsOther tests are specified for stiff mixes and super plasticized mixes. Reference should be made to specialist books on concrete.2.5 TESTS ON HARDENED CONCRETE2.5.1 Normal TestsThe main destructive tests on hardened concrete are as follows.(a) Cube test: Refer to section 2.3.1 above.(b) Tensile splitting test: Refer to section 2.3.2 above.(c) Flexure test: A plain concrete specimen is tested to failure in bending. The theoretical maximum tensile stress at the bottom face at failure is calculated. This is termed the modulus of rupture. It is about 1.5 times the tensile stress determined by the splitting test. The following British standard covers testing of flexural strength.
BS EN 12390:5:2000: Testing Hardened Concrete: Flexural strength of test specimens(d) Test cores: Cylindrical cores are cut from the finished structure with a rotary cutting tool. The core is soaked, capped and tested in compression to give a measure of the concrete
strength in the actual structure. The ratio of core height to diameter and the location where the core is taken affect the strength. The strength is lowest at the top surface and increases with depth through the element. A ratio of core height-to-diameter of 2 gives a standard cylinder test. The following British standard covers testing of cores.
BS EN 12504–1: Testing concrete in structures-Part 1 Cored specimens-Taking examining
and testing in compression.2.5.2 Non-Destructive TestsThe main non-destructive tests for strength on hardened concrete are as follows.
Page 17(a) Rebound hardness testThe Schmidt hammer is used in the rebound hardness test in which a metal hammer held against the concrete is struck by another spring-driven metal mass and rebounds. The amount of rebound is recorded on a scale and this gives an indication of the concrete strength. The larger the rebound number is the higher is the concrete strength. The following British standard covers testing by Rebound hammer.
BS EN 12504-2: Testing concrete in structures-Part 2: Non-destructive testingDetermination of rebound number.(b) Ultrasonic pulse velocity testIn the ultrasonic pulse velocity test the velocity of ultrasonic pulses that pass through a concrete section from a transmitter to a receiver is measured. The pulse velocity is correlated against strength. The higher the velocity is the stronger is the concrete.(c) Other non-destructive testsEquipment has been developed to measure
1. crack widths and depths
2. water permeability and the surface dampness of concrete
3. depth of cover and the location of reinforcing bars
4. the electrochemical potential of reinforcing bars and hence the presence of corrosion
A useful reference on testing of concrete in structures isBungey, J.H. and Millard, S.G., 1996, Testing Concrete in Structures (Blackie Academic and Professional), 3rd Edition.2.5.3 Chemical TestsA complete range of chemical tests is available to measure
1. depth of carbonation
2. the cement content of the original mix
3. the content of salts such as chlorides and sulphates that may react and cause the concrete
to disintegrate or cause corrosion of the reinforcement. The reader should consult specialist literature
2.6 REINFORCEMENT
Reinforcing bars are produced in two grades: hot rolled mild steel bars have yield strength fyof 250 N/mm2; hot rolled or cold worked high yield steel bars have yield strength fy of 460
N/mm2. Steel fabric is made from cold drawn steel wires welded to form a mesh. It has a yield strength fy of 460 N/mm2.
Page 18The stress-strain curves for reinforcing bars are shown in Fig.2.2. Hot rolled bars have a definite yield point. A defined proof stress is recorded for the cold worked bars. The value of Young’s modulus E is 200 kN/mm2. The idealized design stress-strain curve for all
reinforcing bars is shown in BS8110: Part 1 (see Fig.2.2). The behaviour in tension and
compression is taken to be the same.
Mild steel bars are produced as smooth round bars. High yield bars are produced as
deformed bars in two types defined in the code to increase bond stress: Type 1 Square twisted cold worked bars. This type is obsolete. Type 2 Hot rolled bars with transverse ribsFig.2.2 Stress-strain curves for reinforcing bars.
2.7 FAILURES IN CONCRETE STRUCTURES
2.7.1 Factors Affecting FailureFailures in concrete structures can be due to any of the following factors:
1. incorrect selection of materials
2. errors in design calculations and detailing
3. poor construction methods and inadequate quality control and supervision
4. chemical attack
5. external physical and/or mechanical factors including alterations made to the structure
The above items are discussed in more detail below.2.7.1.1 Incorrect Selection of MaterialsThe concrete mix required should be selected to meet the environmental or soil conditions where the concrete is to be placed. The minimum grade that should be
Page 19used for reinforced concrete is grade 30. Higher grades should be used for some foundations and for structures near the sea or in an aggressive industrial environment. If sulphates are present in the soil or ground water, sulphate-resisting Portland cement should be used. Where freezing and thawing occurs air entrainment should be adopted. Further aspects of materials selection are discussed below.2.7.1.2 Errors in Design Calculations and DetailingAn independent check should be made of all design calculations to ensure that the section sizes, slab thickness etc. and reinforcement sizes and spacing specified are adequate to carry the worst combination of design loads. The check should include overall stability, robustness and serviceability and foundation design.
Incorrect detailing is one of the commonest causes of failure and cracking in concrete
structures. First the overall arrangement of the structure should be correct, efficient and robust.Movement joints should be provided where required to reduce or eliminate cracking. The overall detail should be such as to shed water.
Internal or element detailing must comply with the code requirements. The provisions
specify the cover to reinforcement, minimum thicknesses for fire resistance, maximum and
minimum steel areas, bar spacing limits and reinforcement to control cracking, lap lengths,
anchorage of bars etc.2.7.1.3 Poor Construction MethodsThe main items that come under the heading of poor construction methods resulting from bad workmanship and inadequate quality control and supervision are as follows. BS 8110, clause 6.2 gives guidance on many of the aspects discussed below.(a) Incorrect placement of steelIncorrect placement of steel can result in insufficient cover, leading to corrosion of the reinforcement. If the bars are placed grossly out of position or in the wrong position, collapse can occur when the element is fully loaded.(b) Inadequate cover to reinforcementInadequate cover to reinforcement permits ingress of moisture, gases and other substances and leads to corrosion of the reinforcement and cracking and spalling of the concrete.(c) Incorrectly made construction jointsThe main faults in construction joints are lack of preparation and poor compaction. The old concrete should be washed and a layer of rich concrete laid before pouring is continued. Poor
joints allow ingress of moisture and staining of the concrete face.
Page 20(d) Grout leakageGrout leakage occurs where formwork joints do not fit together properly. The result is a porous area of concrete that has little or no cement and fine aggregate. All formwork joints should be properly sealed.(e) Poor compactionIf concrete is not properly compacted by ramming or vibration, the result is a portion of porous honeycomb concrete. This part must be hacked out and recast. Complete compaction
is essential to give a dense, impermeable concrete.(f) SegregationSegregation occurs when the mix ingredients become separated. It is the result of
1. dropping the mix through too great a height in placing. Chutes or pipes should be used in
such cases.
2. using a harsh mix with high coarse aggregate content
3. large aggregate sinking due to over-vibration or use of too much plasticizer Segregation
results in uneven concrete texture, or porous concrete in some cases.(g) Poor curingA poor curing procedure can result in loss of water through evaporation. This can cause a
reduction in strength if there is not sufficient water for complete hydration of the cement.
Loss of water can cause shrinkage cracking. During curing the concrete should be kept damp
and covered. See BS 8110, clause 6.2.3 on curing.(h) Too high a water contentExcess water increases workability but decreases the strength and increases the porosity and permeability of the hardened concrete, which can lead to corrosion of the reinforcement. The correct water-to-cement ratio for the mix should be strictly enforced.2.7.1.4 Chemical AttackThe main causes of chemical attack on concrete and reinforcement can be classified under the
following headings.(a) ChloridesHigh concentrations of chloride ions cause corrosion of reinforcement and the products of corrosion can disrupt the concrete. Chlorides can be introduced into the concrete either during or after construction as follows.(i) Before construction Chlorides can be admitted in admixtures containing calcium chloride, through using mixing water contaminated with salt water or improperly washed marine aggregates.(ii) After construction Chlorides in salt or sea water, in airborne sea spray and from deicing salts can attack permeable concrete causing corrosion of reinforcement.
Page 21(b) SulphatesSulphates are present in most cements and some aggregates. Sulphates may also be present in soils, groundwater and sea water, industrial wastes and acid rain. The products of sulphate attack on concrete occupy a larger space than the original material and this causes the concrete to disintegrate and permits corrosion of steel to begin. Sulphate-resisting Portland cement should be used where sulphates are present in the soil, water or atmosphere and come into contact with the concrete. Super sulphated cement, made from blast furnace slag, can also be used. This cement can resist the highest concentrations of sulphates.(c) CarbonationCarbonation is the process by which carbon dioxide from the atmosphere slowly transforms calcium hydroxide into calcium carbonate in concrete. The concrete itself is not harmed and increases in strength, but the reinforcement can be seriously affected by corrosion as a result of this process.
Normally the high pH value of the concrete prevents corrosion of the reinforcing bars by
keeping them in a highly alkaline environment due to the release of calcium hydroxide by the
cement during its hydration. Carbonated concrete has a pH value of 8.3 while the passivation
of steel starts at a pH value of 9.5. The depth of Carbonation in good dense concrete is about 3mm at an early stage and may increase to 6–10 mm after 30–40 years. Poor concrete may
have a depth of carbonation of 50 mm after say 6–8 years. The rate of carbonation depends ontime, cover, concrete density, cement content, water-to-cement ratio and the presence of
cracks.(d) Alkali-silica reactionA chemical reaction can take place between alkali in cement and certain forms of silica in aggregate. The reaction produces a gel which absorbs water and expands in volume, resulting in cracking and disintegration of the concrete. The reaction only occurs when the following are present together:
1. a high moisture level in the concrete
2. cement with a high alkali content or some other source of alkali
3. aggregate containing an alkali-reactive constituent
The following precautions should be taken if uncertainty exists:
1. Reduce the saturation of the concrete;
2. Use low alkali Portland cement and limit the alkali content of the mix to a low level;
3. Use replacement cementitious materials such as blast furnace slag or pulverized fuel ash.
Most normal aggregates behave satisfactorily.(e) AcidsPortland cement is not acid resistant and acid attack may remove part of the set cement. Acids are formed by the dissolution in water of carbon dioxide or sulphur dioxide from the atmosphere. Acids can also come from industrial wastes. Good dense concrete with adequate cover is required and sulphate-resistant cements should be usedif necessary.2.7.1.5 External Physical and/or Mechanical FactorsThe main external factors causing concrete structures to fail are as follows.Fig.2.3 (a) Partial contraction joint; (b) expansion joint; (c) sliding joints; (d) hinge joints.
(a) Restraint against movementRestraint against movement causes cracking. Movement in concrete is due to elastic deformation and creep under constant load, shrinkage on drying and setting, temperature changes, changes in moisture content and the settlement of foundations. The design should include sufficient movement joints to prevent serious cracking. Cracking may only detract from the appearance rather than be of structural significance but cracks permit ingress of moisture and lead to corrosion of the steel. Various proprietary substances are available to seal cracks.
Movement joints are discussed in BS 8110: Part 2, section 8. The code states that the joints
should be clearly indicated for both members and structure as a whole. The joints are to
permit relative movement to occur without impairing structural integrity. Types of movement joints defined in the code are as follows.
1. The contraction joint may be a complete or partial joint with reinforcement running
through the joint. There is no initial gap and only contraction of the concrete is permitted.
2. The expansion joint is made with a complete discontinuity and gap between the
concrete portions. Both expansion and contraction can occur. The joint must be filled with a
sealer.
3. There is complete discontinuity in a sliding joint and the design is such as to permit
movement in the plane of the joint.
4. The hinged joint is specially designed to permit relative rotation of members meeting at
the joint. The Freyssinet hinge has no reinforcement passing through the joint.
5. The settlement joint permits adjacent members to settle or displace vertically as a result
of foundation or other movements relative to each other. Entire parts of the building can be
separated to permit relative settlement, in which case the joint must run through the full height of the structure.
Diagrams of some movement joints are shown in Fig.2.3. The location of movement joints
is a matter of experience. Joints should be placed where cracks would probably develop, e.g.
at abrupt changes of section, corners or locations where restraints from adjoining elements
occur.(b) AbrasionAbrasion can be due to mechanical wear, wave action etc. Abrasion reduces cover to reinforcement. Dense concrete with hard wearing aggregate and extra cover allowing for wear are required.(c) Wetting and dryingWetting and drying leaches lime out of concrete and makes it more porous, which increases the risk of corrosion to the reinforcement. Wetting and drying also causes movement of the concrete which can cause cracking if restraint exists. Detail should be such as to shed water and the concrete may also be protected by impermeable membranes.(d) Freezing and thawingConcrete nearly always contains water which expands on freezing. The freezing-thawing cycle causes loss of strength, spalling and disintegration of the concrete. Resistance to damage is improved by using an air-entraining agent.(e) OverloadingExtreme overloading will cause cracking and eventual collapse. Factors of safety in the original design allow for possible overloads but vigilance is always required to ensure that the structure is never grossly overloaded. A change in function of the building or room can lead to overloading, e.g. if a class room is changed to a library the imposed load can be greatly increased.(f) Structural alterationsIf major structural alterations are made to a building, the members affected and the overall
integrity of the building should be rechecked. Common alterations are the removal of walls or
columns to give a large clear space or provide additional doors or openings. Steel beams are
inserted to carry loads from above. In such cases the bearing of the new beam on the original
structure should be checked and if walls are removed the overall stability may be affected.(g) SettlementDifferential settlement of foundations can cause cracking and failure in extreme cases. The foundation design must be adequate to carry the building loads without excessive settlement.Where a building with a large plan area is located on ground where subsidence may occur, the building should be constructed in sections on independent rafts with complete settlement joints between adjacent parts.
Many other factors can cause settlement and ground movement problems. Some problems
are shrinkage of clays from ground dewatering or drying out in droughts, tree roots causing
disruption, ground movement from nearby excavations, etc.(h) Fire resistanceConcrete is a porous substance bound together by water-containing crystals. The binding material can decompose if heated to too high a temperature, with consequent loss of strength.The loss of moisture causes shrinkage and the temperature rise causes the aggregates to expand, leading to cracking and spalling of the concrete. High temperature also causes reinforcement to lose strength. At 550°C the yield stress of steel has dropped to about its normal working stress and failure occurs under service loads.
Concrete, however, is a material with very good fire resistance and protects the reinforcing
steel. Fire resistance is a function of member thickness and cover. The code requirements
regarding fire protection are set out below in section 2.9.2.
2.8 DURABILITY OF CONCRETE STRUCTURES
2.8.1 Code References to DurabilityFrequent references are made to durability in BS8110: Part 1, section 2. The clauses referred to are as follows.(a) Clause 2.1.3The quality of material must be adequate for safety, serviceability and durability.(b) Clause 2.2.1The structure must not deteriorate unduly under the action of the environment over its design
life. i.e. it must be durable.(c) Clause 2.2.4This states that ‘integration of all aspects of design, materials and construction is required to produce a durable structure’. The main provisions in the clause are the following:
1. Environmental conditions should be defined at the design stage;
2. The design should be such as to ensure that surfaces are freely draining;
3. Cover must be adequate;
4. Concrete must be of relevant quality. Constituents that may cause durability problems
should be avoided;
5. Particular types of concrete should be specified to meet special requirements;
6. Good workmanship, particularly in curing, is essential. Guidance on concrete
construction such as placing and compaction, curing, etc. are set out in section 6.2 of the code.
2.9 CONCRETE COVER
2.9.1 Nominal Cover against CorrosionThe code states in section 3.3.1 that the actual cover should never be less than the nominal cover minus 5 mm. The nominal cover should protect steel against corrosion and fire. The cover to a main bar should not be less than the bar size or in the case of pairs or bundles the size of a single bar of the same cross-sectional area.The cover depends on the exposure conditions given in Table 3.2 in the code.
These are as follows.Mild: concrete is protected against weatherModerate:concrete is sheltered from severe rain concrete under non-aggressive water concrete in non-aggressive soilSevere: concrete exposed to severe rain, alternate wetting and drying or occasional freezing or severe condensationVery severe: concrete occasionally exposed to sea water, de-icing salts or corrosive fumesMost severe: concrete frequently exposed to sea water, de-icing salts or corrosive fumesAbrasive: concrete exposed to abrasive action Limiting values for nominal cover are given in Table 3.3 of the code and Table 2.1. Note that the water-to-cement ratio and minimum cement content are specified. Good workmanship is required to ensure that the steel is properly placed and that the specified cover is obtained.
| Fire resistance (hour) | Min. Beam b | Rib b | Min. floor Thickness h | Column width Fully exposed b | Min. wall Thickness 0.4%<p<1.0% |
| 1.0 | 200 | 125 | 95 | 200 | 120 |
| 1.5 | 200 | 125 | 110 | 250 | 140 |
| 2.0 | 200 | 125 | 125 | 300 | 160 |
Fig.2.4 Minimum dimensions for fire resistance
2.9.2 Cover as Fire Protection
Nominal cover to all reinforcement to meet a given fire resistance period for various elements in a building is given in Table 2.2 and Table 3.4 in the code. Minimum dimensions of members from Fig.3.2 in the code are shown in Fig.2.4. Reference should be made to the
complete tables and figures in the code.
| Table 2.1 Nominal cover to all reinforcement including links to meet durability requirementsConditions of exposure Nominal cover (mm) | |||
| Mild | 25 | 20 | 20 |
| Moderate | 35 | 30 | |
| Severe | 40 | ||
| Very severe | 50 | ||
| Most severe | – | – | – |
| Abrasive | Nominal cover+allowance for loss of cover due to brasion. | ||
| Maximum free water-to-cement ratio | 0.65 | 0.60 | 0.55 |
| Minimum cement content (kg/m3) | 275 | 300 | 325 |
| Lowest grade of concrete | C30 | C35 | C40 |
| Table 2.2 Nominal cover to all reinforcement including links to meet specified periods of fire resistanceFire Resistance Nominal Cover -mm Beams Floors Ribs Columns Hour SS C SS C SS C | |||||||
| 1.0 | 20 | 20 | 20 | 20 | 20 | 20 | 20 |
| 1.5 | 20 | 20 | 25 | 20 | 35 | 20 | 20 |
| 2.0 | 40 | 30 | 35 | 25 | 45 | 35 | 25 |
| SS Simply supported, C Continuous |
