Review Of Construction Theory

1. 17-08-2022 1

2. 17-08-2022 2

3. Failure Of Structures
17-08-2022 3
• What is a Failure?
• General Definitions
• –Inability of a component, structure or facility to perform its
intended function.
• –Failure does not necessarily involve collapse or rupture.
• –Non-conformity with design specifications or deficient
performance.
• –An unacceptabledifferencebetween the expectedand observed
performance.
• Failure need not always mean that a structurecollapses.
• It can make a structure deficientor dysfunctionalin usage.It may
even cause secondary adverse effects.

4. Failure Of Structures
17-08-2022 4
• Failure Examples
• A floor vibrates when people walk on it.
• One room in the building is not cooled by the air conditioning
system
• A parking area ponds water when it rains
• A new concrete floor spalls
• The foundation moves differentially, causing cracking in the
interior and exterior wall.

5. Failure Of Structures
17-08-2022 5
• Failures may be
• (a) Safetyfailure – Injury, death, or even risk to people:
• Collapse of formwork during concrete placement
• Punching shear failure in flat slab concrete floor
• Trench collapse
• Slip and fall on wet floor
• (b) Functionalfailure– Compromise of intended usage:
• Excessive vibration of floor
• Roof leaks
• Inadequate air conditioning
• Poor acoustics

6. Failure Of Structures
17-08-2022 6
• Failures may be
• (c) Ancillaryfailure– Adverse effect on schedules, cost, or use:
• Delayed construction
• Unexpected foundation problems
• Unavailability of materials
• Strikes, natural disasters, etc.

7. Failure Of Structures
17-08-2022 7
• Causes of structuralfailurein buildingsand civil structures
• 1. Site Selectionand Site DevelopmentErrors:
• Land-use planning errors
• insufficient or non existent geotechnical studies
• unnecessary exposure to natural hazards.
• 2. ProgrammingDeficiencies:
• Unclear or conflicting client expectations
• lack of clear definition of scope or intent of project.
• 3. Design Errors:
• Errors in concept
• Lack of redundancy
• Failure to consider a load or combination of loads, connection details,
• Misuse of computer software, detailing problems including selection of
incompatible

8. Failure Of Structures
17-08-2022 8
• Causes of structuralfailurein buildingsand civil structures
• 3. Design Errors:
• materials or assemblies which are not constructible,
• failure to consider maintenance requirements and durability
• inadequate or inconsistent specifications for materials or expected
quality of work.
• 4. ConstructionErrors:
• Non-conformance to design intent,
• excavation and equipment accidents,
• excessive construction loads,
• improper sequencing,
• premature removal of shoring and formwork, inadequate
temporary support

9. Failure Of Structures
17-08-2022 9
• Causes of structuralfailurein buildingsand civil structures
• 5. MaterialDeficiencies:
• Material inconsistencies
• premature deterioration
• manufacturing or fabrication defects.
• 6. OperationalErrors:
• Alterations to structure,
• change in use,
• negligent overloading
• Inadequate maintenance.

10. Failure Of Structures
17-08-2022 10
• Review
• Causes of structuralfailurein buildingsand civil structures
• 1. Site Selection and Site Development Errors:
• 2. Programming Deficiencies:
• 3. Design Errors:
• 4. Construction Errors:
• 5. Material Deficiencies:
• 6. Operational Errors:

11. Review Of Construction Theory
17-08-2022 11
• Aggregates + Cement + Water + Chemical Admixtures = Concrete
• place of manufacture
• at a construction site as a small batch producedin a portableconcrete
mixer
• at a large batchingplant at the construction site
• transported by concrete mixing truck from a concreteplant some
distance from the construction site.-ready mix concrete-manufacturer
should know about
• intended use of it (i.e. kerb, slab, etc.)
• amount required in cubic meters
• strength required
• slump in mm
• maximum size aggregate (i.e. 14 mm, 20 mm, etc.)
• method of placement (i.e. pump, off the chute, etc.),
• any admixtures required.

12. Review Of Construction Theory
17-08-2022 12
• The proportions of each material in the mixture affect the
properties of the final concrete, as follows:
• As the cement content increases both strength and
durability increase.
• As the water content increases the concrete becomes
weaker; hence, there should just be enough water to make
the mix workable.
• As the water/cement ratio increases, strength and
durability decrease.
• As the fine aggregate increases the mix becomes sticky and,
after compaction, the top few millimeters of concrete
become very sandy.
• As the coarse aggregateincreases the mix becomes bony
and some of the stones can protrude from the surface after
compaction

13. Review Of Construction Theory
17-08-2022 13
• When concrete is placed in the formwork after thorough
mixing ,care must be taken
• not to damage or move the formwork or the reinforcing
steel.
• to ensure that the concrete does not segregate.
• concrete should not be dropped from heights greater than
2.0 meters.
• The formwork is filled by starting to place the concrete
from the corners of the formwork, and from the lowest
level if the surface is sloping.
• Place each load of concrete into the face of the previous
plastic concrete, not away from it.
• Deposit the concrete in horizontal layers and compact
before the next layer is placed

14. Properties of concrete and their control
17-08-2022 14
• Plastic-state concrete
• two most important properties of plastic state concreteare
• workability and
• cohesiveness.
• Workabilitydescribesthe ease with which concrete is mixed,
handled,placed, compacted and finished.
• factors, which affect workability:
• The mix becomes harsherand less workable if the amountof
cement is reducedprovided the amount of aggregate remains
the same.
• The mix becomes more workable if the amount of cementis
increasedprovided the amount of aggregate remains the same.
• However, an excessiveamountof cementproducesa very
stickyand unworkablemix.

15. Properties of concrete and their control
17-08-2022 15
• factors, which affect workability:
• If the aggregate grading, size and shape are considered:
• Well-graded aggregateswith different particle sizes produce a more
workableconcrete.
• Also, well graded aggregates that are smooth, round and as large as
possible improve workability.
• Rough, angularaggregatesproduce less workable concrete.
• Chemicaladmixtures increase the workability of concrete by lubricating
and dispersing the cement particles.
• Never make concrete more workable by just adding water. Increasing
the water content without an increase in cement content lowers the
strength and durability of concrete.
• To make a more workable mix, add more cement(paste), use well
gradedaggregatesand chemicaladmixtures.

16. Properties of concrete and their control
17-08-2022 16
• Cohesivenessmeasures how well the concreteholds together.
• Factorsaffectingcohesiveness,are:
• A mix that has too much water will not be cohesive and may
separate and bleed.
• A dry mix can crumble, with the coarse aggregate segregating
from the cement paste and sand.
• A well graded aggregategives a more cohesivemix.
• Less fine aggregate (sand) gives a bony mix, which tends to
segregate.
• Excess fine aggregate makes the concrete cohesive, but sticky
and difficult to work and place.

17. Properties of concrete and their control
17-08-2022 17
• Hardened concrete
• importantpropertiesof hardenedconcrete
• durability and strength.
• DURABILITYis described as the ability of concrete to resist wear
and tear and other in service conditions without breaking up.
Concrete durability increases with strength.
• Durable concreteis dense and watertight.
• Durability is very important to protect steel in reinforced
concrete.
• COMPRESSIVE STRENGTH is a measure of concrete strength in
the hardened state.
• Concrete is very strong in compression.
• It is not strong in tension because it has a low tensile strength.

18. Properties of concrete and their control
17-08-2022 18
• Durabilityand strengthincrease with
• lower water content,
• higher cement content,
• Higher densities,
• extended moist curing
• correct type of cement.
• Therefore, if water-to-cement ratio is altered by raising water
content, the concrete will be less durable and weaker.
• Proper compaction will also give higher densities and
improve strength and durability.
• Curing time is also important. The longer the concrete is
cured and kept damp, the stronger and more impermeable
and durable it will be.

19. Discontinuities and defects
in concrete structures
17-08-2022 19
• Cracking of concrete
• Cracking affects the appearance of concrete.
• In some cases it affects its structural adequacy and
durability.
• In reinforced concrete cracking allows easier access to air and
moisture which can cause steel to rust and eventually weaken
the concrete.
• Crackscan occur at two stages.
• (a) Before concrete hardens
• Movement of concrete causes these cracks to occur before the
concrete has set. They include:
• plastic shrinkage cracks
• plastic settlement cracks
• cracks caused by movement of the formworks.

20. Discontinuities and defects
in concrete structures
17-08-2022 20
• 1.Crackingof concrete
• (b)After concrete hardens-may be due to
• Drying shrinkage,
• settlement,
• structural cracks, etc.
• They may require structural repair such as high pressure
epoxy injection or other means.
• 2. Spalling
• This occurs when concrete edges or other surfaces chip or
break. Spalling can be repaired by breaking out to sound and
dense concrete then wetting and refilling the area with a
cement material that is then compacted, finished and cured.
• Since this is a visual defect, non-destructive testing is not an
applicable repair technique.

21. Discontinuities and defects
in concrete structures
17-08-2022 21
• 3. Honeycombing
• This is when too much coarse aggregate appears on the surface with
some cavities underneath.
• It occurs as a result of poor compaction or if a bony mix is used with
not enough sand.
• If it only occurs on the surface it can be reprofiled with a render (thin
layer of sand/cement mortar) or a proprietary cement product.
• If cavities exist below the surface, it is more appropriate to break out
to sound and dense concrete and repair as per spalling.
• 4. Dusting
• This is a surface defect that appears as fine powder on the concrete
surface and comes off when brushed
• . It is caused by finishing the concrete before bleed water has dried
out, as well as by inadequate curing.
• It is repaired by applying a chemical floor hardener or bonded
topping.

22. Discontinuities and defects
in concrete structures
17-08-2022 22
• 5. Crazing
• This type of cracking resembles a map pattern.
• The cracks only extend through the surface layer.
• It is caused by minor surface shrinkage as a result of the drying
conditions.
• It is avoided by finishing and curing as soon as possible
• These cracks do not cause any subsequent deterioration of the
concrete.
• If appearance is a problem a surface coating of paint can be
• applied to cover the cracks.
• 6. Rain damage
• Surface pitted or eroded concrete can occur as a result of heavy
rain. It is avoided by covering newly placed concrete with plastic
sheeting when it rains.
• If there is rain damage and the concrete has not hardened it can be
reworked and refinished.

23. Discontinuities and defects
in concrete structures
17-08-2022 23
• 7. Efflorescence
• This is a white crystalline deposit, which appears soon after
completion.
• It is removed by dry brushing and flushing with clean water.
• Efflorescence has no effect on the structural performance of the
concrete.
• 8. Blistering
• Blisters occur when the fresh concrete surface is sealed by
trowelling trapping air or bleed water under the surface.
• It is avoided by delaying trowelling as long as possible and
covering to prevent evaporation.
• 9. Corrosionof reinforcingbars
• Corrosion occurs when the concrete surface cracks allowing
water entry, or if water enters the concrete by diffusion during
carbonation.
• The increase in diameter of thereinforcing bars caused by the
formation of iron oxide (rust) can cause the concrete above the
affected bars to spall off.

24. TESTING OF CONCRETE
17-08-2022 24
• Two simple tests are used to control the quality of concrete:
• SLUMP TEST is used when the concrete is in the plastic state.
COMPRESSION TEST is used when concrete is in the hardened
• state.
• Both tests are used for the quality control of concrete during
manufacture..
• Other tests
• .1. Tensile tests-mainly carried out on the reinforcing bars and stressing
tendons used in concrete construction.
• knowledge of tensile strength is of value in estimating the load under
which cracking will develop.
• The absence of cracking is of considerable importance in maintaining the
continuity of a concrete structure and in many cases in the prevention of
reinforcement corrosion

25. TESTING OF CONCRETE
17-08-2022 25
• 2. Flexuretest
• In a flexure test on a beam the theoreticalmaximumtensile stress
reached in the bottom fibre of the test beamis known as the modulusof
rupture.
• 3. SplittingTest-Inthis test a concrete cylinder, of the type used for
compression tests, is placed with its axis horizontal between the platens
of a testing machine, and the load is increased until failure by splitting
along the vertical diameter takes place.
• 4. Chemicalanalysis-The chemical composition of concrete as
determined from extracted core samples
• Typical information obtainable - cement content, original water content
and water cement ratio . aggregate grading chloride content sulphate
content

26. TESTING OF CONCRETE
17-08-2022 26
• 5. Microscopical examination
• The various microscopical methods, which may be applied to the study of
hardened concrete, are derived from the science of petrography
• identification of ingredients used such as types of cement, aggregates and
cement replacement materials
• estimation of original mix proportion for example cement content,
aggregates content and water cement ratio
• determination of air void content which include entrained air and
entrapped air
• investigation of chemical and durability performance for instance:
• chemical attack
• alkali silica reaction, aggregate or cement paste shrinkage
• frost attack
• carbonation
• leaching
• detection of unsound contaminants
• fire damage.