what is concrete? - trent global · what is concrete? concrete is made by ... water reducers or...
TRANSCRIPT
1
What is Concrete?
Concrete is made by mixing
• Cement
• Fine aggregates
• Coarse aggregates
and
• Water
2
Concrete Admixture
• An additional material, known as admixture, is
sometimes added to modify certain properties of
concrete.
3
Advantages of Concrete
• Good compressive strength
• Protection of embedded steel
• Flexibility of application
5
Cement
• Cement is a fine powder which sets after a few
hours when mixed with water, and then hardens
in a few days into a solid, strong material.
• Cement hardens when water is added.
• Basic constituents for cement are:
– limestone
– clay
6
A Two-Step Process
• Basically, cement is produced in two steps:
• Step 1
– Clinker is produced from raw materials
• Step 2
– Cement is produced from clinker
7
Cement Production Process
• Basic Process
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Hydration of Cement
• When cement comes in contact with water, the
water molecules react with the individual
compounds.
• These products in time produce a firm mass
which give cement its bonding properties,
strength and hardness.
• This process is termed as hydration. The
individual compounds have different rates of
hydration.
9
Heat of Hydration
• Hydration process of cement is accompanied by
heat generation.
• The generated heat in mass concrete may result
in expansion and cracking.
• This can be overcome by using suitable cement
type.
10
Fineness of Cement
• As hydration takes place at the surface of thecement particles, it is the surface area of cementparticles which provide the material available forhydration.
• The rate of hydration is controlled by thefineness of cement.
• For a rapid rate of hydration, a higher fineness isnecessary.
11
Types of Cement
• Ordinary Portland Cement (OPC)
• Rapid-hardening Portland Cement (RHPC)
• Low-heat Portland Cement (LHPC)
• Sulfate-resisting Portland Cement (SRPC)
• Portland Blastfurnace Cement (PBFC)
• Waterproof and Water Repellant Cement
12
Portland Cement
• Portland cement is the most widely used.
• The others being used where concrete with
special properties are required.
13
Cement Storage
• Cement must be kept dry at all times in itspowdery form to ensure that its properties do notchange.
• The shed for storing bagged cement shouldhave water proofed roof and walls and a dry andraised floor, free from ingress of any dampness.
• The storage of bagged cement in the open isnot a recommended practice.
14
Bulk Cement
• Bulk cement is ideal for large projects at which
concrete batching plants are erected.
• However if cement is stored in a silo for a long
period of time (more than 3 months) there is an
increased chance of developing a thin hardened
layer on the top surface, and of cement lump on
the inner surface of the silo.
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First In, First Out
• Have the cement bags stacked so that the
first batch in can be the first out.
• Make sure that the cement really is used
in the same order that it is delivered.
• The important thing with cement is always
to keep it dry.
16
Aggregates
• Aggregates are the inert particles that boundtogether by the cement paste to form mortar.
• The performance of concrete depends largely onthe physical, thermal and chemical properties ofits aggregate.
• Since at least 60 – 80% of the volume ofconcrete is made-up of aggregates, they can bealso be called filler materials.
17
Classification of Aggregates
• The general classification of aggregates is as
follows:
– Heavyweight aggregate
– Normal aggregate
– Lightweight aggregate
18
Functions of Aggregates in Concrete
1. Reduce costs
2. Modify properties of concrete
3. Reduce dry shrinkage
19
Water in Concrete
• Water is used in concrete to react with cementand thus cause it to set and harden.
• It also facilitates mixing, placing and compactingof the fresh concrete. It is also used for washingthe aggregates and for curing purposes.
• The amount of water content is very important todecide the properties of fresh and hardenedconcrete.
20
Admixtures
• Admixtures are substances that are introducedinto a batch of concrete, during or immediatelybefore its mixing, in order to alter or improve theproperties of the fresh or hardened concrete orboth.
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Uses of Admixtures
• to improve the workability of the fresh concrete
• to reduce water content for a given workabilitythereby increasing the strength
• to increase the durability of hardened concrete
• to retard setting time or to increase it
• to impart colour to concrete
• to maintain volume stability by reducing oroffsetting shrinkage during concreting
• to increase concrete resistance to freezing andthawing.
22
Types of Admixtures
1. Air-entraining agents
2. Accelerating agents
3. Retarders
4. Water reducers or plasticizers
5. Super plasticizers
6. Bonding admixtures
7. Water repelling agents
23
Reinforced Concrete (RC)
• Basically made of two materials:
1. plain concrete
2. steel bars embedded in concrete.
• Reinforced concrete is made to combine the
best features of concrete and steel bars.
24
Properties of Reinforced Concrete
Properties Concrete Steel
Strength in tension poor good
Strength in compression good good, but slender bars will
buckle
Strength in shear fair good
Durability good corrodes if unprotected
Fire Resistance good poor – suffers rapid loss of
strength at high temperature
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Composite Action of RC
• The tensile strength of concrete is only about
10% of the compressive strength.
• Therefore nearly all reinforced concrete
structures are designed on the assumption that
the concrete does not resist any tensile forces.
• Reinforcement is designed to carry these tensile
forces.
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Combination of Steel & Concrete
The combination of steel and concrete works
readily for several reasons:
• bond prevents slip of the bars
• proper concrete mixes provide adequate
impermeability of the concrete against water
intrusion and bar corrosion
27
Stress-Strain Relation
• As the load is applied, at first strain increases
linearly with stress and the concrete behaves as
an elastic material.
• After that the curve is no longer linear and the
concrete behaves more like plastic material.
• In the plastic range, if the load is removed, the
deformation would not recover.
29
Concrete Strength
• Concrete generally increases its strength withage.
• A typical variation in strength of an adequatelycured Portland Cement Concrete as suggestedby BS8110 is:
7 days 1 month 2 months 3 months 6months 1 year
20 30 33 35 36 37 N/mm2
• BS 8110 does not permit the use of strengthsgreater than the 28-day value in calculations.
30
Durability of Concrete
The durability of concrete is influenced by:
• the exposure conditions
• the concrete quality
• the cover to the reinforcement
• the width of any cracks
31
Specification of Concrete
• Concrete of a given strength is identified by its‘grade’.
• For example Grade 25 concrete has acharacteristic cube crushing strength of 25N/mm2 after 28 days of curing.
• It is recommended that for normal denseaggregate reinforced concrete, grades of 30, 35and 40 are used.
33
Batching of Concrete
• Batching refers to the process of measuring the
correct amounts of the constituent materials, in
order that the resultant mix will be of the
desired quality.
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Advantages of Using Bulk Cement
• Bulk cement is cheaper than bagged cement.
• A gang of men are not needed to off-load thecement.
• The trouble of ensuring adequate protection ofthe cement during storage is avoided.
• The mixer can always be used to capacitywithout the risk of errors which may result fromsplitting bags.
• Changes in mix proportions are made moreeasily.
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Mixing of Concrete
• The objective of mixing is to coat the surfaces of
all aggregates with cement paste and to blend
all the ingredients of concrete to give a uniform
consistency.
• Machine mixing is generally more economical
and convenient for large quantities and the
concrete produced is also more homogeneous.
36
Transporting of Concrete
• The transport of concrete from the mixing plantto the point at which it is to be placed mustcomply with four main requirements, namely:
– Transport must be rapid so that the concretedoes not dry out or lose its workability duringthe time it elapses between mixing andplacing.
– Segregation must be reduced to a minimumin order to avoid non-uniform concrete. Forthe same reason any loss of fine material orcement or water must be prevented.
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Transporting of Concrete (Contd)
– The transport should be organised so that
during the placing of any particular lift or
section, delays will not result in the
formation of pour planes, cold joints or
construction joints.
– No additional water should be added to the
concrete.
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Time Limit
• In case of ready-mixed concrete, the usual
specified requirement is that the concrete shall
be discharged from a truck mixer or agitator
truck within two hours after the time of loading.
• If non-agitating transporting equipment is used,
the time is reduced to one hour after the time of
loading.
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Method of Transportation
• Various methods are available for transportingconcrete:
– Truck mixers
– Wheel barrows
– Dumpers
– Steel skips and buckets
– Chutes
– Cranes
– Concrete pumps with pipe lines