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Concrete
• Identify & Evaluate the Properties of the
Building Material
What is Concrete
• Composite material which contains a paste to
bind the fine & course aggregates
• Man Made Rock !
What is Concrete
A History of Cement
• Portland Cement
Cement
• Types of Cement
Cement Type GP
Cement Type GB
Cement Type HE
• High Early Strength
Cement Type HE
Cement Type GP
7
28
28
3
1
3
7
56
• Cement Type HE
Cement Type LH
• Low Heat
Used on large civil works to reduce the heat
produced and the Consequential result of loss of
water
Cement Type SR
• Sulphur Resistant
– Where concrete is expected to be in contact with
Sulphates and/or Salts
– Heavy Industrial
– Swimming Pools
Storage of Cement
• If kept dry will last indefinitely
• Moist air or high humidity will allow cement
to start setting
• Stack tightly to stop air circulation
Cement Type SL
• Shrinkage Limited
To reduce cracking that is induced by curing
What is Concrete
Water
• Potable water must be used
• Potable water is water suitable for Human
Consumption
What is Concrete
Aggregates
• Fine – less than 5mm i.e. sand
• Course – greater than 5 mm i.e. Crushed Rock
Fine Aggregate
• Clean
• Sydney Sand
Course Aggregates
• Give Concrete its major characteristics
• Selection of Aggregates will be determined by
required use,
– Exposed Aggregates
– Fire Rating
– Structural Requirements
Course Aggregates
• Clean sands
• Hard rocks such as basalt, granite, diorite, Blue
Metal
• Shales & Slates & Sandstones are unsuitable
Aggregates
• Must be strong enough to cope with
compressive strength
• Hard enough to resist abrasion and wear
• Durable enough to resist the effects of the
weather cycles
• Chemically inert so they will not react with the
cement or other liquids that they may come in
contact with.
Aggregates
• Aggregates should consist of varying in sizes
up to their ordered mix size. Why?
• Less Chance of segregation
• Less chance of air voids
• More workability
Effect of Aggregate Size
• During Tilt Up lifting operations
• A piece of the panel separated striking a rigger
• On investigation it was found that there were
areas of 40mm aggregate.
• The Engineer specification was for 20mm
What is Concrete
Accelerators
• speed up the hydration (hardening) of the
concrete.
• Chloride accelerators have been banned as
they attack the reinforcement.
Retarders
• To slow down the Hydration process
• Sugar is a retardant!
Air Entrainments
• Cause small air bubbles to form.
• Used in severe extremes (Hot/Cold)
• Reduce damage caused by freeze/thaw
• Reduces Concrete Strength
Plasticisers
• Increase workability of Concrete
• Reduce the amount of water need
• Increase strength of Concrete
Pigments
• Colouring of Concrete
• Engineer approval must be sought
The Concrete Mix
Concrete is a Chemical Reaction
• Setting of Concrete is a chemical reaction
called hydration.
• Hydration releases heat- which presents a
problem curing concrete.
• Water is a catalyst
• The loss of water will retard/stop hydration
Water Cement Ratio
• A ratio to determine the amount of water
used in the mix.
• Excess water increases workability
• Excess water reduces strength
Water Cement Ratio
In a mix with correct water/ cement ratio
the cement paste completely surrounds
the aggregates
Water Cement Ratio
If water/ cement ratio is to high excess
water forms in the mix (green)
Water Cement Ratio
When the mix dries out these the leave
voids which may affect adhesion and
cause weakness similar to perforated
paper.
Water Cement Ratio
• May be tested by the Slump test
Properties of Concrete
1. Compressive Strength
2. Tensile Strength
3. Durability
4. Workability
5. Cohesiveness
Properties of Concrete
• Compressive Strength– Concretes ability to resist a crushing force
– Affected by the water cement ration – the
more water the less strength concrete has!
– Concrete has Good Compressive Strength
Properties of Concrete
• Tensile Strength
– Concretes Ability to resist a pulling force
– Concrete has poor Tensile Strength
Properties of Concrete
• Flexural Strength
– Basically a tensile load
– Caused when a side load is placed on a
structural load
– Concrete has Poor flexural Strength
Flexural Loads
Load
Flexural Loads
Concrete will
want to split on
this side
Load
Durability
• The ability of concrete to resist the affect of
water or chemical action.
• This is achieved by
1. Inert Aggregates
2. Correct Water Cement Ratio- to limit voids
3. Dense surface finish to concrete so as to limit
the ability of water to penetrate
Workability
• Is the effort required to handle & place the
concrete – affected by
– Water Cement Ratio – The more water the easier
it will move, but strength will be affected
– Cement Content – Cement as a lubricant and
make the concrete more workable.
– Grading of Aggregates (Boney Mix)– Aggregates
will fit into each other
– Aggregate Shape – Smooth rounded aggregates
will flow more easily
Cohesiveness
• The ability of plastic concrete to hold its form.
– Large aggregate should not sink to the bottom of
the mix during placement and vibration. This is
called segregation.
– The more water in a mix the more likely it is to
segregate
Testing of Concrete
Testing of Concrete
• Water / Cement Ratio Test – On site Slump
Test.
• Compressive Test – Conducted in Laboratory
Slump Test
Tamp / Compact Concrete
Remove Cone
Slump Test Results
Slump Test
Compression Test
• Test designed to test compressive strength of
concrete
• Cylindrical Concrete is tested to destruction
Transporting Concreting
• Concrete should be delivered
– Placed as soon as possible
– Compacted well before the initial set
– Avoid segregation
Transporting Problems
• Delay
– Hydration begins immediately
– No significant stiffening occurs in the first 30
minutes
– If concrete is kept agitated for 90 minutes
concrete will still be suitable to be placed
Drying Out
• If concrete dries out during transportation it
will lose its work ability.
• Site water should only be added under the
supervision of the supplier.
• Dry concrete will also not compact readily.
Methods of transporting concrete
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Wheelbarrows
Hoists
Trucks
Chutes
Pumps
Pipelines
Trucks
• Trucks range from 1.8 mini trucks to 7.5
metres capacity
• When using trucks you may need to specify
– Size of truck i.e 6 wheeler or 8 wheeler
– A cubic metre concrete weighs 2.5t
– A 7.5 metre truck will carry 18.75t of concrete
– This will need to be taken account of when driving
over structures
– Always be aware that after trucks discharge
concrete they actually get higher
Chutes
• A convenient method to transport concrete
• Items that can be used to make a chute
– Roof sheeting
– Bondek
– Plywood
Pumps and pipelines
• Concrete pumps come in two types, line and
boom pumps
Concrete pumps
• When ordering pumps consideration
– A pump’s footprint (a 42m boom pump may need
up to 12m in width to operate properly)
– Generally boom pumps will take up 2 lanes of
road traffic and traffic management plans will
need to be put in place
– Boom pumps can reach approximately 15 floors
after this concrete line pumps can only be used
Placing of Concrete
• Placed vertically and near its final position
• If it needs to be moved it needs to be done
with shovels
• Dropping of concrete should be restricted to
1.8m to avoid segregation
• Sequence should be planned to avoid cold
joints on large slabs
Compacting
• Compaction is required to achieve
– Maximum Strength
– Watertight Concrete
– Fill in Sharp Corners
– Good bond to reinforcement
– Good surface appearance
Inadequate Compaction
• This has a severe effect on concrete strength
Vibration
• Immersion Vibrators
– Large Volume of Concrete
35mm shafts (for use in walls
etc) to 50mm shafts
Only suitable for thin slabs
Form Vibrators
• Clamped on formwork at regular intervals
• Will compact only to 450mm depths
• Used mainly on precast
Surface Vibration
•Placed directly on Concrete Mass
•Max thickness 250mm
•Good for Dry Thickness
•Vibration follows Gravity
•Less Chance of Segregation
Segregation
• The separation of the components of wet
concrete caused by excessive handling or
vibration. The differential concentration of the
components of mixed concrete, aggregate, or
the like, resulting in non-uniform proportions
in the mass
Curing of Concrete
• Hydration requires water to continue
• Hydration generates heat
• Water removal will also cause concrete to
shrink – Tensile Cracking
• Natural effects will also remove water
– Sun
– Wind
Curing Methods
• 2 Types
– Supply of additional moisture
– Sealing the surface
Supply of Additional Moisture
• Ponding
– Edges of slab are built up to flood the slab
– Can only be used on flat surfaces
Supply of Additional Moisture
• Sprinkling
– Can be intermitted or permanent
– Hard to monitor and control if intermittent
– On irregular shapes
– Uses a lot of water
– Usually not allowed
Sprinklers
Supply of Additional Moisture
• Wet Coverings
– 50mm of sand, hessian, straw etc covers slab and
is kept continually moist
Wet Coverings
Sealing the Surface
• A liquid membrane is sprayed on the surface
of the concrete to form a continual barrier
• PVA compounds are the most common
– They allow for work to continue and allow for
other finishes, but has poorest performance
• Other types compounds are more effective
– They allow for work to continue and allow for
other finishes
Curing Compound
Efficiency of Curing Compounds
Effect of Curing
Sealing the Suface
• Concrete is completely covered by plastic so
evaporated water falls back onto slab
Effect of Curing
Effect of Concrete compared to continual wetting for 180 days
Compare Air to 3 & 7 Days
Surface Coatings
• Hardeners
Surface Coatings
Tolerances
Degradation of Concrete
• Chemical
• Mechanical
• Thermal Damage
Degradation - Chemical
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Carbonation
Sulphates
Chlorides
Leaching
Decalcification
Sea Water
Degradation - Chemical
• Carbonation
– Carbon Dioxide in the Air reacts with Calcium
Hydroxide in the Concrete.
– Cracks in the Concrete allow it to enter deeper
into & closer to the reinforcement.
– This process reduces the Alkalinity of the
Concrete. (This is what protects the Reinforcement
from Corrosion)
Degradation - Chemical
• Carbonation
The process of corrosion is initiated by the Acidic
environment the steel in.
These are caused contaminates called IONS.
Alkaline is the opposite to this environment and
will neutralise the effect.
We will cover this in more detail when we look at
Metals.
Degradation - Chemical
• Carbonation
Insert Photo
Degradation - Chemical
• Sulphates
– Attack the Cement Binder.
– Concrete will basically fall apart.
Degradation - Chemical
• Sulphates
– Insert Photo.
Degradation - Chemical
• Chlorides
– Can be used as accelerators
– Chlorides leach Calcium Hydroxide
– Accelerates the Carbonation process
– Attacks the reinforcement
Degradation - Chemical
• Chlorides
– Insert Photo
Degradation - Chemical
• Leaching
– Water penetrating through cracks
– May collect & deposit IONS in these cracks in
proximity to the reinforcement
– This will counteract the Alkalinity
– Cause oxidation of the Reinforcement
Degradation - Chemical
• Decalcification
– Distilled water will leach Calcium out of the
Concrete
– Calcium gives the concrete ductility
– This makes the concrete very brittle and at more
risk to mechanical damage
– A common source of distilled water is steam!
Degradation - Chemical
• Sea Water
– Contains both Chlorides & Sulphates and other
contaminating IONS causing degradation as
outlined above.
– Below water line a protective layer of Brucite is
formed
Degradation of Concrete
• Chemical
• Mechanical
• Thermal Damage
Degradation - Mechanical
• Aggregate Expansion
• Concrete Cancer
Degradation - Mechanical
• Aggregate Expansion
– Caused by poor choice of aggregates that
expansively react in the presence of water.
– These include
• Opal
• Flint
• Quartz
Degradation - Mechanical
• Concrete Cancer
– Concrete Reinforcement for whatever reason
oxidizes
– 2 failures may occur,
• Complete Failure due to failure of the reinforcement.
• Gradual failure as follows
– When steel oxidizes it expands
– This places mechanical stress on the concrete which will
either crack or break away
– This then exposes more reinforcement to the oxidising
environment and the failure continues.
Degradation - Mechanical
• Impact/Overloading
– The structure may suffer a severe impact/
overloading causing immediate failure.
– The structure may suffer a impact/ overloading
causing strain on the structure
• This effects of this strain may cause some minor
damage that may allow the other types of failure to
occur.
Degradation of Concrete
• Chemical
• Mechanical
• Thermal Damage
Degradation of Concrete
• Thermal Damage
– Concrete has good thermal properties and is
typically used to provide protection to other
building materials such as Steel.
– but
Degradation of Concrete
• Thermal Damage
– Up to 300° Normal Thermal Expansion Occurs
– Above 300° the binder begins to shrink due to
water loss but the aggregate continues to expand
causing internal stresses.
– At 400 to 450° C the hydrated cement begins to
decompose.
– Prolonged exposure to > 600° will cause the
concrete to break up
– On cooling uneven contraction will occur cause
crack and or failure exposing the reinforcement
Tolerances
• Tolerances in Placement
Failure
• Korean Collapse
Environmental Aspects