Concrete Test 1 Flashcards
Concrete is like a sponge in that…
Spaces absorb moisture
In freezing conditions water in pours could…
Freeze, increasing in volume and causing cracks
Greater connectivity of pours usually gives
Less durability
Only _____ pours affect durability
Open
Definition of porosity
Overall volume of pours (open and closed)
Definition of permeability
Measure of ease with which water / gas can flow through a material
More permeable / porous concretes are used in places like
Roads, runways, motorways to allow water to drain
Permeability is not the same as porosity, give exmaple…
You could have a high porosity (with closed pours) and low permeability
Usually with concrete the permeability will be…
Relative to the porosity
Definition of absorption
Ability to draw water into voids (open pours only)
Usually porosity is ____________________ to absorption (_____________________)
Not equal to
Due to closed pours not absorbing water
There are how many forms of density - give names
Two
Bulk density
Relative density
Definition of bulk density
Measure of compaction - tells how closely particles are packed - is defined as dry mass of unit volume
Definition of relative density
Same definition as bulk density only volume is only the volume of the solids, no spaces (pours)
Bulk density will have….
A greater volume
Relative density will have
Smaller volume due to solids being crushed
Definition of moisture content
Mass of water in concrete related to its dry mass
When determining mass, concrete samples will be…
Put in an oven to dry / remove moisture then weighed
In highly porous concrete relative density will always be __________ than bulk
Greater
Two types of porosity are
Open porosity
Closed porosity
In most cases, the higher the porosity…
The higher the permeability
Bulk density formula
P = mass / volume (solids + voids)
Relative density formula
P = mass / volume (solids only)
total water absorption is the…
Maximum value moisture content
Moisture content cannot exceed…
The total water absorption
Concrete is composed of (5)
1) cement
2) crushed rock or gravel
3) sand
4) water
5) sometimes chemical admixtures and other materials
Cement + water =
Cement paste
Cement + sand + water =
Cement mortar
Cement + sand + gravel + water =
Concrete
More water used = ______ pores & _________ concrete
More
Weaker
The first British standard for Portland cement was published in
1904
Examples of non hydraulic cements are (2)
1) Gypsum (plaster of Paris)
2) Calcite (lime based cement)
Disadvantage of gypsum and calcite (non hydraulic cement is…
It has high solubilities of calcium hydroxide and gypsum therefore rapid deterioration in moist / wet conditions
Hydraulic cements are (2)
1) More durable
2) hydration products are insolvable - cements set under water
A definition of Portland Cement is….
A hydraulic binder capable of setting, hardening and remaining stable under water
Portland cement consists essentially of…
Hydraulic calcium silicates, usually containing calcium sulphate
EN197-1: Portland cement clinker is a hydraulic material which shall consist of at least _____ by mass of calcium silicates, the remainder consisting of ______ and _______
2/3
TBC
TBC
Raw materials of Portland Cement are (2)
1) 3/4 limestone (calcareous materials - lime bearing)
2) 1/4 clay (argillaceous materials - silica, alumina, iron)
6 stages of production of Portland Cement are;
1) Limestone ground to fine powder and mixed with washed clays (ratio 2:1)
2) Mixture fed into top of rotary inclined kiln, fuel fed in at bottom, giving temperature gradient
3) Water is quickly driven off at top
4) Limestone decomposes to quicklime
5) Some materials (such us aluminium or iron) melt and bind the other materials to form lumps of clinker
6) Clinker is allowed to cool before adding some gypsum and grinding he two to form OPC
Cement abbreviation
CaO = ?
TBC
Cement abbreviation
SiO2 = ?
TBC
Cement abbreviation
AI2O3 = ?
TBC
Cement abbreviation
Fe2O3 = ?
TBC
Cement abbreviation
H2O = ?
Water
Cement abbreviation
CO2 = ?
Oxygen
Cement abbreviation
SO3 = ?
TBC
Chemical composition of OPC
1) Calcium oxide
2) Silica
3) Alumina
4) Iron Oxide
5) Sulphur Trioxide
6) Magnesia
7) Sodium @ Potassium Oxides
1) 60 - 70%
2) 17 - 25%
3) 3 - 8%
4) 0.5 - 6%
5) 1 - 3%
6) 0.1 - 4%
7) 0.5 - 1.3%
Oxides form
Hydraulic compounds
The 4 main hydraulic compounds that can be formed from oxides are
1) Di calcium Silicate
2) Tri calcium Silicate
3) Tri calcium Aluminate
4) Tetra Calcium Alumino Ferrite
Abbreviation for Di calcium silicate is
C2S
Abbreviation for Tri calcium silicate is
C3S
Abbreviation for Tri Calcium Aluminate is
C3A
Abbreviation for Tetra Calcium Alumino Ferrite is
C4AF
The 2 main hydraulic compounds are (include abbreviation)
1) C2S Di-Calcium Silicate
2) C3S Tri-Calcium Silicate
When water is added to hydraulic compounds to start the hydration process what are the 4 main hydration products
1) Calcium Silicate Hydrate (gel)
2) Calcium Hydroxide
3) Ettringite
4) Calcium Aluminate Hydrate
Abbreviation for Calcium Silicate Hydrate is
CSH
Abbreviation for Calcium Hydroxide is
CH
Abbreviation for Calcium Aluminate Hydrate is
CAH
C2S Di-Calcium Silicate reacts _____________ over _____________ with a _________ amount of heat released ______ kJ/kg
Very slowly
Many years
Small
260
C2S Di-Calcium Silicate is a ______ responsible for ____________
Gel
Strength
C3S Tri-Calcium Silicate reacts _________ over _________. Heat being released is _____ kJ/kg
More quickly than C2S
2-3 weeks
500
C3A Tri-Calcium Aluminate reacts ___________ over ________. Heat produced is _____ kJ/kg
very rapidly (flash set)
Few hours
870
To prevent flash set of C3A Tri-Calcium Aluminate _________ is added
Gypsum
Initial phase of hydration process for C3A Tri-Calcium Aluminate produces the hydration product ___________. When it moves onto the normal phase the hydration product is _____________.
Tri-sulphoaluminate (Ettringite)
Mono-sulphoaluminate
C3A Tri-Calcium Aluminate is responsible for _____ ______________
Early strength
Negatives associated with C3A Tri-Calcium Aluminate (2)
1) Concrete is liable to sulphate attack
2) High heat of hydration (mass concreting)
C4AF Tetra Calcium Alumino Ferrite produces _____ kJ/kg heat in hydration
420
Flash set in C4AF Tetra Calcium Alumino Ferrite is controlled by
Adding gypsum
C4AF Tetra Calcium Alumino Ferrite is ________ in its effects which are (2)
Neutral
1) small contribution to strength
2) gives typical grey colour (sole effect)
Hydration rate has the following order
C3A (Tri-Calcium Aluminate) > C3S (Tri-Calcium Silicate) > C4AF (Tetra Calcium Alumino Ferrite) > C2S (Di-Calcium Silicate)
During hydration process ________ forms first, followed by _________ and ___________ with a small change in the total ___________ of the paste
Ettringite
CH (Calcium Hydroxide)
CSH (Calcium Silicate Hydrate)
Volume
The rate of hydration can be increased in the following ways (3)
1) Temperature - for each 10degC increased hydration rate doubles
2) Fineness of cement - more fineness of cement more surface area hydration can take place
3) Change proportion of cement components (e.g. increase C3S and CA)
During the hydration process the following %ages of heat are liberated at different intervals (3)
1) 30% within first day
2) 50% within first three days
3) Remaining over time
As thermal conductivity of concrete is low it acts as an insulator, containing the heat within the concrete and forcing the temperature to rise may cause
Cracking
A way to lower the heat of hydration is to
Adjust the fineness and composition of cement (e.g. increasing C3S and C3A as in type IV cement)
