Concrete Flashcards
Manufacture process of cement
- begins w/ decomp of CaCO₃ @ ~900°C
- leaves calcium oxide CaO + libreates gaseous carbon dioxide (calcination)
- clincering process, CaO reacts @ high temp (1400-1500°C) w/ silica, alumina, ferrous oxide to form silicates, aluminates, ferrites of calcium which comprise the clincer
- clincer ground/milled together w/ gypsum + other additives to produce cement
decomp of CaCO3
CaCo₃–> CaO + CO₂
concrete
construction material composed of crushed rock/gravel + sand bound together w/ a hardened paste of cement and water
basic materials of concrete
- cement
- aggregates
- water
- admixtures
- voids
portland cements
- Ordinary Portland Cement (OPC)
- Rapid Hardening Portland Cement (RHPC)
- Sulphate Resistant Portland Cement (SRPC)
- White Portland Cement (WPC)
Portland cement - raw materials in clinker
- limestone
- iron stone
- sand
- shale
- clay
- other
unsustainability of cement manufacturing
- manif results in high levels of CO₂ output, third ranking producer of anthropogenic CO₂ in world
- 1 tonne of cement prod 780kg of CO₂
- 4-5% of total CO₂ emissions worldwide caused by cement prod
- extraction + processing: landscape degradation
- dust + noise pollution
- energy consumption + vehicle pollution of transportation
anthropogenic
man-made
cement manufacture
- limestone, shale, silica, iron oxides quarried from ground
- rock materials run thru crusher that turns rock into smaller pieces
- crushed limestone + Si + shale + iron oxides mixed together and run thru rotary kiln
- rotary kiln continuously mixes ingredients + “calcines” limestone so that CO₂ is driven off, forming clinker
- clinker ground to fine powder + mixed w/ gypsum (helps moderate how fast cement sets)
- bagged for sale
alternative cementitious (cement-like) materials
- blastfurnace slag cement (GGBS)
- pulverised-fuel ash cement (PFA)
- metakaolin
- rice husk ash
- silica fume
GGBS
- by-prod of iron smelting, quenched slag forms granules
- generally blended with OPC up to 70%
- reduced early age strength + early heat of hydration
- lower carbon footprint
how cements in Europe are classed
-based on perc of portland cement that has ben replaced/substituted
classes of cements in Europe
CEM I - OPC/RHPC (100% cement)
CEM II (65% cement)
CEM III (45% cement)
CEM IV (<45%)
CEM II-IV: OPC with limestone, PFA, or GGBS
when water is added to cement
- each compound undergoes hydration + contributes to final concrete product
- only calcium silicate contributes to strength
- tricalcium silicate: early strength, reacts more rapidly
- dicalcium silicate: late strenth
equation of hydration
Cement + H₂O -> Calcium Silicate Hydrate (C-S-H) + Ca(OH₂) + H₂O
cement compounds
Tricalcium Silicate C₃S
Dicalcium Silicate C₂S
Tricalcium aluminate C₃A
Tetracalcium aluminoferrite C₄AF
Gypsum
setting of cement and hardening of concrete
- a process of crystallization
- crystals form after certain length of time (initial set time), + interlock w/ each other
- cement + water that has crystallized in this way encloses aggregate particles + produces dense material
heat effect on setting and hardening
- speeds up setting + hardening of cement
- cold slows down + can even completely stop process
cement hydration
- setting + hardening results from chem reaction between cement + water, not drying process
- reaction exothermic + irreversible
- heat produced: “Heat of Hydration”
- usually workable up to 2 hours before it begins to set, then harden
- strength gain initially rapid, becoming progressively less
- strength gain continues indefinitely, provided moisture present (CURING)
cement at start of hydration vs at end
- hydration not yet occured, pores filled with water
2. hardening cement paste, majority of space filled with C-S-H
heat of hydration evolution stages
stage 1 stage 2 stage 3 stage 4 stage 5
stage 1
- initial dissolution
- hydrolysis of cement compounds occur rapidly w/ significant temp increase
stage 2
- dormant period
- evolution of heat slows down
- conc in a plastic state which allows problem free transportation + placing
stage 3 + 4
- concrete starts to harden
- heat evol increases due to hydration of tricalcium silicate
stage 5
-slow formation of hydrate products continues as long as water + unhydrated silicates present
cement hydration notes
- when you add water to cement, solid content of mixture increasing due to hydration
- cement particles suspended in water
- hydration continues as long as water is present + there are still unhydrated compounds in cement paste
concrete strength gain with time graph
check slides 1 pg 25
aggregates
- Gravels, crushed rock + sands that are mixed w/ cement + water to produce concrete.
- generally make 50%-80% of concrete mix
coarse aggregates
(stone)
do not pass through a 5mm sieve
fine aggregates
(sand)
pass through 5mm sieve
use of aggregates
- pack efficiently
- reduce spaces
- reduces cost
- modify + improve properties (strength + drying shrinkage)
quality requirements of aggregates
- durability: hard, adequate strength
- cleanliness: free from chem impurities, organic material + dust
aggregate types
- normal density (most gravels + crushed rock)
- lightweight (weak porous solids, good thermal properties)
- high density (radioactive screening)
use of normal density aggregates
used for normal concrete projects
use of lightweight aggregates
insulation
lightweight concrete structures
use of high density aggregates
shielding against nuclear radiation
water - required qualities
- free from impurities. cannot contain sugars, sulphates, chlorides
- sea water must not be used for reinforced concrete
admixtures
- small quantities of additives to conc mix to improve certain properties
- excessive amounts can have adverse effects on concrete
types of admixtures
- accelerators
- retarding agents
- water-repelling admixtures
- water reducing admixtures (plasticisers)
- air entraining admixtures
- superplasticiser
- self-compacting admixture
- foamed concrete
accelerators
- inc rate of strength gain at early stage
- Calcium Chloride CaCl, but may corrode steel
- does not inc final strength
retarding agents
-reduce rate of evol of heat
water-repelling admixtures
- improve impermeability of concrete (basements, water retaining structures)
- no substitute for sound concrete
water reducing admixtures (plasticisers)
- reduces amount of water req for given workability
- calcium ligno-sulphate
air entraining admixtures
- generate evenly dispersed air bubbles in mix
- improves durability against frost + marine environments
superplasticiser
- high flowing concrete
- allows some water removal + thus higher strength
self-compacting admixture
- high flowing concrete
- does not req compacting to get rid of trapped air
foamed concrete
high flowing conc w/ bubbles + without stone
voids (in order of largest to smallest)
- entrapped air
- entrained air
- capillary pores
- gel pores
why are retarding agents necessary
- necessary for large concrete pours
- transport for long distances
- provide time for grooves, curves, other architectural features
- avoid cold joints
cold joints
-plane of weakness in concrete caused by interruption/delay in concreting process
when cold joints occur
when first batch of conc has begun to set before next batch added, so two batches do not intermix
workability
ability of concrete to flow in a mold or formwork
perhaps through congested reinforcement, ability to be compacted to minimum volume
perhaps ability to perform satisfactorily in some transporting operation/forming process
fresh concrete, first 48 hours
- important for performance of concrete structure
- controls long-term behaviour, strength, Young’s Modulus, creep, durability
factors affecting workability
- water content: higher content higher workability (wb)
- fineness of cement: finer cement, faster loss of wb
- aggregate shape: more angular (crushed) more water demand
- grading: uniform grading - better wb
- admixtures
- time: timing of hydration
- temperature: higher temps lead to loss of wb
fresh concrete properties
- workability
- voids
- segregation
- bleeding
- shrinkage
- compaction + honeycombing
- grout loss
- curing
- surface finishes
air voids
air voids when using air-entraining admixture
-should be sufficient air voids within mix to ensure sufficient freeze thaw resistance
removal of air from concrete
agitating
segregation
-Separation of constituents of conc mix caused by excessive handling/vibration/improper gradation of aggregates.
laitance
Coarse aggregates separate towards bottom + cement paste forms a scum on the top
reduce segregation
- inc water content
- reduce water-cement ratio
- inc rate of hydration
dry segregation
- not enough cement + water added
- pebbles/stones separate from mixture
bleeding
- water is lightest material, gravity forces it upwards
- called bleeding
- gets rid of excess water, helps strengthen
water role
- in open air: concrete dries out, shrinks, concrete v fragile @ this stage, causes cracking
- water helps prevent cracking from this shrinking by keeping it moist
causes of bleeding
- too much water (high cement-water ratio)
- less fiens in mix
- poor grading of aggregates
- overworking of concrete
shrinkage
-cracking that occurs when concrete is still plastic
short term: top dries, middle moist, creates tension in top later, cracks
if bleed water replaces concrete lost, no cracking results
causes of shrinkage
-moisture loss from surafece
-
evaporation increased by
- high surface temps
- high wind speed
- low humidity
shrinkage minimised by
- moist aggregates
- cover concrete
- erection of windbreaks + sunshades
- add plastic fibres
compaction
-under and over compaction leads to poor concrete
honeycombing
occurs when voids left in concrete due to failure of mortar to effectively fill spaces among coarse aggregate particles.
honeycombing cause
occurs due to lesser quantity of fine sand leading to harsh concrete mix.
curing
-maintenance of satisfactory moisture content + temp in concrete for period of time immediately following placing + finishing so as to develop desired properties of concrete
grout loss
-cement + water get through
leads to early erosion/corrosion
curing: 2 types
1: keep moisture
2: stable temp
curing: stable temp (thermal curing)
- keeping it warm to prevent overnight freezing of water, (thermal curing)
- it keeps its own heat generated by hydration of cement
curing: putting plastic on top
-traps evaporating water due to bleeding
curing: spraying curing compound advantages + disadvantages
advantage: alcohol based, keeps moist for days
disadvantage: bleeding has to get drop from top
curing: surface finished
- easy to create patterned concretes
- plastic imprints are put on while fresh (using ret agent)
- featured surfaces hide blemishes
hardened concrete properties
- strength development
- Young’s modulus + poisson’s ratio
- creep
- drying shrinkage
- thermal movement
- durability
strength development - what affects it
- temp: heat speeds up strengthening
- age: varies diff for each conc type
- w/c ratio
- compaction
- curing
-can control 4 of it, and test 1 (w/c ratio)
w/c ratio test
- Ireland/UK: cube test
- rest of EU: cylinder test
- America: same cylinder tested sideways
how cube strength is represented
eg. C28/35
28 - cylinder strength
35 - cube strength
3-point bending
-axial failure, tensile failure, stronger in compression than tension
3-point bending
-axial failure, tensile failure, stronger in compression than tension
concrete breaking
- brittle failure
- after yield str, cracks form
- propagates fast + breaks
- concrete breaks suddenly after cracking if not reinforced
strength variability: characteristic strength
below which not more than 5% of test results fall at 28 days old
avg strength + target mean strength
actual avg strength > target mean strength = characteristic strength + 1.64 standard deviations
repeatability
standard deviation of same material tested, by same equip, by same manifacturer
Poisson’s ratio
For concrete Poisson’s ratio is taken as 0.15 - 0.2.
-explains why cubes fail in compression the way they do
what creep depends on
- humidity
- cement cement content
- w/c ratio
- stress
creep failure of concrete + creep recovery graph
check slides 3 pg 19,20
drying shrinkage
- loss of moisture thru evaporation
- 50% of shrink between 1 month-1 year depending on mass
- causes extensive cracking
controlling shrinkage
-reduces moisture, cement, reinforcement, details or fibres
opposite of shrinkage
swelling: material absorbs water
what drying shrinkage depends on
- humidity
- temp of surrounding air
- rate of air flow over surface
- ratio of surface area to vol of conc
- water & cement contents
- curing
thermal movement
- Expansion + contraction on heating/cooling
- Can cause cracking if restrained, eg. due to heat of hydration or daily/seasonal temperature changes
how to get expansion
- If a bar of length L is heated by T°C,
- expansion (e) given by LαT
α = coefficient of thermal expansion, varies between materials
durability of concrete
is its resistance to weathering, chemical attack, abrasion,
frost and fire.
permeability + how to improve it
-should be as low as possible
-improved by:
full compaction
proper curing
low w/c ratio
evaporation/absorption
- loss/gain of liqid from a surface
- depends on ambient temp, wind, sunshine, relative humidity, surface characteristics
diffusion
-liquid/gas movement from a high conc to a low conc
durability - deterioration
-loss of funciton over time
- timber: wet/dry rot leading to strength loss
- concrete: sulphate/acid attack
- steel: formation of rust in presence of oxygen + water
- reinforced concrete: corrosion, esp in sea areas
concrete carbonation
- Reinforcement is protected by highly alkaline pore water in hardened concrete
- CO2 in air neutralises free lime
- If reaction reaches the reinforcement corrosion will occur
- v slow process dependent on diffusion
what diffusion involves
a difference in concentration of a gas/liquid
what permeability involves
involves a flow of a gas or liquid
reinforcing steel and chloride attack
-Slow process dependent on surface absorption + diffusion inside concrete
- attacks steel quickly if chloride ions permeate concrete cover
- sources: sea water, de-icing salts
- keep surface sorptivity low as possible thru compaction + curing + use low w/c ration
durability: fire
- timber: highly flammable, toxic fumes, chars on outside of thick members
- steel: loses strength rapidly at high temps
- plastic: flammable + highly toxic fumes
- concrete: non-flammable but can spall at high temps
reinforced concrete (RC)
-Combination of concrete + steel bars, acting compositely
-Produces strong, durable, versatile building material
-Features best properties of each
material
properties of concrete
strength in tension: poor
in compression: good
in shear: fair
durability: good
fire resistance: good
properties of steel
strength in tension: good
in compression: good, but slender bars buckle
in shear: good
durability: corrodes if unprotected
fire resistance: poor, rapid loss of strength at high temp
why reinforce concrete
- Concrete tends to fail brittle manner (suddenly, without warning – not acceptable for occupants)
- Reinforcing steel takes high tensile loads, the concrete the compression
- When it takes large enough load, yields + becomes plastic (stretches considerably under little increase in load
- ductile material
graphs of concrete and steel compression
slides 4 pg 7
how regular concrete works
- apply a load
- bottom surface in tension at beam centre
- top surface in compression
how reinforced concrete works
- if steel bars located near bottom face (where tension is), beam can take much higher load before failing
- concrete resits tension on top, steel resists tension at bottom
how it works: bond
- steel + conc must act together to transfer tension in concrete into steel
- bonding to round bars using cement paste (“gluing” to surface)
- provide additional bond by having ribs in bars
how bars work
- not by bending with beam
- but by bonding to concrete + stretching as it goes into tension
how it works: anchorage
- Bending bars (into L or U shape) at end of span provides better anchorage (giving it a longer length over which to transfer tension)
- if beam long, normal to use two reinforcement bars overlapped sufficiently, they act as one bar
shear
-failure would probably occur due to diagonal cracking near supports (despite presence of ductile steel)
-known as shear failure – another form of brittle failure
-Need to provide vertical reinforcement to bridge cracks, known as shear links
-have to provide additional longitudinal steel to hold top
end of links in place; nominal size bars called “hangers”
diagram of reinforcement
slides 4 pg 15
under-reinforced beams
-if brought to failure:
- If steel weaker than concrete, steel will yield + stretch significantly – a ductile failure w/ plenty of warning (through cracking on bottom surface)
- Preferred design condition because of this warning
over-reinforced beams
-if brought to failure:
- If additional steel provided to make beam stronger, could lead to concrete becoming the weaker component
- Beam fails suddenly by concrete failure in compression (before steel becomes plastic) – dangerous brittle failure
- Code rules mitigate against this failure method
reinforced columns
-Concrete section in compression but also
some moment – generates tension in part of the section
-Reinforcement provided to take tension + required to take compression
-Now links provided to restrain slender compression elements (vertical reinforcement) to prevent buckling.
-Spacing of links must be such as to prevent this
pre-stressed concrete
- concrete structures tend to be heavy
- Inevitably cracks as steel takes up tensile load
- deliberately pre-stress by compressing it to eliminate tension in bending
way to pre-stress concrete
-by stretching the steel before concrete sets, then release when concrete hard
- adds compressive force to the concrete
- Increased compression in concrete requires higher concrete compressive strength
- No cracking should occur as tension no longer exists under bending
- Deflections smaller + more slender beams can be used
2 methods of applying pre-stress to concrete member
- pre-tensioning: used in factory situations
- post-tensioning: site use
pre-tensioning
-Tendons usually straight
-Steel stressed prior to concrete setting
-When concrete achieved correct strength, steel
released from tensioning device, putting beam in
compression bc bond prevents it from returning to
original length
-release of the tendons transfers compressive force into concrete
=>eliminates tensile stresses under working loads
- Some loss of pre-stress force is inevitable at transfer of load from tendons to concrete
- some longer term losses due to relaxation of tendons over time, drop in temperature (after the heat of hydration), shrinkage + creep in concrete
where pre-tensioning carried out
Usually carried out in factory conditions for precast
units
-Permanent stressing beds constructed
-Long-line production used – similar units produced at the same time
equipment for pre-tensioning
-Tendons anchored to fixed anchor plate at one end of stressing bed + threaded through stop ends of each
individual unit
-Force is applied to tendons by a jack + the tendons are locked off using an anchor
post-tensioning process
-Ducts placed in position prior to concrete pour to predetermined profile.
-Tendons lie inside ducts.
-Concrete cast in mould/ formwork + allowed to harden
-Bond needs to be prevented at this stage to
allow post-tensioning
-When concrete strong enough to take compressive loads, stress tendon + anchor off
-Preformed metal ducts cast into concrete
w/ specially made anchorages
-Need to ensure no concrete grout enters duct
-Ducts need to be accurately located to correct profile + securely anchored in position during concrete pour (would otherwise float)
-once concrete reached sufficient strength, tendons jacked against face of anchorage blocks
-Need to check extension of tendons as will be unable to see movement of the tendon in duct
post-tensioning: notes
- Effectiveness of pre-stressing force is a function of the
force multiplied by the eccentricity
-Can inc efficiency by increasing eccentricity or achieve same pre-stressing effect by applying larger force at given eccentricity
-Curve in profile develops upward camber in concrete section when tendon is tensioned – cancels
out some downward deflection due to beam acting under full working load
post-tensioning and grout
-Ducts are often filled with grout after all tendons are
stressed + locked off
-ensures tendons not subject to corrosion + provides a bond between tendons and concrete
-provides factor of safety against rupture of system
unbonded post-tensioning
-Unbonded post-tensioning requires tendons to be in
greased ducts to provide corrosion resistance
-Unbonded tendons can be re-stressed/replaced
-Create difficulties for demolition as tendons can “blow
out” explosively
concrete
- important to have reached correct strength at transfer
- Accelerated hardening often used in pretensioning for example, using RHPC, accelerators or steam curing
- Elastic deformation occurs under application of pre-compression
- shortens the unit + reduces the stress in the tendon – needs to be accounted for in calculations
concrete - prestressing losses
- thermal movement
- shrinkage
- creep
thermal movement
- When concrete cools down after hydrating, length of element reduces + some prestressing forces in tendons is lost.
- Concrete compression also reduces.
shrinkage
-As the concrete dries out it reduces in length slightly + some of prestressing forces in tendons is lost.
-concrete compression also
reduces.
creep
inelastic deformation due to sustained stress; causes reduction in prestress due to application of sustained compressive stress.
pre-stressed concrete: advantages over RC
- Lighter structures possible
- Savings in foundations, cladding etc.
- Less materials + labour required
- Longer spans
- In buildings, may be possible to increase number of storeys for same overall building height
- Useful for containment vessels due to lack of cracking
pre-stressed concrete: disadvantages over RC
- Need higher quality materials
- More complex technically
- More expensive
- Risk of sudden failure, especially if not lifted properly
- Harder to re-cycle
Explain, using 4 examples, how the choice of concrete constituents
affects its concrete cube compressive strength.
- Cement content - more raises strength
- Rapid hardening raises, GGBS lowers strength
- Rounded aggt lower, crushed angular aggt higher strength
- Larger aggt higher strength
- Water – less implies stronger
- Admixtures: Accelerator higher,Retarder lower strength; plasticiser lower w/c ratio, higher strength, air entrainer or foamer, more air lower strength
Explain, using four examples, how the processes of manufacturing and
taking care of the cubes prior to testing affect its concrete cube
compressive strength.
- Compact fully to remove entrapped air
- Cure to keep hydration going over night
- Do not leave exposed overnight
- Strip carefully as cube still weak and vulnerable
- Put in curing tank within 24 hours
- Thermally cure to enhance strength
- Dry surface before testing
- Turn through 90° before testing
Sketch a typical stress strain curve derived during a concrete cube
compression test for a high strength concrete, showing on the graph
typical peak strength values and Young’s modulus of elasticity values.
Show how we can ascertain from the graph that high strength concrete
is very brittle.
lecture notes
Using a sketch as appropriate, show the typical pattern of failure of a
concrete cube under compression, explaining why the shape is as it is.
lecture notes
On a single graph, sketch typical stress strain curves for concrete, steel
and timber (tested with the grain) when tested in compression, ensure
the differences in strength, stiffness and ductility/brittleness are evident
in your answer.
given in tutorial number 2
Criticise the slump test as a means of measuring workability.
- not accurate
- results are not repeatable
- does not measure many workability features such as cohesiveness, bleeding, finishability, etc.
- result depends on the person doing the test.
If one wished to increase the slump value without causing
segregation, how would one achieve this?
Use a plasticiser and reduce the water content.
explain how:
strength of concrete
varies with time
graph in sample answers
continues increasing mores slowly with time provided water is present
explain how:
water cement ratio of the concrete mix affects the 28
day compressive strength
graph in sample answers
drops dramatically w/ increasing water as no new hydration occurs but creates more caillary voids reducing strength
explain how:
an accelerating admixture might affect
strength development over time.
graph in sample answers
increases early age strength but not later strength
Describe the role of reinforcing bars in reinforced concrete beams,
addressing how tensile cracking, bond, and shear are all accounted for.
in 4th tutorial
Highlight three key structural characteristics of Aluminium and relate them
back to the production and manufacturing processes and explain how these
influence the way that we use the material structurally.
Malleability – how material can be extruded through a
die to form intricate shapes, such as using them for detailed window frames or bicycle frames;
Ductility - to give warning of failure, such as for crowd barrier that can be inspected and replaced or to form aircraft wings with smooth shapes;
Flexibility, with low elastic modulus, for bicycle frames which deforms rather than brakes
Weight, so can be used for light speed bikes, or many parts of aircraft where weight is at a premium to fuel costs.
