Concrete Flashcards
What is the definition of paste?
Cement + water
– Rarely used alone, usually
combined with aggregates
What is the definition of mortar?
Paste + sand (‘fine aggregate’, <5 mm)
– Used to join bricks together, or as a coating
What is the definition of concrete?
Concrete: paste + sand + coarse aggregate
– Coarse aggregate usually gravel, crushed rock, up to a
few cm in size
– Aggregate needs to be unreactive & strong, or can harm
durability & performance – see later lectures
– Aggregate dilutes the paste – cheap, and reduced heat
release – mix (to correct grading) of fine and coarse
helps cohesion & reduces bleed
Why is water important in concrete?
Two main reasons water is important:
– Required in cement hydration reactions
– Makes concrete flow (increased slump)
Why is too much water bad in concrete?
Too much water is bad:
– If there is extra water, it forms extra pores
reduction in durability (more permeable less resistant to chloride penetration, carbonation, sulfate
ingress – see later…)
reduction in strength (more holes in material)
– Can also delay setting/hardening
– Causes bleeding, segregation, plastic settlement
– Increases drying shrinkage
What are the effects of water on strength?
• More water –> less strength
– More porosity gives less strength
– Common to almost all materials
• Water content measured as water/cement mass ratio
– Abbreviated w/c
– For blended cements, often use water/binder (w/b) instead
• Normal ratios are around 0.5±0.2
• Reduce water content with (super)plasticisers – polymer
additives that improve flow properties
When does segregation occur?
• Need a cohesive mix
– Low water content (often with plasticisers)
– Fine aggregate helps avoid segregation
What is plastic shrinkage cracking?
• Rapid water evaporation from the surface makes
the paste shrink – and water bleeds to the surface to enable this to happen
– (evaporation later also causes drying shrinkage cracks)
– Aggregate particles stay in place and restrain the shrinkage – causes cracking/crazing
– Solutions? – avoid drying (!)
– Controlled bleed can reduce cracks, but too much causes cracking
What is plastic settlement?
-Solid aggregate particles can sink through the
paste, leaving water pockets under aggregates and reinforcing bars, and cracks on surface
-Cracks can extend from surface to the first reinforcing bars –this is fatal for durability, because the steel corrodes
very quickly
Other binder for portland cement: Alkali-activated (geopolymer) cements
• Aluminosilicate materials + alkaline solution
(“activator”) – can use blast furnace slag or pozzolans
~60-90% less CO2 emissions than Portland cement
– Main drawback: need for an alkaline solution
– Commercial production in Eastern Europe, China, Australia, increasingly in UK/EU
Other binder for portland cement: Calcium aluminate cement
CAC (also high-alumina cement - HAC, trade
name Ciment Fondu or SECAR)
– Special type of clinker
– Used since 1908 (developed by Lafarge)
– High early strength (90% of final strength after 24 h) –
used in prestressed components
• Sometimes has catastrophic strength loss if used under the wrong conditions
– Banned in structural applications in many parts of the
world
– Very sensitive to water content
– Expensive retrofitting (or demolition) of many buildings has been required
Other binder for portland cement: Magnesium oxychloride cement
• “Sorel cement” (S. Sorel, France, 1867)
• Combine MgO with MgCl2 and H2O
• Main binder phase 5Mg(OH)2ꞏMgCl2ꞏ8H2O
• Very high early strength (>70 MPa after 3-7 days), but sensitive to water (not hydraulic)
– Useful for indoor floors, tiles, artificial ivory, billiard balls
• Variants use sulphate instead of chloride, or zinc instead of magnesium – this can enhance the water resistance
Other binder for portland cement: Bitumen concretes
• Bitumen (asphalt) is a mixture of heavy organic molecules, solid at room temperature
– Naturally occurring or synthetic
• Used to bind stones/gravel together into a solid hardened material (concrete), mostly for roads
– Also (imprecisely) called ‘tarmac’
• Bitumen is not technically a type of cement, but the material made with it is a concrete
– Bitumen is a ‘binder’ (as are the cements we have discussed)
• Use heat (or sometimes chemical solvents) to soften bitumen and make it flowable/workable as desired
Is concrete strong or weak in compression?
Concrete is strong in compression but weak in tension
Why do we need steel reinforcement?
Because steel is strong in tension which concrete isnt
Why does steel reinforcement use mild steel instead of stainless steel?
– Much cheaper
– Passivation chemistry (resistance to corrosion) works better for mild steel – relies on generating an oxide layer on the surface
• Bars often ribbed for better bond to concrete
• Properties specified in
EN 10080 (broadly) and BS 4449 (national details)
More steel isn’t necessarily better
Reinforcement is usually ~3- 5% of cross-section area, but sometimes much more than this is used (badly)
- Too much steel causes congestion where the concrete can’t flow through the gaps to properly compact.
What is prestressing?
Use steel cables to hold the bottom face of a concrete member in compression
– Pre-tensioned (cables stretched, concrete poured, tension released)
– Post-tensioned (concrete poured with a duct, cables
inserted and tensioned)
• Pre-tensioning relies on interfacial bond to steel
• Post-tensioning can have severe problems if the steel corrodes and stress application is lost
Therefore the steel doesn’t curve after loading
What is the main cause of concrete failure?
Steel failure as when steel rusts, it expands, and cracks concrete; the durability of concrete is fundamentally controlled by permeability
How does steel fail (in terms of reaction equations)
Anodic reaction: Fe(0) –> Fe2+ + 2e-
Cathodic reaction: H2O + O2 + 4e- –> 4OH-
Fe2+ + 2OH- – > Fe(OH)2 (This is the start of rust appearance)
Why does chloride make steel failure worse?
Because it corrodes steel; it enlarges the corrosive region
- Fe oxides form a passive film on the steel
- Breakdown of this film leads to Fe corrosion
What is the chemistry of steel corrosion?
• Passive film breaks down if:
– pH drops
– Attacked by chloride
• Service life of concrete often defined as the time taken for the Cl- to diffuse to the steel & initiate corrosion
(– Or some point beyond this when corrosion causes cracking)
Chlorides, acid, carbonation can cause corrosion in the steel; how can we prevent this from occurring?
–> Reduce permeability
- Permeability depends on porosity, porosity
depends on water content
–> Reduce water/cement ratio for better durability
- This is of course an oversimplification, but actually
not a very bad one, and is used in many standards
- Chemical additives (superplasticiser/high range water
reducer) can help reduce w/c while retaining good flow
What is assumed to be the key limiting factor in concrete serviceability life?
Chloride permeability
What helps to keep chloride out?
Dense binder
– Low water/cement ratio
– Lots of C-S-H
– Refined pore structure (small, tortuous pores)
• Pozzolanic reactions really help this in the long term
–> blended cements give good durability
– Producing more C-S-H from portlandite (portlandite doesn’t restrict chloride movement)
– Extra AFm phases help a little (chloride binding slows
down its movement), but not as much as pore filling
by extra C-S-H
When does chloride corrosion occur?
- Cold and warm environments
* Steel rusts, expands, cracks concrete
What are ponding tests?
Testing chloride corrosion:
– Make a concrete cylinder or slab, put a pool of chloride solution (usually NaCl) on top, and wait
– After several months (6-24), measure how far the chloride has travelled into the material
– Use this to calculate the “diffusion coefficient”
What are the advantages and disadvantages of testing for chloride corrosion (ponding tests)?
• Advantage – generally accurate
• Disadvantage – very slow, labour intensive
– Want to get answers faster than this
– Use electricity to force chloride to move faster, and use this to calculate material parameters
What is the rapid chloride permeability test?
• ASTM C1202 – apply a voltage and measure current passed by the specimen in 6 h, use this as a measure of permeability
What is the rapid chloride permeability test actually measuring?
• Test is actually a resistivity test
– Measuring the electrical properties of the specimen, and
assuming that this relates to chloride diffusion
Hybrid methods to test chloride permeability:
• Give the advantage of a 24-hour testing time, but without needing to assume things about the resistivity
of the material
– Can compare different types of cements
– More reproducible (RCPT test has a ±42% error margin
according to ASTM standard)
• But all tests use saturated (underwater) material – no
splashing/drying effects, which are important in reality
What happens when an internal or external sulphate attack occurs?
• Internal sulphate attack
– Excess sulphate (SO4 2-) within concrete (e.g. contaminated aggregates) causing slow accumulation of damage
• External sulphate attack (more common)
– Sulphate from the environment entering material and
causing chemical changes in the cement paste
–>Result is expansion and cracking
What happens in a sulphate attack?
AFm –> AFt conversion is the key mechanism
– External MgSO4 also removes calcium from C-S-H to form soft, low-strength phases, which is doubly damaging!
How can you resist a sulphate attack?
Use slag blends or low-C3A clinker to resist sulphate attack – many cements sold as “sulphate-resistant”
How do you test sulphate resistance
• Immerse the concrete in a sulphate-rich solution
– (usually 5% Na2SO4)
– Measure specimen length regularly
• Testing for conversion of AFm to AFt phases
– Not enough to explain all expansion, but important
– Additional damage from “crystallisation pressure” effects
What are sulphate-resistant cements?
• Low C3A content, or high slag content
– Favour formation of C-S-H rather than AFm phases during hydration, so don’t expand
– High slag content also reduces permeability in the long-term
What is a Thaumasite sulphate attack?
Fairly rare; occurs in cool (4-10 degrees) and wet conditions with both carbonate and sulphate
Thaumasite: Ca3Si(OH)6(SO4)(CO3) . 12H2O
– C-S-H converted to thaumasite becomes ‘mush’ – like porridge
– No strength at all – soft
What are alkali-aggregate reactions?
• Portland cement contains a small quantity of alkalis (mainly K, some Na)
– Remains in the pore solution upon hydration
– Pore solution pH is very high – up to 13.5
• If the aggregate contains reactive (e.g. glassy,
opal etc.) components, it can be attacked by
the pore solution
– Chemical reaction at the aggregate surface
– Sometimes also called ‘alkali-silica reaction’
What are Alkali-silica reaction products?
Makes an expansive, white silicate gel product
How are alkali-silica reaction products identified?
Reaction of alkalis with silica from aggregates causes concrete to expand
• Characteristic ‘map-cracking’ on concrete surface
How do you test for alkali-aggregate?
– Appendix X1.3 of ASTM C33 lists 8 different methods for
combinations of cement, SCM and siliceous aggregates
(plus 3 more for carbonate aggregates)
• UK approach follows BRE Digest 330 (4 parts)
– Limits on alkali content of concrete based on aggregate
reactivity classification (low/normal/high) from rock type
– Concrete prism test (similar to ASTM method below) if
uncertain – 12 months duration
How do you test alkali-aggregate quickly?
What are the advantages and disadvantages of this method?
ASTM C1260, mortar bars in 40 g/L NaOH at 80°C, measure expansion at 16 days
- Because alkali concentration is so high, there is
no influence from the alkalis in the cement
- Advantage is its fast!
- Disadvantage is that its a very aggressive test so often gives false positives
How do you test both aggregate and cement for alkali reactivity?
ASTM C1293 – concrete prism test
– Extra alkali added into the cement (double the normal
limit of 0.60% Na2O or molar equivalent), then store at 38°C for 1 year (to show excessive expansion), or 2 years (to show no expansion problems) –> Slow
-Considered the most reliable test
Carbonation testing
-Interaction in atmospheric CO2 can cause problems; CO2 acts as an acid
Ca(OH)2 + H2CO3 –> CaCO3+ 2H2O, portlandite consumed
– Reduces the alkalinity (pH) of the cement, which can
induce corrosion of the steel reinforcing
– Extreme cases of carbonation can also show damage
(decalcification) in C-S-H phases
– Happens fastest at intermediate humidity (~65%) or
under wet-dry cycling
• Generally want to measure the depth of CO2
penetration into the concrete toward steel
– Rate of ingress under natural conditions is ~mm/yr, so
use higher CO2 concentrations to accelerate the test
How do you measure carbonation?
Phenolphthalein is a useful indicator of pH change
– Pink when conditions are alkaline (pH >12)
– Colourless when pH drops below 9
– Colour change corresponds well to ‘danger levels’ for
alkalinity in concrete leading to steel corrosion
• Measure depth of CO2 ingress after exposure to
elevated concentration, and scale this to predict
performance in natural conditions
What is freeze thaw damage?
When it freezes, water expands ~9%
– salt makes this worse – more dramatic volume change
– external surface of the material is damaged/removed
What is freeze thaw testing?
Freeze-thaw cycle repeatedly (e.g. +4/-18oC every 4 h, or +20/-18oC every 24 h)
– Tens to hundreds of cycles used
– Measure changes in elastic modulus, dimensions, mass (material scaled from surface)
– Sometimes just give a visual rating of damage
How do you protect against freeze thaw?
Put bubbles in concrete; enough space for water to freeze and expand and not damage (< 1 mm, a few % by volume, well spaced)
Comment on different types of testing
• Be careful with curing regimes
• Be careful with sample pre-conditioning
– Blended cements can crack under the harsh drying regimes specified for pure PC
• Be careful with differential ageing of specimens in long-duration tests
– Maturity of the material tested on day 1 will be very different to the material tested on day 730
• Look at precision statements of the tests
What is destructive mechanical testing?
Controlled loading until it breaks
Strength is an extrinsic property
– Depends on the sample, not an intrinsic material property!
• Concrete strength grades specified in compression
– British/European standard BS EN 12390-3
– E.g. C40/50 – 40 MPa cylinder / 50 MPa cube @ 28 days
– Concrete cubes 100-300 mm, or cylinders (aspect ratio 2.0) 100-300 mm diameter
– Cylinders have less restraint on faces, so fail at lower loads – appear to be less strong
– Smaller specimens are stronger; failure is at largest flaw
– Loaded faces MUST be flat
What is the best way to perform mechanical testing?
- Direct compression
- Non-straight samples fail in undesirable ways – discard these results
What is the EU standard for Mortar strength testing?
Cements are standardised (EN 197-1) and sold according to strength grade – e.g. CEM II/A-V 42.5R is a common one
– CEM II/A-V is a type of fly ash blended cement (material
type codes will be defined in detail in the lecture on standards)
– Grade is defined as 28-day compressive strength in
MPa (42.5 here 42.5 MPa @ 28 d)
– On the end of the code, N means normal strength
development, and R means rapid strength development
(criteria are defined by 2-day strengths)
What are the standard Mortars?
Europe uses EN 196-1 for mortars: 1:3 ratio of cement:sand, water/cement ratio 0.50
• Mortar prisms 160x40x40 mm, broken in 3-point flexion (see next slide), then each end tested in compression as a ‘pseudo-cube’
• Details of mixer, moulds, curing etc. all specified
What is tensile/ flexural testing
Use splitting tensile tests; 3-point or 4-point
– Bending tests give strengths ~40-80% higher than
splitting tensile
Correlate the strengths to measurements
Generally correlate it to 28-day strength called fc. IF it contains fly ash or slag use 56 day instead as they gain strength more slowly.
Exponent is between 0.5-0.7, varies between standards
– In Eurocode 2: axial tensile strength fctm = 0.30 fck2/3
fck is the characteristic compressive strength, fcm is the mean
Non-destructive testing
Not always necessary to break the concrete
• Electrochemical testing to check condition of reinforcing steel
– Can detect corrosion currents
• Radiographic or radar-based methods (covermeter)
• Air or water permeability
Surface hardness:
- Schmidt (rebound) hammer
- Windsor (penetration) probe
- Pullout test
How could you test for elastic modulus and the presence cracks and voids?
Ultrasonic pulse velocity – how well does the microstructure of the material transmit ultrasound?
Creep testing
• Concrete creep is mainly caused by C-S-H nanogranules in microstructure
– Sliding over each other (very slowly)
– Becoming compressed/deformed under load
• Long-term test; usual duration 12 months
• Load samples at different ages (2 – 90 d) to get a better understanding of effects of binder maturity
– Concretes with slow-reacting SCMs (particularly fly ash)
can creep much more than CEM I if loaded at early age
• Need to be careful of differential creep in structures (steel and concrete creep differently)
Why is creep in concrete not always harmful?
– Releases some of the stresses in a material, particularly
when loaded in tension
– A material with low tensile strength and low creep will
crack much more than one that creeps more
Types of portland cement; British & European Standard BS EN 197-1
• 27 different types of sub-types of cement, 5 categories:
– CEM I Portland cement (≥95% Portland clinker)
– CEM II Portland-composite cement (65-94% Portland + 1 pozz./limestone)
– CEM III Blastfurnace cement (5-64% Portland + slag)
– CEM IV Pozzolanic cement (45-89% Portland + 1 pozz.)
– CEM V Composite cement (20-64% Portland + slag + 1 other SCM)
Concrete classes
• ‘Strength classes’ also defined for concretes, based on 28-day cube & cylinder strengths
– E.g. C40/50 : 40 MPa cylinder, 50 MPa cube @28d
• ‘Exposure classes’ are used to describe the environments in which concretes are used
– Concrete cubes 100-300 mm, or cylinders (aspect ratio 2.0) 100-300 mm diameter
– Cylinders have less restraint on faces, so fail at lower loads – appear to be less strong
– Smaller specimens are stronger – failure is at largest flaw
– Loaded faces MUST be flat
If concrete has high strength, low w/c, assume …
Good durability
ASTM approach to standards
• Prescriptive standards
– ASTM C150 – Portland cement (defines 5 types of cement): Prescriptive standard for cement composition,
defines clinker composition
– ASTM C595 – Blended hydraulic cements
• Performance-based standard
– ASTM C1157 – ‘Standard Performance Specification for Hydraulic Cement’
Comments on C1157
• Pure performance-based standard
– Very few prescriptive requirements
• Not yet really trusted in practice
• Seen as a way forward for alternative cements – we want to follow a similar route
– Useless to base a standard on tests that don’t apply to the materials we want to test
Examples of innovation in concrete
- Fibre reinforced concrete
- Ultra performance concrete
- Self healing concrete (with CaCO3)
- Nanotechnology
- Additive manufacturing with concrete (3D printing)
How can you test the water content of concrete?
- Slump testing
- Place concrete into steel cone on solid surface in 3 equal layers
- Ensure concrete compacted and carefully lift cone so that concrete slumps
- Measure distance from top of cone to top of slump to nearest 10mm
Name three types of water reducer commonly used in concrete (past paper question)
Melamine sulfonate, lignosulfonate, polycarboxylate
What is bleeding?
When water is pushed up through the concrete due to the settlement of larger particles.
Bleeding is a consequence of plastic settlement.
How can bleeding and plastic settlement be mitigated?
Reduce water content. Use lower slump mix Use finer cements Increase amount of fines in the sand Use supplementary cementitious materials Use air entraining admixtures
In what situation would a slag-blended Portland cement (e.g. the CEM III/B class in BS EN 197-1) be expected to be preferable to the performance of a plain Portland cement (CEM I in BS EN 197-1)?
Slag blended cements would be preferable when using in environments prone to sulphate or chlorine attack.
In offshore/marine developments, sewage and waster treatment facilities.
Better for the environment as using waste material: blast furnace slag.
