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MAT2600 > Concrete > Flashcards

Concrete Flashcards

1
Q

What is the definition of paste?

A

Cement + water
– Rarely used alone, usually
combined with aggregates

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2
Q

What is the definition of mortar?

A

Paste + sand (‘fine aggregate’, <5 mm)

– Used to join bricks together, or as a coating

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3
Q

What is the definition of concrete?

A

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

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4
Q

Why is water important in concrete?

A

Two main reasons water is important:
– Required in cement hydration reactions
– Makes concrete flow (increased slump)

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5
Q

Why is too much water bad in concrete?

A

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

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6
Q

What are the effects of water on strength?

A

• 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

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7
Q

When does segregation occur?

A

• Need a cohesive mix
– Low water content (often with plasticisers)
– Fine aggregate helps avoid segregation

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8
Q

What is plastic shrinkage cracking?

A

• 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

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9
Q

What is plastic settlement?

A

-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

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10
Q

Other binder for portland cement: Alkali-activated (geopolymer) cements

A

• 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

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11
Q

Other binder for portland cement: Calcium aluminate cement

A

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

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12
Q

Other binder for portland cement: Magnesium oxychloride cement

A

• “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

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13
Q

Other binder for portland cement: Bitumen concretes

A

• 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

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14
Q

Is concrete strong or weak in compression?

A

Concrete is strong in compression but weak in tension

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15
Q

Why do we need steel reinforcement?

A

Because steel is strong in tension which concrete isnt

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16
Q

Why does steel reinforcement use mild steel instead of stainless steel?

A

– 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)

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17
Q

More steel isn’t necessarily better

A

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.

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18
Q

What is prestressing?

A

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

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19
Q

What is the main cause of concrete failure?

A

Steel failure as when steel rusts, it expands, and cracks concrete; the durability of concrete is fundamentally controlled by permeability

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20
Q

How does steel fail (in terms of reaction equations)

A

Anodic reaction: Fe(0) –> Fe2+ + 2e-
Cathodic reaction: H2O + O2 + 4e- –> 4OH-
Fe2+ + 2OH- – > Fe(OH)2 (This is the start of rust appearance)

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21
Q

Why does chloride make steel failure worse?

A

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
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22
Q

What is the chemistry of steel corrosion?

A

• 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)

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23
Q

Chlorides, acid, carbonation can cause corrosion in the steel; how can we prevent this from occurring?

A

–> 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

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24
Q

What is assumed to be the key limiting factor in concrete serviceability life?

A

Chloride permeability

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25
Q

What helps to keep chloride out?

A

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

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26
Q

When does chloride corrosion occur?

A
  • Cold and warm environments

* Steel rusts, expands, cracks concrete

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27
Q

What are ponding tests?

A

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”

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28
Q

What are the advantages and disadvantages of testing for chloride corrosion (ponding tests)?

A

• 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

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29
Q

What is the rapid chloride permeability test?

A

• ASTM C1202 – apply a voltage and measure current passed by the specimen in 6 h, use this as a measure of permeability

30
Q

What is the rapid chloride permeability test actually measuring?

A

• Test is actually a resistivity test
– Measuring the electrical properties of the specimen, and
assuming that this relates to chloride diffusion

31
Q

Hybrid methods to test chloride permeability:

A

• 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

32
Q

What happens when an internal or external sulphate attack occurs?

A

• 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

33
Q

What happens in a sulphate attack?

A

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!

34
Q

How can you resist a sulphate attack?

A

Use slag blends or low-C3A clinker to resist sulphate attack – many cements sold as “sulphate-resistant”

35
Q

How do you test sulphate resistance

A

• 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

36
Q

What are sulphate-resistant cements?

A

• 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

37
Q

What is a Thaumasite sulphate attack?

A

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

38
Q

What are alkali-aggregate reactions?

A

• 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’

39
Q

What are Alkali-silica reaction products?

A

Makes an expansive, white silicate gel product

40
Q

How are alkali-silica reaction products identified?

A

Reaction of alkalis with silica from aggregates causes concrete to expand
• Characteristic ‘map-cracking’ on concrete surface

41
Q

How do you test for alkali-aggregate?

A

– 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

42
Q

How do you test alkali-aggregate quickly?

What are the advantages and disadvantages of this method?

A

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

43
Q

How do you test both aggregate and cement for alkali reactivity?

A

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

44
Q

Carbonation testing

A

-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

45
Q

How do you measure carbonation?

A

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

46
Q

What is freeze thaw damage?

A

When it freezes, water expands ~9%
– salt makes this worse – more dramatic volume change
– external surface of the material is damaged/removed

47
Q

What is freeze thaw testing?

A

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

48
Q

How do you protect against freeze thaw?

A

Put bubbles in concrete; enough space for water to freeze and expand and not damage (< 1 mm, a few % by volume, well spaced)

49
Q

Comment on different types of testing

A

• 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

50
Q

What is destructive mechanical testing?

A

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

51
Q

What is the best way to perform mechanical testing?

A
  • Direct compression

- Non-straight samples fail in undesirable ways – discard these results

52
Q

What is the EU standard for Mortar strength testing?

A

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)

53
Q

What are the standard Mortars?

A

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

54
Q

What is tensile/ flexural testing

A

Use splitting tensile tests; 3-point or 4-point
– Bending tests give strengths ~40-80% higher than
splitting tensile

55
Q

Correlate the strengths to measurements

A

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

56
Q

Non-destructive testing

A

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
57
Q

How could you test for elastic modulus and the presence cracks and voids?

A

Ultrasonic pulse velocity – how well does the microstructure of the material transmit ultrasound?

58
Q

Creep testing

A

• 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)

59
Q

Why is creep in concrete not always harmful?

A

– 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

60
Q

Types of portland cement; British & European Standard BS EN 197-1

A

• 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)

61
Q

Concrete classes

A

• ‘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

62
Q

If concrete has high strength, low w/c, assume …

A

Good durability

63
Q

ASTM approach to standards

A

• 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’

64
Q

Comments on C1157

A

• 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

65
Q

Examples of innovation in concrete

A
  • Fibre reinforced concrete
  • Ultra performance concrete
  • Self healing concrete (with CaCO3)
  • Nanotechnology
  • Additive manufacturing with concrete (3D printing)
66
Q

How can you test the water content of concrete?

A
  • 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
67
Q

Name three types of water reducer commonly used in concrete (past paper question)

A

Melamine sulfonate, lignosulfonate, polycarboxylate

68
Q

What is bleeding?

A

When water is pushed up through the concrete due to the settlement of larger particles.
Bleeding is a consequence of plastic settlement.

69
Q

How can bleeding and plastic settlement be mitigated?

A 
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
70
Q

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)?

A

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.

MAT2600 (5 decks)

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