Non-Metals Flashcards
What is masonry?
Blocks of brick held together with mortar
What are the advantages of using masonry?
-Usually locally available stones or clay
-Tends to give high thermal mass
-Usually durable (>500 yrs)
-Usually high fire resistance
What are the disadvantages of using masonry?
-Requires manual labour, hard to mechanise
-Difficult to make very tall structures
-Heavy, needs thick and strong foundation
-Tensile/flexural strength is very limited
-Compressive strength of bricks can also be a limitation
-Unreinforced masonry has low resistance to shearing forces in earthquakes
What material is brickwork?
Usually made from baked clay, iron clay gives a red colour
What is the compressive strength of brickwork?
Can be up to 100 MPa but usually around 20-40 MPa
What is the composition of mortar?
Cement + sand + water
Historically lime was used instead of cement, and is still sometimes used today
What is the strength of mortar under load?
Usually the weakest point under shear load
What is the standard thickness of mortar?
5-10 mm
What are concrete masonry units (CMU) made out of?
Fly ash cement, usually precast with fairly fine aggregates
What does CMU provide a structure?
Light weight and thermal insulation
What is the average compressive strength of CMU?
~20 MPa
What are the structural uses for the materials used to make brick?
-Sand is used for dimensional stability
-Water is needed for effective moulding
-Organic matter or lime accelerates firing
What is the water content of soft mud bricks?
25-30% water - meaning they have more variability
What is the water content of dry press bricks?
~8%, more expensive but give a better product
What is the water content of extruded bricks?
~12%, they are cut by wires from a column of clay, making them very cheap
What is the compressive strength of class A engineering brick BS EN 771?
125 MPa
What is the compressive strength of a normal brick?
15-35 MPa
What is the water absorption of class A engineering brick BS EN 771?
<4.5%
What is the water absorption of normal brick?
~20-30%
Describe efflorescence.
-Moisture travels through masonry and evaporates, leaving salt on surfaces or salt components are present from mortar or from within the clay/concrete
-Avoid this by keeping water out from masonry
-Not usually a problem but aesthetically an issue.
How is glass usually made?
By supercooling a liquid
Compare the theoretical and actual structural properties of glass.
Theoretical flaws could be very strong.
-Theoretical bending strength ~14 000 MPa
-Actual bending strength 20-200 MPa
How is the strength of glass determined?
By the size of the largest flaw
What is the process of tempering glass?
Heating a sheet of glass, then quenching surfaces rapidly
Describe how tempering glass works.
-Quenching puts the surface into compression and the centre in tension, strengthening the surface
-To break tempered glass, a crack must form between the surface region into the core region
-Fracture leaves cubic fragments due to stored strain energy
How is laminated glass made?
A polymer film is sandwiched between two glass sheets
Describe how laminated glass works.
-Polyvinyl-butyral is the most common film used which is rolled out and heated to exclude air and bond polymer to glass
-Protects surfaces and holds in the broken glass fragments is fracture occurs, cracks in a spider web pattern
-Used a lot in car windscreens and other places that are likely to break
How is bullet proof glass made?
Multi-layered tempered glass/polycarbonate laminate, or multiple layers of laminated glass ~20-75 mm thick
Describe how bullet proof glass works.
-Optical properties of materials must be matched
-Polymer layers are much more elastic than glass
How is multiple glazed glass made?
2 or 3 sheets of glass are sealed into a unit, with a gas layer in between
Describe how multiple glazing glass works?
-Sometimes a vacuum can be used if the temperature differential is not going to be too high
-Desiccants are sometimes used in the gas space
-Each interface gives resistance to heat transfer
-Need similar thickness of glass inside and outside to prevent shearing
-Temperature mismatch between inside and outside can cause cracking
-Dimensional changes in units can break seals around unit edges and let moisture in
Describe three properties of cement.
-Binds solid aggregates together by hardening from a plastic state
-Has both adhesive and cohesive properties
-Functions by forming a plastic paste when mixed with water, which develops rigidity and then steadily increases in compressive strength by chemical reactions with water
What is Portland cement?
A hydraulic cement produced by pulverising clinker, consisting of crystalline hydraulic calcium silicates, usually containing one or more of the following: water, calcium sulphate, up to 5% limestone, processing additions
How much of the worlds concrete uses Portland cement?
> 95%
Give the following compounds abbreviations: Calcium oxide (CaO), Silicon Dioxide (SiO2), Aluminium Oxide (Al2O3), Iron Oxide (Fe2O3), Water (H2O), Sulphur Trioxide (SO3), Carbon Dioxide (CO2), Sodium Oxide (Na2O), Potassium Oxide (K2O)
CaO - C
SiO2 - S
Al2O3 - A
Fe2O3 - F
H2O - H
SO3 - S(bar)
CO2 - c
Na2O - N
K2O - K
Describe Alite clinker.
(C3S) Hardens rapidly, develops high strength
Describe Belite clinker.
(Beta-C2S) Hardens slowly, develops high strength
Describe Aluminate clinker.
(C3A) Reacts with gypsum, products are of low solubility
Describe Ferrite clinker.
(C4AF) Similar of C3A Aluminate but not as rapid
Describe Calcium Silicate Hydrate (C-S-H).
-Formed by the hydration of Alite and Belite
-Makes up to 70 wt% of fully hydrated phase
-The major strength contributing phase
Describe Calcium Hydroxide/Portlandite (C-H).
-By product of Alite and Belite hydration (they have more Ca than can be accommodated for in C-S-H)
-Makes up to 20 wt% of fully hydrated phase
-Does not contribute to strength
Describe Ettringite.
-Direct reaction of Aluminate with water leads to flash set (stiffening without strength development) forming aluminate hydrates
-By adding gypsum, aluminate reacts moderately an forms ettringite (Alumina ferric oxide tri-sulphate)
Why is mineral component tat added to cement?
To improve performance and reduce cost
What are supplementary cementitious materials (SCMs)?
Mineral component tat (NOT AGGREGATES)
What is a Pozzolanic reaction?
Silica and alumina component reacts with portlandite to produce more C-S-H
What is the point of carrying out pozzolanic reactions?
C-S-H fills pore spaces at longer age, improving strength and durability
How fast is a pozzolanic reaction?
Generally slower than cement hydration
What happens when cement is substituted for pozzolans?
Early strength is reduced
What physical properties change when the cement is blended?
-Extra C-S-H fills in space, which gives extra strength
-Chemical durability improves
Give some examples of Pozzolanic SCMs.
-Fly ash
-Volcanic earth
-Calcines clays
-Silica fumes
-Rice husk ash
-Waste silicate glass
Give two examples of non-pozzolanic SCMs.
-Limestone
-Blast furnace slag
What are the components of cement paste?
cement + water
What are the components of mortar?
cement + fine aggregates (<5 mm) + water
What are the components of concrete?
cement + fine aggregates + coarse aggregates + water
What are aggregates?
Inert additives which can be bound together with cement to make concrete
What are the three main roles of adding aggregates to a cement?
-Can increase strength
-Can reduce weight
-Higher integrity
How do aggregates change a concretes strength?
-A concrete cannot be stronger than the aggregate, because it fails at its weakest point
-Need good quality, clean, strong aggregates
-Clay contamination tends to have bad effects
How do aggregates change a concretes weight?
Lightweight aggregates for lightweight concretes
How do aggregates change a concretes integrity?
Reduces thermal cracking (dilution of cement)
Why is water important in concrete?
-Required to make cement hydration reactions
-Makes concrete flow (increased slump)
What happens if too much water is added to a concrete?
-Delays setting/hardening
-Forms extra pores, which reduces durability and strength
Describe the flow of concrete?
Shear-thinning, which means it flows more readily when a force is applied, this is why mixing trucks are kept rotating.
Self-compacting concrete flows like a liquid with no vibration, this is achieved by adding chemicals.
What causes the shear-thinning nature of cement?
Needle shape of ettringite crystals
What causes plastic shrinkage cracking?
-Rapid evaporation from the surface makes the paste shrink
-Evaporation also causes drying shrinkage cracks
How can shrinkage and cracking be reduced?
Aggregate particles stay in place and prevent shrinkage that causes cracking/crazing
What causes plastic settlement?
Solid aggregate particles can sink through the paste, leaving water pockets under aggregates and reinforcing bars and cracks on the surface
What is an expected water/cement mass ratio for a regular concrete?
0.3-0.7
Name the main 6 non-Portland cements.
-Calcium aluminate cement (CAC)
-Calcium sulpho-aluminate cement (CSA)
-Magnesium oxychloride cement
-Magnesium phosphate cement
-Alkali-activated (geopolymer) cement
-Bitumen/Tar
What are the physical properties of Magnesium oxychloride cement?
-Very high early strength (>70 MPa after 3-7 days)
-Sensitive to water
-Useful for indoor floors, tiles, artificial ivory and billboard balls
What are the main physical properties of Alkali-activated (geopolymer) cement?
-Can use blast furnace slag or pozzolans
-60-90% less carbon emissions than Portland cement
-Need an alkaline solution
What is Bitumen?
A mixture of heavy organic molecules, solid at room temperature
What are the main physical properties of Bitumen?
-Used to bind stones/gravel together into solid hardened material
-Is technically a binder not a cement
-Use heat to soften and make flowable
Compare how good concrete and steel are in tension and compression.
Concrete is strong in compression but weak in tension, steel is very good in tension
What kind of steel is used for reinforcements and why?
Usually mild (but not stainless) steel, as they tend to not only be cheaper but can be chemically altered to be corrosion resistant, they are often ribbed which creates a stronger bond to the concrete
What are pre-tensioned cables?
Cables are installed, so when concrete is poured over the tension is released - this relies on interfacial bonding to steel
What are post-tensioned cables?
Concrete is poured and then the cables are inserted and tensioned - this can have severe problems is the steel corrodes and the stress application is lost
What is the equation for rusting?
Fe2+ + 2OH- => Fe(OH)2
What causes iron corrosion?
Iron oxides form a passive film on the steel inside the concrete, which breaks down and leads to corrosion
How can corrosion be prevented?
Reduce permeability of concrete
How do you reduce the permeability of concrete?
Reduce the water/cement ratio, by using superplasticisers
What are the specifications for testing in compression?
-Cubes (100-300 mm) or cylinders (100-300 mm diameter, with aspect ratio 2.0)
-Loaded faces must be flat
-Smaller specimens are stronger, because failure occurs at largest flaw
-Cylinders fail at lower loads due to the less restraint on faces
What is an extrinsic property of concrete?
Strength (depends on sample size)
What are the intrinsic properties of concrete?
Density, melting point, temperature
Why are specimens not often tested in tension?
Tension tests are sensitive to specimen geometry, size and loading rate
What is creep testing?
Stressing a material at a load well below failure for a long time
What causes concrete creep?
C-S-H nanogranules in the microstructure sliding over each other and becoming compressed/deformed under load
How should a creep tests samples be arranged?
-Test the samples for around 12 months
-Load samples at different ages (2-90d) to get better understanding of effects of binder maturity
How does creep levels effect cracking?
Low tensile strength and low creep will crack more
What is electrochemical testing used for?
To check the condition of reinforcing steel (detect corrosion currents)
What are radiographic or radar-based methods used to determine?
To identify the location of reinforcement, the condition of the cover concrete, or large cracks
What is the Schmidt (rebound) hammer test?
To see how much the hammer bounces off the surface
What is the Windsor (penetration) prove used for?
To see how far into the surface can a projectile penetrate
What is the pullout test used for?
A bolt is screwed into a drilled hole, and the difficulty to pull it out is measured
What is the ultrasonic pulse velocity test used for?
To see how well the microstructure of the material transmits ultrasound
How can Chloride corrosion be prevented in cold regions?
Salts are applied to concrete for de-icing
What is the main ways to prevent Chloride attack?
-Reduce permeability
-Denser binder
-Pozzolanic reactions help in the long term (by producing more C-S-H from Portlandite)
What effect does chloride have on steel?
Makes it rust
Describe the ponding test for Chloride permeability.
-Concrete cylinder or slab, with a pool of chloride solution on top, and wait several months
-Measure how far chloride has travelled into the material, to calculate the diffusion coefficient
What are the advantages and disadvantages of the ponding chloride permeability test?
-Simple and accurate
-Very slow and labour intensive
Describe the rapid chloride permeability test.
Apply voltage and correlate the current passed by the specimen in 6 hours to chloride diffusion
What parameters does the rapid chloride permeability test depend on?
-Chloride ions present
-Other dissolved ions in the pore solution
What is a disadvantage of the rapid voltage permeability test?
Cannot be used to compare materials with different chemistry, but is useful for quality control of one material
Describe the hybrid method to test chloride permeability.
Use electrical acceleration of chloride mitigation, but split the sample and measure actual chloride depth at the end of the test
What are the advantages of the hybrid chloride permeability test?
-24 hour testing time, without dealing with the resistivity of the material
-Can compare different types of cement
-More reproducible
What are the two reaction pathways for sulphate attack in concrete?
-Internal: Sulphates present within concrete, causing phase evolution after a long period
-External: Sulphates from the environment, entering the material and causing phase changes
Describe a Thaumasite Sulphate attack.
-Needs cool, wet conditions, with both carbonates and sulphates present
-C-S-H is converted to thaumasite
-Concrete loses its strength and becomes soft and ‘mushy’
-Relatively rare worldwide, but important in UK conditions
How do you test for Sulphate resistance in concrete?
-Immerse the concrete in a sulphate rich solution (usually 5 wt% Na2SO4)
-Measure specimen length regularly
-Testing for conversion of AFm to AFt phases
-Crystallisation of sulphate salts can cause additional (physical) damage
What are the properties of Sulphate resistant concretes?
-Low Alite content, or high slag content
-Favours the formation of C-S-H rather than AFm phases during hydration, so does not expand
-High slag content also reduces permeability in the long-term
What is an alkali-aggregate reaction?
When the aggregate contains reactive components, it can be attacked by the pore solution (Portland cement contains small amounts of Na and K that stay in the pore solution)
What are the main Alkali guidelines put in place in the UK?
-Limits om alklai content of concrete based on aggregate reactivity classification
-Most cements sold in the UK are low-alkali
-Blending with slag or pozzolans helps protect from damage
Describe the accelerated testing method (ASTM).
Mortar bars are places in 40 g/L NaOH at 80 degrees, and the expansion is measured after 16 days
What are some advantages and disadvantages of the accelerated testing method?
-Adv: Fast and widely used
-Disadv: Known to give false results (because its an extremely aggressive test)
What are the issues with carbonation?
-Atmospheric CO2 can act as an acid, which reduces the pH of cement which can induce corrosion of the steel reinforcements
-Happens fastest at intermediate humidity (~65%) or under wet-dry cycles
-The main limitation on concrete service life in warm climate areas without marine exposure
How do you measure carbonation?
Measure the depth of CO2 ingress after exposure to CO2 (often at a higher concentration)
Why could carbonation be a positive thing?
It is a possible route to global CO2 reduction, reversing some of the emissions from cement production
What is freeze-thaw damage?
Degradation based on the ingress of water, when it freezes water expands ~9% and even more when higher salt levels are present
What happens during a freeze-thaw test?
-Samples (usually cylinders or slabs) experience freeze-thaw cycles repeatedly
-Tens to hundreds of cycles used
-Measure changes in elastic modulus, dimensions, mass
-Sometimes just give a visual rating of damage
How can freeze-thaw damage be prevented?
-Put appropriately sized bubbles into concrete
-Gives the water space to expand into as it freezes
Name some of the activities covered by the material standards.
-Construction
-Nanotechnology
-Energy management
-Health and safety
What forms can material standards come in?
-Guides
-Codes of practise
-Specifications
-Supporting documents
What are prescriptive standards?
-Specify ingredients and recipes
-If a material is made as specified it is assumed to be okay
-Need to build in safety margins to be conservative
What are performance-based standards?
-Define what a material needs to do, including how to test
-Allow the producer to design a material to meet these requirements
-More scope for innovation, but it takes a lot more work to prove the sufficient performance
What speciality products tend to have purpose-written standards?
-Roof tiles
-Masonry blocks
-Paving slabs
-Tile grout
What is the structure of European standards of concrete?
-Family tree of standards
-Strong cement/concrete industry influence in drafting
-Cascading set of requirements for materials to be used in concrete
-Many testing methods are integrated into material standards
What is the composition of CEM I Portland cement?
> 95% Portland clinker
What is the composition of CEM II Portland-composite cement?
65-94% Portland + 1 pozz./limestone
What is the composition of CEM III Blast furnace cement?
5-64% Portland + slag
What is the composition of CEM IV Pozzolanic cement?
45-89% Portland + 1 pozzolana
What is the composition of CEM V Composite cement?
20-64% Portland + slag + 1 other SCM
What is ASTM C150?
Portland cement
What is ASTM C595?
Blended hydraulic cements
What is ASTM C1157?
Standard performance specification for hydraulic cement
