Non-metals Flashcards
Why are unreinforced masonry walls not used in seismically active regions?
Unreinforced masonry has very little resistance to shearing forces.
What are the positives and negatives of using masonry in construction?
\+) use locally sourced stone/ clay \+) high thermal mass \+) durable (>500 years) -) manual labour, hard to mechanise -) difficult to make tall structures- heavy, low tensile
Which is more expensive, a dry pressed brick or a soft mud process brick? Which is more porous?
dry pressed cost more. less porous porous due to less water content.
What makes many bricks red?
iron, after being baked ≈950’c
Does a high-quality engineering brick show high or low water absorption?
Lower, <4.5% compared to normal, ≈20-30%
Which is more likely to show efflorescence: a brick that is kept completely dry, partially wetted, or immersed in water?
moisture travels through masonry and evaporates, depositing salts on the surface. Avoided by keeping water out.
Why does rapid cooling give a glass in some cases, while slow cooling gives an ordered structure?
Slow cooling allows equilibrium phase change, giving a crystal with ordered structure. Fast cooling creates a non-equilibrium transition.
Float glass is prepared using a bath of liquid tin. What key property does this give to the glass?
Flat surface as it sets on top of the liquid
Why can glass rarely achieve more than 1% of its theoretical strength?
Flawless glass could be very strong. Very weak in tension (always some tension in compression). H2O attack on chemical bonds in cracks makes it weaker under long-term loading (static fatigue).
Why is bullet-proof glass often laminated?
Polymer layers are much more elastic. On impact the glass will shatter but the polymer layer will deform elastically.
Why does a double-glazed window need the same thickness of glass on the inside and outside panes?
- To prevent shearing
- temperature mismatch can cause cracking
- dimensional changes can break seals, allowing moisture to get inside and cause fog and mould
Name the 4 major Portland cement clinker compounds, and describe briefly the role of each in Portland cement hydration
Tricalcium Silicate, C3S (Alite): early strength development, <7 days.
Dicalcium Silicate, C2S (Belite): longer term gain of strength, 28 days.
Tricalcium Aluminate, C3A (Aluminate): controls initial setting time.
Tetrecalcium Aluminoferrite, C4AF: reacts slower giving low early strength. controls melting in the kiln. gives grey colour.
Clinker is interground with another component to make cement. What is this component, and with which of the clinker compounds does it mainly react?
When cement is hydrated, gypsum reacts with tricalcium aluminate, C3A, to hinder hydration and delay the setting time of cement.
What happens when fresh cement paste is dried?
Slows or stops reactions causing cracking and loss of strength.
Identify the 4 main types of hydrate product which are important in Portland cement. Which of these are mainly responsible for: (a) setting, (b) final strength?
(a) Ettringite
(a) Calcium hydroxide (portlandite), Ca(OH)2
(b) Calcium Silicate hydrate, C-S-H
(b) C3A reaction products
Which natural mineral is used as a structural model to describe the C-S-H structure?
Tobermorite
Describe the chemistry of the pozzolanic reaction in an approximate chemical reaction, using cement chemistry nomenclature.
Blend cement with other sources of SiO2 (often waste). These react with CH to form more C-S-H. Results in filling pores at longer age and improves strength and durability.
Define the terms: paste, mortar, fine aggregate, coarse aggregate
Paste: Cement + water
Mortar: Paste + sand (fine aggregate)
Fine aggregate: <5mm
Course aggregate: 15-20mm
Give three reasons why aggregate is added to concrete
Improve strength, durability, fire resistance, control shrinkage and prevent cracking
Give three reasons why adding excessive water to concrete can be undesirable
Required in cement hydration and makes concrete flow. Too much results in:
-form extra pores, reducing durability and resistance
-reduce strength
-delay hardening
causes bleeding, segregation, plastic settlement
-increased drying shrinkage
Describe the process of plastic settlement
Solid particles can sink through the paste leaving water pockets under the aggregate and reinforcing bars causing cracks on surface.
Describe the process of segregation
water separates from the aggregate (non adhesive mix), fine aggregates help to avoid this.
Describe the process of plastic shrinkage cracking
water evaporation from the surface makes the paste shrink. Aggregate particles stay in place and restrain the shrinkage.
Describe the characteristic engineering properties of two non-Portland cements
Calcium Aluminate cement: -high early strength, used in prestressed components -very sensitive to water content Alkali-activated cements: -60-90% less CO2 emissions than portland -need alkaline solution
Does more steel reinforcement always make for a ‘better’ concrete? If not, why not?
Increases the tensile capacity but must not exceed the limit. Concrete must have enough space to flow around it.
What is the difference between pre-tensioning and post-tensioning?
Pre: Use pre tensioned cables to hold the bottom face of the concrete member in compression. (cables stretched, concrete poured, tension released).
Post: concrete poured with duct, cables inserted then tensioned.
Is prestressing of concrete used mainly to enhance its resistance to flexural load, or compressive load?
It adds a greater compressive load to enhance its resistance to fltexural load, tensional and bending forces.
What is a ‘passivating film’ in the context of concrete durability?
As a corrosion prevention method, passive film is a lightweight coat protecting against corrosion. Breaks down if pH drops or if attacked by chloride.
Why is the direct measurement of concrete tensile strength rare? Give two examples of alternative tests
Direct tensile concrete tests are not standardised. More often splitting tensile or bending strengths. ‘Bending tests’ give ≈ 40-80% higher than ‘Splitting tests’
When testing a concrete sample with ends that are not quite flat, will the measured strength be higher or lower than the expected value? Why?
Lower, the load is applied over something less than all of the cross-section area of the specimen. The strength is defined by force per area, so the load at failure will be lower when the force is being applied over a smaller area
Which characteristics of concrete are readily identified through ultrasonic pulse velocity testing?
Elastic modulus
What does a Schmidt (rebound) hammer measure?
Testing the surface hardness of the material. How much does it bounce off the surface?
Describe how the pozzolanic reaction can alter the durability-related properties of a concrete.
Pozzolanic reactions help in the long term:
- blended cements give good durability.
- producing more C-S-H from portlandite (doesn’t restrict chloride movement).
- Extra AFm phases help a bit (binding slows down its movement), but not as much as pore filling by C-S-H.
What is the main symptom of sulphate attack on cement?
External attack is more common than internal. AFm -> AFt. magnesium sulphate removes calcium from C-S-H to form salts. Causes loss in strength, expansion and spalling of surface layers
List two factors which can contribute to Thaumasite formation in concrete
Needs cool, wet conditions, with both carbonate and sulphate.
Describe two different approaches to testing the chloride permeability of concrete, and give one advantage and one disadvantage of each
Ponding test: put a cylinder sample into chloride solution and wait several months. measure distance travelled.
(+) generally accurate
(-) very slow and labour intensive
Rapid chloride permeability test: Apply voltage and measure current passed by the specimen in 6h.
(+) useful for quality control of one material
(-) cant be used to test material with different chemistry
List some non-destructive testing methods
Electrochemical testing to check condition of reinforcement.
Radiographic, where is the steel?
Air or water permeability
Explain the process of creep testing
Long term test (12 months), more noticeable at high temperatures. Load samples at different ages (2-90 days) to gain understanding of effects on binder material.
Which phase in hardened concrete is the first to react with CO2 during carbonation?
CO2 acts as an acid, portlandite is consumed. Reduces the pH which can induce corrosion of the steel.
What are the main identifiable symptoms of alkali-silica reactions in concrete?
Reaction of alkalis with silica from aggregates cause concrete to expand. Characterised as ‘map-cracking’ on concrete surface.
What can be done to protect concrete against freeze-thaw action?
Air entrapment:
-put approximately size bubbles (<1mm, a few % by volume, well spaced) into the concrete. Gives the water space to expand into as it freezes.
Concrete takes up CO2 during, and after, its service life. Can we use this to reverse the emissions footprint associated with its production?
Yes, but carbonation is very slow (mm/yr). Potentially 5-10% reduction
Which EN 197-1 cement types contain the highest, and lowest, contents, of Portland cement clinker?
CEMI >= 95% portland clinker
CEMIII 5-64% portland clinker + slag
CEMV 20-64% portland clinker + slag + other SCM
Concrete standardisation is increasingly moving from a prescriptive basis to a performance basis. Consider which benefits this may be intended to bring to industry and society, and which are the main potential drawbacks associated with this?
Prescriptive:
(+) If built as specified, structure will be safe
(-) Often over use materials and over engineer
Performance:
(+) allow a producer to design a material to meet the specific requirements
(+) more scope for innovation and efficiency
(-) takes a lot more work to prove performance
Name some recent innovations in concrete
- Fibre reinforced concrete: improves tensile strength
- Ultra-high performance concrete: very high compressive strength (>125MPa) and durability
- Self healing concrete: use bacteria to seal cracks
- Nanotechnology: graphene, carbon tubes
- 3D printing
How does production of portland cement contribute to CO2?
CaCO3 -> CaO + CO2
