Glass and Masonry Flashcards
What is Masonry?
Blocks of something solid & strong, usually held together with mortar.
What is the difference between structural Masonry and Veneer?
Structural masonry is when the entire structure is made from masonry, veneer is a masonry front
with steel/concrete/wood structural elements.
Advantages of Masonry
+ Can use locally available stones, or local clay to make bricks
+ Tends to give high thermal mass
+ Usually durable (>500 years in many structures)
+ Usually high fire resistance
Disadvantages of Masonry
- Unreinforced masonry has little resistance to shearing forces in an earthquake
- Requires manual labour, hard to mechanise
- Difficult to make very tall structures
- Heavy – needs thick & strong foundation
- Tensile/flexural strength very limited
- Compressive strength of bricks can also be a limitation
Simplest masonry
Drystone; Oldest form of construction in the world
– Maybe most durable also?
– Interlocking stones (natural or cut)
– Requires skilled crafting to fit stones together
– For housing, filled with soil to block wind/insulate
Describe brickwork
- Bricks usually made from baked clay (~900-1000°C)
- Iron in clay gives reddish colour to most bricks
- Compressive strength of bricks can be up to 100 MPa, but more commonly 20-40 MPa
What is Mortar?
Cement + sand + water
- Lime is still used in many mortars today
- Used to bind bricks together
- Adhesion of mortar onto brick faces is critical
- Often the weakest point under shear load
- Standard mortar thickness ~5-10 mm
- Mortar is less strong than bricks, so don’t want too much of it
Bricklaying
- Bricks are laid overlapping so that the applied load is transferred down through the courses.
- Can also leave a gap in between (cavity wall) for insulation
How is Modern Masonry based on concrete?
Concrete blocks (concrete masonry unit, CMU), often using fly ash cement (cinder block or breeze block)
– Fly ash gives light weight and thermal insulation
– Mostly hollow core, usually ~20 MPa compressive
– A ‘block’ is usually larger than a ‘brick’ (440×215×100 mm normal for blocks in UK)
– Precast, usually fairly fine aggregates used (no very large particles)
– Can also use aerated concrete (very lightweight), ~8 MPa
What is the chemistry of bricks
Clay and sand mixed with water, pressed into moulds, and heated to 900-1000°C
– Some water is needed for effective moulding
– Also add some organic matter & lime to accelerate firing
– ‘soft mud’ bricks have more water (~25-30%), ‘dry press’
bricks less (~8%) – more water means more variability
– Dry pressing is more expensive, but gives a better quality product (nicer finish)
– Extruded bricks (~12% water) are cut by wires from a
column of clay – becoming cheaper & more popular
What are the important properties of bricks and masonry?
Compressive strength – Higher is better (!) – Class A Engineering Brick 125 MPa (BS EN 771) – Normal bricks 15-35 MPa • Water absorption – Lower is better – Class A Engineering Brick <4.5% – Normal bricks ~20-30% • Frost resistance – Measured by mass loss during standard tests (BS EN 772-22) – Measured as high/medium/low
What is Efflorescence and why is it a problem?
Moisture travels through masonry and evaporates, depositing salts on surfaces
– Salt components from mortar (lime) or from within clay/concrete bricks
– Commonly sulfate or carbonate salts
– Avoid this by keeping water out from masonry (damp-course)
– Not usually a structural problem – but ugly
What is the definition of glass?
Definition: solid material lacking long-range chemical
order, usually made by supercooling a liquid
- Zachariasen model (1932)
Glass transition/creation
Cooling (or compressing) rapidly a liquid can give a glassy
material, cooling very slowly makes crystals.
- Liquid-like local structure, disordered, “kinetically trapped”
Can define ‘glass transition’ temperature Tg
- ~500-600°C for soda lime silicate glass, ~1200 for SiO2
High temperatures makes glass…
Flow!
Note no discontinuity in viscosity-temperature plots.
What are glass structures called?
Soda-lime-silicate (Na2O-CaO-SiO2) glass
– Composition ~70% SiO2, ~15% Na2O, ~10% CaO, plus
minor MgO, K2O, Al2O3, TiO2, Fe2O3
What are the mechanical properties of glass?
Extremely brittle
- Flawless glass could be very strong – but engineering
glass isn’t within 1% of this predicted strength
- theoretical compressive strength 14,000 MPa,
actual achieved usually 20-200 MPa
How is glass strength determined?
Governed by size of largest flaw
Glass is very weak in tension
- even a uniaxial compression test induces some tensile stresses
- Also “static fatigue” – H2O attack on chemical bonds in
cracks makes it weaker under long-term loading
How is toughened glass ‘safety glass’ made?
Toughening (tempering): heating a sheet of glass to
above Tg, then quenching surfaces rapidly
– Quenching puts surfaces into compression, centre in
tension - strengthens surface
-Can withstand more flexural load than untreated glass as its surface is held in compression which resists cracking
What is laminated glass?
Glass-polymer composite for enhanced toughness
and fracture resistance
• A polymer film sandwiched between two glass sheets
– Polyvinyl-butyral is the most common; film is rolled on and heated to exclude air and bond polymer to glass
– Protects surfaces, and holds in the broken fragments if
fracture does occur
Multiple glazing
Gives enhanced thermal insulation properties
– Double or triple glazing: 2 or 3 sheets of glass in a
sealed unit, with a gas layer in between
• sometimes vacuum can also be used if temperature differential isn’t ever going to be too high – very rigid
• dessicants are also sometimes used in the gas space
• Each interface gives resistance to heat transfer
• Need to be careful:
– Need very similar thickness glass inside & outside to
prevent shearing
Draw and describe the Float Glass Process
- Molten glass processed at 1050-1200 degrees celsius.
- Passed over liquid bath of molten tin achieving a very flat surface.
What is the name of the longe-term heating process that can help glass evolve to a more ordered state?
Annealing
What percentage of its theoretical strength can a manufactured glass usually achieve?
<2%
Why are dry pressed bricks better than extruded bricks when subject to freeze thaw?
Less porous - compacted and heated
Stronger
What is the Float glass process?
1- Raw materials 2- Weighing + Mixing 3- Melting: Molten glass 4- Float area: Molten Tin 5- Annealing Lehr 6- Inspection 7- Cutting & storage
Disadvantage of seismic resistance
• Unreinforced masonry has little resistance to
shearing forces in an earthquake
• Bam citadel, Iran, was made from adobe
(non-baked clay bricks)
– collapsed in magnitude 6.6 earthquake, 2003
• Modern seismic codes forbid unreinforced
masonry construction in earthquake zones
