Materials 5

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What is durability and design life?
Ability of product to resist attack from environment through design life. Design life is min period where structure performs function without loss of utility or maintenance
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What negative impacts of premature deterioration are there?
Financial: 50% UK construction on maintenance and repair, lost productivity and indirect cost. Environmental: construction waste, more raw materials and energy needed, CO2 emissions in cement manufacture. Quality of life: inconvenience, unattractive
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What are timescales for reinforced cement, portland cement, and lime and pozzolanic cements?
Reinforced concrete:100, portland cement: 180, lime and pozzolanic cements: 2000
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How can reinforcement corrosion happen?
Reinforcement corrosion can be carbonation induced or chlorination induced
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At what potentials and PH's is iron susceptible to corrosion?
At pH 0-6 and potential 0-2, iron is corrosive with Fe2+ most stable form. At pH 4-14 and potential 0-2, iron passive with film of Fe2O3 protecting steel, at negative potential, iron immune (Fe most stable form - corrosion impossible)
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What is in pore solution to protect from carbonation?
Pore solution for OPC protected from CO2, Na+ and K+ (alkali metals), Ca2+, pore solution saturated in Ca(OH)2 ensures pH>12.5, OH- ensures pH~13.7, SO4 2- - low amount
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How is pore solution concentrated and diluted?
Concentrated by drying, diluted by wetting, leaching and carbonation
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What does carbonation corrosion look like and what pH does it occur at? What happens to the bar when carbonation reaches it?
White appearance, at a pH less than 9.5, the cement is carbonated. As carbonation depth reaches rebar, depassivation occurs where bar is subject to corrosion.
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What are carbonation reaction equations?
CO2+H2O -> H+ + HCO3- (carbon dioxide dissolved in pore solution to form carbonic acid). Neutralisation: H2CO3+Ca(OH)2-> CaCO3 + 2H2O (calcium carbonate product)
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What is equation for rate of advance of carbonation front?
x= C*t^0.5 x=carbonation depth, t=time, C=carbonation coefficient
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What influences carbonation coefficient?
C is zero if dry or completely saturated, max for RH 40-60%, and increases with temperature. Influenced by capillary porosity.
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What does chloride induced corrosion look like? How does Cl content vary with depth?
Chloride induced corrosion is brown in appearance. Cl content from surface to rebar drops, can be constant if there is chloride in aggregates.
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Why do more air voids mean chlorination is greater?
More air voids means no hydration products to dissolve so chlorination is quicker.
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What is chloride threshold level and it's relationship with interfacial voids?
Chloride threshold level is minimum amount of chlorine as a percent of cement to initiate steel corrosion. Falls exponentially with interfacial void percent
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How does chlorine make its way into concrete? (penetration)
Chloride diffusion - from high to low concentration, water absorption - water and dissolved salts absorbed into partially dry concrete, permeation - pressure induced flow of water. Penetrability - ease of penetration of ions and fluids into concrete.
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What affects penetrability?
Penetrability influenced by capillary pore volume, size distribution, and connectivity
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Outline the corrosion process and what the anode and cathode release
Cl- present at anode, causes Fe2+ to be released and 2e- electrons. Electrons form current in electrolyte(concrete). 2OH- forms at cathode and reacts with Fe2+ to form Fe(OH)2
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What's the reaction producing rust?
1/2O2+H2O+Fe(OH)2-> Fe2O3nH2O (oxygen and water from cathode)
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What are conditions needed for corrosion from chlorine?
pH <10.5, critical level of chloride present in rebar surface, oxygen and water present.
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What are deterioration phases?
Rust reduces steel area at anode leading to cracking. Depassivation - cracking - spalling *breakingg* structural failure
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Which components of concrete react with sulphates?
Sulphate Na2SO410H20 reacts with Ca(OH)2 to make gypsum. 3(NA2SO310H2O) reacts with C3A to make ettringite. C-S-H reacts with SO4 2- to make thaumasite - characterised by white haloes
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How do you protect against sulphate attack?
Use cements with low C3A, avoid sulphate containing aggregates, ensure SO3 in concrete <4% by weight, use low penetrability concrete. Protective coatings
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What's alkali silica reaction?
Alkalis (Na2O+K2O) 0.5-1,1% cement + Silica (5%) (reactive, present in opal and chalcedony) + additional water -> alkali silicate gel (expansive)
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How do you protect from alkali silica reaction?
Use non-reactive aggregate, low alkali content cement, and protect cement from water
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What is relationship between freeze-thaw resistance and saturation? What are consequences of freezing and thawing?
Freeze-thaw resistance drops very suddenly from 80-20% at 90% saturation. Ice formation increases volume of concrete by 9% leading to cracks
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What are ways to reduce freeze-thaw impact?
Use low penetrability concrete, use air entraining agent, and protect concrete from additional water
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What is abrasion and what affects resistance to it?
Abrasion is scraping of concrete. Resistance from it relies on strength and hardness of surface zone of concrete.
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What protective measures can be taken against abrasion?
Select hard and abrasion resistant aggregate, low cement content (<350kg/m^3), increase concrete strength, use vacuum dewatering on surface to reduce local w/c ratio, provide good curing, use surface hardeners
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Other cards in this set

Card 2

Front

What negative impacts of premature deterioration are there?

Back

Financial: 50% UK construction on maintenance and repair, lost productivity and indirect cost. Environmental: construction waste, more raw materials and energy needed, CO2 emissions in cement manufacture. Quality of life: inconvenience, unattractive

Card 3

Front

What are timescales for reinforced cement, portland cement, and lime and pozzolanic cements?

Back

Preview of the front of card 3

Card 4

Front

How can reinforcement corrosion happen?

Back

Preview of the front of card 4

Card 5

Front

At what potentials and PH's is iron susceptible to corrosion?

Back

Preview of the front of card 5
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