Compaction achieves maximum density. Concrete should be workable to achieve compaction.
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What is impact of air on concrete strength?
Entrapped air reduces strength by 5-6% per 1% increase in air
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What is bleeding and what is a consequence?
Bleeding is where aggregates fall while water bleeds out of concrete. Can cause plastic settlement = cracking as a result of bleeding.
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What's plastic shrinkage? How is it caused?
Reduction in volume of cement paste by hydration, evaporation etc. If water lost exceeds water replenished by bleeding, this leads to plastic shrinkage and cracking.
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What should rate of evaporation be reduced to to reduce plastic shrinkage?
1 kg/m^2/h
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What is impact of curing temperature on strength?\
Strength increases with temp
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What are effects of cold weather concreting and precautions that should be taken?
Delayed setting reduces strength in early ages and delays removal of formwork. Precautions: use rapid hardening cement, insulate formwork, don't place concrete on frozen areas.
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What are effects of hot weather concreting and precautions to be made?
Rapid loss of workability and accelerated set increasing risk of plastic cracking. Reduced long term strength. Precautions: control temperature. In hot, dry, and windy conditions provide shade and wind shield.
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What is adiabatic heat rise and which type of cement gets warmest?
Adiabatic temp rise - no heat loss. Rapid hardening PC reaches greatest temperature.
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What is temp rise per 100kg in concrete.
For cement 300-600 kg/m^3, adiabatic temp rise increases 12 degrees for every 100kg
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What factors influence temperature?
Cement composition/fineness/quantity, concrete dimensions and placing and temp, formwork type, and atmospheric conditions
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What is compressive strength test cube size and results? What factors influence strength?
150-100 mm cube used in UK. Greater rate of loading gives greater strength. Increasing w/c ratio reduces strength and increasing aggregate content reduces strength
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What's ideal temperature for concrete strength?
20 degrees optimum temperature for concrete strength
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Maturity equation?
M= sum(Temperature*time at temperature). Can estimate strength during curing but rule is approximate as strength-maturity relationship relies on mix characteristics, loss of water in curing, adverse effect of high early temp
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What temp and curing method is ideal for strength-maturity relationship to hold?
For wet curing from 5-40 degrees, strength directly proportional to maturity
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At what volumetric strain is minimum vol?
Minimum volume occurs at maximum volumetric strain
As concrete is stretched, cracks grow leading to failure faster. Compressive loading doesn't encourage this. Developing cracks reduce area resisting load so stress in failure zones increases rapidly
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Other cards in this set
Card 2
Front
What's the workability slump test?
Back
Slump test: distance between original height of conical mould and highest point of slumped concrete.
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