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14 Cards in this Set
- Front
- Back
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What is concrete |
• Concrete is the material of choice for many applications due to its unique properties • Cement, water, fine and coarse aggregates are the main constituents of concrete |
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What are properties of concrete influenced by |
• Properties of constituent materials • Mix proportion • Concreting practice |
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Advantages of concrete |
• Strong in compression • Excellent fire resistance • Can be easily formed into desired shapes and sizes • Very cheap and readily available • Low maintenance |
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Disadvantages of concrete |
• Very weak in tension so it is susceptible to cracking • High CO2 emission but low in respect to usage • Low toughness • Weight if compared is high compared to its strength |
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What is good concrete |
Fresh state • Consistent, cohesive and workable Hardened state • Homogeneous • High compressive strength • Impermeable • Durable |
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What is cement |
Cement is an inorganic substance with cohesive and adhesive properties. It is a binder and sets and hardens to hold materials together |
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What are the functions of cement |
• To bind and fill coarse aggregates together • To fill the voids between fine and coarse aggregates particles to make the concrete impermeable • To form a compact mass |
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What is the hydration process and what are the hydrate products? |
Hydration is an exothermic chemical reaction between cement particles and water. · Cement grains start floating about after mixed in water · Mixture is flowable and can be easily changed into a different shape · Cement grains start dissolving · Ions in cement grains start reacting with water to produce new solid call “hydrates” |
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What is the hydration process and what are the hydrate products? |
Hydration is an exothermic chemical reaction between cement particles and water. · Cement grains start floating about after mixed in water · Mixture is flowable and can be easily changed into a different shape · Cement grains start dissolving · Ions in cement grains start reacting with water to produce new solid call “hydrates” |
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Initial vs final setting of cement paste |
Setting time is the change from a plastic state to a solid state. Initial setting: • Beginning of solidification • Refers to loss of consistency and at this point concrete is unworkable • Indicates approximate time limit for handing and placing the concrete Final setting • Time taken cement paste solidifies completely • Concrete completely loses its plasticity |
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What is hardening of cement paste |
Hardening refers to strength gain in a set cement paste and indicates that a useful and measurable strength is developing |
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What cement properties affect hardened paste volume change |
• Hydration – hydrate products (C-S-H or CH) result in a higher volume • Temperature – thermal expansion as temperature increases and thermal contraction when temperature decreases resulting in volume change • moisture content – shrinkage volume decreases due to drying, welling volume increases due to moisture content |
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Describe the 5 steps in hydration process |
- Cement grains start reacting with water - Thin layer of C-S-H is formed around the cement grain causing weak interlocking - Initial set occurs with development of weak skeleton to hold cement in place - Final set occurs as the skeleton becomes rigid and cement particles are locked in place - Spaces between cement grains are filled with hydrates and cement develops strength |
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Describe the stages in kinetics reaction |
- Initial reactions stage: Several dissolution and hydration reactions, duration 10-20 minutes - Induction stage: low reaction rates, 2-4 hr duration at 20C, transporting, casting and finishing, initial setting begins at end of this phase - Accelerating stage: increasing reaction rate, beginning of hydration therefore compressive strength growth begins, duration 5-10 hr - Decelerating stage: reaction rate decreases, products generated during hydration are attached to cement. Water space around particles is filled up with hydrate products, duration 24 hr - Steady state: low reaction rate can take up to years and contributes to strength development |
