Term
| What are the 12 steps involved in Concrete Mix Design? |
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Definition
1. Strength
2. W/C Ratio
3. Stone
4. Air
5. Slump
6. Water
7. Cement
8. Binder Type
9. Admixtures
10. Sand
11. Moisture corrections
12. Batch Proportions |
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Term
| What is the difference between a Pozzolanic and a Hydraulic Reaction? |
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Definition
A Hydraulic cement reacts with water to form CSH and CH. may occur underwater. (Slag, GP, Class C FA)
Pozzolan cements are silliceous materials that react with CH in the presence of moisture to form CSH. (Class F FA, MS) |
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Term
| What causes deterioration in concrete? |
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Definition
Chemical: - Corrosion of reinforcing Steel
-Alkalai-Sillica Reaction
-Sulfate Attack
-Acid Attack
Physical: - Freeze Thaw |
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Term
| How are trees Classified? give description and examples |
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Definition
Hardwood trees (deciduous)
-Broad Leaves
-Fall in autumn
-Oak, Maple, Birch
Softwood (Evergreen)
-Coniferous trees
-Balasam
-Radiata Pine
-Douglas Fir
Also by:
Endogenous - intertwined growth (eg palm, bamboo)
Exogenous - outward growth (most others) |
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Term
| What does the structure of wood comprise of and what are the functions of each component? |
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Definition
Pith - Centre of tree, growing tip
Heartwood - Nonliving cells, resistant to decay (2/3 R)
Sapwood - Transports moisture and stores food (1/3 R)
Cambium - Growth of bark and wood
Inner Bark - Transports sap from leaves
Outer Bark - Protective layer |
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Term
| What causes the yearly rings in trees? |
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Definition
Seasonal variations in growth patterns.
Earlywood is a period of rapid growth in the warmer season and is light in colour. The cells have large diameters and thin walls
Latewood forms in the later part of the growing season and occurs much more slowly. Cells are darker, have smaller diameter but thicker walls. |
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Term
| What is the formulae used to determine radial shrinkage? |
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Definition
S(m2-m1) = ((m2-m1)/FSP)*S(g-d)
where: S(g-d) is the oven dried radial shrinkage
FSP is the fibre saturation point
M(1,2) are the moisture contents before and after respectively |
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Term
| What visual features affect Lumber Grading? |
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Definition
Knots Splitting
Bowing Slope of Grain
Checks Wane
Shake Stain |
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Term
| How do you prevent deterioration in timber? |
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Definition
-Control moisture content
-Treat wood with preservatives
-Reduce UV and weather exposure
-Coatings or paint
-Kiln drying to kill fungi |
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Term
| What factors affect the strength of timber? |
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Definition
-Wet or Dry service conditions
-Treatment
-Load Duration and direction w.r.t. grain |
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Term
| What equation is used to calculate the % strength change for timber under sustained loading? |
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Definition
σ(%) = 91.5 - 7log(t)
where t is time in hours |
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Term
| What are the general incentives toward using SCMs? |
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Definition
Improved workability and durability
Reduced risk of cracking
Reduces Permeability (decreases water exposure)
Continued cement reactions, strength development continues over time
Enhances microstructure |
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Term
| What are the Properties of Fly Ash? |
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Definition
Waster from thermal power stations
Reduces water demand and bleeding
Increases workability
Can increase setting time
Moderate fineness and reactivity
Spherical in shape |
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Term
| What are the Properties of Slag? |
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Definition
Waste from steel manufacturing
Small reduction in water demand
Increases workability and setting time
Angular, crushed shape |
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Term
| What are the Properties of Microsillica? |
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Definition
Waste from ferrosillica industry
Increases water demand
Decreases workability and bleeding
Extremely fine and highly reactive
Risk of ASR |
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Term
| What is the name and properties of C3S? |
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Definition
Tricalcium Silicate
- Hydrates and hardens rapidly
- Responsible for early set and initial strength |
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Term
| What is the name and properties of C2S? |
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Definition
Dicalcium Silicate
- Hydrates and hardens slowly
- Contributes to later age strength (>7 days) |
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Term
| What is the name and properties of C3A? |
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Definition
Tricalcium Aluminate
- Liberates a large amount of heat in first few days
- Contributes slightly to early strength development
- Cements with low percentages are resistant to sulfates |
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Term
| What is the name and properties of C4AF? |
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Definition
Tetracalcium Aluiminoferrate
- Reduces clinkering temperature
- Hydrates rapidly but contributes little to strength
- Colour of hydrated cement (greyness) due to ferrite hydrates. |
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Term
| What are the formulae involved in concrete mix design? |
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Definition
Strength:
F(tm) = F(c) + 1.64(S)
Moisture corrections:
M(batch) = M(ssd)x[1+[Wc-Abs]]
Water corrections:
W(corr) = M(ssd)x[Abs-Wc] |
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Term
What are the half-cell reactions for the corrosion of iron and what are their names?
2Fe + O2 + 2H2O → 2Fe(OH)2 |
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Definition
Anodic Reaction (Oxidation):
2Fe → 2Fe(2+) + 4e(-)
Cathodic Reaction (Reduction):
O2 + 2H2O + 4e(-) → 4OH(-) |
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Term
| What is the Nernst Equation and what result governs the proceeding of a corrosion reaction? |
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Definition
| E = Eo - ((RT/n)/nF)xln(a(red)/a(ox)) where E is potential n is number of mols F is faradays constant T is temperature R is 8.314 a is activity Reactions proceed if E is positive |
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Term
| What thermodynamic property governs the proceeding of a corrosion reaction. |
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Definition
Change in Gibbs free energy must be less than zero for a reaction to proceed.
∆G<0 |
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Term
| What are the requirements for corrosion? |
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Definition
-Electrical circuit with:
-Metallic and electrolytic continuity
-Sufficiently low resistivity
-Moisture
-Oxidising agent
-Available metal |
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Term
| What are the types of metal corrosion? |
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Definition
-Uniform
-Pitting
-Crevice
-De-alloying
-Galvanic
-Intergranular
-Environmentally induced |
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Term
| What regions (corrosion conditions) exist in the Pourbaix Diagram (E vs pH)? How does iron behave in each state? |
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Definition
Immunity - No corrosion takes place, Iron exists in metallic state (Fe)
Corrosion - Where iron exists in a stable ionic state (Fe2+, Fe3+)
Passivity - Stable passive compound (Fe2O3 or Fe3O4) |
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Term
| What role do SCMs play in corrosion protection? |
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Definition
-Increases resistivity (which decreases corrosion rate)
-Reduced chloride threshold concentration but overall are corrosion preventers |
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Term
| What role do pH conditions of concrete play in corrosion? |
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Definition
Alkaline conditions prevail in concrete which form a passive layer on on the embedded steel reinforcement.
Higher pH means less active corrosion occurs. |
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Term
| What causes corrosion of concrete? |
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Definition
Carbonation:
- Lowers the pH of the pore solution (~8pH)
- Leads to general corrosion of steel
Chlorides:
- From de-icing salts, salt water
- Leads to pitting corrosion in steel
- Chloride threshold (Cl/OH ratio~0.6) |
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Term
| What preventative measures can be taken to reduce corrosion in concrete? |
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Definition
- Membranes, coatings or sealers
- Decrease porosity
- Use of SCMs
- Curing
- Low w/c ratio
- High performance concrete |
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Term
| What causes concrete failure when corrosion of steel occurs? What are the failure modes? |
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Definition
Iron oxides expand by 7x the original volume. This results in tension and cracking in the concrete due to expansive pressure.
- Cracking
- Spalling (loses corner)
- Loss of bond
- Loss of section
- De lamination |
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Term
| Moisture Content in Concrete Mix? |
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Definition
Water is required to provide sufficient workability and react with cement to provided strength.
More water is required to lubricate aggregates with angular shapes than rounded smooth particles
SSD is the ideal moisture content of mix constituents as no moisture is added or removed during mixing. |
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Term
| How do impurities in water affect the concrete mix? |
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Definition
Chlorides can cause deterioration of steel bars
Other impurities can also accelerate or decelerate the setting time.
Lowers the strength of the concrete |
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Term
| What are the five stages in manufacturing cement? |
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Definition
1) Quarrying- quarried then crushed
2) Drying the Grinding - storage hall, rawmill, rawmeal silos
3) Kiln Burning & Cooling - Suspension preheaters, rotary kiln, clinker cooler silos
4) Finish Grinding - Gypsum added, cement mill, cement silos
5) Packing - packed or sold in bulk |
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Term
| How does the presence of gypsum affect the hydration reaction |
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Definition
| In the presence of gypsum C3A reacts to form ettingite, which coats C3A grains and prevents rapid reaction. |
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Term
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Definition
| stiffness or sloppines of mix influenced the fluidity of the cement phase (slump) |
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Term
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Definition
| The amount of energy required to place and compact |
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Term
| Water demand should be kept as low as possible too... |
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Definition
| 1) Reduce Cost - more water means more cement 2) Minimise Shrinkage and Creep 3) Control Bleeding and Segregation |
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Term
| What is bleeding? What are it's positive and negative effects |
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Definition
Water rises, solids settle on the bottom.
lenses - trapped water under aggregate
channels - formed by water rising
Positive Effects:
Removes excess water, improves surface finish, Indicates when setting occurs, prevent plastic shrinkage cracking.
Negative Effects:
lenses under aggregate or steel bars reduce bond strength, top bar effect, segregation of mix. |
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Term
| What are some ways to prevent plastic shrinkage cracking? |
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Definition
Pre-wet surfaces
Protect from rapid drying
Avoid extreme weather conditions
Ensure reasonable bleed |
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Term
| Effect of Temperature on Concrete Strength Development |
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Definition
Increased temperature
Increased Rate of Hydration
Increases Strength of Early Age
Decreases Long term Strength |
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Term
| How can we decrease the permeability of porous material? |
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Definition
| 1) Decrease Total Porosity 2) Decrease Connectivity 3) Decrease Pore Size 4) Increase Tortuosity (crookedness of pores) This can be done by 1) Increase the quantity of paste by - decrease water and cement content - improve aggregate grading 2) Improve the quality of the paste - Reduce W/C ratio - Use SCM |
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Term
| How does air-entrainment help to prevent expansive pressures due to freeze thaw cycles? |
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Definition
| Air bubbles provide spaces for water and freezing advances, relieving pressure in the concrete |
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