Concrete Flashcards
What do you understand by carbonation? What could happen as a result?
CO2 in the air penetrate via the pores, a chemical reaction will take place with the calcium hydroxide.
The pH will drop below 9 after the concrete has been totally carbonated.
Corrosion is possible (Dissolution of passive layer)
Mention the type of deformation which is stress and time dependent
Creep (delayed elastic/viscous)
Mention the parameters which determine the quality class of timber independently from the type of timber
- Knots
- Inclination of fibers (wane)
- Pith
- Cracks
- Resin pockets
- Bending
- Insect frass
Mention the two main reaction products by hydration and explain for what they are responsible for
Calcium silicate hydrate (CSH): Strength of the hardened concrete / ensures the strength at the beginning.
Calcium hydroxide Ca(OH)2: Produces the alkaline environment (important for reinforcement / corrosion)
Which protective measures can be taken by concreting in hot weather?
T < 30ºC – 35ºC
To reduce damaging effects one should
- Use ice instead of water
- Cast in the evening
- Use cement with low hydration heat
Measures to reduce the rate of evaporation
- Curing of concrete
- Protect concrete from the sun to prevent cracking during cold nights
- Seal concrete by plastic foils and keep the foils wet
Sketch and label the stress-strain diagram of concrete in compression
Give a ballpark figure for the Modulus of Elasticity
E=210000 N/mm
Mention the measures to avoid bimetallic corrosion
- Avoid coupling od dissimilar materials
- If coupling of dissimilar metals is necessary, choose two that are close together in the galvanic series and electrically
- Insulate dissimilar metals from each other
Mention 2 types of deformation which are stress independent
- Thermal strain
- Shrinkage
- Swelling
- Expansion (chemical)
From which components are the elements (cellulose, hemicellulose, lignin) in timber made of?
- Carbon
- Oxygen
- Hydrogen
What are the main factors influencing the composition of the concrete?
- Compressive strength of cement
- w/c – ratio
- amount of cement paste
Mention at least one technique that may be used to increase the strength and one to increase the ductility of steel?
Increasing of strength with:
- Heat treatment – Hardening
- Cold treatment – Cold rolling, twisting, stretching
Increasing of ductility with:
- Heat treatment – Hardening and tempering
How and why is the concrete compression strength influenced by the water/cement ratio?
The more w/c ratio the lesser sompresive strength, due to the higher porosity of concrete.
Mention and justify the influences on the capillary porosity of fresh concrete
- w/c ratio: the more water the gaps are increasing and the strengrth decreases
- Hydration degree: The reaction products are growing into the pores closing them or at least reducing their size.
- Air pore entrained: Organic admixtures can produce artificial pores
Explain the production of burned bricks
- Grinding and mixing the raw materials
- Forming of the bricks
- Drying of the formed bricks
- Burning of the bricks in tunnel kiln al 900ºC – 1,500ºC for 1-3 days
Mention the quantities which influence the amount of shrinkage
- Time
- Humidity
- Type of concrete (high or normal strength)
- Temperature of environment
Which different kinds of stones do you know?
- Natural stones
- Artificial stones
What is masonry?
Stones + Mortar
Two component system, includes composite materials similar to reinforced concret.
The common materials of masonry construction are brick, stone, marble, granite, travertine, limestone, cast stone, concrete block, glass block, stucco, tile, and cob
Which factors significantly influence the compression strength of concrete?
- w/c ratio
- Cement strength
- Humidity present during curing
- Time
- Temperature
Mention the different types of corrosion
- Uniform attack
- Crevice attack
- Pitting
- Stress
- Bimetallic
Sketch and label the stress-strain diagram of concrete in tension. Also compare it with concrete in compression (curve and significant values)
- Axis with right labelling
- Elastic range of 90% of fct
- Young modulus
- Qualitative right curve
- Tensile strength approximately 1/10fc
- Ultimate tensile strain approximately 1/15 of ultimate compressive strain
Briefly describe at least 2 types of metal corrosion
- Uniform attack: progressive and uniform thinning of the metallic component
- Pitting corrosion: occurs when damages or defects in the protective layer are present (e.g, chloride attack)
- Stress corrosion: occurs in presence of high stresses in sensitive materials or alloys aggressive environments
- Crevice attack: local corrosion due to the different concentration of electrolyte
- Bimetallic (galvanic) corrosion: in the case of two metals with different potentials
From which materials can bricks made from
- Clay
- Shale
- Soft slate calcium silicate
- Concrete
What are the requirements on masonry?
- Load bearing function
- Protection against water
- Sound-insulation
- Protection against fire
- Enclosure of space
There are different ways of measuring the concrete tensile strength
- Direct tension test
- 4-point bending test
- Splitting cylinder test
How is the influence of an increasing fibre-load-angle on the tensile strength of timber?
Decreasing of the tensile strength
What is/are the main problem(s) by concreting in hot weather?
- Increased rate of water evaporation form the fresh mix
- Increase of the early strength but decrease the final strength
Explain briefly the effect of moisture and oxygen on the resistance of timber to Fungi attack, Insects attack and fire
- Fungi need oxygen, with 0% or 100% moisture no attack possible
- Insects need oxygen, with 0% or 100% moisture no attack possible
- Fire need oxygen and low moisture
How are the shape changes of timber due to increasing or decreasing moisture content called?
Swelling and shrinkage
Name the components of concrete
Water + cement + aggregates + (admistures, additives)
What do you understand by metallic corrosion?
The gradual destruction of metals by chemical reaction with their environment.
This means electrochemical oxidation of metals in reaction with an oxidant such as oxygen.
- Two metals with different potentials
- Corrosion increases with increasing difference between the potentials
- They can be connected by an electrolyte, very often also a high humidity (>80%, 80%) will be sufficient to induce corrosion
How does normal concrete and light weight concrete fail under compressive loading? Explain the difference
- Normal weight concrete: strong aggregate / weak matrix
It fails along the grain boundary
- Light weight concrete: weak aggregate / strong matrix
The crack goes trough the aggregate
The relative strength of the matrix and the aggregates determine the crack path (trough / around aggregates)
