Week 3 Flashcards
What is standard cement made of?
Water, fine sand, crushed stone & admixtures
What is the purpose of hardened concrete?
- Resists loads
- Provides strength
- Provides durability
What processes does fresh concrete allow for?
- Transport
- Placement
- Compaction
- Finishing
What are the 5 steps of concrete construction process?
- Mix design
- Transporting
- Placing
- Curing
- Stripping
During the ‘plastic’ stage, how does concrete take form?
Needs formwork
What are the types of concrete?
- Plain (non-structural)
- Reinforced (structural)
- Pre-stressed
What is the purpose for ‘General Purpose Portland Cement’?
General use
What is the purpose for ‘Rapid Hardening Cement’?
To gain early strength
(e.g. when early removal of formwork required)
What is the purpose for ‘Quick Setting Cement’?
To complete construction in short time
(e.g. in water)
What is the purpose for ‘Low Heat Cement’?
To avoid thermal stress in concrete
(e.g. mass concrete)
What is the purpose for ‘Sulphate Resisting Cement’?
To fight against sulphate
e.g. ground floor on sulphate bearing soil
What is the purpose for ‘White Cement’?
Architectural purposes
What is the purpose for ‘Coloured Cement’?
Architectural purposes
What is the purpose for ‘Air Entraining Cement’?
To improve workability
What is the purpose for ‘Hydrographic Cement’?
High workability & strength
(due to water repelling chemicals)
What is the purpose for ‘High Alumina Cement’?
- High early strength
- Resistance to high temperatures
What is the purpose for ‘Masonry Cement’?
- For masonry construction only
- Not structural
What is Type GP cement?
General Purpose Cement
What is Type GB cement?
General Purpose Blended Cement
- Enhanced strength
- Improve workability/pumpability
- Reduce bleeding grouts
- Lower drying shrinkage and creep
- Increase resistance to sulphate/chloride attack
What is Type HE cement?
High Early Strength Cement
What is Type LH cement?
Low Heat Cement
What is Type SR cement?
Sulphate Resisting Cement
What is Type SL cement?
Shrinkage Limited Cement
Water characteristics for cement:
Water must be clean and have set limits of:
- Chlorides
- Sulphates
- Alkalis
- Solids
What are the 2 groups of aggregates?
- Course: crushed rock, gravel or screenings
- Fine: fine and course sands and crusher fines (particle size less than 5mm)
What are the two types of sand?
- Concreting sand (used for concreting)
- Brickies/plasterers sand (NOT used for concreting)
How do aggregates become ‘well graded’?
Range of sizes that fit together well in concrete
How are aggregates measured?
In a sieve
What are the benefits of rounded aggregates?
More workable than angular aggregates
What are the benefits of angular aggregates?
Make concrete stronger
What are the considerations of aggregate selection?
- Strength
(crumbly or flaky rock types (for example, sandstone) must not be used because they will result in a weaker concrete) - Resistance
(to wear and tear and weathering) - Chemically inactive
(with cement) - Free of dirt and clay
(otherwise, the bond between the cement paste (i.e. the mix of cement and water) and aggregates will be weak)
What are the types of admixtures?
- Retarders
- Accelerator
- Air-entraining agent
- Water-reducing admixtures
- Superplasticizers
- Corrosion-inhibiting admixtures
What do retarder admixtures do?
Delay setting time
What do accelerator admixtures do?
Shorten the setting time
(concrete setting time takes longer in colder climates)
What do air-entraining admixtures do?
Increase workability while reducing segregation
What do water-reducing admixtures do?
Used to increase workability without adding water
(by adding them, the water content can be reduced up to 10%)
What do superplasticisers admixtures do?
- Enhances workability without adding water
- Water content may be reduced (by 12-30% by adding them)
What do corrosion-inhibiting admixtures do?
Slow down corrosion in reinforcements
What are the reasons for adding reinforcement steel in concrete?
- To withstand tensile stresses: concrete is weak in tension
(reinforcement steel is used to withstand tensile stresses) - To withstand shear stresses: concrete is weak in shear.
(reinforcement steel is used to withstand shear stresses) - Control cracks: concrete cracks can occur.
(reinforcement steel is used to reduce shrinkage cracks to an acceptable level)
How does cement paste + the nature of aggregates affect the workability of cement?
- Amount of cement paste: a higher amount of cement paste (i.e. water plus cement) in a mix represents a more workable concrete
- Shape of aggregates: rounded aggregates are much more workable than angular aggregates
- Grading of aggregates: the aggregates of a range of sizes are more workable than the aggregates of the same or a few sizes
How is cement workability measured?
Using a slump test
How is the slump test measured?
- Dry slump (20-50mm)
Reduced water: low shrinkage but low workability - Normal slump (60-100mm)
Suitable for general applications - Medium slump (120-160mm)
Increased pumpability for high rise buildings above 130m - Flowing concrete (180mm+)
Enhances placement, saving on labour,
& required for off-form finishes
Typical methods to increase slump of a concrete mix include:
- Add more cement paste
- Use approved admixture (plasticisers)
(DO NOT ADD WATER - leads to porous concrete)
What does the ‘cohesiveness’ of concrete mean?
The degree that the ingredients of a concrete holds together.
What are the 2 setting times?
- Initial setting time:
The elapsed time between water and cement are mixed and the paste starts losing its plasticity - Final setting time:
The elapsed time between water and cement are mixed and the paste has completely lost its plasticity
What are the characteristics of initial setting time?
- Initial setting time 30-45min
- Initial setting time can be delayed by adding retarders (for batching & transport)
- After initial setting time, concrete is not to be disturbed (otherwise the bonds will break)
What are the characteristics of final setting time?
- Final setting time approx. 10hrs
- Structural members (specifically horizontal) are inadequate to support self-weight
- Concrete must remain supported until stripping time
What do structural concrete beams need?
Reinforcement steal (rebar)
e.g. to resist tensile and compressive strength
What are the thermal properties of concrete?
- Low thermal expansion
- Horizontal members are supported on rollers (allowing for expansion)
What are the 4 steps of concrete preparation?
- Coarse and fine aggregates are mixed together
- A measured amount of cement is added to the aggregate mixture and mixed in
- Add a measured amount of water
- Mix all the materials until a consistent concrete (an even mix) is obtained
What are the parameters of standard cement of housing?
- Strength: 20 MPa or 25 MPa
- Class: normal class concrete
- Maximum aggregate size: 20mm
- Slump: 100mm
What are 2 types of cement tests?
- Slump test
- Compression test
What are the 3 types of slump in a slump test?
- True slump:
the concrete subsides while keeping more or less to the shape of the cone - Shear slump:
the top portion of the concrete shears off and slips sideways - Collapse slump:
the total collapse of concrete
How are air bubbles removed from concrete?
- Using vibrators
- Vibrators should extend 150mm into the lowest layer
What is the minimum curing period of concrete?
3 days (can range from 3-7 days)
What is formwork stripping?
Formwork stripping done after the structural element is strong enough to withstand its ‘self-weight’ and ‘construction loads’
What factors affect stripping time?
- Type of formwork
- Type of building element
- Mix design
- Weather
