Lesson 4 & 6 - Cement & Concrete Flashcards
1
Q
Characteristics and uses of rapid hardening cement
A
- Used in roads, pavements, highways and precast structures (slabs, poles, pillars)
- Ideal for early strengthening and repair works so roads/buildings can be made available for use faster
- Has high heat emission rate & develops high early strength - damage from freezing in cold weather can be reduced
- Attains sufficient hardness quickly - formworks can be removed earlier
- Sulfate resistant
2
Q
Characteristics and uses of Sulphate resisting cement
A
- Used in underground structures, concrete in contact with agressive soil
- Lower Tricalcium Aluminate, lower heat of hydration
- Blended cements, where fly ash or slag combined with Portland cement can provide sulphate resistance
3
Q
Characteristics and uses of low heat cement.
A
- Low heat release in early stages, can reduce cracks caused by thermal stress in mass concrete, reduce early water migration and decrease drying shrinkage of concrete.
- Suitable for mass concrete pours ( e.g hydropower stations) where rate of temperature rise and max temperature achieved must be controlled to reduce the risk of thermal cracking.
- Reduced Tricalcium salts so lower strength gain but higher ultimate strength.
- C2S is the main mineral component, with a content of >40%
- Low hydration heat and delay in attaining the max hydration heat
4
Q
Characteristics and uses of White cement.
A
- Used in architectural concrete for its colour
- Application include precast walls, panels, terrazzo surfaces and tile grout
- Careful selection of raw materials containing limited iron oxide and manganese oxide that give cement its grey colour during production
- Suitable raw materials are chalks and limestones with low iron contented and white iron-free clay
- Generally low strength.
5
Q
Characteristics and uses of Calcium Aluminate cement
A
- Strength depends on calcium aluminate hydrates
- Originally developed as a sulphate resisting, high early strength, heat resisting cement.
- Mainly used in non-structural applications in marine and sewer construction.
- Typically designed to gain strength must faster than ordinary Portland cement
- Consists of calcium aluminate that can produce large amounts of heat during the first 24 hours
- Can result in reduced strength and vulnerability to chemical attack when exposed to water for long periods of time at low/moderate temperatures.
6
Q
A
7
Q
What are the traditional cement types?
A
- Ordinary
- Rapid hardening
- Sulphate resisting
- Low heat
- White
- High alumina content
8
Q
What are the requirements of fresh concrete?
A
- Compatibility: Ability to compact with ease in place
- Stability: Segregation and bleeding during
transporting, placing and compacting. - Finishability: Possible to finish the concrete surface
with ease - Consistency: Sufficient consistency for mixing
- Uniformity: Within and between batches
- Flowability: Ability to flow into any shapes of the
mould and between reinforcements - Pumpability: Ability to place concrete by pumping
9
Q
Define ‘workability of concrete’
A
The amount of mechanical work, or energy, required to produce full compaction of the concrete w/o segregation.
10
Q
What are the 6 factors affecting workability of concrete?
A
- Free water content
- Aggregate grading
- Cement content
- Aggregate shape & size
- Temperature and time
- Water-reducing admixtures
11
Q
How does cement content affect workability
A
- Increase in cement content increases workability
- Increases cement paste content
- Concrete will become more cohesive and sticky
12
Q
How do water-reducing admixtures affect workability
A
- Addition of plasticizer increases the workability
- Addition of air-entraining admixture increases the workability
13
Q
How does aggregate grading affect workability
A
- Continuous aggregate grading reduces void content and improves workability.
- Amount of water necessary to wet the aggregate particles depends on the total surface area of the aggregate particles.
14
Q
How does aggregate shape and size affect workability
A
- Low angularity number (rounded and smooth aggregates) produces more workable concrete
- Decrease in the maximum aggregate size of coarse aggregates reduces workability
15
Q
How does temperature and time affect workability?
A
- Increase in concrete temperature reduces workability of concrete
- Increase in the time after mixing reduces workability due to:
- Cement hydration
- Evaporation
16
Q
What is slump loss?
A
Slump loss refers to the reduction of the workability of concrete over time
17
Q
What are the primary causes of segregation in concrete?
A
- Larger max aggregate size and proportion of the large aggregate
- Odd shaped & rough particles
- Dropping fresh concrete from a height
- High specific gravity of coarse aggregate
- Too wet or too dry mixes
- Decreased amount of fines (sand or cement)
- Excessive vibration
18
Q
What are the effects of segregation?
A
- Non-uniformity
- Poor consolidation
- Poor strength
- Honeycombed
- Poor steel-concrete bond
- Increased permeability
- Poor durability
19
Q
How can segregation be controlled?
A
- Extent of segregation can be controlled by choice of suitable grading and care in handling
- The use of finely divided mineral admixtures or air-entraining agents reduces the tendency toward segregation.
20
Q
What is Bleeding?
A
A special form of segregation in which some of the water in mix tends to rise to the surface of freshly placed concrete
21
Q
What are the causes of bleeeding?
A
- Lack of fines
- High free water content
- Water reducing admixture overdose
22
Q
What are the effects of Bleeding?
A
- Creates a porous and weak layer of non-durable concrete
- Creates zones of poor bond between cement paste and large aggregate particles of reinforcement
23
Q
How to control bleeding?
A
- Use finer cement
- Add pozzolans
- Add air entrainment
- Increase proportion of very fine aggregate particles
- Reduce water content
- Use richer mix
24
Q
What is the purpose of testing fresh concrete?
A
- Compliance testing against specification requirement of acceptance of concrete.
- Evaluate trial mixes during designing process and then make mix adjustments if necessary
25
What are the 5 workability tests?
- Slump test
- Compacting factor test
- Vebe test
- Flow test
- Ball penetration test
26
What is the slump test?
- A measure of the behaviour of a compacted inverted cone of concrete under the action of gravity. It measures the consistency or the wetness of concrete.
27
28
Describe slump test procedure?
- Fill inverted cone up with 3 layers of equal volume.
- Rod each layer 25 times
- Scrape off surface and lift cone away
- Measure the distance the concrete has displaced downwards
29
Types of slump and what they mean?
Zero slump:
- Very low workability
True slump:
- Low to medium workability
Shear slump:
- Poorly designed mix
- Less cohesiveness
Collapse slump:
- Very high workability
- Flowing concrete
30
Pros and Cons of slump test?
Pros:
- Widely used
- Simple and inexpensive
- Immediate results
- Specs for concrete in terms of slump
Cons:
- Static test
- Does not provide indication for compatibility
- Only suitable for slumps of med. to high workability
31
What does the compacting factor test measure?
The degree of compaction resulting from the application of a standard amount of work
32
Procedure of compacting factor test?
- Upper hopper filled with concrete
- Bottom door of upper hopper is then released and concrete falls into lower hopper
- Bottom door of lower then released and concrete falls into cylinder
- Excess concrete cut and net weight of concrete in cylinder is measured.
- Density of concrete in cylinder now measured and this density divided by fully compacted concrete density is defined as Compacting Factor
33
Pros and Cons of Compacting Factor Test?
Pros:
- Dynamic test indicating the compactibility
- More sensitive and accurate than slump test, esp. for mixes of medium to low workability.
- Simple and inexpensive
Cons:
- Mixes with same compaction factor do not necessarily require same amount of work for compaction
- Not suitable for concrete with very low workability.
34
What is the Vebe test?
- Measures the remolding ability of concrete under vibration.
35
Procedure of Vebe test?
- Slump cone is placed and filled in the Center of the cylinder
- After removing the slump cone, a glass plate is set atop the fresh concrete
- Time for the concrete to remold is recorded
36
Pros and Cons of Vebe test?
Pros:
- Good lab test esp. for very dry mixes
- Dynamic test; treatment of concrete during test is comparatively closely related to method of placing in practice
Cons:
- Only works for low slump concrete
- Difficult to determine the end point
37
What does the slump flow test measure?
The horizontal spread of a concrete cone specimen after being subject to jolting
38
Flow test procedure?
- A conical mould is used to produce a sample of concrete in the center of a board
- The free edge of the board is lifted against the stop and dropped 15 times
- Flow = diameter of the concrete
- Appropriate for concrete with high or very high workability
39
For each workability, what is the appropriate test method?
Very low:
- Vebe time
Low:
- Vebe time
- Compacting factor
Medium:
- Compacting factor
- Slump
High:
- Compacting factor
- Slump
- Flow
Very high:
- Flow
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