Early age & mechanical properties of concrete Flashcards
What are the processes in concrete?
- Batching
- Mixing
- Transporting
- Placing
- Compacting
- Finishing
- Curing
- Formwork removal
What must fresh concrete have the ability to do?
- Easily mixed
- Easily transported
- Easily placed
- Easily compacted
- Easily finished
What must fresh concrete have the ability to resist?
- Bleeding
- Segregation
What does batching mean?
The batching of concrete refers to the process of measuring and combining the ingredients required for the production of concrete in predetermined proportions.
What is involved in batching and mixing?
- measuring ingredients
- aggregate moisture checked
- drum mixer
In what time is the fresh concrete placed?
1.5 hrs
How can you transport cement?
truck
conveyor belts
pumps
skips
wheelbarrows
How should you place the concrete and why?
Place concrete as near as possible to final its position, mix
remains cohesive, with no segregation.
What should you do with each layer before placing the next?
Make sure uniform and compact
How much of the volume of freshly placed concrete is trapped air?
5-20%
What methods for compaction are there?
Manual ramming
tamping
vibrators (poker, formwork, beam)
What does the air voids in concrete affect?
- Strength
- Density
- Permeability
- Durability
How does trapped air have an affect on bonding?
Affects bonds with rebar and corrosion initiation
What is the purpose of finishing?
To produce flat, level and dense surface
What is screeding?
To strike off excess with sawing action across the surface with a straight edge
What is darby/bull float?
When you smooth down high spots, embedded large aggregates fill small hollows left after screeding
What is floating/trowelling?
With flat wood or metal blades to compact the surface, brings paste to the surface & removes remaining imperfections.
What is brooming?
Uses a rake/broom if skid resistance is required
What are some test methods?
- Slump test
- Flow table test
What is a slump test?
A concrete slump test measures the consistency of a concrete batch to see how easily the concrete will flow
What does a greater slump mean?
More workable
What flow table test?
It is used primarily for assessing concrete that is too fluid (workable) to be measured using the slump test, because the concrete will not retain its shape when the cone is removed.
When flow table test used?
For high or very high workability
What is flow table test useful for?
Good for indicating mix cohesiveness and segregation
How do you work out average spread?
(A+B)/2 (mm)
What are many concrete innovation dependent on?
Rheology
What is roller compacted concrete?
- Has zero slump
- Compacted using a vibrating roller
- Dry enough to prevent sinking but wet enough for distribution
What are the different types of placing of concrete?
- Self compacting concrete
- Pumping concrete
- Shotcrete
- Underwater concreting
- Additive manufacturing
What is the aim of curing?
To keep concrete as nearly saturated as possible
What is needed for curing?
Should start as early as possible and be a continuous
What is curing?
Curing is the process of maintaining satisfactory temperature and moisture conditions in concrete long enough for hydration to develop the desired concrete properties.
What are the methods for curing?
- Impermeable sheets
- Steam curing/autoclaving
- Curing compounds
What does curing promote?
Hydration and microstructure development
What happens when capillary pores become discontinuous?
If capillary pores become discontinuous, concrete will be impermeable and durable
What happens when you drop curing temp?
Strength drops
What is needed for satisfactory strength development?
Not all cement to hydrate
Which type of cement has the highest compressive strength?
Continuously moist
What does poor compaction lead to?
- Reduced strength
- Reduced bond between rebar and concrete
- Increases transport of aggressive agents
- Creates visual blemishes
How much does strength for poorly compacted concrete decrease by?
5-6% for every 1% vol.air
What is segregation?
The separation of the constituent materials in freshly mixed concrete.
What is segregation caused by?
Differences in density
When can segregation be detected?
When handled on site
How can segregation be controlled?
- Improve aggregate grading
- Increase fines content
- Care in transporting, placing & compacting fresh concrete
- Air entrainment – small air bubbles help keep particles in
suspension, reduce sedimentation
What is bleeding?
The process where free water in the mix is pushed upward to the surface due to the settlement of heavier solid particles such as cement and water.
When does bleeding occur?
When rate of evaporation < rate of bleeding
What are the negative effects of bleeding?
- Laitance on surface, dusting - paste easily abraded
- Reduce bond strength between rebar and concrete
How can bleeding be controlled?
- Increase cement fineness & content
- Reduce water content
- Increase fine aggregate content
- Use air entrainment
When does bleeding stop?
When concrete stiffens and sets
Where are plastic settlement cracks common?
In deep sections
What are plastic settlement cracks caused by?
Caused by excessive settlement and bleeding, restrained by large
obstructions
What do plastic settlement cracks look like?
Crescent shaped void beneath rebar, initially filled with bleed water
Where are plastic shrinkage crack common?
In thin sections
What is plastic shrinkage cracks caused by?
Caused by rapid loss of water by evaporation or absorption
What happens when fresh concrete dries and contracts?
Plastic shrinkage
When do plastic shrinkage cracks form?
Shrinkage restrained by inner concrete, induces tensile stress, cracks when exceeds tensile strength
How can plastic shrinkage be controlled?
- Reduce surface evaporation rate to less than 1 kg/m2/h
- Start curing early
- Cool aggregates and mixing water
Why is the compressive strength test the most common test for hardened concrete?
- Used in structural design
- Compliance and quality control
- Formwork removal
- Transfer of pre-stress
- Easy to measure
- Correlates to many properties
- Measured by standard uniaxial compression test
- Cubes or cylinders
What are the compressive strength test results influenced by?
- Ingredients & mix proportions
- Early age conditions
- Test parameters (sample type, size moisture condition, loading rate)
Why is cube strength > cylinder strength?
- Friction between plates of testing machine and specimen produces a restraining effect
- Cylinders (larger H/D ratio) are less affected by end restraint
What are tensile strength test useful for?
- Resistance to cracking
- Shear design
- Assessment of fatigue performance
- Design of highway & airfield pavements, unreinforced structures, dams
What is a disadvantage to tensile strength testing?
More difficult to measure
What is the direct tensile test?
- Axially applied tensile load
- Measures the stress and strain at failure of a specimen
What is the splitting tension test?
- Indirect test
- Load in compression
How does the splitting tension test work?
In this test, a standard cylindrical specimen is laid horizontally, and the force is applied on the cylinder radially on the surface which causes the formation of a vertical crack in the specimen along its diameter.
What does tensile strength increase with?
Drying
What is the flexural test?
It is a measure of an unreinforced concrete beam or slab to resist failure in bending.
How is the max tensile stress calculated?
By finding the modulus of the rupture
How much does the flexural test overestimate the axial tensile strength by?
50-100%
For the flexural test what does tensile strength decrease with?
Drying
What is the relationship between compressive and tensile strength?
As compressive strength increases, tensile strength increases but at decreasing rate
What is the ratio of tensile to compressive strength?
0.1
What is the modulus of elasticity?
- Measures stiffness
- Slope of stress-strain curve
What is the modulus of elasticity used for?
Used to calculate elastic deflection & stresses induced by volume changes
When does ductile failure occur?
Ductile failure occurs when a material is loaded beyond its yield strength and begins to plastically deform for a period before ultimately failing.
Why is a ductile material better?
Ductile is better as it ismore predictable and controllable than brittle fracture, following a well-defined stress-strain curve and exhibiting deformation before failure
What is strength of a concrete a function of?
- Strength of aggregate
- Strength of paste
- Strength of aggregate-paste interface(ITZ)
