Volumetric changes in concrete Flashcards
How can early age change volume of concrete?
- Plastic settlement and plastic shrinkage
- Drying and bleeding -> cracks
What type of material is concrete physically?
Brittle
How can elastic strain change volume?
- Concrete deforms
Only deforms a little so we use micro strains
What is the tensile strain capacity for concrete cracking?
300 micro-strains
What is caused by the movement of water within the microstructure of cement paste?
- Swelling
- Autogenous shrinkage
- Drying shrinkage
- Creep
What does the microstructure of cement consist of?
- Solids
- Voids
- Water
What are the different components of water?
- Free-capillary water (evaporable)
- Physically absorbed gel water
- chemically bound (non-evaporable)
What is calcium silicate hydrate?
Main hydration product in Portland cement binder & source of concrete strength
How much of the paste does calcium silicate hydrate make?
50-65%
What are the features of C-S-H?
- High surface area - contain a lot of water with pore structures
- Morphology varies from fibres to amorphous gel
What is capillary pore water?
- ‘Free’ water, not under the influence of attractive forces exerted by the solid surfaces
- Stored in large voids
What is absorbed water?
- Physically adsorbed onto the surfaces of hydration products. Up to 6 layers of H2O (1.5 nm) can be physically held. Removed on drying < 50% r.h.
What is interlayer water?
Between the C-S-H sheets held by strong hydrogen bonding. Removed on strong drying < 11% r.h.
What is capillary action equal to?
= adhesion + cohesion + surface tension
What is capillary action?
The movement of water within the spaces of a porous material due to the forces of adhesion, cohesion, and surface tension. (against gravity)
What gives a greater adhesion?
Smaller tube size
What does adhesion do?
Adhesion of water to the walls of a vessel will cause an upward force on the liquid at the edges and result in a meniscus which turns upward.
When will capillary action occur?
Occurs when the adhesion to the walls is stronger than the cohesive forces between the liquid molecules.
When does swelling occur?
- Absorption of water by C-S-H
- Water reacts with these silicates and leads to the formation of alkali calcium silicate hydrate.
What are the typical values for cement paste in terms of strain? (swelling)
2000 mircorstrains after 1000 days
What are the typical values for concrete in terms of strain? (swelling)
100-150 mirco-strains after 6-12 months due to presence of aggregates → aggregates do not swell
What is autogenous shrinkage?
Autogenous shrinkage is the uniform reduction of internal moisture due to cement hydration, which is typical of high-strength concrete.
When does autogenous shrinkage?
Occurs in sealed conditions, i.e., no moisture movements in or out of concrete
What is the typical value for strain in concrete? (autogenous)
100 micro-strain after 5 years - normal strength concrete
What is drying shrinkage?
- Concrete exposed to drying externally to the environment
- Water lost to environment
When is water lost from smaller pores in drying shrinkage?
Water lost in larger capillary pores → then smaller pores when relative humidity gets lower
When is cracking caused in drying shrinkage?
Cracking caused when shrinkage restrained
What is the typical value for strain of concrete? (drying shrinkage)
Concrete - 200-1200 micro-strain (partially reversible)
What is the typical value for strain of cement? (drying shrinkage)
4000 με
What is creep?
- Deformation of structure under sustained load
- Gradual increase in strain under
constant stress
When does creep occur?
In hydrated cement
What are the mechanisms of creep?
- Involves internal movement of absorbed water and interlayer water to empty capillary pores
- Sliding and re-arrangement of the C-S-H
- Microcracking at the ITZ also
contributes
How does drying influence creep?
- If concrete dries while under
load, shrinkage and creep
occur simultaneously - Drying increases the magnitude
of creep
What are the negatives of creep?
- Excessive deflection
- Serviceability problems (especially in high-rise buildings and long-span bridges)
- Loss of prestress in prestressed concrete
What are the +ve of creep?
- Reduces stress concentrations induced by shrinkage, thermal movement etc.
- Reduces risk of micro-cracking
- Adding rebar can help restrain deformation
What does cement hydration generate?
Heat
What gives a greater temp rise in concrete?
Greater cement content
What temp can concrete get to?
60-70 degrees celsius
What is the coefficient if thermal expansion?
(α) – change in length per degree
of temperature
How do you calculate thermal movement?
Thermal movement = ∆T × α
What are the main concerns for mass concrete?
a) Temperature rise → expansion
b) Subsequent cooling → thermal shrinkage
c) Cracking if appropriate measures are not taken
What is an example of a massive structure?
Dams
How do you mitigate thermal cracks in mass concrete?
- Using low heat cement and less reactive
- Avoid cement with high specific surface area
- Use increased aggregate content to pack more into the system
- Good curing
What does having no joints lead to?
Uncontrolled cracking
What can you use to construct adequate joints?
Saw
What is the ratio of tensile-compressive strength?
~ 0.1
Why is tensile strength «_space;compressive strength?
- Cracks form orthogonal to direction of load - tensions
- Fracturing is relatively stable and many cracks form before failure - compressions
Where does micro-cracking exist?
Exists at the weak aggregate-paste interface (bond cracks)
What determines number and width of cracks?
Number and width of cracks depend on curing history, bleeding, aggregate characteristics etc…
