Volume changes of concrete Flashcards
What is swelling?
Cement paste or concrete hydrating with an adequate supply of water increases in mass and in volume
Absorbed water forces the gel structure apart
Swelling of cement paste increases much more than concrete that’s hydrated and tails off with time
What’s autogeneous shrinkage?
Hydration with no ingress of moisture leads to autogenous (arising from within) shrinkage
Typical values of autogenous shrinkage for concrete:
- 40 με (= 0.004%) after 1 month
- 100 με (= 0.01%) after 5 years
Shrinkage for paste approximately 10x value for concrete
Autogenous shrinkage increases with increasing cement content
What’s relationship between drying shrinkage and time?
Paste has a similar relationship to swelling, shrinks much more than concrete. Rewetting can improve damage but there’s irreversible drying shrinkage
Classify water in hardened concrete?
Water can be classified under 3 headings:
Capillary pore water - not under the influence of any solid surfaces. Water in aggregate particles is in this state too.
Adsorbed surface water - under the influence of one solid surface, can be treated as a liquid of high viscosity.
Interlayer water - under the influence of two solid surfaces of C-S-H. The water layer is approx. 2 molecules thick and does not have liquid-like properties (best considered as part of the solid structure).
How does relative humidity of water affect water driven off?
Initial loss of free water from capillary pores causes insignificant shrinkage (RH of 80-100% will only cause this)
Continued drying removes adsorbed water from C-S-H and this causes shrinkage - RH between 50-10%
If shrinkage is restrained cracking can occur
<10% of RH causes interlayer water in CSH to be lost
What factors INCREASE drying shrinkage?
Increase in rate of evaporation which depends on:
- RH
- temperature
- velocity of air at surface
- exposed surface area of the concrete
Decrease in restraint provided by aggregate:
- decrease in volume fraction of aggregate
- decrease in aggregate stiffness (E value)
- increase in shrinkage of aggregate
Decrease in size of specimen, component or structure
Decrease in amount of reinforcement
Thermal movement, alpha: what causes temperature variation? What is the formula for thermal movement? What does coefficient of thermal expansion rely on?
Two main causes of temperature variation:
- heat of hydration (Early age heat cycle)
- climate
Thermal movement = temperature change x coefficient of thermal expansion
Causes contraction or expansion due to temp changes
Coefficient of thermal expansion depends on:
- coefficient for hardened cement paste (typically 10-20 x 10-6)
- coefficient for aggregate (typically 4-12 x 10-6)
- volume proportions of cement paste and aggregate
- moisture content of concrete (highest when concrete partially saturated)
Thermal movement may lead to cracking
What is relationship between thermal conductivity (wavelength sign, W/mK) and thermal movement (microstrain/K)?
Thermal conductivity about 1/10 of thermal movement for concrete
What precautions can be made for thermal movement?
Mass structures like dams - Use low-heat cement to control early age thermal cracking.
Water-restraining structures such as tanks and basements - Increase reinforcement to control crack width.
Nuclear pressure vessels - Prestress to avoid cracks
Longspan bridges, roads and airport tunnels - Build in expansion joints, full-width cuts
What is creep? (Elastic movement)
Long term movement due to applied load,
i.e. the increase in strain under a sustained constant stress
after taking into account swelling, shrinkage and thermal movement
How is creep changed when load is removed ?
On a strain-time graph, as load is applied, elastic strain reaches max and then creep increases before sloping off. As load is removed, all elastic strain is recovered but not all creep is, there’s irrecoverable strain
What is mechanism of creep?
Creep occurs in hydrated cement paste.
Exact mechanism is unclear, but probably involves internal movement of adsorbed surface water and/or interlayer water, plus sliding between the C-S-H sheets.
For external exposure creep is typically equal to elastic movement
For internal exposure creep is typically 3 - 4x elastic movement
What are consequences of creep?
Negative
Creep can lead to an excessive deflection of structural members and cause other serviceability problems especially in high-rise buildings and long-span bridges.
In reinforced concrete columns, creep results in a gradual transfer of load from concrete to reinforcement. In eccentrically loaded columns, creep increases the deflection and can lead to buckling.
Creep causes loss of prestress in prestressed concrete.
What are positives of creep?
Positive
Creep may relieve stress concentrations induced by shrinkage, temperature changes, or movement of supports
What are swelling, autogenous and drying shrinkage, thermal movement, elastic movement and creep reliant on?
Swelling: time and moisture dependent, autogeneous shrinkage: time dependent, drying shrinkage: time and moisture dependent; thermal movement: none, elastic moment: stress dependent; creep dependent on all
