Lec 11 Flashcards
Properties of hardened concrete (6)
General
strength
dimensional change
durability
impermeability
fire resistance
In dimensional change, deformation of concrete results come from?
- environmental effect (moisture gain or loss, etc)
- applied stress
both in long term and short term
Main dimensional change (2)
A. https://ibb.co/h2XrDfS
B. https://ibb.co/DzFscCn
Shrinkage (moisture/dry; w/o loading) and Creep (loading)
A. Shrinkage
B. Creep
What is the effect of dimensional change?
After the concrete is mixed and placed -> there is volume changes
- shrinkage
- creep deformation when there is external force after hardening
- SHRINKAGE AND CREEP = time-independent, give cracks (less performance and durability)
What is shrinkage (meaning and types)
- volume changes in concrete due to loss of moisture at different stages for some reasons
- types: plastic shrinkage, autogenous shrinkage, drying shrinkage, and carbonation shrinkage
Explain plastic shrinkage
https://ibb.co/BZHT5G3
- occurs in the plastic stage before it is hardened
- rapid evaporation of water -> humidity -> tensile stress -> shrinkage
- occurs the first 12 hours after placing
- surface cracks (short and irregular), or perpendicular to wind direction, a few centimeters deep from the surface
MASUK
explain Autogenous shrinkage (meaning, factor)
- Meaning: no moisture movement of the cement paste, BUT self-dry and hydration -> loss of water
- low w/c -> internal water consumption and drying
- occur even when concrete has not exchanged and lost water with the environment
- w/c < 0,42 (generally)
HSC and HPC
MASUK
explain Drying shrinkage
- volume shrinkage due to drying mainly due to loss of water
- slowly and gradually formed -> shrinkage rate decreases with time
- 40-80% occur in three months, 60-90% occur in 1 year
- factors: cement type, cement amount (w/c) (high w/c, high shrinkage), proportion (higher aggregate -> less shrinkage), size and shape of structure (decreasing the max aggregate size and coarse -> higher shrinkage), curing environment (less humidity -> higher shrinkage), and reinforcement
put in the order type of material from the smaller dry shrinkage
CONCRETE -> MORTAR -> PASTE
explain Carbonation shrinkage (meaning in reaction, characteristics)
- calcium hydroxide + carbondioxide -> calcium carbonate
- known as neutralization
- add a lil weight and form a porous surface, dust, and color changing
- accelerates the corrosion of reinforcement
- irreversible and superimposed on dry shrinkage -> severe cracks
Explain Carbonation shrinkage (meaning in reaction, characteristics)
- calcium hydroxide + carbon dioxide -> calcium carbonate
- known as neutralization
- add a lil weight and form a porous surface, dust, and color changing
- accelerates the corrosion of reinforcement
- irreversible and superimposed on dry shrinkage -> severe cracks
Explain creep
- elastic strain develops when concrete is loaded (load on the member) -> creep develops with time
- change in deflection occurs rapidly at first, slowing down with time
- load is eventually removed, strain is recovered elastically and recovered by creep, BUT residual strain remains due to the bonding of gel particles
- external forces -> elastic/instant deformation
- Creep -> deformation increases with time under persistent stress
- 2 - 4 times larger than elastic strain
- mainly caused by cement paste
- aggregate not affecting creep
- creep occurs in all load types: compression, tension, shear, and tortion
What is the similarity between shrinkage and creep
- both increase when concrete is air-dried under load
- occur at the same time
Two aspects caused an effect on durability
- physical: percolation/ permeability of water and temperature (high heat hydration), abrasion, erosion, cavitation, freezing, and thawing action
- chemical: dissolution of CH, attacks by sulfates, Alkali-silica reaction (ASR), acid attack, and steel corrosion
IMPORTANT PART:
**chemical -> dissolution of CH, attacks by sulfates, Alkali-silica reaction (ASR)
2 physical causes of deterioration of concrete
- surface wear
- cracking
Explain deterioration by surface wear
- abrasion: dry attrition (erosi) (wear on pavements and industrial floors by traffic)
- erosion: abrasive action of fluids containing solid particles in suspension
- cavitation: loss of mass by the formation of vapor bubbles and their subsequent collapse
Other physical actions that affecting durability
- alternation of drying and wetting: drying -> shrinkage, wetting -> stress and cracking
- freezing and thawing action: freezing -> tensile stress on the outer surface > tensile strength of the concrete -> crack and peel
Explain dissolution of CH and crystalization (chemcial action)
- Ca(OH)2 + CO2 -> CaCo3 + H2O
- crystalization -> spitting out white crystal
- not affect the concrete strength, but cause porosity (loss of durability)
- The most important prevention and control of crystalization is to prevent the occurrence of cracks and prevent water entering the concrete
Explain attack of sulfates (chemical action)
C3A + CSH2 (gypsum) + water -> (crystalization increases volume) -> C6AS3H32 (ettringite) -> (with continuous hydration with C3A dan dissociation) -> C4ASH12
- from ettringite with invaded sulfate radical and water and recrystallized again
- To against sulfate attack -> use cement content with low C3A (cement II and V)
Explain Alkali-Silica Reaction (ASR) (chemical action)
mechanism, effects, necessary conditions, prevention method
contain silica, silicates, and carbonates -> react with alkaline hydroxides. The reaction involves active silica -> alkali-silica reaction
- a gel can destroy bond between aggregate and the hardened cement paste -> absorbs water and swells -> cracking and disruption
- necessary conditions: 1) active aggregate 2) sufficient alkali (K2O + Na2O) > 0,6% 3) presence of moisture
- prevention method: avoid the aggregate having active silicate -> use mineral admixtures and use low alkali cement if contain soluble silica dioxide
Explain acid attack (chemical action)
- attack the calcium hydroxide, converting it into Calcium carbonate and bicarbonate
- the rate attack increases -> decreases pH
- acid (pH <5,5) has corrosive effect
- alkaline substances cause to alkaline reaction
Explain impermeability (properties of hardened concrete)
- flowing water is the main cause (higher w/c -> higher permeability)
- temperature < 0 C -> freeze-thaw damage
- porosity between paste and aggregate interface
- permeability coefficient of concrete is much larger than paste (concrete pores»_space;> paste)
https://ibb.co/cNtJhpH - floating water/cracks between the aggregate and paste -> increase the permeability
Explain fire resistance (properties of hardened concrete)
https://ibb.co/DL26t84
- concrete retain some strength at high temperatures; it will eventually degrade
- the amount of degradation depends on: max temperature, period of exposure, induced temperature gradients, concrete constituents and moisture content, and the size of the element
- concrete (max 500 C for about 2 hours) > steel
- > 500 C -> C-S-H colloid decompose, followed by CH decomposition. 1000C -> limestone decompose
