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Eng Material > Lec 11 > Flashcards

Lec 11 Flashcards

1
Q

Properties of hardened concrete (6)

A

General
strength
dimensional change
durability
impermeability
fire resistance

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2
Q

In dimensional change, deformation of concrete results come from?

A
  1. environmental effect (moisture gain or loss, etc)
  2. applied stress

both in long term and short term

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3
Q

Main dimensional change (2)

A. https://ibb.co/h2XrDfS
B. https://ibb.co/DzFscCn

A

Shrinkage (moisture/dry; w/o loading) and Creep (loading)

A. Shrinkage
B. Creep

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4
Q

What is the effect of dimensional change?

A

After the concrete is mixed and placed -> there is volume changes

  1. shrinkage
  2. creep deformation when there is external force after hardening
  3. SHRINKAGE AND CREEP = time-independent, give cracks (less performance and durability)
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5
Q

What is shrinkage (meaning and types)

A
  • 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
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6
Q

Explain plastic shrinkage

https://ibb.co/BZHT5G3

A
  • 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
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7
Q

MASUK

explain Autogenous shrinkage (meaning, factor)

A
  • 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
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8
Q

MASUK

explain Drying shrinkage

A
  • 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
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9
Q

put in the order type of material from the smaller dry shrinkage

A

CONCRETE -> MORTAR -> PASTE

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10
Q

explain Carbonation shrinkage (meaning in reaction, characteristics)

A
  • 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
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11
Q

Explain Carbonation shrinkage (meaning in reaction, characteristics)

A
  • 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
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12
Q

Explain creep

A
  • 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
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13
Q

What is the similarity between shrinkage and creep

A
  • both increase when concrete is air-dried under load
  • occur at the same time
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14
Q

Two aspects caused an effect on durability

A
  • 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)

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15
Q

2 physical causes of deterioration of concrete

A
  1. surface wear
  2. cracking
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16
Q

Explain deterioration by surface wear

A
  1. abrasion: dry attrition (erosi) (wear on pavements and industrial floors by traffic)
  2. erosion: abrasive action of fluids containing solid particles in suspension
  3. cavitation: loss of mass by the formation of vapor bubbles and their subsequent collapse
17
Q

Other physical actions that affecting durability

A
  1. alternation of drying and wetting: drying -> shrinkage, wetting -> stress and cracking
  2. freezing and thawing action: freezing -> tensile stress on the outer surface > tensile strength of the concrete -> crack and peel
18
Q

Explain dissolution of CH and crystalization (chemcial action)

A
  • 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
19
Q

Explain attack of sulfates (chemical action)

A

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)
20
Q

Explain Alkali-Silica Reaction (ASR) (chemical action)

mechanism, effects, necessary conditions, prevention method

A

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
21
Q

Explain acid attack (chemical action)

A
  • 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
22
Q

Explain impermeability (properties of hardened concrete)

A
  • 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&raquo_space;> paste)
    https://ibb.co/cNtJhpH
  • floating water/cracks between the aggregate and paste -> increase the permeability
23
Q

Explain fire resistance (properties of hardened concrete)
https://ibb.co/DL26t84

A
  • 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

Eng Material (13 decks)

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