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CIMT 348 Midterm Ch.1-3 > Chapter 2: Volume Changes In Concrete > Flashcards

Chapter 2: Volume Changes In Concrete Flashcards

1. Define different types of volume changes in concrete 2. Explain how do these volume changes occur and what factors contribute to concrete volume changes 3. Understand the experimental procedure for each type of these volume changes 4. Analyze concrete mix proportions for potential volume changes 5. Explain techniques to mitigate concrete volume changes 6. Propose repair methods for concrete shrinkage cracks

1
Q

How much does concrete expand when frozen?

A

9%

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2
Q

What is autogenous

A

internal drying of paste (water to reacts with cement dry/harden)

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3
Q

Factors of autogenous

A
  1. self desiccation (internal drying)
  2. doesn’t occur for wet concrete
  3. prominent in concrete with low w/c (lower than .42)
  4. higher water-cement ration leaves more voids/weaker
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4
Q

What is chemical shrinkage

A

reduction in volume and solids and liquids in cement paste resulting from cement hydration. aka chemical reaction

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5
Q

What water curing

A

letting concrete contain moisture while gaining strength

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6
Q

Water curing factors

A
  1. putting water on concrete after placement
  2. prevents drying
  3. apply water on surface
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7
Q

ASTM for Autogenous Shrinkage

A

ASTM C1698 Testing for Autogenous Shrinkage

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7
Q

ASTM Testing for Chemical Shrinkage

A

ASTM C1608 Testing for Chemical Shrinkage

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8
Q

What is Plastic Shrinkage

A

evaporation rate exceeds rate of bleeding, water loss at surface before its set

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9
Q

Plastic Shrinkage factors

A
  1. shrinkage occurring while concrete is still fresh
  2. combination of chemical shrinkage and autogenous shrinkage and rapid bleed water evaporation
  3. plastic = not set
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10
Q

What causes Plastic Shrinkage

A
  1. tensile stress is greater than tensile capacity
  2. low water
  3. high paste/fineness
  4. high scm (silica fume, fly ash)
  5. high air temperature
  6. low humidity
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11
Q

how can you prevent Plastic Shrinkage

A
  1. decrease air temperature (place in morning/night)
  2. increase humidity (fog)
  3. decrease concrete temperature (ice water)
  4. decrease wind (cover/windbreakers)
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12
Q

What is Subsidence (plastic settlement)

A

vertical shrinkage of concrete before initial set

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13
Q

What causes Plastic Settlement

A
  1. bleeding
  2. air voids rise to the surface
  3. chemical/autogenous shrinkage
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14
Q

What is Plastic Settlement Cracking

A

cracking settlement over aggregates and rebar

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15
Q

Plastic Settlement Cracking Prevention

A
  1. stiff formwork
  2. stable subgrade
  3. low slump
  4. low segregation
16
Q

What causes Plastic Settlement Cracking

A
  1. Bad compaction of vibration
  2. high slump
17
Q

2 types of Fibers

A
  1. Microsynthetic
  2. Macrosynthetic
18
Q

Rate of Evaporation

A

if greater than .2= cracking
if between .1 and .2= may or may not crack
if lower than .1= wont crack

19
Q

What is drying shrinkage

A

shrinkage after concrete is finished (dry)

20
Q

what causes drying shrinkage

A
  1. not cured
  2. evaporation of environment
  3. high w/c ratio and paste
  4. type of aggregate (porous)
  5. SCM (fly ash/slag)
21
Q

drying shrinkage factors

A
  1. as it dries, shrinks to center of mass
  2. shrinks .04-.08% when 50% relative humidity
  3. if concrete is retained it cracks
  4. common in flatwork
  5. larger samples have lower shrinkage (hard to dry out)
22
Q

ASTM for concrete Dry Shrinkage

A

ASTM C157: Length change of hardened hydraulic-cement mortar and concrete

23
Q

ASTM C157

A
  1. Free drying shrinkage: caltrans test method
  2. Procedure: 28 days
  3. Store samples in dry air: 50% +/- 4 at relative humidity, 73+/- 3 degrees F
  4. measure length: 1/ week for 64 weeks
24
Q

Lighweight aggregate

A

holds water while mixing, water comes out aggregates while curing, keeping the concrete moist

24
Q

What is internal curing

A

partially replace aggregate with light weight aggregate

25
Q

Differences between Plastic and Drying Shrinkage Cracking

A

Plastic: close together, on the surface(shallow), early in plastic, around aggregate
Dry: through slab (deep), drying while its set, through aggregate, apart

26
Q

How much aggregate is in concrete

A

60-70%

27
Q

Prevent Cracking

A

Mix Design: SCM, lower temp, dense coarse aggregate, reduce w/cm, water, paste
Construction: keep moist, wind breaks, consolidate correctly, lower air temperature, add microfibers, proper jointing

28
Q

Thermal Expansion and Contraction

A
  1. 4-7 millionth per degree F
  2. depends largely on aggregates
  3. CTE= 610^-6
    equation= CTE     initial length * change in temperature
29
Q

What is Curling and Warping

A

out of plane deformation. More common than dyring shrinkage cracking

30
Q

what is curling

A

deformation of slab due to difference in temperature between surface and bottom of slab (hot and cold)

31
Q

what is warping

A

deformation of slab surface profile due to moisture differences between surface and bottom of slab (wet/dry)

32
Q

Control Joint

A

2-3 times thickness of slab, aspect ratio needs to be less than 1.5
ex: 4 inch slab, joints need to be 8ft

CIMT 348 Midterm Ch.1-3 (3 decks)

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