Lecture #2 - Concrete Ingredients - Part 2

Question 1

Name the three different types of failure for a poor mix.

Answer 1

Aggregate, paste, bond

Question 2

What is the ideal failure mode?

Answer 2

Combined failure (aggregates, paste, and bond all fail a bit)

Question 3

Where do normal weight natural aggregates come from?

Answer 3

Natural gravel and sand, weathering and erosion of rocks, sometimes requires a bit of processing

Question 4

What are sound sources of natural aggregates? (rock type)

Answer 4

metamorphic and igenous

Question 5

Define metamorphic.

Answer 5

Recrystallization and reorganization of minerals due to pressure, volume, and termperature changes.

Question 6

Define igneous.

Answer 6

Solidifcation of magma and lava

Question 7

What are unsound sources of natural aggregate? (rock type)

Answer 7

shale and siltstone (sedimentary)

Question 8

Define sedimentary.

Answer 8

consolidation and compaction of sediments

Question 9

What is a sound transport mechanism for natural aggregate?

Answer 9

materials at higher elevations than glacier and are untouched

Question 10

What is a questionable transport mechanism for natural aggregates? Explain.

Answer 10

water transport, smoothed by prolonged agitation in water, harder than less abraded sand and gravel but smooth surface can reduce bond with cement paste

Question 11

How is manufactured aggregate produced?

Answer 11

crushing sound parent rocks or crushing air-cooled blast-furnace slag

Question 12

What are some features of manufactured aggregate?

Answer 12

customizable in terms of grading and higher quality as particle elongatoin and layered flakes can be reduced and less likely to be contaminated by clay minerals and organic material

Question 13

Which has a rougher texture, manufactured or natural aggregate?

Answer 13

Manufactured

Question 14

Which is more uniformly graded: manufactured or natural aggregate?

Answer 14

manufactured

Question 15

How is recyled concrete aggregate made?

Answer 15

Demolishing and removing existing concrete, crushing it in crushers, removing embedded items and reinforcement and dirt, wood, etc., grading and washing

Question 15

What is recycled-concrete-aggregate?

Answer 15

Recycled concrete from buildings, pavements, and other structures used as aggregate to be more sustainable

Question 16

What are the features of recycled concrete aggregate?

Answer 16

higher absorption because the hardened cement paste is more porous, lower specific gravity

Question 16

What do you need to do if absorption is high in your aggregate?

Answer 16

add more water for workability, adjust water for concrete mix

Question 16

Which is less angular, cubical or long: manufactured or natural aggregate?

Answer 16

Natural

Question 17

What is recycled concrete aggregate mostly used for?

Answer 17

pavement reconstruction

Question 18

What might marine-dredged aggregate contain that is not good?

Answer 18

seashells (weak), and salt (ruins)

Question 19

What are the sources of marine-dredged aggregate?

Answer 19

estuaries, seabed

Question 19

What weight of aggregate needs to be prewetted or kept moist?

Answer 19

lightweight

Question 19

Why do we need grading limits?

Answer 19

affects proportions, workability, pumpability, economy, porosity, durability, shrinkage

Question 20

Why would you use marine-dredged aggregate even though it sucks?

Answer 20

if other resources are not available in your region, as you need to use whatever's close by for sustainability and cost

Question 20

What is grading?

Answer 20

particle size distribution of aggregate

Question 21

What kind of curve does single-sized aggregate grading have?

Answer 21

single sharp curve

Question 22

What kind of grading curve does poorly graded aggregate have?

Answer 22

two sharp cruves

Question 23

What kind of grading curve does well graded aggregate have?

Answer 23

two smooth curves

Question 24

What does the ideal grading curve distribution look like?

Answer 24

like a parabola of different sizes

Question 25

Is the volume of paste greater than the voids between aggregates? Why?

Answer 25

yes, to provide workability, cohesiveness of the paste matters

Question 26

What is the fineness modulus?

Answer 26

sum of percent retained on each sieve (except pan)/100

Question 27

Why can't we only have very fine sand as aggregate?

Answer 27

too expensive

Question 28

Why can't we only have very coarse gravel as aggregate?

Answer 28

mix is harsh and unworkable

Question 29

What is the maximum aggregate size?

Answer 29

Smallest sieve that all aggregates pass through (100% passing)

Question 30

What is the nominal maximum aggregate size?

Answer 30

smallest sieve that majority of aggregate passes through (85 to 95% passing), first sieve that retains aggregate

Question 31

What does maximum aggregate size depend on?

Answer 31

Shape and size of concrete member, amount and distribution of reinforcing steel, need to have enough workability so that the concrete can be placed properly without honeycomb or voids

Question 32

Explain why low maximum aggregate size is not economical.

Answer 32

Surface covered by paste increases which causes required water and cement to increase so more cement paste is required to provide enough bond which is expensive.

Question 33

What is air-entrainment?

Answer 33

Microscopic air bubbles in concrete.

Question 34

What are the features of air entrainment?

Answer 34

Increased resistance to freeze and thaw durability, increased resistance to scaling caused by de-icing chemical, improved workability, reduced segregation and bleeding

Question 35

How does air entrainment help vs no air entrainment in concrete?

Answer 35

Water in capillary pores freezes which expands and creates stress in concrete which then tends to crack. Entrained air allows water to expand which relieves stress and prevents cracking

Question 36

What is water content in concrete?

Answer 36

Water in cement paste and water absorbed by aggregates

Question 37

How much moisture is in oven dry aggregate?

Answer 37

none

Question 38

How much moisture is in air dry aggregate?

Answer 38

less than potential absorption

Question 39

How much moisture is in saturated surface dry (ssd) aggregate?

Answer 39

equal to potential absorption

Question 40

How much moisture is in damp or wet aggregate?

Answer 40

greater than absorption, has free water

Question 41

What needs to adjusted to account for the moisture condition of aggregates?

Answer 41

water content

Question 42

Describe how aggregates are prepared for water absorption capacity test.

Answer 42

Put aggregate in water for 24h then dry it with towel then it becomes SSD then put it in the oven

Question 42

Describe how aggregates are prepared for moisture content test.

Answer 42

Sample aggregate from stockpile (can have any moisture condition) is then oven dried

Question 42

Should moisture content and absorption capacity be equal? What do you do if they’re not equal?

Answer 42

Yes. If moisture content is greater than absorption capacity remove water. If moisture content is less than absorption capacity add water.

Question 42

What is bulking?

Answer 42

Increase in total volume of moist fine aggregate over the same mass in a dry condition

Question 42

What is abrasion?

Answer 42

The physical wearing and grading of a surface through friction and impact on the surface