Concrete

Question 1

Hydration of concrete is exothermic. What factor affects the total heat produced?

Answer 1

The volume of concrete produced

Larger volumes generate more heat

Question 2

What happens as the time taken for free water to evaporate increases?

Answer 2

Strength increases

Question 3

What is the standard design drying time for concrete?

Answer 3

28 days

Question 4

Fundamentally, how do you increase concrete strength?

Answer 4

Decrease the number and size of voids

Question 5

What is a standard proportion of aggregates and voids in concrete?

Answer 5

Aggregates: 60-70%
Voids: 1-2%

Question 6

What happens when a concrete mix has too much water? What about when it has too little?

Answer 6

Too much: Large voids form

Too little: Concrete is not workable

Question 7

How much of global CO2 production is due to cement production?

Answer 7

7%

Question 8

How is Limestone typically mined?

Answer 8

With explosives

Question 9

Which parts of the cement process are the most energy-intensive?

Answer 9

Clinker production - requires kiln to reach 1480C

Clay & Limestone grinding

Question 10

Why must Gypsum be added to cement? How much is added?

Answer 10

To delay the hydration process

2-3% by weight

Question 11

What is sometimes used as a cheaper alternative to clinker?

Answer 11

Similar substances which are byproducts of other industrial processes

Question 12

Which chemical structure contributes most greatly to cement strength?

Answer 12

Calcium Silicate Hydrate (C-S-H)

Question 13

Why does cement have a high pH value?

Answer 13

To resist corrosion, such as acid rain

Question 14

What are the 2 types of cement setting failures?

Answer 14

Flash Setting: Too little gypsum is added, hence hydration produces excessive heat which dries & sets the cement too quickly
False Setting: Cement stiffens without generating any heat

Question 15

What are the 3 aggregate weight classes, roughly?

Answer 15

Heavy: 2900-8900kg/m3
Normal: 2100-2550kg/m3
Light: 1450-1900kg/m3

Question 16

How are heavyweight aggregates composed? What are they often used for?

Answer 16

They can contain iron and lead ores

Often used for blast-proof walls/structures

Question 17

How does aggregate shape affect concrete properties?

Answer 17

Sharper aggregates produce stronger structures, but they are harder to cast than rounded aggregates

Question 18

What does saturated surface dry mean?

Answer 18

The aggregate is filled with exactly as much water as it can absorb, no more

Question 19

Why must aggregate be saturated surface dry for mix design?

Answer 19

Dry aggregates cause free water to evaporate too quickly, leaving voids
Wet aggregates increase the total water content of the mix and thereby change the cement/water ratio

Question 20

What are the 3 main aggregate size classes?

Answer 20

Coarse: Average diameter > 4mm
Fine: Average diameter < 4mm
All-In: Both of the above mixed together

Question 21

What are the 2 setting stages of concrete?

Answer 21

Initial set: Gel layers which form on aggregate surfaces expand to overlap with eachother and begin stiffening (occurs about 2 hours into drying)
Finial set: One hour after initial set, gels fully expand to comprise the whole mix, causing the whole structure to stiffen.

Question 22

What are the 2 most important properties of fresh concrete?

Answer 22

Workability and Stability

Question 23

What happens to workability and strength as water/cement ratio (i.e. amount of water) increases?

Answer 23

Workability increases at a decaying rate

Strength decreases at a decaying rate

Question 24

What are the 3 workability tests?

Answer 24

Slump Test
Degree of Compatibility Test
Vebe Test

Question 25

Describe Slump Testing

Answer 25

A mould is used to create a cone of concrete, which is left to sit and slump over time. Slump is measured as the decrease in height of the cone. Collapse or shearing are counted as failures requiring a retest

Question 26

Describe Degree of Compatibility Testing

Answer 26

A tube is filled with concrete and mechanically compacted until it cannot compact any more.
The degree is the original height divided by the compacted height

Question 27

What is the key difference between Slump and Vebe testing?

Answer 27

Vebe testing uses a vibrator table

Question 28

After failure, how is concrete behaviour usually plotted?

Answer 28

Stress against Crack opening

As opposed to Stress/Strain