Midterm 1

Question 1

What are important aspects of sustainable development?

Answer 1

Social, economic and environmental aspects have to be respected for the present and the future generations for development to be deemed sustainable

Question 2

What are examples of inderect energy input

Answer 2

transport of raw materials, processing and manufacturing, extraction

Question 3

what can engineers consider to improve sustainability?

Answer 3

design choices, material source, mode of construction, Durability, maintenance plan

Question 4

three types of energy implicated in construction materials, what aere they

Answer 4

Inderect, direct and reccuring. Inderect is all the energy put in the creation the material and putting it on the market. Direct is when it is used for its purpose. Reccuring is for repairs and maintenance

Question 5

What are the three scales of structure?

Answer 5

macro, micro, nano

Question 6

Name all the categories of engineering materials, and describe them briefly.

Answer 6

Metals and alloys, ceramics and glasses, polymers, composites. Metals are good conductors for heat and electricity, usually used for ductility in structures, they have reccuring and homogeneous microstructure. Ceramics are brittle and have good compressive strength, Electrical resistance, homogeneous microstructure. Polymers are an infinite chain of repeating molecule (hydrocarbons), High degree of plasticity. Composites have a wide range of properties, they consist of two or more engineering materials used together.

Question 7

What is the difference between force and stress

Answer 7

stress is a force applied over a certain area. Force is a certain amount of strenght exerted

Question 8

What makes a material more brittle (list more than one cause)

Answer 8

Low temperature, Higher degree of triaxiality

Question 9

describe fatigue

Answer 9

Fatigue is caused by the existing weaknesses in the material and a cyclic application of a high enough degree of stress that will cause cracks and make them propagate at every cycle, until it reaches capacity resistance and fails.

Question 10

what is the fatigue limit

Answer 10

it is the threshold at which the stress levels are accounted for as a “high enough degree of stress”, therefore getting closer to failure. If the stress isn’t high enough to get past the threshold, failure will never occur

Question 11

describe creep

Answer 11

Long term plastic deformation under sustained loads below yield point. Creep failure is time-dependant and deformation causes additionnal stress on the structure, which leads to deformation beyond the material resistance (gradual resistance)

Question 12

Give the corresponding shortcuts to CaO, SiO2,H2O,Al2O3 and SO3

Answer 12

C, S, H, A, S (with bar on top)

Question 13

What are C-H-S and C-H and how do they contribute to the concrete?

Answer 13

CHS is calcium silicate hydrate, and it contributes to the long term strenght of the concrete. It is dense and amorphous, which helps with impermeability, improving the concrete’s durability

Question 14

What are the three main phases of hydration? (explain each steps)

Answer 14

Dormant period, initial set, curing. Dormant period is when concrete is still workable, adding water will increase w/c ratio. initial set is when crystals grow enough to connect and form a matrix, water is being used for the reaction. Curing is solidifying the matrix, adding water will not affect w/c ratio, it will simply improve the strenght of the crystal matrix.

Question 15

why is concrete weaker at interfacial transition zones?

Answer 15

Wall effect

Question 16

name the three levels of concrete and name what they consist of

Answer 16

paste: cement + water; mortar: paste + fine aggregates; mortar + coarse aggregates

Question 17

What is the advantage of taking into consideration mass and volume for the design and making of concrete?

Answer 17

volume is good for determining the air content of the mixture, mass is more practical as aggregates have variable masses.

Question 18

Why can’t slacking be done under water?

Answer 18

The reaction depends on the evaporation of water

Question 19

What are the main compounds needed to make concrete?

Answer 19

Calcium, iron, silica, alumina, sulfate

Question 20

what is the difference in terms of impact on concrete between tricalcium silicate and dicalcium silicate (Ca3SiO5 vs Ca2SiO5)

Answer 20

tricalcium impacts early strength, while dicalcium impacts ultimate strenght

Question 21

around what temperature are belite crystals formed?

Answer 21

1200C

Question 22

What are the major steps of the production of cement?

Answer 22

acquiring raw materials, processing (crushing, mixing, quarrying), Firring (calcined and burned @ about 1200C, makes clinker), Grinding

Question 23

What does Calicum Silicate and Water give as products? Why is it a generality in concrete making?

Answer 23

It gives CSH (calcium silicate hydrate) and CH (Calcium Hydroxyde). These two elements are determinants in concrete’s strenght.

Question 24

Describe CH’s impact on on concrete throughout hydration

Answer 24

CH is made at higher proportions from C3S, which reacts faster than C2S, hence providing the early strenght of concrete. Once the hydrated cement paste is not reacting anymore, the CH’s microstructure forms capillary pores, which increases permability, which decreases durability. It also reacts with sulfates and can be dissolved, which is a process called leaching, both of which are detrimental to durability.

Question 25

Is porosity directly connected to weakness in the concrete?

Answer 25

It has a close relationship, but the real cause for weekness is the probability of connections between voids, which enables transport of outside elements inside the concrete. The higher the porosity, the higher the chances of the voids to be connected.

Question 26

decribe the size effect.

Answer 26

The chances of having a weakness in the microstucture of an element is proportionnal to its size. Therefore having multiple smaller elements to fill the same volume can be beneficial to decrease the risk of having a weakness in the microstructure.

Question 27

name the effect of tricalcium aluminate(C3A), tetracalcium aluminoferrite (C4AF) and gypsum (CSH2) on concrete’s hydration process

Answer 27

Flux, fast set and sulfate attacks; gives clour to concrete, Flux; prevents fast set by forming ettringite, which is insoluble and slows down the hydration process.

Question 28

what are the the types of concrete and their properties and uses?

Answer 28

type 10: GU - general use
type 20: MH - moderate heat of hydration (moderately less C3S), MS - moderate sulphate resistance, used in arrid environments (moderately less C3A)
type 30: More early strenght (high in C3S-> more CH; or grinded smaller, which makes greater surface area)
type 40: LH - low heat of hydration, used for water (low in C3S and C3A)
type 50: LS - resistant to sulphates ( lowest concentration of C3A - >5%)

Question 29

Sea water has a high concentration of sulfate but is treated like a moderate exposure. Why?

Answer 29

sulfate is soluble to chloride which is also in seawater, therefore the sulphate that will stay in concrete is going to be lesser than the actual concentration of the water

Question 30

describe pozzolans

Answer 30

They are either natural (volcanic remains) or industrial waste (fly ash, silica fume). They cannot react with water alone, therefore cement is still recquired. Once the cement reacts with water, the pozzolans react with the CH and form more CSH, making the concrete more durable! They also lower the heat of hydration and improve workability

Question 31

what are the main steps of concrete production

Answer 31

1: batching
2: Mixing
3: Transport
4: finishing
5: curing

Question 32

define cimentitious SCMs

Answer 32

Cement addition that reacts with water to make aluminum-substituted CSH. The most common is blast furnace slag

Question 33

list three benefits of using SCMs and explain why

Answer 33

economical, because you can cut the amount of cement used, which is the most expensive part of concrete making. environmental, because cement produces CO2, so replacing it is better (also reusing the waste). strenghtens the concrete (more CSH produced!)

Question 34

why would it be advantageous to use a water reducing admixture, and how does it work?

Answer 34

It prevents flocculation of cement grains, which entraps water. This enables the concrete to be designed with a lower w/c ratio.

Question 35

how does plastic shrinkage crackage occur and what are its consequences (and what do the cracks look like)?

Answer 35

When not enough water is added to the curing process, layer dries out and middle doesn’t. Restrained shrinkage leads to cracking due to tensile stress, which are pathways for bad things to get in. Discontinuous shorter cracks

Question 36

How does temperature affect curing?

Answer 36

Higher temperatures can weaken the microstructure, as reactions happen too quickly. Lower temperatures take longer, but the microstructure will be stronger

Question 37

what are the three most influencial factors that affect the mode of failure

Answer 37

degree of triaxility, temperature and strain rate

Question 38

what are the main differences between the 3 point and 4 point flexural tests?

Answer 38

The three point experiences more shear than the 4 point, while the four point experiences a more constant moment that the three point.

Question 39

what are the two types of machines to test fatigue

Answer 39

constant displacement and constant load.

Question 40

What are SN curves?

Answer 40

stress versus number of cycles

Question 41

what are the main transport properties of concrete? Explain the concept.

Answer 41

permeation, absorbtion, diffusion.
Permeation - movement of fluid through pressure gradient
absorbtion - transport of liquids to unsaturated pores due to surface tension.
diffusion - transfer of mass trough ions thanks to concentration gradients

Question 42

why is gypsum important to concrete

Answer 42

It has a retarding effect, it prevents a flash set