Strength of concrete

Question 1

What other properties of concrete can be measure when testing concrete strength?`

Answer 1

Elastic modulus, impermeability, resistance to weathering agents

Question 2

What are the 3 phases when studying the structural behavior of concrete?

Answer 2

1) Coherent morter phase
2) Aggregate phase
3) ITZ

Question 3

What are characteristics of ITZ?

Answer 3

• Large crystals of ettringite and CH with prefferect orientation
• Porous structure.
• IT typically 10-15 um thick around agg.
  *

Question 4

Why is there a higher w/c ratio around large agg. in comparison to the w/c ratio in bulk mortar?

Answer 4

Because in freshly compacted concrete, water film forms around large aggregate .

Question 5

Whats the strength ratio of tensile strength to compressive strength?

Answer 5

Tensile strength is almost 1/10 of compressive strength.

Question 6

How much lower is the permeability of mortar compared to concrete? (with same w/c)

Answer 6

coefficient of permeability of mortar is about 1/100 lower than the coefficient permeability of typical concrete.

Question 7

What are ways to improve ITZ?

Answer 7

• Low w/c ratio
• Silica fume (high surface area)
• Different types of agg.

Question 8

What minerals reduce ITZ?

Answer 8

lime stones. this is because of the calcium composition of lime stone. lime stone is a weak particle however, so it must be crushed to the appropriate size.

Question 9

What is the definition of concrete strength?

Answer 9

The stress required to cause fracture of material.

Question 10

What are some factors affecting terminal compressive strength?

Answer 10

• W/C
• Curing conditions (time, temperature,relative humidity).
• Effect of materials and mix proportions
• testing parrameters

Question 11

How does water content affect compressive strength?

Answer 11

Excess water becomes capillary pores. these pores are weak points in the concrete. A higher water concentration also leads yields weaker ITZ’s.

Question 12

Why does a w/c ratio < .3 yield stronger ITZ’s?

Answer 12

Because ITZ are highly composed of CH and Ettringite crystals ( in exclusive concentration act as weak points). Low water content decreases the size of calcium hydroxide crystals.

Question 13

Is it possible to concrete to gains strength through air-entrainment?

Answer 13

Yes, some lucky batches of low strength concrete with a low cement content may sometimes gain strength through air-entrainment.

Question 14

What is a way to increase concrete durability without increasing the w/c ratio?

Answer 14

• Air-entrainment

* Admixtures

Question 15

Why is there a mandated maximum aggregate size (MSA) to be included with the concrete mix?

Answer 15

Because larger aggregates tend to form weaker transition zones.

Question 16

Why does limestone produce higher strengths in concrete?

Answer 16

because calcium (CaCO3.Ca(OH)2.xH20 is formed at ITZ, strengthening the ITZ’s.

Question 17

At what percent can water be harmful towards concrete strength yet still be used in the mix?

Answer 17

10% biiitch

Question 18

What is the relationship between humidity and concrete strength?

Answer 18

The longer concrete is kept moist (especially during the curing process) the stronger the concrete. Chemical reactions in continuously moist concrete continue to have chemical reactions, (grow in strength) but slowly reach a peak.

Question 19

What is the minimum time curing concrete should be kept moist?

Answer 19

7 days. Pozzolanic concretes require longer curing times.

Question 20

What are some ways to moist concrete while curing?

Answer 20

• Spraying
• Ponding
• Covering concrete with wet sand, sawdust, or cotton mats.

Question 21

How does temperature affect the cement hydration process when two batches are placed and cured in the same temperature?

Answer 21

Up to 28 days, the hotter the temperature, the more rapid the cement hydration, boosting the strength in concrete (granted the concrete is placed and cured at the same temp).

Question 22

How does hydration speed affect the curing process?

Answer 22

The faster the hydration, the stronger the concrete (given that the water content in concrete remains at a constant healthy level).

Question 23

How is strength affected when two identical batches of concrete are placed in the different temperatures but cured at the same temperature?

Answer 23

Up to 2 hours, the concrete that is placed in a cooler environment yields more strength compared to the same batch that is placed at a warmer temperature, then cured at the same temperature. This is because at cooler levels, the microstructure of hydrated cement paste and pore size distribution are more uniform.

Question 24

How is strength affected when two identical batches of concrete are placed in the same temperature but cured at different temperatures?

Answer 24

The concrete curing in the lower temperature will be weaker. sucks for him.

Question 25

How does specimen size affect concrete strength?

Answer 25

The larger the diameter test cylinder, the lower the strength. Larger specimen theoretically introduce more ITZ’s to the surface area of the compressive ram.

Question 26

What are the parameters of testing concrete cylinders?

Answer 26

• Moisture state

* Loading condition

Question 27

What is the required moisture state of testing parameters?

Answer 27

• Specimen must be in moist condition at the time of testing.

Question 28

What is the loading condition of testing parameters?

Answer 28

ASTM says in uniaxial compression test, load should be progressively increased to fail the specimen within 2-3 minutes ( ~35 psi/second).

• the more rapid the loading, the higher the observed strength.

Question 29

Explain the relationship between stress and strain during short-term loading.

Answer 29

Stress-strain have linear-elastic behavior up to 30% of ultimate strength. The ITZ microcracks remain undisturbed when under short-term loading.
* strain = deformation

Question 30

What is the relationship of stress and strain when a specimen is loaded past 30% it’s Terminal strength

Answer 30

The stress begins to rise at a slower rate as the strain begins to rise at a faster rate.

• Between 30%-50% the compressive strength, microcracks within ITSZ show extension.
• NO cracking in mortar matrix yet..

Question 31

What is the relationship of stress and strain when a specimen is loaded last 50%?

Answer 31

The stress begins to rise at a slower rate as the strain begins to rise at a faster rate.

• Between 50%-75%, cracks begin to grow in ITZ.
• At 75% cracks between mortar and ITZ start to connect.
• Between 75%-90% Compressive strength, the stress-strain curve begins to flatten out because the specimen eventually breaks. from here major deformation takes place as the stress required to break the specimen is greatly reduced.

Question 32

What is critical stress?

Answer 32

The stress level at about 75% of terminal compressive strength when unstable crack propagation takes place. The peak of the stress-strain curve before it goes back down due to the increase of strain and decrease in stress.

Question 33

How does the Brazilian (tensile) test work?

Answer 33

As the two long sides of the cylinder experience compressive stress, a transverse tensile stress is produced uniformly along the vertical diameter.

* T = 2P/pi.xLxD 
P = failure load
pi. = 3.14
L = length
D = diameter

Question 34

What is elastic modulus?

Answer 34

The ratio of the force exerted upon the specimen to the resultant deformation (stress/strain)

• Equation
  Ec = stress/strain
  Ec = Wc^1.5 * 33 F’c^.5

Question 35

what is ultimate strength?

Answer 35

Failure load / cross sectional area