exam questions

Question 1

how do the fresh concrete plastic viscosity change with addition of water?

Answer 1

decreases

Question 2

how do the fresh concrete yield stress change with addition of water?

Answer 2

decreases

Question 3

how do the fresh concrete plastic viscosity change with addition of superplasticizers?

Answer 3

decreases

Question 4

how do the fresh concrete plastic viscosity change with addition of stabilizer?

Answer 4

increases

Question 5

how do the fresh concrete yield stress change with addition of superplasticizers?

Answer 5

decreases

Question 6

how do the fresh concrete yield stress change change with addition of stabilizers?

Answer 6

increases

Question 7

witch two practical testing methods can be used for the evaluation of fresh mortar rheological parameters?

name the tests and describe how test results can be correlated with the yield stress and the plastic viscosity of the material.

Answer 7

-slump Test: A smaller measure may indicate a higher yield stress, as the mortar would spread less easily and More fluid and spreadable mortars may have lower plastic viscosity.

-Funnel Test: faster times means that the plastic viscosity and the yield stress are low.

Question 8

what is the main difference between FRC and SHCC in terms fiber volume fraction?

Answer 8

-FRC typically includes a moderate volume of fibers, generally ranging from about 0.5% to 2.5% by volume of the total concrete mix
-SHCC, being a specialized type of cementitious composite, often incorporates a higher volume of fibers compared to conventional FRC. The fiber volume fraction in SHCC can be significantly higher, typically in the range of 2% to 5% or even more.

Question 9

specify one reason for fiber pullout or fiber rupture in SHCC .

Answer 9

really high tensile forces

Question 10

assumed that the specimens in both 3 and 4 point bending tests suffer the same maximum bending moment, which specimen possesses the higher probability to fail at this stress state? provide and explanation

Answer 10

the specimen subjected to a 4-point bending test might have a higher probability of surviving at the maximum bending moment due to a more favorable stress distribution.

Question 11

explain the term fiber saturation point. Give details regarding: moisture range, the influenced properties

Answer 11

-It refers to the moisture content level in wood at which the cell walls are saturated with water, but there is no free water remaining within the cell cavities
-The fiber saturation point typically occurs when the moisture content of wood is around 25-30%

Wood is more dimensionally stable near the fiber saturation point, reducing size and shape changes compared to the green state

Mechanical strength (tensile, compressive, elasticity) is affected by moisture content, with wood generally stronger in the dry state.

Wood is vulnerable to decay and biological damage by fungi and insects in the presence of moisture, highlighting the importance of managing moisture levels.

Question 12

what is the reason for increase in creep deformation as the size of concrete member decreases?

Question 13

creep, similar to shrinkage, is a process characteristic for every cement-based material, such as cement paste, mortar and concrete. define the term ‘creep’ and explain its different to ‘relaxation’. describe one positive and one negative effect of creep. point out the main difference between shrinkage.

Answer 13

-creep is the gradual deformation or strain that occurs over time under a sustained load.
-Creep happens when a constant stress is applied, leading to a gradual increase in strain over time. On the other hand, relaxation occurs when a material is subjected to a constant strain, and the stress gradually decreases over time.
-Negative effects: *increase of structure deformations
*reduction of pre-stressing level
Positive effects: *reduction of eigenstresses, e.g. due
to shrinkage
*reduction of stresses due to restraint
-shrinkage depends more on moisture loss and other factors and creep more in the long term loads.

Question 14

why creep deformation decreases with an increase in concrete age?

Answer 14

primarily due to factors such as increased strength, drying and curing processes, microstructural changes, evolution of pore structure, and stress redistribution within the material.

Question 15

name the advantages of SCC in comparison to ordinary concrete

Answer 15

easier placement, reduced labor, improved durability, enhanced structural integrity, time and cost savings, improved aesthetics, and decreased noise and vibration during construction.

Question 16

what is the effect of lower evaporable water in the sample on basic creep?

Answer 16

It doesn’t affect

Question 17

decide whether a combination of high ductility concrete with conventional steel reinforcement would be appropriate. justify your answer

Answer 17

Yes, can be appropriate, providing enhanced structural performance.

Question 18

FRC can be produced by mixing concrete with different types of fibers.
name four examples:

Answer 18

-steel fibers
-PVA fibers
-glass fibers
-carbon fiber
polypropylene

Question 19

long term measurement of creep deformations of bridges can often give considerably higher values than predicted.
give possible causes for this discrepancy

Answer 19

material aging, environmental factors (such as temperature and humidity variations), construction errors, and uncertainties in load predictions.

Question 20

by adding fibers to concrete, the material behavior shifts towards a less brittle behavior.

Why a ductile behavior of concrete is desirable? name 3 advantages of ductility

Answer 20

• Increase the bearing capacity
  under static load
  -Redistribution of forces
  -Stress redistribution
  -No concrete spalling
• Increase the structural safety
  (early indication of failure)
• High loading capacity under impact loading
  -Security of structure under high stresses
  -Reduction in repair costs

Question 21

what represent the Eutectic point in the carbon-steel diagram and how is its microstructural phase named?

Answer 21

composition and temperature at which liquid steel transforms into pearlite

Question 22

why should alkali-resistant glass be used instead of normal glass in glass textile reinforced concrete? what would happen if normal glass is use?

Answer 22

to prevent alkali-silica reaction, which can cause cracking and compromise the structural integrity. Normal glass is not suitable for GRC as it is prone to this reaction in alkaline environments.

Question 23

name three advantages of SHCC compared to ordinary concrete.

Answer 23

-increase of deformabillity and energy absorption
-dospertion of cracks
-ductility

Question 24

give two examples where application of SHCC is promising?

Answer 24

• structures in earthquake zones
• explosive structures
• elements with excess load
• impact protection structures
• structures under hydroabrasion
  because of cavitations or wear by
  coarse sediments
• coastal fortifications
• piles
• machine foundations

Question 25

in the linear elastic stage of sigma-epsilon curve under uniaxial tension test the stresses are predominantly taken by the matrix.

true or false?

Answer 25

true

Question 26

SHCC is the less durable due to multiple fine cracks compared to ordinary concrete.

true or false?

Answer 26

false

Question 27

A coarse-grained matrix is a requirement for textile reinforced concrete

true or false?

Answer 27

false

Question 28

adding short fibers to textile reinforced concrete reduces the first crack stress

true or false?

Answer 28

False, it’s higher

Question 29

the addition of short PE fibers to TRC composite helps to lower the creep values under compressive loads.

true or false?

Answer 29

False

Question 30

explain and reason if compressive or tensile eigenstresses in the outer regions of a concrete element are more favorable with respect to the durability of

Question 31

how does the choice of the largest aggregate size (16mm) depend on the intended concrete element?
name two aspects

Answer 31

Larger aggregates can reduce workability, influencing ease of mixing, placing, and compacting.
Aggregate size affects the appearance of the finished surface; smaller aggregates may result in a smoother finish, while larger aggregates can contribute to a more textured appearance.

Question 32

explain the purpose of fine grading of inorganic binders.

state two facts

Answer 32

optimizes workability, adjusts water demand, improves strength and durability, and influences heat evolution during hydration, ultimately enhancing the performance of the concrete mix.