Lec 7

Question 1

Why?
• Concrete is brittle in tension but relatively ductile in compression?
• The permeability of concrete containing very dense aggregates will be even higher by an order of magnitude than the permeability of cement paste (same w/c)?
• Mortar is always stronger than concrete (same cement content, w/c and et al.)

Answer 1

Interfacial Transition Zone

Question 2

What is interfacial transition zone

Answer 2

The structure of cement past t the boundary with aggregate particles differs from that bulk cement oaste

Question 3

What role does ITZ play?

Answer 3

stiffness, strength, and permeability of cementitious materials.
In general, this zone has lower density and decreased strength, compared to that provided by bulk cement paste.
ITZ can be referred to as the “weak link” in concrete.

Question 4

https://ibb.co/YhYGBVH

Answer 4

1. Transition zone
2. Bulk cement paste
3. Nonreactive aggregate
4. Interfacial region
5. Bulk paste

Question 5

Microstructure of ITZ

Answer 5

1. There is a water layer on the surface of coarse aggregate, and the local w/c near the CA is higher than the value of hydrated cement paste.
2. Mainly ettringite and CH from hydration of C3S, C2S and C3A
3. Due to the high w/c, large crystals are forming a porous structure. CH is plate-like.
4. C-S-H, CH and ettringite continue to fill the poor structure.

Question 6

What structure of ITZ differ it from bulk paste

Answer 6

⚫ There is less unhydrated cement.
⚫ There is higher porosity (and a lower density). The pores are
generally larger than those in the bulk paste.
⚫ There is less C-S-H.
⚫ There are large, oriented crystals of calcium hydroxide.
⚫ There is generally a greater concentration of ettringite.

Question 7

How much ITZ in typical concrete?

Answer 7

Up 20-40% of the total volume

Question 8

https://ibb.co/GkbDptd
Which image is the failure stress?
Sort the number in the graph from the top!

Answer 8

The 4th

4,3,2,1

Question 9

https://ibb.co/37RmbyW

Answer 9

1. Aggregate
2. Hardened cement paste
3. Transition zone
4. Crack path and direction of propagatiom

Question 10

3 general behaviors

Answer 10

1. Fluidity/consistency: It must be capable of being handled and of flowing into the formwork and around any reinforcement, with the assistance
2. Compatibility: of the air entrapped during mixing and handling should be capable of being removed by the compacting system being used
3. Stability/cohesiveness: The concrete should remain as a homogeneous uniform mass. For example, the mortar should not be so fluid that it segregates from the coarse aggregate.

Question 11

What property of general behaviors when combined together then become workability

Answer 11

Fluidity and compatibility

Question 12

What does workability represent

Answer 12

The ability of concrete to be mixed, transported, and poured under conditions of minimal loss of uniformity

Question 13

Workability is expressed in terms of three separate:

Answer 13

Consistency, mobility, compatibility

Question 14

What factors affecting workability of concrete

Answer 14

Physical properties of cement and aggregate,
mixing ratio,
equipment for mixing,
transporting and pouring,
size and spacing of steel ,
size and shape of structures

Question 15

What are the characteristics of good workability

Answer 15

Adequate amount of cement, appropriate content of fine particles and water, low content of coarse particles

Question 16

For the matrix to have good plasticity, what should we do.

Answer 16

The fines content needs to be higher

Question 17

What is the result of insufficient fines

Answer 17

Will produce rough concrete, easy to segregate, causing difficulty in mixing and pouring

Question 18

1. Smooth natural gravel
2. More edges and slender crushed sandstone
   Which one has better workability

Answer 18

1

Question 19

What happen when the temperature is high?

Answer 19

the concrete setting time may be shortened and the hardening will be accelerated. At this time, additional water is required to maintain a certain workability.

Question 20

What does consistency meaning in workability

Answer 20

indicator of the mobility of concrete, which depends on the proportion and constituent materials

Measured by slump test, the result is called slump

Question 21

What is the general slump of concrete

Answer 21

5 to 18 cm

Question 22

How to increase the slump up to 20-25 cm

Answer 22

by adding high- efficiency water reducing agent or strong plasticizer, and it would become a flowable concrete.

Question 23

How slump loss happened

Answer 23

When concrete is standing still after mixing, the slump will decrease with time due to hydration and evaporation of water

Once the mixing water and the aggregate gradation change

when cement dosage, fineness and brand or room temperature change

Question 24

Can slump represent the quality or workability of concrete?

Answer 24

The slump value cannot fully represent the quality or workability of the concrete, nor is it suitable for comparing the quality of different batches of concrete.

Question 25

What is Bingham model

Answer 25

Related to behavior of fresh paste, mortar, and concrete Flow starts when the applied shear stress reaches a yield stress (τy) sufficient to overcome the interparticle interference effects, and at higher stresses the shear rate varies approximately linearly with shear stress, the slope defining the plastic viscosity (μ).

Question 26

https://ibb.co/ftNZrDV

Answer 26

1. Conventional concrete 2. Segregation zone 3. SCC

Question 27

Test of measurement of workability

Answer 27

1. Slump test 2. Compacting factor test 3. Vebe test 4. Slump flow test (mostly for mortar or self-consolidating concrete) 5. Penetration test

Question 28

Explain slump test

Answer 28

1. The most common test 2. For particles size smaller than 381 mm 3. Has truncated cone (d=100mm, h= 300mm), semi-spherical dome bar (d= 16mm, l= 600mm)

Question 29

The process of slump test

Answer 29

Page 22 1. The cone is filled with concrete in 3 equal layers, and each layer is compacted with 25 tamps of the tamping road 2. The cone is slowly raised and the concrete isnallowed to slump under its own weight 3. The slump is measured to nearest 5 or 10 mm using the upturned cone and slump rod as a guide

Question 30

In traditional concrete, what is Self-Compacting Concrete (SCC)

Answer 30

Does not require tamping, it has the advantages of automatic pouring and reducing construction noise. It can flow through narrow reinforcement gaps or sections to achieve a sufficient filling formwork. (Invented by JP)

Question 31

Types of slumps result https://ibb.co/3WPYD4X

Answer 31

1. Near-zero slump: <2.5 cm, too low water content, poor workability 2. Trye slump: good workability 3. Shear slump: insufficient strength of concrete cones, lack of plasticity and adhesion. Poor workability; less suitable for construction 4. Collapse slump: 18-25 cm. Poorly mixed or too much water. Coarse stay in the middle; placement difficulty 1. True, 2. Shear 3. Collapse

Question 32

Explain compacting factor test https://ibb.co/BjYrY3s

Answer 32

1. Concrete is loaded into the upper hopper 2. The trap door is opened, and the concrete falls into the lower hopper 3. The trap door is opened, and the concrete falls into the cylinder 4. The concrete is struck off level with the top of the cylinder 5. The cylinder + concrete is weighted, to give partially compacted weight of concrete 6. The concrete is fully compacted, and extra concrete added to fill the cylinder 7. The cylinder + concrete is weighed, to give the fully compacted weight of concrete Compacting factor= (weight of partially compact)/(weight of fully compact)

Question 33

What is compacting factor test in short

Answer 33

Test for compaction at standard energy • Test procedure – Fill the top hopper with concrete first, then drop it into the middle hopper, and then drop it into the bottom test mold. – Divided the test density by the fully compacted density to obtain the "compaction factor "

Question 34

Explain vebe test https://ibb.co/tcZy4Jr https://ibb.co/pyVMh67 IMPORTANT

Answer 34

Process: 1. Slump test is performed in containers 2. A clear perspex disc, free to move vertically, is lowered ontol the concrete surface 3. Vibration at a standard rate is applied Vebe degrees in seconds to complete covering the underside of the disc with concrete

Question 35

Vebe test in general

Answer 35

Mostly used in Europe By re-molding Suitable with plastic properties For particles small than 40 mm

Question 36

Explain slump flow test https://ibb.co/jTkMX1n

Answer 36

METHOD WAKTU LAB ENG MATERIAL Flow = final diameter of the concrete Measure the flow capacity under vibration Test method—fill the concrete, then lift the cone, raise it 12.7 mm on the vibration table 25 times in 15 seconds, and then drop it to make the concrete specimen spread-out, and finally measure its flow diffusion value. • Slump is caused by the self-weight of concrete without vibration • The slump flow test uses vibration to force the concrete to deform.

Question 37

Explain penetration test https://ibb.co/XkHgqhw

Answer 37

Use The Kelly Ball Apparatus For fresh concrete (quick checking) • Penetrate the concrete with a penetrator and measure its penetration to assess the consistency of the concrete • Suitable for construction site, the test process is very fast and can be tested on the trolley hopper or on the board of formwork.