Lec 8

Question 1

Involves the larger aggregate particles falling towards lower part of the member

Answer 1

Segregation

Question 2

Process of upward migration of water

Answer 2

Bleeding

Question 3

Segregation and bleeding has one similarity

Answer 3

Occur simultaneously

Question 4

Segregation meaning

Answer 4

The particles in the fresh concrete are separated from the cement paste and lose the overall uniformity, that is, the coarse particles are separated from the mortar to form an inhomogeneous paste.

Question 5

Concrete segregation

Answer 5

1. Heavier and larger aggregates sink
2. Coarse aggregate dispersion uniformly

Question 6

What are related to segregation

Answer 6

Honeycomb, cracks, peeling, other defects

Question 7

How honeycomb exists in concrete

Answer 7

the loss of mortar filling the pores between the coarse aggregate

Question 8

Reason for segregation

Answer 8

• When the paste is very wet or the fines are insufficient
  – High slump concrete is also more prone to segregation than low slump
  – Vibrate too long/intensive during the compacting
  – When concrete is pouring in a formwork, the moving distance of pouring is longer

Question 9

What is bleeding

Answer 9

Concrete without proper consistency cannot hold the mixing water. The mixing water slowly flows out of the paste and rises to the concrete surface.
Type of segregation

Question 10

What will be caused by severe bleeding

Answer 10

causes a lot of water loss and brings finer particles to the concrete surface, creating an nonuniform paste.

Question 11

What affect the content of bleeding

Answer 11

High cement content concrete has less bleeding;

Lack of fine sand concrete has more bleeding

Bad proportions or excessive vibration will increase the bleeding tendency of concrete.

Question 12

https://ibb.co/C1TLy9b

When the compressive strength gets into the highest

Answer 12

At the lowest distance (at the bottom)

Question 13

Reduction of bleeding

Answer 13

1. Add air entrained agent
2. Reduce the amount of mixing water
3. Reduce the water/cement ratio
4. Improve fineness of cement or add fine mineral admixtures

Question 14

https://ibb.co/0mWbrY4

Answer 14

1. Less paste 8. More paste
   Increasing the paste content will normally increase μ and decrease τy
2. Air-entraining agent
3. More water content
   Increasing the water content while keeping the proportions of the other constituents constant decreases τy and μ in approximately similar proportions.
4. Pfa (pulverized fly ash) 7. Ggbs

Replacing some of the cement with pulverized fly ash (pfa, 飛灰) or ground granulated blast furnace slag (ggbs, 爐石) will generally decrease τy, but may either increase or decrease μ, depending on the nature of the cement replacement materials (CRM).

5. Plasticiser 6. Superplasticiser
   Adding a plasticizer or superplasticizer decreases τy but leaves μ relatively constant.
   superplasticizers, which can even increase μ, and can therefore be used to give greatly increased flow properties under self-weight.
6. Less water
   The small bubbles of air produced by air entraining agents provide lubrication to reduce the plastic viscosity, but at relatively constant yield stress.

Question 15

Effect of ambient temperature on the water requirement of concrete

Answer 15

The higher the temperature, the higher the water content

Question 16

Why the workability decreases?

Answer 16

1. mix water absorbed by the aggregate if this not in a saturated state before mixing;
2. evaporation of the mix water;
3. early hydration reactions;
4. interactions between admixtures (particularly plasticizers and superplasticizers) and the cementitious materials of the mitrix.

Question 17

https://ibb.co/tQP0JXq
There 3 informations in this graph!

Answer 17

1. Mix 1 and mix 2 has the same cement content
2. No admixture means has high water content
3. More water means less sensitive to different temperatures