Fresh Concrete (Lecture 6)

Question 1

What are the three main ingredients in portland cement concrete?

Answer 1

Water, aggregate, portland cement

Question 2

What are the desirable properties of fresh concrete?

Answer 2

1) Ease of transport and placement

2) Resistance to bleeding and segregation (too much water and the concrete cannot hold itself together)

Question 3

What are two commonly used methods to measure consistency of fresh concrete?

Answer 3

1) Slump Test- measures the slump after a series of mixing in a cone.
2) Ball penetration test- measures penetration of a 30 lb ball into concrete.

Question 4

What are the accepted slump test results for A) large foundation B) Pavement C) Columns

Answer 4

A) .5-2.5 inches
B) 2.5-5 inches
C) 5-8 inches

Question 5

What is the most important factor that affects the consistency of fresh concrete?

Answer 5

Water content- more water = more slump

Question 6

What other factors affect the consistency of fresh concrete?

Answer 6

Cement content- lowering cement content will produce a harsher mixture which is harder to finish, has better cohesiveness but higher stickiness.
Aggregate size- increasing the max aggregate size increase the slump of fresh concrete
Aggregate angularity and roughness- higher angularity produces lowered slump
Admixture- water reducing air entraining admixtures increase slump

Question 7

What is bleeding?

Answer 7

Bleeding of fresh concrete is manifested by the appliance of water on fate surface after concrete has been placed and compacted. Concrete is weaker on top than bottom. Large amounts of water can get trapped under aggregate and thus be weaker at that location. Latinance (results in a softer surface and dusting.)

Question 8

What are effective remedies to reduce bleeding?

Answer 8

1) Air entrapment and reduce water content
2) Increase proportions of sand and/or cement
3) Use mineral admixtures which reduce water content