Aggregate Properties

Question 1

What is the ideal moisture state of aggregates to use in mixing?
What are the advantages of using a wet vs. an air-dry state?
What is changed in mixture design when using air-dry or wet aggregates?

Answer 1

1. SSD moisture state because there is no excess water, but this state is difficult to obtain
2. Easier to use wet state because excess water is immediately available
3. with air-dry state, the amount of water absorbed is a function of time which makes it more difficult to calculate water content
4. You want to account for the water absorbed or shed by the aggregates in your water/cement ratio and adjust the water/cement ratio accordingly

Question 2

What are the two actions that can be done to cement to increase
early strength and become cement type HE? Explain how
each option works to increase early strength.

Answer 2

1. Increase in C3S content: Produce more CH in cement hydration reaction which contributes to early strength
2. Use a finer grind: Finer grind increases surface area contact which means reaction happens quicker and faster hydration contributes to early strength gain

Question 3

What happens to the compressive strength of concrete if it is
allowed to dry before testing? Why do compressive strength testing standards specify concrete to be completely saturated during testing?

Answer 3

1. When concrete is fully dry, its strength is increased
2. Fully saturated concrete = weakest concrete
   Testing standards want to test the worst-case scenario for a concrete
   specimen and it is easier to test fully saturated concrete (simply
   submerge into water)

Question 4

1. Describe the optimum shape and texture of aggregates in order to:
   a. Improve the workability of fresh concrete
   b. Improve the mechanical properties of hardened concrete?
   Provide justification for your choices

Answer 4

1. Round: low surface area to volume ratio reduces amount of paste smooth: reduces friction between aggregate and paste when mixing
2. Angular: high surface area to volume ratio increases amount of paste required for bonding
   Rough: increases friction between aggregates and paste

Question 5

What are some important aggregate porperties?

Answer 5

1. Size and gradation
2. Shape
3. Texture
4. Density
5. Moisture content

Question 6

What does density affect in concrete?

Answer 6

Choice of aggregates will affect if concrete is lightweight, normal or heavyweight

Question 7

What does size affect in concrete?

Answer 7

Higher maximum size of aggregate decreases amount of paste required

Question 8

How does gradation affect concrete?

Answer 8

1. Continuous gradation (heterogeneous) requires less paste
2. Homogeneous gradation requires the most paste

Question 9

What can an engineer do to reduce the
cementitious paste requirements of concrete by
changing parameters related to aggregates and
why would this work?

Answer 9

1. Increase maximum aggregate size and use continuous gradation