HW7

Question 1

What type of porosity in concrete has the strongest influence on the compressive strength?

Answer 1

Capillary porosity.

Question 2

Concrete is a composite material consisting of hydrated cement paste and aggregates. Draw schematically the stress strain curves for the hydrated cement paste, the aggregate and the concrete. (draw all three stress strain curves in same figure and label your curves).

Answer 2

IMG_1

Question 3

Draw a stress strain curve for a strong concrete specimen as compared to a low strength concrete specimen in the same stress strain diagram. Label each curve and show the f’c on each curve. (put values on the stress axis).

Answer 3

IMG_2

Question 4

List at least 4 factors/parameters that influence the strength of concrete.

Answer 4

W/C, age, curing conditions, air content, specimen size, loading conditions

Question 5

Concrete contains coarse aggregates. List two reasons why coarse aggregates are added.

Answer 5

Improve dimensional stability, increase elastic modulus, decrease cost, reduce thermal stresses

Question 6

Concrete specimens are being tested in compression. Some of the specimens are tested under dry conditions, some under wet conditions. Will all concrete specimens exhibit the same elastic modulus? Please explain

Answer 6

No, wet specimens will have a higher elastic modulus because there is water in pores rather than air. Unlike air, water has a non-negligible stiffness and will thus contribute to the stiffness of the concrete. Additionally, there will be no microcracks in the ITZ for the wet specimens.

Question 7

Why is the chord modulus of elasticity a better representation of the elastic behavior of concrete compared to the secant modulus?

Answer 7

The chord modulus is taken starting at a strain of 50 μm/m and thus bypasses the slight concavity that is often seen close to the origin on a stress strain curve.

Question 8

Where does cracking initiate in normal strength concrete?

Answer 8

In the interfacial transition zone

Question 9

Up to what % of the ultimate strength can matrix cracking be considered negligible?

Answer 9

Up to 50% f’c, microcracking in the matrix is limited to the ITZ and is stable, meaning cracks only grow with increasing load