Homework Questions Related to Wood

Question 1

Why do aggregates reduce the drying shrinkage?

Answer 1

Drying shrinkage takes place in the paste, while the aggregates remain unaffected. Aggregates reduce drying shrinkage by reducing the volume of the paste, which reduces the proportion of the concrete subject to shrinkage. Additionally, the inclusion of aggregates results in increased elastic modulus, which contributes to better dimensional stability and reduced deformation

Question 2

What causes i) drying shrinkage, ii) creep in hydrated cement paste?

Answer 2

i) Drying shrinkage is caused by the removal of adsorbed water where differential relative humidity is the driving force. This occurs when the relative humidity is less than 100% ii) Creep is due to the loss of adsorbed water where applied stress is the driving force. This can occur at 100% humidity, but only when the concrete is stressed

Question 3

Drying shrinkage may not necessarily lead to crack formations. What are the conditions for cracks to form due to drying shrinkage?

Answer 3

Crack formation occurs when tensile stresses are greater than the tensile strength of the concrete. Tensile stresses are present when the concrete is constrained. Free shrinkage (when the member is free to move) does not lead to crack formation

Question 4

Creep of concrete is in general a beneficial property, but may in some cases be undesirable. Give an example where creep is considered i) beneficial and ii) undesirable.

Answer 4

i) Creep can be desirable when there is applied stress and the concrete is constrained. Concrete will experience stress relaxation and the applied stress will be reduced. Creep can also be beneficial when drying shrinkage has taken place, as it will reduce the tensile stresses caused by drying shrinkage. ii) Creep in columns can lead to differential shortening which leads to moment and shear in floor slabs and beams. Creep in floor slabs and beams can lead to sagging, and in prestressed elements can cause loss of prestress.

Question 5

Let’s take the case where we leave the w/c ratio constant but increase the cement content (i.e. reducing the volume fraction of aggregates). i) How will that affect the creep properties of concrete?

Answer 5

Increasing the ratio of paste to aggregate will lead to more creep

Question 6

How will that affect the shrinkage properties of concrete?

Answer 6

Increasing the ratio of paste to aggregate will also lead to higher amounts of shrinkage

Question 7

Along which direction (L, R, T) will the wood exhibit the highest compressive strength? (Please explain)

Answer 7

Wood exhibits the highest compressive strength in the longitudinal direction. In this direction the wood fibers are aligned with the loading direction. The covalent bonds in this direction dominate the strength

Question 8

Along which direction (L,R,T) will it exhibit the lowest compressive strength? (Please explain)

Answer 8

Wood is weakest in the tangential direction. In this direction the weaker, secondary bonds are resisting stress. Also, since the hollow, tubular cells are aligned longitudinally they will collapse when loaded across the cells.

Question 9

Which of the two a) or b) will exhibit the largest change in dimensions due to the compressive load?

Load applied to longitudinal

Load applied to tangential

Answer 9

Load applied to the tangential direction (b) leads to collapse of the cell walls which causes larger deformations than longitudinal loads

Question 10

Will the compressive strength be different if the specimens are tested at 0% moisture content compared to 12% moisture content? Please discuss.

Answer 10

The compressive strength will be higher when tested at lower moisture content, but only when below the fiber saturation point (FSP). As adsorbed/bound water escapes the cells, the wood densifies and leads to higher strength.

Question 11

Why do microfibrils resist shrinkage in the longitudinal direction in wood?

Answer 11

Microfibrils are comprised of mostly crystalline polymers. These crystalline polymers are aligned in the longitudinal direction and restrict shrinkage in that direction. Water is only adsorbed in the amorphous regions, and not the crystalline regions.

Question 12

What are the sizes and compositions of microfibrils?

Answer 12

The microfibrils are on the order of 30-50 Angstroms x 100 Angstroms in cross section, and around 600 Angstroms in length. They are comprised of glucose rings in a linear crystalline polymer surrounded by a cross-linked amorphous polymer

Question 13

Where are microfibrils located?

Answer 13

Microfibrils are located in the cell walls

Question 14

why do knots weaken wood?

Answer 14

Knots cause stress concentrations that lead to reduced strength

Question 15

what causes the formation of knots?

Answer 15

Knots are caused by branches growing outward through the rings of the tree

Question 16

Many alloys form solid solutions. However, some solid solutions are more effective while others are less effective in increasing the resistance to dislocation motion. Explain why.

Answer 16

Alloying elements that are much different in size from the iron atoms will cause large distortions in the crystal structure of the steel. Examples are Beryllium and Tin. Elements similar in size to iron will not provide as much resistance, such as Nickel and Zinc

Indicate in your phase diagram an alloy that could be precipitation hardened.
Composition C is an example of an alloy that could be precipitation hardened