Materials Lecture 8

Pros of Steel

Strong both in tension and compression

Cons of Steel

Limit in shapes, costly, easy to corrode

Pros of Concrete

Less expensive, can be virtually cast into any shapes, corrosion resistant

Cons of concrete

Weak under tension, better in carrying compressive load (still much weaker than steel)

What is concrete composed of?

• aggregates ((60-75% of volume)

  • coarse aggregates

  • fine aggregates

• cement

• water

• admixture (very small amount)

What is the purpose of aggregates in concrete?

• Provide structural stability when it binds with hydrated cement

• Act as filling materials

Example of coarse aggregate

Gravel and crushed stone

Example of fine aggregates

Sand

Two types of voids in aggregates

• permeable voids

  • typically on the surface, and accessible to water

• impermeable voids

  • within the interior of aggregates, inaccessible to water

Bone Dry

No water absorbed (usually achieved through baking in oven)

Air Dry

Aggregates dried in air, with moisture content (MC) unclear

W_moist

weight of water absrobed by aggregate

W_s

weight of dry aggregate

Saturated-surface dry (SSD)

• all permeable voids are filled with water, by the surface remains dry

• moisture content at SSD is called absorption

  • MC at SSD = Absorption

Moist

all permeable voids are filled with water, and the surface is also wet

Specific gravity

weight of material/weight of water of equal volume

V_s

volume of the solid part of aggregate

V_i

volume of the impermeable void of aggregate

V_p

volume of the permeable void of aggregate

γ_w

density of water (1000 kg/m³)

V_pγ_w

weight of water absorbed in permeable voids

v_s + v_i

volume of aggregate not accessible to water or W_sub

Measurement of specific gravity: coarse aggregate

1. coarse aggregates are soaked for 24 hours

2. obtainthe weight of aggregates submerged in water is measured: W_sub

3. Aggregates are dried to the SSD condition and obtain W_SSD

4. Aggregates are dried completely, and obtain W_s

W_sub

weight of aggregates submerged in water or the difference between the weight of dry aggregate W_s and the weight the water with the volume of aggregate inaccessible to water

Measurement of specific gravity: Fine Aggregate

1. Sand is soaked in water for 24 hours, then dried back to SSD condition, and obtain its weight W_SSD

2. Fill a constant-volume flask with water and obtain its weight W_fw

3. Empty flask and then place 500- SSD sand in flask, then fill water to the same volume. Obtain the weight W_fws

4. Take out sand from flask, dry them completely, and obtain W_s

Why is a uniform size distribution bad for concrete?

leads to a large fraction of voids, which is independent of the size of the particles

How to fix large fraction of voids in concrete

mixing particles with different sizes

Moisture Content (MC)

W_moist/W_s