Macroscopic Composites
Relatively large, so two important things to consider:
1. How the load is carried
2. How the properties of composite components vary
Common macroscopic composites used by civil engineers:
1. PCC
2. Steel-reinforced Concrete
3. AC
4. Engineered Wood (Glulams, Structural Strand Board)
PCC
1. Cement paste with aggregate particles with different physical and mech. prop.
2. Aggregate particles act as filler (cheaper than portland cement)
3. Aggregate increases volume stability of concrete (cement shrinks)
4. Bond strength determines strength
5. Bond strength affected by roughness and absorption of aggregate particles
Reinforced PCC
1. Viewed as a composite material
2. Consists of plain concrete and steel rebars
3. Low tensile strength (ignored in designing concrete structures), so rebar is placed in.
4. Steel rebars also used in areas subjected to compression (such as columns), and share load support.
5. Steel reinforcing used in prestressed concrete, where reinforcement is prestressed under tension so that a small cross-section of concrete remains under compression even when externally loaded.
6. Steel rebars are used to control cracking (caused by temperature change).
7. Placement of longitudinal and transverse steel bars at the mid-height of pavement will cause tight cracks (which are not harmful to concrete pavement).
8. Bars have a deformed surface to prevent slipping between steel and concrete.
AC
1. Used in pavements
2. 95% aggregate and 5% asphalt binder, by weight.
3. Traffic loads cause compressive stresses to be supported by aggregate-to-aggregate contact.
4. Asphalt acts as binder to prevent particles from slipping.
5. Asphalt gets soft at high temperatures and brittle at low temperatures.
6. Aggregate doesn’t change its properties with temperature fluctuation.
7. Important to select the asphalt grade that will perform properly within the temperature range in which construction is taking place.
8. Since aggregate is a major portion of the mixture, use aggregate with proper gradation and other properties.
9. Properly designed and compacted AC lasts for a long time.
Engineered Woods
1. Manufactured by bonding together wood strands, veneers, lumber.
2. Doesn’t qualify as a composite according to book’s definition because it consists of components of same material.
3. However, it follows a strength mechanism similar to composites, which is why it is typically considered a composite.
4. Alternating grain orientation of the plies of plywood provides identical properties along length and width and provides resistance to dimensional change under varying moisture conditions.
5. Plywood composite has one-tenth of the dimensional change of solid lumber under any temperature or moisture condition.
6. Engineered wood products include:
a. Plywood
b. Glulam
c. Laminated Veneer Lumber
d. Parallel Strand Lumber
e. Oriented Strand Lumber
f. Wood I-Joists
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