BDCS Notes - Macroscopic Composites

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

BDCS Notes - Microscopic Composites

Microscopic Composites

Two constituent phases of microscopic composites:

1. A Continuous Phase (Matrix)

2. Dispersed Phase (Reinforcing Phase) (Surrounded by Matrix)

The dispersed phase is harder and stiffer than the matrix. The properties of the composite depend on the properties of both component phases, their relative properties and the geometry of the dispersed phase (such as particle shape, size, distribution and orientation).

The Dispersed Phase

Two Basic Classes of Microscopic Composites:

1. Fiber-Reinforced

2. Particle-Reinforced

The classification above is based on the shape of the dispersed phase. The microscopic composites have three different schematics:

1. Aligned Fibers

2. Random Fibers

3. Random Particles

Fiber-Reinforced Composites – Include fibers dispersed in a matrix such as metal or polymer. Fibers have a high strength-to-diameter ratio, with near crystal-sized diameters. That is to say, because of the small diameter of the fibers, they are much stronger than the bulk material. Fibers are much stronger than the bulk form, because they have fewer internal defects.

Classification of Fibers – On the basis of their diameter and character as whiskers, fibers and wires.

Whiskers – Very thin single crystals, have extremely large length-to-diameter ratios. Very strong. High degree of crystalline perfection. Not commonly used in reinforcement, because of high costs and poor bonding, not to mention the difficulty of incorporating them into the matrix.

Fibers – Manufactured from many materials, such as glass, carbon and graphite. Larger diameters than whiskers. Because of low cost and high strength, glass fibers are most common of all reinforcing fibers. Glass fibers are available in several forms suitable for different applications. Common glass fibers include veils, rovings (continuous fibers) and mats. A common fiber-reinforced composite is fiberglass (glass fibers in a plastic matrix).

Wires – Have a larger diameter than fibers. Least used of all three.

Particle-Reinforced Composites – Particles dispersed in a matrix phase. The strengthening mechanism of particle-reinforced composites varies with the size of the reinforcing particles. When the particles are small (in microns) the matrix bears most of the applied load. When the particles are larger than 1 micron, particles act as fillers to improve the properties of the matrix phase and/or to replace some of its volume. Applied load is then shared by matrix and dispersed phases. The stronger the bond between dispersed particles, the larger the reinforcing effect.

The Matrix Phase

Matrix – Polymers (plastics) or metals are used in most microscopic composites. The matrix binds the dispersed materials (particles and fibers) together. It protects the materials from environment and damage.

Benefits of Polymers:

1. Low Cost

2. Easy Possibility

3. Good Chemical Resistance

4. Low Specific Gravity

Shortcomings of Polymers:

1. Low Strength

2. Low Modulus

3. Low Operating Temperatures

4. Low Resistance to Prolonged Exposure (UV Rays)

Metals tend to excel in the areas where polymers are lacking. The metals most commonly used as a matrix phase in composites are aluminum and titanium alloys.

Fabrication –Composites are formed by combining matrix and dispersed material. Fabrication methods are based on the chemical nature of the matrix, dispersed phases and temperature to form, melt and cure the matrix.

Pultrusion – An automated process for manufacturing fiber-reinforced composite materials into continuous, constant-cross-section profiles.

Pultrusion Process:

1. Reinforcement Material

2. Resin Bath

3. Heated Die

4. Puller

5. Saw

6. Finished Product

Civil Engineering Applications:

1. Structural Shapes

2. Concrete Reinforcement (Instead of Rebar)

3. Tanks

4. Industrial Flooring

5. Trusses and Joints

6. Walkways and Platforms

7. Waste Treatment Plants

8. Handrailings

9. Plastic Pipes

10. Light Poles

11. Door and Window Panels and Frames

12. Electrical Enclosures

13. Strengthen and Wrap Columns and Bridge Supports

Fiber-Reinforced Concrete – Fiber can impede the progression of cracks in concrete. They also increase the tensile and flexure strength of concrete so that a more efficient structural member can be designed (up to two to three times the flexural strength of unreinforced concrete). The material under increased load does not fail abruptly, but yields gradually because of the fibers.

Entrained Air – A component in a microscopic composite material. Entrained air increases the durability of concrete since it releases internal stresses due to freezing of water within the concrete. For a same water-to-cement ratio, however, air bubbles reduce the concrete strength by about 20%. Entrained air helps workability in concrete, the water-to-cement ratio can be reduced to compensate for some of the strength reduction.

BDCS Notes - Properties of Composites

Properties of Composites

Composite materials are affected by:

1. Component properties

2. Volume fractions of components

3. Type and orientation of the dispersed phase

4. Bond between dispersed phase and matrix.

“The properties of the components can be viewed as the weighted averages of the properties of the components.”

Assumptions generally made to simplify the analysis of composite materials:

1. Each component has linear, elastic and isotropic properties.

2. A perfect bond exists between dispersed and matrix phases (no slipping).

3. The composite geometry is idealized and the loading pattern is parallel or perpendicular to reinforcing fibers.

LOADING PARALLEL TO FIBERS

Isostrain Condition – When loads are applied to aligned fiber-reinforced composites parallel to fibers, matrix and fiber phases deform equally.

e_c = e_m = e_f = e

e_c = total strain

e_m = composite

e_f = matrix strain

e = fiber strain

And the force applied to the composite F_c is the sum of the force carried by the matrix F_m and the force carried by the fibers F_f:

F_c = F_m + F_f

F_c = F_m + F_f

Thus, σ_cA_c = σ_mA_m + σ_fA_f

_{σi = stress of component i}

A_i = area of component i

BDCS Notes - Composites

Composites

The need for materials with properties not found in conventional materials, combined with advances in technology, have resulted in combining two or more materials to form composite materials.

Qualities of Composites That Can Be Improved:

1. Strength

2. Stiffness

3. Specific Weight

4. Fracture Resistance

5. Corrosion Resistance

6. Wear Resistance

7. Attractiveness

8. Fatigue Life

9. Temperature Susceptibility

10. Thermal Insulation

11. Thermal Conductivity

12. Acoustical Insulation

Fiberglass – An example of a composite material that is useful for civil engineering. Strong, stiff and corrosion resistant. Can be used to make concrete reinforcing rebars to replace corrosive steel rebars. These combinations of properties are formidable and cannot be found conventionally.

Other Examples:

1. Straw to strengthen mud in masonry construction

2. Metal armor

3. PCC

4. Fiber-reinforced concrete

5. Car metal for new automobiles

6. Fiber-reinforced and particle-reinforced plastics

7. Naturally formed composites (such as wood (cellulose and lignin))

8. Bone (protein and collagen)

Composites are classified as either:

Microscopic – Include fibers or particles in sizes up to a few hundred microns.

Macroscopic – Constituents of much larger size, such as aggregate particles and rebars in concrete.

BDCS Notes - Wood Production

Wood Production

Trees are harvested in fall or winter, because water content and environmental concerns (with regards to fires) are ideal then. Wood is harvested from forests as logs, and transported to saw mills to be cut into dimensional shapes.

Dimension Lumber: Wood from 2 inches to 5 inches. Sawn on all four sides. Common shapes are 2x4, 2x6, 2x8, 2x10, 2x12, and 4x4. Surfacing generally removes ¼ to 3/8 in. per side. Used for studs, sill and top plates, joints, beams, rafters, trusses and decking.

Heavy Timber: Wood larger than 6 inches in one dimension. Sawn on all four sides. Common shapes are 4x6, 6x6, 8x8 and larger. Surfacing generally removes 3/8 in. per side. Used for heavy frame construction, landscaping, railroad ties, and marine construction.

Round Stock: Posts and poles used for building poles, marine piling, and utility poles.

Engineered Wood: Products manufactured by bonding together wood strands, veneers, lumber, and other forms of wood fiber to produce a larger and integral composite unit. Types include structural panels (plywood, oriented strand board, composite panels), glued laminated timber (Glulam), structural composite lumber, and composite structural members.

Specialty Items: Milled and fabricated products that reduce on-site construction time, including lattice, handrails, spindles, radius edge decking, turned posts, etc.

Sawn Wood Production: (1) Sawing into desired shape, (2) seasoning, (3) surfacing, (4) grading, (5) preservative treatment (optional).

Surfacing (planing) of wood products can be done before or after drying. Post drying surfacing is superior, because it removes small defects developed during the drying process. When surfaced before seasoning, dimensions are slightly increased to compensate for shrinkage during seasoning.

Cutting: Harvested wood is cut into lumber and timber at saw mills using circular saws, band saws, or frame saws. The most common patterns for sawing a log are plain (slash), quarter, and combination sawing.

Categories of Sawing:

1. Flat-sawn, 45^o or less

2. Rift-sawn, 45 ^o to 80 ^o

3. Vertical- or edge-sawn, 80 ^o to 90 ^o

Flat-sawn boards have desirable exposure of grain for decorative applications. Flat-sawn boards tend to distort more than vertical-sawn boards in response to moisture fluctuations. Therefore, vertical-sawn boards are used more for structural applications.

Plain-sawing is rapid and economic straight-cutting of lumber whereas quarter-sawing maximizes the amount of vertical-sawn cuts.

Seasoning: Green wood contains 30% to 200% moisture by the oven-dry weight. Seasoning removes excess moisture from wood. For structural wood, recommended moisture is 7% in the dry southwestern states to 14% in the damp coastal regions. Framing lumber has an average moisture content of 15%.

Wood is seasoned by air and kiln drying. Air-drying is inexpensive, but slow. The green lumber is stacked in covered piles to dry. These piles of lumber are made of successive layers of board separated by 1-inch strips so that air can flow between layers.

Typically, it takes three to four months to dry wood.

After air-drying, the lumber may be kiln dried. A kiln is a large oven where all variables can be closely monitored. Drying temperatures in a kiln range from 20 C to 50 C, typically requires 4 to 10 days.

Drying to quickly results in cracking and warping.