Factors controlling performance of composites
• Physical properties of reinforcement and matrix
• Strength of bond between reinforcement and matrix
• Fibers distributed through given cross section, reinforcement bars only placed where required
• Fibers are short and closely spaced, reinforcing bars are continuous
• Small reinforcement ratio when compared to reinforcement bars
Reinforcement Ratio =Area of Reinforcement/Area of Concrete
Advantages • Easily placed
–Cast
–Sprayed
–Less labor intensive than placing rebar
• Can be made into thin sheets or irregular shapes
• Used when placing rebar is difficult
Disadvantages
• Efficiency factors as low as .4 2-D(spray placement method), or .25 3-D placement (casting method)
• Not highly effective in improving compressive strength
Types of Natural Fibers
• Sisal (Castro, 1981)
– From AgaveSisalana in Mexico
– Durability problems caused by chemical decomposition
in alkaline environment
• Coir (Balaguru, 1985)
– Coconut husks
– Very durable to natural weathering
– Increases modulus of rupture of concrete (MOR)
• Bamboo (Ghavami 2005; Rodrigues, 2006)
– E is very similar to that of concrete
– Susceptible to volume changes in water
– Increases ultimate tensile strength and MOR
• Jute (Balaguru, 1985)
– Grow in India, Bangladesh, China, and Thailand
– Increases tensile, flexural and compressive strengths,
as well as flexural toughness
• Akwara (Balaguru, 1985)
– Abundant in Nigeria
– No dimensional changes due to variations in water
– Alkali resistant
– No changes in flexural or compressive strengths
– Impact strength 5 to 16 times greater than unreinforced
cement matrix
• Elephant Grass (Balaguru, 1985)
– Very durable – good rot and alkali resistance as well as
small dimensional changes
– Increases flexural and impact strength
History of Natural Fiber Reinforcement
• Egyptians used straw in making mud bricks 1200-1400BC (Exodus 5:6)
• 2500 BC asbestos fibers used in Finland to make clay pots (Active Asbestos )
• Hornero bird native to South America builds nests out of straw and clay (Mehta, 2006)
• Replacement for asbestos
Current Uses • Fiber Cement Board
– Siding
– Backer board
– Roofing materials
– Non pressure pipe
• Buckeye Technologies
– UltraFiber 500
– Slab on grade concrete
– Precast concrete
– Decorative concrete
– 85% crack reduction
– Improved hydration
– Improved freeze/thaw resistance
Wood Pulp Fiber Composition
• Cellulose –(C6H12O5)n
– n degree of polymerization
– 600-1500 for commercial wood pulps
– Determines the character of the fiber
– As cellulose increases fiber tensile strength and E increase linearly
• Hemicelluloses – polysaccharides of five different sugars
– Easier to degrade than cellulose
– Highly variable with fiber type
• Lignin – complex polymer
composition
– Binds wood together
– Found in the middle lamella
– Used in concrete as a set retarder
• Extractives
– No physical structure
– Give properties such as color, odor, taste
– Some can be incompatible with concrete
Fresh Properties
• Workability
• Setting time
• Cement hydration
• Fiber clumping/consolidation
• Shrinkage
– Plastic
– Free
– Drying
• Internal curing and autogenous shrinkage
Durability Improvement
• Pressure treatment on fiber cement board
• Reduction in w/c ratio to decrease porosity
• Addition of SCMs eliminated degradation
due to wet/dry cycles (Mohr, 2005)
– 30%, 50% Silica Fume
– 90% Slag
– 30% Metakaolin 235
– 10% SF / 70% SL
– 10% MK235 / 70% SL
– 10% MK235 / 10% SF / 70% SL
• Chemically coated fibers
Conclusions
• Natural fibers offer many benefits for
reinforcement
– Low cost and abundant
– Renewable
– Non hazardous – replacement of asbestos
• Can improve characteristics of concrete
– Increase flexural strength and toughness
– Increase impact resistance
– Reduce shrinkage and cracking
– Improve durability by stabilization of
microcracks and decrease in permeability
Future Research
• Sources of pulp fibers
– Thermomechanical fibers
– Paper mill residual solids
• Optimal fiber ratios for specific uses
• Durability
– Additional research on freeze thaw
– Wet/dry cycle effects
• SCM addition
