O U T L I N E C O N S T R U C T I O N M A T E R I A L S C O M P O S I T E C O M P O S I T E. Introduction & History

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1 O U T L I N E Introduction & History C O N S T R U C T I O N M A T E R I A L S FIBER-REINFORCED COMPOSITES 2010 Praveen Chompreda, Mahidol University Fiber-Reinforced Polymer (FRP) Fibers and Matrices Properties Types & Applications FRP in Structural Retrofit Durability & Failure Fiber-Reinforced Cement Composite (FRCC) Concrete vs. FRCC Fibers Usage Advantages & Disadvantages Mechanical Properties Structural Applications 2 C O M P O S I T E C O M P O S I T E Composite generally means the material that has two or more distinct constituent materials. The constituents generally have significantly different physical and mechanical properties The properties of composite are different from the properties of constituents Composite allows us to create a material with properties not otherwise found in one material. Strength, stiffness, fatigue Specific weight Corrosion resistance, wear resistance Thermal insulation, thermal expansion, thermal conductivity Basic composite consists of 2 phases Continuous Phase or Matrix Dispersed Phase or Reinforcing Phase Properties of composite depends on the properties of both phases, their relative properties, and geometry and distribution of the dispersed phase Matrix Dispersed Phase (particle, randomly distributed) Matrix Dispersed Phase (fiber, continuously aligned) Matrix Dispersed Phase (fiber, randomly aligned) 3 4

2 C O M P O S I T E Composite can be classified as Microscopic and Macroscopic. However, the scale can be quite arbitrary. Microscopic Composites: Fiber-Reinforced Composites (Aligned or randomly oriented) Particle-Reinforced Composites Macroscopic Composites Concrete (Aggregate in Cement Paste Matrix) Asphalt Concrete (Aggregate in Asphalt Cement Matrix) Reinforced Concrete (Rebars in Concrete Matrix) In this class, we will talk about Fiber-Reinforced Composites I N T R O D U C T I O N The concept of fiber-reinforced composite has been around for a long time The use of straw to reinforce sun-dried brick or adobe houses Asbestos were used to reinforced cement mortar Horse hairs were used to reinforced plaster 5 6 I N T R O D U C T I O N I N T R O D U C T I O N Only in the past 50 years that the composite technology has been extensively studied The fiber-reinforced composites are widely used in aerospace industry to save weight Sport industry also manufacture their latest equipments with fiber composites This figure shows the increase in the height jumped in the Olympics when the pole material was changed from hickory wood to bamboo, aluminum, and finally glass fiber reinforced composite. 7 The entire fuselage of the Boeing s newest 787 Dreamliner jet will be built with carbon fiber Lance Armstrong's seven Tour de France victories can be attributed partly to his carbon fiber bicycles 8

3 T Y P E S O F F I B E R C O M P O S I T E S T Y P E S O F F I B E R C O M P O S I T E S The use of fiber-reinforced composite in civil engineering was widely explored only in the past 20 years There are two types of fiber-reinforced composite used in civil engineering applications Fiber-Reinforced Polymer (FRP) Reinforcement Fiber-Reinforced Cement Composite (FRCC) or Fiber- Reinforced Concrete (FRC) Fiber-Reinforced Polymer (FRP) Continuous aligned fibers in resin or epoxy matrix Brittle fiber in ductile matrix Matrix generally have lower strength and stiffness than the fiber Improvement is in the precracking behavior. Matrix transfer stress to fibers. Fiber-Reinforced Cement Composite (FRCC) or Fiber- Reinforced Concrete (FRC) Short randomly-oriented fibers in concrete/ cement matrix Ductile fiber in brittle matrix Fiber generally have higher strength than the matrix. The stiffness may be more or less Improvement is in the postcracking behavior. Fibers help reduce cracking in the matrix by bridging the cracks T Y P E S O F F I B E R C O M P O S I T E S FRP Ductile Matrix Ductile Matrix Ductile Fiber Brittle Fiber Fiber-Reinforced Composites Brittle Matrix Brittle Matrix Ductile Fiber Brittle Fiber I Fiber Reinforced Polymer (FRP) Reinforcement Fibers and Matrices Properties Types & Applications Advantages & Disadvantages Durability & Failure FRCC 11 12

4 F R P FRP is the use of fiber embedded in the plastic matrix as an internal or external reinforcement Fibers are usually stronger in one direction but brittle in the other Matrix keeps fibers aligned together in the strong direction, transfer the stresses to the fiber, and protect them from environment Matrix generally have much lower strength than the fiber Strength of composite is low in the perpendicular direction to fiber F R P T Y P E S 4 broad types of FRP found in civil engineering applications Fabric Sheet Plate/ Laminated Bar Mesh Fiber Matrix F F F R P F A B R I C F R P F A B R I C FRP Fabric Fibers are woven into a fabric. Available in rolls. Can be cut into any shape and size. Require field application of resin matrix Carbon Fiber 1 st Resin Coat Epoxy Putty Filler Primer 2 nd Resin Coat Protective Coating Applications Structural repair/ strengthening Advantages Easy to install, requires minimal special equipments More aesthetically pleasing than other types of repair Light weight Corrosion resistance Can wrap around complex shapes Source:

5 F R P P L A T E / L A M I N A T E FRP Plate/ Laminate Fibers are impregnated in hard resin from the factory. Attach to structure directly using special epoxy adhesives May contain several layers of fibers Unidirectional Multi-Directional F R P P L A T E / L A M I N A T E Applications Structural repair/ strengthening Advantages Easier and faster to install than fabric type More aesthetically pleasing than other types of repair Light weight Corrosion resistance Source: F R P B A R FRP Bar Fibers are aligned in one direction and impregnated in resin into a bar shape, similar to steel reinforcing bar Available as both prestressed/ nonprestressed bars FRP Reinforcement Bar Applications Primary internal reinforcement External posttensioning Advantages Corrosion resistance No electromagnetic effects (for rooms with sensitive equipments, such as MRI) Source: ICRI (2006) FRP bar in bridge construction 19 Source: 20

6 F R P M E S H FRP Mesh Fiber composites are made into small thread and woven into a mesh, which can be used to reinforce thin sections F R P M E S H Applications Thin-shell constructions Overlay repairs of concrete surface Benefits Lightweight Corrosion resistance Source: F I B E R M A T E R I A L S Several materials may be used as fibers Glass Fibers Carbon-Fibers Synthetic Fibers (Aramid, Kevlar, Spectra) F I B E R S Most fiber materials have very high tensile strength Most also have linear-elastic tensile stress-strain behavior up to failure Fiber Modulus of Elasticity (GPa) Ultimate Tensile Strength (MPa) Glass Carbon Aramid Kevlar Source: Source: Source: 23 Steel (Reference)

7 F I B E R S M A T R I X Matrix can be made up of various thermosetting and thermoplastic polymers: Polyester Resin Epoxy Resin Nylon The property of matrix and fiber must be compatible Elastic Modulus/ Poisson Ratio Tensile Strength Coefficient of Thermal Expansions Chemically compatible with fibers In addition, the matrix should have the following properties Good adhesive properties to the fiber Thermal stability Low curing shrinkage Durable Relatively inexpensive F R P I N S T R U C T U R A L R E T R O F I T F R P I N S T R U C T U R A L R E T R O F I T FRP fabric and plates are widely used for structural retrofits Repair damages Increase load capacity of structure Bond to concrete surface using epoxy resin Generally used in: Beams Slabs Columns Source:

8 R E T R O F I T T I N G S Y S T E M ( S H E E T S ) Surface preparation is essential for a good bonding Carbon Fiber sheet is bonded using resin adhesive Protective layer is for durability and aesthetic Carbon Fiber 1 st Resin Coat Epoxy Putty Filler Primer 2 nd Resin Coat Protective Coating R E T R O F I T T I N G P R O C E D U R E S ( S H E E T S ) Surface Preparation Cleaning of surface Apply Primer Coat Apply Putty Filler to fill out the voids Application of CRFP Sheet Apply first resin coating Attach the CFRP sheet and remove the backing paper Apply second resin coating Protective Layer Apply the protective coating R E T R O F I T T I N G P R O C E D U R E S ( S H E E T S ) Surface Preparation Cleaning of surface Apply Primer Coat Apply Putty Filler to fill out the voids Application of CRFP Sheet Apply first resin coating Attach the CFRP sheet and remove the backing paper Apply second resin coating Protective Layer Apply the protective coating R E T R O F I T T I N G P R O C E D U R E S ( S H E E T S ) Surface Preparation Cleaning of surface Apply Primer Coat Apply Putty Filler to fill out the voids Application of CRFP Sheet Apply first resin coating Attach the CFRP sheet and remove the backing paper Apply second resin coating Protective Layer Apply the protective coating 31 32

9 F R P R E I N F O R C I N G S C H E M E S D U R A B I L I T Y & F A I L U R E S Although FRP can be used effectively to increase the strength of existing structure, some considerations are needed: Reduced Ductility of structure Bond Failure Sudden failure of FRP Long-Term Durability D U R A B I L I T Y & F A I L U R E S FRP increases the strength but reduced the ductility (i.e. smaller deflection at failure) in CFRP retrofitted system Load-Deflection Relationship for Flexural Specimens 2 Layers of CFRP Sheet 1 Layer of CFRP Sheet D U R A B I L I T Y & F A I L U R E S Bond Failure may occur before the FRP reaches its tensile strength due to: Insufficient bonding length Improper application technique (surface was not cleaned, wrong type of glue, air bubble, etc ) Different thermal expansions between concrete, glue, and FRP system Glue deteriorated over time Strong impact causes local failure and leads to delamination Load (kg) Reinforced Concrete Beam (Control) Deflection (mm.) 35 36

10 D U R A B I L I T Y & F A I L U R E S Failure of the FRP-Retrofitted system by tensile rupture of fibers can be quite sudden Must designed such that sufficient yielding of steel reinforcement occurs before FRP ruptures This requirement and the delamination problems limit the maximum amount of FRP that can be used D U R A B I L I T Y & F A I L U R E S One of the primary reason why FRP is popular is that it does not corrode like steel, leading to low maintenance and repair costs. However, under certain chemical, thermal, and mechanical loadings, the durability of FRP composite may be degraded. Currently, very little data is available on the durability of FRP-retrofitted system Common durability problems: UV causes deterioration of Epoxy Resin Glass fiber may be damaged by Alkali Temperature cycles causes degradation of bond between epoxy resin and concrete C O N C R E T E V S F R C C s II Fiber-Reinforced Cement Composite (FRCC) Concrete vs. FRCC Fibers Usage Advantages & Disadvantages Mechanical Properties Structural Applications Concrete is very weak in tension (tensile strength is about 10% of its compressive strength) Microcracks (seen under microscope) develops early, even at 10-15% of the ultimate load In order to use concrete in a structural system, something else has to resist the tensile force Large Scale Reinforcement Bar (Reinforced Concrete) or Strands (Prestressed Concrete) Meshes (Ferrocement) Small Scale Discontinuous Fibers (FRCC) 39 40

11 F R C C s FRCC is a composite containing randomly-oriented fibers in concrete/mortar matrix Fibers help resisting the tensile force after matrix cracking Fibers are introduced during the mixing Fiber Matrix (Mortar or Concrete) A P P L I C A T I O N S Typical applications Precast components Slab on ground Pavement/ Runway Shotcrete Special applications Blast-resistant Seismic resistant structure Source: Matrix Fiber 41 Source: 42 A P P L I C A T I O N S B E N E F I T S The benefits of using FRCCs are through: Strength Durability Construction Process Source: Note that performance benefits depends on the types of fiber, types of matrix, and the amount of fibers used. Fibers are not equal. Source: Source:

12 B E N E F I T S Improvements to the strength Increase tensile and flexural strength of mortar/concrete. Thus the structure is more difficult to crack Decrease crack widths and damages Increase energy absorption capacity suitable for seismic applications C O N C R E T E V S F R C C s Tensile Stress-Strain behavior of FRCC and Concrete TENSILE STRESS B E N E F I T S Large Crack! B E N E F I T S Improvements to the durability Limit plastic and drying shrinkage cracking Control crack width Less corrosion problems (crack is too small for water to penetrate through) Damage in Well-Designed Reinforced Concrete Beam Damage in 2% Fiber FRCC Beam Due to the smaller crack widths developed in FRCCs, damage is much smaller than regular concrete 47 Mortar Steel FRCC 48

13 B E N E F I T S Improvements to the construction process Reduce or eliminate the need of steel reinforcements Saving in labor cost and time to tie the steel Easier to pour the concrete Easier to repair (for example, after earthquake) due to small damage occurred Reduce the number of construction joints in large slabs L I M I T A T I O N S High cost of fibers (will be lower if mass-produced) Higher cost of matrix than typical concrete (smaller and lesser aggregates, need more superplasticizer to ensure workability) Difficulty in mixing Difficulty in estimating mechanical property values for the design (large variability with different fibers and matrices) No standard or widely-accepted test methods for mechanical properties More research data are needed in some areas Difficulty in demolition and recycling Joints are undesirable in warehouses or factories F I B E R S F I B E R & M A T R I X Several materials may be used to make fibers Metals Synthetic polymers Natural Fibers However, not all of the fibers can develop good behavior Fibers must be used with a compatible cement-based matrix (strength must not be too high or too low) in an appropriate amount (not too much, not too little) 51 52

14 F I B E R S Typical properties of Fibers T Y P E S O F F I B E R Monofilament Fiber is the fibers that are separated from one another, such as steel fiber Fiber Assemblies or Bundled Fibers are usually bundles of small fibers, such as glass, carbon, Kevlar, and natural fibers. They stay bundled even when dispersed in matrix F I B E R G E O M E T R Y Diameter, d Length, L Aspect Ratio is the ratio between the length (L) and the diameter of fiber (d). L Aspect Ratio= d Long fiber is the fiber with high aspect ratio. F I B E R D I S P E R S I O N Fibers may be dispersed in matrix as 1D, 2D, or 3D They may be aligned in particular direction or random 3D dispersion requires the member dimensions to be much larger than the fiber length If one dimension of the member is small, the alignment may be 2D 55 Source: Bentur and Mindess (2007) 56

15 F I B E R D I S P E R S I O N Uniformity of dispersion depends on the mixing efficiency, fiber dimensions, amount of fiber, and matrix consistency Excessive vibration may cause fibers in thick member to align in 2D distribution M E C H A N I C A L P R O P E R T I E S O F F R C C s Volume Fraction v fiber V= f v composite F A C T O R S A F F E C T I N G P R O P E R T I E S C O M P R E S S I V E S T R E N G T H There are several factors affecting the mechanical properties of FRCC materials Fiber Geometry (Bond Characteristic) Diameter Length Shape Fiber Materials Tensile Strength Modulus of Elasticity Bond Strength Amount of Fiber in a unit volume Volume Fraction Number of Fibers/ Space between Fibers Strength of the mortar/concrete matrix Orientation of Fibers in Matrix Size of member relative to fiber size (Size Effect) Due to various factors affecting the behavior, a laboratory test is needed to evaluate its mechanical properties for a given mixture and fiber Empirical formula are available but are limited to few types of fibers Tested method is similar to typical concrete using cylindrical specimen 59 60

16 C O M P R E S S I V E S T R E N G T H Compressive strength of FRCC having fiber content less than ~3% is not substantially higher than that of concrete However, we can see a significant increase in ductility (ratio of ultimate strain to yield strain) C O M P R E S S I V E S T R E N G T H Compressive strength of very high-volume content FRCC (called SIFCON) is significantly higher than the concrete due to the very high percentage of fibers (8-25%) T E N S I L E S T R E N G T H Tested by Direct Tension Test T E N S I L E S T R E N G T H Tensile Stress (psi) PE 2.0% PE 1.5% SH 1.5% Concrete Strain Tensile Stress (MPa) For low volume of fiber, the direct tensile strength of FRCC is the same as that of matrix cracking strength For higher volume of fiber, the direct tensile strength of FRCC can be more than that of the matrix Significant increase in ductility at any volume content 63 64

17 T E N S I L E S T R E N G T H T E N S I L E S T R E N G T H Average Tensile Stress (MPa) L 24 L 32 L 40 S 24 S 32 Different fiber shapes produce different FRCC tensile behaviors Fiber Content (kg/m3) Tensile strength is not significantly affected by fiber content in the low range B E N D I N G S T R E N G T H B E N D I N G S T R E N G T H 1600 Tested using small beams in a 3- point or 4-point bending setups Torex, V f = 2.0% Spectra, V f = 1.5% Bending strength can increase if the fiber content is high enough Significant increase in ductility EQUIVALENT BENDING STRESS, psi Control Torex 1.6% PVA#13 1.6% Spectra 1.6% PVA, V f = 2.0% DEFLECTION AT MIDSPAN, in

18 B E N D I N G S T R E N G T H 5.0 B E N D I N G S T R E N G T H Flexural Strength (MPa) L 24 L 32 L 40 S 24 S 32 FA 24 Toughness Index I5 4.5 L 24 L 32 L S 24 S 32 FA Fiber Content (kg/m 3 ) 20 Another method for bending strength test is by using 800-mm round panel specimen (75 mm thickness) Suitable for sprayed concrete (shotcrete) L 24 L 32 L 40 Fiber Content (kg/m 3 ) Flexural Strength Changes very little with the fiber content (at low levels) for any kind of mixtures. However, Toughness Index and Energy Absorbed clearly increase with an increase in fiber content. Energy Absorbed at L/150 (J) 16 S 24 S 32 FA Source: ASTM (2007) Fiber Content (kg/m 3 ) Flexural Strength & Toughness S H R I N K A G E Tested under 3 point support and measure load vs. deflection to calculate energy absorption Fibers slightly reduce the amount of free plastic and drying shrinkages However, fibers significantly reduce the shrinkage cracks Source: ASTM (2007) 71 72

19 S H R I N K A G E S H R I N K A G E Restrained plastic shrinkage cracking test Comparison of maximum crack width under restrained plastic shrinkage 73 Control 0.6% Carbon Fiber Source: Wongtanakitcharoen (2005) 74 S H R I N K A G E M I X I N G F R C C s Drying shrinkage cracking test using ring-type specimens 75 76

20 M I X T U R E D E S I G N F I B E R C H O I C E S Things to consider when designing an FRCC mixture Types of Matrix (Mortar/Concrete) Workability Fiber Types Amount of Fibers needed Fiber sizes Concrete Matrix Need to be smaller aggregates (3/8 or less) Fiber volume fraction ranges from % Mortar Matrix Maximum fiber volume fraction for conventional mixing method is about 2-3% Low-Strength/ Low Modulus Microfibers PP, Some PVA and PE For control of plastic shrinkage Steel Macrofibers Steel fiber of any shapes For temperature and drying shrinkage crack control Increase flexural and tensile strength and/or ductility HPFRCCs High-Strength/ High Modulus Microfibers Carbon, Spectra (PE) For control of plastic shrinkage as well as increase strength/ ductility HPFRCCs F I B E R C H O I C E S Microfibers, due to its high surface/volume ratio, provides effective plastic shrinkage crack control Low modulus fibers are not very effective in controlling shrinkage cracking at later ages To obtain ductility and strength benefits, fiber must be high strength (also high modulus is preferred) and bonds well with the matrix A M O U N T O F F I B E R Amount of fibers required for specific applications depends on the performance of the fiber Consult manufacturer s technical documents Past experiences with similar project Factors to consider Performance level needed (the more fiber, the better) Workability of high-volume content FRCCs (varies, depends on the shape, size, and geometries of the fiber) Congestion with steel reinforcement 79 80

21 Amount of Fibers Needed For plastic shrinkage crack control using microfibers: % by volume For temperature and drying shrinkage crack control (secondary reinforcement) using steel macrofibers: % by volume M I X I N G Steel Fibers Dry mix all the aggregates (sand, fly ashes, silica fume) Add Portland Cement and dry mix Add fibers and dry mix Slowly add water and superplasticizer and mix well Synthetic Fibers or Steel Fibers Dry mix all the aggregates (sand, fly ash, silica fume) Add Portland Cement and dry mix Add water and mix well Slowly sprinkle fibers and superplasticizer while mixing For applications requiring strength and toughness (such as slab on ground) using steel or high strength microfibers: % by volume High-Performance applications using steel or high strength microfibers: 1.5~25% by volume W O R K A B I L I T Y Workability tends to decrease as the volume fraction of fiber increases For the same volume fraction and fiber type, FRCC with longer fibers has lower workability than that with shorter fibers In stiff mixes, fibers may not be able to dispersed uniformly and may lumped together For volume fraction of fiber of about 1% or more, superplasticizer is generally required. Workability may be tested using Slump method, or inverted slump cone R E F E R E N C E S Balaguru, P. N. and Shah, S. P. (1992), Fiber Reinforced Cement Composites, McGraw- Hill, New York, 530 pp. Chompreda, P. (2005), Deformation Capacity and Shear Strength of Fiber Reinforced Cement Composite Flexural Members Subjected to Displacement Reversals, Thesis, University of Michigan, Ann Arbor, MI, 186 pp. Chompreda, P., Jarurattanapong, P., Suriyamongkol, P. (1999), Strengthening RC Beams with Carbon Fiber Sheet, Term Project Report, Chulalongkorn University, Thailand, Courtney, T. H. (2000), Mechanical Behavior of Materials, 2 nd Edition, McGraw-Hill, New York, 733 pp. Hollaway, L. (1993), Polymer Composite for Civil and Structural Engineering, Blackie Academic & Professional, London, 259 pp. Nawy, E. G. (2000), Fundamentals of High-Performance Concrete, 2 nd Edition, John Wiley and Sons, New York, 441 pp