Advanced Composite Materials (ACMs) can be defined as a combination of materials appropriately arranged using reinforcing fibers, carefully chosen matrixes, and sometimes auxiliary materials like adhesive core and other inserts. These combinations after proper manipulation and processing result in a finished structure/item with synergistic properties i.e. properties achieved after fabrication cannot be obtained by individual components acting alone. The ACMs can be classified in different categories on the basis of microstructures, multiphase, reinforcements, manner of packing fibers layered compositions, method of composition, matrix system processing methods, etc.
The basic components of ACMs are (i) Reinforcement (fibers) (ii) Matrix (iii) Honeycomb core/adhesives ( for sand witched structures ). The great variety of fibers materials in various forms, shapes, and sizes have been recently developed for use in ACMs and in the construction industries. Steel, glass, carbon, Aramid (kevlar), boron, silicon carbide, silicon nitrates, alumina fibers are some of the commonly used high-performance reinforcement fibers in ACMs. The reinforcements may be called by different names according to sizes such as Whisker ( < 0.025 mm ), fiber ( 0.025 – 0.8 mm ), Wire ( 0.8 – 6.4 mm ), rod ( 6.4 – 50 mm ) and bar ( > 50 mm ).
In general, the continuous filamentary type reinforcement is important from a structural application point of view. It is the reinforcement that is primarily responsible for the mechanical properties of ACMs. Usually, all the reinforcements (fibers) are stronger in tension than steel but weak in shear ( i.e. brittle ) requiring the filler material (Matrix) relatively strong in shear which will protect reinforcement against abrasion or environmental corrosion. Matrix also helps in distributing the load from reinforcement, absorbing energy, reducing stress concentration, and preventing crack propagation. Thermosetting and thermoplastic types of organic polymers are used as Matrix ( e.g. epoxide, phenolic, polyamide resins, etc.).
Some of the important fibers used as reinforcement in ACMs along with their characteristic properties are discussed briefly.
i) Carbon/Graphite Fibers
Carbon fibers are produced from petroleum pitches in large volumes. These are low cost, low modulus. The current technology for producing carbon fibers generally centers on the thermal decomposition of various organic precursors, Rayons polyacrylonitrile (PAN), pitch, polyesters, polyamide polyvinyl alcohol, polyvinyl chloride, polyp-phenylene, and phenolic resins have all been considered and investigated as potential precursor materials for producing carbon fibers. These are produced by heat-treating the precursor to temperature up to 10000c in an inert atmosphere.
Carbon fibers are very small in diameter and also manufactured as continuous mats, brails, continuous straight fibers. These are high strength, high modulus, low density, lightweight, and have significant cost and handling advantage, outstanding creep, and fatigue resistance. Pultruded carbon-reinforced composites are noted for their lubricity, wear resistance, the capacity of heat dissipation, and resistance to alkaline and soil solutions. Carbon fibers in general are not affected by moisture, atmosphere, solvents, bases, etc. Table No 1 Shows carbon fiber properties.
ii) Glass fibers
On a specific strength (i.e. strength to weight) basis, glass fiber is one of the strongest and most commonly used structural materials. Some Lab tested fibers has shown strength upto 6896 MPa and commercial grades range from 3448 – 4830 MPa. The continuous glass filament are manufacture by two basic process i.e. by marble melt process and direct melt process respectively. To minimise abrasion related degradation of glass fibers, surface treatments (sizings) are applied prior to gathering of fibers in to strands. Commonly glass fibers are round and straight and have diameters ranging from G (9-10.2 mm) to T (22.9 – 24.1 mm ) are used. The glass fibers are available in different forms like continuous form, woven roving, surfacing mats, three dimensional and multidimensional (such as 5-D, 7-D, 11-D ) etc. There are several glass fiber types with different chemical compositions providing the specific physical/chemical properties.
E-glass ( calcium aluminoborosilicate composition) is best for general purpose structural use. S-glass (magnesium aluminosilicate composition) is a special glass with higher tensile strength and modulus, good heat resistance, strong resistance to acids. These properties make S-glass fibers suitable choice in ACMs to be used in elevated temperature and humid environments. C-glass has good chemical stability in chemical corrosive environments. T-glass fiber exhibits improved performance over E-glass such as 36% increase in tensile strength, 16% increase in tensile modulus, increased heat resistance, improved impact, electrical, thermal and chemical resistance properties. R-glass (magnesium-lime-aluminosilicate ) has higher tensile strength and modulus compared to E-glass and gives higher resistance to fatigue, aging temperature and corrosion. A-glass, ECR-glass, AR-glass, D-glass are some types of glass fibers.
Table No. 2 : Glass fiber properties.
| Â |
E-glass |
R-glass |
D-glass |
S-glass |
|
Density (g/cm3) |
2.60 |
2.55 |
2.16 |
2.49 |
|
Tensile strength (Mpa) |
3400 |
4400 |
2500 |
4580 |
|
Tensile modulus (Gpa) |
73 |
86 |
55 |
86.93 |
|
Elongation at break (%) |
4.5 |
5.2 |
4.5 |
5.4 |
|
Filament diameter |
3-14 |
3-14 |
3-14 |
-- |
iii) Aramid Fibers
Aramid fibers ( aromatic polyamide ) was introduced by Dupont under the name Kevlar. The aromatic polyamide are believed to be made by solution polycondensation of diamines and diacid halides at low temperature. The structure of aramid fiber is anisotropic gives higher strength and modulus in the fiber longitudinal direction. Aramid is resistant to fatigue, exhibits good toughness and general tolerance characteristics. Applications of aramid fiber in civil structures include ropes, cables, prestressing tendons, pipes, walls etc. Table No 3 shows properties of Aramid fibers.
Table 3. Properties of Aramid fibers
| Â |
Polyester |
MONEX |
Kevlar29 |
Kevlar49 |
Teflon |
|
Density 9/cm3 |
1.38 |
1.38 |
1.44 |
1.45 |
2.15 |
|
Tensile Strength(MPa) |
900 |
670 |
2700 |
3500 | Â |
|
Tensile Modulus(GPa) |
18 |
60 |
135 |
133 |
-- |
|
Elongation at break % |
10-15 |
20-30 |
4 |
2.5 |
20-30 |
|
Filament diameter |
10-12 |
-- |
-- |
-- |
20 |
iv) Linear organic Fibers
This fiber may become one of the major reinforcement for civil and building structure in future. This high strength and high modulus organic fiber can be produced by arranging the molecular structure of simple polymero to become straight during manufacture. The properties of this fiber includes rigidity (240 GPa), lower density (0.97), tensile modulus (117 GPa) and tensile strength (2.9-3.3 GPa)
v) Other notable high performance fibers are boron and silicon carbide fibers (Sic). Ceramic fibers (including oxide and non oxide) are also developed. Other organic fibers available are acrylic, nylon, polybenzimidazole (PBI), polyester, polypropylene and teflon.
ACMs AS REINFORCEMENT, CABLES AND TENDONS
1. ACMs VERSES STEEL
Technical properties of ACMs depends upon type of reinforcing fibers, their form, style, proportion, direction etc. The characteristic properties of ACMs bars when compared with that of a high strength steel (ref fig I) can be summarised as:
i) All the ACMs are stronger than steel.
ii) ACMs with carbon fibers has same stiffness as steel.
iii) ACMs with carbon fibers are stiffer than ACMs with aramid fibers which inturns are stronger than steel.
iv) Stress strain carves of all ACMs are linear up to failure.
v) All ACMs have less ductility and unpredictable plastic behaviour.
vi) Due to their higher strengths and lower stiffnesses ACMs component develop much larger strains within elastic limits as compared to steel.
vii) The ACMs has low bond strength which can be overcome by providing mechanical anchorages and surface treatment to ACM bars.
The ACMs properties like corrosion resistance, lightweight, high tensile strength, high fatigue strength, withstanding extreme environmental attacks, zero electrical conductivity, high impact resistance, smooth and fine finishing with various colors, easy fabrication, handling, erecting, vibration-damping characteristics makes it far superior, efficient and effective and reliable construction material in place of steel.
2. ACMs as cables and Tendons.
The application of ACMs in bridges as cables and tendons has been reported from different countries. As ACMs cables are much lighter, stronger than conventional steel cables much longer distances can be spanned by ACMs cable. In coastal areas where corrosion of steel cables is a very big problem, ACMs cables prove an efficient and effective alternative. The state-of-the-art application is cable-stayed bridges. The pultruded cables and tendons of ACMs supporting concrete decks and girders in bridges render the structures stronger and durable than steel.
The following are some of the case studies reported on use of ACMs as cables and tendons.
- Pultruded cables using several hundred parallel carbon fibers composite wires (6 mm dia. pultruded from 500000 carbon fibers) using epoxy resin system. Bundled wires are embedded in a polymer matrix, non carbon radicals are then burned off in an inert atmosphere and replaced by carbon. These pultruded cables can yield two to three times the specific strength of steel and could last more than 100 years, permit longer span cable-stayed bridges ( up to 6 to 9 km ). The steel cables would snap under their own weight over such a distance.
- The idea of joining Europe and Africa across the Strait of Gibraltar is being under serious consideration by using a carbon fiber composite cable-stayed bridge ( ref fig.2 ).
- The Ullenbergstrace bridge in DUSSEL-DORF and Marienfelde bridge both in Germany is the first kinds of post-tensioned bridges incorporating tendons using glass fiber ACMs. Both the bridges are two spans each ranging between 20 to 25 m. Difficulties were observed in attaching the anchorages to the tendons. Slipping out and breaking out of tendons prematurely was reported. As the modulus of elasticity of glass fiber ACM, the tendon is low leading to a strain as high as 2% at working stress level during initial prestressing which produces very long elongation of prestressing tendons and should be accommodated by long anchorages and enough space behind anchorages.(refer fig 3, 4 )
- Pretensioned slab on girder bridges and post-tensioned using aramid fiber ACM tendons and carbon fiber ACM tendons have been reported from Japan.
- The Heavy Assault bridge made for the U.S. Army is to be carried in three jointed sections on an armored vehicle, unfolds hydraulically to create a 106ft. span, and supports the 70-ton load. Twelve carbon-epoxy chords about 38 ft long and 4x5 inch in cross-section support the structure.
- ARAPREE is a composite prestressing tendon consisting of aramid filaments and an epoxy matrix. The mechanical properties of the rectangular strip (20 mm x 1.5 mm) with 50% fiber volume are: tensile strength 2800 MPa, Young’s modulus 125-130 GPa, failure strain 2.4%, density 2.4, Relaxation is 15 to 20%.
- AFRP ROD from Teijin is another concrete reinforcing tendon using Technora aramid fibers. Mechanical properties of a typical 6 mm diameter rod with 65% fiber volume are tensile strength 1862 MPa, tensile modulus 52.9 GPa, failure strain 3.7%, relaxation 7-14%.
- CFCC is the carbon fiber composite cable from Tokyo Rope Manufacturing Company. PAN-based carbon fiber is impregnated with epoxy. The Mechanical properties of typical cable (Seven strands, dia. 0.49 inch ) are: tensile strength 2118 MPa, tensile modulus 137 GPa, specific weight 1.5, elongation at break 1.57 %, relaxation loss up to 2.46%, creep 0.04%, bond stress 7.2 MPa.
3. ACMs as Reinforcement
It has been reported that ACM reinforced bar behave in the same manner as that of steel bars in the slabs and beams1. Due to their less young’s modulus deflection was considered as a limiting criteria in case of ACM reinforced beams. In slabs ACM bars are used as reinforcement in the form of composite grids, when compared with steel grid, the maximum load supported by slab reinforced with ACM grid was observed to equal or more than slab reinforced with steel grid2. Slabs when reinforced with 3-D continuous carbon fiber and loaded exhibited non linear behavior and reduction in stiffness in post cracking stage3.
Kajima-FRC reported a type of composite concrete called 3R-FRC in which 3-D fabric, made by weaving the fiber rovings in three direction is impregnated by epoxy and cured, and is employed as main reinforcement. The fiber is a hybrid of PAN based carbon, aramid and vinylon fibers. NEFMAC is also a kind of composite reinforcement for concrete. A hybrid of continuous carbon, glass and aramid fibers is impregnated with resin and formed into mesh enabling thinner section of concrete to be used.
COST ECONOMICS
In general composite products for main reinforcement of concrete, cables and tendons are more expensive than steel on the basis of weight for weight thereby prohibiting the use of ACMs extensively in structural applications. However, weight is no logical basis for cost comparisons. A more rational basis should be a strength. The corrosion resistance, nonmagnetic properties, low electrical conductivity, weather durability, lightweight and other properties of ACM’s may play an important role for engineers to select ACM reinforcement instead of conventional steel. If the cost of corrosion rehabilitation and repairs of R.C.C. structures is to be considered, ACMs cost could be comparable with steel. With the increased amount of usage, adopting a good design scheme, increase opportunities for application, sophistication in technology the cost of ACMs will be decreased considerably in future.