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Transcript of Advanced Construction Materials
ADVANCED CONSTRUCTION MATERIALS
DUCTILE HYBRID FIBER REINFORCED POLYMER (D-H-FRP)REINFORCEMENT BARS FOR CONCRETE
STRUCTURES
ADVANCED CONSTRUCTION MATERIAL
DUCTILE HYBRID FIBER REINFORCED POLYMER (D-H-FRP)
REINFORCEMENT BARS FOR CONCRETE STRUCTURES
ABSTRACT:
Reinforced concrete structures especially pavements and bridge decks that constitute vital
elements of the infrastructure of all industrialized societies are deteriorating prematurely.
Structural repair and upgrading of these structural elements have become a more economical
option for constructed facilities especially in the United States and Canada.
This paper describes the scaling requirements, tensile strength, creep-rupture, and energy
absorption capabilities of a second generation ductile hybrid FRP reinforcement developed in the
form of a ductile hybrid bar (D-H-FRP) which simulates the stress-strain characteristics of
conventional steel reinforcement .
The developed hybrid FRP bar possesses a ductile behavior, which is intrinsic to its
hybrid fiber composition and the hybrid geometric architecture of its fibers. The ductility indices
were based on definitions of ductility according to displacement, rotation, and energy
considerations. The tensile strength, bond strength, and flexural interaction in concrete beams,
cost analysis, applications, advantages and limitations and conclusion have been described.
An in-line braiding and pultrusion process was developed to produce high quality D-H-
FRP bars up to 10 mm nominal diameter. Using the results from the tensile tests and fracture
analysis, the stress-strain behavior of the D-H-FRP bars was fully characterized. The developed
analytical model closely predicted the stress-strain behavior of D-H-FRP bars.
INTRODUCTION:
DEFINITION FOR FRP COMPOSITE BARS:
Composites are a combination of a reinforcement fiber in a polymer resin matrix, where
the reinforcement has an aspect ratio that enables the transfer of loads between fibers, and the
fibers are chemically bonded to the resin matrix.
DESIGN CONCEPT OF THE DUCTILE HYBRID FRP (D-H-FRP)
Ductility is the ability of a material or structural member to undergo large inelastic
deformations without distress. In the extreme event of a structure loaded to failure, it should be
able to undergo large deflections at or near its maximum load carrying capacity. This will give
forewarning of failure and prevent total collapse and may save lives. In statically indeterminate
concrete structures, ductility of the members at the critical sections is necessary to allow moment
redistribution to take place. In order to achieve ductility in reinforced concrete structures without
using conventional steel rebar, a new design methodology was introduced identify suitable
fibrous composite materials that mimic the stress-strain characteristics of steel.
Taking advantage of the design flexibility and the wide availability of manufacturing
capacity in the industry, braided structures were employed as the primary fiber architecture for
the construction of the reported ductile hybrid frp rebar system. The methodology of braiding, as
detailed by ko, is a well established technology which intertwines three or more strands of yarns
to form a tubular structure with various combinations of linear or twisted core materials. By
judicious selection of fiber materials and fiber architecture for the braided sleeve and the core
structure, the load-deformation behavior of the braided fibrous assembly can be tailored.
the sleeve structure may be a tough aramid (e.g., kevlar) filamentous structure for example
whereas the core structure could be high modulus carbon fibers to provide the desired high
stiffness. The rib effect, as commonly incorporated in steel rebars to increase the bond strength
between the rebar and concrete, can also be built into the sleeve structure during the braiding
process. By proper combination of the braided fibrous assembly with a protective resin matrix
system to form a composite material system, the stress transfer from the rebar structure to the
fibers can be controlled. The end product of this hybridization of material systems and fiber
architecture is a second generation composite rebar which high initial resistance to tensile
deformation has followed by a gradual failure process.
RESULTS OF 10 mm DIAMETER D-H-FRP BARS UNDER UNIAXIAL
TENSION
Tests were conducted to obtain the tensile strength of 10 mm D-H-FRP bars. The core
(Thornel P-55S) provides initial stiffness while the yielding of the braided aramid fibers provides
ductility [4].
A 50 kn capacity universal test machine was used .Two Linear Variable Differential
Transformers (LVDT) gauges were attached to measure extension. Two series of tests were
conducted, one on 5 mm diameter bars and the other on 10 mm diameter bars.
The load was then plotted versus strain. The specimens had end tabs to prevent material
crushing and stress concentrations at the supports. The end tabs for the 5 mm specimens were a
chopped fiber composite with glass fibers and epoxy resin. The 10 mm diameter specimens were
embedded in 76.2 mm by 152.4 mm concrete cylinders, confined by a plastic sleeve and steel
rings. This provided enough lateral confining pressure to prevent failure at the supports.
The load versus strain for both the 5 mm and the 10 mm diameter specimens. All of the
specimens, both 5 and 10 mm diameters, show a ductile mode of failure with a bilinear (trilinear)
load-strain curve. The linear (elastic) portion of the curve was repeatable for all specimens, with
the 10 mm diameter specimens having a slightly higher initial modulus. As shown, however, the
transfer efficiency is not 100 % from the 5 mm bar to the 10 mm bar. A first level approximation
for the transfer properties from the 5 to 10 mm bar is a factor of four. This is based on the bar
area which is proportional to the square of the diameter. However, the ultimate load was fairly
high (15.66 to 20.69 kn) for the 10 mm bar compared with the 5 mm bar (4 to 6.1 kn) and the
ultimate strain was between 1.5 % and 3.0 %. These curves show a significant amount of
toughness for a composite material, which results in energy absorption. The 10 mm bar has
increased toughness due to the larger fiber content which increases the crimp in the yarn
geometry.
CREEP-RUPTURE TEST:
Data on the creep-rupture capacity of materials is needed for proper life-cycle design
of structures. The creep-rupture strength and behavior is well known for materials such as steel,
but is less defined for newly manufactured composite materials.
For steel, a sustained tensile load of 75% or lower of the ultimate load is considered
safe for creep rupture failure. For the proper life-cycle design of reinforced-concrete structures
with D-H-FRP reinforcing, it is mandatory to know the creep-rupture strength for the 50-year
design life of the structure.
Creep is a viscose elastic effect that is manifested as a polymer deforms under constant
load. For steel, this effect occurs at elevated temperatures, but for polymers, creep can occur at
room temperature. Since the D-H-FRP is designed with life-cycle costs in mind, the long-term
time- dependent properties of the material must be fully quantified.
A test was conducted to determine the creep-rupture behavior of 5 mm diameter D-H
FRP bars. The bars were mounted in the test machine and subjected to a sustained static load
until failure (Fig. 5). The load to weight ratio is 1:10. The ends of the bar were embedded in 50.8
mm by 101.6 mm (2" by 4") concrete cylinders.
The load was based on a percentage of the average ultimate tensile load, P ult. The maximum
sustained load and time to failure were then recorded for each specimen. Fig. 6 shows a plot of
the load ratio Prupture/Pult (rupture load to ultimate load ratio) versus time until failure. Four load
levels were used in the test based on percentages of the ultimate tensile capacity of the material.
From tension tests, the ultimate capacity of the material is 5.34 kn .
The load levels chosen were 3.56, 4.00, 4.23, 4.45 kn. These values correspond to 67 %,
75 %, 79%, and 83 % of the ultimate tensile load, respectively. To initially get the system in
horizontal equilibrium, an additional 0.31 kn of preload was added.
CREEP-RUPTURE CAPACITY OF 5 MM D-H-FRP BARS:
The data shows decreasing load ratio with increased failure time. Two specific failure
times are noted in Fig. 6. First, failure at 50 years is shown. The load ratio value at this time is
0.69or 69 % of the ultimate load. The load ratio value at 106 hours is 0.61 or 61 % of the
ultimate. These values are very promising when compared to other results from creep tests. IT IS
found that sustained stress is limited to 60% of the short-term ultimate strength for GFRP.
Reported no loss in strength for 50% of the ultimate for CFRP and GFRP. Therefore, the
obtained results are in line with other FRP materials.
ENERGY ABSORPTION CAPACITY OF 5-mm DIAMETER D-H-FRP:
It is crucial for a structure to be able to absorb energy, especially during an earthquake
event. Therefore, a ductile reinforcing material is needed for reinforced concrete structures. Steel
provides this ductility for R/C elements, but many FRP’s are linear elastic to failure and do not
possess significant energy absorption capacity under reverse cyclic loading.
Contrary to this, however, the D-H-FRP developed shows significant energy
absorption capacity. The bars were embedded in square concrete columns of lengths 3150 and
3937 mm. Four D-H-FRP bars were placed as the longitudinal reinforcing. These were confined
with hoop reinforcement called stirrups which prevented buckling of the longitudinal reinforcing
and provided shear reinforcement. The column was loaded at its tip and was cycled through both
tensile and compressive loads. Both tip deflection and base rotation were measured.
The result of a reverse cyclic load test. As seen in the load cycles, the energy absorption
capacity of the D-H-FRP material is significant. The hysteresis loops illustrate two key points.
First, the D-H-FRP bars show energy absorption capability, unique when compared with most
composite materials. Second, the ductility of the D-H-FRP is further quantified by this energy
absorption capacity. This is particularly valuable in design and retrofit of reinforced concrete
structures in regions of high seismicity.
STRENGTH COMPARISON:
Using a combination of high modulus carbon (Thornel P-55S) and aramid (Kevlar 49)
fibers, 5 mm diameter bars with a Young's modulus of 202 gpa (same as that of steel
reinforcement compared on a total fiber cross-section basis), a bi-linear stress-strain
tensile curve with a definite yield, an ultimate strength higher than yield and an
ultimate failure at between 2 % and 3 % strain were obtained.
Excellent bond characteristics were obtained by integrating ribs into the braided
jacket to increase the mechanical interaction at the bar to concrete interface. Results
have been presented from concrete beams 50 mm x 100 mm in section and 1.2 m long
reinforced with ductile hybrid FRP bars which produced ductility indices that were
essentially the same to those of a companion steel reinforced beam.
COSTS OF D-H-FRP REBAR’S:
Costs for D-H-FRP bars are typically 3 times that of conventional reinforcement or 50%
that of stainless steel bars, which results in an additional cost of about 4-8% for a typical
highway bridge project. However, there are several other issues that need to be considered in a
cost/benefit analysis, these include:
The amount of bar required for reinforcing structures in view of the different mechanical
properties.
The effect of redesign and reduction in concrete usage on whole component cost.
The reduction in weight of the reinforcing bars and time saved during construction of
cages.
The reduction in weight of section and subsequent reduced crane age and transportation
costs. It has been estimated that typically a 15% weight saving in pre-cast sections may
be anticipated due to cover reduction, although some case study analyses have indicated
significantly higher savings are achievable (soil nail spreader plates potentially from 200-
47kg).
From a through-life perspective, there is no reliance on the concrete quality, low
permeability or membranes to protect the reinforcement and therefore the maintenance costs for
this reinforcement are negligible. This can significantly reduce the maintenance and repair costs
for UK structures (currently estimated at £500 million pa) potentially freeing ups a significant
portion of this money for new capital projects.
In the UK in 2005 about 250 tonnes of non-ferrous reinforcement was specified. The UK
market is estimated at 15,000 tonnes, roughly a £50 million pa industry. In the USA, significant
volumes have been reported, in excess of 5000 tonnes, and in Japan similar quantities have been
recorded.
ENVIRONMENTAL CONSIDERATIONS:
There have been conflicting studies relating to the comparison between producing steel
and D-H-FRP products. Overall it is felt that these two materials have similar environmental
effects with the difference that steel is more readily recycled at the end of life. However, FRP
products are designed to last longer and this advantage may therefore be challenged.
When considering the application of D-H-FRP reinforcing bars in concrete however,
several advantages are evident; the potential reduction in concrete cover required results in lower
utilization of cement. Cement produces 20 times more CO2 emissions during processing than
steel or D-H-FRP reinforcing bars. In addition, concrete structures containing steel reinforcement
are difficult to recycle since the steel needs to be extracted prior to crushing. This is not the case
with FRP reinforcing bars. Lighter weight products also provide the opportunity to reduce
emissions during transportation.
Furthermore, the visual environment is enhanced since there is no rust staining or concrete
spalling associated with the use of FRP reinforcing bars.
D-H-FRP reinforcing bars have no toxic or other fume emissions during use.
APPLICATIONS:
There are four main types of application where D-H-FRP reinforcing bars are suitable
alternatives to steel, epoxy-coated steel and stainless steel bars:
REINFORCED CONCRETE EXPOSED TO DE-ICING SALTS:
D-H-FRP bars can eliminate the corrosion problems and reduce maintenance and repair costs
in northern climates where massive quantities of de-icing salts are used every year on roads and
pavements. Applications include, parking structures; bridge decks, parapets, curbs, retaining walls and
foundations, and roads and slab-on-grade foundations.
Road bridge, USA Slope reinforcement
STRUCTURES BUILT IN OR CLOSE TO SEAWATER:
Corrosion of steel reinforcement is a common problem in structures built in or near
seawater. Possible applications are: quays, retaining wall, piers, jetties, caissons, decks, piles,
bulkheads, floating structures, canals; roads and buildings, offshore platforms, and swimming
pools and aquariums.
Marine reinforcement Tunnel reinforcement
APPLICATIONS SUBJECTED TO OTHER CORROSIVE AGENTS:
Chemical processing industries as well as domestic or industrial wastewaters constitute
major sources of corrosion for steel reinforcement. Typical applications include: wastewater
treatment plants, petrochemical plants, pulp and paper mill and liquid gas plants pipelines and
tanks for fossil fuel, cooling towers, chimneys mining operations of various types, nuclear power
plants, and nuclear waste facilities.
APPLICATIONS REQUIRING LOW ELECTRIC CONDUCTIVITY OR
ELECTROMAGNETIC NEUTRALITY:
Using steel bars in applications where low electric conductivity or electromagnetic
neutrality is needed often results in a complex construction layout, if such use is possible at all.
Potential applications for D-H-FRP here are:
Aluminum and copper smelting plants,
Manholes for electrical and telephone communication equipment,
Structures supporting electronic equipment such as transmission towers for
telecommunications,
Airport control towers,
Magnetic resonance imaging in hospitals; railroad crossing sites, and
Military structures with requirement for radar invisibility.
D-H-FRP REINFORCING BARS OFFER SEVERAL ADVANTAGES:
Non corrosive and totally resistant to chloride ion attack
Tensile strength (1000mpa) is 1½ - 2 times that of steel
Lightweight – easy to handle and transport, only 25% of the weight of equivalent size
steel reinforcing bar
Low modulus (45gpa) compared to conventional materials
Non-conductive and are highly effective electromagnetic and thermal insulators
Excellent fatigue resistance and perform well in cyclic loading situations
Good impact resistance. They resist sudden and severe point loading
Magnetic transparency and are not affected by electromagnetic fields They are
excellent for use in MRI and other types of electronic testing facilities
High Strength to weight ratio
Impervious to pests and woodpecker attack
Design – Tailor material properties, some systems are modular
NON-MAGNETIC STRUCTURES:
Maglev train in Japan Fencing with no electromagnetic shadow
Environmentally safe – no leaching of toxic chemicals into soil
LIMITATIONS OF FRP REINFORCING BARS INCLUDE:
The inability to bend the product on site (although this is possible with thermoplastic
FRP bars); there are no available coupling systems.
They have longer transfer lengths.
High partial safety factors currently applied.
CODES AND SPECIFICATIONS:
FRP reinforcing bars offer many advantages over other concrete reinforcing products.
The properties of FRP reinforcing bars are different from those of steel reinforcement. The
design of concrete reinforced with FRP reinforcing bars is different in many cases. Design
engineers should consider the feasibility of reinforcing concrete with FRP reinforcing bars,
keeping in mind the following basic points for design:
Direct substitution of FRP reinforcing bars in concrete members designed with steel bars
is not possible in most cases.
Lower modulus of elasticity and shear strength of FRP reinforcing bars will limit
application to short spans of secondary structural elements.
Glass FRP reinforcing bars is limited to a maximum sustained stress of 20% of the
guaranteed design tensile strength.
Glass FRP reinforcing bars applications are limited to the reinforcement of concrete and
are not to be used as a pre-stressing or post-tensioning element.
CONCLUSIONS:
The Ductile Hybrid Fiber Reinforced Polymer (D-H-FRP) bar shows much promise
in the design of new reinforced concrete structures and retrofitting existing structures, especially
in areas of moderate to high seismicity.
The bar has been successfully scaled up to the 10 mm nominal diameter size, the
equivalent to a conventional steel number 3 bar. These larger D-H-FRP bars show good tensile
properties, and a bilinear load-strain curve, with significant post-elastic deformations.
The energy absorption capacity of the D-H-FRP bars is significant, with a large
amount of hysteresis. The long-term mechanical properties also show significant promise,
specifically the creep-rupture properties. The D-H-FRP is adequate for the life-cycle design of a
reinforced concrete structure.