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.