FRP in Civil Engineering

Elisabetta MonaldoPhD student

Meccanica dei Materiali e della FratturaComplementi di Scienza delle Costruzioni

What is a Fiber Reinforcement Polymer (FRP)? FRP is a composite which combines fibers and matrix. Fiber-reinforced polymer composites (FRPs) are made by combining tough plastics with strong fibers such as fiberglass or carbon to produce new materials that offer light weight, corrosion resistance, and improved strength.

Fibers: - Provide strength and stiffness; - Their quality, orientation and shape affect the final product. Matrix: - Coats the fibers; - Protects the fibres from abrasion; - Transfers stresses between the fibers.

FRP Materials

Lightweight

Easy to install

High Strength

5x steel

Corrosion resistant

Durable structures

Highly versatile

Suit any project

What kind of FRP products are available?

• Plates (rigid strips)

• Sheets (flexible fabric)

• Bars

• Profiles

CFRP uni and multi-directional laminate

FRP pultruded profiles

And, what kind of fibers are used?

- Carbon

- Glass

- Aramid

Glass fiber is by far the most predominant fiber used in the reinforced polymer industry and among the most versatile. Although melting glass and drawing into fibers is an ancient technique, long continuousfiber drawn from glass was introduced in the 1930’s by Owens-Corning as glass wool and given the popularname fiberglass. Fibers made from glass are manufactured in many varieties for specific uses. It typically has a silica contentof greater than 50 percent, and the composition with different mineral oxides give the resulting product itsdistinct characteristics.Of all the fibers available for structural strengthening and reinforcement, E-glass is by far the most used and is the least expensive. E-glass – Alkali free, highly electrically resistive glass made with alumina-calcium borosilicates. E-glass is known in the industry as a general-purpose fiber for its strength and electrical resistance. It is the most commonly used fiber in the fiber reinforced polymer composite industry

Aramid fibers are high strength and high elastic modulus fibers that are better known for their use in bullet- and fire-resistant clothing. Aramid is a synthetic fiber made from the polymer aromatic polyamide. It was first introduced in the 1960s as a meta-aramid and later as para-aramid.The terms meta- and para- refer to the location of chemical bonds in the aramid. The chemical bonds of a para-aramid are aligned in the long direction of the fiber. Meta-aramid bonds are not aligned but are ratherin a zigzag pattern and therefore will not develop the higher tensile strength of para-aramid bonds.

And, what kind of fibers are used?

- Carbon

- Glass

- Aramid

What else

Environmental sustainability- biodegradable- non-toxic

Cost competitiveness- Production technologies- Local raw materials

Properties:• Good strength and stiffness

properties also at high temperatures.• Operating temperature range:-300 to 700 °C.• Long-term durability.• High acid and solvent resistance.• Low water absorption.• Remarkable heat and sound insulation properties.• Good processability.• Eco-compatibility and recyclability, resulting a

high-performance green inorganic material.

Basalt fiberBasalt is a dark-coloured mafic extrusive rock and it is the most widespread of all igneous rocks, resulting in more than 90% of all volcanic rocks. The rate of cooling of molten lava strongly influences the microstructure of this rock: when the rate is low, basalt structure exhibits an almost regular atomic arrangement, whereas an amorphous (uncrystallized) structure arises when the solidication rate is high.From a mineralogical point of view, basalt is mainly composed of three silicate minerals: plagioclases, pyroxenes and olivines. Its chemical composition depends on the nature of the magma it derives from; in each case, silicon dioxide (SiO2) is the main component, followed by Al2O3, FeO +Fe2O3, CaO, MgO, Na2O, K2O, TiO2, and so on. The percentage of chemicalcomposition of basalt is strictly related to the geographic area.

synthetic matrices

• Thermoplastic : (PE, PVC,PP)• Termosetting : epoxy resin

natural matrices

• Starch• PHB: Polyhydroxybutyrate (belonging to the polyesters)• PLA: Poly(lactic acid) or polylactic acid or polylactide

Matrices

Applications:

Applications:

Novel structures Strengthening and retrofitting

Fiberglass, FRP PolycarbonateRoofing Structures

Wrapping FRP fabrics around columns increases the shear and axial strength to improve ductility and energy dissipation behavior.

Scandinavia's first composite bridge - the Fiberline Bridge in Kolding, Denmark, was officially opened on 18 June, 1997. The new bridge, matching the strictest safety standards, has shown that GFRP composites may represent the building material of the future for lightweight, maintenance-free bridges.

Novel structures:

“The various compositions of FRP enables products to be tailored to meet the diverse requirements of different market sectors such as new build, refurbishment, private and public sectors. A major advantage is the ability to meet the most stringent of fire, security and sound insulation standards. This is achieved most effectively when FRPs are combined with other materials (mainly as core materials) such as steel, thermoplastics or recycled plastics to provide the required properties cost effectively. Social and economic benefits include improved thermal efficiency of the home, and enhanced durability, increasing replacement intervals.”

compositesuk.co.uk

Fiber Reinforced Polymer composite systems can be used to increase the strength of existing beams, slabs, walls, columns, piles, chimneys and structural connections. Strengthening applications include seismic retrofit, strengthening due to corrosion, change of use or in some cases, the correction of an existing defect due to a construction or design error. FRP may be applied to strengthen reinforced concrete and pre-stressed/post-tensioned concrete, steel, masonry and timber.

• Seismic Retrofit• Concrete Restoration• Increase load capacity• Change of use• Non-Intrusive/Rapid Repair• Earthquake protection

Strengthening and retrofitting of existing structures

FRP application systems

Wet lay-up system - The system is installed on the surface element while the resin matrix is still wet, and the polymerization occurs on site; - Saturate sheets with epoxy adhesive, then place on the surface element and press with a roller; - Used with flexible sheets; - Multiple layers can be used.

Pre-cured system;- Used with FRP plates or laminates; - Used for surface bonded plates near surface mounted reinforcements; - Not as flexible for variable structural shapes; - The pre-cured laminates should be placed on or into the wet adhesive; - Can be placed on surface or inside the element.

FRP reinforcement techniques

Externally Bonded Reinforced (EBR)

Near Surface Mounted (NSM)

Type Application Fiber Direction

ConfinementAround

columnCircumferential

FlexuralTense side face

of the beam axis of beam

Along long.

ShearSide face of

beam (u-wrap)

Perpendicular

to long. axis

of beam

Strengthening and retrofitting of existing structures

Flexural behavior of RC beams

Verifying soundness and effectiveness

of externally bonded FRPs as

flexural reinforcement of RC beams,

distinguishing different fiber types.

Main assumptions:

• perfect bond between steel bar/concrete,

• perfect bond between FRP/concrete,

• beam cross-sections remain planar during the overall deformation process.

Constitutive laws:

Concrete Steel FRP

F/2 F/2

L

h

b

Asc

Ast

𝛿

d

Flexural behavior of RC beams

𝑁 = න𝑦𝑚𝑖𝑛

𝑦𝑚𝑎𝑥

ሺ𝑏 𝑦 𝜎𝑐ሺ휀 𝑥𝑛, 𝑦))𝑑𝑦 + 𝐴𝑠𝑡 𝜎𝑠 휀 𝑥𝑛, 𝑥𝑛 − 𝑑 + 𝐴𝑠𝑐 𝜎𝑠 휀 𝑥𝑛 , 𝑥𝑛 − 𝛿 + 𝐴𝐹𝑅𝑃 𝜎𝑓 휀 𝑥𝑛, 𝑥𝑛 − ℎ

𝑀 = න𝑦𝑚𝑖𝑛

𝑦𝑚𝑎𝑥

ሺ𝑏 𝑦 𝑦 𝜎𝑐ሺ휀 𝑥𝑛, 𝑦))𝑑𝑦 + 𝐴𝑠𝑡 𝜎𝑠 휀 𝑥𝑛, 𝑥𝑛 − 𝑑 𝑥𝑛 − 𝑑 ൯+ 𝐴𝑠𝑐 𝜎𝑠 휀 𝑥𝑛, 𝑥𝑛 − 𝛿 𝑥𝑛 − 𝛿 + 𝐴𝐹𝑅𝑃 𝜎𝑓 휀 𝑥𝑛, 𝑥𝑛 − ℎ ሺ𝑥𝑛 − ℎ

Translational equilibrium along the beam axis:

Rotational equilibrium around the neutral axis:

Flexural behavior of RC beams

RC beams reinforced with different FRPs

CarbonCarbonGlassAramidBasaltHempFlax

FailurePossible failure modes of FRP strengthened RC beams include shear failure, flexural failure (concrete crushing and FRP rupture) and FRP debonding. The most common failure mode of flexural beams strengthened with FRP is debonding of the FRP reinforcement. When a reinforced concrete beam with an externally bonded FRP plate is subjected to flexural loading, high tensile and bond shear stresses develop in the concrete near the adhesive layer. In general, debonding of the FRP plate from the concrete is due to these high stresses.The weakest part in the bond surface is the concrete layer close to the surface. Cover delamination or FRP debonding can thus occur if the interfacial stresses cannot be sustained by the concrete. Debonding initiation generally takes place in a region of high stress concentration at the concrete-FRP interface. These regions include the ends of the FRP reinforcement and those around the shear and flexural cracks. Therefore codes and guidelines concentrate on two debonding failure modes:• plate end debonding• midspan debondingThe first mode is the failure that originates near the plate end and propagates in the concrete either at the cut-off point of the plate ((1) plate end shear failure) or at the last crack ((2) anchorage failure at last crack). The second mode is the failure that is caused at a shear or flexural crack and then propagates from such a crack towards the plate end.

Debonding failure:

- Experimental study

- Design issues

Debonding test configurations

FRP debonding: experimental campaign

Aiming to investigate about debonding failure mechanisms occurring in BFRP-concrete systems, an experimental campaign was conducted at the Laboratory of Structures and Materials of the University of Rome “Tor Vergata”. In particular, more than 40 debonding tests on BFRP-concrete specimens, obtained through the in situ application of BFRP sheets on prismatic concrete block by means of fresh epoxy resin, were performed.

FRP debonding: experimental campaign

Debonding test between Basalt-FRP sheets and concrete specimens

Fixed support

Mobile loading cell

INSTRON 4482

FRP debonding: experimental campaign

Debonding test between Basalt-FRP sheets and concrete specimens

Each BFRP-concrete specimen was vertically oriented, with the concrete block placed between two horizontal rigid steel plates actioned through an universal testing machine (equipped with a loading cell of 100 kN). A displacement-driven approach was adopted, by setting a constant loading velocity equal to 0.1 mm/min. A steel cylinder was inserted into the U-shaped BFRP eyelet, representing a fixed support that constrains the BFRP layer, and thereby that indirectly applies upon it the pulling load. As a result, the loading scheme corresponds to a self-equilibrated system of two opposite forces parallel to the bonding interfaces. Due to the steel cylinder, the pulling load equally distributes between the two sides of the specimen where the BFRP layer is attached.

push-pull double shear test

push-pull double shear test

FRP debonding: experimental campaign

Debonding test between Basalt-FRP sheets and concrete specimens

BFRP sheets were equipped with several strain gauges per side aligned along the loading direction and centered with respect to the BFRP width.

In detail, at least three strain gauges were placed along the BFRP length for each bonded region and one in the non-attached zone, the latter being adopted for giving a reference control strain measure.

Experimental campaign: results

As regards the failure mode, BFRP mainly experienced a typical mode-II debonding failure, associated to the detachment of a thin layer of concrete. Due to possible preparation defects or sample imperfections, seven BFRP-concrete specimens reached failure as a consequence of the BFRP fracture close to the cylindrical support.

Experimental campaign: results

Load-slip curves

Strain patterns

debonding load: the maximum transmissible load F inducing the debonding failure

effective bond length: the minimum value of the bond length ensuring the maximum bearing capacity of the joint

FRP debonding

FRP debonding

are mainly based on traditional FRPs,

do not specifically refer to BFRP-concrete systems

Technical guidelines

kg is equal to 0.063 mm or 0.077

mm for FRP preformed or cured in

situ systems, respectively.

c1 is equal to 0.64 and it has

been obtained by referring to a

calibration procedure based on

experimental data associated

to only CFRP-concrete systems

FRP debonding

Aim

Verifying soundness and effectiveness of some technical guidelines, for predicting:

debonding force F

effective bond-length Leff

distinguishing different FRP fiber types and FRP applications

carbon, glass and basalt precured and in situ systems

FRP debonding: database

Carbon-FRP(precured)

Carbon-FRP(in situ)

Glass-FRP(precured)

Glass-FRP(in situ)

Basalt-FRP(in situ)

Experimental data against technical indications

F

Fth

Fexp

experimental data

bisector line

regression line

0

The slope of the regression line is

determined via a least-square fitting

procedure and results in:

t : technical standard

f : fiber type

p : implementation procedure

Comparative assessment

Carbon-FRP concrete system

(precured)

FTheoretical estimates based on CNR-DT200R1/13 and

Fib Bulletin 14/01 better agree with experimental

measures than SIA 166/2004

Comparative assessment

Carbon-FRP concrete system

(in situ application)

In the cases of carbon-based FRP more reliable estimates

are obtained are obtained by referring to Fib Bulletin 14/01

F

Comparative assessment

Glass-FRP concrete system

(precured)

Although indications provided by SIA 166/2004 furnish a

safe mean assessment of the debonding load, corresponding

discrepancies between theoretical and experimental results

reach higher values, e.g. in the order of 20% - 30% if GFRP-

concrete systems are considered

F

Comparative assessment

Glass-FRP concrete system

(in situ application)

For all technical indications, the theoretical values of the

debonding force are in better agreement with the experimental

data for precured systems rather than for in situ systems

F

Comparative assessment

Basalt-FRP concrete system

(in situ application)

all technical indications fail in producing an effective estimate

of the debonding load when BFRP-concrete systems are

considered, the regression-slope discrepancies ranging from:

19% (Fib Bulletin 14/01) to 46% (SIA 166/2004)

F

Original and proposed correction factors for the evaluation of the:

- debonding load

- effective bond length

A novel set of values for the empirical correction coefficients occurring inthe analyzed models has beenproposed, specifically accounting for fiber type and FRP implementation procedure.

label 'Fs’:F equal to C, G or B, for indicating carbon, glass or basalt fibers, respectively.s equal to p or i , for indicating the FRP application, precured or impregnated systems, respectively.

Conclusions

E. Monaldo, F. Nerilli, G. Vairo. Basalt-based fiber-reinforced materials and structural application in civil engineering (to appear).

E. Monaldo, F. Nerilli, G. Vairo. Effectiveness of some technical standards for debonding analysis in FRP-concrete systems, Compos Part B: Eng 160(2019): 254-267.

Nerilli F., Vairo G. Experimental investigation on the debonding failure mode of basalt-based FRP sheets from concrete. Compos Part B: Eng 153(2018), 205-216.

E. Monaldo, F. Nerilli, G. Vairo. Technical standards for debonding in FRP-concrete systems: an experimental contribution for basalt-FRP, 9th International Conferenceon Fibre-Reinforced Polymer (FRP) Composites in Civil Engineering (CICE 2018), July 2018, Paris, France.

Nerilli F., Vairo, G. Strengthening of reinforced concrete beams with basalt-based FRP sheets: An analytical assessment. In AIP Conference Proceedings 2016 (Vol.1738, No. 1, p. 270016). AIP Publishing.

For further details:

Conclusions

Contact us:Giuseppe Vairo vairo@ing.uniroma2.itElisabetta Monaldo elisabetta.monaldo@alumni.uniroma2.eu