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MODIFIED EPOXY FOR FIBRE REINFORCED CONCRETE POLYMER STRENGTHENING OF…
MODIFIED EPOXY FOR FIBRE REINFORCED CONCRETE POLYMER STRENGTHENING OF CONCRETE STRUCTURE
INTRODUCTION
Deterioration of concrete structures are due to :
overloading
corrosion of reinforcement
fire
environmental exposure
inadequacy of design
lack of maintenance
Repairs are needed to :
extend the service life of structure
ensure safety and serviceability
FYP :
one of the materials used for repairing public infrastructure which enhance the load bearing capacity of structure
has high specific strength and stiffness, lightweight, recyclability, environmentally-friendly ease of handling, high durability against corrosion, reduced manufacture time
the performance FRP depend on the material, adhesive substrate performance and adhesive bond interface
FRP STRENGTHENING
FRP commonly used when a structure is required to carry additional load or when there's a deficiency in existing designs
Applications of FRP :
improving flexural properties for a structure
improving shear capacity and compression member
FRP composites composed of continuous fibre embedded in a thermoset resin matrix (epoxy, vinyl ester or polyester resin) that hold fibres and transfers the load to them.
3 common used FRP for strengthening :
carbon ( has been widely used)
glass
Aramid
2 different form of FRP arrangement :
uni - directional ( has higher tensile strength and elastic modulus and most suitable for increasing the structural capacity of an element)
bi - directional
Techniques used :
pregregs are adhesively-bonded as prefabricated elements on the concrete structure
dry FRP is applied through wet-lay up onto the concrete structure
EPOXY AS A BONDING AGENT
Epoxy functioned as coating, self-levelling floor, bonding of prefabricated elements in bridges, grouting and structure repair
Epoxy is thermosetting polymer which offers excellent chemical, mechanical and good insulating properties, and a good environmental and chemical resistance over wide range of temperature
Epoxy is part A (resin) + part B (hardener) which is mixed with different ratio suggested by epoxy manufacturers
Curing temperature plays a crucial role as one of the most important properties of epoxy is glass transition temperature (Tg).
Tg exposed to mild external temperature ----> adhesion between FRP and concrete will be reduced.
Tg exposed to moderate temperature (above Tg) of non-fully cross-linked thermoset polymer ----> cause post-curing which can increase Tg, stiffness and strength of the resin
Tg value < 75C even if the cross-linking has been completed through post - curing procedures,
The adhesive plays an important role in bonding the fibres and concrete together
Epoxy can cause health problem therefore it cannot be applied in humid condition and also can creates waste disposal problem
CURING OF EPOXY FOR FRP APPLICATION
Low viscosity of epoxy resin can be formulated and at room temperature when fully cured, it exhibits excellent mechanical properties and environmental resistance
The polymerisation of part A give rise to a rigid network-type structure and occurs when part B present as a curing agent
Polymerisation depend on
:check: heat / radiation on the ingredient
:check: curing mechanism
Curing stages :
Gelation
:star: stage whereby the resins transforms from liquid to rubbery state.
:star: The flowability of resin reduces due to formation of cross-linking polymers and become insoluble but swell as it is imbided in the solvent
Vitrification
:star: resin change from rubbery to a glass state ( glass transition phase)
:star: molecules are linked to each other creating a 3-D network of epoxy
Method of curing epoxy :
:!: Cold curing (use in FRP strengthening)
epoxy resin left in ambient temperature is able to achieve acceptable mechanical properties in a reasonable curing time.
Aliphatic amines act as curing agents which can react with epoxy at low temperature
:!: Heat curing
pre-cure step : cures at low temperature
post-cure step : cures at high temperature
:!: Ultraviolet light
FRP - CONCRETE INTERFACE
Long curing time in cold curing techniques exposed to a humid environment which affects the bonding properties of FRP-concrete interface
Exposure to high temperature and high humidity can cause :
bond degradation
affect the mechanical and durability of the entire strengthening system
It is recommended that FRP must be installed at a temperature between 4C to 49C to avoid set time and saturate workability at high temperature
Among wet/dry and hot/cold cycles in 15% saltwater for FRP in marine environment, bond degradation is the greatest under the wet/dry cycles which caused by moisture absorption by the epoxy that is detrimental to bond durability
Direct tension pull-off test is a simplified method that can be used to assess the bond behavior of epoxy, FRP and concrete.
Test is carried out by applying FRP sheet on a concrete substrate, and then the steel plate (dolly) will be bonded with similar epoxy over FRP sheet.
Groove will be made around the bonded FRP and dolly will be pulled by applied load
This table shows the failure mode from the test :
Failure mode G is the most desirable whereby the failure occurs entirely on the concrete structure and not in FRP or epoxy
CONSISTENCY OF MODIFIED EPOXY
Important criteria of the consistency of modified epoxy :
Mixing procedures
Method to ensure the homogeneity of the combined materials
Method to assess the flowability or viscosity of the mixture before tested for other properties
Amine Terminated Butadiene Acylonitirle (ATBN) and Carboxyl Terminated Butadiene Acrylonitrile (CTBN) have been used by to produce modified epoxy
ATBN + hardener then heated before adding epoxy resin
CTBN + epoxy resin then heated before combining with hardener
Epoxy cannot be mixed in large quantities because the heat from the combined part A & B which promotes speed of hardness in addition to the nature of its nature
CNT
was used commercially available epoxy resin (in liquid form of CNT)
steps :
:check: mechanical stirrer was used to ensure homogeneity and then was subjected to Ultra Sonicator for 30 minutes
:check: Ultrasonication : method to achieve homogenisation and stability of the mixture especially for nanomaterials
:check: Then hardener was added and mixed
*if sand is added later after the hardener & resin, there can be a decrease of the mixture workability
Producing epoxy mortar by using sand and sand washing waste :
10% - 25% amount of epoxy used (depend on the workability condition)
Automatic mixer mix the dry sand and sand washing waste for 2 minutes
the prepared epoxy was then poured into the dry mixture and mix for 2 minutes
Consistency of epoxy resin can be determined using test method for the flow of hydraulic mortar ( ASTM c1437 / ASTM C230)
steps :
:check: Truncated cone is filled with epoxy on the flow table, then raised the top and drop it by rotating the cam
:check: Remove the cone, rotate the cam so that the table can be raised and dropped 25 times in 15s
:check: Record the initial and final diameter of flow after the cone is removed
*This method helps to control the flowability of the mixer when a supplementary materials is used before assessing further properties (strength)
HARDENED PROPERTIES IN MODIFIED EPOXY
Brittle : so modification made to neat epoxy should be able to toughen it by improving its ductility and strength
Hardened properties of cured epoxy will be assessed through its compressive and tensile strength which should lies in its bond strength if the repaired structure, so the problem like thermal incompatibility to the concrete substrate should be avoided
Based on the investigation of bond strength of FRP in environmental exposure, it is confirmed that the bond strength increased followed by strength of loss of modified epoxy resin (using nano-silica filler)
The use of sand modified epoxy for structural strengthening with CFRP (ratio of sand-to-epoxy ranges from 0 - 1.5) cause the increase of stiffness, ductility and toughness of modified epoxy
The use of sand washing waste as a filler for epoxy resin can obtain better mechanical properties (compressive strength, flexure strength, and modulus of elasticity) and increase the bond strength
Depending on the epoxy & fibre types, there are improvement in bond strength and ultimate slippage (cohesion failure was either at the concrete or interface between the concrete and FRP)
FUTURE ALTERNATIVES FOR EPOXY RESIN
Building & construction industries have been expecting to grow to USD 2696.2 million by 2025 due to the increasing infrastructure development around the world
The use of epoxy resin as an adhesive to FRP is widely used for repair and rehabilitation of deteriorated concrete structure
FRP depends on petrochemical-based resins which compromises the sustainability of FRP and epoxy is produced from non-renewable sources and can cause human toxicity
New FRP should have good flexibility/stiffness, thermal expansion, water vapor diffusion, impact resistance, and excellent adhesion to the concrete substrate
Epoxy can improve in the sense of durability (which is depend on the proper bonding between substrate and repair material), as it should be able to deliver a long lasting repair over its service life with no change in its inefficiency, therefore it should have UV resistance, resistance to alkaline conditions and have wide range of temperature tolerance
Epoxy should also have low shrinkage (which can prevent loss of bond & cracking), low permeability as it can helps protect against penetration of carbonation and chloride ions and strong adhesion
To achieve environmental sustainability, a renewable source of epoxy have been studied such as vegetable oil, sucrose, lignin as they are highly available and safe to human and animal
