INTRODUCTIONConcrete deterioration is mostly associated withcorrosion of steel reinforcement.Therefore the natural choice for repair would beimpermeable highly alkaline cement basedmaterials analogous to the passivating nature ofhighly alkaline cement matrix as in good qualityconcrete.However some situations demand repaircompositions containing polymers as admixturesfor cementitious systems or as high strengthbinders.
TYPES OF POLYMERS USEDPolymers used in concrete repair consist of twodifferent types of materials:1. Polymers used to modify cementitioussystems;2. Reactive thermosetting resins: epoxy,polyester and acrylic resin systems.
POLYMER MODIFIED CEMENTITIOUSSYSTEMSPolymers used as admixtures for cementitioussystems are milky white dispersions (latex) inwater.Such mortar with latex provide alkalinepassivation protection of steel.The polymer latex acts in several ways:Water-reducing plasticizerImproves bond between repair mortar andconcreteReduces permeability of repair mortar to waterActs as integral curing aidIncreases tensile & flexural strength of mortar.
POLYMER MODIFIED CEMENTITIOUSSYSTEMSUnmodified cementitious mortar sets andshrinkage occurs. Micro cracks are in the orderof 4-5 micrometers.Addition of polymer latex controls micro crackwidths to 1-2 micrometers.Polymers form ‘elastic bands’ across the microcracks, increasing tensile & flexural strengths,and reducing permeability of mortar.Different types of polymer latexes used asadmixtures for cementitious materials are:Polyvinyl acetates (PVAc)Styrene butadiene (SBR)Polyvinylidene dichloride (PVDC)Styrene acrylics.
POLYMER MODIFIED CEMENTITIOUSSYSTEMSPVDC latexes are not recommended for repairmortars for RCC because of the possibility ofrelease of free chlorides in the long run.On-site blending:Polymer modified mortars are blended fromsand, cement, latex and water on site.Lack of adequate quality control may result inunsatisfactory mortars.Pre-blended mortars:Complete packs of latex and pre-blended sand& cement available.This mortar is used for repair of Reinforcedconcrete where cover to be replaced is more than12mm, but less than 25-30mm.
RESIN REPAIR MORTARSFor relatively smaller repair areas, cover lessthan 12mm Resin Mortars are used.In this case protection to steel reinforcementdepends on total impermeability.Resin Mortar = Reactive Resin + Gradedaggregate.Commonly used Resins: Epoxy, Polyester,Acrylic.These are Thermosetting Resins (they do not meltor flow like thermoplastics when heated, but losestrength gradually).Resin mortars are supplied as 2/3 componentsystems: Resin, Hardener & Filler.
EPOXY RESINSEpoxy resins consist of reactive resin which canbe considered as ‘hooks’ and a hardener (alsocalled curing agent) considered as ‘eyes’.Correct proportioning and thorough mixing areimperative while using epoxy resins.The curing (hooking up) of epoxy resins in anexothermic reaction.Rate of cure is temperature dependent.
UNSATURATED REACTIVE POLYESTERRESINSIn polyester resin systems, the liquid resincomponent contains both ‘blocked hooks’ and‘eyes’ in the right proportion, intimately mixed.The hardener (or catalyst) is required just toinitiate the ‘unblocking’ of the hooks.This results in a chain reaction that continuesthroughout the resin system.The cure of the polyester resin system is alsoexothermic.
UNSTAURATED ACRYLIC RESINSAcrylic resin systems form high-strengthmaterials by similar chemical cure mechanismsto the unsaturated reactive polymer resinsAcrylic resins are based on monomers of very lowviscosity, hence very high filler loadings arepossible and mortars tend to exhibit lessshrinkage.
SAFE HANDLING OF RESINSEpoxy, Polyester and Acrylic resins requirehandling with care.Contact with resins, especially epoxy resins cancause skin irritation.Polyester and Acrylic resins contain volatileconstituents which are flammable.Most acrylic resins are highly flammable withflash points below 10 degree Celsius, and vapoursgiven off can cause toxic reactions.
SELECTION OF RESINSEpoxy resins are most widely used in concreterepair.Polyester and Acrylic resins are used for small-area repairs and where rapid strengthdevelopment is required.In most repair situations, the polymer-basedrepair material is bonded directly to concrete orother cementitious material.
POLYMER BONDING AIDSWhen applying conventional concrete, sprayedconcrete or sand/cement repair mortars, bond isoften a problem.Water loss at the interface between repair mortarand the prepared concrete prevents properhydration of cement matrix at the interface.An epoxy resin or polymer latex bonding aid canbe used to bond green uncured concrete tohardened concrete.Many systems are available. Instructions have tobe followed carefully.
REPAIR OF CRACKS/RESIN INJECTIONCracks in concrete greater than 0.3mm have tobe sealed to prevent ingress of moisture,oxygen,etc..It is imperative to establish the cause beforedeciding on the appropriate methods/materialsfor the repair.Only then is it possible to restore the structureto its original tensile/shear strength.Low viscosity epoxy, acrylic or polymer resinsspecifically developed for repairing cracks maybe used.