election of concrete repair materials is based on evaluation of type of damage, types of materials to be used for repair and the local condition. Concrete repairing material must be compatible with the concrete being repaired.
Variety of materials is available for repair of concrete structures. The choice of such materials is based on performance and cost. These materials are low viscosity polymers, very rapid setting cement, special concrete for overlays apart from Portland Cement mortar or concrete.
The selection of concrete repair materials should be made based on following properties :
Bond with concrete
Strength development of material with concrete(compressive, flexural and tensile)
Co-efficient of thermal expansion of the material
Co-efficient of permeability of the material
Stress development at interface whether on shrinkage, temperature change, alternative cycles of wetting and drying
Corrosion resistance property of the material
Durability of such concrete repair material
appearance of finished surface
speed of concrete repair
Basically, the concrete repair materials can be grouped into:
i) Cementitious System
ii) Polymer Modified Cementitious System
iii) Polymer Concrete System
iv) Reactive Thermosetting Resin System
Following are the some of the common repair materials used for repair or rehabilitation or strengthening of the concrete structures:
(a) Unmodified Portland Cement Mortar or Grout:
Portland cement mortar or grout is the most common repair materials used for repairing damages to concrete structures. It is selected because it is readily available and has low cost. This material consists of ordinary Portland cement and suitable aggregate. Cement mortar is generally used for small repair works and cement concrete are commonly selected where large area is to be repaired.
(b) Latex Modified Portland Cement Mortar or Concrete:
This repair material is used to prevent chloride attack on concrete structure due to use of low water-cement ratio. This is same as ordinary Portland cement mortar or grout with addition of a latex emulsion. The strength of this material is same as ordinary mortar or grout.
ingress can be reduced due to lower water cement ratio.
Synthetic latex that have been found useful in Portland cement mortars or concrete are polyvinyl acetates, acrylics, styrene – butadene and vinylidene chloride. The last three are particularly suitable for wet environment.
The addition of latex modifier influences strength and durability of cement. The use of this material should be based on the service conditions of the structure.
Latex modifier concrete recommended for sections up to 30mm deep, should have 1:3-3.5 as ratio of cement and fine aggregates. Water ratio should be 0.3 with latex solid cement ratio of 0.1 to 0.2 by weight. Latex modifier concrete recommended for sections deeper than 30mm should have proportions of 1 part of cement to 2.5-3 parts fine aggregate to 1.5-2 parts coarse aggregate.
(c) Quick Setting Non-shrink Mortar:
Cracks on concrete surface due to shrinkage of concrete is repaired by this material. It has good bond with old concrete. The use of suitable admixtures combined with this repair material also increases strength and improve bond and workability while reducing curing time.
(d) Polymer Concrete:
Most popular polymer concrete used is an epoxy concrete system with curing agents or methylmethacrylate monomer with an inhibitor and promoter. Epoxy system is widely available in formulated repairing materials. This repair material can be customized as per requirement for use in repair of different types of concrete damages.
Concrete Repair Materials Commonly Used:
According to ACI-546 report, a low slump Portland cement concrete admixed with accelerating admixture (ASTM C 494 – Type F) is recommended for use in repair of partial depth patches along with some bonding agent. Bonding agent may be of LATEX-cement slurry or any epoxy system. W/C ratio of concrete mix shall be less than 0.45 and maxi mum size of aggregate shall be less than 1/3 of the patch depth. Concrete shall be laid while the bonding agent is still tacky.
Polymer modified mortars are readily available in the market in pre-batched conditions and very easy to mix and apply in the field. Such materials should have compressive strength around 25 N/mm2 and flexural strength around 5 to 6 N/mm2. Small patch repair having a depth less than 13mm may be done using such mortar.
Around 4 to 6% (by weight of cement) SBR latex may be used to modify Portland cement-sand mortar or concrete. Such mortars will attain high flexural and bond strength and have minimum shrinkage on drying after application. For patch repair of depth 13-32 mm, latex modified mortars and over 32mm depth – Latex modified concrete is recommended by ACI-546.
Epoxy mortar or concrete may be used for repair of spall when time of repair is very short. ACI 503 report gives necessary guidelines for use of epoxy system in concrete superstructure. Epoxy system should conform to ASTM C 881-87. Type, grade and class must be chosen to satisfy job conditions and requirements. Epoxy mortars generally consists of 15-20 parts silica sand which shall be mixed slowly with a premixed resin-hardener formulation by using a slow speed drill fitted with a stirrer. Mortar shall be laid after application of an epoxy primer and shall be trowel finished within its pot life.