Besides being of compatible properties, repair materials for cement, concrete/mortar shall also be easy to apply and require no attention after the repair has been applied.
- Required Properties of Concrete Repair Materials
- Low Shrinkage of Concrete Repair Materials
- Requisite setting/hardening Properties
- Workability of Concrete Repair Materials
- Bond with the Substrate
- Compatible Coefficient of Thermal Expansion
- Table – 1: Coefficients of Thermal Expansions of Commonly Used Construction/Repair Materials
- Compatible Mechanical Properties and Strength
- Relative Movement, if Expected
- Low air and water permeability
- Durability and Bio Non-degradability
- Non Hazardous / Non –Polluting
Required Properties of Concrete Repair MaterialsThe essential parameters for deciding upon a repair material for concrete are:
- Low shrinkage properties
- Requisite setting/hardening properties
- Good bond strength with existing substrate
- Compatible coefficient of thermal expansion
- Compatible mechanical properties to that of the substrate
- Should allow relative movement, if expected particularly in case of sealing of cracks or dealing with expansion joints.
- Minimal or no curing requirement
- Alkaline character
- Low air and water permeability
- Aesthetics to match with surroundings
- Durable, non degradable or non-biodegradable due to various forms of energy life, UV rays, heat etc
Low Shrinkage of Concrete Repair MaterialsCementitious repair materials shrink with passage of time. Most of the shrinkage generally takes place in the initial period from the time of casting to 21 days. Therefore, repair material in its original form, if used for repair of concrete/mortar, is likely to get either delaminated due to debonding or develop shrinkage cracks on its surface due to shrinkage strains and stresses. Shrinkage cracks so developed in the repair patch would allow the easy access to atmospheric air and water, which would be harmful for concrete and reinforcement. It is therefore essential that the low shrinkage property of material shall be looked for while selecting a material for concrete repair. Cementitious materials need additional non-shrink compounds so as to be effective in achieving the desired property. Therefore the formulation of the patch mortar incorporates, in the cement matrix, several special chemicals to mitigate the shrinkage. Using low cement content and low water cement ratio will also reduce the drying shrinkage.
Requisite setting/hardening PropertiesIt is desirable that the repaired structure shall be put to use at the earliest possible to reduce the downtime of plant, machinery, building or road. It is therefore, essential that repaired patch shall harden in the minimum possible time. However, in exceptional cases, it could also be essential to have the slow setting property as a desirable property for repair material. Such situation could be where more working time is required to work on repair materials or the repair process is intricate that more working time is required.
Workability of Concrete Repair MaterialsThe repair material is to be applied by the field workers and hence its acceptability by them is very important. The property desired by the field workers is good workability. Hence optimum workability is to be achieved without sacrificing the other desirable properties by use of suitable additives/admixtures.
Bond with the SubstrateThe bond strength of repair patch with the substrate is essential to have a successful repair system. If it is felt that the bond strength of the repair material with the base material is inadequate or less than the strength of the base material, then some other suitable means could be explored to improve the bond strength between the repair material and substrate. These could be use of:
- Surface interlocking system
- Mechanical bonding
Compatible Coefficient of Thermal ExpansionThe difference in volume change because of temperature variations can cause failure either at the bond line or within the section of lower strength material. Therefore, in the areas exposed to temperature variations, the patches of the repair should have same coefficient of thermal expansion to ensure that no undue stresses are transferred to bonding interface or the substrate. Due to similar coefficient of thermal expansion, cementitious materials are preferred over epoxy materials. Coefficients of thermal expansions of commonly used construction/repair materials are given in table 1.
Table – 1: Coefficients of Thermal Expansions of Commonly Used Construction/Repair Materials
|S. No||Material||Coefficient of thermal expansion in 10-6/oC|
|(i) Igneous rocks||8 to 10|
|(ii) Lime stones||2.4 to 9|
|(iii) Marbles||1.4 to 11|
|(iv) Sand and sand stones||7 to 6|
|(v) Slates||6 to 10|
|(vi) Steel and iron||11 to 13|
|3||Bricks and brickwork||5 to 7|
|4||Cement mortar and concrete||10 to 14|
|5||General purpose non-shrink Cementitious micro-concrete||10 to 12|
|6||Polymer modified mortar concrete||10 to 12|
|7||Epoxy mortar/concrete||20 to 25|