Concrete Repair Materials and its Requirements.
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. The 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
Cementitious 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 de-bonding 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 Properties
It is desirable that the repaired structure shall be put to use at the earliest possible to reduce the down time 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.
The 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 Substrate:
The 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
A variety of adhesives, in the range of epoxies, polymer modified cement, slurries including unmodified polymer applications are available. The selection depends upon available open time for bonding etc, which are being specified for different applications.
Compatible Coefficient of Thermal Expansion:
The 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 usedconstruction/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|
Compatible Mechanical Properties and Strength
The hardened material shall have compatible mechanical properties or rather slightly better strength than that of base material. This property is desirable to ensure uniform flow of stresses and strains in loaded structures. It is well known that the elastic modulus of two concretes would be different for different crushing strength so if repair concrete is having strength much different than the base, it could lead to non-uniform flow of stresses and may result in an early failure of the repair patch. For example, if M20 grade of concrete has been used in original construction, the grade of the repair material shall neither be less than M20 nor higher than M25.
Relative Movement, if Expected:
It is desirable that the repair material shall not have any curing requirement after the repair has been applied or even if its is required, it should be minimal to ensure that the repair patch hardens and attains the desired strength without much post-repair-care. Only epoxies don’t need any curing. Other material applications need nominal to moderate curing, which need to be specified. Such materials have several other merits, which are to be kept in view, while making a selection. Curing compound can be applied over cementitious materials used for repairs but after examining its compatibility.
In case of RCC, it is important to maintain the alkalinity of concrete around reinforcement with its pH above 11.5 from corrosion protection point of view. In this context, it is necessary for the repair material to have chemical characteristics such that it does not adversely affect the alkalinity of the bas concrete at a later date. The chemical characteristics of the repair material and it shall be examined beforehand. In addition, the pH of reinforcement protection applications, bonding coats and that of the repair material must also be similarly alkaline. This would ensure inbuilt compatibility.
Low air and water permeability:
Permeable material allows easy permeation of environmental chemicals including carbon dioxide, water, oxygen, industrial gases, vapours etc. It is essential that repair materials should have a very low air/water permeability to provide protection to the reinforced concrete against ingress of harmful chemicals.
It is desirable that colour and texture of the repair material should match with the structure and give aesthetically pleasant appearance. If needed, this could be achieved through appropriate finishes.
Economics is important while considering various options for repair materials, but cheaper repair material should not be selected at the cost of performance characteristics.
Durability and Bio Non-degradability:
The repair material selected should be durable under its exposure conditions during the service life against chemical attack, resistant to any form of energy like UV rays, infra-red rays, heat etc and should be bio-non-degradable.
Non Hazardous / Non –Polluting:
The repair materials should not be hazardous to field workers, however adequate safety measures are required to be taken for repair materials, which are hazardous to workers involved with their application etc. These should also be environment friendly.