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Essential Requirements of Repair Materials

Er. Parbhakar Dwivedi
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1. Low Shrinkage

It is well known that the 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, cementitious 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 of atmospheric air and water which could be harmful for concrete and reinforcement.

It is, therefore, essential that the low shrinkage property of repair 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.

2. 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 trials or the repair process is intricate that more working time is required.

3. Workability

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.

4. 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 bond strength between repair material and substrate, e g use of

• Adhesive,

• Surface interlocking system, and/or

• 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 during which material remains tacky or plastic.

5. Compatible Coefficient of thermal Expansion:

The difference in volume change because of temperature variation can cause failure either at the bond line or within the section of lower strength maternal. Therefore, in the areas exposed to temperature variations, the patches 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 expansion of commonly used construction/repair materials.

Coefficient of Thermal Expansions of Commonly Used Building Materials

S. No. Material Co-efficient of thermal expansion in 10-6 /%
1 Stones
(i) ligneous rocks 8 to 10
(ii) Lime stones 2.4 to 9
(iii) Marbles 1.4 to 11
(iv) Sand& sand stones 7 to 16
(v) Slates 6 to 10
2 Metals
(i) Aluminium 25
(ii) Bronze 17.6
(iii) Copper 17.3
(iv) Lead 29
(v) Steel and iron 11 to 13
3 Bricks and brick work 5 to 7
4 Cement mortar and concrete General purpose non shrink concentitiou micro- 10 to 14
5 concrete 10 to 12
6 Polymers modified mortar/concrete 10 to 12
7 Epoxy mortar / concrete 20 to 25

The hardened maternal 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 earth failure of the repair patch. For example, it M-20 grade of concrete has been used in original construction, the grade of the repair material shall neither be less than M-20 nor higher than M-25.

7. Resilience & Elasticity

Particularly in case of sealing of such cracks where movement is expected or at expansion joints, the repair material selected shall be resilient and elastic to be able to absorb the anticipated relative movements of the structure without any signs of distress or crack.

8. Minimal or no curing Requirement

Lt is desirable that the repair material shall not have any curing requirement after the repair has been applied or even if it 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.

9. Alkalinity

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 base concrete at a later date. The chemical characteristics of the repair material and its after effect on the pH of RCC 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.

10. Low Air & Water Permeability

Permeable material allows easy permeation of environmental chemicals including carbon dioxide, water, oxygen, industrial gases/vapors 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 environmental chemicals.

11. Aesthetics

It is desirable that colour and texture of the repair material should match with the structure and give aesthetically pleasant appearance. If need be, this could be achieved through appropriate finishes.

12. Cost

Economics is important while considering various options for repair materials but cheaper repair material should not be selected at the cost of performance characteristics.

13. Durability &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 ultra violet rays, infra-red rays, heat etc. and should be bio non-degradable.

14. Non-Hazardous/Non- Polluting

The repair material 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.

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