Repair & Maintenance Techniques for RCC Building Facades

The building façade or the envelope is the primary line of defense against water entering the living spaces. A typical building envelop today consists of:

• Combination of RCC Frame and Masonry (CMU, Aerated Blocks or Clay Bricks)
• RCC only (Aluminium Formwork Cast Walls or Precast Walls)
• Combinations of RCC Frame and Glass or Aluminium Glazing

In this article, we have covered the solutions that are normally seen for the first two types of building façades/envelopes. The most common signs of deterioration seen on RCC/RCC frame/masonry building are:

• Biological growth (plants/fungus)
• Salt and efflorescence attack
• Dampness and wetness (outside to inside or in some cases from wet areas to façade)
• Cracks and delamination in plasters and paints/coatings
• Leakings and damaged chajjas and wind breakers
• Structural deterioration of columns and beams

The concrete structure becomes vulnerable to ingress of water and other aggressive elements due to the action of the mechanisms listed above. These problems can only be identified by being vigilant and by conducting inspections from time to time. Figure 1.

If neglected, then the problems manifest into serious defects like spalling concrete and corrosion of reinforcement. In some cases, the degree of corrosion is so high that the repair becomes structural in nature. Such repairs are normally costly but are necessary for preserving the structures. The reports of maintenance and the records should be preserved to determine the suitability as well as the durability of the repair materials. In addition to having knowledge of structural behavior, a rehabilitation expert should also have a thorough knowledge of materials science, not only in terms of advantages, but also the disadvantages and the limitations of the materials employed therein.

New generation materials and methods in restoring or repairing structures have become an invaluable part of this idea. Understanding these materials is key to selecting the correct methodologies to address defects in the building structures.

The Process of Remediation

When a structure shows signs of distress or deterioration, the following steps should be taken in principle. EN 1504 recommends the steps for the entire procedure for repairs from assessment to maintenance. A guidance note from the Concrete Society, UK recommends:

1. Assessment of damage to the structure
   1. Present condition
   2. The design approaches
   3. Exposure and environmental conditions the structure is exposed to
   4. Assessing the conditions and studying construction records
   5. Usage conditions and history
   6. Intended future use
2. Choose Options of Repair Considering
   1. Intended Use, Design and Service Life
   2. Required Performance Characteristics to be specified by Structural Engineer
   3. Long-term performance of repair
   4. Opportunities for additional protection and monitoring
   5. Acceptable number and costs for future repair cycles
   6. Future maintenance and access costs
   7. Properties of methods of preparing existing substrate
   8. Final appearance of the repaired structure
3. Choose the appropriate repair principles based on EN 1504: These principles address the methods for repair of both concrete and methods for mitigating corrosion of the reinforcement.
4. Choose the appropriate methods of repair in accordance with the principles mentioned in EN 1504.
5. Choose appropriate materials for repair conforming with the principles and characteristics mentioned in EN 1504.
6. Specify ongoing requirements such as maintenance of records of repair, maintenance schedules and periodic evaluation of residual life of the structure.

In these steps, the choice of procedure, base pre-treatment, and materials for concrete repair will depend on the existing degree of deterioration, the mechanism of deterioration, its causes and anticipated future stress. The reasons for evident defects or damage must be explored as thoroughly as possible. Following this, a thorough Q&A system needs to be established to ensure the durability of repairs.

Steps for Restoration

To successfully restore the building façade aesthetically as well as from a durability perspective, the following steps are needed:

• Removal of moss and biological growth and plants from the façade surface
• Leak Treatment: Cementitious Injection Grouting using Cement, Epoxy or PU Grout
• Treatment of joints [Polymer Modified Mortar, Injection Grouts and Reinforced Coating System]
• Addressing general damages spalling/honeycombs using polymer modified cosmetic mortar for masonry pointing and repair of RCC elements. [In special cases of highly damaged elements, it is better to get advice of a structural engineer and use structural repair products].
• Crystalline Waterproofing for damp walls [exterior as well as interior].
• Plastering to finished surface, by adding special plaster integral waterproofing admixture such as to the plaster.
• Application of Anti-Carbonation Protective Coating to the external façade.

Removal of Moss and Biological Growth

The first essential step would be the removal of moss and biological growth from the building façade as that would indicate the true damage to the envelope and prevent further growth of fungus. This would include removal of any plants growing cracks and crevices. Existing biological growth should be cleaned and removed from the structures using an appropriate cleaner and the surfaces should be treated with a bio-growth resistant sealer to avoid the recurrence of moss. For most applications, a solution of an oxygenated cleaner like SS – KleenStain SP or a biological cleaner like SS – BioKleen SH can be used for cleaning. A bio- resistant like SS – BioStop Sealer can help protect the structure from further biological growth. Figure 2.

Sealing of Cracks and Voids in Structure (Structural or Leaks)

The sealing of cracks in concrete is a special measure in itself, which becomes absolutely essential in the event of the following widths of cracks: More than 0.3 mm in dry areas/rooms or more than 0.2 mm on buildings in the open or more than 0.1 mm where the structure has a high corrosion risk. Low-viscosity epoxy-resins can be used to treat fine cracks (0.1 to 1 mm). For bigger cracks (0.4 to 2 mm) mineral injection grouts are also available, Figure 3. This injection must be carried out before application of the final treatment. After completion of diagnosis and selection of materials for injection, the work of injection passes through following stages:

1. Preparation of the crack
2. Location of points for injection
3. Surface sealing of cracks
4. Injection of resin proper
5. Removal of packers and plugging
6. Removal of sealing material
7. Final surface treatment after injection resin/grout hardens

Prior to selection of a material and method to remediate a crack or void, the characteristics of the defect need to be clearly assessed. The properties to be assessed include:

• Need for Crack Filling: Enhance load transfer or stop water/dampness/ingress through cracks
• Depth of Crack: Surface or deep cracks
• Crack Width
• Crack Movement: Moving or non-moving
• Condition of the Crack: Dry or damp / actively water bearing

Based on these properties, the correct injection material from Cement, Epoxy, PU or Coatings needs to be selected. Table 1 given above can act as a guideline for the selection of the correct injection grouting material.

Treatment of Joints and Transitions

Joints inside and outside the structure need treatment to prevent ingress of water through those joints. Joints can be first grouted with a cement based expansive grouting material. Joints then have to be V-Grooved and filled with a polymer modified mortar. The treated area should then be over0coated with a flexible 2K cementitious system, fortified with an alkali-resistant glass fibre mesh. In case of treating outside or inside corners in wet areas, the corners and interfaces should be treated using a thin high tenacity, special coated 3-ply tape, having a high extension, in cross direction and excellent toughness lengthwise. Figure 4.


The tape is normally composed of polypropylene non-woven fabric sand- wiching an ageing resistant thermoplastic elastomeric fabric. The flashing tape is resistant to chemicals such as 3% HCl, 35% H2SO4, Citric Acid, Lactic Acid, KOH [3%, 20%], Na Hypochlorite, Saltwater [2%]. The tape should be resistant to water pressure > 1.5 bar. The flashing tape shall be applied to all horizontal and vertical corners and pipe penetrations by adhering it using a Cementitious, Thin Bed Tile Fixing Adhesive.

Treating General Spalling/ Honeycombs using Polymer Modified Mortar

Small areas and patches less than 100 mm thick are usually repaired with hand / trowel or spray applied polymer repair mortars. Some of these products are proprietary products. On the other hand, these mortars can also be site batched using polymer additives. In most cases, a pre-bagged manufactured ready-to-use polymer modified mortar is preferred. The premixed/manufactured PCC for repairs to be used should have the following properties:

• Air Content: ≤ 3%
• Compressive Strength: ≥ 45 MPa
• Bond Strength to Concrete: ≥ 2 MPa
• Chloride Ion Content: 0.05%
• Capillary Absorption: ≤ 0.5 kg/m2.h0.5
• Adhesive Bond Strength to Concrete: ≥ 2 MPa

These mortars are most suitable for repairs when the section to be replaced ranges from a depth of 5 mm to 50 mm. In case of depths more than 100mm need to be placed, additional layers may be needed. Follow manufacturer’s recommendations for mixing, placement, compaction and curing. In case of large structural elements, a ready to use non-shrink micro-concrete can be used.

To achieve a visually uniform surface and to provide additional preventive protection, the repaired concrete surface should be smoothed over with a cosmetic mortar. This is done with a fine polymer modified, concrete cosmetic or fine sand and a mixing liquid composed of water and or the polymer component.

Crystalline Waterproofing for damp walls [exterior as well as interior]

To prevent passage of water, dampness and salts across the walls, applying a two component-Cementitious Crystalline Coating System for Waterproofing of Cement Bound Substrates and Masonry, over the prepared / repaired wall surfaces will add another layer of protection. This will stop the passage of water and visible dampness across the walls. Figure 5.

Plastering to Finished Surface, by adding Special Plaster Integral Waterproofing Admixture

With high demands on durability, the thought process should turn to preventing water ingress through the living space, by providing the correct plaster as an envelope to the building. Modern Plaster Additives like SS – PlastProof IL are a combination of selected additives designed to impart the produced plaster with specific properties relating to water resistance, stability, sag resistance, open time, workability and water-retention. This innovation combines many benefits into one well designed judicious product, which improves the following properties in any wall / ceiling plaster:

• Waterproofing
• Excellent Bonding, even to concrete blocks
• Thixotropy
• Smooth Finish
• Minimal to Zero Wastage
• Lesser Rebound
• Improved Sprayability
• Minimal labour requirement for Finishing
• Ability to use thin plaster layers for finishing

This solution has the robustness to convert any mortar, to a sprayable, easily finishable, durable wall plaster. The robustness of this formulation, allows it to be used with multiple raw material sources including crushed sand, manufactured sand, GGBSF, Flyash, Natural sand, PPC and other base raw materials for plaster.

Application of Protective Anti-Carbonation Coating to the external façade

On completion of the work described above, the entire concrete surface must be provided with a final seal or final coating. This is mandatory to maintain status quo of the corrosion. Such surface treatments perform several duties at the one time. Firstly, all the concrete is protected from further stress due to aggressive pollutants in the air and from progressive carbonation. In case of exposed surface, it may also be imperative to use hydrophobic impregnations for protecting the structures from water ingress.

The coatings to be applied on concrete should not be selected solely for aesthetics but also on protection criteria. It is possible to have both aesthetic and protection judiciously combined in a single well-designed coating. Figure 6 shows the properties and requirements of general protective coatings for concrete and cover the concrete protection solutions in the section above. According to International literature, the following properties are most essential for an excellent concrete protection system.

Breathability

It is that property which enables the water vapor to move in and more importantly out of the concrete with the fluctuation of temperature and humidity. Mineral based coatings or high-performance acrylic polymer modified anti-carbonation elastic elastomeric coatings, provide much higher breathing capacity than most resin-based coatings, and thus can protect structures longer.

Impermeability to Water, CO2 and other gases

Protective coatings should be permeable to water vapor, but impermeable to water. Good protective coatings provide a sound physical and chemical protection barrier to prevent corrosion in concrete, ensuring durability. These properties can be verified by adequate testing to ensure quality of coatings.

Crack Bridging

This is an important property as the coating should be able to negotiate expansions and contractions in the structure due to temperature or dynamic loads. It should also effectively seal and bridge over cracks, to ensure no water or gases enter concrete through them.

Ultraviolet (UV) ray’s resistance

For exterior application, it is mandatory that the coatings should be UV stable. Correctly designed protective coatings remain flexible even after exposure to sunlight without degrading.

Conclusion

These methods can be followed for durable repair of RCC Building Façades. The system from assessment to completion of repair and further maintenance should be rigorously followed for durability. Protective coatings should also be used as they help improve the durability of repairs as well as that of the structures. It should be remembered, protect the concrete and the steel will take care of itself.

References:

1. EN 1504 (Parts 1 to 10)
2. Repair Guidance: Note No. 4 – Scope of EN 1504, Concrete Society UK
3. IRC SP 40
4. IRC SP 80
5. Concrete Repair Guide ACI 546R-04
6. Technical Report No. 38: Patch Repair of Reinforced Concrete by Concrete Society UK