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Guidelines for use of Construction Chemicals for Repair and Rehabilitation of Structures

Reinforced Concrete, a perfect combination of Concrete and Reinforcing Steel, has long been a material of choice for construction on account of its structural

strength, mouldability and durability. This makes it attractive for use in varied exposure conditions. Vulnerability of rein-forced concrete to deterioration has been a cause of deep concern. Concrete Cover plays a key role in this phenomenon. The process of deterioration begins almost immediately after the casting. From material science point of view the durability of concrete structure is a direct function of achieving speci-fied cover not only in dimensions but also in quality. Consid-ering a mix of complicated construction, quick completion deadlines, speed of construction and economy, it becomes virtually impossible to cast specified cover to reinforcement. The outermost layer of the reinforced concrete becomes the weakest one. If formworks are not impermeable and slump of concrete is not adequate, the concrete does not cover the reinforcement optimally to render it passive to corrosion. This facilitates deterioration of reinforced concrete and the subsequent need for repairs.

Since the cover zone is the first line of defense to inhibit corrosion, concrete repair and rehabilitation is recreation of cover with the highest protection quotient. We therefore need to use the latest technologies like polymers, fibres, etc. to lower the permeability, minimize the cracks and provide ad-equate bonding, to stop further deterioration. Given actual working conditions, it is difficult to create a cover that will be resistant to carbonation or chloride ingress. Construction Chemical technology today provides the material science to address this difficulty.

A combination of materials from corrosion resistant coatings for the reinforcement, to bond coats, cement or resin bound polymer-modified repair mortars, micro-con-cretes, fairing coats and protective coating systems offered by the construction chemicals industry should be strategi-cally used to form a durable repair system, that can signifi-cantly enhance the life of the structure being repaired.

Cause of Deterioration

It has been established that the increasing environmen-tal stress is one of the key factors causing concrete damage. Figure 1 shows this process, due to carbonation attack. It is recognized that steel embedded in a heavily alkaline medium like concrete with pH-values from 9 upwards will not rust.

Sunny Surlaker Head of Admixture Division, MC-Bauchemie India Private Limited

The concrete’s hydration process ensures the concrete’s al-kalinity, producing a pH-value of more than 12.6, which ren-ders the steel surface passive to corrosion.

Due to this, even the occurrence of small cracks (up to 0.1 mm in width) or blemishes in the concrete need not necessarily lead to damage. Environmental influences and carbon dioxide in particular, will reduce the concrete’s pH-value (carbonation) and will remove the passivating effect. In conjunction with existing humidity, the result is corrosion of the reinforcement.

The Reason why the deterioration process begins can be ascribed to varying causes, which frequently overlap. Figures 2a and 2b taken from EN 1504 provides an excellent guide-line summary of possible causes of concrete deterioration.

The first points emphasized in the statistical record of causes of damage are poor workmanship on the part of the concrete producer and environmental stress. With regard to poor workmanship, the principal factors concerned here are an excessive water/cement ratio, defective concrete cover and defective curing. Taking these factors into consideration, specifiers should issue more stringent guidelines in respect of the quality of the concrete cover. Particular importance should be attributed to sufficient curing of the concrete with a view to ensuring its durability. Other factors, of course in-clude higher mechanical, chemical, thermal and biological loads, the solution for which is specifying the correct protec-tive coatings for the concrete.

Figure 1

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The Process of Remediation

When a structure shows signs of distress or deteriora-tion, the following steps should be taken in principle. The condition survey is to be conducted and the field investiga-tions are to be done. The steps are as under:

- Preliminary Investigation, detailed Investigation- Diagnosis- Laying out specifications for repairs- Selection of Materials- Surface Preparations- Actual Repairs- Periodical maintenance- Maintenance of Reports etc. for future repairs

In these steps, the choice of procedure, base pretreat-ment and materials for concrete repair will depend on the existing degree of deterioration, the mechanism of deterio-ration, its causes and anticipated future stress. A correct diagnosis must therefore be arrived at prior to repair. The reasons for evident defects or damage must be explored as thoroughly as possible. Following this a thorough QA system needs to be established to ensure the durability of repairs. To provide building owners and architects with a high level of reliability in terms of durable protection and repair work, we have developed “Quality Assurance Systems” based on the following criteria:

- Long years of experience, professional competence and technical equipment on the part of the executing agency

for assessment of defects, damage and for establish-ment of repair measures.

- Depending on environmental or structural stress con-cerned and on extent of damage, protective and repair system adapted to the structure in question and involving suitable and reliable products for the repair of concrete structures will be recommended from a range of suc-cessfully tested and established materials.

- Experienced professional firms who are thoroughly fa-miliar with the material concerned should only carry out protective and repair work. For these reason, we pass on the experience and knowledge of its staff to professional applicators. The company will advise such firms before and during the performance of repair work.A well-established repair system for concrete structures

in building construction is described in detail below. Hints are also given on alternatives frequently required due to con-ditions at the structures in question. In Brief Figure 3 shows the procedure of effective concrete repair. The steps are dis-cussed in detail below.

Figure 2a

Figure 2b

Figure 3

Correct use of Construction Chemicals for Durable Repair – The Materials and their Correct Usage

Surface Preparation

The existing base must firm stable and free from oils, and fats, impurities of all kind including form/mould oil resi-dues must be removed along with any cement laitance. Me-chanical Cleaning or wire brushing is an effective water-free method to clean reinforcement. Use of acidic rust removers and water is not recommended to protect the reinforcement from further corrosion. Their use needs very careful control as they need to be completely removed from the reinforcement and the concrete base prior to application of the corrosion control system. If these materials are left on the substrate, they may cause further damage to the system and therefore their use is not recommended. The base should be lightly moistened before commencing work and should be slightly damp but still absorbent and never saturated with water.

Protection of Exposed Reinforcement

All exposed reinforcement cleaned to bare metal must be protected immediately after mechanical cleaning by us-ing a suitable corrosion protection Coat. This material can either be a polymer modified one-component corrosion in-hibitor and bond coat, or in harsher conditions, a zinc rich resin based material like MC-DUR ZKE can be used. Figure 4 shows

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a simple application of the one-component corrosion inhibi-tor coat.

Treatment of Cracks

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 in the size range of 0 to 0.1 mm or above. An ultra-fine cement suspension with a suitable plasticizer and non-shrink additive is suited for use for crack widths of size 0.05 mm to 0.25 mm or slightly above. Fine, non-shrink cement grout can be used suitable for crack widths of 0.2 to 0.8 mm or slightly above.

These materials produce a structurally perfect bond to transfer loads in the case of non-moving cracks. This in-jection must be carried out before application of the final

treatment. These materials cannot be injected into moving cracks or injected against pressurized water. Incase of very wet cracks or pressurized water, suitable PU based injection materials can be used. Figure 5 shows the materials for use for different crack widths.

Bond Coat

All defective areas to be filled are first pre-treated with a suitable polymer modified or epoxy resin-based bond coat. The coarse repair or fill material is worked wet-in-wet over the bond coat. Figure 6 shows application of the bond coat prior to coarse repair mortar / fill material.

Figure 4

Figure 5

Figure 6

Coarse Repair Mortar or Micro-Concrete

These materials are applied wet-on-wet onto the bond-coat, while the adhesive bond is still fresh. These materials can be classified into different categories including but not limited to:

a. Polymer Modified Spray Applied Coarse Repair Mortarb. Polymer Modified Trowel Applied Coarse Repair Mortar

(Ready-to-Use One-Component)c. Cement Sand Trowel Applied Coarse Repair Mortar

modified with polymer emulsion on sited. Cementitious Micro-Concretee. Epoxy-resin modified Trowel applied or poured repair

mortar

It should be remembered that all these materials need thorough mixing prior to application. In all cases, the powder component should be thoroughly dry-mixed and then added to the liquid component and never vice-versa. The material should be mixed with a mechanical slow-rotating paddle mixer until homogenous and lump-free. The Manufacturer’s recommendations on water to be added to the mortar must be followed. Hardened material should not be mixed and re-used and also the applied mortar should not be overworked once it is dry. In case larger thicknesses of repair need to be undertaken, these can be done in several steps. Recommen-

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dations on layer thickness for each mortar, as per the manu-facturer, should be followed. Subsequent layers should be applied when the first layer has stiffened sufficiently, but is still green. If dry, the first coat should be slightly- prewetted and a bond coat has to be applied prior to the second layer. All coarse repair systems need to be suitably cured with wa-ter or curing compounds (these are recommended). Some of the applications are shown in Figure 7.

have a better flexural strength to allow its use in Repair-ing Structural Elements. These materials can be poured into structural elements or can be mixed with less wa-ter and used as a non-shrink trowel applied mortar with very high compressive and flexural strengths. These are best suited to fill large honeycombs, for jacketing, or for pours into beams, slabs or vertical shear walls. These can also be used to repair very high strength concrete (> 80 MPa).

e. Epoxy-resin modified Trowel applied or poured repair mortar: These materials can be used in highly aggres-sive environments with strong chemical loads. Particu-lar care should be taken in the mixing of the resin com-ponent according to the manufacturer’s instructions. As always, the powder should be slowly added to the resin during mixing and not vice-versa. Epoxy resins are sen-sitive to moisture and therefore, the substrate moisture contents should be regulated prior to application of the epoxy-resin repair mortar. Suitable bond coats / scratch coats should be applied prior to the epoxy-mortar.

Fine Filling / Fairing Coat

To achieve a visually uniform surface and to provide ad-ditional preventive protection the repaired concrete surface should be fine-filled. This is done with a fine polymer modi-fied- concrete cosmetic and a mixing liquid composed of wa-ter and/or a polymer component. Depending on the structure involved, it can be used in overhead work or as a fine-filler under elastic, crack-bridging systems. Figure 8 shows the application of fine cosmetic mortars.

Figure 7

Some further precautions / recommendations for each of the types of coarse mortar to be used are as follows:

a. Polymer Modified Spray Applied Coarse Repair Mor-tar: Recommended water contents should be followed. These materials do not always need a bond-coat prior to application.

b. Polymer Modified Trowel Applied Coarse Repair Mortar (Ready-to-Use One-Component): Most of the precau-tions are listed above.

c. Cement Sand Trowel Applied Coarse Repair Mortar modified with polymer emulsion on site: For Site-Mixed mortars, the type of polymer to be used must be thor-oughly vetted. It should be remembered that “Polymer” is a generic name and Latex means white colour liquid. Both are misguiding words. Every white liquid is not a Polymer / Latex and every Acrylic/SBR is not suitable as repair admixture. Also a higher Solid Content does not mean better product. This being said, the properties re-quired of the repair mortar should be clearly specified. These specifications can include properties such as air-content (the lower the voids in the mortar, the more du-rable will be the repair), compressive strength, and more importantly the flexural strength and bond strength to the substrate.

d. Cementitious Micro-Concrete: This is a versatile mate-rial that can be used very effectively in repair strategies. Micro-Concretes are a class of construction materials that consist of a dry mixture of cement, mineral aggre-gates and admixtures and additives. They are factory-made, dry-stored and protected from the weather. These materials are processed by Mixing water at the site and mixing to a flowable / pourable consistency. Micro-Con-crete’s have all the advantageous material properties of non-shrink grouts, and are additionally formulated to

Figure 8

Impregnation to provide Hydrophobic Qualities

As an additional, protective measure, particularly in the case of porous substrates susceptible to fractures (e.g light weight concrete), a hydrophobic impregnation agent can be applied up to saturation point to the surfaces first treated by a fine finish. This impregnation will ensure that there will be no infiltration leading to more extensive damage if hairline cracks from or if the surface protection is subjected to me-chanical damage.

Concrete Surface Protection, Carbonation inhibitor, Co-loured finishes: 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 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. Figures 9 to 11 show suit-ably protected concrete and masonry structures.

These coating systems must have to a high CO2 resis-

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tance if they are to be effective in protecting against carbon-ation and, on the other hand, they must not have a negative effect on the buildings water vapor diffusion rate.

Some performance properties that are normally re-quired of an able protective coating are:

1. High Diffusion resistance to CO2

2. Suitable Diffusion resistance to H2O vapour, when desig-nated as breathable coatings

3. High Resistance against water penetration4. Resistance to chlorides (ponding)5. UV Resistance6. Crack Bridging ability under weathered conditions

Figure 12 illustrates these properties.

Systems on a Two- component Epoxy/ PU Resin Base:

Figure 9: Cooling Tower Systems based on reaction resins have proven to be best for structures subjected to severe chemical stress- for e.g. in production areas, water clarification plant, factory floors, multi-storey car parks and many other structures. It should be noted here that systems of this kind, based on two-com-ponent reaction resins, show a thermal expansion behavior different from that of concrete. Allowance must be made for this fact in the repair of structures in the open air and of ob-jects exposed to severe stress from changes in the repair of structures in the open air and of objects exposed to severe stress from changes in temperature.

Conclusion

Corrosion damage to the reinforcement in concrete and the resultant spalling of sections of concrete along with oth-er concrete surface damage is far more serious than being just an eye-sore: it constitutes a serious risk to the useful life of the concrete structure and even become a danger to occupants and passer-by. The guidelines provided above in-dicate possible ways of providing reliable repair with well-established products and systems.

In addition to quality of the repair material, professional handling of these materials and a knowledge of concrete technology, construction physics and their inter relationships on the part of those applying the treatment is of decisive im-portance. It is therefore essential to reserve the execution of concrete repairs solely to experienced and trained pro-fessional firms. It has also been shown that, besides careful execution and observation of all specifications relating to the durability of concrete structures, preventive protection can provide added assurance.

Particularly worthy of note in this connection is curing of the fresh concrete with products which will provide long-term protection against CO2 and other air pollutants, thus preventing carbonation of the concrete surface. Reference is also made to proven protective systems for structures espe-cially at risk, like hydrophobic impregnations for preventive protection, or anti carbonation coats and the various liquid plastics for protection of concrete against corrosive materials, acids, salts, and other chemical effects -severe chemical at-tack.

In conclusion, properly planned and completely executed repair works can help prolong service life of most reinforced concrete structures. w

Figure 10: Masonry-Heritage

Figure 12

Figure 11