Dr. Hermann Lutz, Wacker Polymers, Germany

Concrete is a very versatile, long-lasting and durable building and construction material if applied accordingly. Unfortunately, in the past and even today, repeated disregard of the fundamental principles of concrete and structural concrete application did and still does lead to severe and serious damages in the building industry. For successful concrete renovation, dispersible polymer powders show extremely good results and guarantee high impermeability against water, CO² and penetration of other pollutants.

The cost of repairing concrete structures has dramatically increased during the last 30 years in all industrial countries. In Germany, approximately 20% of the costs of the volume of structural concrete work is attributed to the repair and maintenance of existing buildings and structures.

The degradation of structural concrete is caused by corrosion of the steel reinforcement due to chemical processes often occurring over a long period. One of the main causes is the carbonation of concrete. Acidic carbon dioxide from the atmosphere and other aggressive media (such as SO2, acid rain) neutralises the alkalinity of the concrete. If the alkaline environment of the steel reinforcing no longer exists, the steel starts to corrode and due to its volume increase, causes spalling or splitting of the concrete overlaying the steel. A secondary cause of corrosion is the penetration of free chloride ions into the concrete, leading to chloride ion attack on the steel.

In the construction industry, concrete repair work can be classified in two types:
  • Repairing concrete which does not contain steel reinforcement and which does not have load-bearing functions. Repair is done for aesthetic reasons (cosmetic repair work) only, with so-called patching mortars.
  • More important and more relevant for the building industry is the
  • Repair and reconstruction of damaged reinforced and load-bearing concrete structures in order to maintain and reconstitute their structural stability. This is done in stages with different kind of mortars which are part of a so-called concrete rehabilitation system (typical applications: repair work and rehabilitation of bridges, parking decks, tunnels, etc).

Damage to Reinforced Concrete Structures

The stability of reinforced concrete as a composite material is based on the fact that the concrete cover protects the steel against corrosion, in addition to the close similarity in the thermal expansion of the steel and concrete. Provided that chlorine ions are either absent or present only in very low concentrations, steel reinforcement only undergoes corrosion when the pH value is less than 8-9 and moisture and oxygen are present. The pH value in the capillary pores of the hardened cement is approximately 13, depending on the presence of calcium hydroxide and also sodium- and potassium hydroxide produced during the hydration of the cement. This high pH value serves to passivate the iron reinforcement, so that no corrosion can occur.

Alkaline anti-rust protection of the steel reinforcement is jeopardised when either the concrete is not protected against the penetration of aggressive media, and/or the cover is too thin or/and the concrete itself is not dense (impermeable) enough. It is also being increasingly threatened by the rising level of corrosive substances in the atmosphere. Carbon dioxide and sulphur dioxide impair concrete's protective properties (lowering the pH value) through carbonation and the formation of salts. High chloride concentrations stemming from the use of de-icing salts are often found near bridges and other road engineering structures. If sufficiently concentrated, chlorine ions can cause pitting corrosion of steel even under alkaline conditions.

Causes of Damage to Concrete Structures

There are four basic factors which damage concrete structures:
  • Structural factors For example incorrect structural calculations, badly formed construction and movement joints, incorrectly calculated reinforcement.
  • Concrete related factors Unsuitable type and content of cement, inappropriate composition of the concrete (e.g., too high water/cement ratio), unsuitable or non-ideal particle size distribution of aggregates, unsuitable cement additives/admixes or incorrect amount of additives/admixes.
  • Poor workmanship In particular, too thin concrete coverage and/or too low density of the concrete. Also, poor curing, poor surface structure (release agent from formwork etc.), mistakes during the pouring of concrete.
  • External influences on concrete: Static or dynamic overload, fire and chemical damage (chemical damage includes pitting corrosion of the steel reinforcement by chloride ions or aggressive salts), neutralisation of alkalinity by acids (strong carbonation; environmental influences such as CO2 , SO2 and water); alkali reaction of the cement with silicates (alkali-silicate reaction, very rare).

Diagnosing the Causes

Before permanent and durable renovation and repair of concrete structures can be done, the causes of damage must be determined. Finding these causes is all the more difficult as there are usually many interacting factors involved. It is generally not possible to link firmly a particular type of damage with specific symptoms or reasons. The use of a checklist of possible symptoms is very helpful in order to identify the possible causes, such as color variations of the concrete, discoloration, pores, voids, pockets of loose gravel, cracks, spalling, rust marks, concrete cover. Physicochemical test methods are often employed as well.

Once the causes and the extent of the damage have been determined, the structure in question must be carefully prepared for repair and rehabilitation. Basically, a clear distinction must be drawn between so-called concrete re-profiling ("concrete cosmetics") with patching mortars and concrete replacement, which is intended to preserve or restore a building's structural stability. Concrete rehabilitation is subject to special requirements and specific regulations in order to match the characteristics and functions of the old concrete and concrete structure.

Repair and Rehabilitation of Damaged Concrete

Patching mortars for reprofiling and cosmetic repair are mainly based on dry-mix mortars and are not part of a whole system. Usually, cement-based mortars are used for indoor and outdoor applications, whereas gypsum-based products are only used for some specific indoor applications (cosmetic repair). Patching mortars are used to repair defective or damaged areas of mineral surfaces without taking on a load-bearing function, e.g., for stopping small holes, voids, cracks and cavities in order to restore the original dimension (cosmetic reparation). Typical applications are patching mortars for walls, ceilings, floors, steps of staircases, etc.

The most important technical requirements for such patching mortars are:
  • good workability and good wetting of the surface
  • simple and consistently easy to apply
  • good adhesion to all construction substrates to be repaired
  • high durability (and abrasion/wear resistance if subjected to direct wear or load)
  • sufficient flexibility (to reduce the risk of crack formation)
  • low shrinkage
  • water repellent effect for outdoor applications
Preconditions for the corrosion of the reinforcing steel are the reduction of the alkaline passivation capacity of the concrete by carbonation and the penetration of moisture in combination with oxygen, CO2 , SO2 or Cl2 to the steel surface (except pitting corrosion due to halogen salts). Due to the higher volume of rust compared to steel, the process of the corrosion usually causes cracking of the concrete and finally spalling of the concrete covering the steel reinforcement.

In order to be able to guarantee the durable and reliable repair of structural concrete, three main fundamental requirements of a concrete rehabilitation system must be fulfilled simultaneously:
  • restoration of the corrosion protection of the steel reinforcement (alkaline environment)
  • restoration and reprofiling of the concrete structure including its load-bearing functions
  • restoration of the durability of the whole construction (protection against weathering and environmental damage caused by CO2 , SO2 , Cl2 , de-icing salts, etc)
In order to meet all these requirements, for patching mortar as well as for the rehabilitation of reinforced structural concrete polymer-modified cement concrete as a prepacked dry-mix mortar is the most convenient solution. They can be applied by hand, in a wet or even a dry spraying process. Depending on the dosage, polymer modification with VINNAPAS dispersible polymer powders improves the workability of the mortar, its wetting of the substrate, the adhesion to all kind of substrates, the flexural strength, the abrasion resistance, the flexibility (lower modulus of elasticity), the durability and the water repellent effect (with special grades of VINNAPAS dispersible polymer powders).

Influence of VINNAPAS dispersible polymer powders on the characteristics of concrete rehabilitation mortars

VINNAPAS® is the commercial name for thermoplastic polymer binding agents which are produced by spray drying special water based polymers. In 1953, Wacker Chemie invented dispersible powders, and today WACKER is the world's largest producer of dispersible polymer powders. In combination with mineral binders (mainly cement), VINNAPAS® dispersible polymer powders have been used in the construction industry with great success for 50 years now. The main applications of polymer-modified premixed mortars are as construction and tile adhesives, exterior insulation and finish systems, self-leveling compounds and grouts, as well as plasters, repair mortars and sealing slurries.

Cementitious mortars as repairing compounds for concrete offer, if formulated correctly, such important advantages:
  • high compressive strength
  • high water resistance
  • excellent resistance to weathering
  • and in addition, they are relatively low priced.
However, cementitious systems have several weaknesses, including incomplete hydration of the cement in thin-layer applications, poor adhesion to many substrates (prewetting or the application of a primer is required), poor resistance to aggressive media such as acid rain and an extremely low deformability or high modulus of elasticity.

The modification of a cementitious mortar with VINNAPAS dispersible polymer powders as a film-forming thermoplastic resin combines the advantages of a cementitious mortar with the advantages of a polymer binder. Both types of binders are ideal complements to the two individual binders and provide characteristics, which cannot be achieved by one of the two separately. Cement needs water to develop its strength, whereas the polymer binder only forms a film when the mortar with the re-dispersed VINNAPAS polymer powder loses water. The use of VINNAPAS powders in dry-mix mortar for the rehabilitation of concrete structures is, in many cases, a must in order to meet the technical requirements and improves the mortar performance.

The following technical characteristics are improved:
  • better water retention (better hydration and strength development of the mortar)
  • better workability
  • improved wetting of the substrate
  • improved adhesion on the substrates through the polymer binder
  • improved abrasion resistance and higher toughness
  • higher flexural strength and flexibility
  • reduction of the water/cement ratio (higher compressive strength)
  • reduction of the modulus of elasticity (lower tendency for shrinkage and cracking)
  • improved impermeability of the mortar (reduced penetration of CO2, SO2and salts)
  • significant improvement of the durability

Improvement of rehabilitation mortars with VINNAPAS dispersible polymer powders

The best adhesion strength to sandblasted steel and to concrete is obtained with a polymer modified reprofiling mortar and priming the substrate with a polymer modified primer or key coat. Without the application of a primer, the adhesion on concrete is still high enough due to the modification with VINNAPAS powder, whereas the adhesion on steel is not high enough. Using a purely cementitious reprofiling mortar not modified with a polymer binder, the adhesion to concrete is not sufficient and there is no adhesion at all to steel.

If the mortar is applied by the dry shotcrete process, the water is mixed with the dry mortar only in the jet and the mortar is then projected immediately to the surface. Despite this extremely short mixing and almost no maturing time, the VINNAPAS dispersible polymer powder re-disperses quickly and completely enough in order to improve the tensile adhesion strength and the flexural strength in almost the same magnitude compared with its conventional application by hand. Even for the shotcrete mortar applied in the dry process there is a very significant increase in adhesion bond strength if modified with VINNAPAS powder.

When the morphology of such polymer modified concrete repair mortars is studied by a scanning electron microscope (SEM), large continuous areas of polymer films can be noted, independent whether the mortar is applied by hand or with the dry shotcrete process.

The polymer films are concentrated along the capillary cavities and voids in the mortar, which are responsible for its poor flexural strength compared to its compressive strength. From such pictures it is evident that the dispersible polymer powder do indeed act as a binder for the cement and aggregate particles and that the polymer film will also overbridge voids and cracks. Since the polymer films are coating the walls of the pores and cavities in the mortar like a sort of a covering membrane, it follows that capillary absorbency and penetration of liquids and gases will be reduced, since their transport takes place via these pores. In addition, the polymer film acts as a coupling agent between the interface and the mortar, thus being responsible for the adhesion of the mineral mortar to difficult substrates such as steel reinforcement.

In principle, the mortar for the fine stopper or levelling mortar must meet the same requirements as the reprofiling mortar. Usually, this mortar should contain higher amounts of VINNAPAS polymer powder in order to reduce the modulus of elasticity more efficiently (the modulus of elasticity of a coating should always be lower than the modulus of the layers below it), to improve the impermeability (e.g. carbonation resistance) and the abrasion resistance. As a final protective layer, different types of coating materials can be applied, e.g., dispersion paints or cementitious waterproofing sealing slurries.

Practical Steps for Concrete Rehabilitation Work

For concrete rehabilitation systems, different kind of mortars with different characteristics and functions are required. For the reinforced steel, primer and adhesion promoter must be used. To repair the concrete, adhesion promoter slurries (primer or key-coat) are used. Afterwards, restoration and reprofiling mortars are applied, followed by a fine stopper or smoothing mortar. Finally, a protective and finishing coat is applied.

1. Preparing the surface

All cavities, loose and weakened parts of the old concrete must be removed by hacking, scrubbing, sandblasting or waterblasting. Areas contaminated with chloride and carbonated areas around the reinforcement must be removed totally. The reinforcement itself must be completely derusted.

2. Priming and protecting the exposed steel reinforcement

Today, derusted reinforcing steel is often covered with an epoxy resin primer, generally two coats being required. Sand is sprinkled over the wet primer to act as a key for the repair mortar. This technique is very laborious and care must be taken to ensure that the coating of epoxy resin is totally free from blemishes. Practical experience has shown that the adhesion of cementitious mortars to sanded epoxy resin primers on the structural steel is not as good as to cementitious slurries. Poor adhesion to the steel can give rise to crack formation on the surface, promoting cracking of the repair mortar. In addition, any epoxy resin applied unintentionally to the adjacent concrete will form there a closed structure to which the reprofiling mortar will show bad adhesion. Moreover, recent research has shown that repairing small areas of structural reinforcing steel with coats of synthetic resins can create an electrical anode under the resin coat and severe corrosion can result from the formation of such electrical "macro-cells".

All these problems can be overcome by priming the steel reinforcement and the concrete with a slurry-like cementitious product which should contain a fine particle sized sand and a synthetic binder (resin) in the form of a VINNAPAS dispersible polymer powder. This anti-corrosive protective coating and concrete key-coat will adhere very well to the steel reinforcement as well as to the old concrete to be repaired. In addition, there is no formation of electrical macro-elements and the reprofiling mortar will adhere well to this cementitious mortar. The synthetic resin/cement ratio and the water/cement ratio of the slurry should be similar to those of the repair mortar. Depending on the formulation used, good results can be obtained from resin/cement ratios of 0,05 to approx. 0,12. Despite the polymer modification, the concrete substrate should still be pre-wetted before the application of the cementitious primer slurry, but no free water should be left on the surface.

3. Repair mortar (reprofiling mortar)

The reprofiling mortar must perform all the functions of the former concrete before its damage, including protection of the steel reinforcement and the load-bearing functions of the concrete structure. When formulating the coarse reprofiling mortar a standard (Portland) cement is usually used as hydraulic binder. The use of higher quality Portland cements will result in slightly faster setting reactions and a more rapid increase in strength. On the other hand, such mortars suffer from the disadvantage of higher water demand, higher shrinkage, and a reduced pot live. The dosage of the cement in the prepacked dry-mix mortar must be adjusted to adapt the modulus of elasticity to that of the concrete to be repaired.

Unfortunately, on job sites, the repair work is often not carried out exactly according to the recommendations of mortar manufacturers. Therefore, mortars which do not fail if they have been mixed with slightly more gauging water should be used. For that reason many reprofiling mortars contain fibres, which can, to a certain extent, prevent the formation of microcracks especially during setting and the early hardening phase.

Although prewetting of the damaged areas is recommended, under practical conditions on the building sites neither prewetting is carried out nor in some cases adhesive slurries are used. In these cases the reprofiling mortar must have excellent bond strength to concrete. Good adhesion even to dry concrete surfaces can be achieved by polymer modified dry-mix mortars containing VINNAPAS dispersible polymer powders.

4. Fine stopper

Repaired concrete areas generally must be covered with a layer of fine stopper, levelling or smoothing compound of approx. 1-4 mm thickness. This layer imparts an even, smooth and pore-free surface to the rough reprofiling mortar and old concrete substrate. In principle, fine stopper mortars must meet the same requirements imposed on the reprofiling mortar. They have mainly the same constituents but with a maximum particle size of 0,2 – 0,6 mm.

Extremely good results are obtained if the cementitious fine stopper dry-mix mortars are modified with VINNAPAS dispersible polymer powders. VINNAPAS powders improve the workability, prevent thin layers form drying too rapidly and from cracking, they improve the adhesion, the abrasion resistance, and increase the strength and the impermeability to water.

Fine stopper mortars with a high impermeability or water repellent effect should be preferred even if a protective finishing coat will be applied. Such water-repellent or impermeable mortars can be formulated by modifying them with hydrophobic VINNAPAS powders. Apart form the usual advantages conferred by VINNAPAS powders on the mortars, these hydrophobic VINNAPAS grades impart long-lasting water repellency to the mortar, thus preventing the penetration and transport of water soluble gases and salts.

5. Finishing or protective coat, top coat

Once all stages of priming the reinforcement, applying the reprofiling mortar and the fine stopper are completed, the fourth and final step in a concrete rehabilitation system consists of applying a protective coat. This final coat is an integral part of the rehabilitation system and should be as impermeable as possible in order to prevent the penetration of water and aggressive media form the atmosphere and environment (CO2, SO2, salts). It also helps, together with the other parts of the rehabilitation system, to preserve the alkalinity of the structure and the repaired area. Thus, carbonation is delayed, leaching is prevented and substances that promote corrosion of the steel reinforcement are not able to penetrate into the concrete.

In order to fulfil their functions, these coats must be impermeable and resistant to weathering. Mostly crack overbridging paints, cementitious waterproofing sealing slurries or dispersion paints based on VINNAPAS dispersions are used as the protective finishing coat.

Summary

VINNAPAS dispersible polymer powders of WACKER POLYMERS have now been used successfully worldwide for decades to improve the technical performance of mortars employed under many different climatic conditions to repair and rehabilitate structural concrete. These modified mortars used for reprofiling are part of a comprehensive system which includes anti-corrosive coatings for the steel reinforcement, bonding bridges (primers) to improve the adhesion of the reprofiling mortar and surfacing mortars (fine stopper) –all of them being polymer modified cementitious materials. Subsequently applied finishes and protective coating systems, mostly polymer based building products, serve as a protection against further damages and for aesthetic reasons.

With its VINNAPAS® dispersible polymer powders, WACKER has been the global market and technology leader for fifty years in the field of polymeric binders for modifying cementitious systems. More than a million metric tons of VINNAPAS® polymer powders have been sold worldwide since they were first launched. Typical applications are ceramic tile adhesives and building adhesives, jointing and grouting mortars, thermal insulation mortars, self-levelling mortars and screeds, concrete repair mortars and mortars for concrete rehabilitation systems, all types of plasters, renders and mineral finishing coats, lime cement-paints and cement-free powder paints, cementitious waterproofing sealing slurries, joint fillers and trowelling and smoothing compounds.

WACKER first produced polymeric binders in powder form for the construction industry back in 1957. These thermoplastic polymers derived primarily from vinyl acetate and ethylene are produced by spray drying special aqueous polymer dispersions. This revolutionized working methods in construction, since, for the first time, a one-part, polymer-modified cementitious system was available that only required water to be added on site. To this day, the system represents a much simpler way of working, with substantial cost advantages.

The benefits that VINNAPAS® bestows on the end product include easier processing, excellent anchorage to all substrates, increased flexibility and flexural strength, and enhanced weathering resistance. Another advantage is that VINNAPAS® polymer powders do not contain plasticizers and film-forming aids, and therefore have low emissions levels.
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