Repair of concrete and its rehabilitation with the help of polymer-modified mortars

The CMD and the Directors of Sunanda Global recommend that polymer-modified cement concrete mortars are best suited for the repair of concrete and rehabilitation jobs.

The broad steps involved in such jobs are described in detail. The need for standardization and incorporation of these products into the relevant Indian Standards is also highlighted.

Factors affecting the deterioration of reinforced and prestressed concrete structures and the mechanism of deterioration are well understood today.

Major factors responsible for deterioration are faulty design, unsuitable materials, improper workmanship, overloading, and exposure to an abnormally aggressive environment.

Concrete degeneration usually manifested in the form of carbonation, corrosion of reinforcing steel, cracking, spalling, excessive deflection, etc.

For an effective method of repair of concrete and its rehabilitation, the following points need thorough consideration, besides the considerations of structural parameters.

i) Ascertaining the extent of corrosion and carbonation.

ii) Near-total removal of a corroded portion of steel.

iii) Application of a corrosion-resistant barrier film on the reinforcement steel (preferably, the such film should inhibit further corrosion.)

iv) Application of a proper bond coat that assures good bonding.

v) Rendering a robust and passive carbonation-resistant polymer-modified / polymer concrete cover of proper generics, wherever necessary.

vi) Applying a protective seal coat on the entire surface to avoid any aggressive attack of chemicals.

All the above aspects are vital in the chain of various steps of any repair job, and neglecting to overlook them can spoil the repair job. Classical methods for concrete repair, like replastering, jacketing and gunning, etc., often prove unsatisfactory because these measures do not adequately take care of the above points.

It is found that in the traditional repair of concrete, the problems recur very fast, and detailed investigations in many cases have revealed that those results are basically due to two significant factors that are corrosion of steel not being totally removed and arrested effectively and bonding between new concrete and old concrete being inadequate.

The advent of chemicals for the repair of concrete

Construction chemicals for concrete repair were used initially in the form of rust-removing and passivating solutions, generally phosphatizing and complexing chemicals. With such materials, the life of repaired structures did improve, but only to a limited extent.

However, failures still occurred, and those pertained to failure in interfacial bonding. In the initial stage, bonding chemicals were in the form of epoxy resins, which, although solid, posed problems due to their two-pack operational difficulties and workmanship limitations. This led to the development of single-pack aqueous polymers mixed with cement, the striking example of which is SBR latex or PVA emulsion 2.3.

Limitations were, however, observed in the use of SBR or PVA, such as degradation due to any form of energy, particularly ultra-violet radiations, heat radiations, etc⁴. Some lower-grade polymers seem to leach out in wet conditions⁵.

It made researchers lookout for better materials, and SBR was slowly replaced by modified SBR, modified acrylates, and pure acrylates, in that order. Today, all over the world, this order is being followed, and the best possible results are obtained.

The necessity of a good repair material is further felt while replacing the spalled concrete, or in other words, making up the lost concrete cover section.

Earlier, a pure epoxy mortar was used, but besides being cost-prohibitive, its use involved the operational drawbacks mentioned earlier. This shifted the focus to single-pack water-based, cement-compatible polymers.

They are used very conveniently with good results, with a sizeable cost reduction in the overall cost of repairs. Thus, today’s scenario in the field of repairs encompasses construction chemicals, which have become inevitable worldwide. Many enlightened consultants in India have also been convinced about the profitable use of construction chemicals and incorporate them into their specifications.

In this background, the aspect of paramount importance, which is wanting, in many instances, is appropriate collaboration and interaction between structural engineers, consulting engineers, contractors, clients, and, more importantly, material scientists.

Material scientists must thoroughly apprise consulting engineers and contractors about the entire range of construction chemicals and polymer properties so that the same can be done more knowledgeably.

This paper attempts to elaborate on the suitability of various chemicals depending on their generics and their action mechanism. This discussion follows the steps mentioned earlier.

Near-total removal of corrosion on reinforcement steel for the repair of concrete

Total removal of the corroded portion from the embedded steel can not be practically achieved, although highly desirable. Hence, one can only achieve near-total removal of corrosion products.

This cannot be achieved only by mechanical means…. sandblasting exposes fresh metal surfaces more prone to corrosion and reduces the section, and wire brushing proves inadequate⁷.

Water pressure jetting is yet practiced in India on a large scale. Hence one has to resort to chemical cleaning and passivating. Neither chemical cleaning-cum-passivation nor rust-converting processes are permanent relief from corrosion.

Hence, a protective barrier film is generally applied to the treated steel. One of the options is to use epoxy liquid on the bars, which is a challenging setting that becomes almost plastic-like, resulting in a substantial loss of bond with subsequent concrete. It is reported that as much as 40 percent of bonds are lost⁶.

To stop this bond loss, fine quartz sand is sprinkled on the wet epoxy many times. In addition, this treatment being only a barrier film does not do anything to create non-corrodible conditions around the steel.

Hence, any slight physical damage to the film can reset the corrosion process. However, despite these drawbacks, epoxy treatment for bars is more effective than any other coating treatment like zinc chromate priming, etc⁷.

To overcome the shortcomings, a slurry of water-based polymer emulsions and cement is coated on steel these days. This mixture is highly alkaline and keeps the environment around steel alkaline, even though the cover subsequently gets carbonated. This situation helps significantly maintain a passive (Y) Fe₂O₃ film on the steel, thereby preventing corrosion.

Besides, this film being cement-based, it is a compatible material with concrete and does not result in any loss of bond strength making structural engineer’s work easy.

Moreover, the film is challenging and flexible, and the cement does not peel off as it would otherwise happen. In addition to being a one-pack polymer system, hardening unused material or a set of materials due to delayed use, etc., is more or less eliminated.

For reasons mentioned before, pure acrylates, modified acrylates, modified SBR, and SBR is utilized as concrete modifiers, preferably in that order.

These materials are readily available indigenously, and for proper generic selection and subsequent effective use, material scientists or polymer chemists should interact with the engineers more rigorously.

Application of a bond coat for good bonding between old and new concrete

In several cases, traditional concrete repair is executed by replastering more concrete jacketing or oven uniting. It is often seen that the new concrete/mortar mass separates from the old concrete.

This happens due to dissimilar behavior patterns of the old, already-set concrete and the subsequent new concrete undergoing stresses and strains while stiffening, mainly due to shrinkages.

To an extent, this setback is negated by using steel wire mesh. Though the wire mesh helps distribute shrinkage stresses evenly, it may add more corrosion problems. To overcome this, galvanized wire mesh can be used, which may prove costly.

Hence, years of experience have taught the specification makers to use a bond coat that ensures mechanical monolithic between old and new concrete. However, sometimes inappropriate quantity/quality of bonding coat, together with the shortcomings in workmanship, results in undesirable performance.

Liquid epoxy in tacky situations is found to be a fantastic bonding coat. Sometimes, if a vast area is to be concreted, like, jacketing of all 4 sides of the column, or when due to the negligence of workmen, the time difference between the application of the epoxy bond coat and subsequent placement of new concrete increases. It results in the epoxy being partially or fully set, and consequently, it acts as a debonding agent rather than a bonding agent.

In cases like this, separation cracks at the interface can be observed. This is not because of the failure of the material but due to the two-pack epoxy not being utilized properly. Hence, user-friendly material is necessary, and this should preferably be a one-pack system.

Bonding polymers based on polymer latexes, when used along with cement, give excellent adhesion to the old and new concrete. There is a substantial cost reduction, too and the polymer’s one-pack nature keeps the surface’s tackiness for a long time.

It also helps keep the conditions around the exposed steel and exposed concrete usually alkaline, therefore helping prevent corrosion of steel and carbonation of the adjoined concrete. Those latexes mentioned earlier for steal protection are suitable for bonding purposes too.

Rendering a robust and passive carbonation-resistant polymer-modified/polymer concrete cover

Giving a new cover to a repairable structure should be done only wherever essential, and the temptation to expose the entire surface… even if a part of it is unaffected by solid concrete, should be avoided. Judicious negation of degenerated concrete is, therefore, necessary.

If the replacement is done by unmodified concrete, it can deteriorate due to carbonation and chemical attacks, howsoever wall-made and controlled it may be.

In the beginning stages of adopting polymers to repair concrete fields, epoxy was the only reliable material for making up the lost concrete.

It is a solid material and can easily give compressive strength of 60 10 BO N/mm² and high tensile strength of 20 to 30 N/mm². More likely, epoxy mortars, under the banner of polymer mortars, are unaffected by chemical attack or carbonation. However, the following few points made engineers and material scientists ponder alternatives to this system.

i) Most of the reinforced concrete to be repaired of the strength of 15 to 20 N/mm². Hence, how right is its introduction in intermittent pockets of a greatly strengthened mortar?

ii) The cost of epoxy repair can be high, mainly if large areas are to be rehabilitated

iii) If the faulty application is one, basically due to the two-pack nature of epoxy and negligence on the part of the laborers, bonding, and integrity of mortar suffer, resulting in undesirable behavior of the concrete.

iv) Epoxy mortars are easily affected by fires or fire-prone areas, wherein the mortar catches fire readily¹. This leads to the loss of earlier repairs, and the fire risk increases.

v) If user-friendly, one-stack polymer cementitious mortar is used, then such mortar will be more compatible with the existing reinforced concrete members and have good properties like chemical resistance, carbonation resistance, etc.

vi) Preparation of cementitious polymer mortar is easy for construction workers since it is a plain cement mortar in which the polymer is to be mixed. This does not require specialized training, and hence problems of poor workmanship can be minimized.

Due to the above points, the trend is shifting in favor of polymer-modified cementitious mortars, which have improved chemical and physical properties compared to ordinary cement/concrete mortar.

The names and properties of the polymers used in these mortars are given in Table 1. The cost of these mortars is approximately 33 percent of the cost of epoxy mortar.

The polymer mortar cover is 10 to 15 mm thick in the above steel and this thickness itself can significantly protect against additional chemical attacks or following carbonation. The remaining part of the cover can be finished with well-controlled plain cement/concrete mortar to get the proper level.

Such judicious use of polymers can further reduce the cost of rehabilitation without sacrificing performance. In many countries, such mortars find significant applications in repairing bridge decks where the cost of repairs due to corrosion would otherwise be colossal.

Application of a protective seal coat on the entire surface

This treatment on the concrete surface becomes necessary to avoid future damage due to continuous environmental attacks. Repairs are generally carried out on a patchwork basis, and chemical attacks are ineffective wherever polymer mortars or polymer-modified cementitious mortars are used.

However, the seal coat application becomes vital for the entire area to protect the remaining areas from attacks and avoid subsequent repair expenses. Usually, these protective seal coats are appropriately pigmented to simultaneously take care of the protection and aesthetics of the structure.

Table 1: Properties of polymer-cement mortars

Polymer/cement ratio on a weight basis		0	0.2	0.4
Adhesion to concrete, N/mm2	Dry	0.07	2.0	3.4
	Wet	0.3	1.0	1.4
	Wet	0.0	1.4	2.1
Adhesion to steel, N/mm2	Dry	0.0	2.0	1.6
	Wet	0.0		1.3
Tensile strength, N/ mm2	Dry	3.0	6.0	4.3
	Wet	1.8		3.9
Compressive strength, N/ mm2	Dry	56	50
Flexural strength, N/ mm2	Dry	7.1		10.6
	Wet	5.8		9.6
Effect of chemicals on dry flexural strength after 6 months immersion, N/mm2
Untreated		7.2	13.2
10 percent potassium hydroxide		6.1	12.3
10 percent magnesium sulfate		4.3	13.2
5 percent lactic acid		5.9	8.0
5 percent hydrochloric acid		0	2.2
Effect of extremes of temperature, N/mm2
Untreated		7.1		10.6
After 60 freeze/thaw cycles at -18° C (in 10 percent brine)		0		10.4
After 1 year at 70° C		5.2		14.3
Adhesion to concrete (dry), N/mm2
Untreated		0.1		3.4
After 6 months at 70° C		0		2.6
Shrinkage during drying
Water/cement ratio		0.40	0.34	0.30
percent shrinkage		0.07	0.02	0.01
water vapor penetration, g/m2/24 hrs		46.9	38.1	1.9
water penetration with Revinex 29 Y 40 in mortar, kg/m2/24 hrs		100	35	0

Several coatings, such as polyurethane, epoxy, alkyds, chlorinated rubbers, and acrylic emulsions, can be used for this purpose. However, selection should be made keeping the following points in mind.

i) Adhesion to the surface

ii) Compatibility with the alkalinity of the surface

iii) Breathing capacity at the same time as the coating (coating should be impermeable enough)

iv) Resistance to various aggressive attacks

v) Expected longevity of the treatment

vi) Capacity to absorb irregularities of the surface like slight dampness or imperfect cleaning of the surface

vii) Ease in application and availability of color shades

After considering all these parameters,s and users can select appropriate material. However, for obvious reasons, the inclination these days is to use water-based coatings rather than solvent-based ones.

Conclusion and recommendations

The discussion so far brings out that using construction chemicals to repair concrete and its rehabilitation is inescapable for obtaining long-term results. Thus, the repair of concrete becomes a mixed topic to be studied by civil engineers and material scientists.

Like concrete and its aggregates, unless polymers/chemicals are equally well understood, it would be challenging to arrive at the best solution. Today, numerous monomers and polymers are available in the marketplace, and most look alike.

This necessitates a better understanding of how the co-matrix of cement and polymer behaves in the case of polymer-modified cement mortars. Understanding its behavior vis-a-vis earlier cases is essential if it is polymer mortar. Not to mention that economics is also one of the governing factors.

All over the world, considerable research and standardization are being carried out in the field of construction chemicals in general and polymer concrete/polymer-modified concrete in particular.

As repairs and rehabilitation of old structures are becoming a significant business in India, it is high time that we adopt these developments at the earliest and without reservations at all levels and without restricting their use to a select few privileged structures.

After accepting the need for polymer-modified concrete/mortars and other construction chemicals, it is to be noted that there is no mandatory or recommendatory reference for these products in any Indian code or standards.

On the contrary, a review of international codes reveals the existence of several mandatory codes like DIN (German), BS (British), JIS (Japanese), and recommendatory standards like ACI, which have amply dealt with these topics.

To exercise proper quality control from a materials point of view and its performance point of view, it is necessary to frame such standards, even if recommendatory to begin with. In the meantime, a reference in the existing codes, I.S.: 9103, I.S.: 456, or any such relevant standard, is much needed in this worthwhile exercise.

References

• LEE, H and NEVILLE, KRIS, Handbook of Epoxy Resins, McGraw – Hill Book Company
• 2) Mine H Japanese patent 124,131, January 28, 1977
• OKAUICJO A and MURVIS, Japanese patent, 487, 204, December 3, 1964.
• SOLOMATOV, V.I. Reference (21), 39 (1967)
• TAYLOR O.Z. and DRAKE R.S. Reference (13), 30-39 (1961)
• CAIMSJOHN and ABDULLAHRAMLI, ACI Materials Journal, Vol.91, No.4, pp. 331, July-August 1994.
• STEELY CARROLL N. and KELLY THOMAS EWP., Concrete surface Preparation Coating and Lining, and Inspection, National Association of Corrosion Engineers, Houston 1991.
• BRETT, M.A. and BRETT, A. O., Electrochemistry Methods and Applications, Oxford University Press, 1993 pp. 361.

Abbreviations –

1. S. B. R. –  Styrene Butadlene Rubber.
2. P. V. A. – Polyvinyl Acetate.

By Dr. S. K. Manjrekar, Ishita Manjrekar, Sourabh Manjrekar