Grouting is one of the age old tools which is becoming increasingly useful with the progress in material science. A large spectrum of new materials – organic polymers as well as inorganic chemicals – are being used in structural and non – structural repair of concrete structures. An attempt has been made in this paper to discuss the properties and types of grouts, the application techniques and economics in relation to the selection of appropriate repair material.

Grouting is one of the oldest techniques used in repair and rehabilitation of civil engineering structures. The technique has been refined over years on account of the availability of newer and better materials on one hand, and advancement in the equipment used for grouting on the other.

In the initial stages, cement-based grouts were used. However, their limitations were soon realized. Newer and newer materials and material combinations were tried. With a large spectrum of new materials available today, it is now possible to achieve better results of grouting. However, it is important to choose a right type of grout material. The following factors play an important role in the right choice of grout materials.


Workability:

While ease and safety in pouring and handling are of paramount importance, the grout should preferably get cured under ambient conditions. It should not decompose on atmospheric exposure and have a bad odour.


Wetting capacity:

The grout must spread freely in the confines and evenly bridge the surfaces. Anionic, cationic or non-ionic surface active agents are used to enhance the wetting capacity. The non-ionic variety is generally preferred.


Dimensional stability:

After consolidation, when the grout solidifies in the contours of the enclosure, it should not undergo any dimensional changes which, if any should be negligible. In cement-based and solvent-based polymers (like epoxies) some shrinkage does occur. To offset such shrinkage, cementitious grouts are modified by addition of expansion-causing admixture. Such “gas-forming” admixtures many times lead to corrosion due to “hydrogen embrittlement” or stress corrosion. (This is, however, not true for grouts based on expansive cement). In case of polymeric grouts, it is advisable to use already-polymerised water-based grouts, rather than cross-linkable monomers. In the case of former, volume changes are negligible.


Adhesion:

Good adhesion to surfaces is an essential requirement of a grout. Generally, solvent-based grouts are poor in adhesion to wet surfaces, while water-based emulsion-cement comatrix grouts exhibit good performance in such adhesion. In addition, mechanical properties such as tension and flexure are better in the latter type.


Strength:

The grout must have sufficient compressive strength, but not in excess of that of the adjacent concrete material, in case of structural concrete repairs. It can be less, in case of permeability-reducing or “damp-proofing” operations. However, in a particular case of anchor bolt fastening, load aspects like deadweight stress for machinery, normal allowance for dynamic load, suggested safety factor and additional heat factor together demand a higher compressive strength requirement. In addition to compressive strength, flexural strength of the grout is also important.


High elastic modulus:

This modulus by definition is the ratio of stress over strain and hence higher elastic modulus indicates less strain for any given stress. It is reported that 103N/mm2 is a good target for minimum acceptable elastic modulus at elevated temperatures. For comparison, it may be noted that most plastics have elastic moduli in the range 1 to 7 x 103N/mm2. In the present works, this property of high elastic modulus was of importance and were chosen.


Chemical resistance:

Many times, the internal matrix of concrete gets weakened and eroded due to attack of chemicals such as compounds containing sulphates and chlorides. Sometimes, it can be other aggressive chemicals too. Thus, the grout chosen should be tailor-made so as to combat the existing aggressive conditions or else it should be generally of an inert nature. Polymer grouts are helpful in such situations due to their resistance to chemicals.


Long-term integrity:

Some grout materials may get decomposed with passage of time resulting into a change in mechanical properties. These changes are not advisable for its successful service life. Polymer materials particularly like epoxies are known to change their intra-molecular arrangement with temperature changes and are susceptible to catch fire ate moderately higher temperatures. Comparatively, polymer latex and cement-based comatrix grouts are more useful in such eventualities. Due to the presence of cement the comatrix is not readily susceptible to temperature changes and the simultaneous presence of polymer offers the desired flexural properties to the matrix.


Resistance to corrosion:

Sometimes grouting is to be done in areas where congested steel reinforcement is present. In such a case or any other case where steel is around, the aspect of corrosion also has to be taken into serious consideration. For example, gas-forming expansive grouts can give rise to corrosion due to “hydrogen embrittlement”. Although pure polymers are quite satisfactory in these aspects, some water-based polymer latexes can create problems due to the nature of emulsifying agents used. Emulsification of SBR latexes are particularly known to be done with anionic surface-active agents and due to the anionic character these polymer latexes can be less corrosion resistant as compared to the ones which have non-ionic surface-active agents such as acrylate copolymer latexes. In addition, a point may be noted that SBR polymers are less stable to various forms of energy than acrylate copolymer latexes which do not decompose easily.


Economics:


Finally, cost implications along with cost-benefit ratio are also important factors in any civil engineering operation. While selecting an appropriate material, the attempt should always be to offer little more than the requirement at the most competitive price. This necessitates thorough study of the problem, its demands and appropriate solutions based on the sound study of material science. Thus, material scientist and civil engineer have to coordinate with each other so that the best use of appropriate materials can be made. For example, the structural requirements may demand high compression and flexural properties in a grout and an apparent obvious choice would be an epoxy grout. But, this otherwise excellent grout, may prove ineffective, if there exists leakages, dampness in structures to be repaired. This is because the grout may not show good adhesion or during the passage, setting of resin may take place owing to an obstacle such as a piece of aggregate and further cavity may not be filled at all. Further, if the temperature is high then the grout may get decomposed. This indicates that possibly use of acrylate based polymer latex modified cement grout would be more useful as it will be workable in wet conditions without loss of adhesion and being a one-pack system it can flow smoothly throughout the passage of the cavity. Being cement based it would not appreciably decompose with temperature variations and most importantly if would give the desired compression and flexural strengths. Thus, proper considerations on the basis of developments in material science can offer better performance. In addition, cost of acrylate-modified cement grout would be considerably lower than epoxy or polyurethane polymers.