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.
