GROUTS & GROUTING

1.0 : GROUTS & GROUTING
SCOPE
This booklet covers the fundamental concepts and theories that apply to precision grouting in the construction industry. It is instructive, informative and represents an important benchmark for specifies and users.

1.1: Definition of a grout
“A fluid material which is designed to be introduced into a cavity for the purpose of filling it and which will subsequently harden to give specific physical properties.”

1.2: Function of a grout
The function of a grout is to fill cavities. It is essential that the grout fills the cavity (eg. A gap between a base plate and substrate) completely and permanently.

The shape, size and situation of the cavity or cavities will dictate:
The method of introducing the grout.
Its fluid characteristics (such as its ability to be poured, pumped or injected)

The reasons for filling the cavity or cavities will dictate the characteristics required of the hardened grout, such as: To provide or restore continuity of strength or transfer of load across the cavity (eg. Compressive strength, resistance to impact and shear.)
To provide a seal against either water ingress or passage into or across the cavity (eg. Impermeability to water).

1.3 Table 1: Examples of grouting needs
The table below is designed to be read from left to right. It describes what are the essential properties that grouts need to exhibit when applied in a particular way to fulfill a function in different situations to fill various needs.

Non-shrink and durable properties are essential to all the cases described below.

When the method of application is using an injection system, then it is probable that some pressure of pumping would be necessary.

2.0 PRINCIPAL GROUT TYPES
2.1 Cement and sand-cement grouts
The simplest grout used in the construction industry is a mixture of cement and water. It is frequently used where relatively small and narrow cavities need to be filled and where no load transfer is required. It is also used in soil stabilization.

A note of caution!
There are however some disadvantages that are encountered with the use of unmodified sit batched cementitious grouts:

-› High shrinkage factors, which worsen with excess water.
-› Poor flow characteristics, which are only improved by the addition of
   water leading to bleed and segregation.
-› Unreliable strengths, which are variable with, repeat mixes.
-› Variable performance due to site labour and variable quality of raw
   material.

There are several alternatives available in the market place that can be employed in order to overcome the basic problems and restrictions mentioned above.

-› These alternatives can be classified into three categories:
-› Non-shrink grout admixtures
-› Cement based non-shrink grouts
-› Epoxy resin based grouts

2.2 Non-shrink grout admixtures
When a plasticising admixture with expansive properties is added to a site-batched mixture of sand/cement and water, higher mobility of flow characteristics will be obtained using less water.

The expanding action, normally imparted by an aluminium flake content (generating discrete
hydrogen bubbles by reacting with cement alkali), will compensate for the plastic or settlement shrinkage-which is already reduced by lowering the water demand.

-› To summarise, there are three main advantages derived from this

    system:
-› Improved flow and cohesion characterisics-plasticising action.
-› Less water in mix-plasticiser reduces the need for the original water

   content.
-› Non-shrink-expansion aid compensates for natural volume change of

   cement.

There are two limitations:
-› Variable performance of the grout because of site batching.
-› Vairability of the raw materials.

2.3 Pre-packaged cement grouts
Pre-packaged cement grouts are factory made materials utilising accurately measured amounts of controlled constituents. Such grouts give precise performance characteristics, both in the fluid and hardened states.

Pre-packaged cement grouts may incorporate plasticisers to facilitate pouring and pumping; expansive agents to ensure non-shrink characteristics and the aggregate content may be varied to meet a range of cavity sizes and needs.

The aggregate quantity, quality and grading is carefully controlled.

Grouts may also incorporate accelerators to improve early strength gains.

-› The advantages of using such a system are:
-› Precise strength, flow and shrinkage characteristics.
-› Consistent performance from factory controlled product.
-› Pre-packaged. Convenient to use. Only water addition required.
-› Ability to eliminate segregation and bleed.

The limitations of using pre-packaged grout systems are:
-› Precise water addition requirement which is a site labour dependency but

   also necessary under site batching conditions.
-› On very large jobs, special mixing equipment is necessary.
-› Cost will be higher than site batched grout.

2.4 Resin based grouts
Resin based grouts are invariably factory-made materials utilising either epoxy or polyester resins.

2.4.1 Epoxy resins
Epoxy resin based grouts form the principal materials for cavity filling purposes. They consist of a base resin and a hardner which, when mixed together, react chemically to create strong and durable materials. They have high impact and chemical resistance and are adhesives in their own right.

2.4.2 Epoxy resin based grouts
Resin based grouts are usually supplied in a 3-pack from consisting of:
Resin ; Hardner ; Aggregate (filler).

It is essential that all the resin and hardner supplied are thoroughly mixed together. Otherwise the chemical reaction may not be complete and the resulting material will not develop the expected properties.

The resin/hardner ratio is critical and accurate factory measuring and packing is essential. The basic materials are therefore supplied in pre-packaged form. Selected inert aggregates may be added to reduce exotherm and cost.

The advantages in using the system are:
(i) High strengths and impact resistance is achieved capable of withstanding high and dynamic loading.
(ii) Adhesives create bond between cavity surfaces.
(iii) Non-shrink.
(iv) Excellent flow characteristics-with a wide range of cavity sizes possible.
(v) Pre-packaged and ready to use when mixed.
(vi) High chemical resistance in most circumstances.
(vii) Rapid strength gain which reduces equipment down time.

The limitations in using the system are:
(i) Higher costs when compared with cementitious grouts.
(ii) Temperature sensitive conditions must be suitable.
(iii) Possible wastage total pack must be utilized.
(iv) Mechanical mixing equipment required.
(v) Risk of high exotherm in large mass sections.
(vi) Greater operator diligence in mixing.

2.4.3 Temperature limitation
A minimum temperature of 5⁰C is necessary for the curing process to commence. Once it begins it will speed up or slow down with subsequent increases or decreases in temperature and stop completely if temperature falls below 5⁰C.

The process will only re-start with a rise above this figure but may not reach the same strength as grout cured continuously at 5⁰C.

2.5 Polyester resins
Polyester resins are generally faster reacting and more tolerant of temperature and water presence than epoxies. They do, however, shrink during curing and hardening. For this reason their use is confined principally to highly filled mortar applications, such as anchoring and small
area repairs.The FAIRSCREED range of repair compounds and FAIRGROUT range of chemical anchor resins, are based on polyester chemistry.

2.6 Comment
Pre-packaged cementitious and resin based grouts are necessarily of higher costs than site mixed materials on a volume for volume basis.

The reduction in potential failures of grouted structures using a pre-packaged product will always outweigh the costs of potential down time for remedial work due to grout failure of a site mixed material.

3.0 MECHANISM OF CEMENTITIOUS GROUTS

3.1 Grout Mix
A grout mix may contain one or more of the following ingredients combined with water:

-› CEMENT
-› PLASTICISER
-› EXPANDING AGENT
-› FILLER

The interaction between each of the ingredients gives the flow and mechanical properties of the fluid and hardened grout. By examining the properties of each ingredient an indication can be obtained of their effect in use and it is also easier to draw comparisons between different grouts and grout products.

3.2 Types of cement
Ordinary Portland Cements (OPC) are normally employed in grouts. Other types of cement, such as Sulphate Resistant Cements may also be used.

3.3 Bleed & segregation
It must be noted that bleed and segregation are problems that commonly arise when excess water is added to site batched grouts in order to achieve the required flow.

Factors that affect bleed and segregation are:

-› Water/Cement ratio
-› Aggregate grading
-› Gaseous expansion systems

These factors are accounted for in the formulation of pre-packaged cementitious grouts.

3.4 Graph 2: Flow vs Water/Cement ratio relationship and the effect of the addition of a plasticiser

Graph 2 shows that a reduction in W/C ratio reduces the ability of a non-plasticised grout to flow (B to A).

Water Cement Ratio [Graph 1].

Clearly the requirement of high strength and flowability are directly opposing. Unless the amount of flow shown by measurement A (Section 3.5 Graph 2) is sufficient to do a specific job of work, then there is no alternative except to add more water until the grout is fluid enough. This means accepting a lower strength in the hardened material.

3.5 Graph 2: Flow Vs Water/Cement ratio relationship and the effect of the addition of a plasticiser
 

Graph 2 shows that a reduction in W/C ratio reduces the ability of a non -plasticised grout to flow (B to A).

Water Cement Ratio Graph 2

In order to get sufficient flow with site batched cement/water or sand/cement/water grouts containing no plasticiser, the site operative adds more water and thereby reduces the eventual strength of the grout.

The use of suitable plasticiser enables these opposing factors to be overcome. It works in exactly the same way as any effective concrete plasticiser: by aiding dispersion of the cement particles and enabling the water to work more efficiently.

Less water is therefore needed to create the same workability or mobility of the grout.

From Graphs 1 and 2, it can be seen that by using a plasticiser, both optimum grout flow and optimum strength can be achieved. This is because the water necessary to give the high flow measurement B reduces from W/C2 to W/C1. In other words, a plasticiser makes it possible to produce a grout, which has good flowing properties and a high-hardened strength.

3.6 Expansion
The usual purpose of using a grout is to completely fill a cavity. For example, effective grouting under a machine base plate should result in permanent contact of the hardened grout with the underside of the base plate. Loads on the base plate will then be evenly transferred to the grout and the grout will provide support.

However, certain features of simple cement systems make it difficult to achieve complete and permanent contact.

When in the plastic state, simple sand cement grouts can be prone to problems of bleed/segregation and plastic shrinkage. The former can result in a layer of bleed water forming on the grout surface. When this water has evaporated a void is formed between the hardened grout surface and the underside of the base plate preventing effective load transfer. Plastic shrinkage leads to the formation of cracks in the grout, which is again detrimental.

As the grout hardens beyond the plastic state, drying shrinkage occurs as a gradual process over a longer time period. The fundamental cause is that all cementitious materials shrink in volume as they dry. This longer-term shrinkage can also lead to loss of contact with the underside of the base plate.

Expansion or shrinkage compensation additives are used to overcome both plastic shrinkage and drying shrinkage. Other features of the mix design, such as aggregate grading, are also critical in overcoming bleed/ segregation and other problems.

3.6.1 Expansion plastic state
The mechanisms of bleed/segregation and plastic shrinkage are related and it may be difficult to differentiate between them if problems occur while the grout is in the plastic state. Both problems can result in a loss of contact, either partial or total, with one or more of the surfaces of the void with which the grout should be making contact.

An expansion aid is used to help counteract these effects. This effectively helps to fill the void completely and maintains pressure between the grout and the walls of the cavity until the grout has hardened.

A commonly used expansion aid is finely divided aluminium flake. The reaction between aluminium and the alkaline pore water released from cement particles generates hydrogen gas and hence the expansive force. This reaction can take place relatively quickly and the expansive process can be complete before the grout is fully placed. It is therefore important that the rate of gas generation is controlled so that expansion occurs gradually. The expansive force must be exerted while the grout remains fluid and continues upto the point at which it has formed sufficient structure (at about initial set).

The selection of the type and grade of aluminium is important. Only aluminium flakes with the right degree of fineness and specially coated to slow down the reaction, will give the appropriate rate of expansion.

In some countries, notably France, it is maintained that using hydrogen as the expansion medium for grouts can lead to hydrogen embrittlement of high tensile stressed cables and other elements. The use of aluminium in cable duct grouting has therefore been vetoed in those countries.

In other countries, such as Canada, grout additives containing aluminium have been used since the 1960s for cable duct grouting with no apparent problems of hydrogen embrittlement to date.

There is now a general trend towards the elimination of base metal additives such as aluminium or iron (see later) from all cementitious grouts. One alternative way of producing expansion is to use active charcoal/carbon. Due to its surface chemistry the charcoal particles will adsorb gases from the atmosphere onto their surfaces. When mixed with water it is claimed that these gases are released to provide expansion.

Another method is to incorporate chemicals, which produce nitrogen when the grout is mixed with water. The advantage of this more advanced system is that the nitrogen is inert and could not lead to problems of hydrogen embrittlement. Furthermore the rate of reaction can be controlled so that the expansion occurs gradually until the grout reaches initial set.


3.6.2 Expansion hardened state.
In the hardened state it is necessary to generate some form of internal stressing to compensate for drying shrinkage stresses.

One method is to incorporate small particles of iron in the grout. Moisture in the hardened grout eventually causes oxidation of the iron. This creation of rust results in a solid stage expansion. However further oxidation also causes unsightly surface and then continue progressively through the grout. Grouts with a ferrous metal content are therefore undesirable.

Alternative additives in cement grouts are expansive cements, which are based on calcium sulphoaluminate. On reacting with water, ettringite (calcium sulphoaluminate hydrate) is formed as long needle shaped crystals. It is thought that the internal stress created by the formation of these ettringite crystals counteracts the forces generated by drying shrinkage.

3.6.3 Note of caution
Section 3.6.1 and 3.6.2 have described various additives for cement grout systems. It should be noted that unless available as a pre-blended proprietary grout additive (for example FAIRADD) it is strongly advised that these materials are not used as additives for site batched sand/cement grouts.

Many of the materials discussed above are used in complex formulated pre-packaged grout products where additive levels are rigorously controlled and the product has been taken through a carefully planned quality control test procedure. Incorrect addition levels of these materials can lead to, for example, excessive expansion and subsequent disintegration of the grout with disastrous results.

3.6.3 Free expansion.
It is important to understand that if a grout, which contains a gaseous expansion system, is allowed to expand freely with no restraint there will be a loss in compressive strength of the hardened material of approximately 6 % for every 1% of volumetric expansion.

The grouting process should ensure that a cavity is filled and the surfaces of the cavity will prevent the grout from expanding further. This restrains the grout so that the strength is maintained.

In practice, for example when pouring a grout under a base plate, a proportion of the upper grout surface will remain exposed and will therefore not be restrained by the base plate. For practical considerations this is unavoidable but the areas of exposed grout should obviously be kept to a minimum.

When carrying out laboratory testing, such as the measurement of compressive strengths, it is essential that the test cubes be fully restrained to prevent the expansion from occurring. Measurement on unrestrained cubes can give an indication of lower strengths than would be obtained in practical grouting jobs.

The question of restraint when related to hardened state expansion systems is more complex. The efficiency of the expansion system in controlling cracking, which results from drying shrinkage, can depend on the degree and type of restraint provided and will not be discussed in further detail.

3.7 Filler
Fillers, in the form of graded sands, are included in the formulation of the pre-bagged cementitious grouts in order to enhance flow characteristics and reduce shrinkage and costs.

3.8 Graph 3: Effect of voids on Strength of Grouts
The Grout strength is shown as a relative strength.

Example:
If the 100% strength is say 60 N/mm², then a 5 % void content will result in a drop sto 42 N/mm² (extrapolated at 70% of strength).

Additional aggregate may be required to fill out the prebagged grout for use in large cavities in order to minimise postential exotherm.

To minimise the risks of Alkali Silica Reaction, non-reactive aggregates should be used, such as crushed limestones.

3.8 Summary
By understandingthe function and relative importance of these five possible ingredients it becomes easier to make comparisons between alternative products or mixes and in particular, to compare FAIRMATE precision grouts with competition and make the proper choice of product based on sound technical judgements.

Loss of potential orders have undoubtedly occurred because grout samples have been tested incorrectly.

4.0 : MECHANISM OF EPOXY GROUTS

4.1 Epoxy grout
An epoxy based grout mix contains:

-› EPOXY RESIN BASE
-› HARDNER
-› FILLER

4.2 Resin viscosity
The inherent flowing properties of an epoxy grout depend upon viscosity and this in turn is very much affected by temperature. A resin which is fully mobile at a temperature of 20⁰C may
become very slow pourig and slow flowing at 5⁰C.

Such a “standard temeprature response” resin would present serious problems for grouting where low temperatures occur.

To deal with this practical problem of temperature range a “low viscosity/temperature response” is necessary. This can be attempted by modifying the resin phase of a standard resin but this will
also lower the mechanical properties of the hardened grout which is undesirable.

Alternatively, special”low viscosity/temperature response” resins are obtainable and the ideal
grout, based on these, will maintain full mechanical properties and gibe good low temperature
flow characteristics.

4.3 Graph 4: Viscosity/Temperature relationship of epoxy resins
Graph 4 illustrates two response curves. A standard viscosity/ temperature response resin
curve (A) and a low viscosity/ temperature response resin curve (B).

The viscosity increase of these special resins at 5⁰C. is limited to 50 % of the value at 25% of the
value at 25⁰C. (ie.there is an increase from 4 to 6).

A “standard temperature response” resin increases in viscosity by 600 % over the same temperature range. (Ie. There is an increase from 4 to 24).

4.4 Resin Curing
All epoxy resin systems depend upon mixing a predetermined quantity of hardener with the base resin in order that the essential chemical reaction will occur. A temperature of 5⁰C or higher must prevail for continued chemical reaction.

During the curing cycle, the resin gradually increases in viscosity changing from a liquid to a 'grease-like' consistency and finally to a solid. The liquid resin has a chain-like structure with active centres on the chain. When the hardener is added it links across the active centres joining one chain to another to form a three-dimensional network and thus changing the resin into a solid.

Dependent upon temperature, this process produces a solid material within a few hours and approximately 90% of its ultimate strength after 24 hours @ 20⁰C.

Strength gain does in fact continue for some seven days after placing.

An epoxy resin grout is normally considered to be fully cured after 7 days at 20⁰C and for this reason strength and other characteristics are usually measured at that age and temperature.

4.5 Function of a hardner
Apart from its obvious function, the hardener component in an epoxy resin system has an important effect on the characteristics of the resulting grout.
The hardener system controls the extent to which the resin will tolerate water or wet surfaces in its liquid state and its resistance to various chemicals or to creep in the hardened state.

The hardener is therefore a vital component and the ratio in which it is used with the resin base must be exact. For that reason it is essential that all the hardener supplied in a pre-packed system is mixed with all the resin.

No attempts to use part packs should ever be made, otherwise failures will occur (even if an apparently hard product is formed).

The chemical reaction generates exothermic heat during the early period after mixing. In naturally high temperature conditions (such as the Middle East) pre-cooling of the components should be carried out to reduce this effect. Materials should, for example, be stored in air-conditioned areas (or refrigerators) and only brought out immediately before mixing.

In hot climates the use of epoxy resin grouts should be confined to the coolest time, even if this means working at night. The work place should be protected from direct sunlight, if necessary, by temporary shading.

Conversely, at low ambient temperatures, materials should be maintained in a warm (15 - 25⁰C) store to facilitate mixing and ensure good initiation of reaction.


4.6 Filler
Inert fillers are incorporated in the mix in order to reduce the cost and heat output, or give particular characteristics where large cavities are to be filled.

Fillers of the correct type, grading and strength form part of pre-packaged resin grout systems.

The resin base and hardener must always be mixed together first and the filler only added to the thoroughly mixed liquid resin components.