Selecting the correct grout

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21 August 2019 - 11:57, by , in Grouting, Comments off

A grout is a stable material which, after having been applied in a dry-pack, plastic, flowable or fluid state, will set to fill a void. After setting, the grout provides permanent, strong and total infilling of the original void to the required performance criteria.

 

What is important when choosing a grout?

Before we consider the technical properties and the installation of grouts we need to be mindful of the particular project we are working on. It is important to ensure we don’t under specify a product but also allow for the use of lesser performing products in less critical applications to provide a more cost effective installation. For example, while the grout used to install a multi-million dollar gas turbine needs to perform a similar function as a grout used under a tiltslab wall panel – the cost difference between a high performance grout and a lesser performing grout is insignificant compared to the cost of shut-down and replacement of the grout if the grout under the turbine fails to perform.

Grout manufacturers like Parchem have broad ranges of grouting products to enable specifiers to select the correct grout for their particular application. There is no one grout that is appropriate for every application – so what is important to each application?

 

Effective Bearing Area (EBA)

The aim of most grouting applications is to evenly spread the weight and transfer the load of a machine being installed over the full area of its baseplate on to the foundation below. Achieving 100% Effective Bearing Area (EBA) is ideal but a figure which is rarely met. Although not covered in any Australian Standard, ASTM C1339-02 provides a guide to EBA where after a polymer (resin) grout is flowed under a plate and the grout hardens, the plate is removed and the grout surface wire brushed to expose any surface air bubbles or voids. A visual estimate is then made of the percentage of the grout top surface that is in contact with the plate. As part of the subsequent report the percentage bearing area is expressed as:

High (greater than 85%); Medium (70 to 85%) and Low (less than 70%)

Visual guide of 85% EBA

 

grouting of a bearing pad where only around 30% EBA was achieved with a cement based grout – fortunately the issues were investigated and rectified for the subsequent pours.

 

Shrinkage

In our definition at the beginning of this document, we mentioned “total infilling of the original void”. If the grout we install under a base plate shrinks significantly, it is possible that the grout could shrink away from the underside of the baseplate and therefore would not be supporting the baseplate at all (in the worst case scenario). Shrinkage can also lead to cracking in the grout which can be detrimental to the installation.

Looking first at cement based grouts, these materials are essentially fine concretes – cement, aggregate and admixtures in a bag – to which we add water on site. Concrete shrinks – so does a cement based grout. What we need to do is minimise or compensate for this shrinkage, hence the term “Shrinkage Compensation”. Rather than trying to totally eliminate shrinkage, we can limit it to a point where it becomes insignificant to the final result on site. Two types of shrinkage exist in cement based grouts – shrinkage in the “plastic” phase and “long term drying shrinkage” in the hardened phase of the grout. Several processes on site as well as admixtures within the material can contribute to minimising shrinkage in cement based grouts. They include:

Moisture loss during application – losing water from the mix during and after application will result in a volume change – shrinkage in the plastic phase. This can be limited by restricting unrestrained (open) surfaces to stop evaporation and by pre-soaking the concrete foundation to stop water in the grout being absorbed into dry concrete. Subsequent moisture loss during the curing of cement based grouts is also a problem and should be minimised with the use of curing compounds or wet hessian/burlap onto exposed surfaces once the grout has achieved initial set, then onto other surfaces once forms are stripped.

Gas generating admixtures – chemicals are formulated into the grout which react when the grout is mixed to produce small bubbles of gas (hydrogen or nitrogen) which essentially make the grout expand around 2% while it is still unset (plastic). This again is shrinkage compensation in the plastic phase. The gas does not affect the strength of the grout and generally occurs during the first 30 minutes after mixing/ placing. These types of grout were previously known as Class A grouts (SAA Misc Pub MP20,Part3-1977). Some markets (underground mining in particular) are sensitive about hydrogen production in grouts due to the aluminium powder used in the grout to generate the gas. There is also a belief that hydrogen embrittlement may occur when a hydrogen producing grout is used near high tensile steel – using a nitrogen producing grout negates this.

Expansive cements – grouts in the higher performance spectrum will generally contain some amount of expansive cements which help compensate for shrinkage after the grout has set and initially hardened. This technology is normally aligned with gas producing expansion technology to produce what is known as “Dual Shrinkage Compensated” grouts. These grouts were previously known as Class A/C grouts (Class C if they only expanded in the set/hardened phase).

Shrinkage in resin based grouts – depending on the type of resin being used, shrinkage can be significant in resin based grouts. Polyester based formulations are quick to set but can exhibit up to 8% shrinkage which makes the technology unsuitable for most grouting applications. Well-formulated epoxy resin based grouts will only exhibit minimal shrinkage (around 1%), – but this could still be enough to create potential problems when used in large volumes and deep sections. Epoxy resin based grouts are generally formulated to be used within a range of gap sizes to ensure shrinkage is controlled. One of the features of epoxy based grouts is their ability to bond to the concrete foundation and the base plate. During the initial cure time of the grout when shrinkage is likely, the grout is likely to be bonded to the base plate and this will reduce the possibility of the grout shrinking away or even cracking under the base plate. Any tension built up at the time should “relax” out over time due to the plastic nature of epoxies. Experience has shown that even when shrinkage cracks occur in the unrestrained grout around the perimeter of a base plate grout, the cracks rarely continue for more than 20mm or 30mm under the baseplate.

 

The image on the left shows a significant crack in an epoxy grout which was initially a concern. However, investigation revealed the crack was only in the perimeter grout and didn’t extend under the baseplate. The crack was later sealed with a low viscosity epoxy mix to stop water infiltration and for appearance’s sake.

 

 

 

 

 

Strength

Grout strength is tested according to the Australian Standard AS1478-2005 and the figures quoted by manufacturers are established under controlled conditions which may differ from the results often reported for site prepared samples. There are several types of strength which relate to grouting which are described below in simple terms;

Compressive strength – the ability of the grout to resist the weight of the machine or item being placed on it; typically a static downward force. Most grouts these days will exceed 50MPa at 28 days, some reaching over 100MPa. A grout should achieve well above the compressive strength of the concrete is it placed on but there seems little advantage in placing 100MPa grout on 40MPa concrete foundations other than possibly the associated increase in tensile strength of the grout (discussed below). What can be more important is how quickly a grout gains its strength. Compressive strength development time is an important property particularly when turn-around times are critical to the project (for example in the case of machinery replacement). High Early Strength (HES) grouts can achieve over 30MPa in just 2 hours, which may be sufficient to allow a machine to go into service subject to the supervising Engineer’s approval. However, HES type grouts may only achieve a 28 day strength of around 50MPa. High ultimate strength grouts, 90MPa plus, can be a compromise by achieving reasonable high early strength, perhaps 30MPa in 24 hours, while still achieving over 65MPa in 7 days.

 

Compressive strength of cement based grouts should be tested in accordance with AS1478.2- 2005 (supersedes AS2073-1977) using maximum 75mm restrained metal cubes.

Tensile strength – the ability of a grout to hold together – not to break apart. One property is measured as Indirect Tensile Strength according to AS 1012.10 -2000. Depending on the application a grout may need higher tensile strength if the installation is subject to significant vibration or dynamic loads (as opposed to a static downward load). Such an application may be a drop forge or crane rails where there is a force cycling up and down or sideways forces being exerted.

 

 

Whilst cement based grout can have high compressive strength, they will by nature have relatively lower tensile strength compared to epoxy resin based grouts which may be more suitable in these applications. The table below gives some comparative strengths representative of products in the market:

 

Tensile bond strength of epoxy grouts is also much higher than that of cement based grouts – the epoxy grout can virtually glue the baseplate to the concrete foundation (making any future removal of a machine much more difficult).

Strength in cement based grouts is achieved essentially by using high cement content (typically >50%) and mixing at low water to cement ratios. Varying the water content per mixed bag of grout is a useful feature of cement based grouts as it allows different consistencies of the grout to be achieved without significantly affecting the strength of the finished product (providing this is done within the scope recommended by the manufacturer).

Strength in resin/epoxy grouts is achieved by laboratory formulation of the raw materials – something which must be accurately carried through when using on site by ensuring products are mixed in their correct proportions. Epoxy grouts which are poorly mixed or mixed out of ratio, will not cure to their full potential strength and will often have a sticky surface. Although adding extra hardener may, up to a point make the epoxy set faster, the final cured product will be more brittle, reducing the dynamic load capability and because they are mixed our of ratio, the grout surface will remain tacky.

 

Workability

Different situations will require different methods of application of the grout. Some situations such as grouting under a wall panel or small base plate for example, will favour installation by dry packing or trowelling in the grout. Larger baseplates typically will best be grouted using a flowable consistency grout and formwork. The workability or consistency of the grout needs to match the requirements of the application.

Most cement based grouts have the ability to be used as dry pack (very stiff mortar) through to flowable consistency. The difference in consistency is achieved simply by varying the amount of water which is mixed with the dry powder. There will be an upper and lower limit to the amount of water which can be added to achieve the stated properties of the product and within each product the more flowable consistency will have a slightly lower strength than the stiffer mortar consistency – a result of the water to cement ratio changing.

Some of the higher end grouts will be more specific in their application and therefore more specific in their consistency. High Flow grouts are designed for use as flowable or fluid materials rather than trowellable mortars where they will tend to slump due to the plasticisers in the formulation.

The workability/consistency of cement based grouts should be measured in accordance with AS1478.2. There are three different tests depending on the target consistency required.

  • The test for stiff mortar types uses a mechanical drop plate test to measure spread and slump, similar to a slump test for concrete. This test is referenced to ASTM C230.
  • The test for a flowable grout uses a flow trough which measures the ability of a grout to flow a distance. This is a more relevant test for flowing grouts under baseplates.
  • The test for a fluid grout uses a flow cone and measures the time taken for a specific quantity of grout to pass through the flow cone orifice. This is the test for maximum fluidity and only the highest performing grouts will pass this test without bleed water appearing.

Further information on these tests and interpreting the results is given in Section 4.

Retention of flow characteristics is sometimes overlooked but is often critical to achieving a successful installation. In the real world problems occur: pumps stop, breaks occur in the pouring process, etc. If a grout has good flow retention then the grout should be able to re-establish a flow once the problem is solved within a reasonable time. Grouts with poor flow retention will tend to gel if the flow is not continuous, and re-establishing flow of the grout will be difficult if not impossible.

On critical large grout pours, contingencies should be established prior to the work commencing in order to have a “plan B” if the grout stops flowing under a base plate for example. Strapping is a common problem solver; when the grout stops flowing, metal strapping is pushed into the grout and slowly worked through the grout to help re-establish flow. This can be a simple yet effective solution. Under no circumstances should vibrators be used to make the grout flow; these will “shock” the water out of the grout like bleed water from concrete and this water will accumulate below the baseplate, seriously compromising the effective bearing area.

 

Other things to consider

Chemical Resistance

Around 50% of the formulation of most cement based grouts is normal GP Portland cement, which is not resistant to common acids often encountered in processing plants. As such, cement based grouts are generally not suitable for grouting in these environments and epoxy based grouts are typically better choices where chemical resistance is required.

Cement based grouts in industrial applications may be over coated with chemical resistant epoxy coatings to protect the grout from chemical attack.

Temperature Resistance

Epoxy resin based grouts are generally not resistant to long term exposure to temperatures above 70°C unless they are specifically formulated for such an environment. Cement based grouts can typically handle much higher temperatures (up to 250°C), providing they have been properly applied and allowed to fully cure before exposure.

 

Specifying the grout to be used

It is important that any requirements for a particular grout are clearly set out in the project specification. Too broad a specification will allow inferior materials to be used and too narrow a specification can be time consuming for a contractor to track down the right product, unless it is nominated by brand and product name.

For example, a typical performance based specification for large equipment baseplates which have predominantly compressive loads would be:

Grouting Requirements

  1. To the nominated area(s) (specify details and areas of application), grouting must be carried out using a pre-packaged, nonmetallic and chloride free, dry powder blend of cements, graded fillers and chemical additives proprietary grout.
  2. It is to be mixed with clean potable water to the required consistency. The plastic grout must not bleed or segregate. The storage, handling and placement of the grout must be in strict accordance with the manufacturer’s instructions.
  3. A positive volumetric expansion up to 3% shall occur while the grout is plastic by means of a gaseous, hydrogen free system. Additionally the grout is to be formulated to compensate for longer term expansion in the hardened state.
  4. It shall exhibit Flow Characteristics when tested to AS 1478.2.2005 of 10 – 30 seconds using the flow cone procedure.
  5. It shall be formulated to exceed the following characteristics when mixed with 3.7 litres water to one 20kg bag of the specified grout, at fluid consistency, when tested @ 23°C:

 

Compressive Strength (AS 1478.2.2005)
1 day – 30MPa
7 days – 45MPa
28 days – 60MPa

Indirect Tensile Strength (AS 1010.10 – 2000)
1 day – 2.5Mpa
7 days – 4.5MPa

Modulus of Rupture (Flexural Strength) (AS 1012.11 – 2000)
1 day – 3.0MPa
7 days – 9.0MPa
28 days – 10MPa

Conbextra® HF manufactured by Parchem Construction Supplies meets the performance criteria and is approved for this application.

 

A typical performance based specification for less critical grout under small structural steel columns would be:

Grouting Requirements

  1. To the nominated area(s) (specify details and areas of application), grouting must be carried out using a pre-packaged, nonmetallic and chloride free, dry powder blend of cements, graded fillers and chemical additives proprietary grout.
  2. It is to be mixed with clean potable water to the required consistency. The plastic grout must not bleed or segregate. The storage, handling and placement of the grout must be in strict accordance with the manufacturer’s instructions.
  3. A positive volumetric expansion up to 3% shall occur while the grout is plastic by means of a gaseous system.
  4. It shall be formulated to exceed the following characteristics when mixed with 3.8 litres water to one 20kg bag of the specified grout, at fluid consistency, when tested @ 23°C:

 

Compressive Strength (AS 1478.2.2005)
1 day – 30MPa
7 days – 40MPa
28 days – 60MPa

Conbextra® GP manufactured by Parchem Construction Supplies meets the performance criteria and is approved for this application.

 

Source: parchem – FOSROC constructive solutions

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