The importance of shotcreting

Playing an essential role in the mining, building and civil construction industries, shotcrete is a concrete or mortar mixture that is conveyed through a hose and pneumatically applied at high speed onto surfaces.

Shotcrete is an exceptionally versatile material that can be easily and quickly applied to provide a cost-effective means of construction.

Once applied, it forms a bond to a number of materials, including rock, masonry and steel.

The integration of shotcrete in the mining industry has changed the way underground operations are carried out, offering enhanced safety, efficiency and adaptability to varying geological conditions.

Its continuous evolution and adaption underscore its critical role in mining practices across Australia.

The Australian Mining Review speaks with Australian Shotcrete Society committee chair Dr Stefan Bernard about Australia’s shotcreting industry and how it compares to the rest of the world.

AMR: What is the Australian Shotcrete Society and can you tell us about your role as committee chair.

SB: The Australian Shotcrete Society is a technical society within Engineers Australia devoted to improving shotcreting practice for both civil and mining applications.

It is comprised of industry professionals focused on the ongoing development and promotion of shotcrete as a construction material.

A key activity of the society is the provision of guidance to designers and practitioners on current best practice for shotcreting.

The society engages with other international bodies such as the American Shotcrete Association to maintain international integrity in the understanding and identification of challenges within the industry.

The main thing the society does is organise workshops and seminars to foster education about shotcrete, but we also produce documents such as the Recommended Practice for Shotcreting in Australia, which is the most commonly used document for shotcrete in Australia.

I was the founding chairman back in 1998 and I worked to create the society soon after the American Shotcrete Association was formed (also in 1998).

A number of people in Australia decided to create the country’s own society and I’m the only original member still part of the society.

AMR: What are the methods of application for shotcrete?

SB: Spraying is the most common method of applying shotcrete and this is usually done with a conventional concrete pump, plus the addition of an air compressor to supply compressed air at the nozzle.

Shotcreters use a swing-arm pump to supply the concrete, although occasionally people use a peristaltic pump.

The swing-arm pump uses pistons to pump the concrete whereas the peristaltic pump uses a tube that is squeezed with rollers to push the concrete forward.

There’s also another form of shotcreting called dry mix shotcreting where you use dry powdered mortar without water and the dry mix is then blown through a hose and water is added at the nozzle.

While this was the original type of shotcrete used back in 1914, it’s still used today for applications where you need to be able to turn the shotcrete on and off frequently, or where you have access issues and it’s difficult to carry a heavy hose around full of concrete.

Instead, you have a light hose that just has air and powder flowing through it and it’s a much easier way of spraying.

AMR: How important is it to adhere to industry-specific standards and guidelines?

SB: There is no standard for shotcrete in Australia and the two main standards for structural concrete in Australia, AS3600 and AS5100.5, do not include shotcrete.

Instead, we have guidelines that are commonly used and these are produced by the Australian Shotcrete Society.

Otherwise, we have the American Concrete Institute Committee 506 Reports which are a set of guidelines and while this only really applies in the US, it can be used internationally.

There are also some European documents such as the EFNARC Guideline that are occasionally used in Australia.

We generally use a mixed bag of guidelines in the absence of national standards.

AMR: How does the consistency of the concrete vary depending on where it’s applied?

SB: The consistency of shotcrete depends on two things, the consistency of batching of the concrete (which is the responsibility of the concrete supplier) and the consistency of the spraying (which is the responsibility of the sprayer).

The spraying can be done by a manual sprayer who physically holds the hose, or it could be a remotely controlled sprayer which is a machine that’s powered by someone with a joystick controlling where the spraying is done.

AMR: What are the differences between manual and automatic spraying?

SB: Almost 90% of the concrete sprayed in Australia is sprayed remotely using remotely controlled machines, accounting for almost the entire mining industry, plus the civil tunnel construction industry.

Manual spraying is only used for swimming pools, basements and slope stabilisation and it’s usually done with a much lower slump concrete, you would typically use around 70mm slump.

Remote spraying is used with a much more fluid concrete, typically around 180mm to 220mm slump, and a set accelerator is used to stiffen the fluid concrete and prevent it falling off vertical and overhead surfaces.

Most machine-applied shotcrete is sprayed with fibre reinforcement, whereas manually sprayed shotcrete is usually sprayed with mesh or bars.

The mesh or bars are placed first and then the manual sprayer comes along and fills in the space between with concrete.

This is done because it’s very difficult to encapsulate the bars when you’re using a machine.

You almost never see shotcrete applied with a machine when you’ve got bars present, it’s just too difficult.

AMR: How can shotcreting be improved and modernised to meet the demands of the constantly changing mining industry?

SB: The most pressing issue is the effective training of sprayers.

At the moment, training is all over the place and there are many different schemes for training. While it’s slowly improving, there is no national training program for shotcrete nozzlemen.

We use foreign guidelines, for example there is the EFNARC accreditation scheme in Europe and the ACI nozzleman certification program in the US.

These two foreign programs are the most commonly used in Australia and these are applied inconsistently.

We also have many companies that do their own training and certification independently of any international guidelines and this is why we end up with quite ad hoc training of sprayers.

There also tends to be an over-reliance on high performance shotcrete in some applications as people expect a bit too much of the material.

For example, people are specifying higher levels of toughness in fibre reinforced shotcrete that are difficult to achieve consistently and this is a challenge.

I think we’re reaching the limit of performance for shotcrete and it’s causing more frequent failures in quality control due to excessive expectations.

The most common expectation is post crack toughness or residual strength in bending and that’s typically measured using the ASTM C1550 panel test or the European EN14651 beam test.

These two tests are commonly used in Australia and they measure post-crack performance

As some projects have very high requirements on the performance of the mix, they’re getting failures.

There needs to be an improvement in education or in procedures to either achieve these requirements consistently or possibly lower the requirements a bit to make sure we don’t get such frequent failures.