By Warren Mahoney
Shotcrete testing is important because it ensures that the Shotcrete you are delivering is meeting the specification and will do the job the technical engineer has designed it to do. You need to ensure that you are supplying to that standard as a concrete producer and also as an applicator.
About the Author
Warren Mahoney is regarded by many as one of Australia’s leading authorities on Shotcrete. He has worked with Shotcrete in mining since 1995 and is currently Asia Pacific Technical Manager, Ground Support for BASF Australia. Warren is one of the founding committee members of the Australian Shotcrete Society (AuSS), founded in 1998. He was one of the steering committee members for the development of the “Recommended Practice of Shotcreting in Australia.”
Careful attention needs to be paid to compressive strength. In the early stages, this is anywhere from two to four hours for re-entry work. Final strength is measured at twenty-eight days for long-term performance.
When you come to be testing for strength, you are looking at a couple of factors. You want to know the strength in the early stage, in the ultimate stage, and the performance of fibres that are added to the concrete. This is measured in the form of joules, which is an energy rating. How many joules do they want in the Shotcrete for a given rock mass classification? That is how you know what will be required of your Shotcrete.
One of the most common problems I see with testing is the quality of the specimens. Often, the specimens that are sprayed or poured are not done correctly. This means that you can get poor results from your testing. These specimens will not be a reliable indication of the quality of the Shotcrete on the face itself.
We would advise people about the correct procedures to adopt when preparing specimens. A lot of those specifics are recorded under a certain standard. There’s a known best procedure or practice to adopt. The challenge is educating the personnel onsite to ensure that they are following those particular procedures.
If preparing specimens is a problem, people can certainly get in touch with people like myself or could get in touch with relevant testing authorities like the Australian Shotcrete Society (AuSS). In addition to this, they could speak with component concrete producers. Some of the standards are typical of what would be used for normal standard concrete.
Testing can be carried out in many cases to Australian Standards. Probably the biggest shotcrete testing company in Australia is K&H Geotechnical Services. Some universities and other independent testing authorities also do Shotcrete testing.
Identifying A Problem Site
I’ve been involved with certain mine sites for example, where panels have to be sprayed and I’ve had calls from the testing companies who say to me, “Look, Warren, we’ve got a major problem with the strength results of these particular cores from this particular site.” Then I would typically ask for photographs of the specimens. I can see from the photographs that the specimens are just absolutely shocking. In some cases they can’t be tested at all, they are that bad.
I was on a site only recently and had to do some training on how to prepare some particular type of test panels for that site. They’d tried to test their panels but they were getting terrible results. I could immediately see the quality for panels and that the specimens were terrible. I’ve done the training onsite and now things have improved significantly.
The Safety Message
The first thing we do during our onsite training is to stress the importance of doing your specimens to a standard. We explain why they need to be done correctly and the ramifications if done otherwise. In other words, if you don’t prepare the specimens correctly, you just don’t know how that Shotcrete is going to perform in a real-life situation.
In most cases, I generally say to the trainees, “Look, we take Shotcrete for ground support seriously. If we all don’t take it seriously, that Shotcrete’s going to fall down, and it’s going to hurt somebody if not kill somebody. So we’ve got to take it seriously. The only way to verify that you’ve got your concrete up there in good order is through testing.”
We set the scene and stress the safety aspect of getting it right. Then we go into the procedure of how you prepare panels or cylinders, how you cast them and leave them for certain periods of time before they can be moved, how you strip them, how you cure them, and then how you transport them.
Training can go for an hour or up to 6 hours in the training theory. We also go underground and get the crew to actually spray some panels so we can see how well they are done and then instruct them accordingly. It is important for us to see if they are doing it right and to coach them on the spot. Most people, once they understand the importance of correct procedure, will try to do it right.
Variables That Can Compromise Quality
There are a couple of factors that can compromise the quality or economy of Shotcrete.
Having the nozzle too close or too far from the surface, having it at an incorrect angle, too much away from the perpendicular when you spray can cause excessive rebound.
Lack of adequate air pressure and air volume can be a major problem. Poor quality or poorly maintained Shotcrete equipment, and that includes also poorly maintained agitator trucks that can have excessive buildup inside the agitator bowl or suffer from worn fins. This means that you may not get adequate mixing of your Shotcrete prior to spraying.
Calibration of Accelerator Dosing Pump
There may be circumstances where the accelerator dosing pump in the Shotcrete rig is not calibrated correctly. In this circumstance, you don’t know the correct amount of accelerator that you should be spraying.
Most of the advancement in mining use of Shotcrete is through drill and blast, whereas in tunnels it’s through road headers or TBM. In tunnels, you’ve got a fairly nice, even profile that you’re spraying up against. By contrast in tunnels, when you blast something, you’ve got a lot of overbreak, which is like pockets, deep pockets. You haven’t got a nice circumference, and it’s those pockets that can use excessive amounts of Shotcrete. Theoretically, you’re only after, say, 50mm thickness of Shotcrete to cover that circumference in the tunnel, in the mine. You just follow your overbreak, follow the rough pattern until you get to the required thickness of Shotcrete.
A lot of applicators see these areas of overbreak, and they have the tendency to want to fill them in to make it all look nice. Well, in a mining environment we’re not at an equestrian event, we’re at a rodeo. It doesn’t have to look fancy. It just has to perform. That is one issue. This is an example of how the condition of the mine or the profile does play a part in the correct use of Shotcrete.
The thickness of shotcrete will vary from site to site. It’s up for the engineer to work out the amount of load that’s on his rock, as opposed to the thickness of the Shotcrete to be used. Shotcrete in a single application can go to 200mm or 300 mm thickness. Typically in mining, they’ll spray between around 50mm to 75mm thickness of Shotcrete. Sometimes they can go +100, but 50 to 75 is typical.
If cracks occur in a Shotcrete surface there will be three options. Firstly there might be a re-spray of another layer over the affected area at some later stage. It may be moving because of the massive rock movement behind it, so they may install bolts or cables into that area to secure the ground. On other occasions, they may want to inject into that strata where that crack is to knit that ground back together with injection resins.
Mines usually use a combination of approaches. It is rare that you’ll come across bolt-less Shotcrete. It has been done but that would only be used on very good ground. Typically the bolts are there to pin back very bad, blocky ground, and the Shotcrete is there to cover and distribute load and to stop weathering. One of the great things about Shotcrete, it stops the air from getting into the rock, which can cause weathering.
Shotcrete Setting Time
One of the most common questions people ask is to do with the setting time. People might say, “Look, that Shotcrete is not going off quick enough.” In other words, they want the strength of the Shotcrete to develop much quicker so they can get back under it with personnel at a faster rate. Mining is all to do with production. They want that to harden and to develop strength as quickly as possible so the next cycle can start. Crews are keen to get their drilling jumbos back in, start drilling and blasting. That is why it is so common to have questions about speeding up the process.
We often say, “Let’s look at the type of Shotcrete accelerator that you’re using. Let’s look at the dose rate that you’re using it at.” We could also say, “Let’s look at your mix design. Do you have enough cement in the mix? Is the mix design adequate for a much faster strength developing Shotcrete?” Quite often we do a lot of work with the mix design including water addition. Sometimes people might add too much water to the mix. The more water that is added to the mix, the slower it will take to develop strength and setting times in the early stages, and the lower the ultimate strength will be.
After speed of setting the next most common pressure is about cost. Mining companies are always under pressure to cut costs and this includes the cost of Shotcrete. The cost has certainly come down as economies of scale have kicked in. However, it is unlikely to fall much further unless certain companies cut their margins to a point where it won’t be worth being in the business.
After setting time and price the next most common question is to do with waste. During spraying, a certain percentage of your Shotcrete rebounds off the wall and ends up on the floor. Whatever is on the floor is wasted. That’s dollars sitting on the ground.
This can occur when the angle during spraying is too far off the perpendicular, or it can be due to a very poor mix design. In other words, an insufficient balance of coarse aggregate V’s sand. The percentage of coarse aggregate may be too high, so that might need to be adjusted. It could also be the accelerator that could be causing it.
The effectiveness of Shotcrete testing and application depends on how well trained personnel are. Mining companies can use their own experienced people onsite, to train up new blood coming on. They can come to companies like BASF for training. The Australian Shotcrete Society (AuSS) will certainly run training programmes. We do it often. There are other private training providers, such as Edvirt who use Shotcrete simulators. BASF hold two workshops in Switzerland every year. These are attended by people from around the world.
It is very pleasing to see an overall improvement in the standards of Shotcrete testing and application. I look back now, compared to what it was like ten, fifteen or even twenty years ago things have improved enormously. That’s not to say that we should ever be complacent about the performance of Shotcrete. We should always be checking to make sure that standards continue on a path of steady improvement.
I am particularly pleased to see how much the rebound issue has improved significantly. When we first started Shotcrete, rebound would result in 25 – 30% of the mix ending up on the floor. Industry average these days is around about 5% to 8%. That is a significant improvement but it still needs to be monitored regularly. Re-entry times for strength development compared to years ago has also improved significantly.
Slump retention refers to the process where we nurse the concrete from the point of batching to the point of discharge underground. When wet Shotcrete first came out for underground use, people were reluctant to use it, because they only had about one hour to maybe two hours of open time to spray it. Now in the modern world, we can put concrete to sleep for three days and wake it up at any time in that three-day period to use it, so there’s a lot of control, a lot of discipline over the performance and the use of concrete. These days we can tell concrete what to do rather than it telling us what to do.
Australia’s Role In Shotcrete Development
Australia is one of the leading countries in the world for Shotcrete. Australia has led the way in Shotcrete use for mining applications. That’s not to say that Shotcrete wasn’t used in other countries before Australia. It certainly was. I’m talking about wet Shotcrete, not dry Shotcrete. Australia was most probably the first to take it on in such volume. Australia then became one of the leading countries in the world in mining to actively promote the use of Shotcrete for ground support. Australia led the way in the robotic application for mining. Robotic Shotcrete machines were around before then for tunnel application, but Australia were the leaders in developing robotic equipment for mining. Australia still leads the world in the use of wet Shotcrete.
Shotcrete Use in Australia
On average over the last ten to fifteen years in Australia 80% of wet Shotcrete for use in ground support would go into mining and 20% would be used in tunneling. At the moment it’s most probably a little bit different because there’s so much tunneling work going on in Australia, particularly in Sydney. There’s a lot of Shotcrete being used in tunnels, so the ratio at the moment may be out slightly. It may be 75% mining, 25% tunnels.
Shotcrete testing is a critical process to ensure that Shotcrete supplied onsite matches the specifications. If problems occur there are specialised individuals and organisations who can step in to train crews to improve their testing procedures. Mine safety depends on the level of training personnel receive and their willingness to follow well-established procedures.