Similarities
The Shotcrete association and other groups prefer to think of Gunite as a sub-category of shotcrete. Both are ways of placing concrete. The end result is still concrete. Technically, the two types of Shotcrete are “Dry mix” and “Wet mix”. I think the industry types avoid the word “Gunite” because it was a proprietary term (100 years ago), but most people (non-lawyers) just refer to “dry mix” as “Gunite” and “wet mix” as “Shotcrete” even if they know the technical terms.
In both cases, the mix (wet or dry) is pushed thru a hose with a nozzle at the end and a gunner can direct the concrete. In both cases, the concrete is shot onto a single-sided form (or preexisting concrete) where it builds up.
In both cases, concrete is added without leaving cold joints, even if the previous concrete was there 100 years sooner. You can find many articles or white papers where microscopes were used to try and find the cold joint in sample cross-sections.
Price seemed similar in both cases, at least in my area. But I have been told that the shotcrete crews often charge more because the equipment is more expensive and there is more competition.
The Crew
In my case, there were significant differences between my shotcrete and gunite crews that affected everything from the reliability of the quotes to the quality of the work. Those differences would have existed even if both crews used the same method. I’ll try to factor out the crew differences below. Generally speaking, it should be possible to get the same sort of quality finish with either method.
In most cases, if you end up with shotcrete or gunite will depend on what the crew you hire is using.
If you ask a crew which is better, you can bet the answer will depend on the equipment that they have.
Differences
The primary difference is that gunite blows a dry concrete mix thru the hose and the gunner adds water at the nozzle. The concrete is actually fully mixed only when it hits the wall. Shotcrete pumps already mixed concrete thru the hose and the gunner injects air at the nozzle to shoot it onto the wall.
The primary concern with gunite is that you are dependent on the skill of the gunner and crew. It isn’t hard to find stories of gunite pools that fall apart due to badly mixed concrete.
The primary concern with shotcrete is that you are at the pace and mercy of the mix trucks.
The Strength
The compressive strength of the concrete starts with the mix. The shotcrete mix is controlled by the mixing truck and includes lots of additives (such as fly ash or glass fibers) that can improve the properties of the concrete. The gunite rig only allows you to mix bags of concrete with sand in a much less precise way. I assume the gunite rig could use bags of high strength concrete, but those cost 50% more.
I have read in many places that gunite is stronger than shotcrete, but most do not explain why it should be. Primarily, I think this is based on the idea was that using less water would result in stronger concrete. Since dry gunite doesn’t need to worry about pumping the concrete thru the hose, it can use much lower slump concrete. That may be true if the Shotcrete crew were somehow limited to a very basic cement mixture, but they are not. Instead, the shotcrete mix can include fly ash or advanced admixtures that help with flowability thru the hose or other desirable properties. There is also the idea that smaller aggregates (such as sand) result in stronger concrete than coarser aggregates (such as the peas stone associated with shotcrete). I don’t know why shotcrete primarily uses peastone, but I supposed they could use sand if they wanted to.
After working with the resulting concrete on my site for quite a while, I can tell you that the shotcrete was much stronger (officially 8900 psi) than the gunite (maybe 2500 or 3000 psi).
Concrete consistency
Mix companies are very good, computer controlled good, at sending out trucks with very well mixed and consistent concrete recipes. I trust their ability to give me good concrete over any gunite crew. After watching the gunite rig for a while, I would double down on that argument. Lets look at all the potential trouble spots.
The gunite rig has two hoppers. One for sand, the other for dry cement. The rate that the sand or cement falls down to the bottom of these affects the mix. They use vibrators and mixing arms to try and keep the powder moving downward, but wat the video and you will see they often used sticks to keep things moving.
Actually, variability in the moisture content of the sand before being dumped in the hopper would also make a difference. Even the way it is dumped matters. I noticed our gunite guy (Bert) would carefully shake the sand off the skid steer bucket so it would fall nice and loose into the hopper. A less skilled guy may leave you with more lumps.
At the bottom of the hoppers, augers push the constituents together. Presumably, this rate can be adjusted by the gunite crew. I think we had a good crew, but I would trust the mix company more. There were a few times when one of the augers stopped mixing properly and the mix became either too sandy or cement-y and they had to shut down the rig and sort it out. This could be noticed by either the gunner or the rig crew, but I wonder about smaller changes that would cause alarm but would still affect consistency. For instance, could one section be 3000 psi and another be only 1500 psi? Hard to tell.
Then the inclined auger mixes on the way up. Not sure how well it does, but it looked pretty good.
After blowing thru the pipe, water is added right at the gun. The gunner controls the ratio and I am sure he does pretty well for the majority of it, but I noticed that there are some issues when the compressor blasts dry powder out without enough water and it never set up properly.
I also found some small pockets of gunite, especially in shadow regions, where the mix was poor and crumbles to the touch.
One good counter-argument is that the shotcrete crews are relying on a mix plant that may be many miles away and may see a difference in consistency between jobs in different locations. Gunite crews mix their own and don’t have to worry about where they are.
Slump
The slump of the gunite is lower than for the shotcrete. It is difficult to know how much of my experience is more due to the crew than the method, but I have read about the lower slump in other places, so I think it belongs on the method side. Where I have pipes or window bucks, it is very clear that the shotcrete slumped down and away from these. I used quite a lot of hydraulic cement filling slump gaps after shotcrete. With gunite, I saw very little slumping under objects and didn’t do any filling. Low slump also helps when you don’t have these obstacles.
Low slump also means less shrinkage and less cracking. But actually, in my case, I have not noticed cracking in either my shotcrete or gunite sections.
I don’t like to think about if the shotcrete slumped around the rebar also. On the other hand, it could “flow” to better suround rebar. Again, the skill of the gunner matters here.
Coarseness
Typically speaking, Gunite uses sand and Shotcrete uses peastone. I am not expert enough to know why that is. I assume either method could use either aggregate, but there must be a reason why this is usually the case. If someone knows, please comment.
Using the finer sand means that the gunite may be a little stronger than the same mix with pea stone. It also means that the concrete will be easier to finish, etc.
When shooting on screen and lath (like I did), the pea stone more quickly blocks any holes and prevents blow thru.
Pace
The pace of shotcrete is pushed by the cement mix trucks. Mix trucks arrive with 8 to 11 yards of concrete that needs to be put up in a limited time or dumped out. Lead time on the trucks is also a problem because you need to know if you will need another 8 yards 1.5 hours from now or not. You can’t stop to calmly sort out an issue. You can’t just do a few more yards before quitting time, you need to think in full truckloads. In contrast, gunite crews can start or stop the mixer at any time. Any cement bags that are not used up can simply be taken to the next job. Any cement left in a concrete transit mix truck (and you always should order more than you think you will need) gets dumped as waste.
This is why lower volume shoots are usually done with the dry mix method.
However, even in my case which I wouldn’t consider “low volume”, using gunite significantly reduced my stress levels, waste, etc.
Weight
The weight of the hose also makes a difference. The heavy shotcrete hose is not something that the crew wants to move around much. This makes them less likely to load things in a balanced way and leads to them putting up too much in one spot before moving to the next. The gunite crew was much more able to put up a little on at a time. This kept the loads more balanced, resulted in far fewer blowouts, etc. It also enabled the gunite crew to work much longer days, probably twice as many hours of shooting per day compared to our shotcrete crew. The gunite crew was also able to dedicate fewer people to just helping move the hose.
Rebound
I had been told that “rebound”, concrete that bounces off the wall, and needs to be thrown out (wasted), is greater with gunite than with shotcrete, but this has not been my experience so I left it off the chart.
In my case, I saw much more concrete wasted with the shotcrete method in terms of rebound, slumping off, or simply not used because something went wrong with the pump or they still had more in the truck when they were done shooting the wall, etc.
Most of the piles wasted concrete shown in our gunite video were actually trimmings from cutting the excess off the wall to shape it precisely. That is more about the gunner spreading it on fat and it was worth it to get such a nice final finish. I hardly saw any rebound.
Similarly for the rumor about dust levels. You can watch the videos, but I didn’t feel like I saw much more dust with the dry method. Maybe that is related to the skill of the operator.
Independence
The gunite rig is somewhat more independent than the transit mix truck dependent shotcrete rig. A transit mix truck usually needs to empty within 90 minutes. This didn’t affect me very much because I am within a reasonable distance of several transit mix companies, but it could be a major factor for someone building in a more remote location. As travel time increases from the mix plant, the only thing the operator can do is dilute it (or add retarding admixtures), which weakens it. If you have dirt roads or other access issues, heavy concrete trucks might get stuck or have other problems. Bad traffic or a bridge out could cause serious concrete wasting delays.
More?
There are probably more differences. I’ll come back and add them as I think of them. Feel free to comment or email me if you have ideas.