"The thing about architectural concrete is, it gives you all kinds of options. You can make gently flowing structures, curves. You can do straight lines. You can do beautiful structures in different colors. We can do all kinds of things with concrete. It's limited only by the imagination of the people who are designing and building the concrete.
When you do architectural concrete correctly the first time, you can have a very cost effective, very good looking material that's going to last a long time, but if you screw the concrete up initially, it can be very expensive to pull it out, replace it or try to patch it.
Concrete is the only thing that we're doing on-site where we're starting with one set of materials: Rock, sand, cement, water, reinforcing steel forms. When the time comes for us to take the forms away, we have a totally different material than we started with. We now have concrete. We no longer have rock, sand, cement and water. That makes the concrete construction process a little bit different from any other process that we see on the construction site. That's why it takes a little bit of extra effort to understand that concrete construction process. Once we understand the process, it makes it easier for us to do concrete right the first time.
Central to everything is the project drawings and specifications. If the project drawings and specifications aren't right, the rest of the project is not going to be right. As we change the shape of one of the pieces of the puzzle, we have to change other pieces of the puzzle so that we maintain a complete puzzle."
(This is transcription of some of the highlights of a presentation by Jay Shilstone, an American Concrete Institute Fellow, Chairperson of ACI 304 and on the 303 committee. Jay is a concrete technologist at Command Alkon in Plano, Texas. Command Alkon is the maker of Command Series and Command QC, which are software that's used in concrete quality management and production. This article is one of the best we have found, available publicly on the internet, that explains the proper use and application of architectural concrete form release agent.)
"For example, say the reinforcing steel has to go closer together, we're in a seismic zone. That means we're going to have to change the concrete mix. We may have to go to a smaller maximum aggregate size, which means we're dealing with new materials. We'd have to re-proportion those materials differently. If we can't get a one-inch stone between the reinforcing steel, how are we going to get an inch and a half vibrator between the reinforcing steel. We may have to change the shape of the puzzle for the vibrators. Then, if we're going to be using form vibrators on the concrete, we may have to change our form work to beef it up so that it will handle those form vibrators. All these parts are connected. If we change the shape of one part, we have to change the shape of the other parts.
The problem is, many times on the construction site, we have people that are familiar with one part. We have the pumper who knows how to pump the concrete. We have the form work sub, who knows how to assemble the form work, but does the form work sub ever talk to the pumper? Not in my experience. Does the guy who is erecting the reinforcing steel ever talk with the testing lab about the concrete mix and what's going to happen to it as it goes over the reinforcing steel? Not in my experience. There are a lot of different parts to the puzzle that have to come together and different people on the job site have to understand the functions of others so that we can bring this entire construction process together and get a harmonious product.
Central to the system is the project drawings and specifications. Again, if the drawings and specs aren't right, then the project won't be right.
The summary I wanted to make about concrete is, it's the ultimate in functional artistry. We can make a product that looks good that follows a function, but we have to understand the concrete, both at its plastic state and its hardened state. We have to understand the concrete process. To properly design a structure, we have to know the materials that are available. Then, we have to recognize the limitations of the material and the process and we have to understand our process of creating the architectural concrete.
Concrete is something unlike any material that we are using on the construction site. The designer can use his imagination to create this idea, but then it's up to the contractor to take that idea and turn it into reality using the types of equipment and materials and form work and consolidation and what have you that is going to help reveal that idea. Architectural concrete is something where we have to have true teamwork so that the constructor can work with the designer to make sure that they can get the design that is desired. Once you understand the process, it makes it a lot easier to do that.
The first thing is drawings and specs, prior proper planning prevents poor performance. The reason I say that is there was a study done by the British Building Research Establishment back in 1975, that of 500 buildings surveyed around the world, 60% of the problems in the buildings originated from the design.
The architect and the engineer need to start off understanding the materials of the process. Then, they need to produce a design that's constructable. Back in the late 70s and 80s, I believe, ACI actually had a committee called "The constructability committee". It was, I think, a board task group at the time. My father was involved in that, but the idea being, architects and engineers should design structures that are inherently buildable. If you design a structure that's buildable, then it's a lot less expensive to build it. If you design a structure that's not buildable, then it's very expensive to build it and you're going to have problems on the job.
One of the best ways to help determine if a project is buildable or not is to have a pre-bid conference so that the contractors are fully aware of the intent of the architect and they can express their concerns back to the architect and say, "We're going to have problems with this. Have you considered this?" You want to have a dialog going back and forth. With architectural concrete, more than with any other type of concrete, you want to have a true partnership.
Then, of course, once you've got the bid and you're getting ready to start, you want to have a pre-construction conference so that everybody knows the details of how things are going to be built and in what sequence and what's going to be the impact of certain construction requirements on the aesthetics and also what are the important factors of the aesthetics so that the contractor is going to be able to address those points and make sure that they're done properly.
Then, finally, the best way to assure the contractor can build the project as designed is to do a full-scale mock-up. Let's see. There we go. There's the full-scale mock-up. You want it to be full-scale with all the reinforcing steel, the exact form work, the exact footing layouts that you would have in the normal structure because by doing this, you can determine when you're going to have problems out in the field. Do all of the components come together? Does the reinforcing steel impact block-outs and so on? The only way that you can adequately do this is through a full-scale mock-up.
Another thing that architects and engineers should consider is the impact of different parts of the construction process on the finish that they're trying to achieve. The Construction Specification Institute, back in 1974, created the CSI monograph on cast-in-place architectural concrete. One of the things that it has is this grid. Across the top of the grid, it shows the different types of finishes that can be achieved: Sandblasted, as-cast and so on. Down the side, it shows the different aspects of the concrete construction process that are going to impact that finish.
A word of warning, exposed concrete is the most difficult finish to achieve. A lot of designers think, "We're just going to cast the concrete, take the forms off and whatever is there, that's what we're going to take." Then, the first forms come off, they look at the concrete and say, "Oh my gosh, that's not what I wanted. What do I do now?" The only thing you can do is either paint it or tear it down and start over again. What I find that most designers want, most architects want when they say, "Exposed concrete", thinking it's not going to be architectural concrete, it's going to be exposed concrete, so it's going to be cheaper. They don't want exposed concrete. They want smooth as-cast architectural concrete, which is the most difficult finish to achieve.
A word of warning, those of you that either design for exposed concrete or who have to build exposed concrete, what you are asking for and what you are going to receive may be two entirely different things. You have to understand the distinction between the two.
Whenever somebody tells you that something is "Almost" like something else, you should usually run, don't walk. If exposed concrete is "Almost" like architectural concrete, that's a problem. Run away from those things, don't walk. If you want architectural concrete, specify architectural concrete.
The same thing with superplasticized, self-consolidating concrete. If somebody has a concrete mix that they say is "Almost" like self-consolidating concrete, those are the concrete mixes that I've had the most problems with on jobs. Yeah they may flow, but they segregate. They're difficult to handle. I either design self-consolidating or design fluid concrete, but don't design almost self-consolidating concrete. Exposed concrete is the most difficult finish to achieve. Be aware of that.
There is always a certain degree of variability. There are going to be imperfections. If you want to design a structure to contain those imperfections and to embrace that variability, then you've got a great medium here. If you want to design a structure that's perfectly smooth and uniformly colored, then you're designing for paint. Be prepared. You want to embrace the characteristics of the concrete, understand those characteristics in order to understand how that material is going to impact your ultimate design.
Now, after project drawings and specifications, we start looking at the ingredients, the raw materials going into concrete, the color of the cement that we're going to have, the color, the shape and the size of the aggregate that we're going to have, the gradation, even the water that we're going to have. If you're in an area where they're not able to use potable water, you may actually have water that's going to contain materials that can color your concrete. We want to take these mixture ingredients, put them together in a form that's going to create the finish that we want to achieve.
Now, let's look at reinforcing steel. This is always the fun one. How are you going to get not only a one and a half inch vibrator but even a half inch rock particle between those pieces of reinforcing steel when they're touching each other. Especially when you get into seismic zones, you see conditions like this all the time, not just with reinforcing steel, but with post-tensioning strand and with block-outs.
This is one of the places where people say that BIM is going to help us, building information modeling, because we can do conflict avoidance using BIM techniques, but we have to make sure that our BIM models are going to accurately reflect what is going on in the structure. Otherwise, we wind up with something like this where the concrete can't get between the reinforcing steel and we wind up with these gaps that have to be patched or filled in. We have to look at reinforcing steel congestion, how that's going to impact not only our ability to place our concrete mix, but our ability to consolidate the concrete mix. BIM is not the entire answer.
For example, we have tolerances on our ability to bend reinforcing steel. The larger the reinforcing steel, the greater the radius is going to be for the bend. Plus, we also have a tolerance that we're allowed to vary the manufacturer of the reinforcing steel.
You can see here that we've got, these steel bars are all within tolerance for the particular application they were designed. They're supposed to go into a one-inch topping slab. It's not going to happen.
We've got to have the right cover on the reinforcing steel. Otherwise, we have failures like this and like this, but at the same time, we need to make sure that the concrete is constructed properly. We don't improperly use a chair like this, which is going to wind up leaving two long rust marks on the concrete as the cover, the thin film of concrete over that chair is eroded away, and then the chair starts to rust. We have to understand what the objectives are, but we also have to understand how what we're doing in the field impacts the quality of the concrete. We don't want conditions like this.
Also, the reinforcing steel directs the vibrator to a location. If we've got vertical bars right here in between the horizontal bars, which is something usually detailers don't like to do, they want to have the vertical bars on the outside. If the vertical bars are on the inside, then it makes it a lot easier for us to get the vibrator down between the vertical bars. We don't have the horizontal bars blocking us.
The reinforcing steel is going to direct where that vibrator is going to go. If the concrete has to be vibrated and the steel is blocking access to the interior of the concrete, the vibrator operator is going to stick his vibrator between the reinforcing steel and the form work and give us a burn like this.
The next thing we want to look at is form work. Now, a lot of people say that concrete is a modular material. Concrete is not a modular material. Concrete is a plastic material. It takes the shape and the appearance of whatever it's formed against. It's form work that is a modular material. If we understand that, then we can make use of that to develop less expensive buildings. If we have a building where we have a module that's repeated over and over again, we can have much less expensive concrete form work, which means much less expensive concrete, but if we have a system that has a whole lot of curves, every face is different from every other face, that is not a modular system. It may be more aesthetically pleasing in some cases and worth the money, but it's going to be a much more expensive option.
Concrete will always mirror the form work. If we have a patch in our form work, if we have a form butt joint, then we will have concrete that looks like that patch or that form butt joint. You've got to understand, the material that you're forming against is going to be representative of what your actual appearance is going to be.
You'd be amazed how many architectural concrete specifications I see that say, "BB form plywood or other approved material." BB form plywood is for structural concrete, never intended for architectural concrete, but many times you'll still find it allowed in an architectural concrete project and you wind up with concrete that looks like it's been formed against BB form plywood.
We have a lot of materials we can use instead. We can use plastics, we can use elastomerics, we can use wood forms if they're good quality, we can overlaid wood forms, high density or medium density. All of these different materials are going to give us different appearances on our concrete. A sealed wood form is going to have a different appearance than an unsealed wood form.
We've done cases where we've done very elaborate form work and then used boat builders to put fiberglass over the entire form work so that we have a totally impervious concrete form work, no butt joints to work with, to deal with so we wind up with very uniform concrete.
There are lots of possibilities about form work. Some of them are more expensive than others. Some of them are going to be more reusable than others. For example, steel forms are very expensive initially, but if you've got a system that repeats throughout a structure, you can use a steel form and become very cost effective on that project. For example, I believe One Main Place was built using steel forms.
Looking at release agents. A lot of people think that if a little bit of release agent is good, a lot of release agent is better. That's not the case. Too much release agent will kill the surface of the concrete.
In this particular case, after they applied the release agent, they walked on these beam forms. You can bet, I know for a fact that when they removed these beam forms and you looked up, you could see every one of those footprints on the beam, on the soffit of the beam. You've got to understand the impact of the release agent.
In this particular case, we had too much release agent. You can see these dark splotches. What these dark splotches were is ... There we are. The dark splotches are where we had too much release agent, which delayed the set of the concrete. When they stripped the forms, it actually pulled the skin of the concrete off. Where we have white or lighter colored concrete, the skin was left on. Where we had the blotches, that's where we had too much release agent and the skin was pulled off.
Here's another example of having too much release agent on the right versus adequate release agent on the left. We need to work at sealing form joints. Not only form butt joints when we put two plywood panels together, and we can do that either with foam tape or in this case polyethylene tape or other techniques that you can use to try, like caulk, silicone caulk, to minimize water penetration through those form joints.
Then, we've got placement techniques. We want to be able to place the concrete by different methods, whether it be by bucket, by pump, by conveyor belt. We need to make sure that we don't have segregation. This is from an ACI document on avoiding segregation. We don't want the concrete bouncing off the reinforcing steel causing rock pockets or sand pockets.
Consolidation, we have different types of consolidation equipment that's going to give us different impacts, different types of motors that are going to give us different effects. For example, these backpack motors that some people have for portable vibrators. Those are usually direct-drive vibrators. They operate at a very low frequency. Those typically will result in segregation. We want high-frequency vibrators that are going to have what's called "Radius of influence".
We need to make sure we have appropriate overlap between the vibrator insertions. Usually, concrete is extremely under-vibrated, not over-vibrated like a lot of people are concerned about.
One of the things that we did as part of our study on architectural concrete is, we cast some columns of concrete that one form face was plexiglass. Then, we put white cement concrete into it. Gray cement is too dark. It won't work. You put white cement concrete in and you vibrate the concrete and you watch these bubbles come up. You can see right here, I've got a very large bubble. These bubbles, we find, rise at the rate of about one to two inches a second. If you've got a three-foot lift, then you need to vibrate that lift of concrete between 18 and 36 seconds.
Normally, the vibrator operator throws his vibrator down and then brings it back up and that's all it gets. It gets about maybe of the quarter of the vibration it actually needs. When the vibrator tip gets above the air bubbles, the air bubbles stop moving. The vibrator tip needs to go all the way down to the bottom and come up to the top. Like I said before, we want to insert the vibrator vertically so we don't get segregation.
You can see here, a proper vibration train. We place the concrete, we have one vibrator operator that's leveling out the concrete, we have an inspector behind him and then a second vibrator operator that comes along and vibrates the concrete until all the air bubbles have left.
This is a little video on how to vibrate architectural concrete. You take the vibrator, take it all the way down to the bottom of the concrete, and then you start to pull it out with a churning motion. You want to raise the tip of the vibrator about one to two inches a second. You can see how slowly I'm coming out of the concrete at this point. It's coming up the rate of about one to two inches a second. The churning motion, when you push down, it actually pushes the air bubbles up. There's another mechanical motion that helps remove those air bubbles from the concrete. We keep coming up at the rate of about one to two inches a second. I'm actually going a bit faster here for the purposes of the video. Then, when you get to the top of the lift, you actually want to take the vibrator out rapidly. Otherwise, the vibrator will start to churn air back into the concrete.
Then, when you come in for the second lift, you want to go all the way down into the previous lift of concrete and then start doing your churning motion. Make sure that you do a lot of churning there at the line between the first lift and the second lift so that you knit those two layers together and you avoid a lift line later on.
A vibrator is rated for a radius of influence. If a vibrator has a radius of influence of 18 inches, you want to do your insertions at about 80% of the diameter of influence. Say the radius of influence was 15 inches. That makes the diameter 30 inches. 80% of 30 is 24 inches, so you want to insert at 24-inch centers. If we don't do it properly, we wind up with problems, lift lines, honeycomb and leakage and air bubbles.
Then, finally, we have the management, the testing. Make sure the testing lab provides the test results to the concrete producer. It's now part of ASTMC-94 and 318. The concrete producer is required to receive the test results for his concrete. That's a very important thing that needs to be considered, but I know many a concrete producer who has had to pull out good concrete because of bad testing. This is not a good concrete test cylinder. You can see we have the vertical fracture instead of the nice double cones or today using neoprene caps, the sheer cones and so forth.
It all goes back to the process, making sure all of the parts fit, understanding the process. If you change the shape of one part, you have to change the shape of another part to make sure the process fits.
Murphy's Law: Nothing is as easy as it looks. Everything takes longer than you expect. If anything can go wrong, it will and at the worst possible moment. I know a perfect concrete, architectural concrete projects, where the only blemish on the concrete surface is right in front of the president's parking spot. That's where it's almost always going to happen." Learn more.
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