Interlayer Reinforcement in Shotcrete-3D-Printing
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| Anzahl der Teile | 12 | |
| Autor | 0000-0003-2392-5439 (ORCID) 0000-0001-8626-918X (ORCID) | |
| Lizenz | CC-Namensnennung 3.0 Deutschland: Sie dürfen das Werk bzw. den Inhalt zu jedem legalen Zweck nutzen, verändern und in unveränderter oder veränderter Form vervielfältigen, verbreiten und öffentlich zugänglich machen, sofern Sie den Namen des Autors/Rechteinhabers in der von ihm festgelegten Weise nennen. | |
| Identifikatoren | 10.5446/56108 (DOI) | |
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00:00
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Transkript: Englisch(automatisch erzeugt)
00:00
Yeah, welcome to my presentation today. I'm Niklas Point and I am delighted to present you recent results of an experimental study we performed on interlayer reinforcement in shotcrete 3D printing. In this study, we want to investigate the effect
00:21
of different accelerator dosages on the resulting bond behavior. As you all know, in the field of additive manufacturing, reinforcement integration represents a highly relevant topic. When we look at a traditional way of casting or traditional way of fabricating reinforced concrete elements,
00:43
the integration of reinforcement is very simple. You place your reinforcement in the formwork, you fill the formwork with concrete and the result is a reinforced element. When we now look at the additive manufacturing, everything is different. We have our additive manufacturing process,
01:01
we have the reinforcement element and we have to bring these two things together. So we have to rethink the integration of reinforcement and when we develop new strategies, we should take some fundamentals into account. We want to maintain the process automation and we don't want to reduce our geometrical degrees of freedom
01:24
this novel technology gives us and we should more of a use this innovative character of our additive manufacturing and create novel strategies of reinforcement integration and for example novel reinforcement arrangements to save material.
01:44
And the key is that we have to gain a good bond between the reinforcement element and the concrete so that we can produce elements with high mechanical performances. In the literature you find several approaches
02:00
on the integration of reinforcement. You can basically distinguish between strategies where the concrete supports the reinforcement, so the concrete serves as a supporting structure for reinforcement or the other way around where the reinforcement supports the concrete or incremental strategies like my colleague Cala Matteo has presented before.
02:24
In my presentation I want to focus on the interlayer reinforcement and you can find this here in the upper left. Interlayer reinforcement is a very simple and fast reinforcement method. You can conduct this in parallel to a printing process,
02:42
so you have to conduct this in parallel to the printing process because you use this layered characteristic of your additive manufacturing process and place the reinforcement inside the interlayers. So you can use conventional reinforcement materials, you are very flexible in the rebar diameter, in the rebar material itself
03:01
and when you combine this reinforcement method with an adaptive path planning, so where the layers are not only horizontally oriented but they are oriented in an angled manner, you can easily produce force-flow compliant reinforcement layers.
03:22
And as the title of my presentation showed you, I want to perform this study or I want to perform these experiments with short grid 3D printing, so let me introduce short grid 3D printing first. Short grid 3D printing, short SC3DP,
03:41
is an additive manufacturing method that can be categorized in the material jetting category according to the violin classification. The material is applied with high kinetic energy as you can see in the picture here on the left-hand side. And this leads to a very good interlocking between the layers
04:02
and a good compaction of the concrete you are depositing. And you can change a lot of process parameters. You can change nozzle distance, you can change nozzle angle and you can change different volume flow rates. And with help of this you can change the strand geometry for example.
04:23
You can say that short grid 3D printing is an additive manufacturing method for the production of large scale elements and a good example is this double curved reinforced 2.5 meter high wall that was printed a few years ago at our digital building fabrication laboratory.
04:42
And as I told you that there are a lot of process parameters you can change, this was exactly the point where the motivation of this study started. Because when we now look at the interlayer reinforcement, we have a lot of parameters that could affect the resulting bond.
05:01
And in this illustration here you see some of them, so the parameters can be material related, for example the accelerator dosage or process related, like for example nozzle distance or the different volume rates like the volume airflow.
05:21
And in this study I want to focus on the accelerator dosage. We use accelerator to control the structural build up of our material and with this we can control the building rate. So we are changing the rheological properties of our material and we ask ourselves ok this could have an effect on the bond behavior of integrated rebars.
05:47
So we produced specimens at our digital building fabrication laboratory with the short grid process and you can see a sketch or illustration of these specimens here on the right hand side.
06:00
Basically these specimens consist of six layers, so we printed three layers then we placed reinforcement bars perpendicular to the printing direction and then we covered it with six additional layers. We tested three different accelerator dosages, 0%, 2% and 4% and the bond length was limited to 5 times the rebar diameter, which was 12 mm,
06:28
so we have a deep fine bond length of 6 cm. We cut the specimen here in fresh state and we gained different specimens out of it. Three pull out specimens that were printed, one specimen with a carbon bar
06:44
that we will later use for computed tomography scans and we produced reference specimens in moulds where we cast the material. As you can see here there is also formwork because we covered the printed specimens with concrete
07:03
so that we get better comparability for the geometry and we have a more uniform load introduction for our mechanical tests. However there is no additional bond because this plastic sleeve here was longer than the formwork
07:20
so we have no additional bond here. Here are some impressions from the printing process. On the left hand side you see the printing process of the third layer here with our shotcrete nozzle at the DBFL and on the right hand side you see a specimen and an important detail is that we have a height adjustable support structure here
07:43
that was fixed to the specimen plate so that the rebar could not tilt after it was inserted into the interlayer. So to sum up, we want to evaluate, we want to investigate the effect of accelerator dosage
08:01
on the resulting bond properties to start the feedback loop and to see the limits of this reinforcement method. Because we want to see the effect of accelerator dosage we firstly have to check what is the effect of the accelerator on our rheological properties so we performed fresh concrete investigations.
08:24
Then we want to see what is the effect of on a bonding zone and so we conducted computed tomography scans to look at the bonding zone and see the quality of the bond. And finally we are interested in the mechanical performance of our specimens so we performed pullout tests according to Rylem RC6 28 days after manufacturing.
08:46
So let's start with the fresh concrete properties. We performed this test with a shot-grip penetrometer. This shot-grip penetrometer measures penetration resistance and we did this directly after the printing process. So we have a zero minute value
09:02
and a zero minute value gives us information on the material properties at this time where the rebar was inserted and the material properties of the material to the time when it covers the rebar. This penetration resistance can then be recalculated to a yield stress
09:25
in accordance to Lotens et al. 2009. And here on the right hand side you see the result. On the x-axis you see the accelerator dosage and on the y-axis the yield stress. And you see between 0 and 2% there is no significant change
09:43
in the yield stress for the zero minute value. So don't think that the structural buildup is the same but it's a zero minute value. And when we increase the accelerator dosage to 4% we see a significant increase of the yield stress.
10:01
Here it's about 20-20 kPa and for 4% more than 60 kPa. But does this change in yield stress? So yield stress has an effect on the bond strength and the bond quality. So to answer this question we have to look inside our specimen
10:21
and as I told you before we use computed tomography. Within the analysis of our computed tomography scans we defined a region of interest. This region of interest has a circular shape around our rebar and a diameter of 32 mm. Our rebar is a diameter of 12. So we are looking at the area of 1 cm around our rebar.
10:44
And the length of this region of interest, so it's a volume, was set to 60 cm, what is equal to 5 times the rebar diameter, so the bond length for the puller test. And then we used grayscale analysis
11:00
and we calculated the volume of air of our concrete and the rebar and we can define a void content which is equal to the volume of the voids divided by the volume of the region of interest minus the volume of the rebar and here you can see the results.
11:21
For 0 and 2% the void content is in the same range, it's about 2 volume percent. However, when we look at the specimen with 4% accelerator the void content is as tris as high as the values for the 0 and 2%.
11:43
So let's remember the results from the yield stress and we can correlate this and we can see that with an increase in the yield stress we have an increase in voids. So are these voids located homogeneously around our rebar?
12:01
This was the next question and so we looked at the bonding zones. And no, they are not evenly distributed over the rebar diameter. In this picture A here you can see the space between the ribs of the rebar from the top view and the bottom view.
12:21
The top view, that's the side that was facing the nozzle and you see that there are not a lot of voids here, some small, but when we look at the bottom side there are huge voids under it. So especially for 4% accelerator, so a material with a higher yield stress, voids are increasingly identified in the bottom zone.
12:45
And you can also see this here in a cross-sectional view, here are voids under our rebar. And the second location we could define or we could find was the existing voids under or above the ribs.
13:02
And this could be based on rebound of the material. So now we want to see what is the effect of these voids on our mechanical performance. And so we performed this polar test according to Rylem RC6.
13:20
The geometry of our specimen is a cube with an edge length of 20 cm. The bond length is here located at the bottom of this. You fix your cube to the testing machine frame and then you pull this rebar out of it. The bond length, as I told you before, was defined to 6 cm.
13:43
And we performed this experiment displacement controlled with a defined displacement rate of 0.02 mm per second. We performed this experiment for printed specimens and for cast one. So let's start with the result for the cast experiment.
14:03
And we see no significant effect of the accelerator on the bond strength and this is a very nice result. Because we have a homogeneous bonding zone, we placed our concrete in the formwork and we vibrated it. So we see there is no significant effect of this accelerator
14:23
in the range up to 4% on our fresh concrete properties. So here is the bond strength and here the accelerator. So now let's take a look on our shotcrete 3D printing results. And we can see two things.
14:42
The first thing is that the results of our shotcrete 3D printed specimens are always above the cast runs. So the bond strength is higher than in a traditional cast process and this could be due to this high compaction of the material
15:00
within the printing process. And the second thing is that we see an effect of the set accelerator on our bond strength and this can be correlated to the void content. So for 0% we have 26.2% and this decreases to 22.8 MPa for 4% accelerator.
15:28
And that's a reduction of about 13%. So when we now bring this together with the void content, we can say that traditionally our bond strength is related to this increasing void content
15:44
so when you have more voids in your bonding zone, then the forces of the pull-out have to be transferred through a smaller contact area so you have peak stress over your bond length and so earlier failure. So to sum the results of our experimental study up,
16:04
the integration of interlayer reinforcements represents a very promising, fast and simple integration method and we have very good results in the bond strength with the effect of accelerator on the bond strength.
16:22
However, up to 2% accelerator we could create very homogeneous bonding zones and regardless of the accelerator dosage, the results show higher values than for conventionally cast specimens. That's from my side. Thank you very much for your attention
16:42
and if you have some questions don't hesitate to contact me or to ask me now or later via mail.