Magnetic alignment of microsteel fibers
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| Author | 0000-0003-3243-0251 (ORCID) | |
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Lecture/ConferenceMeeting/Interview
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ConcretePanel painting
Transcript: English(auto-generated)
00:00
Hello everybody, so I would, as Mikkel mentioned, just show you the latest research that I have been working on. Many of you maybe have seen some presentations of mine about the magnetic alignment of microsteel fibres in UHP FRC or in UHPC, Ultra High Performance Concrete and I would like to divide my little presentations into three parts like
00:27
process, material tests and applications. And so let's just jump in with, I hope the film works with the film. Okay, so this is quite an old approach but just to show you what it's all about. So what the topic is, is that you use magnets in order to align steel
00:48
fibres according to the force of flow that you expect to have in a certain concrete element. And what you see here is a translucent gel, so to just visualise how the process
01:06
works. And so this was in smaller scale several years ago. And to apologise maybe this is not all of it, or maybe 99% of it is not 3D printed. But maybe there is some
01:22
time left for me to work on this as well. But it is sort of, yeah, additively manufactured in the way that you additively design strands in the material, but with a formwork, I'm sorry. But maybe this will change. So about the microsteel fibres that I was looking at
01:42
in the last couple of months, they were different types. As you can see, they differ in length, especially like microsteel fibre MSF, like 4mm 6.8, then the mix of it and the 11.1, and the diameters are super small, like 0.13. And also I had a look on the recycled fibres,
02:03
which is quite interesting, I think, for several applications as a substitute for the quite expensive microsteel fibres. So just to explain a little bit about the recycled fibres, so
02:21
they come from tyre recycling and they are about only 7% of the costs of the normal fibres that you can buy. So this is really interesting because they are more or less like garbage, they are treated like garbage, like raw material steel. So this is a really nice opportunity to see whether they work or not. And you can see here, this is the
02:45
cost share of the fibres that I use, of the brand new fibres. So you can see this is the cost of UHPC that I used with a 2.5 volume percent, which is quite decent, which is a normal
03:00
percentage. And over 50%, almost 60% are the fibres in terms of cost. So this is quite a lot. And so when we can save fibres here, or when we can at least make the fibres work in a more efficient way, then I think we can step ahead of those who have to pay so much money for
03:22
that low material properties. And the microsteel, the recycled fibres here, they are not so slender, so they are a little bit thick and small. So actually, the ideal slenderness of the
03:40
one that's used today is 60, so length divided by diameter. And so those fibres that you can get here out of the recycling industry, it's rather about between 20 and 40 most. Okay, so they are not that good, not so efficient, but cheap. So the magnets. So I designed several
04:01
magnets and also simulated them in order to know what forces I can expect. So these are the magnets, the magnet fields, and here little fibres, you can see how the forces go in the fibres and attract them or manipulate them so they can actually swim through the still
04:20
liquid matrix. And I had a look on different magnets, like permanent magnets here, neodymium magnets, and also electric magnets, but in this presentation I would like to show only neodymium magnets that I focused on then. And so I measured a lot of these magnets, so to get the measure of the magnet flux density, to know how to make this relationship
04:46
between the actual magnet force and the effect of the fibres flowing through the matrix as one parameter. And this is one of my end-effectors that I can couple to
05:02
robots, or in my case because of corona I use only a CEC machine at my private workshop. And this was one of, this is so to say for the next coming tests, the one that I used a lot. And the special thing about it is it has two magnets and you can actually switch between the magnets and two magnetic fields. So you can on the one hand attract the fibres to the magnet
05:27
so they are more or less perpendicular to the formwork and then in the second step you can rotate them again and align them more in parallel to the formwork surface. So that worked actually quite okay. And here's one of the parameter studies that I did. So the parameters
05:47
that I focused on here was the distance of the magnet to the formwork which is quite crucial because the magnet force decreases a lot when you go away from the fibres
06:00
inside of the formwork, then the speed of the robot and the thickness of the element. So you can see here for example I will write the distance of the magnet and the speed is the same I think. And you can see when we go far away from the magnet then most of the fibres will do something
06:21
but will maybe not be attracted to the side where the magnet is moved on the surface. So this then is beneficial for introducing sort of tensile zones within one section and in several cases this is exactly what we need to have a concentrated tensile zone and
06:45
to increase so to say the distance between the tensile zone and the compression zone so to say. And these are some of the parameters I don't know maybe you can see here this is the neutral
07:01
axis so this would be the tensile zone then in the element subject to bending loads and the concentration is here very much on this side so you can see 29 percent 92 percent of sorry in the tensile zone and only eight percent are in the compression
07:20
zone so this could be beneficial for bending applications. And there's one just to explain a little bit about the difference between chaotic distribution and aligned distribution. So when you think you want to reach sort of an alignment well around the other way
07:48
when you have a chaotic distribution then only three percent of the fibres are in the direction of the force that you intend to have in this member. So these are just the three percent
08:02
of the fibres that are in perfect orientation and this increases of course when you say okay 45 degrees is also okay for me a deviation from the trajectory line let's say in the member then okay you have 45 percent. And to find out what is the best angle I did some
08:22
pull out tests and you can see here these are little nozzles 3D printing but the fibres were glued in and to grab them and pull them out and so surprisingly for me it was that as you may expect that zero decrease is the best bonding in the matrix the best
08:50
bonding but it's not it's up to 45 percent and when you have a fibre 45 percent in the crack it's you see you need more force to pull it out. So that's quite interesting but there is
09:04
a counteracting effect which is when you have to imagine you have a crack and there's fibre going through and then when you have a sort of projected length of the fibre this length decreases of course and the likeliness that the fibre hits or that the crack hits
09:22
the fibre decreases. So these effects are sort of counteracting and in literature they say it's more or less then it doesn't matter a lot because this one here is a plus so to say and this is a minus so but I'm going more into detail in some research that I'm doing here what is really the best but at least it shows that it doesn't really have to be super straight
09:46
straight but it can also be right 45 percent and then you also have great improvement of the tensile strength or flexural bending strength of the member. So these were tests that I
10:04
examined and I cannot of course show a lot of them but maybe I will concentrate here on the tensile tests and so as you have seen before when you put or when you pull the fibres in the tensile zone it's a great effect when you have an element that you
10:23
want to need or that you need to bend but if you want to get rid of this effect of pulling the fibres to one side of the section then of course centric tensile tests will show more honestly or more honest what the effect is. So I designed some tensile test bones
10:45
and yeah I will show later but in the way anyway I also would like to point that all of these tests just to summarize quickly the increase of f max of the maximum force you can you can put on or you can apply on these elements will increase depending on the test setup
11:03
and it will rise on the bending test between 30 and 70 percent shear test only 15 to 25 I have to look what went wrong there I expected more and centric tensile test 20 to 90 percent depending on the amount of the fibre content so 1 percent is then 30 percent and 4.5 percent for example
11:24
would be 70 percent so with increasing fibre content the method gets more efficient. So this is the test setup I would like to show today and there are no regulations about it so I made something up that I could produce and measure so they are not really tests that are
11:44
exactly for my dimensions in material and this is then the formwork a water jet cut formwork and here these clamps are the test setup and the results quickly are that these red ones here are the magnetically treated
12:04
elements or specimens and the blue ones are the reference ones the chaotic ones and what you can see is for example here when you have 3.5 volume percent like quite a high volume percent you increase disproportionately high like almost 90 percent increase of the tensile strength between
12:25
chaotic and magnetically treated and this here is 4.5 volume but recycled fibres that I showed before so you can see there is an effect but on a lower level but at least you also say okay I use more recycled fibres and treat them with a magnet and then they are about
12:46
like 3.5 percent of the super expensive fibres on the more or less same level but you need then to have this application with a magnet so that is one of the results I would like to point out here and this is on the left side
13:09
micro CT scans of a chaotic fibre distribution on the right side with magnets and down here you can see oh my god sorry oh my god sorry and
13:25
yeah I mean maybe you don't see so much difference but what I can explain is then when you see a lot of long white lines it's good because they are so to say perpendicular or they are in parallel to the forces where we pull and this is also the difference here
13:45
between the stress deformation diagram and also here you can see they are really concentrated around 0 degrees like parallel to where we actually pull the specimens okay and so this is
14:06
not perfect this is quite rough but anyway I would like to point out the economic and ecological saving potentials so when we reduce 40 percent which is quite of an average that I calculated
14:20
with all the tests let's say 40 percent is quite realistic to reduce the fibre content we can reduce the amount of the cost of the entire uh UHPC mixture the one that I use with expensive fibres by 24 percent so this is quite a lot I guess so you can you can now go to your company and then calculate a little bit of how expensive magnets are and then this
14:46
robot is and so on and how quickly you can really at the end save money with this investment of using a magnet I mean for the applications where it's really suitable also sure and the same amount like 50 plus percent is also the effect of the fibres that need to be for the
15:07
production of the fibres so it's more or less comparable so more or less you could say okay it's also about 25 percent let's say saving of the global warming potentials when you just reduce the fibres and even more when you would just use recycled fibres this is much more because
15:25
those recycled fibres also had this process behind with that very energy intense and so this is really worth to think about to use these fibres and to get a certificate maybe for this fibre somewhere okay application quickly so
15:49
these are the applications that in a bigger scale I did but also manually and only partly but at least showing that these were elements that we produced at the ITE and so this is a
16:07
thin walled coffered ceiling with a wall thickness of only 15 millimetres but the construction itself is of course big and the spans are huge so with this little scale that the magnets work
16:21
in you yet can produce huge elements so this you have to produce them in a hollow way and then this is what UHPC or UHFRC is made for anyway so you can make a bigger construction that's what I'm trying to say and also these are visions of course rendering so
16:41
according to the flow of forces and maybe some shell segments that you could think to produce in this way and here two more funny applications maybe so these are claims made of UHFRC this is a little several years ago two years ago or something
17:02
but in order to use the flexibility of this material with the fibres aligned in it you could you can also think about new applications for this new hybrid material like clamps you normally would make out of metal yeah so this worked actually quite nice
17:23
and maybe for several applications it's a funny idea or maybe it's just an idea okay and this is this is an another idea this is also a little older but just to show you and here's a little bit 3D printing also on the right hand side so it's not 3D printing what I'm trying this is 3D
17:43
printing but but not with concrete but what I'm showing you now is the snap fits made of UHP UHP FRC using this idea of having these you will see in a second these clips inside of it reinforced with magnets with a high percentage of micro steel fibres
18:04
and so this you would normally also make out of steel if you want to make it anyway and so you can make it now maybe better than before with UHP FRC and so this was a test
18:21
and on the right hand side maybe you see how you can also implement these snap fits into other 3D printed elements because I guess one one topic or one one problem maybe for for 3D printed elements is also the precision of the joints you know you have to mill them or you have to
18:41
do something so this could be some some idea to to say okay there there are two robots one is holding it and the other one is spraying it and then we have one one super precise snap fit easy to to to assemble on the side and to to use this technique
19:00
and so this is an outlook I also built this but I couldn't find a photo but the idea here is to have sort of a metric metric feeder and and you pump in concrete and here you have your your magnet and in order to prevent cold joints you can somehow um persuade the fibers to to to stand vertically on on the different layers and stick out and
19:27
and to get a better bonding with the with the next layer so this is something that I because we are here at the edit we are talking about additive manufacturing and so this is something that I have to work on in the future also okay thank you