Uni-axial tensile test of textile reinforced concrete (TRC) panel

Uni-axial tensile test of textile reinforced concrete (TRC) panel


Hi all, my name is Sachin Paul I am doctoral
research scholar working in building technology and construction management department of
IIT Madras, my area of research is on textile reinforced concrete, I will be explaining
what is textile reinforced concrete and how a uni-axial tensile test is done for textile
reinforced concrete and what are the factors that we need to consider while we are doing
a uni-axial tensile test. This test can be also done for other materials
with fine grain aggregates, basically mortars of fine grain high strength concrete can be
also tested, there is limitation for testing on larger aggregate concrete but this is basically
as per RILEM standards of 2016 RILEM test methods of 2016 for uni-axial test of textile
reinforced concrete, okay. Now I will explain to you what a textile reinforced
concrete is, so a textile reinforced concrete is a concept where we are combining a normal
concrete with textiles as reinforcement. Now what is the advantage of this is that
the textiles that we use are either carbon, polymeric or glass fibre textiles, these does
not corrode in an aggressive environment so it is a corrosion free element. So, we can have a very thin element structural,
both structural and non-structural elements with high capacity and it can be used for
very thin application, exterior applications and he has very high durability since the
corrosion is not happening this material, okay. So here with me I have an example of how what
a textile is this is glass fibre textiles these are E glasses coated E glass textiles. So, these textiles come in a role of 50 meters,
usually have a width of 1 meter. So, we incorporate this in fine grain concrete,
why fine grain concrete is because when you have textiles of small measures you need aggregates
to be very small such that you could easily penetrate to the small gaps of the textiles. So that is why we are basically using a fine
grain concrete. So this is a strip of textile that we have
cut out from this roll this is what we use for reinforcement for uni-axial tensile test
specimens, this final specimen that we fabricate after will be looking something like this,
the dimension of the specimen will be 60 mm width, 10 mm thickness and a length of 500
mm, this is actually a four layered textile, so we have four layers of textile inside it
with a spacing of 1.5 mm each. What we are trying to do is do uni-axial tensile
test with respect to this. The maximum size of aggregates that we use
in this is around 0.6 mm, so it is a very fine grain concrete. So, the problem a uni-axial tensile test is
the gripping is the primary problem how to grip the specimens. So, what we use is an aluminium plates that
is attached to the end such that the crushing failure does not happen during the gripping
process. These are specimens that is attached with
aluminium plates so we cut out rectangular aluminium plates on the gripping area and
we ebbed them or we fix them with epoxy regions and we let them cure for 1 day, then we are
doing the test after that. So, this prevents the crushing failure of
the support due to the pressure that builds up during the testing process or gripping
process. Now gauge length for the uni-axial tensile
test is basically a 200 mm gauge length as per RILEM standards. So, we will be using a combination of two
axial extensometer one is axial extensometer, another one is a video extensometer and with
combination of this we will be trying to find the strain variation or the displacement in
this material with respect to the load acting on the element. This is a 100 mm axial extensometer so what
we are trying to do this is a contact type axial extensometer so we will be attaching
the axial extensometer to a 100 mm gauge length on the specimen and this is a dual axial extensometer
that means it has two axial extensometers and what we get is average of both the displacement
from the two sides, okay. So, it (gets) gives you an average displacement
that is happening in the two phases of the specimen. So here is a setup for uni-axial tensile test
for TRC, we use two systems for taking out the displacement or the strain of the element
one is the axial extensometer, so this is an axial extensometer of 100 mm gauge length
as I have said this is a dual axial extensometer where you have two setups for the axial extensometer
and you get an average of the two displacement at the two phases. We also have a video extensometer a video
extensometer is a non-contact type extensometer, so this is a video extensometer, the video
extensometer has two sets of cameras, the video extensometer detects the contrast between
white and dark spacers in any image and converts that into pixel and whatever displacement
that is happening with respect to those pixels it records as a strain or displacement, okay. So basically, I have marked 4 points or 4
positions in the gauge length of the specimen, the first one is 100 mm gauge length which
is equal to the axial extensometer, why? This is to detect whether the video extensometer
ratings are as same as that of what we observe in axial extensometer, the second one is a
gauge length of 200 mm where (the) that is as per the standards of RILEM for testing
of textile reinforced concrete. So, the video extensometer (detects) can detect
between any given 4 points or 5 point or any n number of points given in the framework
of the video. So, once we have tested for example if we
need to find the gauge length or displacement between the gauge length of 200 mm you can
test between the gauge length of 200 mm and then after testing you can actually go back
to the video and then find the difference or the displacement between any other gauge
lengths provided in the frame of the video. So that is one of the advantages with a non-contact
type extensometer, the other advantage is that this material can crack anywhere in the
specimen. So, for a contact type extensometer when you
crack when you have crack near to the extensometer it causes distortion in the ratings, this
can be avoided in cases of video extensometer, also for a material like textile reinforced
concrete you will have multiple cracking that is happening in the gauge length and that
can be very well captured by the video extensometer. So, we are trying to see the interface of
the axial extensometer and civic testing machine and we will see how the video is captured
and how it is dictated and how the strain measurement is taken out by the video capture,
okay. So in the screen what we can see here is 4
points representing the 4 dots we have in the specimens, the first two dots is a gauge
length of 100 mm and the second two sets of dots is the gauge length of 200 mm, the red
lines indicates the marking points in the specimen by the video extensometer, what video
extensometer measures is the contrast between the white and the dark so you can select the
zone of interest and that zone of interest will be marked as the point of reference for
the displacement for the video. Right now, I have selected two points of 100
mm gauge length, now after testing if I need to find the gauge length or displacement in
a gauge length of 200 mm, I can shift to the 200 mm gauge length and do the test again
without repeating the test just by rerunning the software just by rerunning the video. So that is one of the major advantages of
(axial extensometer) video extensometer where you can do ‘n’ number of repeated iterations
with different gauge lengths with a single test. So, we will start the experiment. You can see the force and the displacement
.You can see multiple cracking forming in the gauge length of the specimen. Now this is where the influence or the advantage
where non-contact type video extensometer comes into play, since the cracks can form
anywhere, you can also form in the position of an LVDT or a strain gauge. So, a non-contact extensometer helps us to
take data out even if you have a mechanism like a multiple cracking. So, we have completed the test now and we
will look at the stress strain plot of the TRC element and see what are different characteristics
that is involved in a TRC element subjected to tensile stress. So, this is a basic plot of force versus strain
percentage strain we can see two different slopes for the graph, first slope is representative
of the matrix properties and the first slope is ended by the first crack formation in the
specimen. So, the initial portion of the curve in a
textile reinforced concrete test or when textile reinforced concrete is subjected to a tensile
stress is represented by the properties of the matrix itself. Once it starts cracking, it forms multiple
cracking and it stabilizes at some stages, the stabilization process says that further
crack formation is not possible in the specimen. Once the crack has stabilized then the strain
hardening happens in the specimen, this slope of the strain hardening zone is the representative
of the property of the textile. So, it basically depends upon the property
of the textile and the volume fraction of textile that is in the specimen. The graph can be basically divided into two
segments, the first segment till first crack of the specimen is dominated by the properties
of matrix the stiffness is directly proportional to the stiffness of the matrix itself and
the second portion is the strain hardening area or the strain hardening zone where the
properties of the textile is predominant. Once the multiple cracking has happened then
all the tensile stress is taken by the textile. So, we have discussed the specimen preparation
procedures, how to mount the specimen, the gripping procedure, how the end plates are
fixed, how we use a video extensometer and axial extensometer to take out the displacement
in the specimen and also the stress strain response of TRC in uni-axial tensile test. So that is how, thank you from me.

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