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Experimental study on fiber-reinforced concrete panels subjected to shear, Experiments on FRC - Tesi

Tesi di laurea "Experimental study on fiber-reinforced concrete panels subjected to shear", in Tecnica delle costruzioni, Corso di laurea magistrale in ingegneria edile-architettura dell'università di Brescia.
Tesi sperimentale sul comportamento di SFRC e MSNFRC.
Experiments on FRC.

Materia di Tecnica delle costruzioni relatore Prof. F. Minelli

Anteprima

ESTRATTO DOCUMENTO

4. EXPERIMENTS ON FRC

the length of the core linking to the secondary. The frequency is usually in the range 1

to 10 kHz.

Following Figure 4.3 show an internal part of one element of LVDTs:

Figure 4.3: Internal section of a LVDT.

As the core moves, the primary's linkage to the two secondary coils changes and causes

the induced voltages to change. The coils are connected so that the output voltage is the

difference (hence "differential") between the top secondary voltage and the bottom

secondary voltage. When the core is in its central position, equidistant between the two

secondaries, equal voltages are induced in the two secondary coils, but the two signals

cancel, so the output voltage is theoretically zero. In practice minor variations in the

way in which the primary is coupled to each secondary means that a small voltage is

output when the core is central.

When the core is displaced toward the top, the voltage in the top secondary coil

increases as the voltage in the bottom decreases. The resulting output voltage increases

from zero. This voltage is in phase with the primary voltage. When the core moves in

the other direction, the output voltage also increases from zero, but its phase is opposite

to that of the primary. The phase of the output voltage determines the direction of the

displacement (up or down) and amplitude indicates the amount of displacement. A

synchronous detector can determine a signed output voltage that relates to the

displacement. 131

4. EXPERIMENTS ON FRC

The LVDT is carefully designed with long and slender coils to make the output voltage

essentially linear over a wide displacement that can be several hundred millimeters

(several inches) long.

The LVDT can be used as an absolute position sensor. Even if the power is switched

off, on restarting it, the LVDT shows the same measurement, and no positional

information is lost. Its biggest advantages are repeatability and reproducibility once it is

properly configured. Also, apart from the uniaxial linear motion of the core, any other

movements such as the rotation of the core around the axis will not affect its

measurements.

Because the sliding core does not touch the inside of the tube, it can move without

friction, making the LVDT a highly reliable device. The absence of any sliding or

rotating contacts allows the LVDT to be completely sealed against the environment.

Different number of LVDTs were used in the experimental program, due to the

specimen used, and to the type of test adopted.

For the panels, a total of six LVDTs were used on each face (two in the x-direction, two

in the y-direction, one in the horizontal direction and one in the vertical direction).

These have a stroke of ±15 mm or ±24 mm based on availability in the laboratories.

The number of LVDTs used for the tests was chosen from the thesis of Jimmy Susetyo,

that used the same number: two in the x-direction, two along the y-direction and one for

each diagonal of the panel, to have the correct measurement of displacement and strains

of the specimen.

4.4 LED Target

The following points are about the LED targets and the Metris K610 3D camera, used

during the experimental tests of the panels.

The optical measuring CMM is available in a portable and a mobile configuration. As

such it is the ideal tool for on-site measurement and troubleshooting task for

applications requiring a large measuring volume.

Systems can be also integrated in existing metrology rooms by fixing the camera to the

ceiling, wall or space frame. 132

4.

. EXPERIMENTS ON FRC

The K-Scan

Scan MMDx is a walk-around

walk around scanning solution combining the digital Model

Maker MMDx laser scanner with the portable K-series

K series optical CMM. K-Scan

K is ideally

suited for on-site

site 3D digitizing tasks requiring minimum se

tup and fast results.

Operating the scanner with a laser stripe width up to 200 mm is easy and efficient. The

dense point clouds that are acquired can be graphically analyzed in Focus software, or

3

in party packages. Figure 4.4: Setting Camera.

The initial alignment of a work piece is monitored by 3 LEDs mounted directly on the

part. The camera-to-

-part

part position is constantly monitored and updated, avoiding the

need for leap-frogging.

frogging. As such it is possible to measure objects on vibrating work floor

or parts that move.

K-Series cameras,

, as the one used in the experiment tests, measure the position of

infrared LEDs by means of linear CCD cameras. Through triangulation, the 3D position

of each LED is calculated. 9 LEDs are built into the handheld SpaceProbe, an

ergonomically designed device that enables an inspector to measure the actual 3D data

of an inspection part in single point or scanning mode. The optical measuring CMMs

are available in a portable

portab and a mobile configuration.

The following Figure 4.5

4. shows how the Metris Camera works: 133

4. EXPERIMENTS ON FRC

Figure 4.5: Metri K610 3D camera.

When large objects, such as a construction, need to be measured, it is possible to expand

the measurement volume with the walk around reference.

Following Table 4.1 shows the different technical options of the camera used along the

study: Measurement Single Point Volumetric Temperature

Volume Accuracy Accuracy Range

17 m³ Up to 37 µm Up to 60 µm 15-40°C

K610 Table 4.1: Technical Data of the Camera.

In this case of experiment, a total of 19 target were used, 16 as a grid on the specimens

and 3 on the yellow frame of the panel as a reference plane for calculations.

LED target were used in combinations with the 3D camera, because they provide a

measurement of localized strain behavior, so readings were taken along the all test.

The readings from LED targets can be also averages and compared with the LVDT

reading for data verification.

These targets were fixed to the concrete surface of the panel, and the configuration of

the target as well the numbering of them is depicted in following Figure 4.6: 134

4. EXPERIMENTS ON FRC

Figure 4.6: Arrangements of LED Targets for the Panel tests.

LED target were connected to the strobe unit and the Camera that reads the position of

LED targets, and strobe unit were hooked into the computer, to have the correct reading

of the deformations of the panel (the accuracy of the reading is 20 µm).

4.5 Specimens Description

As previously described, four types of different specimens were tested: cylinders, dog

bones, prisms and panels.

At each casting set, one panel was prepared, three dog bones, eight or nine cylinders and

two prisms, with the following strength as shown in the following Table 4.2: 135

4. EXPERIMENTS ON FRC

Number Number Number Number

Set ID of Dog of of of

(MPa) Bones Cylinders Prisms Panels

DC-P1 71.7 3 9 2 1

DC-P2 62.1 3 9 2 1

DC-P3 50.9 3 9 2 1

DC-P4 64.0 3 9 2 1

DC-P5 54.3 3 9 2 1

Table 4.2: List of specimens.

In the following paragraph there are all the indications for all the specimens prepared

and then tested: as said, two types of fibers were used, plastic fibers for DC-P3 and DC-

P5, whereas steel fibers were used for DC-P2 and DC-P4. The first set, DC-P1, was

made of plain concrete, as a parameter control.

4.5.1 Concrete Mixing Procedures

Two different casting procedures were used, for two different test sets: one for the plain

concrete, and one for the Fiber Reinforced Concrete (FRC).

The plain concrete was done to have one specimen for control and to make a

comparison with the FRC.

The mixing process of the plain reinforced concrete was as follows:

1. The coarse aggregate was mixed into the mixer for 30 seconds;

2. All sand and cementitious materials were added to the mix, and the process

continued for 3 minutes;

3. One-third of the required superplasticizers (150mL) and half of the required

water were added to the mix. The mixing process continued for three minutes;

4. The mixing process was stopped for two minutes; 136

4. EXPERIMENTS ON FRC

5. One-third of the required superplasticizers (150mL) and one-quarter of the

required water was added and the mixing process continued for other two

minutes;

6. The remaining superplasticizer (150mL), retarder (~75mL) and water was added

and the mixing process continued for two minutes;

7. The consistency of the concrete was then observed and, if required, small

amounts of superplasticizer and water were added and mixed (50mL of

superplasticizer and 1.0L of water were added);

8. The concrete mix was then loaded into a wheelbarrow and the specimens were

cast.

Differently from the plain concrete, the mixing process of the fiber reinforced concrete

was as follows:

1. The coarse aggregate was mixed into the mixer for 30 seconds;

2. All sand and cementitious materials were added to the mix and the process

continued for 3 minutes;

3. One-third of the required superplasticizers (150mL) and half of the required

water were added to the mix. The mixing process continued for three minutes;

4. One-quarter of the required water was added and the mixing process continued

for another minute;

5. The mixing process was stopped for one minute;

6. One-eighth of the water was added and the mixing process continued for another

minute;

7. The remaining superplasticizers (300 - 400 mL) and one-eighth of the water

were added to the mix, and the compost was mixed for another minute;

8. The consistency of the concrete was then observed and 50mL of

superplasticizer, and 1.0L of water were added and mixed for one minute;

9. The fibers were then gradually added the concrete mix. The mixing process

continued until it was observed that all fibers had been uniformly dispersed in

the concrete mix (two minutes); 137

4. EXPERIMENTS ON FRC

10. The concrete mix was then loaded into a wheelbarrow and the specimens were

cast;

11. Moist curing using plastic covered wet burlap for 7 days for the panels,

dogbones and modulus of rupture prisms and 1 day for cylinder;

12. The cylinders were then demoulded and placed in curing chamber at 100% RH

for 7 days

4.5.2 Cylinders

The purpose of the cylinder compression test was to evaluate the compressive strength

and compressive behavior of the Fiber Reinforced Concrete (FRC). For this purpose, at

each casting stage, eight or nine specimen were prepared.

The diameter and the height of the cylinders were 150 mm and 300 mm, respectively.

Cylinders were stored in a curing room (a room with a 100% relative humidity) for 7

days and left in ambient temperature for the rest to the days until their testing: they were

tested on 7 days, 28 days and on panel test day (if not sufficiently close to 28 days)

Following Figure 4.7 shows a concrete cylinder being tested under compressive load:

the cylinder compression tests were conducted using a MTS testing machine that used

the displacement transducers of the machine as a means of controlling test.

The maximum capacity of the machine was 4,500 kN (1,000 kips): two Linear Variable

Differential Transducers (LVDTs) were affixed to the opposing sides of the cylinders to

continuously measure concrete strains during the tests, and the gauge length of these

LVDTs was 250 mm. 138

4. EXPERIMENTS ON FRC

Figure 4.7: Cylinder compression test.

4.5.2.1 Casting Procedure

A pre-casting procedure was done, to prepare all the moulds that were needed to prepare

the concrete cylinders: nine 6” cylinder moulds and lids cleaned with water and, after

drying, were oiled using old hydraulic to allow for an easier demoulding.

After that the pre-casting has been done, cast took place with 6 steps as follows:

1. The concrete was poured into the moulds in 3 lifts, tamping each lift with a

tamping rod 25 times in a circular pattern. If the concrete was FRC, a small

form vibrator was used instead tamping;

2. Tamping rod was used to bang the side of the mould and the top was finished

using trowel;

3. The cover was oiled and placed on top of the finished specimen; 139

4. EXPERIMENTS ON FRC

4. The moulded cylinders were covered with wet burlap and plastic and left to cure

for one day;

5. On the next day, the cylinders were demoulded and then placed in a curing room

at 100% RH for the next 6 days;

8

6. On the day after casting the cylinders were removed from the curing room

and left to cure in ambient conditions.

Following Figure 4.8, Figure 4.9 and Figure 4.10 show different steps of casting the

concrete cylinders: Figure 4.8: Second step of casting.

Figure 4.9: Filling Concrete in moulds. 140

4. EXPERIMENTS ON FRC

Figure 4.10: Mould ready for curing.

4.5.2.2 Test Procedure

8

Not before the day after casting, the ends of the cylinders were ground flat using a

cylinder grinder (plain concrete) or the ends were cut (FRC). In both cases, about 10

mm were removed.

If the cylinder was saw cut (FRC), the ends were hooded using a sulfur capping

compound to create a level surface. The hood was typically 5 - 7 mm in thickness.

Cylinders were tested on 7 days, 28 days and on panel test day (if not sufficiently close

to 28 days).

7 day cylinders were tested using a Forney load-controlled testing machine, just to

ensure that the compressive strength of the concrete seemed to be as expected.

For 28 day and test day cylinders, the full stress-strain curve was obtained using the

MTS machine.

This was done using a 250 mm gauge length LVDT mounting rig that was centered on

the specimen with the help of a ruler.

The specimen was centered under the cross head; LVDTs were then mounted into the

rig and zeroed with a stroke of ±5mm. 141

4. EXPERIMENTS ON FRC

The test then commenced at a loading rate of 0.005 mm/sec, until the maximum stroke

of the LVDTs was reached. Pictures were taken periodically throughout the course of

the test.

Following Figure 4.11 shows a Cylinder specimen under testing:

Figure 4.11: Cylinder specimen under testing.

4.5.3 Dog Bones

As discussed in previous chapters, adding fibers into a concrete brittle matrix has been

found to increase the tensile strength of concrete and to improve the post-cracking

behavior, toughness and ductility of the usual brittle concrete matrix.

It was expected that the uniaxial tension tests would aid in determining the tensile

strength, tensile stress-strain relationship and the tensile stress-crack width relationship

of FRC such that these relationship could be implemented to model the behavior of

FRC.

A standard specimen has not been formulated for the uniaxial tension tests: RILEM

Technical Committee 162 (2001) has developed a recommendation for the uniaxial

142

4. EXPERIMENTS ON FRC

tensile test on the steel fiber reinforced concrete (SFRC). Their recommendation

involves testing a notched SFRC cylinder with a nominal cross sectional diameter of

150 mm and a circumferential notch with a dept and a width of 15 mm and 2-5 mm,

respectively. Following the procedures of Jimmy Susetyo, it was decided to use an

unnotched dog-bone shaped specimen instead of the recommended notched cylinder

specimen, in this experimental program.

Reasons are as follows: as previously discusses, the crack in a notched specimen will be

forced to occur at the notch, due to the stress concentration and the reduction of the

cross-sectional area at the notch. In an unnotched specimen with a uniform cross

section, cracks would develop at the weakest point along the length of the specimen.

However, bond failure related problems due to stress concentrations at the interface

between the concrete and the steel loading plate are common. The use of an unnotched

dog-bone shaped specimen allow the development of cracks at the weakest points, along

the length of the specimen, while minimizing the possibility of bond failure between the

steel loading plate and the concrete specimen by lowering the stress at the interface.

Dimensions and details of the dog-bone shaped specimen used in the experimental

program are given in the following Figure 4.12:

Figure 4.12: Uniaxial Tension Test. 143

4. EXPERIMENTS ON FRC

Dimension of midsection of the specimen were 100 x 70 mm, resulting in an effective

area of 7000 mm². The dimension were chosen such that some degree of random fiber

orientation within the specimen could be achieved while maintaining portability of the

specimen and resemblance to the condition in the concrete panel specimens, as done in

the thesis of Jimmy Susetyo.

Chosen of 70 mm dimension was done to reflect the thickness of the panel, following

the experiment done by Jimmy Susetyo, whereas the 100 mm dimension was chosen to

ensure the specimen portability while allowing some degree of random orientation.

The recommended minimum dimension for fiber reinforced concrete was at least three

times the fiber length (ASTM C1018, 1997).

A 50x50 MW9.1/9.1 wire mesh was also embedded in the end regions of the specimens,

as shown in the following Figure 4.13: this was done to ensure that the cracks would

occur in the middle portion of the specimens.

Figure 4.13: Wire mesh in the end region of the uniaxial tension test specimens. 144

4. EXPERIMENTS ON FRC

4.5.3.1 Casting Procedure

A pre-casting procedure was done, to prepare all the moulds that were needed for the

concrete cylinders, and it was made of eight steps.

The most difference following the thesis of Jimmy Susetyo are done by the point 3, 4, 5

and 6:

1. Three dog-bone forms were cleaned and assembled with a particular attention to

the dimension, to be sure that all is correct;

2. A ¾” threaded rod was the screwed into the end blocks such that the rod reached

65 mm into the specimen, with a nut on the outside to hold the rod in place;

3. Using a ruler, the threaded rods were verified to be in line with each other and in

the centre of the concrete specimen (in terms of height and width inside the

form);

4. A nut was placed on the end of the rod inside the form, to help with stress

transfer;

5. The forms were then oiled, and no oil was placed on the threaded rod;

6. Two wire meshes for each end of the dogbones were cut to the correct size,

meant to provide some reinforcement for the end large cross-section of the

specimens.

Once that the pre-casting procedure has been done, the casting was made by the

following steps:

1. Pour in 3 lifts, using a form vibrator to ensure good distribution each time;

2. At the end of vibration for each lift, one of the wire meshes was placed into the

end regions of the dog-bone (as showed in previous Figure 4.13). The wire

mesh was placed under the threaded rod for the first lift, and on top of the

threaded rod for the second lift;

3. Once full, the form was vibrated once more and the top surface was finished

using a trowel. An attempt was made to ensure that any fibers around the edge

of the specimen were removed to prevent sharp edges once the concrete had

cured;

4. The form was then covered with wet burlap and plastic and let cure one day; 145


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DETTAGLI
Corso di laurea: Corso di laurea magistrale in ingegneria edile-architettura (a ciclo unico di durata quinquennale)
SSD:
Università: Brescia - Unibs
A.A.: 2013-2014

I contenuti di questa pagina costituiscono rielaborazioni personali del Publisher ale.baselli di informazioni apprese con la frequenza delle lezioni di Tecnica delle costruzioni e studio autonomo di eventuali libri di riferimento in preparazione dell'esame finale o della tesi. Non devono intendersi come materiale ufficiale dell'università Brescia - Unibs o del prof Minelli Fausto.

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