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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 MTS machine was equipped with two sets of universal joints: one set was bolted to

the actuator, and the other was bolted to the loads cell while the free ends of the joints

were threaded to allow a simple connection between the specimen and the machine. The

specimen was instrumented with a set of four Linear Variable Differential Transducers

(LVDTs), one for each side of the specimens. The LVDTs provided continuous

measurement of the displacement over the gauge length throughout the test.

(a) (b)

(c

c) (d

(d)

Figure 4.15: Uniaxial Tension Specimen used in experimental tests: (a) Specimen

dimension; (b)LVDTs configuration used; (c) Susetyo end-blocks;

end blocks; (d) experimental end-

end

blocks.

Jimmy Susetyo used three different types of LVDTs configurations, shown in the

previous Figure 4.14 and due to difficulties encountered along his studies, it was chosen

to adopt just only one of these: difficulties were related to cracks occurring outside the

147

4.

. EXPERIMENTS ON FRC

gauge length. So, working on Jimmy Susetyo’s experience,

experience

, one of three LVDTs set

configuration was used as shown in the previous Figure 4.15.

Jimmy Susetyo also adopted end-blocks

end blocks to transmit the load from the machine to the

t

specimen, gluing the steel end blocks to the specimen shape; in this experimental

programs, steel end-blocks

blocks were not used. As previous view, a ¾” threaded rod was the

screwed into the end blocks ending 65 mm into the specimen,

, with a rod on the outside

outsid

of the form to hold the rod in place: this solution was found to be more appreciable (and

easier) than the use of the steel-end

steel blocks.

Unfortunately, at the beginning of the test, a bad failure was obtained outside from the

LVDTs on the larger part of the specimen, so non-measurable;

measurable; it was then decided to

use experimental end-blocks:

end blocks: steel plates that permit the correct transmission of the load

to the specimen, without primary failure. This experimental procedure was new: steel

plate at the end of the specimen, as showed in the previous Figure 4.14,

4. and detailed in

the previous Figure 4.15,

4. , were connected to round cylinders where the dob-boned

dob

change

ge width, with two rods per size and it was found to be a good way to transmit the

load as shown in Figure 4.16

4. and in Figure 4.17:

Figure 4.16:

4. Detailed view of the experimental end-blocks.

end- 148

4.

. EXPERIMENTS ON FRC

Figure 4.17:

4. Detailed view of the experimental end-blocks.

end-

The ideal configuration of Susetyo’s LVDTs set, was the configuration 1, in which the

only displacement monitored was that in the mid-section

mid section of the specimens, where the

strain distribution was uniform. However, it was found by Susetyo that, in some

specimen,

men, cracks occurred outside the mid-150

mid 150 mm observation region, preventing data

collection of the post-

-cracking behavior.

So LVDTs configuration 2 and 3 were introduced to overcome this difficulty.

The preparation of the specimens was made by following the

th e next steps:

1. Instrumentation locations were marked off using a ruler: lines were drawn for

the instrumentation mounts, to be sure that it could be installed in the direct

centre of each face of the specimen, with a gauge length of 300 mm or 150 mm;

2. The specimens

ecimens were then painted;

3. Instrumentation mounts were then epoxied on to dogbones using a rig designed

to ensure the correct gauge length is maintained.

While the procedures to tests the specimens was as follows:

1. The dog-bone

bone was loaded into the MTS machine; 149

4. EXPERIMENTS ON FRC

2. LVDTs with a stroke of ±5 mm were then attached to the mounts and set to 3

mm (so as to allow 8mm of stroke in the LVDTs). The LVDTs were then

zeroed;

3. Loading was then commenced at a rate of 0.001 mm/s;

4. The test was run without changes until first cracking and then until the

maximum load post-cracking was achieved;

5. At this point, the loading rate was gradually increased to a maximum of

0.01mm/sec;

6. If the LVDTs approached saturation, they were reset;

7. At around 20% of the maximum post-cracking load, instrumentation was

removed;

8. Fibers across the main failure crack were then counted and catalogued. Cross-

sectional dimensions adjacent to the failure crack were measured.

4.5.4 Prisms

Prisms were made to investigate, with the bending tests, the tensile behavior of FRC

when subjected to bending moments and to evaluate the flexural toughness parameters

of the FRC.

As for the dog-bones, it was possible to make notched and unnotched specimen; the use

of an unnotched specimen, as done in the experiment tests, enables the cracks to occur

at the weakest point along the region of the constant moment.

Two different type of tests can be done to observe the behavior of this type of test: the

ASTM C1018 Standard Test, using a non-slotted beam under four-point loading and the

RILEM Technical Committee 162 (RILEM TC162), using a notched beam under three-

point loading.

The presence of a notch in the specimen, however, forces the crack to occur at the

notch: this simplifies the observation of the crack development and enables the

measurement of the crack mouth opening displacement. This weakness in the form of a

150

4. EXPERIMENTS ON FRC

notch introduced in the specimen may not reflect the actual flexural strength of the

concrete.

So, the use of an unnotched specimen was chosen, although the crack mouth opening

displacement cannot be measured in the ASTM test because the location of the cracks is

unknown prior to their occurrence.

Moreover, the ASTM C1609 tests evaluate the flexural performance using toughness

indices, calculated from the load-deflection curve as the ratio of the area under the load-

deflection curve up to a certain prescribed deflection to the area under the load-

deflection curve up to the first-crack deflection.

The experimental program uses a 152 x 152 x 533 mm (6 x 6 x 21 in) beam specimen.

Figure 4.18 shows the loading arrangement of the specimens used in the ASTM C1609

tests: loading points were at 152 mm from the supports.

Figure 4.18: Loading arrangement of the ASTM C1609 tests.

Figure 4.19 shows the dimension of the specimens used in the ASTM C1609 tests: 151

4.

. EXPERIMENTS ON FRC

Figure 4.19: The ASTM C1609 test specimen: (a) Specimen dimension; (b)

Instrumentation.

4.5.4.1 Casting

asting Procedure

The instrumentation for these tests consisted of a set of two Linear Variable Differential

Transducers (LVDTs), one for each side of the specimen at the mid-span,

mid as indicated in

previous Figure 4.19.

.

The preparation of the specimens was made by finding two modulus of rupture prism

steel forms, cleaning them and then covering them with oil.

Once that the pre-casting

casting has been done, the following

f

ollowing steps of casting were done as:

152

4. EXPERIMENTS ON FRC

1. The specimens were poured in 2 lifts, and were vibrated with a form vibrator

between each lift to ensure good consolidation;

2. Excess fibers sticking out of the edges of the top surface were removed to

eliminate sharp dangerous edges after the concrete had cured;

3. The specimens were covered with wet burlap and plastic and let cure one day in

form;

4. On the next day, specimens were demoulded and again covered with wet burlap

and plastic and let cure for 6 more days;

5. The burlap and plastic were removed and the specimens were and left to cure in

ambient conditions afterwards.

4.5.4.2 Test Procedure

These specimens were prepared to be tested as follows:

1. Locations of 4 point loading were marked on all 4 sides of the specimen;

2. Along the line of the support points, the centre height was marked on the front

and the back. This is the location where the testing rig (for the LVDTs) was to

be attached to the specimen;

3. Prisms were then painted;

4. The midspan of the prism was then found and two instrumentation mounts (one

in the front and one in the back) were epoxied to the specimen as reaction points

for the LVDTs.

The tests procedure was made by the following steps:

1. The specimen was loaded into the MTS1000 test machine;

2. The testing rig was attached to the prism;

3. Using a level and a ruler, adjustments were made to ensure the testing rig was

centered and leveled;

4. LVDTs with a stroke of ±8 mm were mounted onto the rig and set to a value of

4 mm, to allow for a total of 12 mm of stroke for the test (the specification

requires that the test is terminated after a deflection of L/60 is achieved. This is

7.5 mm for these specimens); 153

4. EXPERIMENTS ON FRC

5. The cross head was lowered to ensure the prism is in correct location (centered

in the machine and on the supports);

6. Loading was then started at a rate of 0.006 mm/s;

7. The test continued until peak, taking pictures periodically. The test was

continued until at least the 7.5 mm midspan deflection was achieved. Loading

rate was increased gradually after peak up until a maximum of 0.02 mm/sec;

8. The specimen was removed from the machine, and the cross sectional

dimensions adjacent to the failure fracture were measured.

4.5.5 Panels

Panel tests were conducted to investigate the behavior of fiber reinforced concrete

(FRC) members under in-plane shear loading, and to compare their behavior to

conventionally reinforced concrete (RC) members.

Panel tests were necessary, in addition to uniaxial tension tests, to better understand the

overall behavior of FRC members: uniaxial tension tests are primarily conducted to

investigate the tensile behavior of the concrete and are usually done with a smaller scale

specimen, which hinder to use of conventional steel reinforcement commonly found in

reinforced concrete structures. By conducting tests on a larger scale, conventional steel

reinforcement can be incorporated and the interaction between overall understanding of

the behavior of FRC can be obtained.

Tests were conducted using the Panel Tester Machine at the University of Toronto,

which was developed by Vecchio (1979): it was designed to apply various in-plane

loading conditions to an 890 x 890 x 70 mm concrete panel.

Two types of panel configurations were used in fabricating the 890 x 890 x 70 mm

concrete panels: the first configuration was used for the control panels, in which

conventional steel reinforcement was provided in both the x- and y-directions.

The second configuration was used for FRC panels, in which conventional steel was

provided only in the x-direction.

Following Figure 4.20 shows the design of the control panels: forty D8 deformed wires

were provided for the x-direction reinforcement, giving a total reinforcement area of

2063 mm², which equated to a reinforcement ratio of 3.31%. In the y-direction, ten D4

154

4.

. EXPERIMENTS ON FRC

deformed wires with a total reinforcement area of 260 mm² were provided, which was

equal to a reinforcement ratio of 0.42%. The reinforcement was connected to the shear

keys

ys by splicing them with a 5/16” threaded rods that were bolted to the shear keys.

The x-direction

direction reinforcement ratio was chosen, as done in the thesis of Jimmy Susetyo,

to provide an adequate post-cracking

post cracking resistance of the panel: a premature failure may be

damage all the experiments, soon after cracking. Therefore, a significant reinforcement

in the x-direction

direction was needed: 3.31% was a reinforcement ratio sufficient to guarantee

an adequate post-cracking

cracking resistance. Note: all dimensions in mm

Figure 4.20: Design of the control panel. 155

4.

. EXPERIMENTS ON FRC

About the reinforcement in the y-direction,

y direction, was higher than the required 0.2% minimum

prescribed by CSA A23.3-04

A23.3 04 (2004) but it was chosen to follow the correct and exactly

procedure that Susetyo

yo did in his experimental program.

The FRC panels were similar to the control panel, but there was an exception of the y-

y

direction reinforcement, as shown in the following Figure 4.21:

: no wires were adopted

along the y-direction

direction and the load applied was transmitted to the specimen with 5/16”

threaded rods, bolted to the shear keys. Note: all dimensions in mm

Figure 4.21:

4. Design of the FRC panels and shear keys used. 156

4. EXPERIMENTS ON FRC

The same splicing was used to connect the threaded rods to the x-direction

reinforcement.

Same 40-D8 deformed wires were used for the x-direction reinforcement, which

correspond to a total reinforcement area of 2062 mm², and a reinforcement ratio of

3.31% (as in the control panel).

Nuts and washers were attached to the ends of the threaded rods in the y-direction to act

as a mechanical anchorage, in order to enhance the connection resistance.

4.5.5.1 Casting Procedure

The first part to cast a complete panel, is the construction:

1. Reinforcing bars were cut to the appropriate lengths (1050 mm for full

reinforcing bars (D8 or D4) and alternating 225 or 255 mm for long and short

dowels respectively);

2. End of all bars were ground, polished and threaded;

3. The bars were then wiped to remove any dirty parts;

4. The casting plate was cleaned and oiled;

5. 20 shear keys were then placed on the table, aligned and bolted securely into

place;

6. Each bar was locked to the end of the shear keys using a double nut connection;

7. Holes gaps between shear keys were filled with plasticize to complete the form.

Once that the mould is completed, the casting take place:

1. The concrete was poured in 3 lifts, vibrating each lift with the form vibrators

attached to the casting table;

2. Tamping rods or vibrators were used to aid in concrete consolidation,

particularly around the teeth of the shear keys;

3. The top surface was finished using a piece of 2 x 4 and a trowel;

4. The panel was covered with wet burlap and plastic and let cure for 3-4 days on

the table;

5. At this time, the panel was removed from the table and placed on a pallet, still

8 day after casting;

wrapped in burlap and plastic until the 157

4. EXPERIMENTS ON FRC

6. At this point, the burlap and plastic were removed and the specimen was left to

cure in ambient conditions afterwards.

Following pictures Figure 4.23, Figure 4.24, Figure 4.25, Figure 4.26 and Figure 4.27

shows different steps of casting panels DC-P5:

Figure 4.22: Steps of casting panels DC-P5.

Figure 4.23: Steps of casting panels DC-P5. 158

4. EXPERIMENTS ON FRC

Figure 4.24: Steps of casting panels DC-P5.

Figure 4.25: Steps of casting panels DC-P5. 159

4. EXPERIMENTS ON FRC

Figure 4.26: Steps of casting panels DC-P5.

Figure 4.27: Steps of casting panels DC-P5.

4.5.5.2 Test Procedure

Following steps talk about the test preparation and the installation of the strumentation

on the panel, and of the panel on the machine element tester:

1. First, on the back face (finished face), the LED grid and LVDT mounting points

were marked;

2. Then, 3/16” pilot holes were drilled into the specimen at locations of LVDT

mounts;

3. The panel was painted using one part white paint and one part water;

4. Using epoxy, 10-32 threaded rods were then fastened into the holes. These rods

were later used to screw on the LVDT mounts;

5. Panel was flipped over so that the front face (formed face) was now facing up.;

6. Steps 1-4 were repeated, but the LED grid was not needed on the front face;

7. The rigid links were first extended and bolted onto the panel to ensure correct

positioning of the specimen;

8. The hydraulic cylinders were extended two at a time (the horizontal and vertical

jack that meet at the same shear key) and bolted to the panel with a nut and

washer;

9. Instrumentation was applied to the panel; 160

4.

. EXPERIMENTS ON FRC

10. LVDTs were installed into the mounts and set as close to 0 mm as possible.

Extension rods were applied using

usi ng shrink tubing and a heat gun;

11. LED targets were hot-glued

hot glued to the specimen surface and the wires

w were

connected to the strober box;

12. All necessary hoses and pressure lines

l ines were hooked up and checked;

13. The load maintainer was used to apply 500 psi of biaxial tension to the specimen

in the machine to remove any out of straightness in the system;

14. The out of plane tension rods were attached to the specimen, tightening to the

end of the bolts at the panel side and then being loosely attached

at to the yellow

back frame;

essure was let off the specimen;

15. The pressure

16. One last check was performed to ensure all systems

system s were working correctly.

Following Figure 4.28

28 shows how was the disposition of LVDTs instrumentation used

for these experiments, on the panel:

Figure 4.28:

4. Configuration of the LVDTs for panel tests.

Testing

esting requires the following steps: 161


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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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