Experimental study on fiber-reinforced concrete panels subjected to shear , Materials and Behaviours

Workability of concrete is mainly affected by consistency i.e. wetter mixes will be more

workable than drier mixes, but concrete of the same consistency may vary in

workability; it is also used to determine consistency between individual batches.

The test is popular due to the simplicity of apparatus used and simple procedure.

Unfortunately, the simplicity of the test often allows a wide variability in the manner

that the test is performed. The slump test is used to ensure uniformity for different

batches of similar concrete under field conditions, and to ascertain the effects of

plasticizers on their introduction.

According to European Standard EN 206-1:2000 five classes of slump have been

designated, as tabulated in the Table 3.5 below:

Slump class Slump in mm

S1 10 - 40

S2 50 - 90

S3 100 - 150

S4 160-210

S5 ≥220

Table 3.5: European Classes of Slump.

The slump test is referred to several testing and building codes, with minor differences

in the details of performing the test. 82

3. MATERIALS and BEHAVIORS

In the United States, engineers use the ASTM standards and AASHTO specifications

when referring to the concrete slump test: American standards explicitly state that the

slump cone should have a height of 0,30 m (12-in), a bottom diameter of 0,20 m (8-in)

and an upper diameter of 4-in. The ASTM standards also state in the procedure that

when the cone is removed, it should be lifted up vertically, without any rotational

movement at all. The concrete slump test is known as "Standard Test Method for Slump

of Hydraulic-Cement Concrete" and carries the code (ASTM C 143) or (AASHTO T

119).

In the United Kingdom and in mainland Europe, the standards specify a slump cone

height of 300 mm, a bottom diameter of 200 mm and a top diameter of 100 mm. The

British Standards do not explicitly specify that the cone should only be lifted vertically.

The slump test in the British standards was first (BS 1881–102) and is now replaced by

the European Standard (BS EN 12350-2). The test should be carried out by filling the

slump cone in three equal layers with the mixture being tamped down 25 times for each

layer.

In this experimental program, slump test is shown in the Table 3.6:

Type Slump

⁄

Batch of Test

ID (MPa) (%) Fibers [mm]

DC-P1 71.7 0.0 - - 70

DC-P2 62.1 1.0 79 Steel 200

DC-P3 50.9 2.0 67 MS 70

DC-P4 64.0 1.0 79 Steel 136

DC-P5 54.3 2.0 67 MS 162

Table 3.6: Workability data for each test set.

As Table 3.6 shown the results obtained, it is possible to see that there are many

differences between test slump in the plain concrete and in the fiber reinforced concrete,

probably obtained from the consistency of the concrete, different damp of the contents,

different dimension of the aggregates composing the material, humidity in the air, .. 83

3. MATERIALS and BEHAVIORS

So many factors can influence the result of the slump test, but the main difference is

between the steel fibers and the macro-synthetic one: it can be plausible that probably

one influence come from the percentage of fibers in the matrix.

The following pictures shows the different types of slump test done in lab:

Figure 3.4: DC-P1 Slump test.

Figure 3.5: DC-P2 Slump test.

Macro-synthetic fibers are not very dense, so they float to the top of the concrete. They

are also very longer and harder to move around then the steel one. 84

3. MATERIALS and BEHAVIORS

Figure 3.6: DC-P3 Slump test.

Figure 3.7: DC-P4 Slump test.

Figure 3.8: DC-P5 Slump test. 85

3. MATERIALS and BEHAVIORS

3.2.3 Reinforcing Steel

Reinforcing steel was provided and D4 and D8 types of reinforcing steel were used only

for the shear panel specimens: deformed wires, with no transverse reinforcing steel for

FRC panels. Steel coupon tests were conducted on the wires and the results are

presented on the following Table 3.7:

#

!" # %

!"

$ $ &

(x ) (MPa) (x ) (MPa) (MPa)

2.67 484.3 22.71 624.4 183,850

D4 2.43 466.4 39.64 605.4 192,515

D8 Table 3.7: Description of Reinforcing Steel Bars.

These types of bars were chosen from a different types and measure, to continue the

work from Jimmy Susetyo, and to have the same steel wires. So this steel wires were

chosen D8 on x-direction with the percentage of 3.31%, to prevent the yielding before

the crack in the concrete, and D4 with a percentage of 0.41% to have a low shear

transfer reinforcement ratio on the plain concrete, whereas on FRC panels there are steel

wires only in the longitudinal direction, as the desire was to compare the behavior and

failure obtained using low amount of transverse conventional steel reinforcement to that

of SFRC and MSNFRC. Following Figure 3.9 shows the reinforcement steel wires used

during casting the panels:

Figure 3.9: Steel Reinforcement wires. 86

3. MATERIALS and BEHAVIORS

These steel bars were cut from a long wire (generally rebar are fabricated from a hot-

rolled mild steel), and 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).

Ends of all bars were ground and polished to remove sharp edges and approximately 50

mm on each end of each deformed wire bar was threaded using either an M8 dye (D8

bars) or M6 dye (D4 bars).

The bars were then wiped with acetone to remove any oil or dirt.

3.2.4 Fibers

Fibers are added to the concrete matrix to give better tensile behavior, crack control

characteristic of the concrete, post-peak compressive response, post-cracking tensile

response, toughness and ductility of concrete than in the plain concrete.

There are several properties that fibers should have.

First, fibers should have a much higher tensile strength than the concrete matrix to be

able to carry the tensile stress that member is subjected to, without an immediate brittle

fiber fracture after the concrete has been cracked.

Second, fibers should be able to carry a high tensile strain in order to provide significant

toughness: high tensile strain will allow the fibers to significantly deform without

fracturing, and this will lead to a high energy absorption capacity of the concrete. This

factor is particularly important in hooked-end steel fibers: a high tensile strain will

ensure the straightening of the hooks during the fiber pullout without fracturing while

these fibers are expected to remain elastic in the straight portion since a stable fiber

bond stress-slip response is required to achieve a satisfactory composite behavior.

Third, fibers should have a higher elastic modulus than the matrix to allow greater

proportion of the applied load to be carried by the fibers prior to cracking and to reduce

the post-cracking composite strains when the fibers carry the entire load.

Fourth, the Poisson’s Ratio of the fibers should be sufficiently low to overcome

problems associated with fiber debonding at the fiber-matrix interface.

Fifth, fibers should be elastic and not prone to creep, to avoid problems related to stress

relation prior to cracking and time dependent strains after cracking. 87

3. MATERIALS and BEHAVIORS

Lastly, fibers must be physically and chemically compatible with the concrete matrix,

which has inherent moist and alkaline characteristics.

In the following Table 3.8 there are shown the material properties of various type of

fibers and fibers that satisfy the mentioned requirements, which are steel fibers and

several of the synthetic fibers. Strain

Diameter Young’s Tensile

Specific at

!"

Fiber Type Modulus Strength

(x Gravity Failure

(MPa) (MPa)

mm) (%)

Steel 101.6 - 7.8 200,000 345 – 1,725 3.5

101.6

High tensile 10.2 - 7.8 160,000 2,070 5.0

Stainless 330.2

Glass

E 10.2 2.50 72,000 3,450 4.8

alkaliresistant 12.7 2.70 80,000 2,480 3.6

Synthetic

Polypropylene

Monofilament 101.6 - 0.90 5,000 450 18

203.2

Fibrillated 508 - 0.90 3,450 550 - 760 8

4,064

Polyethylene 25.4 - 0.96 5,000 - 200 - 3,000 3 - 80

1,016 172,400

Polyester 10.2 - 1.38 10,000 - 550 - 1,170 10 - 50

76.2 17,200

Acrilic 5.1 - 17.8 1.18 18,000 205 - 1,000 28 – 50

Nylon 0.90 1.14 5,170 965 20

Aramid

Kevlar 29 11.9 1.44 62,000 3,620 3.6

Kevlar 49 10.2 1.44 117,200 3,620 2.5

88

3. MATERIALS and BEHAVIORS

Carbon

I (high 0.5 -

7.6 1.90 380,000 1,790

modulus) 0.7

III (high 1.0 -

8.9 1.90 230,300 2,620

strength) 1.5

Natural 20.3 - 1.50 10,000 - 305 - 905 -

Wood 119.4 40,000

cellulose

Sisal < 203.2 - 13,000 - 280 - 565 3 – 5

26,000

Coir 101.6 - 1.12 - 19,000 - 115 - 200 10 – 25

406.4 1.15 26,000

Bamboo 50.8 - 1.50 33,000 - 350 - 505 -

406.4 40,000

Elephant grass 431.8 - 4,940 180 3.6

Table 3.8: Selected fiber types and properties (Daniel, 1991)-

Natural fibers are excluded from research of structures because their relatively low

modulus of elasticity and tensile strength, and also because they have a high absorption

rate, making the matrix more subject to shrinkage or swelling, and also subjected to

deterioration due to alkali attach.

Glass fibers are not ascertained about their long-term durability, so they’re not used in

construction; aramid and carbon are too expensive to be used.

Studies are carry forward for polypropylene fibers and for steel fibers, that successfully

reduce both plastic and drying shrinkage, resists to alkalis, moisture and acids, and

they’re relatively inexpensive.

Previous studies found that a fiber volume fraction less than 0.25% was found to not

significantly improve the behavior of the brittle concrete matrix, whereas a fiber volume

fraction higher than 2.0% was deemed to cause a significant reduction in the workability

such that special mixing processes and placing methods were necessary.

The workability decreases with increasing aspect ratio, as shown in Figure 3.10, in

practice it is very difficult to achieve a uniform mix if the aspect ratio is greater than

89

3. MATERIALS and BEHAVIORS

about 100 and, in

n general, the problems of both workability and uniform distribution

increase with increasing fiber

fibe length and volume.

Figure 3.10: Workability versus fiber

fibe content for matrices with different maximum

aggregate sizes..

So the addition of fibers to concrete generally produces improved ultimate tensile

strength, toughness and ductility properties, and provides a good crack control

mechanism.

As indicated previously, fibers

fibe s are added to concrete not to improve the strength, but

primarily to improve the toughness, or energy absorption capacity. Commonly, the

flexural toughness is defined as the area under the complete load-deflection

load curve in

flexure; this is sometimes referred to as the total energy to fracture. Alternatively, the

toughness may be defined as the area under the load-deflection

load deflection curve out to some

particular deflection, or out to the point a